Steel Tempering Process

Steel Tempering

Tempering is a heat treatment process in which quenched steel is reheated to a temperature below A1, held at that temperature, and then cooled. Tempering determines the steel's microstructure and properties during use. The purpose of tempering is to stabilize the microstructure, eliminate quenching stresses, and improve the steel's ductility and toughness, achieving an appropriate balance of strength, hardness, ductility, and toughness to meet the varying performance requirements of various workpieces.

 

 

Steel tempering can be divided into three categories based on the tempering temperature.

 

Low-Tempering (150-250°C)

The microstructure after low-temperature tempering is tempered martensite, which consists of supersaturated α phase and coherent ε-Fe2.4C. Its morphology retains the flake or lath-like structure of quenched martensite.

 

The primary purpose of low-temperature tempering is to maintain the high hardness (58-62 HRC) and wear resistance of the quenched martensite while reducing quenching stresses and brittleness. It is primarily used in various high-carbon steel cutting tools, gauges, cold stamping dies, rolling bearings, and carburized workpieces.

 

Medium-Tempering (350-500°C)

The microstructure after medium-temperature tempering is tempered troostite, which is composed of unrecrystallized acicular ferrite and dispersed, extremely fine flaky or granular cementite. Its morphology remains the lamellar or lath-like structure of quenched martensite.

 

The primary purpose of medium-temperature tempering is to achieve a high yield strength ratio, high elastic limit, and high toughness. The hardness of tempered troostite is 35-45 HRC. Medium-temperature tempering is primarily used for processing various springs and forging dies.

 

High-Tempering (500-650°C)

The microstructure after high-temperature tempering is tempered troostite, which is composed of recrystallized ferrite and uniformly distributed, fine-grained cementite. Due to the recrystallization of the ferrite, the flaky or lath-like structure of the quenched martensite is lost, becoming polygonal granular. Simultaneously, the cementite aggregates grow.

 

The main reason for tempering deformation is the residual hardness or structural changes generated during tempering and quenching, that is, shrinkage due to the elimination of tensile stress and expansion due to the elimination of compressive stress. This includes some shrinkage of the carbides precipitated in the early stage of tempering, large shrinkage during the condensation of carbon iron, expansion of the residual austenitic iron transformed into loose iron, and expansion of the residual austenitic iron transformed into ductile iron, which leads to deformation of the workpiece after tempering. Prevention methods include:

(1) pressure tempering treatment;

(2) using a hot bath or air quenching to reduce residual stress;

(3) correction by mechanical processing;

(4) preventing deformation.