Stages of Tempering

Carbon Lowering

• The first stage of tempering takes place in the steel as it is heated up to temperatures as high as 480 degrees Fahrenheit. Some types of steels may enter the second stage at temperatures lower than this. During this stage, the carbon content in a particular type of structure in the steel known as martensite is reduced. Martensite is the part of the steel structure that looks like very slender needles when examined microscopically.

Austenite

• This phase ranges all the way up to around 570 degrees Fahrenheit. When the steel is initially quenched before the tempering process begins, austentite structures are formed in the steel. Austenite starts to break down during the second phase of the tempering process. This process moves the structure of the steel towards the final, hardened product.

Cementite and Ferrite

• At temperatures that range from 480 degrees Fahrenheit to 660 degrees Fahrenheit, the third stage of tempering occurs. During the earlier stages, transition carbides are formed in the structure of the metal. By this time, the altering martensite and the transition carbides begin to form new structures. The result is cementite and ferrite. Cementite is the basis for carbon steel, as it is made from the combination of iron and carbon.

Cementite Alteration

• The fourth stage of tempering takes place at all temperatures above the end of the third stage and may continue into very high heats. During this stage, the shape of the cementite particles in the steel changes. The particles become rounder and more like tiny spheres instead of retaining a crystalline structure. Not all steels and alloys will be heated to the heights required for fourth-stage tempering, as sometimes the resulting decrease in tensile strength is not desirable.