Effect of Alloying elements in Steel, TTT diagram and Phase transformation diagram

It’s a long-standing tradition to discuss the effect of alloying elements in steel to achieve better properties like Nickel make steel tougher and chromium makes steel harder. This study would require investigations of a large number of ternary phase diagrams over a wide temperature range. However, to summarize, Scientist has divided alloying elements into three classes i.e. ferrite stabilizer elements, austenite stabilizer elements, and carbide former elements.

With the addition of the above alloying elements group, four types of changes would possibly occur in the phase diagram including expanding austenite region, contracting austenite region, etc. So, in this article, we will give a brief introduction to steel classification, phase diagram variants, and the effect of alloying elements in steel. It’s better for your understanding, If you read the Article on TTT diagram of steel to learn about the basics of the TTT diagram and various transformation terms that will be used in this article.

Steel Classification

Steel classification is important and largely depends upon carbon percentage and alloying elements added in steel. Importance of alloying elements in steel can be understood from steel classification given below;

Steel is divided into two major groups;

1. Carbon Steel
2. Alloy Steel

Carbon Steel: Carbon steel is steel with carbon percentage up to 2% with no ternary alloying addition.

Carbon steel is further divided into low carbon, medium and high carbon steel.

Alloy Steel: If one or more ternary alloying elements along with carbon are present in steel, than it is termed as Alloy steel. More precisely, steel with addition of alloying element along with carbon to bring some positive effects in steel is an alloy steel.

Alloy steel is also divided into Low alloy, and high alloy steels.

Effect of Alloying elements in Steel and Phase diagram

Alloying elements are added in interstitial position, and creates strain field around it. This distortion in crystal structure and presence of strain field prevents dislocation movement making steel stronger and harder.

For a complete understanding of the effect of alloying elements effect in steel, it is best to study the behavior of phase diagram due to the addition of alloying elements in steel.

To study effect of alloying elements in steel, it is best to understood study article on TTT diagram in steel and martensitic transformation and Widmanstatten transformation to have brief idea about, “How these phase diagram works?”. As we have explained earlier, ternary phase diagram prevails due to addition of alloying elements, that’s why iron binary phase diagram undergoes following classification;

1. Open gamma field
2. Closed gamma field
3. Expanded gamma field
4. Contracted gamma field

Type -1: Open Gamma Field

It can be seen in above picture of open gamma field; gamma region is expanded by raising gamma to delta transformation region and by depressing gamma to alpha transformation region.

Elements which have this type of effect of phase transformation are Nickel, Manganese, Cobalt, and inert metals such as platinum.

If we add Nickel and Manganese in fairly high amount, then it is possible to stabilize austenite at high temperature which can be clearly seen in picture that A3 and A1 line completely vanishes at high concentration of alloying elements. With austenite stable at high room temperature, Austenitic steel are developed using high concentration of Nickel and Manganese.

Type – 2: Expanded Gamma Field

This group of alloying elements is somewhat similar to open gamma field in few concepts like, similar to open gamma field, it depresses gamma to alpha transformation to lower temperature, and raises gamma to delta transformation to high temperature but range of existence of gamma field gets limited.

Most important elements in this group are Carbon and Nitrogen. Elements like Copper, Zinc and Gold have similar effects.

The expansion of homogenous gamma region proceeds till 2% carbon and 2.8% nitrogen. With further addition of these alloying elements, gamma field become inhomogeneous and new products gets nucleated as seen in picture.

Type – 3 Closed gamma field

Within this group, added elements contracts the gamma region to small area called gamma loop. Within this group, elements added raises the alpha to gamma transition to high such high temperature that delta and alpha fields become continuous. With the fair addition of these elements, the gamma field can be avoided by the direct transition from delta to alpha region during cooling.

Most common elements belong to this group are Aluminum, Beryllium, and Phosphorus. Carbide forming elements like Titanium, Vanadium, Molybdenum, and chromium also help contract gamma field by forming the carbides and reducing required carbon.

