疲劳载荷对17CrNiMo6喷丸强化层残余应力与组织的影响

高强度渗碳钢17CrNiMo6常用喷丸强化工艺来改善抗疲劳性能,但疲劳载荷会使喷丸强化层的残余应力与组织发生改变,削弱了强化效果。采用X射线衍射方法研究了其残余应力、半高宽和残余奥氏体随疲劳载荷周次的变化规律,同时研究了喷丸前后表面形貌、硬度和组织的变化。结果表明,喷丸与二次喷丸强化层残余应力场在疲劳载荷前100周次内发生了应力松弛,松弛幅度分别为50%与33%,之后基本保持稳定。半高宽随载荷周次的增加分别下降了5%与7%,而残余奥氏体含量变化不大。此外,喷丸处理增加了材料表层残余压应力,细化了晶粒尺寸,使疲劳寿命提高了11%;二次喷丸可进一步增加材料表层残余压应力,细化晶粒尺寸并改善表面形貌使材料疲劳寿命提高了23%。因此在评估喷丸工艺强化效果,预测喷丸处理后零部件的疲劳寿命时,宜采用松弛后的残余应力作为衡量参数。

Effects of Fatigue Load on Residual Stress and Microstructure of 17CrNiMo6 Shot Peening Strengthened Laye

High-strength carburized steel 17CrNiMo6 is often strengthened by shot peening to enhance its fatigue resistance. However, the effects of shot peening can be diminished by fatigue load which can change the residual stress and microstructure of the shot peening strengthened layer. The variations in the residual stress, the full width at half maximum (FWHM) and the retained austenite were investigated by XRD under the fatigue load, and the variations in the surface roughness, hardness and microstructure before and after the shot peening were also studied. Results show that 50% of stress relaxation occurs in the shot peening strengthened layer and 33% in the re-shot peening strengthened layer during the first 100 cycles. Then the residual stress stays stable in the following circles. The FWHM declines by 5% and 7% with the increase of load cycles, but the content of the residual austenite changes slightly. While the shot peening enhances the surface compressive residual stress, refines the grain size and increases the fatigue life by 11%, the re-shot peening can further enhance the surface compressive residual stress, refine the grain size, reduce the surface roughness, and increase the fatigue life by 23%. Therefore, it is appropriate to use the residual stress after relaxation as a parameter to evaluate the effects of shot peening and to forecast the fatigue life of the parts strengthened by shot peening.