What are the Causes and Hazards of Surface Defects in Bearing Materials?

1. Material surface folding

The main reason for the material surface folding is: steel in the rolling, forging process produced by the flying edge, burr, bump and sharp edges, etc., in the continued rolling, forging is pressed into the internal steel and the formation of cracks called folding. Folded cracks are different from material cracks in that they have an arc or linear crack shape under the skin and are generally at a certain angle to the surface of the steel, and the presence of oxide skin can be observed in the folded cracks through microscopic observation and decarburization on the sidewalls of the cracks. Since the folding crack only appears in the local area on the steel, the loss caused is not as serious as the material crack, but once the folding crack exists on the finished bearing, the impact on the service life of the bearing is the same as the material crack.

2. Pockmark

The reason for the pockmark is the bearing steel in the rolling process, due to billet or steel oxidation, inevitably produce a certain thickness of oxide skin on the surface of the steel. When the oxide skin off or is removed, the surface of the steel leaves a pit of varying size is called a pockmark or pockmark. For cold processing using spheroidized annealed steel or cold-drawn steel wire, if its surface pockmark (pit) depth is equal to or greater than the amount of parts specified in the processing stay, it can not guarantee the processing quality of bearing parts.

3.scarring

Due to the ingot surface slag, sub-surface bubbles, etc. in the forging, rolling process, resulting in the formation of thin flat layered defects on the steel surface is called scarring. This defect is not allowed to exist in the current bearing steel material standards explicitly.

4. Surface decarburization

Decarburization refers to the bearing steel material in the heating process, due to the chemical reaction with air or other oxidizing medium, and the steel surface layer of carbon content is reduced or completely lost phenomenon. Bearing steel materials in the forging, rolling process, as well as in the spheroid annealing are inevitably to occur in a certain decarburization situation. Therefore, the decarburization depth of the steel surface is strictly regulated in the standards of bearing steel materials at home and abroad. If the depth of decarburization layer of bearing steel exceeds the standard range, not only increases the amount of bearing manufacturers and production costs, reducing labor productivity, and if the bearing parts are processed and formed, the surface of the decarburization layer is not thoroughly removed, will cause the parts in the heat treatment after quenching and tempering hardness does not meet the requirements of the relevant standards, but also easy to produce quenching cracks in the quenching process. If the decarburized layer is retained to the finished bearing is still not removed, not only reduce the hardness, strength and wear resistance of bearing parts, and will greatly affect the bearing life.

5. Steel plate delamination

Steel plate delamination defects are caused by the presence of shrinkage residue in the slab, foreign slag or steel, there are more, more concentrated non-metallic inclusions, resulting in the rolling process of these defects can not be well welded together.

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