The most intuitive understanding of surface hardening is that it can improve the surface hardness of materials and enhance wear resistance. But it is also an effective way to improve the fatigue strength of materials. Do you know why? Today we will take a look at the mechanism behind this.
There are three main reasons for the most common nucleation of fatigue cracks, as shown in the figure above. One is slip dislocation on the surface of the material, and the other two are caused by internal defects of the material, such as internal foreign matter inclusions, grain boundary gaps (such as shrinkage holes), etc. Today we discuss the effect of surface hardening on fatigue strength. Assuming that the material has no defects, slip dislocations are the focus of consideration.
When the material is subjected to external force, slip occurs on a microscopic scale, and the slip bands produce extrusion and intrusion by sliding against each other. As shown in the figure below, these slip bands and the adjacent areas of extrusion and intrusion act as stress concentration points, where cracks originate and develop. Because shear stress is dominant, cracks usually appear and expand at a 45° angle. Compared with the internal grains, the grains on the surface of the parts are more likely to form defects earlier due to the lack of support from the surrounding grains.
When we surface harden the parts, the hardness of the material surface is greatly improved, and the hardness is directly related to the strength of the metal. The increase in hardness means that its ability to resist plastic deformation has increased. Compared with soft materials that have not been hardened, the hardened material is relatively less likely to produce slip dislocation movement. If you observe the microphotograph, you will find that there is basically no obvious extrusion or intrusion on the hardened surface. Therefore, there is no stress concentration caused by slip dislocation on the metal surface, and the possibility of crack initiation is greatly reduced. This is the main reason why surface hardening can improve the fatigue strength of the material. Of course, there is also a secondary reason. After quenching and hardening, the surface of the material can usually form residual compressive stress on the surface. This compressive residual stress also helps to resist the initiation and expansion of fatigue cracks, thereby improving the fatigue life of the material. However, there are two points to remind everyone at the end: First, if the structural design of the parts is unreasonable or the quenching process control is unreasonable, cracking or microcracks may occur during the hardening process, which will become the source of fatigue failure. Second, if the hardened parts need to be finished, these processes may also cause microcracks (such as grinding cracks) or change the hardness distribution, reduce residual compressive stress or even transform it into tensile stress, which will become some unfavorable factors affecting fatigue strength.(文章来源：iMechanics机械)
