When the steel ball is fully quenched into martensite and heated to 150 ° C - 170 ° C tempering, as the tempering temperature increases, according to its internal structure changes, it is divided into four stages:
1) decomposition of martensite; 2) transformation of retained austenite; 3) transformation of carbides; 4) change of e-phase state and accumulation of carbides
Secondary hardening: When 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 the secondary hardening image.
Tempering purpose: reduce or eliminate the quenching stress, improve the toughness and plasticity, and obtain the appropriate combination of hardness, strength, plasticity and toughness to meet the performance requirements of the steel ball.
Tempering characteristics of carbon steel
The mechanical properties of hardened steel after tempering are often measured by hardness. In the case of not completely hardened, the difference in hardness along the cross section of the workpiece decreases with the increase of tempering temperature and the tempering time. The tempering characteristics of alloy steel are basically similar to those of carbon steel. However, for steels with secondary hardening, it is not possible to simply use M parameters to characterize the degree of tempering.
Low temperature tempering is also called "stress relief tempering". The tempering temperature range is 150-250 degrees Celsius, and the tempered tissue is tempered martensite. Steel has high hardness and high wear resistance, but internal stress and brittleness are reduced. For parts that have been carburized and surface hardened, the hardness after tempering is generally 58-64 HRC.
The tempering time should include the heating time required to uniformly reach the tempering temperature according to the cross section of the workpiece and the time required to complete the structural transformation according to the M parameter to achieve the required tempering hardness. If the internal stress is removed, different tempering temperatures should be considered. The time required for the lower stress relaxation.
The low temperature tempering time based on stress relaxation should be longer than the listed data and can be as long as several tens of hours. For secondary hardening type high alloy steel, the tempering time should be determined experimentally according to the carbide transformation process. When there is more retained austenite and is eliminated by secondary quenching, the number of tempering should also be determined.
After tempering, the steel balls are cooled in the air. To prevent cracking. After tempering, oil cooling should be carried out to suppress temper brittleness. Under the condition of preventing cracking, oil cooling or water cooling may be performed, and then a low temperature supplementary tempering is performed to eliminate the internal stress generated by the rapid cooling.