We studied an important concept of the Critical cooling rate in the TTT diagram. It was stated, slower cooling rate results in Pearlite and Cooling rate higher than CCR results in Martensite. While quenching thick steel castings, the heat transfer rate decreases as we move towards the center which causes a cooling rate lower than CCR resulting in Pearlitic formation in the center of steel castings. This variation in microstructure and percentage of martensite formation can be described by the term Hardenability of Steel. In this article, the main things we are going to focus on are; What is the hardenability of steel, how is hardenability measured, factors affecting the hardenability of steel, Jominy End Quench Test.
Follow the below article for a better understanding of the concept of hardenability of steel;
- Role of quenching media on Martensite formation
- Role of Critical Cooling rate on the martensitic formation
- Role of Austenitization temperature on martensite formation
- Microstructure development in case of equilibrium cooling in a Phase diagram
What is the Hardenability of Steel?
Hardenability of steel can be defined as,
“Hardenability of steel is depth to which structure contains a minimum 50% of martensite in its microstructure.”
Hardenability of steel is considered very important as it gives the idea of the quenching rate needed for steel to achieve the required properties. With this term, we can qualitatively evaluate maximum hardness that can be achieved in thick castings using standard quenching methods in the center and surface of the steel.
Hardness VS Hardenability of Steel
The idea of hardness is very different yet relatable to Hardenability. With a variety of hardness, one can get the idea of hardenability.
Hardness can be considered as steel’s property which is resistance to penetration. While hardenability is materials ability or potential to achieve maximum hardness by thermal treatment.
Hardness is localized resistance to load measurement property in case of steels. Equipment’s we can use for hardness measurements are the Vicker hardness test, Rockwell hardness test, and Brinell Hardness test.
To achieve maximum hardness in steel, the following methods can be used which are also explained in the TTT diagram;
- All carbon must be present in solution form in Austenitizing temperature critical Cooling rate is achieved
- Absence of retained austenite
- Avoid Auto tempering of steel
All these factors must be achieved at some time in the center and surface to achieve uniform hardness. With center not being able to achieve CCR completely, chances of pearlite and retained austenite formation increases giving lower hardness. This lowers the ability of steel to be hardened in the center.
This variation of hardness in the center and surface of steel give rise to the idea of hardenability of steel. Several factors like alloying elements, quenching media, the thickness of castings limit the cooling rate in the center, and resultantly generates Pearlitic microstructure.
With hardenability, depth is measured up to which microstructure contains a minimum 50% of martensite.
Why can’t we achieve maximum hardness in the center of castings?
The cooling rate depends upon quenching media.
Better the quenching media, the faster the cooling rate. As castings get thick, the cooling rate in the center decreases. To get a higher cooling rate, very severe cooling is required. This severe cooling can result in distortion or warping in steel. So, hardenability of steel defines the ability to harden without distortion. We can check hardenability of particular steel using Jominy End Quench Test.
Case Study – Hardenability of 1040 steel
1040 steel is hypo eutectoid steel containing 0.40% carbon. This carbon steel can be an excellent choice for hardening. You can study the microstructure of 1040 steel in the Phase diagram article.
While quenching 1040 steel, martensite will form on the surface. As we move towards the center, martensite percentage decreases and pearlite concentration increases ultimately achieving complete pearlite in the center of steel provided cooling rate is slower than CCR.
The martensite phase has a hardness of 65 HRC. While, on the other hand, pearlite has a hardness of 40 HRC. Microstructure which contains 50% martensite and 50% pearlite has a hardness of 54 HRC.
When we measure the hardness of thick casting from surface till center, variation in hardness is observed as given in the picture;
From hardness values, we can estimate the hardenability of 1040 steel.
At surface hardness, 64 HRC indicating complete martensite and, in the center, 40 HRC indicating complete pearlite. At some point while moving from surface to center, 54 HRC is observed indicating 50% martensite and 50% pearlite.
There is a horizontal line intersecting the hardness curve which explains the region divides into two. One region contains a hardened steel region while the other region contains the Pearlitic region.
The position where 50% pearlite and 50% martensite is obtained as the maximum depth up to which steel has hardened with given quenching medium.
