On the measurement of residual stress in induction hardened parts

摘要

It is well known that induction surface heating followed by rapid quenching generally increases the fatigue life of steel components subjected to bending loads by significantly postponing the micro-crack nucleation and propagation processes.The phase transformation volume change combined with severe thermal gradients leave a hard surface layer under relatively high and deep compressive residual stresses.In this paper, residual stress measurements are done on induction hardened AMS6414 martensitic steel (aerospace grade of AISI4340) cylinders using two techniques: the so-called contour method and X-ray diffraction.For both methods, induction hardened parts raise many challenges.The contour method hardly describes high stress gradients near the surface while the diffraction technique accuracy appears limited considering the strong microstructural variation and the high depth of the stresses to be measured.For the contour method, a CMM and an optical pen using the confocal chromatic imaging principle were used to measure the surface after precision WEDM cutting.The effect of data filtering and smoothing on the calculated stresses are discussed.For X-ray analysis, the effect of stress relaxation during layer removal and analysis technique is explained.The difference between the residual stress measurements done with the two techniques is discussed with emphasis on both the surface and the in-depth measurements.