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Bainite in Steel | Guide for Upper and Lower Bainite | Bainitic Transformation

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Bainite in Steel is a platelike microstructure formed between 150-500oC. This microstructure is explained by edger Bain and requires controlled cooling in-between pearlite and martensitic formation. The heat treatment process used for bainite formation is termed Austempering. Bainitic steel has several applications in auto and rail parts.

What is Bainite?

Bainite in steel is a platelike non-lamellar mixture of ferrite and cementite formed between 150oC – 450oC. The microstructure is divided into upper bainite and lower bainite with a better combination of mechanical properties. The most common applications of bainite include lightweight car bodies, auto, and rail parts, and more.

“Bainite steel is non-lamellar microstructure opposite to pearlite that evolves when two phases i.e. ferrite and iron carbide grow at different rates”.

Definition of Bainite

History of Bainitic Steel

Bainite in steel intrigues researchers as it provides an amazing combination of mechanical properties without lots of alloying addition. A lot of research has been carried out within the bainitic structure in the 1900s. 

Bain in 1930 started experimenting with the isothermal cooling in-between pearlitic transformation range and martensitic transformation range. Within this region, experiments by Bain revealed acicular, dark etched structures different from martensite and pearlite. 

The structure revealed at that point was similar to the below figure; 

Bainite microstructure

Toriano referred to this microstructure as Austempering structure in 1940. 

Researchers and colleagues from Bain’s lab started calling this dark-etched bainite to commemorate the efforts of Bain in presenting the first bainitic microstructure. 

Later researchers postulated the microstructure as initially flat plates of ferrites nucleates along specific crystallographic planes within austenite.

With ferrite ejecting from carbon, carbide particles are formed within ferrite plates. The formation of cementite and ferrite plates is random and non-lamellar opposite to the lamellar structure of pearlite. 

The high and low range of bainite steel is later on termed as Upper and Lower bainite with a subsequent difference in properties that you will found below.

Morphology and Characteristics – Bainite Microstructure

Pearlitic growth occurs at common nucleation or high energy points like triple points and grain boundaries. This pearlitic transformation involves coupled growth of ferrite and cementite and the composition of both phases compliments each other’s growth. This means extra carbon from ferrite is accommodated within cementite. 

In the case of bainite, the process of ferrite and cementite nucleation and growth takes place inseparable stages. Firstly, ferrite growth takes place and later on, precipitation of carbide takes place. 

We will explain both processes in form of separable stages. Stages are termed as follows;

1. Growth of Ferrite

2. Precipitation of Carbide

Growth of Ferrite

Whether it’s upper or lower side of said phase, it is an aggregate of plates of ferrite that are separated by martensite, untransformed austenite, or cementite. The aggregation of those ferritic plates is termed as sheaves. A single ferritic plate in the sheave is called a sub-set. 

Sub-sets are not isolated rather they are interconnected in 3 dimensions. All sub-sets are crystallographically aligned with each other.

The shape of sheaves is wedge type with a thicker side starting from the austenitic nucleation site. Sub-sets have lathe-type morphology that is more prominent near the austenite site. 

Subsets are nucleated to a specific size and have limited growth. Every new sub-set is nucleated over a previously nucleated one. The exact morphology can be seen below figure;

Lathe type ferrite is the main type of structure within bainitic microstructure where subsets have untransformed austenite and martensite present in between them. This lathe type of ferrite formation is complex proves and involves lots of variables to be processed.

One of the major factors to consider during shape transition within the structure is the yield strength of the parent phase and rapid radial growth of sub-sets.

How is Bainite formed in Eutectoid Steel?

The formation of a bainite plate involves lattice shear resulting in surface distortion mainly surface tilts and resulting accommodations. As opposed to the martensite plate where the formation is fast, bainitic plates forms slowly and continuously and the growth is retarded by the time required for the diffusion process. 

Incubation Period

At the start of the reaction, there is an incubation period where no transformation occurs after which the reaction is initiated.  

Transformation Step

During the transformation, the uniformly dispersed carbon atoms in austenite concentrate in localized regions leaving a carbon-free matrix. It is not an athermal reaction.  

The reaction involves compositional changes and requires the diffusion of carbon at a certain time at the transition temperature. 

The difference in Formation of Upper Bainite and Lower Bainite

In plain carbon steels, diffusion of carbon in austenite provides the required activation energy for the formation of upper bainite, while the diffusion of carbon in ferrite activates the formation of lower bainite.  

The formation of both upper and lower bainite occurs due to successive nucleation of individual plates and the multiple nucleation processes control the growth rate of these plates. Some theories suggest that the relief of transformational strains controls the growth of the bainite phase and the transformation is shear-type transformation.  

The formation of lower bainite is controlled by shear stresses as it was observed that the first ferrite lath formed was supersaturated with carbon much more than the upper bainite ferrite. 

Difference between upper bainite and lower bainite

Upper bainite

Upper bainite is formed at a temperature ranging from 300-500 ͦC. The lath-like ferrite elements are arranged in packets or sheaves with layers of carbides between ferrite plates. The higher temperature permits the excess carbon to partition before it can precipitate in ferrite. The carbide precipitates are formed from austenite grains high in carbon and the ferrite plates are free from any carbide precipitates. The carbide precipitates have around 6.7% carbon. 

Structure Evolution in Upper Bainite

When a bainite lathe grows in upper bainite, the high diffusivity of carbon allows partitioning of carbon between ferrite and austenite, hence, formed the low carbon content (< 0.03%) ferrite, results in the enrichment of carbon in austenite. 

Thus, instead of precipitation inside the laths, carbides having enough carbon precipitate out at the lath boundaries in austenite. In low carbon steels, the carbides are present as discrete particles at the lath boundaries. While in high carbon steels, these carbides form continuous stringers. 

You can see the exact difference in the picture given below, where upper and lower bainite structures are discussed in combination. 

what is the difference between pearlite and bainite microstructure?

The structure of upper bainite is quite fine and resembles pearlite, however, in pearlite alternate plates of ferrite and carbides are formed while in upper bainite, pockets of carbide precipitates are formed by the rejection of excess carbon by ferrite in between ferrite plates.

Lower Bainite   

In lower bainite formed at a temperature ranging from300-500 ͦC, the structure is relatively coarse and carbide precipitates have a hexagonal structure having high carbon ~ 8.4%. As opposed to upper bainite, lower bainite has two kinds of carbide precipitates.  

Due to reduce transformation temperature the diffusion is slower providing enough time for carbon o precipitate inside supersaturated ferrite. Some of the carbides precipitate out from carbon-rich austenite in between the ferrite plates also the same as upper bainite. 

Bainite VS Martensite

Difference between Lower Bainite and Martensite

The structure of lower bainite resembles martensite but the studies show that the carbides in lower bainite have the same crystallographic orientation while in the tempering of martensite variants of cementite with different crystallographic orientation are formed. 

Structure evolution in Lower Bainite

The ferrite forms as plates not lath and have a broader structure and have a higher dislocation density. When the ferrite plates nucleate at austenite grain boundaries, secondary plates are also formed within grains from primary plates. The carbides are precipitated out within the ferrite plates during the transformation. 

The carbides in lower bainite have a rod or blade-like morphology and are aligned almost parallel to each other. As the reaction continues the precipitation and consequent, lowering of carbon content in austenite provides a driving force for transformation. 

What quenching method produces 100% bainite?

The process uses to achieve a fully bainitic structure in steels is called austempering. It is an isothermal heat treatment process different from conventional heat treatment where a part is heated above 843°C and then quenched in oil or water at room temperature.

Heat treatment defects may occur when we quench steel from austenitizing temperature, we have provided common remedies for those defects.

In austempering, the part is heated to a temperature of 843°C to 954°C and then quenched in a molten salt bath kept at 232°C to 399°C. In a salt bath, heat is transferred by conduction combined with convection, which results in the formation of a fully bainitic structure throughout the whole section thickness. The bainitic transformation temperature is higher than the martensite start (Ms) temperature and hence the temperature of the salt bath is higher than the Ms temperature. 

TTT Diagram for bainite - Austempering process for bainite - bainite formation - Bainite cooling process

The immersion time in the bath depends on the thickness of the part, hardness and material chemistry, and section thickness. Generally, immersion time decreases with increasing transformation temperature, and an increase in carbon content increases the transformation time at the same transformation temperature. 

Bainite Properties

The bainitic steels have general high strength and good toughness attributed to the distribution and morphology of carbide particles in the matrix. For very wide sections, the formation of bainite structure is ideal to achieve superior properties as a constant transformation temperature permits the formation of similar microstructure and properties over a wide range of cooling rates.

  • Strength: Bainitic steel has a variety of strength range from lower strength like pearlite to higher strength like martensite. After annealing, ferrite of bainitic steel has excess leftover carbon which responsible for high strength due to more carbide precipitation. However, the fine grain structure is the main factor responsible for high strength in bainitic steels.
  • Hardness: The hardness of upper and lower bainite is like that of pearlite and tempered martensite, respectively. For hardness measurement, Rockwell, Vicker and Brinell hardness tester are commonly employed.
  • Toughness: Lower bainite has higher toughness than upper bainite due to the presence of finer cementite particles. The small carbides are not cracked, and if they crack the critical size of the defect is not possible to achieve and hence brittle failure is prevented

Bainite VS Martensite

The microstructure of martensite and bainitic steel appears to be the same and even the properties of tempered martensite and bainite. However, the morphology of both plates of steel is different when observed using a transmission electron microscope.

