Industry Knowledge

When the steel ball is fully quenched into martensite and then heated to 150°C-170°C for tempering, as the tempering temperature increases, according to the change of its internal structure, it is carried out in four stages:

1) Decomposition of martensite; 2) Transformation of retained austenite; 3) Transformation of carbides; 4) Change of e-phase state and aggregation and growth of carbides.

Secondary hardening: When the steel contains more carbide-forming elements, alloy cementite or special carbides are formed in the temperature zone of the fourth stage of tempering (about 500-550 °C). The precipitation of these carbides will increase the hardness again, which is called secondary hardening.

Tempering purpose: reduce or eliminate quenching stress, improve toughness and plasticity, and obtain an appropriate combination of hardness, strength, plasticity and toughness to meet the performance requirements of steel balls.

1. Tempering properties of carbon steel

 The mechanical properties of quenched steel after tempering are often measured by hardness. In the case of incomplete hardening, the difference in hardness along the workpiece section decreases gradually with the increase of tempering temperature and the prolongation of tempering time. The tempering properties of alloy steel are basically similar to those of carbon steel. However, it is different for steels with secondary hardening phenomenon, and the degree of tempering cannot be simply characterized by the M parameter.

2. Low temperature tempering

Low temperature tempering is also known as "stress relief tempering". The tempering temperature range is 150-250 degrees Celsius, and the structure after tempering is tempered martensite. Steel has high hardness and high wear resistance, but reduced internal stress and brittleness. For parts that have been carburized and surface quenched, the hardness after tempering is generally 58-64HRC.

3. Tempering time

The tempering time should include the heating time required to uniformly reach the tempering temperature according to the workpiece cross-section and the time required to achieve the required tempering hardness according to the M parameter to complete the microstructure transformation. If the elimination of internal stress is considered, different tempering temperatures should also be considered. time required for stress relaxation.

For the low temperature tempering time mainly based on stress relaxation, it should be longer than the data listed in the table, and the length can be up to tens of hours. For the secondary hardening type high alloy steel, the tempering time shall be determined by tests according to the carbide transformation process. When it contains more retained austenite and is eliminated by secondary quenching, the number of tempering should also be determined.