Roller bearing life and reliability of the bearing steel with a large degree of quality is closely related. Due to the characteristics of bearing steel, the quality of smelting
requirements than the general industrial steel to be much more
stringent, such as steel chemical composition, purity, organization and
uniformity.
First, strict chemical composition requirements
General
bearing steel is mainly high-carbon chromium bearing steel, that is,
about 1% carbon content, adding about 1.5% of chromium, and contains a
small amount of manganese, silicon eutectoid steel. Chromium
can improve the heat treatment performance, improve the hardenability,
uniformity of the organization, tempering stability, but also can
improve the anti-rust properties of steel and grinding performance. However,
when the chromium content exceeds 1.65%, quenching will increase the
retained austenite in steel, reduce the hardness and dimensional
stability, increase the carbide heterogeneity, reduce the impact
toughness and fatigue strength of steel. To this end, high-carbon chromium bearing steel in the chromium content is generally controlled at 1,65% or less. Only by strictly controlling the chemical composition of the bearing
steel in order to obtain the heat treatment process to meet the bearing
performance of the organization and hardness.
Second, high-precision size requirements
Rolling
bearing steel requires high dimensional accuracy of steel, this is
because most of the bearing parts have to be pressure forming. In
order to save material and improve labor productivity, the vast
majority of bearing rings are forging molding, steel ball is cold
heading or hot rolling molding, small size of the roller is also cold
heading forming. If the steel size accuracy is not high, it can not accurately
calculate the size and weight of the material, but can not guarantee the
quality of bearing parts, but also likely to cause damage to equipment
and mold.
Three, especially the strict requirements of the purity
Steel's
purity refers to the amount of non-metallic inclusions contained in the
steel, the higher the purity, the less the non-metallic inclusions in
the steel. Bearing
steel in the oxide, silicate and other harmful inclusions are leading
to early fatigue bearing off, significantly reduce the bearing life of
the main reasons. In
particular, the brittle inclusions are the most harmful, due to the
process of easy to peel off from the metal matrix, seriously affecting
the bearing parts after finishing the surface quality. Therefore, in order to improve the bearing life and reliability, it is
necessary to reduce the content of inclusions in bearing steel.
Four, strict low magnification organization and micro (high power) organization requirements
Low
magnification of bearing steel refers to the general loose, central
loose and flat, micro (high power) organizations, including steel
annealing organization, carbide mesh, ribbon and liquid chromatography. Carbide liquid is hard and brittle, and its hazards are the same as those of brittle inclusions. Mesh
carbide reduces the impact toughness of steel and makes it
inhomogeneous, prone to deformation and cracking during quenching. The banded carbides affect the annealing and quenching tempering tissues as well as the contact fatigue strength. Low and high power organization of the pros and cons of rolling
bearing performance and service life have a great impact, so in the
bearing material standards for low, high power organizations have strict
requirements.
5, particularly stringent surface defects and internal defects requirements
For
bearing steels, surface defects include cracks, slag, burrs, scarring,
scale, etc., internal defects include shrinkage, bubbles, white spots,
severe loose and segregation. These defects for the bearing processing, bearing performance and life
have a great impact in the bearing material standard clearly does not
allow these defects.
Six, particularly stringent requirements for carbide nonuniformity
In
the bearing steel, if there is a serious distribution of carbides
uneven, in the heat treatment process is likely to cause uneven
organization and hardness, steel unevenness of the contact fatigue
strength have a greater impact. In
addition, the serious carbide unevenness is also easy to make the
bearing parts in the quenching and cooling cracks, carbide unevenness
will lead to lower bearing life Therefore, in the bearing material
standards, different specifications of steel are clear special requirement.
Seven, particularly stringent surface decarburization requirements
In
the bearing material standard on the steel surface decarburization
layer has strict rules, if the surface decarburization layer beyond the
standard range, and in the heat treatment before the process did not
remove all of them, in the heat treatment quenching process Easy to produce quenching cracks, resulting in scrap parts.
Eight other requirements
In
the bearing steel material standard also on the bearing steel smelting
method, oxygen content, annealing hardness, fracture, residual elements,
spark inspection, delivery status, logo, etc. have strict requirements.
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