Effect of the Depth of Decarburized Layer in SKL15 Tension Clamp on Fatigue Strength

Yeun-Chul Park,Chang-Beom An,Hyoung-Bo Sim,Mancheol Kim

doi:10.3390/app11093841

Abstract

The surface of a quenched and tempered spring steel may have a decarburized layer from which the carbon component has been reduced. The fatigue strength of the decarburized layer is low compared to the base metal, which can easily develop fatigue cracks. Recently, fatigue failure was reported in the tension clamp (SKL 15) of the DFF-300 rail fastening system during use on one urban transit route in South Korea. As a result of measuring the depth of the decarburized layer of the SKL 15 tension clamp where the fatigue failure occurred, a decarburized layer thinner than the manufacturer’s maximum allowable decarburized layer was found in one of the eight tension clamps. To check the depth of the decarburized layer where the fatigue crack may have initiated, the decarburized layer was assumed to be the initial crack, and fatigue crack initiation was assessed based on the linear elastic fracture mechanics. The manufacturer’s maximum allowable decarburized layer depth of 0.2 mm may result in fatigue cracks.

Highlights

By comparing the stress intensity factor range (∆KI ) and fatigue threshold stress intensity factor range (∆Kth ), we propose a maximum depth of decarburized layer at which the fatigue crack may not occur in the case of the route under investigation
The depth of the decarburized layer of the tension clamp in which the fatigue crack occurred was measured, and the depth of the decarburized layer at which a fatigue crack could occur was confirmed via a linear elastic fracture mechanics (LEFM)-based method
All but one had a decarburized layer depth greater than 0.2 mm, which is the maximum allowable decarburized layer depth suggested by the manufacturer, and the fatigue failure occurred even in the tension clamp with a decarburized layer depth thinner than the allowable depth

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Many SKL15 tension clamps, which are used in the DFF-300 fastening system in South Korea, have been reported to have experienced fatigue failures during service. As a result of an in-depth investigation of this fatigue failure, it was confirmed that the depth of the decarburized layer was deeper than that suggested by the manufacturer and excessive stress occurred during train passage [1]. In the case of the tension clamp in the rail fastening system, heat treatment is performed to increase the strength of the metal, which could create the decarburized layer on the surface of the clamp. According to Frost et al [2], a decarburized layer occurred on the surface of the forged or quenched and tempered steel This treatment caused decarburization on the surface, producing a soft layer with a lower strength than the base metal; that is, the decarburized layer. By comparing the stress intensity factor range (∆KI ) and fatigue threshold stress intensity factor range (∆Kth ), we propose a maximum depth of decarburized layer at which the fatigue crack may not occur in the case of the route under investigation

Result of Decarburized Layer Measurement

Decrease in the Fatigue Strength Due to Decarburized Layer

Section Force Generated in the Tension Clamp Due to Train Running

Findings

Conclusions