Heat treatment of bearing steel parts

Forging process and preliminary heat treatment

Bearing steel forging process is one of the important means to ensure the quality of the final product, after foreign steel ingot casting, removal of the end of a certain length pouring cap mouth, then carries on the diffusion annealing, in a timely manner after the length direction of the steel ingot to a certain degree of compression, make the ingot to extend transverse direction, and then gradually to the length direction of the polyhedron forging, the dimensions of the final forging into need it to change the internal organization structure of steel ingot, avoid the occurrence of banded carbide groups such as defects.

The cost of this process will be higher, but it has a good original structure and is well prepared for the subsequent heat treatment, which is also the advantage of foreign bearing steel. According to the needs of large ingot billet into various specifications of the blank, its bar, plate (block) wood, wire or pipe, etc., the supply needs to be spheroidized annealing treatment, the ring through the rolling process also needs to be spheroidized annealing. The initial forging temperature should not be too high, and the billet after the final forging should be spread out or spray cooled. When there is reticulated carbide, it can be solved by normalizing process, and good quality can be closed from the forging to provide a good billet for the final heat treatment.

Baoding heat treatment equipment for bearing steel parts

The heat treatment of bearing steel is generally classified as inner ring, outer ring, roller, ball, needle, etc. Among them, the heat treatment process of inner and outer rings is relatively complex, and the technical requirements are very high. Their materials are generally: GCr15SiMn, GCr15, GCr18Mo, GCr15SiMo, GCr4, etc. The corresponding materials are selected according to the technical requirements, size and effective thickness of the product. Their heat treatment equipment is generally controllable atmosphere multi-purpose furnace, controllable atmosphere bottom charging multi-purpose furnace, controllable atmosphere conveyor belt roller furnace, mesh belt furnace, roller furnace and vacuum quenching furnace. This is suitable for heat treatment of small and medium sized bearings and will be discussed. For large bearing inside and outside rings will generally use box furnace, trolley furnace, well furnace, bell jar furnace, etc.

1. Multi-purpose controlled atmosphere furnace, multi-purpose bottom charging furnace and roller furnace

For the effective thickness of 10 mm or bearing steel quenching of internal and external ring, using fast light or overspeed quenching oil quenching, the center of the organization is difficult to meet the technical requirements, especially some enterprise of product required by organizations to grade 1 or less, therefore need to use in martensite gradation of nitrate quenching or bayesian isothermal quenching, only in this way can meet the technical requirements for quenching. The heating equipment is controllable atmosphere multi-purpose furnace, bottom charging multi-purpose furnace or roller furnace, in the use of sealed box type multi-purpose furnace, the sealing of the door must be reliable, the fire sealing device of the door to be good.

In terms of output comparison, the quantity of roller furnace is much higher than that of multi-purpose furnace and bottom charging multi-purpose furnace. There are two kinds of nitro - salt quenching grains: open type and closed type. Where the requirements of nitro salt quenching furnace, after the furnace will not be easy to stop, because nitro salt cooling after the start needs a long time and a greater power, is generally the use of heat preservation function. Therefore, the power consumed by maintaining the whole production line is large, and the furnace type should be selected with a clear target and careful selection.

2. Controlled atmosphere mesh furnace

For effective thickness < 10 mm of inner and outer bearing ring heating quenching with multi-purpose furnace, bottom loading multi-purpose furnace, roller furnace, mesh belt furnace, but with more basic use and mesh belt furnace roller furnace, the selection of the quenching cooling medium, according to the size of the effective thickness and technical requirements, can choose respectively fast bright quenching oil and step quenching oil to solve.

Mesh belt furnace using range is very wide, it is divided into a muff cans and no two furnace muffle tank type, the former is early eighty from Swiss Sally variety (SAFED) company to introduce technology, through the digestion and absorption after change become domestic muffle tank mesh belt furnace, the width of the muffle tank is in commonly 200 ~ 500 mm, used for quenching, carburizing and carbonitriding and shallow. According to the technical requirements of the product, the mesh belt furnace with muffle tank can be made into a variety of specifications of the furnace, when the use of muffle tank mesh belt furnace abroad, is basically applied to the heat treatment of precision parts, such as the whole set of stamping needle bearings assembled carbon nitriding or carburizing. Large muffle pot mesh belt furnace muffle width can achieve 900mm, for precision elastic parts, clocks and other parts of the quenching muffle pot width can also achieve 100mm.

In foreign countries, the mesh belt of muffle tank mesh belt furnace can be partially renewed after partial damage, but the domestic mesh belt has been changed into a folded structure, the mesh belt once there is damage to the local replacement is difficult, generally is the whole replacement. With muffle tank mesh belt furnace tank better sealing, the furnace atmosphere control is good, can ensure the long-term stability of the furnace atmosphere, the bearing products made without poor decarburization phenomenon, muffle tank in the service life of about 5 years, the furnace life of about 30 years. No muffle tank can also be a mesh belt furnace quenching, infiltration depth of 0.3 mm or less shallow carburizing or nitriding heat treatment, but to precise control in furnace atmosphere carburizing and carbonitriding long-term stability sometimes has the certain difficulty, when used only for the protection of the atmosphere when quenching is basically can achieve the goal of a stable, without muffle tank in shallow carburizing furnace life for 10 years or so commonly, it's difficult to find the furnace cracking, when used for quenching furnace life longer.

3. Controllable atmosphere drum furnace

For bearings such as steel ball, roller, needle, etc., in order to obtain the hardness uniformity of the rolling body in the heating and quenching, it is necessary to make the rolling body in the heating along with the rotation of the roller, the rolling body itself is also in the rotation and forward, the roller furnace needs to pass a controlled atmosphere. In a massive heating roller should choose drum of controlled atmosphere furnace automatic production line, the heat resistant steel roller diameter of 1000 ~ 2000 mm, tube wall welding have a certain height of heat resistant steel spiral of continuous, it can make the roller when rotating drum side flip side to go forward, like this rolling body is heated evenly, this type furnace is automatic feeding, feeding, on the other side discharging, rolling element when the tail out slowly falling into a quick bright quenching oil or water soluble agent directly cooling quenching medium.

For the heating of the rolling body with more varieties and less batches, it is better to use the octagonal olive-shaped protective atmosphere drum furnace, which is a manual feeding, feeding in front of the quenching. Do not recommend the use of mesh belt furnace heating quenching.

4. Vacuum heating and quenching furnace

Higher product technical requirements for the small stainless steel bearing and high carbon chromium bearing precision parts can be used in a vacuum quenching process, such as gas oil quenching double smothering vacuum furnace is a better choice, its operating environment friendly, according to the material defect layer or charging way different, choose different cooling rate of the vacuum quenching oil, vacuum quenching parts with uniform hardness, good fatigue resistance, high wear resistance, long service life, after quenching distortion less bright and clean, etc. Under the condition of not causing the alloy element volatilization during vacuum quenching, the vacuum degree is higher, the residual oxygen and water vapor in the furnace will be reduced a lot, the bearing ring quenched out is also very bright, keep the metal natural color, but the vacuum degree should not be chosen too high, otherwise it will cause the surface of the ring off chromium and other defects.

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