Residual stresses measurement of induction hardened railway S38C axles using neutron diffraction

中性子回折による高周波焼き入れS38C鉄道車軸の残留応力測定

Qin, T. Y.*; Zhang, H.*; Zhou, L.*; Ao, N.*; 徐 平光   ; Su, Y. H.  ; Wu, S. C.*; 菖蒲 敬久  

Qin, T. Y.*; Zhang, H.*; Zhou, L.*; Ao, N.*; Xu, P. G.; Su, Y. H.; Wu, S. C.*; Shobu, Takahisa

The S38C railway axles are developed for Shinkansen (Bullet Train) through high-frequency induction hardening and the residual stresses are introduced into the axle with a depth of several millimeters. Residual stresses seriously affect the fatigue mechanical strength and fatigue life of engineering structures, and the accurate determination and the optimization control of the three-dimensional (3D) residual stress distribution of high-speed railway S38C axles are increasingly necessary for improving the axle service life. Here, the residual stresses of the S38C axle samples were measured by using the RESA (residual stress analyzer) angle dispersive neutron diffractometer at JRR-3 (Japan research reactor No.3). The large axle samples were electro-discharge machined into: (a) 3 mmH3 mmA20 mmR comb-shape stress-free coupons with a comb spacing of 1 mm to relieve the transformation induced residual stresses and determine the stress-free lattice spacing, d0; (b) sectioned bar sample 120mmA15mmR 15mmH) and sectioned fan sample (91mmR158mmH15mmA) to well measure the stress distribution in limited beam time. The residual stress distribution of the large axle samples was obtained in three directions (axial(A), radial(R) and hoop(H)) from the surface to the core. Unlike previous studies that only focused on the axial residual stresses of the axle, we found that the hoop residual stresses also have considerable values, which verified the necessity of 3D residual stress testing. Subsequently, according to the residual stress field and basic mechanical properties of the S38C axle obtained by experiments, the 3D residual stress field is reconstructed in the real axle by using an iterative technique. These results are much valuable to reveal the fatigue failure behavior of high-speed railway axles after surface strengthening and to optimize the non-destructive inspection interval.