Difference in Microstructure

When observed under a light microscope, due to low reflectivity the microstructure of bainite appears darker than martensite. Similarly, the properties of both plates of steel are different from each other when studied in detail. Scaling of microstructure is required before you start studying it. Check our guide on microstructure scaling that can help you standardize microstructure and make them easy to understand for everyone.

Martensitic Structure Evolution

The heat treatment process for achieving martensitic structure requires heating of steel in austenite region and then quenching rapidly in oil or water or salt bath through martensite start (Ms) to finish (Mf) temperature. The transition is continuous and quenching is performed rapidly to avoid all other transformations. The tempering process is done by reheating at a low temperature where transformation is avoided but the stresses are reduced by the formation of small carbides and carbide plates at a higher temperature.

How to make Bainite?

On contrary, bainite form by an austempering process in which steel is heated to austenite range quenched to an intermediate temperature in a molten salt bath and held rather than cooling to room temperature. It is an athermal process, which results in a bainitic structure having excellent toughness. The martensite forms by diffusionless transformation whole in bainite carbides are formed by diffusion of carbon atoms.

Martensite is more prone to heat treatment defects than Bainite in steel

The formation of bainite by austempering process avoids the risk of quench cracking as stresses are less severe to cause cracking. When the bainitic transformation starts the part has more likely to have an equal distribution of temperature throughout leading to a homogenous structure at once.

As martensite formation is diffusionless transformation and rapid quenching to room temperature may cause uneven distribution of stresses, so it is difficult to avoid quench cracking in martensitic steels. 

Comparison of Properties of Bainite with Martensite

Bainite steels have somewhat superior wear resistance and toughness as compared to martensite at the same hardness may be due to the presence of a large fraction of carbides or some retained austenite.

Absence of Temper Embrittlement in Bainite

Similarly, the temper embrittlement phenomenon is not present in bainitic transformation unlike martempering. In a range of transition temperatures, plates of martensite are formed instead of lath, which results in a brittle structure. Whereas, during bainitic transformation, the structure formed is composed of laths instead of plates, even in high carbon steels, which prevents embrittlement.  

why pearlite bainite and martensite dont appear on diagram?

While I was researching for Bainite on Internet, I came across this question. So, Lets discuss this.

Pearlite is product of equilibrium reaction that involves slow and uniform cooling. While Martensite and bainite requires non-equilibrium conditions that are discussed above. Since, Iron-Iron carbide diagram is equilibrium reaction-based diagram, so we can not present martensite and bainite over it. However, pearlite is very much visible there. For martensite and bainitic phase, diagram followed in TTT diagram in steel.

What are 3 differences between martensite and bainite?

First difference is microstructure within both phases. Second difference is absence of temper embrittlement in case of bainite. Third difference is in properties. Bainitic region has higher wear resistance and toughness, while martensite has higher hardness. Detail is discussed above.

Why are bainite, spheroidite and tempered martensite not in the fe-c diagram?

Bainite, Spheroditie, and tempered martensite are the product of a non-equilibrium reaction that requires fast cooling and athermal heat transfer. While the Fe-C diagram is based on equilibrium reactions, that’s why these phases can not be presented in that diagram.

Application of Bainitic Steels

Due to their excellent mechanical properties, bainitic steels have widespread applications.

  • They have good weldability, good formability and high strength, and high toughness. 
  • They are readily used in the automobile industries to replace martensite for the fabrication of camshafts or crash reinforcement bars as they are more economical.
  • The low carbon bainite is also used in strength structural components in the aviation industry and for making, pressure vessels, and boilers.
  • The good creep resistance enables bainitic steels to use in power generation industries.
  • Due to higher strength, high carbon bainitic steels are used as mandrel bars, railway wheels or tires, back-up rolls, and many more.

If you want to learn about high carbon steels, we recommend checking out 1095 steel, 1080 steel and 1095 cro-van steel.

AISI 1080 Steel – Complete Information – Composition, Properties, and Applications

1080 steel is considered one of the common spring steel grades used in piano wires. Article here summarizes all important aspects related to 1080 high carbon steel including properties, composition, heat treatment, manufacturing, microstructural evolution, and much more.

What is 1080 Steel?

ASTM AISI 1080 steel is high carbon fully pearlitic steel having a carbon percentage between 0.75-0.88%. After heat treatment, this steel exhibits an optimum combination of elastic properties, high hardness, and low ductility. 1080 high carbon alloy finds applications in piano wires, springs, shafts, and in other automotive and suspension parts.

1080 Steel Composition

As you can see from AISI 1080 designation, it’s a pure alloying element with the minor addition of Mn. A detailed composition is given below;

ElementsWt%
C0.75-0.88
Mn0.60-0.90
P<0.05
S<0.04

1080 Carbon Steel Properties

This steel is eutectoid steel containing a lamellar structure of pearlite. Mainly this steel is used after rolling and subsequent heat treatment because of its enhanced elastic properties, and fatigue resistance.

Physical Properties

PropertiesUnits (metric)
Melting Point1430 C
Density7.85 g/cm3

Mechanical Properties

HB is used for the Brinell hardness test, HR is used for the Rockwell hardness test and HV is used for the Vicker hardness test.

1080 Mech PropertiesAs RolledCold Drawn + Spherodized AnnealedHot RolledNormalizedOil Quenched     
Hardness, Brinell 293192229293363
Hardness, Knoop 319214252319392
Hardness, Rockwell B 99919699100
Hardness, Rockwell C 3111193140
Hardness, Vickers 309201241309384
Tensile Strength, Ultimate 965 MPa675 MPa772 MPa1035 MPa1270 MPa
Tensile Strength, Yield 585 MPa515 MPa425 MPa550 MPa869 MPa
Elongation at Break 0.120.10.10.1240.121
Reduction of Area 0.170.40.250.2770.344
Modulus of Elasticity 205 GPa205 GPa205 GPa200 GPa205 GPa
Bulk Modulus 160 GPa160 GPa160 GPa160 GPa160 GPa
Poissons Ratio 0.290.290.290.290.29
Shear Modulus 80.0 GPa0.4580.0 GPa80.0 GPa80.0 GPa
Izod Impact 7.00 J80.0 GPa7.00 J

Thermal Properties

1080 steelThermal Properties in Metric Unit
Coefficient of Thermal Expansion11.0 µm/m-°C
Specific Heat Capacity0.490 J/g-°C
Thermal Conductivity47.7 W/m-K

1080 Spring Steel Rolling

This steell is mainly used after the rolling or forging process to be used in wires, springs, or automotive parts. The rolling process not only reduces surface area but also increases tensile strength due to plastic deformation. 

Plastic deformation refines microstructure and also reduces interlamellar spacing in pearlite. This refined microstructure and reduced interlamellar spacing increase tensile strength and fatigue resistance. With such refine microstructure, crack propagation is also controlled. However, in applications where sufficient ductility is pre-requirement, heat treatment processes are employed.  

AISI 1080 Steel Heat treatment

Common processes employed on 1080 cold rolled steel Normalizing, annealing, and hardening. 

Normalizing

Normalizing 1080 steel is an important process because it refines microstructure. This refined microstructure makes this steel suitable for springs and shafts.

The normalizing temperature for this steel is between 800-850oC.

Full Annealing

Incase of annealing, there is no sub-critical annealing region and the composition is closer to the eutectoid point. So, the Full annealing temperature of this specific grade of steel is lower as compared to other grades. 

79-850o C is the optimum annealing range for 1080 high carbon colled rolled steel. Holding time for thickness less than 75mm size sample is 1 hr with addition half-hour for each 25.4 mm increase in sample size.

Quenching 1080 Steel

Hardening is extremely important to achieve desired hardness in the microstructure. To achieve a hardness of 65HRC measured on Rockwell hardness tester, water or bring quenching is used on 1080 grade.

Hardening temperature is similar to the above heat treatment processes i.e. 800-820oC.

Tempering 1080 Steel

Tempering of 1080 steel is extremely important to relieve quenching stresses. There are lots of quenching defects that can generate which can be removed in process of tempering. 

The effect of tempering can be seen in the chart below. We explained the role of tempering on the microstructure of 1080 steel and its hardness in the article, “tempering steel process”. Give it a look, you will find all answers.

After tempering for 2 h at °C (°F)Rockwell C hardness, HRC
205 (400)57
260 (500)55
315 (600)50
370 (700)43
425 (800)41
480 (900)40
540 (1000)39
595 (1100)38
650 (1200)32

In this chart above, steel is Normalized at 885 °C, and water quenched from 800-815 °C.

AISI 1080 Steel weldability

Steel starting from0.8% carbon has a major portion of cementite. These types of steel are not recommended for welding.

Microstructure Evolution

As mentioned earlier, this steel is eutectoid and, during cooling, this steel transforms into complete pearlite and no secondary phase i.e. ferrite or cementite nucleates. The right process of microstructure evolution can be studied in the annealing section

The most important aspects of the 1080 grade microstructure are austenite initial grain size, interlamellar spacing, and strain hardening to determine resultant properties.

1080 carbon steel vs W2 

1080 has a carbon percentage of 0.8% while, in the case of w2, it is 1.1%. W2 steel is low alloy high carbon steel with small additions of V, W, and Cr. With the addition of these alloying elements, high surface hardness, moderate toughness is achievable in W2 making it ideal for knives and swords. On the other hand, this steel is plain high carbon steel which is most suitable for cutting tools, cold drawn wires, and springs. 

