Tempering Steel Process– Microstructure, Color Chart and Alloying elements effects

Steel is one of the most common structural materials used in industry with applications ranging from fighting jets to the small needle, from hard, wear-resistant grinding media to highly plastic beverage cans. With such diverse applications, the material requires an alteration in microstructures to achieve certain criteria. That’s why phase transformations in steel are of prime importance. For getting an idea about steel transformations, Follow the TTT diagram in steel. Tempering steel after hardening is important for quench microstructures which contains a considerable amount of martensite. Martensite generates stresses in the material making it prone to cracks during working. Before reading this post about tempering steel process for as-quenched steels, go through the posts mentioned below;

1. Effect of Austenitizing temperature on Martensitic transformation in steel to develop an idea about Martensite structure, importance of heating in Austenitic region, and hardness of Martensite.
2. Role of Quenching media in Martensitic transformation gives an insight on critical cooling rate. It gives an idea about heat extraction capacity of Water, Brine solution, water and oil and their effect on percentage of martensitic transformation in steel.
3. TTT diagram in steel explains region of martensitic transformation, importance of cooling rate, possible of retained austenite in microstructure and possibility of bainite formation in steel.
4. Alloying elements in steel delay diffusion-based transformations. This increases the chances of martensitic transformation in thick sections. Effect of grain size, carbon content and alloying elements on Phase diagram, TTT diagram and transformations are main theme of this article.

Tempering Steel after hardening Definition

Tempering steel process after hardening is the heat treatment process carried on as-quenched steel to a temperature that is lower than a lower critical temperature line to induce ductility and toughness.

In general, heat treatment is carried out to bring balance between hardness and toughness. With an increase in time and temperature, the hardness of steel decreases. Martensitic transformation is always accompanied by stresses. That’s why post-quench heat treatment can be used for stress relieve as well. We can summarize the main task of tempering or post-quench heat treatment into four points;

1. To relieve internal stresses developed during martensitic transformations
2. To bring balance between hardness and toughness
3. To transform non-magnetic retained austenite into magnetic ferrite
4. To bring dimensional stability in the microstructure

Before discussing changes in the properties of steel due to the process of post-quench heat treatment, we will first discuss Tempering steel Stages.

Tempering Steel Process Stages

Before the start of the post-quench heat treatment process, it is better to bring some insight from the article, “Martensitic transformation” about martensitic structure. After quenching mild steel from Austenitizing temperature, some fraction of austenite converts into needle-like martensite while rest remains as retained austenite. Martensite is a hard phase that generates stresses in the material. These stresses prevent their use in routine applications without stress relieving. Retained austenite developed bring softness in material as well. Tempering Microstructure looks like below;

From start tempering microstructure, post-quench heat treatment stages can be divided into five sections based on microstructures taking place in the above microstructure. The five Post-Quench heat treatment stages i.e. Preliminary, First, Second, Third, and Fourth stage are represented in below picture as P, 1, 2, 3 and 4;

It is a general perception that all tempering steel process results in loss of hardness but, as prevalent in the picture above, hardness increase is also observed. One must note the important fact that all these stages are not absolute and results are superimposed. Alloying elements may shift the stages and hardness behavior as well. The above behavior, in general, can be explained in five bullet points depicted in a picture as a, b, c, d, and e;

1. An increase in hardness is observed during this stage. This is attributed to the formation of epsilon (€) carbide.
2. Hardness in this stage slowly declining after reaching a peak as epsilon carbide increases hardness. With temperature increase, retained austenite is converting into ferrite and carbides. Formation of softer phase ferrite and coarsening of carbides is causing hardness declining trend.
3. The softening stage involves coarsening and loss of coherence by carbides.
4. Increase in hardness due to precipitation of alloy carbides initially dissolved in the matrix
5. Again, the softening stage due to coarsening and loss of coherence by carbides

Preliminary Stage – Tempering Steel Process

We already know one simple fact i.e. Martensite is the metastable state of iron form during non-equilibrium fast cooling. During fast cooling, carbon atoms are mostly located in high strain areas and within the matrix phase. When steel temperature is in between room temperature and 100oC, carbon atoms start moving around.

In order to lower energy, carbon atoms start migrating to dislocation areas and low strain energy areas. They also start forming clusters in martensite and modulated structures before epsilon carbide formations.

First Stage

The first stage of the tempering steel process exists between 100-250oC. During the first stage, carbon starts nucleating from martensite and results in the formation of epsilon carbide. With depletion go carbon from carbon saturated BCT martensite, it transforms into low carbon martensite.

