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Bearing knowledge: introduction to bearing steel and its heat treatment process

Bearing knowledge: introduction to bearing steel and its heat treatment process

When it comes to bearings, everyone may know better, but they may not know much about the materials they are made of. Today I will introduce to you what is bearing steel. Bearing steel refers to the steel used to make balls, rollers and bearing rings, and it is also a special steel material. Bearing steel is also called high-carbon chromium steel, with a carbon content of about 1% and a chromium content of 0.5% to 1.65%. Bearing steel can be divided into six categories: high-carbon chromium bearing steel, chromium-free bearing steel, carburizing bearing steel, stainless bearing steel, medium and high temperature bearing steel and antimagnetic bearing steel.

Bearing steel has high and uniform hardness and wear resistance, and high elastic limit. Bearing steel is one of the most stringent steel grades in all steel production. The requirements for the uniformity of the chemical composition of the bearing steel, the content and distribution of non-metallic inclusions, and the distribution of carbides are very strict. Bearing steel can also be used to make precision measuring tools, cold die, machine tool screws, such as die, measuring tools, taps and precision parts of diesel oil pumps.

The heat treatment process of bearing steel is divided into two main links: pre-heat treatment and final heat treatment. GCr15 steel is the most widely used bearing steel, a high-carbon chromium bearing steel with low 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. Annealing

(1) Complete annealing and isothermal annealing: complete annealing is also called recrystallization annealing, generally referred to as annealing. This annealing is mainly used for castings, forgings and hot-rolled sections of various carbon steels and alloy steels with hypoeutectoid composition, sometimes Also used for welding structures. Generally 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 steels and alloy tool steels (such as steel grades used in manufacturing cutting tools, measuring tools, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching.

(3) Stress relief annealing: Stress relief annealing is also called low temperature annealing (or high temperature tempering). This annealing is mainly used to eliminate residual stress in castings, forgings, welded parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, it will cause the steel parts to deform or crack after a certain period of time or in the subsequent cutting process.

2. Quenching

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. Salt water quenched workpieces are easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that are not hardened, but it is easy to cause serious deformation of the workpiece and even cracks. The use of oil as the quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively large stability of undercooled austenite.

3. Tempering

(1) Reduce brittleness and eliminate or reduce internal stress. There is a large internal stress and brittleness of steel parts after quenching. If they are not tempered in time, they will often deform or even crack.

(2) Obtain the required mechanical properties of 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 through appropriate tempering to reduce the brittleness and obtain the required The toughness and plasticity.

(3) Stabilize the size of the workpiece.

(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 carbides in the steel and reduce the hardness to facilitate cutting.

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