Heat Treatment of Alloys

Annealing reduces the hardness, improves machinability, facilitates cold forming, produces a desired microstructure, and alters mechanical properties. Annealing normally means full annealing, which involves heating the steel – which has a structure of ferrite plus carbide – to its austenitizing temperature, leaving it at that temperature (soaking) for a predetermined time to allow full transformation to austenite, then cooling it slowly through the critical temperature. The resultant structure is still a mix of ferrite plus carbide, but coarser, more ductile, and tougher. 

The austenitizing temperature will vary with the carbon and alloy content of the steel, as will the cooling rate. There are numerous grades of steel, from simple low-carbon steels and alloy steels, through the more complex tool steels and martensitic stainless steels.  Each type has its own optimum austenitizing temperature. This is where the art and science of heat treatment find their way into the capable hands of the master heat treater, who has years of experience with the steels and their furnaces. This mix of art and science will inevitably find its place in all heat treatment methods.

Normalizing, as annealing, involves heating a steel above its transformation temperature to allow complete transformation to austenite, leaving it there long enough to achieve a uniform temperature throughout its mass, then cooling it in still air in a uniform manner. This results in a pearlitic structure in the steel. The normalizing process may be used to bring about the following changes: 

• Grain refinement and homogenization of the microstructure, to improve a response to hardening.
• Improvement of machinability.
• Modification and refinement of cast structures.
• Attainment of desired mechanical properties.

Parts with complex shapes that may crack or distort in liquid quenching may be normalized and tempered, providing the resultant properties are acceptable.

The hardening process involves, again, heating the steel to the austenitizing temperature and soaking to allow transformation to austenite. Each carbon and alloy steel has its own austenitizing temperature. As hardening suggests, the process serves to harden the steel, thus increasing its durability, yield strength, and tensile strength. The cooling rate from the austenitizing temperature, a function of the steel’s chemistry, hence its hardenability, should ensure transformation to martensite, that hard, brittle phase that is responsible for the hardness of steel. Hence hardening uses water, oil, or air cooling, depending on the steel’s hardenability. Hardening and the formation of martensite results in a brittle steel, lacking in ductility and toughness. Thus, steel is tempered after hardening. Martensitic stainless steels (part of the 400 series) may be annealed and hardened and tempered in a similar manner to some carbon and alloy steels.

Tempering involves heating previously hardened or normalized steel to a temperature below the transformation (to austenite) range, and cooling it at a suitable rate. The process modifies the microstructure and mechanical properties of hardened steel, with the martensite transforming to tempered martensite or to carbide particles, depending upon the tempering temperature. The tempering temperature of a given steel will determine the structure,  properties, and machinability of that steel. In this process, the experience of the person doing the actual heat treating, and their experience and knowledge of the steel and the furnace, is vital.

Steel is doubtless the major engineering material amongst metals and alloys. But it is far from the only one. The range of heat treatments of such nonferrous alloys as those of aluminum, titanium and nickel, is not as varied as those for steels. The major processes are annealing, solution heat treatment, aging, or precipitation hardening.

Solution heat treatments in nonferrous metals are performed to allow solution of all phases that subsequently participate in the precipitation hardening process. Parts are water quenched after the solution treatment to keep the solid solution condition intact. Aging, or precipitation hardening, is carried out after the solution treatment to allow the secondary hardening phases to precipitate, and to give the desired properties.

Annealing of nonferrous materials is used to soften alloys after cold work, with a reduction in hardness and an increase in ductility.

This basic summary of heat treatment generally covers a whole wide range of grades of steel and the mechanical properties that may be thus obtained. AMFG stays up to date on the practical aspects of alloy processing, of which heat treatment is a very significant part.