Regarding W2, read detailed review in article, W2 tool steel.

1080 steel VS 1095 steel

1095 steel has more carbon as compared to 1080 steel. High carbon leads to higher hardness and low ductility. 1095 steel is specifically used where high hardness and wear resistance is required like knives and cutting tools. While 1080 grade has good elastic properties with optimum hardness making it ideal for cold-drawn rods or piano wires.

FAQ

What is the hardening temp for 1080?

The hardening temperature for 1080 grade is 815o C. You can use water or brine solution for quenching.

How strong is 1080 steel at 56 HC hardened?

You can see the tempering chart in this article to see the exact process to achieve this hardness with tempering.

How to read it diagram for 1080?

Detailed description and microstructure evolution according to ttt diagram is explained in ttt diagram in steel.

What products are made of 1080?

The most common products made of this steel are piano wires, hand tools. 

What is the carbon content of 1080 grade?

Carbon content of 1080 grade are 0.75% to 0.88%.

What is the chemical composition of 1080?

1080 is plain high carbon iron alloy with C-0.75-0.88%, Mn-0.6%, and trace elements of P, and S.

1080 steel uses

1080 is a widely used high carbon grade with applications in cutting tools, automotive parts, and high tension wires.

  1. One of the most common applications of this steel is piano wires.
  2. Other than piano wires, this steel is used in spring clamps, antennas, springs, and leaf springs.
  3. This steel is also used in the broad field of cutting tools i.e. hand tools, ring rolling tools, and hitting tools. 
  4. In the automotive sector, this sector finds uses in the shaft and many other spare parts.

W2 steel Composition, Properties, Heat treatment, Applications, and Comparison with Other Steel Grades

W2 steel review is needed for all knife users because it’s perhaps one of the most notable blade steel if high hardness and edge retention are prerequisites. W2 steel knife is respected by knife users because of the points we mentioned in our review. 

If you are interested in w2 tool steel, then I hope I answer all your thoughts in this review. If you still have questions, you can always comment and we will try to reply as brief as possible.

What is meant by ‘W’ in W2 tool steel?

W is basically the term used for specific alloy steels that are water hardenable. During quenching of these alloys, the quenching medium used is water to generate maximum hardness and refined microstructure. Other than W2 alloy, W grade has two more variants including W1 and W5 steel. You can read the review on W1 tool steel if you want to compare both. 

What is W2 steel?

W2 steel is water hardenable high carbon low alloy steel with a carbon percentage above 1%. This steel is most commonly used in knives, cutting tools, and large blades due to an excellent combination of high hardness, optimum toughness, and refined grain size. Out of all W-grade steels, W2 is most preferable for knife making. 

W2 carbon content is above 1% resulting in high hardness. Chromium addition in this steel resulted in increased edge retention and hardness in blades and knives. 

This blade steel is called high carbon low alloy steel and consists of the following elements;

ElementsWt%
C0.85 - 1.50
Si0.10 - 0.40
Mn0.10 - 0.40
P<0.020
S<0.020
Cr<0.15
Ni<0.20
V0.15 - 0.35
Mo<0.10
W<0.15

You can see effect of each element on properties of steel  in our article, the effect of alloying elements in steel”.

W2 Steel Properties

W2 carbon steel is used in places where a good combination of high hardness and toughness is required. Because of high carbon and water quenching, we have high hardness with Rockwell hardness approaching 60 HRC in certain cases. 

Due to the nature of water hardenable structure, applications of w2 blade steel are limited to rather simpler shapes like knives. That’s why w2 alloy is most commonly known for blades and knives. 

All properties mentioned below in the table are for annealed w2. After performing certain quenching and tempering treatments, you can increase the hardness of w2 steel to 58 HRC – 62 HRC.

w2 steel composition

Physical PropertiesUnits (metric)
Melting Point1430 C
Density7.72 - 7.8 g/cm3
Mechanical PropertiesUnits (Metric)
Poisson ratio0.3
Elastic Modulus203 - 211 GPa
Shear Modulus79 GPa
Hardness (Brinell)202 HBN
Hardness (Rockwell)14-18 HRC
W2 SteelThermal Properties in Metric Unit 
Coefficient of Thermal Expansion1 - 1.3 E-5 /K
Specific Heat Capacity420 - 460 J/kg-K
Thermal Conductivity17 - 42 W/m-K

There is Brinel hardness mentioned which can be measured using Brinell hardness tester.

How do you heat treat w2 tool steel?

W2 is considered high carbon and low alloy, and after the manufacturing process right set of treatments is necessary to achieve the right hardness and desired physical appearance for final use.

The process of heat treatment in the case of w2 knife steel involves normalizing, annealing, machining, stress-relieving, and then hardening followed by tempering.

After forging this steel, we will use the process of annealing, normalizing to refine microstructure and homogenize the distribution of carbides. With these two heat treatments, structural changes are expected and that’s why machining is performed after these heat treatments. 

After machining and final shaping, stress-relieving and hardening are performed to achieve the right hardness and knife characteristics. 

W2 steel heat treatment

Normalizing W2 steel katana

We are going to shed some light on the purpose of annealing and normalizing in the case of low alloy w2 grade. For detailed literature on both processes, have a look into the Annealing process and Normalizing Process. 

Here, the purpose of the normalizing process is to produce fine and uniform microstructure with the right distribution of alloy carbides. After manufacturing, the structure is irregular and it requires the right type of adjustment to obtain optimum condition so it can be hardened to maximum properties. 

w2 tool steel heat treatment cycle

Normalizing conditions are enlisted along with annealing conditions in the below chart.

how to anneal w2 tool steel?

Annealing is a process of heating steel to the right temperature and allows uniform distribution of microstructure. After uniform distribution, steel is cooled slowly. This may increase grain size and lowers hardness. With lower hardness, machining becomes easier. 

Regarding the microstructure, annealed steel will have carbides distributed in a ferritic matrix. Spherodize annealing of w2 grade is used to lower the hardness and make w2 grade machinable. You can see the right process of spherodize annealing of w2 tool steel in the chart below;

You can see annealing and normalizing conditions for w2 below;

ConditionsValue
Normalizing Temp C790-925
Annealing Temp C760-790
Cooling Rate during Annealing (oC/h)22
Hardness after annealing163-201

Stress relief annealing after machining is performed to remove stresses and restore microstructure. You can read it in more detail in annealing types. Holding time should be 1-2 hours at right stress relief annealing.

how to harden w2 tool steel?

Hardening of w2 blade involves a set of three operations as we previously explained in the Quenching process i.e. Austenitizing, Quenching, and Tempering. 

Austenitizing is critical in the case of low alloy tool steels because of problems like distortion, or loss of ductility. Read more in detail in Defects in steel during heat treatment. 

Within high carbon steel, carbides get dissolved resulting in the austenitic microstructure. Austenitic temperature is optimized in such a way as to minimize austenitic grain growth and dissolution of carbides. 

The fine structure even after austenitization is attributed to carbides of vanadium and tungsten. These carbides dissolve at very high temperatures and that is why they prevent abnormal grain growth in austenite in the case of w2 grade.

Quenching results in a fine microstructure consisting of martensite and retained austenite. The lesser the retained austenite, the harder the w2 tool blade. 

Tempering steel is a very important step in all heat treatment operations. Temper w2 steel has high hardness and edge retention because of secondary hardening phenomena. This can be studied in detail in the tempering steel process. 

W2 steel Characteristics

Hardness

W2 steel is hardness after the process of tempering reaches 60 HRC i.e. 58 HRC – 63HRC. High hardness is attributed to alloying elements like Vanadium and tungsten which not only refine microstructure but also give secondary hardening to w2 steel during tempering. 

Machinability

W2 tool is high carbon and low alloy and it contains cementite microstructure in its original non-heat-treated condition. Because of such brittle content, w2 grade is not machinable. That’s why the process of annealing is developed to perform final machining before hardening. 

As discussed above, the annealing process converts cementite structure into carbides in a ferritic matrix. This structure is relatively easier for a machine. 

After the hardening process, the hardness of 60 HRC prevents any material operations like machining.

Edge Retention

Because of high hardness, the blade edge will not become dull over time faster than other low carbon steel. Edge retention is excellent of w2 high carbon steel because of provided 63 HRC hardness.

Sharpness

Blade made of w2 tool steel is extremely hard and that also makes it difficult to sharpen. The only time sharpening of the blade is possible is after a process of annealing when the hardness of the w2 tool is at a limited value i.e. 15 HRC.

Toughness

W2 alloy is known for high surface hardness and good toughness in the core region. In this way, good impact resistance is also achieved. However, increase in hardness to achieve high edge retention and wear resistance, there is a compromise on toughness. To keep the optimum value of toughness, it is recommended to have hardness lower than 63 HRC in w2 knife steel.

Corrosion Resistance

The corrosion resistance of carbon steel is low and the same is true for w2 alloy. To achieve a high level of corrosion resistance, you need high percentages of chromium. We recommend checkout 1095 cro-van steel or 8CRMoV and 9Cr18MoV steel if corrosion is one of the deciding factors. 

W2 steel Characteristics
Resistance to decarburizationHighest
Hardening ResponseShallow
Amount of distortionHigh
Resistance to crackingMedium
Approximate Hardness (HRC)50-64
MachinabilityHighest
ToughnessHigh
Resistance to softeningLow
Resistance to wearLow to medium

Is W2 steel good for knives?

