Improvement of Safety and Reliability of SCM440 Steel with Induction Hardening

Abstract

In this study, structural SCM440 steel was used to investigate harmless crack size using compressive residual stresses by induction hardening (IH). The fatigue limits of base metal (BM), quenching–tempering (QT), and IH specimens were obtained. The harmless crack size (ahml) was evaluated using the fatigue limits, threshold stress intensity factor using the Ando equation, and the sum of the stress intensity factor using the Newman-Raju and API-RP579 equations. Because as the crack depth increases, the compressive residual stress rapidly decreases, the harmless crack was determined from the intersection of depth for all aspect ratios (As). However, the outermost surface crack did not intersect because the compressive residual stress (σr,s) on the surface is always present. The ahml values based on BM and QT are 1.04−1.45 and 1.02−1.39 mm, respectively. These values can be evaluated as the ∆Kth(l) of a long crack. ahml did not significantly depend on As. If the crack detected after nondestructive inspection (NDI) is not surface modified after repair, then NDI1 with a very high resolution must be performed.