Manifestations of fatigue defects in gray cast iron

In order to effectively improve the fatigue wear resistance of gray cast iron, it is necessary to understand the fatigue wear failure form and its formation mechanism, and then eliminate the factors conducive to the formation of fatigue defects to improve its fatigue wear resistance. The common rolling contact fatigue wear failure mainly has the following three forms:

(1) Pitting

Pitting corrosion is a typical feature of contact fatigue wear. Generally, the primary fatigue crack originating from the surface extends obliquely to the sub surface (at an angle of 10-30 degrees with the surface), extends to a certain depth and then turns to the surface. The material above the crack breaks and falls off to form a small vacancy, which is called pitting corrosion. The mechanism of pitting corrosion is explained by dislocation theory as follows: under the action of shear stress, dislocations move in the sub surface layer and form accumulation at obstacles such as non-metallic inclusions or grain boundaries; Because the direction of shear stress changes constantly in the rolling process, the direction of dislocation movement will also change constantly. The mutual cutting of dislocations will produce holes, and the continuous accumulation of holes will form holes and finally form cracks. Therefore, the basis for crack generation can be expressed as follows:

Where τ Represents the critical shear stress; γ Is the surface energy + plastic deformation work of crack propagation to adjacent grains; E refers to the elastic modulus of the material; D is the average grain size; Β Is a constant, which depends on the degree of triaxiality of positive pressure.

(2) Spalling

The formation mechanism of spalling and pitting corrosion is roughly the same, both of which belong to the form of contact fatigue failure, but their specific manifestations are different: spalling cracks generally originate in the deeper part of the sub surface layer (such as up to hundreds of microns), expand in the direction parallel to the surface, and finally form flake metal spalling, whose size is much larger than the size of pitting corrosion.

It is generally believed that the spalling crack is caused by the cyclic shear stress of the subsurface, so the maximum shear stress should be considered first when analyzing the formation mechanism of spalling τ Or orthogonal shear stress τ The size of YZ and the forming area. In addition, the change of material properties in the subsurface layer should also be considered. In the process of machining, due to work hardening, the hardness in the sub surface layer will exceed that of the substrate, and the closer it is to the surface, the higher the hardness. The research shows that assuming that the yield strength or fatigue strength of the material is directly proportional to the hardness, it is considered that the crack will be under shear stress τ Ratio of hardness to HV( τ / HV) is the controlling factor of spalling failure.

(3) Delamination

After the delamination theory was published in 1973, it was quickly responded by the tribological circles in various countries, and was continuously improved in the subsequent research to form a more mature delamination theory. The basic viewpoint of delamination theory is derived from the metal sliding contact model: when two sliding surfaces contact each other and the micro protrusion on the hard surface acts on the soft surface, the contact point of the soft surface will undergo a cyclic load, resulting in overall plastic deformation and the formation of dislocation; However, the dislocation density on the surface is often lower than that on the inside because the dislocation on the surface disappears due to the action of image force; When the micro protrusion moves on the surface repeatedly, the dislocation plug is preferentially formed at the highest dislocation density, and develops into vacancy. In addition, defects such as inclusions and second phase particles in metal materials are often the area of fatigue crack formation. When vacancies gather to a certain extent, they appear in the form of macro cracking, that is, the so-called fatigue microcrack. After the crack is formed, it generally propagates parallel to the surface and further expands under the action of each micro protrusion. When the crack propagates to a certain critical size, the crack propagation direction tends to the surface, and the material between the crack and the surface falls off as flake wear debris under the action of shear stress.