W2 grade sounds pretty good because of its high hardness and optimum toughness value. However, this is not everything in the knife checklist, we must consider other factors as well.

W2 high carbon steel has a high hardness value and optimum toughness value which indicates it can handle a lot of damage in terms of cutting or impact. That is why the most common use of w2 grade is found in cutlery, surgical and diving knives. 

In the case of fillet, kitchen or travel knives like crkt minimalist bowie hunting knife, corrosion resistance, and good wear resistance is of much important value. The knife will become dull or rusty fast and sharpening this knife is difficult because of its high hardness. 

That’s why this knife is not recommended for fillet or kitchen. However, you must have seen lots of knife users still using fixed blades made of w2 grade. Well, that’s because the heat treatment is simple and it is available in the market at a lower cost.

is w2 tool steel good for knives

w1 vs w2 steel

W1 steel and W2 steel both belonged to W grades. The structure of W2 is finer and harder as compared to w1 grade because of the addition of Vanadium and Tungsten. Vanadium is the main reason in the steel for high hardness and higher edge retention. W2 is more suitable for knife making than w1 grade.

W2 steel VS D2 steel

D2 steel is one of the most common cold work tool steel. Carbon percentage and hardness are similar to discussed steel. In D2 steel, after hardening and tempering, the hardness value is equivalent to 58HRC which is almost similar to this steel. One additional benefit that D2 steel has is corrosion resistance because of 11 percent Chromium. Chromium makes steel rust-free. D2 steel is more suitable for knife making than W2 tool steel provided use is in kitchen, fishing, or hunting.

W2 VS 1095 steel

1095 steel is high carbon steel with no additional alloying element. While W2 variant is also high carbon alloy but also has a small addition of alloying elements like Cr, V, W, and Mo. These alloying elements produce more fines and tougher structures with similar hardness as compared to 1095 steel. Regarding knife making, W2 alloy is better in application than pure 1095 grade.

W2 Steel VS Damascus

Damascus steel is simple forged high carbon steel similar to 1095 steel. In terms of application in the field of cutting blades and knives, w2 variant offers a better package having high hardness, better toughness, and good impact resistance than Damascus steel.

Common FAQ

how heavy is w2 steel?

This steel has a density equivalent to simple iron. It is as heavy as any other steel blade or knife.

How long to quench W2 steel?

You can see the complete heat treatment cycle we shared here. There is no break till reaching room temperature. So, Quenching this steel in water is carried out until it reaches at least 200oC to avoid any structure and minimize retained austenite formation.

How good are W2 steel swords?

They are pretty good. The reason is a good combination of high hardness and optimum toughness. It can resist any impact or damage and also edge retention offered by structure is suitable for sword marking. You can use this steel in sword-making comfortably.

How good is w2 steel?

W2 tool contains high carbon with minor additions of Cr, V, and W. Heat treatment of this steel results in high hardness and good toughness because of carbon and other alloying elements. Knife and blades made of this steel can handle a good beating. This steel is considered good in the field of sword making, cutlery, and similar applications.

What can be used in place of W2 steel?

This steel is mainly called high carbon steel and there are a variety of other options available in the market that offer similar or better properties. We recommend checking out 1095 cro-van steel, which has good hardness along with a good amount of corrosion resistance. 

What made out of W2 steel?

 This steel is most commonly known for its extensive use in making blades. Fixed blades made out of w2 tool steel are used in making swords, cutlery, and surgical knives.

What is carbon content of W2 steel?

This high carbon steel has a carbon content of between 0.85 – 1.5%.

9Cr18MoV Steel Review – Properties and Knives

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Looking to buy an awesome EDC Knife or interested in Damascus Blade, you must have read about 9Cr18MoV stainless steel. We perform detailed research on this topic and now writing a 9Cr18MoV steel review. 

We are trying to go in-depth regarding this topic and going to cover as much as possible from composition to common blade knives. The comparison with other blade steels will also be discussed to help you make a decision.

What is 9Cr18MoV Steel?

9Cr18MoV steel is 440B Chinese high-quality stainless steel with 0.9% Carbon and 18% Chromium. High Hardness i.e. 58-60 HRC and Excellent Corrosion Resistance make this steel ideal for surgical blades, stainless steel knives, and cutlery. 

High Carbon and ample addition of chromium, vanadium, and molybdenum make this steel better in edge retention compared to other blade steels. Along with edge retention and durability, this steel also has high corrosion resistance. Common applications included are blades, surgical blades, cutlery, and other items.

9Cr18MoV high carbon stainless steel Composition

The composition consists of 0.9% Carbon and 18% Chromium along with minor additions of molybdenum and vanadium. 

ElementsWt%
C0.9
Si0.8
Mn0.8
P<0.04
S<0.03
Cr18
Mo1 - 1.3
Ni0.6
V0.07 - 0.12
FeBalance

Steel Equivalent

440B is closed to 9Cr18MoV knife with higher Vanadium and Molybdenum. Both have a comparable hardness and tensile properties and can be used interchangeably. 

You can see more steel grades being equivalent to Chinese 9Cr18MoV stainless steel in the picture below; 

9cr18mov stainless steel equivalent grades

9Cr18MoV Properties

Physical Properties

PropertiesUnits (metric)
Specific Heat Capacity460 J/kg - K
Thermal Conductivity29,3 /m-K
Electrical Resistivity at 20 C0.65
Density7.70 g/cm3

9cr18MoV steel hardness

This steel hardness is in the range of 58-60 HRC.

The presence of 0.95% C is one of the prominent reasons behind its hardness. With the increase in carbon, during hardening, martensite concentration increases resulting in higher hardness as explained in the TTT diagram of steel

The presence of vanadium helps in increasing hardness as the carbide of vanadium does not dissolve during austenitizing. Austenitizing process during heat treatment is performed for homogenization and austenitic phase formation. 

All alloying elements have a direct impact on hardness which are explained in a detailed manner in the “Tempering steel process”.

Edge Retention

Edge retention is the ability of a knife to retain its shape and not becoming dull during regular use. Edge is a small microscopic region of the knife that is most under pressure and that’s why this region needs to be the hardest to resist all types of stress. 

Hardest does not mean complete brittleness, however, the knife will be less tough than those having lower carbon content. 

Maximum hardness in the edge region is achieved using a combination of carbon and other alloying elements. 9Cr18MoV high carbon stainless steel edge retention is attributed to its high carbon and vanadium content.

Toughness

Toughness is the ability of a knife to resist shock and impact loading. There is always a trade-off between hardness and toughness. Due to better edge retention, and high hardness, this steel has poor toughness. 

Chinese grade has a small addition of Nickel and Molybdenum which slightly improves the toughness and tensile strength of steel. Chinese are improving this grade with modern metallurgy to bring this steel reaches a high level of toughness without losing much hardness.

Corrosion Resistance

9Cr18MoV knife is highly rusted resistant. 

Stainless steel is commonly used for outdoor applications where chances of corrosion and rust are high. For hunter knives, mainly knives used are stainless. Stainless steel prevents any oxidative attack due to layers of Chromium, Nickel, and Molybdenum. 

Normally, stainless steel is stainless if the chromium percentage is above 11.1%. Within 9Cr18MoV stainless steel, the percentage of chromium is 18% and it is sufficient for a knife to be rust-resistant. 

9Cr18MoV stainless steel Sharpness

This steel is very hard and sharpening hard blades is extremely difficult especially for newbies. High-quality knife sharpeners like smith knife sharpener are required for the finishing and sharpness of the knife. 

Is 9Cr18MoV Steel good for knives?

9Cr18MoV steel is low price stainless steel knife with high edge retention and hardness compared to other cheap alternatives. Due to lower toughness and tensile strength, this knife will not be able to wit withstand a large amount of beating as in the case of hunting and camping. 

If I have to categories this knife, I say it’s an ideal kitchen knife mostly used for high hardness and high edge retention. However, it’s not recommended to use it for camping, hunting, or any other similar purpose.

Common 9Cr18MoV Steel

Civivi Praxis Knife

sCivivi is a newer WE Knives that are affordable and considered a high-quality EDC Style knife. WE is a popular brand among EDC knives, which are good in manufacturing quality blades for hunting and outdoor activities. 

This Praxis knife is also one impressive product with a solid G10 handle and 9Cr18MoV high carbon stainless steel blade. The ball bearing action is extra smooth and it is popular among the masses for outdoor activities. 

You can check the price on amazon.com by clicking on the picture below;

Schrade SCH305 Knife

Schrade is one highly trusted brand among hunters. This knife is also considered impressive because of its beautiful ergonomics and amazing performance. The handle is a favorite part which is designed with dual thumb studs and finger guard. I would highly recommend this knife if you are going camping. 

The blade of this knife is also made of 9Cr18MoV stainless steel and It perfectly combines high corrosion resistance and edge retention. 

9cr18mov vs D2

D2 steel and 9Cr18MoV steel, both, are popular among knife manufacturers. D2 steel is high carbon steel with no extra addition of Cr or Mo, that’s why it is prone to corrosion. While 9Cr18MoV steel is highly corrosion resistant due to the addition of Cr, and Mo. 

D2 steel is harder as compared to 9Cr18MoV steel but is not recommended for outdoor activities like hunting or camping. A knife made of D2 steel is only suitable for kitchen activities and it requires extra to prevent rust during use. 

