Many people know about bearings, but they don't seem to know much about bearing steel. The so-called bearing steel refers to the steel used to make balls, rollers and bearing rings. It is also a special steel material. Based on years of experience, tell you the use characteristics of bearing steel and some matters needing attention in the use process.

What is bearing steel? What are the characteristics of bearing steel?

Steel used to make balls, rollers and bearing rings. Bearing steel, also known as high carbon chromium steel, contains carbon ω C is about 1%, chromium content ω Cr is 0.5% - 1.65%. Bearing steel is divided into six categories: high carbon chromium bearing steel, chromium free bearing steel, carburized bearing steel, stainless bearing steel, medium and high temperature bearing steel and anti magnetic bearing steel.

Bearing steel has high and uniform hardness, wear resistance and high elastic limit. The requirements for the uniformity of chemical composition, the content and distribution of non-metallic inclusions and the distribution of carbides of bearing steel are very strict. It is one of the most stringent steel types in all steel production.

Bearing steel is a kind of steel used to manufacture balls, rollers and sleeves of rolling bearings. It can also be used to make precision measuring tools, cold stamping dies, machine tool lead screws, such as dies, measuring tools, taps and precision coupling parts of diesel engine oil pumps. Bearing steel is the steel used to make balls, rollers and bearing rings.

Heat treatment process of bearing steel

The heat treatment process of bearing steel includes two main links: pre heat treatment and final heat treatment. GCr15 bearing steel is the most widely used bearing steel with less chromium content. GCr15 bearing steel has high and uniform hardness, good wear resistance and high contact fatigue properties after heat treatment.

annealing

(1) Complete annealing and isothermal annealing: complete annealing and weighing crystallization annealing are generally referred to as annealing. This kind of annealing is mainly used for casting, forging and hot rolled profiles of various carbon and alloy steels with hypoeutectoid composition, and sometimes for welded structures. Generally, it is often used as the final heat treatment of some unimportant workpieces or as the pre heat treatment of some workpieces.

(2) Spheroidizing annealing: spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel used for manufacturing cutting tools, measuring tools and molds). Its main purpose is to reduce hardness, improve machinability and prepare for quenching in the future.

(3) Stress relief annealing: stress relief annealing, also known as low-temperature annealing (or high-temperature tempering), is mainly used to eliminate the residual stress of castings, forgings, weldments, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, the steel parts will be deformed or cracked after a certain time or in the subsequent machining process.

quench

In order to improve the hardness, the main methods are heating, heat preservation and rapid cooling. The most commonly used cooling media are brine, water and oil. The workpiece quenched by brine is easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that cannot be hardened, but it is easy to cause serious deformation and even cracking of the workpiece. The use of oil as quenching medium is only applicable to the quenching of some alloy steel or small-size carbon steel workpieces with high stability of undercooled austenite.

tempering

(1) Reduce brittleness and eliminate or reduce internal stress. There is great internal stress and brittleness after quenching. If not tempered in time, the steel will often deform or even crack.

(2) Obtain the mechanical properties required by the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the requirements of different properties of various workpieces, the hardness can be adjusted through appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.

(3) Stable workpiece size

(4) For some alloy steels that are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing), so as to properly aggregate carbides in the steel and reduce the hardness, so as to facilitate cutting.

Basic requirements for quality of bearing steel

① Strict chemical composition requirements.

General bearing steel is mainly high carbon chromium bearing steel, that is, hypereutectoid steel with carbon content of about 1%, chromium of about 1.5% and a small amount of manganese and silicon. Chromium can not only improve heat treatment performance, hardenability, microstructure uniformity and tempering stability, but also improve rust prevention and grinding performance of steel.

However, when the chromium content exceeds 1.65%, the residual austenite in the steel will be increased after quenching, the hardness and dimensional stability will be reduced, the heterogeneity of carbide will be increased, and the impact toughness and fatigue strength of the steel will be reduced. Therefore, the chromium content in high carbon chromium bearing steel is generally controlled below 1,65%. Only by strictly controlling the chemical composition in the bearing steel can the structure and hardness meeting the bearing performance be obtained through the heat treatment process.

② Higher dimensional accuracy is required. For the hot-rolled annealed bar forged on the high-speed upsetting machine, higher dimensional accuracy should be required.

Rolling bearing steel requires high dimensional accuracy, because most bearing parts have to be pressure formed. In order to save materials and improve labor productivity, most bearing rings are forged, steel balls are formed by cold heading or hot rolling, and small-size rollers are also formed by cold heading. If the dimensional accuracy of steel is not high, the blanking size and weight cannot be calculated accurately, the product quality of bearing parts cannot be guaranteed, and it is easy to cause damage to equipment and molds.

③ Especially strict purity requirements.

The purity of steel refers to the number of non-metallic inclusions in steel. The higher the purity, the less the non-metallic inclusions in steel. Harmful inclusions such as oxides and silicates in bearing steel are the main reasons for early fatigue spalling and significantly reducing bearing life. In particular, brittle inclusions are the most harmful. They are easy to peel off from the metal matrix in the machining process, which seriously affects the surface quality of bearing parts after finishing machining. Therefore, in order to improve the service life and reliability of bearing, the content of inclusions in bearing steel must be reduced.

④ Strict requirements for macrostructure and microstructure (macrostructure).

The macrostructure of bearing steel refers to general porosity, central porosity and segregation. The microstructure (macrostructure) includes annealing structure, carbide network, band and liquid precipitation of steel. Carbide liquid is hard and brittle, and its harm is the same as that of brittle inclusions. Network carbide reduces the impact toughness of steel and makes its structure uneven, which is easy to deform and crack during quenching. Banded carbides affect the microstructure of annealing, quenching and tempering and contact fatigue strength. The advantages and disadvantages of low and high magnification structure have a great impact on the performance and service life of rolling bearing. Therefore, there are strict requirements for low and high magnification structure in the bearing material standard.

⑤ Particularly strict requirements for surface defects and internal defects.

For bearing steel, surface defects include cracks, slag inclusions, burrs, scabs, oxide scales, etc., while internal defects include shrinkage, bubbles, white spots, serious porosity and segregation, etc. These defects have a great impact on the processing, performance and service life of the bearing. It is clearly stipulated in the bearing material standard that these defects are not allowed.

⑥ Strict requirements for carbide non-uniformity.

In bearing steel, if there is serious uneven distribution of carbides, it is easy to cause uneven microstructure and hardness in the process of heat treatment. The uneven microstructure of steel has a great impact on the contact fatigue strength. In addition, the serious non-uniformity of carbide is also easy to cause cracks in the bearing parts during quenching and cooling, and the non-uniformity of carbide will also reduce the service life of the bearing. Therefore, in the bearing material standard, there are clear special requirements for steels of different specifications.

⑦ Strict requirements for the depth of surface decarburization layer.

There are strict provisions on the decarburization layer on the steel surface in the bearing material standard. If the decarburization layer on the surface exceeds the specified range of the standard and is not completely removed in the processing process before heat treatment, quenching cracks are easy to occur in the process of heat treatment and quenching, resulting in the scrapping of parts.

⑧ Other requirements.

The bearing steel material standard also has strict requirements on the smelting method, oxygen content, annealing hardness, fracture, residual elements, spark inspection, delivery status and identification of bearing steel.