Stainless steel spring tempering method
When the stainless steel spring is completely quenched into martensite and then heated to 150°C-170°C, the tempering temperature rises according to its internal structure and is divided into four stages: 1) martensite Decomposition; 2) Transformation of retained austenite; 3) Transformation of carbides; 4) Changes in the state of e-phase and aggregation and growth of carbides.
Secondary hardening: When the hot coil spring steel contains more carbide forming elements, it forms alloy cementite or special carbides in the tempering fourth stage temperature zone (about 500-550°C). Precipitation will make the hardness increase again, called the image of secondary hardening.
The purpose of hot coil spring tempering: to reduce or eliminate the quenching stress, improve toughness and plasticity, to obtain the appropriate combination of hardness, strength, plasticity and toughness to meet the performance requirements of hot coil springs.
Stainless steel spring tempering time control tips:
1. Tempering characteristics of carbon steel
The mechanical properties of stainless steel spring quenched steels are often measured in terms of hardness.
In the case of incompletely hardened, the hardness difference along the workpiece section gradually decreases with the increase of tempering temperature and tempering time. The tempering characteristics of alloy steel are basically similar to that of carbon steel.
However, for steels with secondary hardening, it is not possible to simply use the M parameter to characterize the degree of tempering.
2, low temperature tempering
Low-temperature tempering of stainless steel springs is also called "eliminating stress tempering." The tempering temperature range is 150-250 degrees Celsius, and the tempered structure is tempered martensite.
Steel has high hardness and high wear resistance, but internal stress and brittleness are reduced. Carburized and surface-hardened parts, the hardness after tempering is generally 58-64HRC.
3, tempering time
The tempering time of the stainless steel spring shall include the time required to reach the tempering temperature evenly according to the cross section of the workpiece and the time required to complete the transformation of the microstructure according to the M parameter required to reach the required tempering hardness. If the elimination of the internal stress is taken into account, the difference should be considered. The time required for stress relaxation at the tempering temperature.
The low-temperature tempering time, which is mainly based on stress relaxation, should be longer than the listed data, and can be as long as several tens of hours. For the secondary hardening type high alloy steel, the tempering time should be determined by tests according to the carbide transformation process. When more retained austenite is contained and quenched by secondary quenching, the number of temperings should also be determined.