۱٫ Introduction

۲٫ Surface integrity customization

۳٫ Rotary bending fatigue tests

۴٫ Discussion

۵٫ Conclusions

۶٫ References

This works aims at identifying the influence of surface integrity parameters on fatigue life of a 15-5PH. The effect of residual stress profile, surface roughness and microstructure are investigated. Various cutting and superfinishing processes (turning, ball burnishing and belt finishing) are used so as to reach various engineered surface integrities and as a consequence to highlight the influence of each surface integrity parameter. Rotary-bending tests have been carried out on samples to determine the average fatigue strength at 2 million cycles. It is shown that the influence of a deep compressive sublayer is by far more important than the ones of surface roughness or microstructure. The so-called ‘white-layer’ brings also a slight improvement of fatigue resistance.

Introduction

The 15-5PH is a martensitic stainless steel widely used for critical parts in the aeronautic or energy fields. The table 1 below presents its composition. In these sectors the machining sequences are critical and cannot be changed easily by authorities’ certifications. The reason for this is that the machining operations modify the surface integrity parameters responsible of the fatigue strength of the steel and the parts and slight changes in the process can have major influences on the performances and lifetime. [1], [2]. [3], [4] In previous works Chomienne [5] has machined customized parts trying to identify the influence of roughness and surface residual stress on fatigue behavior. The picture below (figure 1) presents the 4 sets of surface characteristics that were tested. In the article, the conclusions were that surface compressive residual stresses associated with a very low surface roughness increases the fatigue strength. Nevertheless, a higher roughness seems to have a non-negligible effect on the fatigue strength even if there are large compressive residual stresses. In fact it seems that the improvement of fatigue strength due to compressive residual stresses decrease with an increase of surface roughness.