9Cr18MoV steel knife is mainly used for hunting and camping due to its stainless nature. 

9cr18mov vs 440c

440C and 9Cr18MoV steel have comparable compositions and can be used interchangeably. 440c is USA ASTM grade equivalent to Chinese grade 9Cr18MoV. Both sheets of steel can be used for outdoor activities. You can also see the equivalency chart above. 

9cr18mov vs 8cr13mov

9Cr18MoV steel has more carbon and Chromium as compared to 8Cr13MoV steel. 9Cr has more hardness and better edge retention but less tough as compared to 8Cr13MoV steel. For the composition and properties of 8Cr13MoV steel, you can see more in article 8Cr13moV steel

9cr18mov vs 14c28n

14c28n has less carbon and chromium as compared to 9Cr steel. It has a lower hardness and edge retention as compared to the 9Cr knife.

Conclusion

Knife made of this steel is considered impressive and recommended for outdoor, EDC, and kitchen use. This steel has impressive edge retention, high hardness, and good corrosion resistance and this makes this knife ideal for outdoor activities unless you want to give this knife a large beating. Due to lower toughness, extra abuse may cause this knife to fail due to lower tensile strength. Other than that, a knife made of this steel is a great pick for overall use. 

D6ac Steel – Composition | Properties | Applications

D6AC steel is vacuum melted, medium carbon, low alloyed, and ultra-high-strength steel. This grade is used for high strength applications like aircraft structure with strength levels up to 280,000 psi. With proper tempering, yield strength and hardness of this grade can reach up to 1750 MPa, and 53HRC respectively.


D6AC Steel Composition

ElementsWeight %
C0.46
Cr1.1
Mn0.75
Ni0.6
Mo1
Si0.25
V0.1
FeBase

D6AC Steel Properties

Physical Properties

PropertiesUnits (metric)
Melting Point1426 C
Specific Gravity7.8 g/cc
Modulus of Elastic Tension32

Mechanical Properties

We used the Rockwell Hardness test for the measurement of the Hardness of D6AC steel.

Tempered D6AC Mechanical PropertiesTempering Temp - 316 C Units (metrics)Tempering Temp - 510 C Units (metrics)
Yield Strength1724 MPa1345 MPa
Poisson Ratio0.27-0.30.27-0.3
Elastic Modulus210 GPa150 GPa
Reduction Ratio23 %25 %
Elongation at break7 %7 %
Hardness (Rockwell C)5346

Thermal Properties

D6AC steelProperties
Thermal Conductivity (W/m. K)30 /m-K
Specific Heat Capacity0.116 J/g - C
Mean Coef of thermal Expansion7

D6AC Steel Heat treatment

D6AC heat treatment cycle starts from the normalizing or annealing process. With annealing or normalizing, uniformity in structure and removal of previous stresses is carried out. Both Annealing and Normalizing for this grade of steel are carried out in the gamma region.

After removing stresses, hardening treatment is carried out. With the hardening process, carbides of Mo, and V nucleate at grain boundaries provide extra strength to the material. After soaking at hardening temperature, steel is immediately quenched, not allowing carbides to dissolve. This results in ultra high strength in D6AC steel.

Hardening treatment makes this grade prone to quench cracking. To prevent this defect from occurring, this grade is immediately tempered at temperatures given above in a table. The tempering process removes all stresses and also makes this steel ultra-hard as a result of secondary hardening. 

The heat treatment cycle for Annealing, Normalizing, Hardening, and Tempering for this steel grade is explained in the picture below;

d6ac heat treatment

Forging

This steel grade is forgeable. Forging for this specific steel grade is carried out between 1090oC to 1230oC. For minimum finishing, the forging temperature should be at least 930o C. After forging, the steel part is furnace cooled up to 540oC, and later it is air quenched to room temperature.


Welding

Thin sections of D6AC are welded by TIG welding while GMAW can be applied on the thick section with filler rods of the same steel grade. Preheat steel parts which are to be welded up to 200oC to avoid weld cracks. In the case of restrained weldments, proper post-weld heat treatment should be applied.

Post welding treatment is carried out by heating this steel grade to 600F, air-cooled to 300F and then it is stress relived at 1000F. After stress relieving, normalizing and re-tempering processes are conducted to achieve desired mechanical properties.


Machining

Low alloy steel can be easily machined. The same goes for D6AC steel. Machining of this grade is carried out in normalized temper condition. Heat-treated steel grades that have yield strength of 170MPa should pass from the stress-relieving step after the machining process. Duration for stress relief is 3 hours at the annealing temperature.


D6AC Steel Applications

D6AC Steel have many structural and defence applications. The fields in which this structural steel is employed are vehical, airplane, pressure vessel, power generations and many more..

If we just summarized them, following are the possible places where this alloy steel is employed;

  • Fasteners
  • Boiler rods for support
  • Turbine fasteners
  • Connecting ROds
  • Mechanical Parts like gears and shaft
d6ac steel applications

What is 430 Stainless Steel | Properties and Applications

430 stainless steel is ferritic steel containing chromium as major alloying elements and belongs to the category of non-hardenable steels. This steel grade provides an excellent combination of corrosion resistance and practical mechanical properties. Common Applications of this steel are Automatic Screw machines, Dishwasher linings, and lashing wires.

430 Stainless Steel Composition

ElementsWt%
C< 0.12%
Cr14 - 18 %
Mn< 1.0%
S< 0.030
P< 0.040
Si< 1.0%
FeBase

430 Steel Properties

Physical Properties

PropertiesUnits (metric)
Melting Point1425 - 1510 C
Solidus1425 C
Liquidus1510 C
Maximum Service Temperature, Air815 C
Density7.80 g/cm3

Mechanical Properties

We used VickerBrinell, and Rockwell Hardness test for measurement of Hardness of 430 stainless steel.

PropertiesUnits (Metric)
Yield Strength310 MPa
UTS517 MPa
Poisson ratio0.27 - 0.3
Elastic Modulus200 GPa
Elongation at Break0.3
Hardness (Brinell)155
Hardness (Vicker)162
Hardness (Rockwell B)82

Thermal Properties

430 steelCTE Liner (µm/m - C)
100 C10.4
315 C11
540 C11.3
650 C11.9
815 C12.4
430 steelproperties
Thermal Conductivity (W/m. K)26.1 /m-K
Specific Heat Capacity0.461 J/g - C

Electrical Properties

430 SteelElectrical resistivity (ohm - cm)Magnetic Permeability
20 C6.0E-5600 - 1100

430 stainless steel Heat treatment

Ferritic Stainless steel, normally, is non-hardenable. It means their hardness will not increase during quenching. Only heat treatment applicable is annealing. It is carried out by heating at 815oC for 30 minutes and, furnace cool to 600oC and, from there, quick air cooling.

Slow cooling below 600oC results in embrittlement of this grade steel so it is normally avoided.

Fabrication

Elongation at break shared in mechanical properties describes the final length to initial length after breakage after the tensile test. This indicates the ability to resist shape change in a material without fracture and used for indication of the degree of bending of steel.

This steel grade has a large elongation at break i.e. 25% which indicates higher ductility and lower work hardening rate. So, this steel grade can readily be shaped without an increased risk of brittle cracks or fractures. Sub-critical annealing may be used for rigorous cold-working.

430 steel Welding

For welding this grade steel, it should be preheated to 150oC. After welding, post-weld annealing can be used to reduce brittleness in this steel grade. Recommended filler rods for this type of steel are 430, 308L, 309, and 310.

Find fillers for welding of 430 SS on Alibaba.

430 welding

430 Stainless Steel Machining

430 stainless steel is considered easily machineable as compared to 300 series of Stainless steel. The reason for easy machining is high elastic modulus. High elastic modulus material does not stretch or bend easily, resulting in better machining.

Elastic modulus is a term that describes deformation in the material in response to force applied. If deformation is less than that means the material is less gummy and has a higher elastic modulus. So, in this case, the elastic modulus of 430 steel is 200 GPa which is considered high as compared to 300 series steel’s elastic modulus. 

The best way to machine this steel grade is to process it in stain hardened condition when a material is already stressed. In stressed condition, deformation will be less and this steel grade will easily break off in pieces. In an annealed state, soften structure will tend to gall resulting in slightly difficulty machining.

Heat Resistance

This steel grade can resist high-temperature oxidation up to 820oC. However, it will become brittle on cooling. This problem can be alleviated using proper post-annealing.

Corrosion resistance

Grade 430 steel belongs to the class of ferritic steels and all ferritic grades have excellent corrosion resistance (stress-corrosion). They are resistant to several chemicals, organic and nitric acids. Corrosion resistance further levels up if this steel is used in polished condition. In the case of pitting corrosion resistance, it is comparable to 304 steel.

430 Stainless Steel Applications

Steel 430 is used in lots of decorative applications where chances of stress corrosion cracking are high. Common applications are;

  • Dish Washer Liners
  • Refrigerator Cabinet Panels
  • Stove Trim Rings
  • Fasteners
  • Fuel Linings
430 stainless steel applications

FAQ

What is 430 Stainless Steel?

430 stainless steel is non-hardenable, rich in chromium stainless steel. They possess excellent corrosion and heat resistance along with optimum formability and mechanical properties. They can be used in chemical applications along with fuel linings, fasteners, and many more.

What stainless steel food grade is best 430 or 403?