How is hardenability measured
Hardenability of steel is measured by the following methods;
- Grossman’s Critical Diameter Method
- Jominy End Quench Test
- Fracture Test
- Use of chemical composition for hardenability
Grossman’s Critical Diameter Method
Grossman’s method provides a direct approach to measure to a critical diameter up to which center portion contains a minimum 50% pearlite and 50% martensite.
Up until now, the following points have been mentioned;
- Cooling rate decreases as we move from the surface of the steel to center.
- Percentage of martensite decrease with a change in cooling rate as we from the surface to the center.
- The depth up to which 50% martensite is present is considered a hardened region while the rest of the region which contains higher pearlite percentage is considered unhardened region.
With the Grossman method, we can choose several diameter rods of specific composition and perform quenching. Now we can measure hardness in the center and surface.
We can identify the hardness for 50% martensite and 50% pearlite region.
Now Grossman’s critical diameter is the maximum diameter which contains hardness incenter equivalent to the hardness of structure containing 50% martensite and 50% pearlite.
This critical diameter can help in identifying the maximum diameter which can be hardened using specific quenching media. With a change in composition and quenching media, this critical diameter is subject to change.
With an increase in diameter, variation in hardness can be seen below. The structure which contains hardness throughout the cross-section above the horizontal line is uniformly hardened steel.
You can further study the Grossman critical diameter from the wonderful book given below;
Jominy End Quench Test
Jominy end quench test is a precise and less costly method to find a critical diameter for specific steel composition.
Here, we use a standardized bar to heat it to austenitizing temperature and soak it there for homogenization. After soaking, the water jet stream is used to quench one side of the bar which results in a progressive decrease in cooling rate from the quenched side to the hot side. The depth up to which 50% martensite is obtained is termed as Hardenability depth.
The experiment specimen is 0.1m equals to 4 inches in length and 25 mm equals to 1 inch in diameter cylinder-like structure. (Fig below). The type of steel is normalized to eradicate disparities in micro-configuration due to aforesaid heated work) and again austenite mostly at a heat having a temperature of 795 to 930 C° equals to 1460 to 1710 F.
The examination specimen is rapidly disseminated to the experiment faucet (Fig. below), which makes steel to be quenched by drizzling a steady ebb of moisture over the one edge of the specimen (Fig. below). The refrigerating ratio changes along with the size of the sample and is very immediate at the quenched side and at which water blows sample, up to at the end which is comparable to air replenishing is very slow.
The pudgy sample is accordingly ground straight parallel to its length on contrary aspects to a chasm of 380 microns to eliminate the decarburized substance. Attention must be put up with that abrasions do not warm up the specimen, as tempering can be caused by this, and it may cause the softening of steel and steel to become fragile.
Hardness is computed at lengths from the end, which is quenched, commonly at 1.5 millimeters length for steel of alloy and about half of 1.5mm for steels of carbon, starting up as near as feasible to end which is quenched. The hardness reduces with the extent from the end which is quenched. The area or length where a high ratio of martensite forms has very high hardness along its length. If there is lower hardness so we can say that there must be the formation of pearlite/ferrite mini structures or bainite.
The Jominy End quench test is considered most feasible method for quantitative estimation of Hardenability.
There are two phases under discussion; One is Martensite and the other is Pearlite. Pearlite has a ductile fracture appearance while Martensite has a brittle fracture appearance due to variation in hardness. Ductile and brittle fracture appearance can be seen in the picture.
Steel at the surface appears to have a brittle fracture due to martensite formation. With moving towards the center and crossing the 50% martensite and 50% pearlite region, a sudden shift towards ductile fracture takes place. This sudden change is accompanied by a sudden transformation shift.
This fracture test can be used only in those cases where the change from martensite to pearlite is sudden and we can identify the region of maximum depth.
Use of chemical composition for hardenability
Although we have tried till now tried to establish maximum depth experimentally by calculating hardness or microstructure observation, but we can also estimate qualitatively with chemical composition.
For a set carbon percentage and grain size of steel, hardenability can be estimated qualitatively. You can observe in this link that with an increase in carbon percentage and grain size, hardenability depth increases which indicated higher martensitic formation. This concept is also explained in the Effect of alloying elements on the TTT diagram.
Alloying elements also influence the martensitic formation and hardenability depth. One of the most important features of alloying elements is that they delay the high-temperature transformation thereby increase the chances of martensitic formation.
All alloying elements quantitively affect the hardenability curve differently as we can see below in the figure. It must be understood that all alloying elements effect is independent of others.