Steel with large wt.% of above added elements are not suitable for heat treatment due to absence of gamma to alpha region. These means, “Martensitic transformation is not possible with these alloying additions”.

Type – 4 Contracted gamma field

It is somewhat similar to expanded gamma field in a concept that gamma loop is strongly contracted but is also accompanied with compound formation as shown in picture.

Elements like boron along with carbide formers Tantalum, Zirconium, and Niobium have major contribution in this group.

Similar to close gamma field, heat treatment of steel is not possible with alloying elements of this group.

Classification of Effect of Alloying elements in Steel

We have discussed four variants of iron-iron carbide phase transformation diagram provided carbon content remains same while alloying elements concentration is varied.

Based on evaluation of these diagrams, alloying elements in steel are broadly classified as;

• Austenite stabilizer elements (e.g., Ni, Mn, Co, Pt)

We have discussed earlier; these elements are responsible for open gamma field region. Application of these type of steels is found in Austenitic steel like Hadfield steel.

• Ferrite stabilizer elements (e.g., Si, Mo, Cr, Al, Be, Zr, Ti, V)

With addition of ferritic stabilizers, gamma loop shrinks and maximum matrix contains ferrite. Steel are normally called ferritic steels. Application of ferritic steel is transformer sheet material which is made up of 2% Si low carbon steel.

• Carbide forming elements (e.g., Cr, Mo, W, V, Nb, Ta, Ti, Zr)
• Carbide stabilizer elements (e.g., Cr, Mn, Si, Zr, Hf, Mo)
• Nitride forming elements (e.g., Cr, Al, Ti, Ta, V, Nb)

Details of Alloying elements and their behavior in steel will be discussed later in the article.

Effect of Alloying element in steel on Eutectoid Point

Eutectoid transformation normally in plain carbon steel requires 0.8% carbon and 768^o C temperature. This is one of very important and widely used solid state transformation converting austenite into pearlite upon cooling. With addition of austenite stabilizer elements, ferrite stabilizer elements, and carbide forming elements, eutectoid point on phase diagram changes position not only on temperature scale but also on carbon scale.

With austenite stabilizers, austenite stabilizes at lower temperature as well which indicates eutectoid transformation will occur at lower temperature. While carbide forming elements will have affinity towards carbon thereby altering required carbon percentage for eutectoid reaction. This alters carbon percentage for eutectoid transformation.

Variation in required temperature and Carbon percentage with alloying elements can be shown by the picture below;

Distribution of Alloying Elements in steel

Well, we have clear understanding of how these elements are beneficial in determining final microstructure in steel. Since, iron has BCC and FCC structure in working temperature range and solubility of interstitial atoms is limited. So, with lots of alloying elements added in steel, it’s possible that some elements remain in steel as soluble, some form carbides and some exists as inclusions.

Normally, in Commercial steels, alloying elements are present in following forms:

• In free state, as separate entity like particle of Platinum etc.
• As intermetallic with iron or other alloying elements
• As inclusion or oxide or sulfide
• As a carbide compound
• In form of homogenous solution in iron

For understanding alloying elements behavior within iron, we will divide alloying into two groups;

1. Inability to form carbide (e.g Si, Ni, Al, Cu and Co)
2. Carbide forming elements within steel (e.g. Mo, V, Cr, W and Ti)

Inclusions

Alloying elements which don’t form carbides or intermetallic, they will be dissolved as homogenous solid solution. Austenite and Ferrite have limited solubility for alloying elements. For example, Cu can be maximum dissolved in iron up to 7%, if it exceeds this percentage it will exist as metal inclusion. In case of nitrogen, maximum solubility limit is up to 0.015% and remaining nitrogen will exist as inclusion or will form nitrides with other alloying elements like V, Al and Ti.

