According to FAG formation mechanism of grinding badness layer of bearings working surface, the main factors influencing the grinding metamorphic layer is a function of grinding and grinding force.The following analysis about FAG bearing failure reason.

1. FAG bearing grinding heat

In FAG bearing grinding, grinding wheel and workpiece contact zone, consume large amounts of energy, to produce a large number of grinding heat, local transient high temperature caused by grinding area.On the basis of the theory of linear motion heat source heat transfer formula derivation, calculation or by the method of infrared ray and measured experiment under the condition of instantaneous temperature thermocouple method, can be found in the grinding zone within 0.1 ~ 0.001 ms instantaneous temperature up to 1000 ~ 1000 ℃.The instantaneous high temperature, which would have made certain depth on the surface of the work surface to produce high temperature oxidation, amorphous group, high temperature tempering, secondary hardening, and even burn cracking and other changes.

(1) the surface oxide layer

Under the action of transient high temperature of the surface of the steel and the oxygen in the air, rises into extremely thin (20 ~ 30 nm) iron oxide thin layer.It is worth noting that the oxide layer thickness and surface grinding metamorphic layer is the total thickness test results in corresponding relation.This shows that the thickness of oxide layer, is directly related to the grinding process is an important symbol of grinding quality.

(2) the amorphous layer

Instantaneous high temperature parts surface grinding area to molten state when molten metal molecular flow is evenly coated to the work surface, and the matrix metal cooling at breakneck speed, formed a very thin layer of amorphous tissue layers.It has high hardness and toughness, but it is only about 10 nm, it is easy to be removed in the precision grinding processing.

(3) high temperature tempering layer

Transient high temperature can make the surface of the grinding area within a certain depth (10 ~ 100 nm) was heated to a temperature higher than tempering heating temperature of workpiece.In the absence of austenitizing temperature reached, with the improvement of heated temperature, the surface will produce one by one and heating temperature corresponding to the organization transformation, or high temperature tempering tempering hardness.The higher the heating temperature and hardness of the decline is more severe.
(4) the hardening layer on the second floor

Instantaneous high temperature heat the workpiece surface layer when the grinding area to the austenitizing temperature (Ac1) above, then the austenitizing layer of tissue in the subsequent cooling process, the quenching into martensite structure anew.Where the secondary hardening burns of workpiece, the second time hardening layer must be under very low temperature tempering hardness layer.

(5) the grinding crack

The secondary hardening burns will make the workpiece surface layer stress changes.Secondary hardening zone is in compression state, the high temperature materials back to the fire area underneath there are the largest tensile stress, this is the place where the most likely to occur crack core.Crack is the most easily spread along the original austenite grain boundary.Serious burns can lead to crack of the grinding surface artifacts scrap (takes the crack).

2. The FAG bearing the metamorphic layer formed by the grinding force

In the process of grinding, the workpiece surface layer will be grinding wheel cutting force, compression force and friction force.And, in particular, the role of the latter two, make the workpiece surface layer formed directional strong plastic deformation and strain hardening layer.The metamorphic layer is bound to affect the surface residual stress changes.

(1) cold plastic deformation layer

Every moment in the process of grinding, grinding grain is equivalent to a cutting edge.But in many cases, the rake Angle of cutting edge negative, in addition to abrasive cutting action, is to make the surface under compression (plough), make the left obvious plastic deformation of workpiece surface layer.The deformation layer deformation degree as the degree of grinding wheel dull increases and grinding feed.

(2) thermoplastic deformation (or high temperature deformation) layer

Grinding heat in the transient temperature of the work surface formation, makes a certain depth of the workpiece surface layer elastic limit fell sharply, even to the point of elastic disappear.The work surface on the grinding force, especially under the action of compression force and friction force, caused by free stretch, subject to the conditions of the matrix metal, the surface is compressed (plough), caused a plastic deformation in the surface layer.High temperature plastic deformation under the condition of the grinding process is constant, with the temperature increasing of workpiece surface.

(3) the work hardening layer

Sometimes use micro hardness and metallographic method can be found that caused by the machining deformation of surface hardness.

In addition to grinding machining, casting and heat treatment heating caused by the surface decarburized layer, if again later processing has not been completely removed, residual on the workpiece surface will cause surface softening metamorphism, result of bearing early failure.

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