Epsilon carbide formation takes place by nucleation and growth process involving nucleation along martensitic sub-grain boundaries. During the growth process, epsilon form thick film around martensitic plates with carbon continuously diffusing out of martensite.

It must be understood that the formation of epsilon carbide and transformation of high carbon martensite into low carbon martensite is not a single step simultaneous process. With an increase in time and temperature for tempering steel after hardening, conversion takes place slowly.

Initially, precipitated epsilon carbides are coherent with a martensitic matrix with habit planes i.e. (1011) e and (101) a.

The final product of the first-stage tempering steel process is epsilon carbides embedded in low carbon martensite. During this stage, phases are coherent or partially coherent. Carbon diffusion is also taken place to reduce internal stresses. Matrix is, now, low carbon martensite. This low-carbon martensite with reduced stresses and precipitates of epsilon carbide makes steel in this stage harder and tougher.

Second Stage

As the temperature is increased from 200-350o C, the second stage of the tempering steel process starts. We have shown in the as-quench microstructure above, retained austenite is also the product of martensitic transformation as achieving the Final martensitic transformation line is very difficult due to a large number of stresses generated and required cooling rate. This retained austenite is stabilized due to uneven carbon distribution and stresses developed during martensitic transformation.

We can say, retained austenite is a metastable product, which will convert into carbides and ferrite upon heating the structure in the second stage of tempering steel after hardening.

We see a continuous decline in hardness value from peak hardness obtained during the first stage. For measurement of hardness, equipment’s like Brinell hardness tester, Vicker hardness tester, and Rockwell hardness tester are employed.

This is attributed to the fact that; previously nucleated carbides are slowly moving towards the coarsening stage and results in the incoherent microstructure. All type of stresses gets relieved during stage which lowers the hardness despite the fact that carbide is nucleating from soft austenite phase.

Third Stage (Tempering Steel Process)

The third stage of post-quench heat treatment starts from 300oC. Within this stage, all products of the first and second stages are replaced by ferrite and cementite.

We have discussed in the first stage, that, high carbon martensite is diffusion carbon out of the matrix to form low carbon martensite and epsilon carbide. At this stage, low carbon martensite diffuses out left-out excessive carbon to form the ferrite matrix. This results in a softer matrix giving rise in toughness. 

On one side, we have low carbon martensite converting into ferrite giving complete ferritic matrix. On another side, the coarsening of graphite is occurring converting into spheroids of cementite to reduce interfacial energy. At the end of the stage, we have a spheroidal cementite matrix distributed in the ferritic matrix.

It is pertinent to mention that, for mild steel, no further changes will happen except the coarsening of spheroids reducing the hardness of steel. For alloy steel, carbide forming elements will start nucleating as carbide out of ferritic matrix giving rise to secondary hardness in Fourth Stage.

Fourth Stage of Tempering steel after hardening

We mentioned below that there are two types of alloying elements are there. One class contains carbide forming elements and another class contains non-carbide forming elements.

Non-carbide forming elements will have very little influence on tempering behavior.

Alloy carbides are very stable and diffusion of alloying elements is almost negligible during the start of post-quench heat treatment stages. After reaching the temperature of 400oC, alloying elements diffusion speeds up and carbides of metalloids start nucleating at the expense of cementite, thereby creating the effect of secondary hardness within Alloy steel.

Tempering Steel Color Chart

With a rise in tempering temperature, oxide on the surface of steel starts forming on the surface. This layer of oxide surface thickens with rising temperatures. Due to the interference process, light reflecting from the oxide surface may give color against different oxide thicknesses.

In earlier times, tempering steel color was used to determine tempering temperature. One important point to note is that tempering steel color appears above 200^o C. Below this temperature, oxidation of steel occurs but thickness is very less resulting in no tempering steel color.

Since tempering steel color appears against the thickness of the oxide film, it should be pertinent to mention that, along with a rise in temperature, an increase in time may also affect tempering color.

In order to remove this anomaly, tempering temperature is noted instant the color appears or after 1 or 2 minutes.

Tempering steel color chart used are shown below;

Classification of Tempering steel Process

During post-quench heat treatment after hardening stages, there are three trends we followed;

1. Increase in Hardness: During the start of post-quench heat treatment after hardening, epsilon carbide nucleates and increases hardness. The first stage is termed as Low-temperature tempering.
2. Increase in Toughness: With martensite converting into ferrite and carbides converting into spheroids, toughness increases along with machinability of steel. The second stage is termed as Medium tempering temperature.
3. Increase in Secondary hardness: In alloy steels, carbide precipitation delays resulting in secondary hardness. This stage is termed as a high tempering temperature.

