Quenching heat treatment arrangement after carburization The workpiece must be heat treated after carburization. The purpose is to: improve the strength, hardness and wear resistance of the surface of the carburized layer; improve the strength and toughness of the core; refine the grains ; Eliminate network cementite and reduce the amount of retained austenite. The quenching heat treatment is generally arranged as follows:
1. Direct quenching method: The direct quenching method refers to the method in which the workpiece is carburized and then cooled in the furnace (or pre-cooled out of the furnace) to 760-860°C and then quenched directly. The purpose of cooling down with the furnace or pre-cooling out of the furnace is to reduce the internal stress and deformation during quenching. At the same time, it can also precipitate some carbides in the high-carbon austenite, reduce the carbon concentration in the austenite, and thereby reduce the residual residue after quenching. Austenitic, obtaining higher surface hardness.
2. Reheating and quenching: After carburizing, the workpiece is cooled to austenite and completely transformed, possibly into ferrite/pearlite, or martensite structure, and then reheated to the desired Quenching temperature, then quenching. This method can obtain a structure with finer grains. In addition, an intermediate simultaneous fire can also be arranged, and some cutting processing can be performed before the final quenching and heating, such as cutting off part of the carburized layer. In order to avoid excessive deformation caused by repeated heating, preheating can be performed once or several times for workpieces that are prone to deformation.
3. Tempering: After the carburized parts are quenched, they are then tempered between 150~250℃. For non-alloy steel, the tempering temperature is generally 150-180°C, and for alloy steel it is 160-200°C. After this treatment, tissue stress can be reduced while maintaining favorable compressive stress in the outermost layer. In addition, tempering improves the grindability of carburized and quenched parts and reduces grinding crack susceptibility. Within this temperature range, the reduction in hardness is up to 5 hrc, mostly 1-3 hrc. Different authors have different opinions on the effect of tempering on wear resistance and fatigue strength.
4. Heat treatment of high-alloy steel carburized parts: for high-strength alloy carburized steel, especially chromium-nickel steel (such as 12crn13a, 12cr2n14a, 20cr2ni4a, 18cr2ni4w, etc.). Due to the high content of alloying elements, when the surface carbon content reaches 0.8%-1.0% after carburization, direct quenching will significantly increase the amount of retained austenite, seriously affecting the service life of the parts. For this type of steel, the heat treatment after carburizing must try to reduce the amount of retained austenite, and attention must also be paid to improving the machinability. Generally, the process scheme of adding an intermediate high-temperature tempering process after infiltration and before reheating and quenching is adopted.
(1) If the primary quenching method is used, a high-temperature tempering should be added before quenching;
(2) If the secondary quenching method is used, a high-temperature tempering process should be added before the first quenching. (or normalizing) and before the second quenching, a high-temperature tempering is added to soften the surface of the carburized parts and reduce the hardness to about 30 hrc to improve the cutting performance and reduce deformation during subsequent quenching.
The high-temperature tempering temperature is generally 640~680℃, and the temperature is maintained for 3-8 hours. The carburized parts are quenched after high-temperature co-fire. The surface layer is quenched martensite + a small amount of retained austenite. Carbide, the core is low carbon martensite structure; after low temperature tempering, the surface layer is tempered martensite + less retained austenite + carbide, and the core is low carbon tempered martensite. Intermediate high-temperature tempering of high-alloy steel also has the effect of reducing the amount of retained austenite after final quenching. For example, after carburizing 18cr2ni4w steel, it needs to undergo high-temperature tempering 2-3 times, and generally a higher tempering temperature (680-700℃) is used, and each time it is kept warm for 3 hours.