Heat Treatment of Steel

Various types of heat treatment processes are used to change the following properties or conditions of the steel:

Improve the toughness

Increase the hardness

Increase the ductility

Improve the machinability

Refine the grain structure

Remove the residual stresses

Improve the wear resistance

The following are the general reasons for heat treatment:

Hardening (Steels can be heat treated to high hardness and strength levels. The reasons for doing this are obvious. Structural components subjected to high operating stress need the high strength of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation.)

Tempering (As-quenched hardened steels are so brittle that even slight impacts may cause fracture. Tempering is a heat treatment that reduces the brittleness of a steel without significantly lowering its hardness and strength. All hardened steels must be tempered before use.)

Softening a Hardened Structure (Hardening is reversible. If a hardened tool needs to be remachined, it may be softened by heat treatment to return it to its machinable condition. Most steels weld better in their soft state than in their hardened state; softening may be used to aid weldability.)

Recrystallization (If a metal is cold worked, grains or crystals deform, become elongated, and in doing so harden and strengthen a metal. There is a limiting amount of cold work that a particular metal can be subjected to. In rolling of steel into thin sheets, you can only reduce the cross-sectional area so much before it gets too hard to roll. At this point it would be desirable to return the grains to their original shape. Heat treatment can accomplish this. The transformation of cold-worked grains to an undistorted shape is called recrystallization. Very large coarse grains can also be refined by recrystallization.This type of heat treatment is essential if a steel is to be subjected to severe cold working in rolling, drawing, etc.)

Stress Relief (One of the most frequent reasons for heat treatment is to remove internal stress from a metal that has been subjected to cold working or welding. Stress relieving is a heat treatment used to remove internal strains without significantly lowering the strength. It is used where close dimensional control is needed on weldments, forgings, castings, etc.)

Hot-Working Operations (Most metal shapes produced by steel mills are at least rough shaped at elevated temperatures. Heat treating is required to bring the rough metal shapes to the proper temperature for hot-forming operations.Forging, hot rolling, roll welding, and the like are all performed at temperatures of sufficient magnitude as to prevent the formation of distorted grains that will harden the metals. Hot-working operations require dynamic recrystallization which is achieved by working at the proper hot-work temperatures.)

Diffusion of Alloying Elements (One of the criteria for hardening a steel is that it have sufficient carbon content. Low carbon steels can be hardened, at least on the surface, by heat treating at an elevated temperature in an atmosphere containing an alloying element that will diffuse into the steel and allow surface hardening on quenching. Carbon is frequently diffused into the surface of soft steels for surface hardening. Using this same principle, elements such as chromium, boron, nitrogen, and silicon can be diffused in the surface of a steel for special purposes.)

Figure 1 shows major types of heat treatment processes.

Figure 1. Types of Heat Treatment Methods