Page 409 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 200 I Elsevier Science Ltd. A11 rights reserved 393

Hydrogen embrit tlement failure of hot dip galvanised high

tensile wires

N.K. Mukhopadhyay, G. Sridhar, N. Parida, S. Tarafder, V.R. Ranganath"

National Metallurgical Laboratory, Jamshedpur 831 001 7, India
Received 30 July 1998; accepted 13 September 1998

Abstract

A case of failure of high carbon eutectoid steel wires is investigated. During the production stage, i.e. cold
drawing and subsequent coiling/stranding operations, the wires of - 4.0 mm diameter failed by central
splitting along longitudinal planes. Microscopic examination, fractography and mechanical tests along with
acoustic emission monitoring were carried out on the wire samples. The experimental results confirm that
the failure is related to hydrogen embrittlement which has been characterised by fractography, strain rate
sensitivity and susceptibility to delayed fracture as indicated by acoustic activity. It has been argued that the
cohesive energy model for hydrogen embrittlement, where hydrogen reduces the bond strength and cohesive
strength ahead of pre-existing cracks, can explain the failures observed in the present case. It appears that
improper pickling and subsequent baking processes, during the final stages of drawing operations, are
responsible for the hydrogen related failures.

Keywords: Hydrogen embrittlement; Galvanised wire; High tensile wire; Acoustic emission

1. Introduction

Several types of wire rod failures are reported in the literature, mostly due to inclusions, strain
ageing, hydrogen embrittlement, liquid metal embrittlement, improper control of microstructure
etc. resulting in bulging, flaking and splitting. Splitting failures may occur spontaneously when the
material is embrittled along a longitudinal plane and the residual stresses generated during the
drawing and handling operations are relieved. Cold drawn high carbon steel wires are amenable
to being more weak along the longitudinal plane as compared to the transverse planes due to the
fibre-like deformation of pearlitic phases in the longitudinal direction [l]. It has been observed that
metallurgical embrittlement may also take place due to interfacial segregation of hydrogen (H) in
high strength steels, which leads to delamination/decohesion type failures [2]. A short discussion

*Corresponding author. Tel.: +91 657 426091; fax: + 91 657 426527; e-mail: vrr@csnml.ren.nic.in

Reprinted from Engineering Failure Analysis 6 (a), 253-265 (1999)