Food grade means still must have high crossing resistance, resistance against stress corrosion cracking, organic and nitric acids. All these qualities are present in 430 steel due to very high chromium content making it one of the best food-grade steel in the market.

How good is 430 stainless steel?

430 is the best food-grade material with magnetic characteristics. It has good corrosion resistance with practical sheet bending ability. This steel can be used in the presence of strong acids and also at high temperatures like 800oC. 

How to anneal 430 stainless steel?

Annealing of 430 steel is carried out by heating it at 820oC and soaking it there for 15 minutes. After homogenizing, it is slowly cooled up to 600oC and after that quickly air-cooled to avoid embrittlement.

Stainless steel 430 what is it made of?

Stainless steel 430 is made of 16% Chromium, 0.1% Carbon, 1% Manganese, 1% silicon and 0.03% sulfur.

Why can 430 stainless steel be attracted to a magnet but 316 cannot?

430 stainless is ferritic steel while 316 steel is austenitic steel. The ferritic structure is magnetic while the austenitic structure is non-magnetic. Due to the ferritic structure of 430 steel, It is attracted to a magnet.

What is the difference between 304 and 430 stainless steel?

304 and 430 are different in terms of structure. 304 steel is austenitic while 430 steel is ferritic. Due to this 304 is non-magnetic while 430 is magnetic. Due to higher chromium content in 430 as compared to 304, it can be used in food processing applications. 

Steel Heat Treating – Types and Applications

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Steel Heat treating is a process which involves cooling and heating of a metal substance at usually high temperature and conditions. It is useful for softening, hardening, and changing physical properties. Moreover, you can manufacture various metal structures like glass by passing it through different thermal techniques.

This steel heat treating is performed under certain conditions and passes through seven main processes before changing metals’ mechanical attributes. 

Annealing heat treatment

            The first process through which metal passes through is termed as Annealing. The procedure involves both cooling and heating processes. As a result, the microstructure increases, thus ensuring a change in electrical and mechanical properties.

This steel heat-treating process heats the metal at a critical temperature. After this, the metal can cool down slowly at room temperature and pressure.

To study Annealing in detail, Follow the microstructure development in Annealing and, also, types of annealing.

Most importantly, this process is carried for electrical and dimensional stability. The benefits result in the softening of metal for natural fabrication processes. The hardness is reduced, and the ductility property of the solid mineral is enhanced.

Steel annealing types all
Steel annealing types all

Moreover, the Annealing boosts up the electrical conductivity with changes in mechanical state. Sometimes, the Annealing metal is also allowed for the thermal process in hot furnaces. This process must be performed at low cooling temperature and pressure. This whole process of heating, cooling, and then repeating is continuously performed for 4 to 8 hours.  

Normalizing – Steel Heat treating

After done with Annealing, the metal is passed through the process of Normalizing. The Normalizing process is part of the steel heat treating method.

In this method, the solid metal is heated above 40* degree Celsius. As a result, the Normalizing process forms pure grain homogeneous structures that have more unique properties. Most importantly, the process is only applicable to ferrous metals, and it is different from Annealing.

Hypo eutectoid steel annealed microstructure
Normalized Structure – Steel heat treating

The process for scaling is mentioned in article, “Scaling of Microstructure“.

The main goal or stress Normalizing is to reduce stress from the heat-treating prestressed machining, and casting induced stress is also removed by this method. The temperature goes up, then the critical temperature and then allowed to cool down in the air.

This technique should be performed with all safety precautions and in perfect desired conditions for accurate results. Besides, the Normalizing method is most common in plate mills for the mass production of forging material. This process should only be done on ferrous metals or alloys like steels. It is the second part of the heat treating of steel.

Quenching heat treatment

The third process of heat treatment of steel is named as Quenching. This process is also called hardening due to the characteristics of metals. In this technique, the solid metal is first heated above the conditions and then quickly allowed to cool down.

This process is termed as rapid cooling for maintaining the mechanical or physical properties. This ensures that the structure of homogeneous metal remains constant.  Quenching is recommended for ferrous alloys, which harden the metal and decrease the ductility. After heating above the critical temperature, the metal can cool down by passing it through the nitrogen air, water, or any polymer.

Microstructure of martensite and martensitic transformation
Microstructure of Martensite after Quenching – Steel heat treating

This process is used for hardening the metal as in the previous method of Normalizing, and the steel was softened. The Quenching process has one drawback as it can make the metal brittle. However, this issue can be resolved by heating it again.

Hardness is simple approach to measure properties of quenched microstructure. Vicker hardness is non-destrutive hardness method while Rockwell is commonly used industrial hardness method. In case of non-destructive hardness test, Brinell hardness test is used.

Case Hardening – Steel Heat treating

The next process is called Case hardening. This is used to solidify alloying metal solid, thus ensuring greater efficiency in wear resistance of machine parts. Moreover, this process is a thermochemical diffusion that allows the dissemination of nitrogen or carbon on the upper surface of alloying metals.

The Case hardening is the fourth step in Steel heat treatment and is used to harden alloying metal substances through diffusion. The diffusion of carbon forms a thin layer of a harder alloy on the surface of metals.

This Case hardening technique is completed in 20 to 30 minutes of the constant procedure. This formed metal is more complicated, with strong forces as compared to the process of Carburizing.

The benefits of Case hardening are producing more durable, more stringent, and potent substances. This increases the life span of solid metals together with the quality of being easily weld due to more flexibility attributes. The alloying metals are more malleable with hardness higher than produced in Annealing or normalizing.

Carburizing – Steel Heat treating

After Case hardening, the fifth process of steel heat treatment is called as Carburizing. Before diving deeper, let us know the basics of Carburizing.

This process is the same as Case hardening because it is also performed for making the alloying metal hard and tough. In this technique, the heat treatment involves the absorption of iron and steel. The carbon layer formed in Case hardening is absorbed with the iron alloy’s help for making the metal more durable. This process is also known as Case hardening and should be performed under the same conditions and temperatures.

carburizing - Steel heat treating
carburizing – Steel heat treating

Likewise, case hardening, this technique also forms a more pliable and flexible alloying metal substance. Carburizing is divided into three types named as Gas carburizing, liquid carburizing, and vacuum carburizing. These three types are based upon the raw materials it uses for the absorption of carbon alloy. After this, the metal is heated with carbon monoxide and charcoal, etc.

Nitriding

The Nitriding is the sixth process of heat treatment of steels. As Carburizing uses carbon alloy to make the metal hard, this Nitriding process diffuses nitrogen gas on the surface of the solid metal substance. The nitrogen gas absorbs on the surface of the metal and makes it sturdy and more robust. This technique is recommended for low steel alloy, aluminum, and titanium.

The Nitriding process is divided into two types. Firstly, the diffusion through gaseous nitrogen gas named Ammonia and secondly with plasma ion of nitrogen. This Nitriding process is performed in 48 hours for completing one cycle. It increases wear resistance and abrasion quality of the solid metal. Most importantly, it enhances bending and contact fatigue properties with strong forces of metals. This process is carried out with strong safety precautions and under certain conditions and temperatures.  

Stress relieving

The extreme stress heat treating steel is named as stress relieving. This stress is used for removing induced stress in the alloy or metal. This stress is formed due to different processes like machining, rolling, and Annealing, etc.

The metal is firstly heated at the high temperature, and pressure is then allowed to cool down in the air. Finally, this process is essential for the proper heat treatment of steel alloy. In this procedure, the metal becomes pure free of any induced or unwanted stress on its upper layer. The last process is not much time consuming and is most accessible of all.

What does heat treating do to steel?

Heat treating is applied on steel to optimize grain structure for specific properties, relieve internal hardness, creating hard case and tough core for impact applications. Depending upon cycle given during heat treatment, steel properties can be controlled.

What temperature do you heat treat steel?

Mostly, heat treatment temperature lies in region of austenite phase. With carbon percentage, appearance of austenite may very resulting in variation of heat treatment temperature. To understand importance of heat treatment temperature, follow annealing article which can explain importance austenite phase in achieving steel properties.

What are three stages of heat treatment?

What are three stages of heat treatmeIn general, three stages of heat treatment are comprised of heating, soaking and cooling of steel. During soaking, steel structure is homogenized for optimum properties throughout microstructure.

What is heat treatment process?

Heat treatment is a heating and cooling process employed on steel for achieving optimum properties and homogenized microstructure.

Does steel weaken with heat?

No, steel gets softened with heating as stresses are released and dislocations can move easily. This softening can be considered weakening but proper heat treatment can harden the structure.

How do you harden steel after heat treat?

Heat treatment process is employed for hardening steel structure. During heat treatment, after soaking steel in austenite region, it is quenched in water, brine or oil which drastically increase hardness of steel. Details can be studies in Effect of austenitizing temperature and Quenching media on hardening of steel.

What is difference between hardening and tempering?

Hardening of carbon steel improves the hardness of materials due to the formation of martensite. It is carried out by cooling the steel from the austenite region in oil, water, or brine solution. On the other hand, Tempering is employed after the hardening process to induce ductility and toughness of quenched microstructure. The temperature of tempering is lower than that of the hardening process.

Why is heat treatment important?

Casting of steel and primary metal working processes induce various defects like coarse microstructure, segregation of impurities, softness and stresses. All these defects produced by initial manufacturing process can be recovered or removed using heat treatment process.

What is normalizing heat treatment?