If we have Critical Diameter of Di than it will be multiplied by factor F for each of alloying elements as given by the formula below;
The alloying elements like Phosphorus and Sulphur are considered impurities and their multiplying factor is only 1 If u must consider.
Here is a case study from the tables mentioned above for understanding;
Factors affecting hardenability of steel
We are discussing a lot of stuff about hardenability and its dependence upon Pearlitic and martensitic formation. So, we are going to look into some factors which check the Pearlitic formation thereby increases the chances of martensitic formation giving better hardenability.
Why does austenite grain size increase hardenability?
Austenitization is very important in the quenching process. The formation of martensite depends a lot on the austenite phase as explained in the article, “Effect of austenitization temperature in Steel”.
At the start of austenitization, the structure contains a fine and large quantity of austenite grains. These large numbers of austenite grains have large grain boundaries per unit area which increases the chances of heterogenous nucleation giving rise to Pearlitic formation.
So, giving more soaking time on austenitization temperature increases in austenite grain size. This increase in austenite grain size lowers the chances of Pearlitic formation and raises the hardenability of steel.
Although an increase in austenite grain size gives better hardenability this process is not recommended for practical use, because higher grain sizes have their demerits like lower impact strength, lower ductility, and higher chances of quench cracking.
How does carbon content affect hardenability?
Practically, carbon steels have lower hardenability. In plain carbon steel, there is no hindrance to diffusion of atoms and the normal quenching method can not achieve CCR. This is why manganese and silicon are added to shift the TTT curve to the right side and delay the diffusion transformation giving easier martensitic formation.
If we consider only the carbon percentage effect on the hardenability of steel, hardenability first increases as we move up to 0.8% and then it decreases from 0.8% to 2%.
Up to 0.8% of steel, an increase in carbon content results in more chances of martensite formation. In 0.2% carbon steel, the carbon percentage is too less for martensite formation. Martensite is a high carbon phase having a BCT structure. If the carbon percentage is already on the lower side, chances of martensite formation decrease.
After 0.8% Carbon, annealing or austenitization takes place in the Inter-critical annealing region. This region consists of austenite plus cementite at the Austenitization temperature. The presence of cementite limits hardenability as only austenite gets the chance to form martensite. With more carbon, the cementite percentage in microstructure increases.
What alloys are particularly effective in increasing the hardenability of Steel?
The prime purpose of alloying elements is to delay the diffusion-based transformation giving better hardenability.
Alloying elements play a variety of roles in steels which have a direct effect on properties of steel. We are going to discuss a few cases.
Austenite transformation takes place by nucleation and growth at the expense of pearlite, cementite, and ferrite. After reaching austenitization temperature, austenite forms on the ferrite region, but soaking time is needed for complete homogenization and carbon distribution.
Time for homogenization is more than time for austenite formation.
Alloying elements if present will not dissolve at homogenization temperature due to higher carbide and nitrides stability. Each carbide and nitride have a higher temperature of dissolution in austenite.
If carbide remains undissolved in austenite, then it will reduce the carbon content in solution and also restricts the grain growth. We have studied above that increased grain size and carbon content favors hardenability.
Cobalt favors diffusion-based transformation due to ferrite stabilizer nature and results in lower hardenability. Cobalt is normally avoided in steels which needed deep hardenability.
Vanadium and Niobium carbides are very stable. They take as much as 1050oC and 1150oC respectively for dissolution in austenite. The process of dissolution will start at this temperature and we have mentioned above that homogenization will take more time than nucleation.
So, soaking at such high temperatures results in excessive grain coarsening and oxidation of steel. Excessive coarsening results in lower impact resistance and many other negative properties which diminishes the purpose of heat treatment. That’s why it is best avoided to use niobium or vanadium in steel needed deep hardenability.
Effect of Boron
Boron is considered a very important element in deep hardenable steels. If it stays in a solution of austenite grains, it diffuses itself towards grain boundaries and restricts the nucleation of pearlite giving hardenability with lower cooling rate.
Boron is reactive towards nitrogen and oxygen. So, deoxidizers must be added along with boron to completely remove nitrogen and oxygen for boron to do its trick.
Boron can be added as little as 0.002% to 0.005% and can have an equally strong effect as 1.5% Molybdenum. They are preferably used with low carbon steels.