Non-destructive residual stress distribution measurement in nano-structured ultrahigh-strength gear steels

The well-established enhanced fatigue performance associated with beneficial compressive residual stresses has been broadly applied in the development of new engineering materials, particularly gear and bearing steels. Residual stress enhancement processes, such as shot/laser peening, have also been investigated to maximize their benefits on fatigue strength. However, the measurement of residual stress distributions still heavily relies on the conventional X-ray technique, involving destructive material removal, tedious data correction and time-consuming data collection, which slows new material design and process optimization. To overcome this problem, we employ novel, non-destructive synchrotron techniques with high-energy x-rays to measure the distribution of residual strain/stress in a laser-peened, ultrahigh-strength gear steel. This study will assist in process optimization, to achieve the desired residual stresses for selected applications. X-ray measurements were performed at the 1-ID beamline at the Advanced Photon Source (APS), Argonne National Laboratory. An x-ray energy of 76 keV and conical slit were used to create a diffraction volume of ∼ 20 x 20 x 150(micro)m3. An area detector was placed after the conical slit to collect diffraction over a plane encompassing (nearly) the axial and normal strain directions. Cylindrical specimens (76 mm long, 9.525 mm diameter) were rotated during the measurement to ensure a sufficiently large number of grains were irradiated. The steel, FerriumC67(regsign), was designed utilizing thermodynamics-based strengthening models to achieve a new level of case hardness (67 HRc) and good core toughness, employing a 3nm M2C carbide dispersion. After heat treatment, C67 was laser peened and subject to rolling contact fatigue (RCF) screening tests under the extreme Hertzian contact stress of 5.4 GPa. Both regions away from ('untested') and under wear tracks were studied for comparison. Four BCC reflections from martensite ((200), (211), (220) and (222)) were recorded and (211) was used for residual strain analysis. Strain components ((varepsilon)11, (varepsilon)12 and (varepsilon)22) were obtained and the axial ((varepsilon)11) is plotted in Fig. 1 for unpeened and laser peened C67 samples. Large compressive axial strains were observed near-surface after peening. After cyclic loading the surface strain relaxed but a sub-surface maximum was formed, attributed to yielded material from the extreme cyclic loading. These strains were converted to stresses (not shown) via elastic constants and assuming equibiaxial strain ((varepsilon)33 = (varepsilon)11).