The main process of preheating and final heat treatment of bearing steel heat treatment process. GCr15 steel is the most widely used bearing steel, high carbon chromium bearing steel with less alloy content and good performance. GCr15 bearing steel has high and uniform hardness, good wear resistance and high contact fatigue performance after heat treatment.
(1) Complete annealing and isothermal annealing: Complete annealing, also known as recrystallization annealing, is generally referred to as annealing. This annealing is mainly used for casting, forging and hot-rolled profiles of various carbon steels and alloy steels with sub-eutectoid compositions. Also used to weld structures. It is often used as a final heat treatment for some unimportant workpieces or as a pre-heat treatment for certain workpieces.
(2) Spheroidizing annealing: Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel cutting tools, measuring tools, and steels used in molds). Its main purpose is to reduce hardness, improve machinability, and prepare for later quenching.
(3) Stress relief annealing: stress relief annealing, also known as low temperature annealing (or high temperature tempering), is mainly used to eliminate residual stresses in castings, forgings, welded parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, it will cause deformation or cracking of the steel after a certain period of time or during subsequent cutting operations.
In order to improve the hardness, the main form is through heating, heat preservation and rapid cooling. The most common cooling media are brine, water and oil. The salt-quenched workpiece is easy to obtain high hardness and smooth surface. It is not easy to produce hard spots that are hard to quench, but it is easy to deform the workpiece seriously and even crack. The use of oil as a quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively high stability of supercooled austenite.
(1) Reducing brittleness, eliminating or reducing internal stress, there is great internal stress and brittleness after quenching of steel parts. If it is not tempered in time, it will cause deformation or even cracking of steel parts.
(2) Obtain the mechanical properties required for the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted by appropriate tempering to reduce the brittleness and obtain the required Resilience, plasticity.
(3) Stabilizing the workpiece size
(4) For some alloy steels that are difficult to soften by annealing, high-temperature tempering is often used after quenching (or normalizing) to properly aggregate the carbides in the steel and reduce the hardness to facilitate cutting.
Basic requirements for quality of bearing steel
1 strict chemical composition requirements.
Generally, the steel for bearing is mainly high carbon chromium bearing steel, that is, hypereutectoid steel containing about 1% of carbon, adding about 1.5% of chromium, and containing a small amount of manganese and silicon. Chromium can improve heat treatment performance, improve hardenability, microstructure uniformity, tempering stability, and improve the rust resistance and grinding performance of steel.
However, when the chromium content exceeds 1.65%, the retained austenite in the steel is increased after quenching, the hardness and dimensional stability are lowered, the carbide non-uniformity is increased, and the impact toughness and fatigue strength of the steel are lowered. For this reason, the chromium content in high carbon chromium bearing steel is generally controlled below 1,65%. Only by strictly controlling the chemical composition of the bearing steel can the microstructure and hardness satisfying the bearing performance be obtained through the heat treatment process.
2 Higher dimensional accuracy requirements, for the use of hot-rolled annealed bars forging on high-speed upsetting machines, the dimensional accuracy should be higher.
Steel for rolling bearings requires high dimensional accuracy of the steel because most of the bearing parts are pressure molded. In order to save materials and increase labor productivity, most of the bearing rings are forged, the steel balls are cold-rolled or hot-rolled, and the small-sized rollers are also cold-formed. If the dimensional accuracy of the steel is not high, the size and weight of the blank cannot be accurately calculated, and the product quality of the bearing parts cannot be guaranteed, and the damage of the equipment and the mold is easily caused.
3 particularly strict purity requirements.
The purity of steel refers to the amount of non-metallic inclusions contained in the steel. The higher the purity, the less non-metallic inclusions in the steel. Harmful inclusions such as oxides and silicates in bearing steel are the main causes of early fatigue spalling of bearings and significant reduction in bearing life. In particular, brittle inclusions are the most harmful, and they are easily peeled off from the metal substrate during processing, which seriously affects the surface quality of the bearing parts after finishing. Therefore, in order to improve the service life and reliability of the bearing, it is necessary to reduce the content of inclusions in the bearing steel.
4 Strict low-fold tissue and microscopic (high magnification) tissue requirements.
The low-magnification structure of bearing steel refers to general looseness, center looseness and segregation, and microscopic (high-power) microstructure includes steel annealed structure, carbide mesh, ribbon and liquid analysis. Carbide liquefaction is hard and brittle, and its hazard is the same as brittle inclusions. The reticulated carbide reduces the impact toughness of the steel and makes it unevenly organized, and is easily deformed and cracked during quenching. Banded carbides affect annealing and quenched tempered structures and contact fatigue strength. The advantages and disadvantages of low and high-magnification organizations have a great influence on the performance and service life of rolling bearings. Therefore, there are strict requirements for low and high-fold organization in bearing material standards.
5 particularly strict surface defects and internal defect requirements.
For bearing steel, surface defects include cracks, slag inclusions, burrs, crusting, oxide scales, etc. Internal defects include shrinkage cavities, bubbles, white spots, severe looseness and segregation. These defects have a great influence on the processing of the bearing, the performance and the life of the bearing, and it is clearly stated in the bearing material standard that these defects are not allowed.
6 strict carbide non-uniformity requirements.
In the bearing steel, if the serious carbide distribution is uneven, the unevenness of the structure and hardness is easily caused during the heat treatment process, and the structural heterogeneity of the steel has a great influence on the contact fatigue strength. In addition, severe carbide inhomogeneity can easily cause cracks in bearing parts during quenching and cooling. Carbide inhomogeneity can also lead to reduced bearing life. Therefore, in the bearing material standard, the steel of different specifications is clear. special requirement.
7 strict surface decarburization depth requirements.
In the bearing material standard, there is strict regulation on the decarburization layer of the steel surface. If the surface decarburization layer is beyond the scope of the standard and is not removed during the processing before the heat treatment, it is in the heat treatment quenching process. Quenching cracks are easily generated, resulting in the scrapping of parts.
8 other requirements.
In the bearing steel material standard, there are strict requirements on the smelting method of bearing steel, oxygen content, annealing hardness, fracture, residual elements, spark inspection, delivery status, and marking.