high quality slew bearing factory

1.Cold treatment process and function of slew bearing

First, the role of cold treatment
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Precision slew bearings have high requirements for dimensional stability; after quenching of the slew bearing (especially staged quenching), the internal structure still retains a lot of retained austenite. This retained austenite is an unstable structure.

During the process, changes will continue to occur, thereby changing the accuracy of the bearing. For this reason, a cold treatment process is used, which can reduce the retained austenite in the structure and slightly increase the hardness of the part.

2. Cold treatment process and requirements
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1. Stay at room temperature after quenching: After quenching, be sure to cool the inside and outside of the ferrule evenly to room temperature before cold treatment, otherwise it is easy to crack, and immediately after cooling to room temperature (generally no more than 30min), otherwise it will stop the austenite to the horse. The transformation of the body.
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2. Cold treatment temperature: The temperature of cold treatment is mainly based on the martensite transformation termination temperature Mf of the steel, and the influence of cold treatment on mechanical properties and manufacturability should also be considered.

For GCr15 steel, use -70℃ for cold treatment; when there are restrictions on ferrules or equipment with low precision requirements, the cold treatment temperature can be selected as -40～-70℃; for super precision bearings, the temperature can be between -70℃～-80℃ Cold treatment in between. Subcooled temperatures affect slew bearing impact fatigue and contact life.
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3. Cold treatment and heat preservation: Although the transformation of a large amount of martensite is completed when it is cooled to a certain temperature, in order to make the surface and core of a batch of ferrules reach the cold treatment temperature evenly, a certain heat preservation time is required, generally 1 to 1.5 h.
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4. Tempering after cold treatment: The ferrule is placed in the air after cold treatment, and its temperature is slowly raised to room temperature and then tempered in time. The temperature rise cannot be too fast, otherwise it is easy to crack; if the tempering is timely, otherwise the large residual stress inside the ferrule will cause the ferrule to crack, generally not more than 2h.

2.What are the factors that affect the fatigue life of slew bearing steel?

1. The effect of nitrides on fatigue life
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Some scholars have pointed out that when nitrogen is added to the steel, the volume fraction of nitrides decreases. This is due to the reduction in the average size of the inclusions in the steel. Limited by technology, there are still a considerable number of inclusion particles smaller than 0.2 in. counted.

It is precisely the existence of these tiny nitride particles that has a direct impact on the fatigue life of slew bearing steel. Ti is one of the elements that form nitrides. It has a small specific gravity and is easy to float. There will also be a part of Ti left in the steel to form multi-angular inclusions.

Such inclusions are likely to cause local stress concentration and fatigue cracks, so it is necessary to control the occurrence of such inclusions.

2. The effect of oxides on fatigue life
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The oxygen content in steel is an important factor affecting the material. The lower the oxygen content, the higher the purity and the longer the corresponding rated life. The oxygen content in steel is closely related to oxides. During the solidification process of molten steel, the dissolved oxygen of aluminum, calcium, silicon and other elements forms oxides.

The oxide inclusion content is a function of oxygen. As the oxygen content decreases, the oxide inclusions will decrease; the nitrogen content is the same as the oxygen content, and also has a functional relationship with the nitride, but because the oxide is more dispersed in the steel, it acts as a fulcrum with the same function as the carbide. , so it has no destructive effect on the fatigue life of steel.
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Due to the existence of oxides, steel destroys the continuity of the metal matrix, and because the expansion coefficient of oxides is smaller than the expansion coefficient of the slew bearing steel matrix, when subjected to alternating stress, it is easy to generate stress concentration and become the origin of metal fatigue.

Most of the stress concentration occurs between oxides, point inclusions and the matrix. When the stress reaches a sufficient level, cracks will occur, which will rapidly expand and destroy. The lower the plasticity of the inclusions and the sharper the shape, the greater the stress concentration.
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3. The effect of sulfide on fatigue life
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Almost all of the sulfur content in steel exists in the form of sulfides. When the sulfur content in the steel increases, the sulfide in the steel increases accordingly, but because the sulfide can be well surrounded by the oxide, the influence of the oxide on the fatigue life is reduced, so the number of inclusions has no effect on the fatigue life.

Certainly, it is related to the nature, size and distribution of inclusions. The more inclusions, the lower the fatigue life must be, and other influencing factors must be considered comprehensively.

In bearing steel, sulfides are dispersed and distributed in a fine shape, and are mixed with oxide inclusions, which are difficult to identify even by metallographic methods.

Experiments have confirmed that on the basis of the original process, increasing the amount of Al has a positive effect on reducing oxides and sulfides.

This is because Ca has a fairly strong desulfurization ability. Inclusions have little effect on the strength, but are more harmful to the toughness of the steel, and the degree of damage depends on the strength of the steel.