Application effect of heat treatment technology
(1) the application scope of GCr15 steel is expanded. Generally, the effective wall thickness of GCr15 steel ring is less than 12mm when it is quenched. However, due to the strong cooling capacity of nitrate when BL is quenched, the effective wall thickness of the ring can be extended to about 28mm if measures such as stirring, crosslinking and adding water are adopted.
(2) stable hardness and good uniformity
The BL change is a slow process, generally GCr15 steel 4 h, GCr18Mo steel need 5 h, ring in long isothermal nitrate, on the surface of the core organization transformation almost simultaneously, so the hardness of stability, uniformity, good general BL GCr15 steel after quenching hardness in 59 ~ 61 HRC, uniformity of 1 or less HRC, unlike quenching ring wall thickness slightly bigger when there is low hardness, soft point, poor uniformity.
(3) reduce quenching and grinding cracks
In the bearing production of railway and rolling mill, due to the large size and weight of the sleeve ring, the M structure is very brittle when oil is quenched. In order to obtain high hardness after quenching, strong cooling measures are often taken, resulting in quenching microcracks. When BL is quenched, because BL tissues are much more ductile than M tissues, and the pressure stress up to -400~ -500mpa is formed on the surface, which greatly reduces the tendency of quenching cracks. In the grinding process, the surface pressure stress counteracts part of the grinding stress, which reduces the overall stress level and greatly reduces the grinding crack.
(4) the service life of the bearing is improved
To withstand large impact load of railroad, rolling mill bearings, etc., after M quenching when using main failure forms is: inner sleeve cracking when assembling, use in the process of impact outer ring, inner ring fracture, falling guard and isothermal quenching bearing with good impact toughness, surface compressive stress, whatever the assembly inner sleeve craze, or using Chinese and foreign set of guard metal shed and inner sleeve fracture tendency is greatly reduced, and can reduce the edge stress concentration of roller. Therefore, the average life and reliability are improved after isothermal quenching compared with that after M quenching.