Quenching, annealing and forging heat treatment processes for 9Cr18 and 9Cr18Mo steel bearing parts

The influence of heat treatment on the quality of workpieces is crucial. It is directly related to the quality, wear resistance and service life of the workpiece. Improper heat treatment will greatly affect the performance of the workpiece and its use, thereby affecting the operation of the machine. Therefore, understanding the heat treatment process is very necessary for our work. Today, I will briefly introduce the quenching and forging heat treatment processes of 9Cr18 and 9Cr18Mo steel bearing parts.

9Cr18 and 9Cr18Mo steel are high carbon, high chromium martensitic stainless steels. This type of steel has high hardness, elasticity, wear resistance and excellent corrosion resistance after heat treatment (quenching, cold treatment, tempering). It mainly manufactures bearing rings and rolling elements that work in corrosive media. This type of steel can also be used to make high-temperature-resistant bearings. The tempering temperature is ≤410 degrees. Beyond this tempering temperature, the corrosion resistance drops sharply, so attention should be paid to it during use.

1. Forging and annealing. During the forging process, due to the poor thermal conductivity of this type of steel, the complex carbides in the steel dissolve slowly into austenite at high temperatures, so the forging heating rate should not be too fast. And because the steel has good hardenability, the cooling rate after forging should be slow, and it should be cooled in lime, hot sand or an insulating box (furnace). The structure of the forging does not allow overheating, overburning, twins, and coarse carbide networks caused by high forging stop temperatures and slow cooling rates. The normal forging structure should be composed of martensite, austenite and primary and secondary carbides, and the grains of steel should also be small. After annealing, it should be inspected according to the JB1460 standard. The quality requirements are as follows:

(1) The microstructure must not contain twin carbides.

(2) The depth of the decarburized layer shall not exceed 2/3 of the minimum machining allowance on each side before quenching. The measurement of the decarburized layer after annealing of hot stamped steel balls shall be carried out on the grinding surface of the test piece perpendicular to the cross section of the ring.

(3) Hardness 197-241HBS (indentation diameter 4.3-3.9mm).

2. Quenching and tempering. Quenching is usually performed in a salt bath furnace, vacuum furnace or electric furnace with a holding atmosphere to avoid decarburization. When heating in a salt bath furnace, it is necessary to prevent corrosion pitting on the surface of the parts. The quenching heating temperature is generally selected from 1050 to 1100 degrees. During heating, it must be preheated at 800 to 850 degrees and then heated to the quenching heating temperature. The preheating time is generally twice the quenching heating and holding time, and the holding time is calculated based on the effective thickness of the part.

The crushing load value of the steel ball after quenching and tempering shall not be lower than the specified value.

The microstructure after quenching and tempering is composed of cryptocrystals, fine martensite, residual primary carbides, secondary carbides and retained austenite. The rating is based on the second level chart of JB1460 standard, second, third, fourth and sixth. The first-level structure is a qualified structure, and the first-level underheated structure, the fifth-level overheated structure and the seventh-level twin carbide structure caused by forging overheating are not allowed to appear.

For bearing parts working in corrosive media, corrosion resistance inspection is required if required. This inspection is generally carried out with artificial seawater or dilute nitric acid aqueous solution. For bearing rings operating at low temperatures, dimensional stability inspection still needs to be carried out. During the inspection, measure the size of the ferrule before assembly and place it at a low temperature of -180 to 200 degrees for 1 to 1.5 hours. After taking it out, measure its size at room temperature. The size change should be within the qualified range.

Surface decarburization, corrosion pits, oxide scale and other defects after parts are quenched and tempered must be removed during the grinding process.

This structure is mainly formed when the forging heating temperature is too high.

Heat treatment of the workpiece will greatly improve the performance and service life of the workpiece. However, after using the equipment, don't forget to perform maintenance and cleaning on the equipment, which will greatly increase the service life of the equipment.