How To Control The Quenching And Re-ignition Process Of Forgings

The special steel used to make rolling bearings is called rolling bearing steel, usually referred to simply as bearing steel. Rolling bearings work in extremely demanding conditions, essentially under high load, high speed and sensitivity. The main forms of failure are fatigue and wear. Therefore, rolling bearings are usually required to have high and uniform hardness and wear resistance, high contact fatigue strength, high elastic limit and a certain degree of toughness, good dimensional stability and a certain degree of corrosion resistance. In addition, there are other special requirements for bearings operating under special conditions, such as high temperature performance, corrosion resistance, etc. In order to meet the performance requirements of bearings, the correct chemical composition, high metallurgical quality and reasonable processing process are the three main keys for bearing steel. Among them, heat treatment plays an important role in improving the in-house quality of bearings and prolonging their service life.

The selection of quenching and re-ignition process parameters has a great impact on bearing performance and fatigue life. The quenching temperature of high carbon chromium bearing steel is 830-860 degrees C, after oil quenching, a fine and uniform orthocular grain size (5-8 grades) can be obtained, and the carbon content in the solid solution is generally 0.8 degrees. 5%-0.6% (mass fraction), with a small, evenly distributed granular carbides on the reclusive marmaid substation with a content of 7%-8% (mass fraction) and a small amount of residual Austrogen. Such tissues provide the highest hardness, bending strength and toughness in performance. For example with double-fine treatment, that is, the carbides are first ultra-finely treated, and then the use of lower temperature quenching and re-ignition, forgings can obtain carbon and grain at the same time refining effect. For example, after the ultra-fine treatment of GCr15 steel carbides, after 820 degrees C heating quenching, 170 degrees C tempering, to obtain double-fine tissue, grain size of 11 grades, hardness value with the length of insulation time between 55-63HRC changes, the quality can reach the level of imported bearings.

If the quenching temperature is too low or the insulation time is too short, the carbides in the Austrome solid solubility is too small, then the hardness is insufficient after quenching, on the contrary, if the quenching temperature is too high or the insulation time is too long, the concentration of carbon and chromium in the Austile body increases, then the quenching will obtain a large marster, and will reduce the toughness of the steel.

Bearing steel quenching cooling should be sufficient to avoid insufficient cooling, part of the Austite to the beech body transformation, reduce the hardness of steel quenching. The oil temperature for quenching and cooling is 50-80 degrees C, stirring and cooling, and cooling should be uniform.

Bearing steel quenching should be timely tempering, in order to improve the stability of the organization and size of forgings, improve the performance of metal. Tempering temperature is generally 160 degrees C, insulation 3h or longer, insulation after air cooling. The tissue after the fire is the receding marma body plus the carbides distributed by the dispersion distribution, and a small amount of residual Austrogen. The hardness after the return is between 62-66HRC. The basic reason for the instability of shaft forging size is the presence of internal stress and residual ausome, and the common measures to increase tissue stability and reduce stress are cold treatment and additional re-ignition. The cold treatment temperature is -40-80 degrees C, and the cold treatment time is calculated at 60min per 25mm insulation. Additional tempering is often after grinding, immediately at a temperature lower than the original tempering temperature of 20-30 degrees C and then re-ignition, such as 120-150 degrees C insulation 3-5h, to eliminate stress, stable tissue. Bearings with particularly high accuracy requirements are to be re-ignited 2-3 times after coarse, fine and fine grinding, at a time of 15-24h.