The size of carbides that form during tempering are much smaller than the larger carbides that we see with high carbon steels and tool steels. However the carbides that form during tempering are much smaller than even those small carbides. They are certainly much larger than the nm transition carbides. However, it is evident that the carbides formed during tempering are still much smaller than the larger carbides formed at higher temperatures in high carbon steels, tool steels, and stainless steels.
Because tempering is a diffusion-controlled process, the degree of tempering is controlled both by temperature and time. The higher the temperature, the faster the diffusion of elements and therefore more rapid tempering. However, lower temperatures with longer holding time can also lead to the same degree of tempering. C is a constant which they found varied with composition and they determined that it was Therefore hardness is controlled more strongly by temperature because time is on a log scale.
Okay, this article ended up even longer than I anticipated. A lot is going on during tempering. When the steel is heated to sufficiently high temperatures, the carbon precipitates out of martensite as carbides and the martensite recovers and recrystallizes, reducing its tetragonality and dislocation density. The carbides can also contribute to hardness through precipitation strengthening. Changes to retained austenite also occur because it decomposes to bainite or destabilizes and transforms to martensite.
Temperature is more important than time with tempering as time is on a log scale, where much longer times are required at lower temperature to reach the same level of hardness. I covered very little about the effect of tempering on toughness; that will come in future tempering articles.
There are also some practical aspects of tempering I did not cover such as the recommendation by some to quench after tempering. After the background given in this article, I can now cover some other concepts, such as bainite formation as the process has similarities to tempering and therefore we have a basis upon which to describe bainite.
I could write an article about the effect of silicon on toughness as it primarily affects toughness through its effects on tempering. There are also articles about cryo processing of steel which claim that cryo promotes the formation of eta transition carbides to improve wear resistance. Therefore, I can write articles about that aspect of cryo processing since I can reference this article for anyone that needs to understand what type of carbides these articles are referring to.
Steels: processing, structure, and performance. Asm International, Antonsson, eds. Springer handbook of mechanical engineering. Tool Steels. Beachwood, Ohio: American Society for Metals, Hribal, and L. AIME : View all posts by Larrin.
In general longer time at higher temperatures makes the carbides more stable as they become coarser and therefore more stable. However, in either case if the steel was heated to a yet higher temperature they would continue to coarsen.
Hi Larrin. I had some D2 tool steel heat treated. It is softer than I would like. Why is it important to anneal the steel, instead of just re-heat treating it? I figured once the steel is at austenizing temperature, does the previous annealing process have any benefit?
Re-austenitizing can lead to grain growth, excess retained austenite, or other issues. Annealing resets everything to avoid those problems. Re-austenitizing can work but it is not ideal. Multiple austenitizing cycles when done intentionally for grain refinement are done differently. Great work Larrin. Would you consider writing an article on whether precipitation hardening results in better properties for knife steel?
As a result, the swords were strong, but brittle. Their lack of toughness meant that they could not absorb much of an impact before fracturing. Want to learn more about steel metallurgy? See our metallurgy courses page.
Tempering is used to improve toughness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite. At these temperatures the martensite decomposes to form iron carbide particles. The higher the temperature, the faster the decomposition for any given period of time. The micrograph shows a steel after substantial tempering.
The black particles are iron carbide. Untempered martensite is a strong, hard, brittle material. Once this temperature is reached, it is held there for a specified amount of time. The exact temperature and time depend on several factors such as the type of steel and desired mechanical properties.
To get the steel to its critical temperature, some type of heating device must be used. Common devices include gas furnaces, electrical resistance furnaces, or induction furnaces. Often, this heating is done in a vacuum or with an inert gas to protect the steel from oxidation. Once the furnace achieves the desired temperature, a dwell time occurs. Following the dwell time, the furnace is shut off and the steel is allowed to cool at predetermined rate.
Tempering steel after a hardening process allows for a middle ground of hardness and strength. This is achieved by allowing the carbon diffusion to occur within a steel microstructure. When steel is hardened, it can become excessively brittle and hard.
However, when not hardened, the steel may not have the strength or abrasion resistance needed for its intended application. Tempering also improves the machinability and formability of a hardened steel, and can reduce the risk of the steel cracking or failing due to internal stresses.
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