The quality of the surface and surface layer of the bearing

The failure modes of bearings include surface contact fatigue, abrasive wear, adhesion wear and corrosion wear. They always occur on the working surface and surface layer of the bearing. Obviously, the quality of the working surface layer is crucial to the reliability and service life of the bearing.

The study of bearing working surface quality includes: surface topography analysis; surface material and surface metamorphic layer analysis; surface stress state analysis; surface wear state and corrosion state analysis, etc.

Due to the influence of cold and hot processing conditions and lubricating media, the microstructure, physical, chemical, and mechanical properties of the bearing working surface are often very different from its core. The surface layer with changes in the microstructure, physical, chemical, and mechanical properties of the bearing surface is called the surface metamorphic layer. If the surface deterioration layer is caused by the grinding process, it is called the grinding surface deterioration layer. The analysis of the degraded layer of the bearing working surface is the main component of the bearing surface quality analysis, and of course it is also one of the important components of the bearing failure analysis.

According to the formation mechanism of the grinding degraded layer on the working surface of the bearing, the main factors affecting the grinding degraded layer are the effects of grinding heat and grinding force.

1. Grinding heat

In the grinding process, a large amount of energy is consumed in the contact area between the grinding wheel and the workpiece, and a large amount of grinding heat is generated, resulting in a local instantaneous high temperature in the grinding area. Using linear motion heat source heat transfer theory formula to derive, calculate or apply infrared method and thermocouple method to measure the instantaneous temperature under experimental conditions, it can be found that the instantaneous temperature of the grinding zone can be as high as 1000-1500℃ within 0.1-0.001ms. Such instantaneous high temperature is sufficient to cause high-temperature oxidation, amorphous structure, high-temperature tempering, secondary quenching, and even burn and cracking of the surface layer at a certain depth of the working surface.

 (1) Surface oxide layer

An extremely thin (20-30um) thin layer of iron oxide. Is formed on the surface of the steel under the action of instantaneous high temperature, It is worth noting that the thickness of the oxide layer has a corresponding relationship with the total thickness of the surface grinding metamorphic layer. This indicates that the thickness of the oxide layer is directly related to the grinding process and is an important indicator of grinding quality.

 (2) Amorphous tissue layer

When the instantaneous higtemperature of the grinding zone makes the surface of the workpiece reach a molten state, the molten metal molecule flow is evenly coated on the working surface, and is cooled by the base metal at a very fast speed, forming a very thin layer of amorphous state Organizational level. It has high hardness and toughness, but it is only about 10nm, which can be easily removed in precision grinding.

 (3) High temperature tempered layer

The instantaneous high temperature in the grinding zone can heat the surface to a certain depth (10-100um) ，which is higher than the tempering heating temperature of the workpiece. In the case of not reaching the austenitizing temperature, as the heated temperature increases, the surface layer will undergo a re-tempering or high-temperature tempering structural transformation corresponding to the heating temperature, and the hardness will also decrease. The higher， the heated temperature, the greater， the decrease in hardness.

 (4) Secondary quenching layer

When the instantaneous high temperature in the grinding zone heats the surface layer of the workpiece to above the austenitizing temperature (Ac1), the austenitized structure of this layer is re-quenched into a martensitic structure in the subsequent cooling process. For all workpieces with secondary quenching burns, the secondary quenching layer must be a high temperature tempered layer with extremely low hardness.

(5) Grinding cracks

The secondary quenching burn will change the stress of the surface layer of the workpiece. The secondary quenching zone is in a compressed state, and the material in the high-temperature tempering zone below it has the greatest tensile stress. This is where the crack core is most likely to occur. Cracks are most easily propagated along the original austenite grain boundaries. Severe burns will cause cracks (mostly cracks) on the entire grinding surface and cause the workpiece to be scrapped.

2. Metamorphic layer formed by grinding force

In the grinding process, the surface layer of the workpiece will be affected by the cutting force, compression force and friction force of the grinding wheel. Especially the effect of the latter two makes the surface layer of the workpiece form a highly directional plastic deformation layer and a work hardening layer. These metamorphic layers inevitably affect the changes in the residual stress of the surface layer.

(1) Cold plastic deformation layer

In the grinding process, each abrasive grain is equivalent to a cutting edge. However, in many cases, the rake angle of the cutting edge is negative. In addition to the cutting action, the abrasive grains also squeeze the surface of the workpiece (plough action), leaving a significant plastic deformation layer on the surface of the workpiece. The degree of deformation of this deformation layer will increase with the degree of blunt grinding of the grinding wheel and the increase of the grinding feed.

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

The instantaneous temperature formed by the grinding heat on the working surface makes the elastic limit of the surface layer of the workpiece at a certain depth drop sharply, even reaching the level of elasticity disappearing. At this time, the working surface layer is freely stretched under the action of grinding force, especially compression force and friction force, and its surface is compressed (plough) due to the limitation of the base metal, causing plastic deformation on the surface layer. Under the condition of the same grinding process, the high temperature plastic deformation increases with the increase of the surface temperature of the workpiece.

(3) Work hardened layer

Sometimes microhardness and metallographic methods can be used to find that the hardness of the surface layer increased due to processing deformation.

In addition to grinding, the surface decarburization layer caused by casting and heat treatment heating will remain on the surface of the workpiece if it is not completely removed in the subsequent reprocessing, causing the surface to soften and deteriorate, and promote the early failure of the bearing.

 (1) Cold plastic deformation layer

In the grinding process, every tick is equivalent to a cutting edge. However, in many cases, the rake angle of the cutting edge is negative. In addition to the cutting action, the abrasive particles are subjected to the squeezing action (plough action) on the surface of the workpiece, leaving an obvious plastic deformation layer on the surface of the workpiece. The degree of deformation of this deformation layer will increase with the degree of blunt grinding of the grinding wheel and the increase of the grinding feed.

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

The instantaneous temperature formed by the grinding heat on the working surface makes the elastic limit of the surface layer of the workpiece at a certain depth drop sharply, even to the extent that the elasticity disappears. At this time, the working surface layer is freely stretched under the action of grinding force, especially compression force and friction force, and the surface is compressed (more ploughed) by the limitation of the base metal, causing plastic deformation in the surface layer. The high-temperature plastic deformation increases with the increase of the surface temperature of the workpiece under the condition of the same grinding process.

 (3) Work hardened layer

Sometimes microhardness testing method and metallographic methods can be used to find that the hardness of the surface layer increased due to processing deformation.

In addition to grinding processing, the surface decarburization layer caused by casting and heat treatment heating, if not completely removed during subsequent processing, remains on the surface of the workpiece will also cause surface softening and deterioration, leading to early bearing failure.