Normalizing heat treatment is employed for refining the grain structure. During this process, steel is heated in austenite region than it is cooled in air for increasing hardness.

Conclusion

In conclusion, these were the main seven techniques of steel heat treatment. All these methods have different producers and form different solid metal type. The heat treatment process is known for the manufacture of glass substance. Changing of physical and mechanical properties will help you to maintain the structure of the metal alloy. These seven processes should be used one by one and incorrect order.

All techniques have their own desired temperature and pressure conditions to be carried out. Start Stress annealing, it finally ends on the stress-relieving step. The metal or steel becomes tight, hard, and, most importantly, stress-free at the end of the procedure.

Degree of Polymerization of Polymers

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The Degree of polymerization (DP or Xn) is defined as number of repeating monomer units in the polymers.

The Degree of a polymerization is calculated by taking ratio of molecular weight of the polymer and molecular weight of the repeat unit.

Degree of Polymerization Formula

What is Polymerization?

Polymers derived from word “poly” means many and “mers” mean parts, are long chain molecules formed by repetition of many small simple chemical repeating units.

The repeating units are termed as monomers. These monomers can either repeat linearly or in the form of chains which are connected as branches to form a three-dimensional network. The monomers are linked with one-another via chemical reactions by a process called polymerization.

Types of Polymerization

In 1953, Paul Flory classified polymerization reactions in two groups; step growth polymerization (also called condensation reactions) and chain polymerization (also known as addition reactions).

Types of Polymerization
Types of Polymerization

Step Polymerization

In Step Growth Polymerization, bi-functional or multifunctional monomers react to first form dimers, then trimers, longer oligomers and then long chain polymers.

The mechanism of step polymerization is just like a group of people reaching out each other with their hands to form a human chain, in case of polymers the reactive sites act as two hands which join with one another to form a polymer chain.

As there can be more than one reactive site on a monomer so there is a high extent of polymerization resulting in a high molecular chain. Polyesters, polyamides, polyurethanes etc. are few common examples of step polymerization.

Difference between Step growth and Chain Growth Polymerization
Difference between Step growth and Chain Growth Polymerization

Chain Polymerization

In Chain polymerization, the reaction takes place by an active center which is a free radical or ion to form a macromolecule. The polymer growth takes place at the end points of the chains hence it is also termed as addition polymerization.

The mechanism of reaction involves three steps, initiation, propagation and termination.

Many common polymers are formed by chain polymerization such as polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polyimide (PI), polymethyl methacrylate, polyvinyl acetate.

How to Calculate Degree of Polymerization

Degree of polymerization can be calculated by dividing polymer’s molecular weight to repeating unit’s molecular weight.

Carothers Equation for Degree of Polymerization

Classification of Degree of Polymerization

The Degree of Polymerization is classified in two forms, the number-average degree of polymerization and weight-average degree of polymerization.

The number-average degree of polymerization is the weighted mean of degrees of polymerization, weighted by the mole fractions (or the number of molecules) of the polymer. It is experimentally determined by measurements of the osmotic pressure of the polymer.

The weight-average degree of polymerization is the weighted mean of degrees of polymerization, weighted by the weight fractions (or the overall weight of the molecules) of polymer. It is experimentally determined by measurements of Rayleigh light scattering by the polymer.

Degree of polymerization (DP) of Step-growth polymerization

In step-growth polymerization, the DP is calculated by Carothers’ equation  which is defined for linear and branched molecules.  For linear polymer formed by the reaction (usually by condensation) of two monomers in equimolar quantities, the equation of DP is:

Carothers Equation for Degree of Polymerization

In Linear polymerization where one monomer is in excess quality the equation is modified as:

Degree of Polymerization for linear polymerization

Where, r is the stoichiometric ratio of reactants, as the excess reactant is generally placed in denominator so r < 1 always.

The excess reactant is generally added to control the DP of polymers for a given value of p. When the limiting reagent monomer is added in limit of complete conversion, p → 1 and

Limiting Reagent monomer and DOP

When a monomer has functionality greater than one, it results in branching of polymer. In this case, the DP of polymers will depend on the average functionality of the monomer fav. The fav for a given reaction can be calculated as:

Formula for Functionality of Monomer

The modified Carothers equation for branched step polymerization will be:

Modified Carothers Equation for DOP

Degree of polymerization (DP) of Chain-growth polymerization

The degree of polymerization of chain-growth polymerization, cannot be calculated by Carothers.

At the beginning of reaction long chains are formed and longer reaction times have little effect on the average molecular weight it only increases the polymer yield. Hence, the DP of polymers given by formula below, is associated with the kinetic chain length, which is the average number of monomers polymerized for each chain initiated.

What is Polyamide Fabric?

What is Polyamide Fabric?

Firstly, we are going to answer one of the most important questions of the series, “What is Polyamide fabric?

Polyamide fabric is a synthetic polymeric fabric formed by the condensation polymerization reaction of an amine (-NH2) and a carbonyl (C=O) containing monomer. Polyamides are soft as silk with high elasticity and strength. Synthetic polyamides are commonly used in Parachutes, undergarments, motorcycle garments, tires and conveyor belts, carpets and coated fabrics.

The repeating chains of these two monomers are linked together by an amide linkage. An amide functional group has the formula -CONH2. Polyamide fabric refers to a wide range of fabrics that are manufactured from monomers.

Structure of polyamide linkage
Structure of polyamide linkage

Common Commercial fabrics

The most common polyamide fabric is Nylon, Perlon, and Aramid, etc. DuPont ™ Corporation was the first company to manufacture this fabric in the mid-1930s. The main advantage offered by these fibers is their elasticity, but the customers have also ranked it as soft material.

Polyamide fabric is also known for its military applications. Kevlar, which is a type of Aramid Fabric, is used in bullet-proof jackets because it is light-weight, heat resistant, and stronger than steel.

With high mechanical strength, high heat resistance, wear-resistance, high elasticity and dimensional stability, Polyamide fabric have a wide variety of applications for example:

  1. Parachutes
  2. Fine stockings
  3. Undergarments
  4. Carpets
  5. Outer sports garments
  6. Motorcycle garments

Unlike other fabrics (for instance Rayon and Viscose), these fabrics are easy to handle, these fabrics do not lose their strength when wet and can easily be ironed but they cannot be dry cleaned.

Types of Polyamide Fabric

There are two main types of these fabrics:

  1. Polyamide 6, this fabric is usually known as Perlon
  2. Polyamide 6, 6, this fabric is commonly known as Nylon

The number(s) after the term polyamide indicates the number of carbon atoms that are present in each molecule of the polymer so the name acts as structural formula.

Polyamide 6 structure
Polyamide 6 structure
Polyamide 6, 6 structure
Polyamide 6, 6 structure

Manufacturing Process

The monomers for the manufacturing of Polyamide fabric are commonly extracted from petroleum oil or crude oil. The origin of the monomers for these fibers shows that its manufacturing method cannot be an environmentally-friendly process as petroleum is one of the major sources of pollution. Petroleum oil is also the origin material for many other polymeric materials. This process of polyamide fabric manufacturing is known as a melt spinning process.

The monomers used for the production of fibers are hexamethylenediamine (HMD) and adipic acid (ADA). A chemical reaction takes place when these two monomers and water are added to a reaction vessel.

Hexamethlene Diammonium adipate
Hexamethlene Diammonium adipate

The reaction yields Hexamethylene diammonium adipate, commonly referred to as the “nylon salt” solution. Water is evaporated and removed from the salt solution. After the removal of water, the salt is heated until it melts.

Other additives like a slurry of TiO2 pigments and water can be added to the salt to deluster the fibers. This molten salt polymerizes in the polymerization reactor to form polyhexamethylene adipamide. The polymerization step takes place at 275OC. It is then passed through a metal spinneret with a process called extrusion. The molten salt hardens as soon as it passes through the spinneret to form these fibers. These fibers are collected on a type of spool called a bobbin.

These fibers are then stretched to increase their elastic and mechanical properties. Then the molecules of the polymer are arranged in a parallel structure through a process called drawing. After drawing, these fibers are loaded onto another spool and are ready to be spun into fabric. In most applications, the polyamide fabric is blended with other fabrics to enhance its properties and applications.

Manufacturing process of polyamide fabric
Manufacturing process of polyamide fabric

During the extrusion process, the molten nylon salt is cooled by using a large quantity of water. This water is contaminated and needs to be disposed of safely. Production of polyamide fibers offers other environmental effects as well, for example, Nitrous oxide (N2O), which is a powerful greenhouse gas and contributes to the depletion of stratospheric ozone is produced during its manufacturing.

Moreover, the dust and fumes formed as by-products can irritate the nose, throat, and skin of a worker. Thus, amide manufacturing has a high impact on the environment as well as the workers working in production plants.

Follow these articles for characterization of polyamide fabric and their processing;

  1. FTIR analysis of Aramid
  2. Thermal Analysis of Aramid
  3. Blending of Polyamide products to improve processability

Applications of Polyamide Fabric

After these amide fabrics are obtained, they are used for a wide variety of applications because these fabrics offer many properties. It was invented as an alternative for Silk in order to be used as common commercial fabric. These applications will be the main reason you should be familiar with the concept of, “what is polyamide fabric”.

  • Tires and conveyers belts

Heavy-duty tires and conveyers belt reinforced with polyamide fabric can withstand high temperatures and fast curing cycles. The superior performance is because of its high-temperature stability, high strength, and elasticity.