Oxide Formation

Elements which form inclusions are problem for steel as they are not as hard as iron and they will make structure soft, but some inclusions are very beneficial in improving performance of steel for instance, oxide forming. Iron is made up in blast furnace which can have considerable amount of oxygen in it. This oxygen reacts with iron to form of iron carbide and reduces properties. Some, elements which dissolves in steel and have more affinity towards oxygen act as deoxidizers and removes oxygen. Some carbide forming elements like V and Ti also reacts with oxygen to form oxides which pin the grains to form fine microstructure generating harder steel.

Carbide Formation

The elements which form carbides in steel can exist like solid solution or carbides. The distribution of carbides depends upon of alloying elements concentration and carbon percentage. If steel contains high alloying element percentage, then all carbon will be used for carbide forming elements but alloying elements in form of inclusion will be still part of microstructure.

Effect of Alloying elements in Steel TTT diagram

We have characterized effect of alloying elements in steel as austenite stabilizer elements and ferrite stabilizer elements and carbide forming elements. Each element will have considerable effect on TTT diagram curves and will affect any transformations taking place.

We can say, in general, alloying elements in steel will affect kinetics of all transformations including Pearlitic, baintic, and martensitic transformations. Follow TTT diagram in steel to study these transformations…

Normally all transformations, either diffusion or diffusion less, depends upon critical cooling rate.

The Critical cooling rate is termed as a cooling rate associated with a cooling curve tangent to C shaped TTT curve is termed as a Critical cooling rate. Any cooling rate which is faster or equal in magnitude to CCR will produce martensite.

Following factors Critical cooling rate of TTT diagram;

• Grains Size
• Carbon Content
• Alloying elements

With increase in carbon content or percentage of alloying element or grain size results in shift of c-shape curve towards right making it feasible for martensitic formation.

Grain Size: Fine grain size will have more grain boundary area and more nucleating points results in easier nucleation of pearlite.  This promotes diffusion-based transformation. With Coarse grain size, triple points and grain boundary is very limited giving few numbers of nucleating points for Pearlitic formation resultantly delaying the Pearlitic transformation. This causes shifts of c-shape curve towards right.

Carbon Content: With the increase in carbon content, chances of martensitic formation increase. This is due to ease in achieving hardenability in steel due to high percentage of carbon. Important point to consider is that increase in carbon decreases start of martensitic transformation temperature “Ms”.

Alloying Addition: Different alloying elements will have different effect on steel TTT diagram. One common effect that all alloying elements will have on TTT diagram is shifting of c-shape curve towards right.

With the addition of alloying elements, diffusion process slows down and time required for Pearlitic formation increases. This can be understood by following figure;

Other important effect of alloying elements depends upon ferrite stabilizer and austenite stabilizer elements.

Effect of Austenite stabilizer elements on TTT diagram

With the Austenite stabilizer elements like Ni, Mn, Si, eutectoid transformation goes down or open gamma field curve is followed making austenite stable at a lower temperature. This causes merging of pearlite plus bainite region as pearlite transformation also gets delayed or depressed with lowering of eutectoid transformation region.

Effect of Ferrite stabilizer elements on TTT diagram

When the ferrite stabilizer elements are added in steel like Cr, Mo, V, etc. austenite region shrinkage and eutectoid transformation line go up separating the Pearlitic and bainitic region. With the addition of ferrite stabilize elements, not only diffusion slows down which shifts the pearlite region towards the right but also separates pearlite and bainite bay. This helps in easier control of bainitic transformation.

Summary (Effect of Alloying elements in Steel)

With all literature above, it becomes clear that effect of alloying elements in steel is of prime importance in achieving optimum properties.

Few common observations are alloying elements that delay the diffusion-based transformation thereby reducing the critical cooling rate requirement for diffusionless transformations. An increase in grain size also promotes martensitic transformation. Increase in Carbon percentage also promotes martensitic formation.

Effect of Alloying elements in steel discussed can be summarized below;

References

1. Article on, “The Effect of Alloying elements in Steel” by Total Materia
2. Article on, “The influence of Alloying elements in Steel” by the industrial heating
3. Presentation on Engineering Materials
4. Article by Substech on Influence of elements addition on steel.