According to the composition mechanism of the grinding transformation layer on the working surface of self-aligning roller bearings, the main factors affecting the grinding transformation layer of self-aligning roller bearings are the effects of grinding heat and grinding force.

1. Grinding heat

In grinding, a lot of energy is consumed and a lot of grinding heat occurs in the contact area between the grinding wheel and the workpiece, forming an instantaneous high temperature in the grinding area. By using the heat transfer theory formula of linear motion heat source to derive, calculate, or measure the instantaneous temperature under experimental conditions using infrared and thermocouple methods, it can be found that the instantaneous temperature in the grinding area can reach up to 1000-1500 ℃ within 0.1-0.001ms. Such instantaneous high temperature is sufficient to cause high-temperature oxidation of the surface layer at a certain depth on the work surface, including various modifications such as amorphous structure, high-temperature tempering, secondary quenching, and even burns and cracks.

(1) Surface oxide layer

The steel surface under instantaneous high temperature reacts with oxygen in the air to form an extremely thin (20-30nm) layer of iron oxide. It is worth noting that there is a corresponding relationship between the thickness of the oxide layer and the total thickness of the surface grinding deformation layer test results. 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 organizational layer

When the instantaneous high temperature in the grinding area causes the surface of the workpiece to reach the melting state, the molten metal molecules are evenly coated on the working surface and cooled by the base metal at an extremely fast speed, forming an extremely thin layer of amorphous structure. It has high hardness and resistance, but it is only around 10nm and can be easily removed in precision grinding.

(3) High temperature tempering layer

The instantaneous high temperature in the grinding area can heat the surface to a certain depth (10-100nm) higher than the tempering temperature of the workpiece. Without reaching the austenitizing temperature, as the heating temperature increases, the surface will undergo layer by layer structural changes corresponding to the heating temperature, such as re tempering or high-temperature tempering, and the hardness will also decrease. The higher the heating temperature, the more severe the decrease in hardness.

(4) Two layer quenching layer

When the instantaneous high temperature in the grinding zone heats the surface layer of the workpiece above the austenitizing temperature (Ac1), the austenitized structure of that layer is re quenched into martensitic structure during the subsequent cooling process. Any workpiece with secondary quenching burns must have a high-temperature tempering layer with extremely low hardness below the secondary quenching layer.

(5) Grinding cracks

Secondary quenching burns will cause stress changes in the surface layer of the workpiece. The secondary quenching zone is under pressure, and the high-temperature tempering zone material below it has the maximum tensile stress, which is the most likely location for crack centers to occur. The simplest way for cracks to propagate is along the original austenite grain boundaries. Severe burns can cause cracks (mostly cracking) to appear on the entire grinding surface, resulting in the scrapping of the workpiece. 

2. Deformation layer formed by grinding force

During the grinding process, the surface layer of the workpiece will be affected by the cutting force, tightening force, and friction force of the grinding wheel. Especially the role 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 will inevitably affect the modification of residual stress in the surface layer.

(1) Cold plastic deformation layer

During the grinding process, each moment of the abrasive grain is equivalent to a cutting edge. However, in many cases, the front angle of the cutting edge is negative, and the abrasive particles, in addition to the cutting effect, make the surface of the workpiece subject to compression (plowing effect), 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 grinding dullness of the grinding wheel and the increase of grinding feed rate.

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

The instantaneous temperature formed by grinding heat on the working surface causes a sharp decrease in the elastic limit of the surface layer of a certain depth of the workpiece, and even reaches the degree of elastic disappearance. At this moment, the surface layer of the homework is relaxed and expanded under the action of grinding force, especially tightening force and friction force. It is constrained by the base metal, and the surface is tightened (plowed), forming plastic deformation in the surface layer. High temperature plastic deformation increases with the increase of surface temperature of the workpiece, while the grinding process remains unchanged.

(3) Work hardening layer

Sometimes, microhardness and metallographic methods can be used to observe an increase in surface layer hardness caused by machining deformation.

In addition to grinding, the surface decarburization layer formed by casting and heat treatment heating, if not completely removed in subsequent processing, will also form surface softening and transformation on the workpiece surface, promoting the early failure of self-aligning roller bearings.