유도 경화한 SCM440강의 미세 조직, 경도, 깊이 및 무해화 균열 크기 평가

Abstract

In this study, an made annealed base metal (BM) specimen and quenching-tempered QT specimen with SCM440 steel were used. These specimens were subjected to induction hardening (IH) at frequencies of 100, 50, and 10kHz while varying the induction coil moving speed to 30, 15, and 10mm/s, respectively. The IH specimen was used to evaluate the effects of before-IH process structure, induction coil travel speed, and frequency. In addition, to determine the harmless crack size of SCM440 steel, the hardness, residual stress, and fatigue limit were determined using BM and QT specimens and BM and QT specimens with IH. The harmless crack size was evaluated using the sum of the fatigue limit, threshold stress intensity factor by the Ando equation, and stress intensity factor by the Newman-Raju and API-RP579 equations. The induction coil travel speed had a large effect on the structure and surface hardness owing to the heating temperature, while the frequency had a small effect owing to inductance loss. The effect of frequency on the total case depth and effective case depth had a significant effect on the current penetration depth owing to the skin effect, and the induction coil travel speed also had a significant effect. Because the structure before IH affects the Ac3 transformation point and austenitization, it significantly affects the structure, surface hardness, and hardening depth. Because the residual stress increases rapidly as the deepest point of the crack becomes deeper, harmless cracks were determined at the deepest point of the crack for all As, regardless of the BM and QT specimens. Meanwhile, surface cracks did not develop on the outermost surface because the compressive residual stress always acted on it. The harmless crack size was 1.02~1.35mm in the BM specimen and 1.04~1.45mm in the QT specimen, and this size could be evaluated as the threshold stress intensity factor of the long crack. The harmless crack size is minimum at the aspect ratio (As) of 1.0 and maximum at (As) 0.1, but is almost similar regardless of the BM and QT specimens, and the dependency on As is small because of the constant harmless crack size. The deeper the residual stress depth, the larger the harmless surface crack size. If NDI detection cracks are not peened after repair, NDI must be performed with good resolution. When peening after repair, the minimum harmless surface crack size is large. Therefore, for safety reasons, an 80% harmful surface crack size must be detected, repaired, and peened. This improves NDI reliability and reduces the number of maintenance points. If NDI inspection is performed and peening is not performed, microcracks must be detected and repaired to ensure safety.