High carbon chromium bearing steel heat treatment method
“GCrl5, high-carbon chromium bearing steel containing 1.0% C and 1.5% Cr, came out in 1901 and was the first to be included in the standard in 1913 in the United States. At present, most of the high-carbon-bearing steel grades included in the standards by countries all over the world are developed through appropriate adjustments of Mn, Si, Cr, Mo, and Al element content on the basis of GCrl5. For high carbon chromium bearing steel, the main heat treatment methods are:
martensite quenching, tempering
The martensite quenching process of high carbon chromium-bearing steel is as follows: the bearing parts are heated to 830 ~ 880 ℃ for 0.5 ~ 1 h, and then quenched in oil. After quenching, it should be tempered immediately to eliminate internal stress, improve toughness, and stabilize structure and size. In order to eliminate the grinding stress generated during the grinding process, and to further stabilize the structure and size, it is necessary to perform an additional tempering after the grinding process.
The structure of martensite after quenching is composed of martensite, retained austenite and undissolved carbides. The content of retained austenite is generally 6% to 15%. Retained austenite can improve toughness and crack propagation resistance, and its existence is beneficial to the performance of the material.
bainite austempering
High-carbon chromium-bearing steel is quenched at 230-250°C for 2 to 4 hours after isothermal temperature. Its structure is composed of lower bainite, retained austenite, and undissolved carbides. As the quenching temperature increases, the bainite bars become longer; the isothermal temperature increases, the bainite bars become wider, the carbide particles become larger, and the angle of intersection between the bainite bars becomes smaller, tending to be arranged equally, forming Similar to the structure of upper bainite; the amount of bainite after austempering increases with the extension of the isothermal time.
Studies have shown that: the impact toughness of the bainite structure is about 3 times higher than that of the conventionally quenched low-temperature tempered martensite structure; the impact toughness of the martensite structure is increased by 30%-50%, and the fracture toughness is increased by 20% than that of the martensite structure tempered at the same temperature; The wear resistance is lower than that of the martensite structure that is quenched and tempered at low temperature, and is close to or slightly higher than that of the martensite structure that is tempered at the same temperature.
composite structure quenching
In order to synthesize the advantages of martensite and bainite, heat treatment scholars have studied the quenching process of bainite-martensite composite structure, that is, the bearing parts are heated to the temperature of Ac1～Accm for a period of time, and then transferred to cooling In a quenching medium (oil or salt bath) with sufficient capacity, the austenite in the workpiece will be partially transformed into lower bainite, and finally, continue to be cooled to a certain temperature below the martensite (Ms) point to make the remaining austenite in the workpiece Most of it is transformed into martensite.
Bainite-martensite composite structure after quenching is composed of lower bainite, martensite, a small amount of retained austenite, and a small number of undissolved carbides. This is a new quenching technology with significant advantages and broad application prospects, which is still under development.