Tires and conveyers belts
Tires and Conveyers belts
  • Carpeting

The carpet industry was revolutionized when DuPont invented Bulk Continuous Filament (BCF) process to impart bulk to nylon for making carpets.

  • Coated Fabrics

Nylon fabrics can withstand a temperature of 200OC so it is used to coat other polymeric materials.

You can further study Polyamide fabric over WIKIPEDIA.

Other applications of polyamide fibers consist of:

  • Women Stockings
  • Bulletproof vests
  • T-shirts
  • Leggings
  • Shorts
  • Stretch pants

VISCOSE VS RAYON – Ideal Semi-Synthetic fiber

Rayon and viscose are cellulose-based semi-synthetic fabrics. Both the artificial fabric provides comfort like natural fiber, easily breathable features, and smooth texture. The “Viscose VS Rayon” is difficult to postulate but one thing which appears foremost is the ability of Rayon to absorb oil and body sweat which leave spots on Fabric. Rayon, on one side, shrinks while washing in the water while Viscose, on the other hand, stretches. Topic covered here are; Manufacturing Methods, Advantages, and Disadvantages, Environmental Impacts to a clear difference between Viscose and Rayon.

Overview

Both types under discussion are cellulose synthesized semi synthetic fiber. The origin of cellulose in these fabrics is from wood pulp, bamboo trees or directly from cotton. The natural source of the raw material makes it difficult to characterize it as either a natural or synthetic fabric even though the manufacturing process is man-driven.

Rayon and viscose are very inexpensive fabrics and, they are known for their soft and silky touch. The manufacturing process of Rayon was patented in 1855 by a well-known Swiss chemist, Georges Audemars. The commercial production of these fibres started in 1905 by Samuel Courtaulds and Company, Ltd. The cellulose in these fabrics is depolymerized and then regenerated again through chemical reactions. We have detailed their benefits and drawbacks to compare both the fabrics.

Cellulose structural formula is given below;

Cellulose Structure
Cellulose Structure

Viscose VS Rayon

  1. Viscose, as synthetic fabric, is manufactured by driving cellulose only from the pulp, while Rayon drives cellulose from wood pulp, cotton litter, and a bamboo tree.
  2. Rayon is flammable fabric, while Viscose characterizes as inflammable.
  3. Viscose fabric feels like silk and difficult to die while Rayon is easy to die.
  4. Body sweat does not leave spots on Viscose, on the other hand, it gets absorbed into Rayon fabric and leaves visible spots.
  5. Rayon shrinks on washing while Viscose stretches in water.

What is Rayon Fabric

Rayon fabrics
Rayon fabrics

Manufacturing Process

The primary ingredient is cellulose, cellulose is a linear polymer of β-D-glucose repeating units with the empirical formula (C6H10O5)n. The following manufacturing method of Rayon has been in use since the early 1900s. The manufacturing process starts with processed cellulose which is extracted from wood pulp. The wood pulp must contain 87% to 97% cellulose. The cellulose is immersed in caustic soda to remove impurities and convert it into alkali cellulose.

Manufacturing process of Rayon
Manufacturing process of Rayon

Manufacturing of these semi-synthetic fabric follows concepts of polymers. You can study about common polymerization techniques used in Aramid in Article, “Aramid Synthesis”.

The reaction for this conversion is as follows:

(C6H10O5)n + nNaOH → (C6H9O4ONa)n + nH2O

This solution is pressed between two rollers to remove the excess liquid. The pressed sheets are shredded to form a white crumb. This white crumb is depolymerized to an extent. The rate of depolymerization is controlled by temperature, air and the presence of other additives like metallic oxides and hydrides. It is then reacted with Carbon Disulfide to form Sodium Cellulose Xanthate. This process is called Xanthation and it takes place in vats under controlled temperatures of 20OC to 30OC.

(C6H9O4ONa)n + nCS2 → (C6H9O4O−SC−SNa)n

The color of Sodium Xanthate Cellulose is yellow and it is called Yellow Crumb. The yellow crumb is then dissolved in Sodium Hydroxide solution and allowed to age for 4 to 5 days.

Rayon filaments are produced from the aged Yellow crumb solution by treatment with a mineral acid, for example, Sulfuric Acid. In this reaction, the xanthate groups are hydrolyzed to recover cellulose and discharge dithiocarbonic acid that later breaks down to carbon disulfide and water.

[C6H9O4-OCS2Na]2n + nH2SO4  → [C6H9O4-OH]2n +2nCS2 + nNa2SO4

H2COS2 → H2O + CS2

The filaments are then washed and bleached to remove any remaining acid or impurities and are converted into a fabric that can be cut to desired shapes and size.

Advantages of Rayon

Rayon provides comfort just like natural fabric, with a smooth texture and soft-touch, rayon has other advantages as well as some limitations.

  • Inexpensive fabric
  • Smooth texture and silk-like touch
  • Very comfortable
  • Breathable fabric
  • Easily dyed
  • Blended with other fabrics
  • Highly absorbent fabric
  • Drapes well

Disadvantages

Some of the limitations of this semi synthetic fiber are:

  • It wrinkles easily unless treated
  • Difficult to iron
  • Sensitive to heat and Is very flammable
  • Washing it is difficult because it shrinks
  • May always need dry cleaning
  • It weakens when it is wet
  • Toxic chemicals are used during its manufacturing

Disposal and biodegradability

The biodegradability study of various fabrics containing cellulose in aerobic soil was conducted by Mary Warnock. The study concluded that the rate of biodegradation depends on the crystallinity of Cellulose.

Pure rayon, being the least crystalline out of pure Cotton and pure Tencel®, degraded at a much higher rate. The presence of air also increased the rate of degradation. In 2007, 10 million tons of textile waste was buried in landfills, the anaerobic conditions in these sites resulted in slow degradation rates of the fabrics.

A survey in 2014 found that 56.9% of the total fabrics in the ocean were Rayon. It was found in the deep ocean areas. In oceans, the degradation rate depends on the wettability of the fiber, the more hydrophobic it is, lesser will be the degradation rate.

Enviromental effects

Highly toxic chemicals are used during the manufacturing process of Rayon for depolymerizing and regeneration of cellulose. Even though the CS2, NaOH and H2SO4 are removed before it reaches the customer but the removal process of these toxic chemicals is very hazardous for the workers in Rayon producing factories. They are at a high risk of brain damage, heart diseases, stroke and nerve damage. Once these chemicals are dumped into waterways, then the whole community is at a risk of being poisoned.

What is Viscose Fabric

Viscose Fabric - Viscose VS Rayon
Viscose Fabric

Manufacturing Process

To make viscose fibre, sheets of pure cellulose are immersed in NaOH to form alkali cellulose. The solution is pressed between rollers to remove excess liquid. It is then shredded and left to depolymerize in the presence of air and heat in metal containers. The crumbled substance is called white crumb which is then reacted with CS2 to produce yellow crumb. The yellow crumb is known as Cellulose Xanthalate.

Product is then reacted with NaOH to form a honey-like viscous substance known as viscose. This viscose is allowed to age for some time. After ageing the viscose is filtered and degassed to remove any impurity and undissolved particles. It is then extruded through a spinneret; upon exit it reacts with H2SO4 to form viscose filaments.

Manufacturing process of Viscose - Viscose VS Rayon
Manufacturing process of Viscose – Viscose VS Rayon

Viscose VS Rayon comparison | One has singular source while Rayon has multiple cellular source.

Advantages of Viscose

  • Silk-like feel
  • Lightweight
  • Breathable, even better than cotton
  • Very luxurious and cheaper
  • Vivid colors on dying
  • No trapped body heat
  • Soft and comfortable
  • Less sensitive to heat
  • Biodegradable

Disadvantages

Some of the limitations associated with this fabric are:

  • difficult to wash
  • weaker when wet
  • vulnerable to stretching and difficult to recover
  • not recommended for use in home furnishings because it stretches
  • absorbs sweat, body oils, and water, which leaves spots on the fabric
  • Shrinks when washed

Disposal and Biodegradation

The biodegradation of viscose was studied by many researchers in different media including freshwater, soil, marine water etc. A recent study conducted in marine water conditions showed that viscose fully degraded after 35 days. But it can take much more time if disposed in landfills.

Environmental Effects

In 2017, a researcher reported that manufacturers of viscose fabrics were dumping untreated wastewater in local lakes which had a devastating effect on local people. CS2, which is used during the manufacturing of viscose, is considered as a powerful solvent. It will cause very serious health issues like insanity. Moreover, NaOH can cause corrosion, skin burn and eye damaged if inhaled, ingested or if it makes any contact with skin.

FAQ – Viscose VS Rayon

Viscose VS Cotton

Viscose fibre shares many properties of cotton, for example, its comfort and touch are like cotton but it is much cheaper than cotton.
Viscose differs from cotton in many regards, viscose is not as strong as cotton, its wet strength is much weaker than cotton, viscose is semi-synthetic but cotton is completely natural fabric

How to wash Viscose

Washing viscose fabric is a bit tricky. The safest method of washing is hand washing in cold water. Evenly agitate soap in cold water and immerse the fabric for 30 minutes. Press the water out of the fabric and let it dry in its natural shape. Normal cleaning mostly requires dry cleaning as it is a very delicate fabric

Is rayon breathable?

Rayon has very thin fibers that allow air to pass through easily, preventing the fabric to stick to your body during hot and humid weather