GCr15 is a hypereutectoid high carbon chromium bearing steel. with the development of equipment manufacturing industry, the requirements for its comprehensive mechanical properties are getting higher and higher, and there are many related researches and discussions. Through double refining process, the bending strength, impact toughness and wear resistance of GCr15 steel are improved by 29.7%, 100% and 35% respectively. After carrying out different austempering tests on GCr15 steel, the multiphase structure of martensite and lower bainite is obtained, and the impact toughness, bending strength and deflection are improved. The results show that the impact toughness of the material decreases after cryogenic treatment. However, there is little research on the comprehensive influence on the properties of GCr15 steel after cryogenic treatment. Therefore, the researchers made a comprehensive study on the changes of microstructure, retained austenite content, mechanical properties and wear properties of GCr15 steel after cryogenic treatment for different time, in order to provide reference for production practice.
See table 1 for the main chemical compositions (mass fraction) of GCr15 steel for test:
Firstly, the raw materials are pretreated by isothermal spheroidizing annealing: after 790℃×5h, the raw materials are cooled to below 300℃, discharged from the furnace and air cooled. Then, the processed samples were quenched at 820℃×20min and oil cooled. After quenching, the samples were randomly divided into four batches, in which one batch was directly tempered at low temperature, and the other three batches were immersed in liquid nitrogen within 10 minutes after quenching and cooling, and then taken out after soaking for 1 ~ 6 hours. After heating to room temperature, they were tempered at low temperature. Low temperature tempering process is 160℃×1.5h .. The microstructure morphology was observed and analyzed by OLYMPUSPGM3 optical metallographic microscope. Hardness of samples was measured on HRD-150 Rockwell Hardness Meter. Impact tests at 200N and 120 were carried out by using JBN-300B pendulum impact tester, and the impact energy Aku under different treatment conditions was measured. It is found that the hardness increases slowly with the extension of cryogenic time. Compared with the samples without cryogenic treatment, the hardness increases by 1.6HRC and the impact energy decreases by 0.5J after cryogenic treatment for 6 hours. The reason is that after cryogenic treatment, the retained austenite in GCr15 steel obviously decreases, the content of martensite structure increases, the degree of lattice distortion decreases, the lattice constant decreases, and the martensite structure decreases averagely, and more uniform and fine hard carbide particles are dispersed and precipitated on the martensite matrix, which produces precipitation strengthening effect and improves the hardness of steel. The retained austenite structure changes to martensite, which reduces the absorption of impact energy. The wear resistance of the samples after cryogenic treatment is obviously improved, and it gradually increases with the extension of cryogenic treatment time.
In the test, the strengthening methods of GCr15 steel include phase transformation strengthening and precipitation strengthening. During cryogenic treatment, retained austenite transforms to martensite, and supersaturated C atoms in martensite matrix precipitate twice, that is, the combination of transformation strengthening and precipitation strengthening improves the hardness and wear resistance of steel. At the same time, compared with the samples without cryogenic treatment, the volume fraction of carbides is obviously increased and a large number of fine carbides are precipitated, which can also improve the wear resistance of steel.
To sum up:
(1) Cryogenic treatment can improve the hardness of GCr15 steel, but the impact toughness decreases slightly;
(2) The retained austenite in GCr15 steel was transformed into martensite during cryogenic treatment, and the retained austenite content decreased greatly after cryogenic treatment for 6 hours.
(3) After cryogenic treatment, the isothermal spheroidizing annealing pretreatment of GCr15 steel can be obviously improved.