There are two main factors affecting the grinding metamorphism layer of sliding bearing pedestal, namely the grinding force and grinding heat.
1. Grinding force
During grinding, the surface layer of the workpiece is affected by the cutting force, compression force and friction force of the grinding wheel.In particular, the latter two role, the workpiece surface layer form a highly directional plastic deformation layer and the work hardening layer.These metamorphic strata inevitably affect the change of residual stress in the surface layer.
(1) Cold plastic deformation layer
In the grinding process, each moment of grinding grain is equivalent to a cutting edge.However, in many cases, the cutting edge of the front Angle is negative, in addition to the cutting effect, grinding grain is to make the workpiece surface under the pressure (plough action), so that the workpiece surface leaves a significant plastic deformation layer.The deformation degree of the deformation layer will increase with the increase of grinding wheel blunt degree and grinding feed.
(2) Thermoplastic deformation (or high-temperature deformation) layer
The instantaneous temperature formed by grinding heat on the working surface makes the elastic limit of the surface layer of a certain depth of workpiece drop sharply, and even reaches the degree of elastic disappearance.At this time, the working surface layer under the action of grinding force, especially compression force and friction force, caused by free extension, subject to the restriction of the matrix metal, the surface is compressed (more plough), in the surface layer caused by plastic deformation.The high temperature plastic deformation increases with the increase of surface temperature of the workpiece under the condition that the grinding process remains unchanged.
(3) Work hardening layer
Sometimes microhardness and metallographic method can be used to find that the surface hardness increases due to machining deformation.
2. Grinding heat
In the grinding process, the grinding wheel and the workpiece contact area consumes a lot of energy and generates a lot of grinding heat, resulting in local instantaneous high temperature in the grinding area.It can be found that the instantaneous temperature in grinding zone within 0.1 ~ 0.001ms can be as high as 1000 ~ 1500℃ by deducing and calculating the theoretical formula of heat transfer with linear motion heat source or measuring the instantaneous temperature under experimental conditions by using infrared method and thermocouple method.Such a transient high temperature is sufficient to produce high-temperature oxidation, amorphous structure, high-temperature tempering, secondary quenching, and even burn cracking at a certain depth of the surface layer.
(1) Oxidation layer on the surface of sliding bearing housing
The surface of the steel under the action of instantaneous high temperature interacts with the oxygen in the air and rises to a very thin (20 ~ 30nm) layer of iron oxide.It is worth noting that there is a corresponding relationship between the oxide thickness and the total thickness of surface grinding metamorphic layer.This indicates that the oxide thickness is directly related to the grinding process and is an important marker of grinding quality.
(2) Amorphous tissue layer
When the surface of the workpiece reaches the melting state due to the instantaneous high temperature of the grinding zone, the molten metal molecular flow is evenly coated on the working surface and cooled by the matrix metal at an extremely fast speed, forming an extremely thin layer of amorphous structure.It has high hardness and toughness, but it is only about 10nm and is easily removed in precision grinding.
(3) High-temperature tempering layer
The instantaneous high temperature of the grinding zone can heat the surface to a certain depth (10-100nm) above the tempering temperature of the workpiece.In the case that the austenitizing temperature is not reached, with the increase of the heating temperature, the surface layer by layer will produce the retempering or high-temperature tempering tissue transformation corresponding to the heating temperature, and the hardness will also decrease.The higher the heating temperature, the more the hardness drops.
(4) The second quenching layer of sliding bearing pedestal
When the surface layer of the workpiece is heated to above the austenitizing temperature (Ac1) by the instantaneous high temperature of the grinding zone, the austenitized structure of the layer is re-hardened into martensite structure in the subsequent cooling process.Where there is a second quenching burn of the workpiece, the second quenching layer must be a very low hardness of the high-temperature tempering layer.
(5) Grinding cracks
The secondary quenching burn will change the surface stress of the workpiece.The secondary quenching zone is in the state of compression, and the materials in the high-temperature return fire zone below it have the maximum tensile stress, which is the place where crack core is most likely to occur.The crack propagates most easily along the original austenite grain boundary.Severe burns can result in cracks (often cracking) on the entire grinding surface, resulting in the workpiece being scrapped.