Low Tempering Temperature (Tempering Steel Process)

Hardened steel has a large number of internal stresses which can result in quench cracks and various distortions and warping mechanisms. Low tempering temperature will be used to relieve internal stresses. This results in lower brittleness without losing much hardness and a slight increase in toughness. This type of treatment is given to plain carbon steels and low alloy steels and this results in tempered steed having high cutting ability, abrasion and wear resistance.

Low temperature tempering steel process is carried out in a salt bath and furnace which has the facility for air cooling. Tempering oil is silicon oil which is used for low temperature tempered steel. For surface hardened steel like carburized steel, low tempering temperature is preferred.

Medium Tempering temperature

Troosite tempering microstructure is the main product of medium-temperature tempering. Temperature range is 250^oC – 500^oC. Troosite is a highly elastic microstructure with hardness in a range of 40-50 HRC.

Troosite tempering microstructure if water quenched from tempering temperature results in compressive stresses within the steel structure. These compressive stresses increase the endurance limit. The combination of a high elastic matrix, medium-range hardness, and high endurance limit makes transit microstructure ideal for spring steel, coil and dies.

High Tempering Temperature

High strength and toughness are the main characteristics of this tempering temperature range. The structure of this type developed during this post-quench heat treatment range is termed as Sorbite. Sorbite is a tempering microstructure used for machine tools. At this post-quench heat treatment range, all types of stresses included internal and residual gets relieved and material is ready for post-tempering processes.

Effect of Alloying Elements on Tempering Steel process

In alloy steels the temperature at which transformations take place changes as a result of which the rate of softening is retarded. The transition carbides (e.g. -iron carbide) and the supersaturated martensitic structure becomes stable at higher tempering temperature and the precipitation and growth of cementite are also delayed.

Effect on the Formation of Iron Carbides

At the early stages of the post-quench heat treatment steel process, the changes in structure are quite difficult to follow but alloying additions render the transformation delaying the initial structural changes. Some alloying elements stabilize the iron carbides at higher tempering temperature i.e. iron carbide is stable up to 400°C in steels with the addition of 1-2 wt% Si which slows the nucleation and growth and further increase may stabilize it even at higher temperatures.

In contrast, few elements like Mn and Ni decrease the stability of supersaturated iron carbide solid solution. Furthermore, the addition of alloying elements e.g. Cr, Mo, W, V, Ti, Si, also maintains the tetragonality of martensite which vanishes by 300°C in plain carbon steels, stabilizing it at 450°C and even as high as 500°C.

The addition of alloying elements delays all stages of tempering steel process as the coarsening of cementite at 400-700°C in the fourth stage of post-quench heat treatment, can be delayed effectively when Si, Cr, Mo, and W are present in steels. These steels can have higher hardness with good toughness even at higher temperatures and can be used in high friction applications. These alloying elements hold the fine Widmanstatten precipitation of cementite even at higher temperatures either by segregation at the carbide/ferrite interface or by making the place into the cementite structure.

The Formation of Alloy Carbides (Secondary Hardening)

Alloy steels containing Mo, V, W, Ti, Cr at higher alloy concentrations when tempered in the range 500-600°C , fine alloy carbides precipitate out of structure resulting in secondary hardening. This secondary hardening is just like the age-hardening process in which coarse cementite is replaced by fine carbides with increases in strength of steel even ore than as-quenched martensite.

Chromium: Cr is a strong carbide former even 0.5% Cr can retard the softening during post-quench heat treatment and 4% forms carbide resulting in secondary hardness. Steels with12 wt% Cr have higher secondary hardness due to formation of Cr₇C₃ additionally, Cr₂₃C₆ nucleate at the same time but at austenite grain boundaries and at ferrite lath boundaries

Molybdenum: The effect of Mo is more pronounce in maintain higher hardness and strength at higher temperatures as compared to Cr even in small amounts.

Vanadium: It maintains the hardness even at 550°C when added in a small amount to steels. The V₄C₃ is a stable and high strength carbide which replaces cementite at higher tempering temperature.

References

1. “Effect of Alloying elements on steel” by Mehran Maalekian, TU Graz
2. An Article on “Tempering of Steel: Stages and Classification” by engineering notes
3. Thesis on, “Second stage of Tempering” By Shoulu Chang, University of Wollongong