Of the various thermal processing methods for steel, heat treating has the greatest overall impact on control of residual stress and on dimensional control. This article provides an overview of the effects of material- and process-related parameters on the various types of failures observed during and after heat treating of quenched and tempered steels. It describes phase transformations of steels during heating, cooling of steel with and without metallurgical transformation, and the formation of high-temperature transformation products on the surface of a carburized part. The article illustrates the use of carbon restoration on decarburized spring steels. Different geometric models for carbide formation are shown schematically. The article also describes the different microstructural features such as grain size, microcracks, microsegregation, and banding.

This article describes the advantages of martempering and the use of oil and salt as quenchants in the martempering process. It also discusses safety precautions to be followed by an operator and reviews the steels that are suitable for martempering. The article provides information on the importance of controlling process variables in martempering, including austenitizing temperature, temperature of the martempering bath, time in the bath, salt contamination, water additions to salt, agitation, and the rate of cooling from the martempering bath. It also describes specific situations in which distortion problems have been encountered during martempering. The article contains tables that indicate typical applications of martempering in salt and oil by listing commonly treated steel parts and giving details of martempering procedures and hardness requirements. The article also lists equipment requirements for oil and salt martempering of steel.

Tempering of steel is a process in which hardened or normalized steel is heated to a temperature below the lower critical temperature and cooled at a suitable rate, primarily to increase ductility, toughness, and grain size of the matrix. This article provides an overview of the variables that affect the microstructure and mechanical properties of tempered steel, namely, the tempering temperature, tempering time, carbon content, alloy content, and residual elements. Tempering after hardening is performed to relieve quenching stresses and ensure dimensional stability of steel. The article discusses the embrittlement problems associated with tempering. Four types of equipment are used for tempering, namely, convection furnaces, salt bath furnaces, oil bath equipment and molten metal baths. Special procedures for tempering are briefly reviewed.