Hydrogen Embrittlement Failure of a Galvanized Washer
- PDF / 997,797 Bytes
- 6 Pages / 595.276 x 790.866 pts Page_size
- 21 Downloads / 209 Views
TECHNICAL ARTICLE—PEER-REVIEWED
Hydrogen Embrittlement Failure of a Galvanized Washer Xiao-lei Xu • Zhi-wei Yu • Haixuan Yu
Submitted: 11 July 2012 / in revised form: 28 November 2013 Ó ASM International 2014
Abstract A galvanized washer used for a locomotive impeller broke into three pieces after an accumulative service of 4–5 h. The washer is fabricated from 42CrMo steel and the fracture surfaces reveal intergranular fracture morphology with microvoid coalescence observed on the facets. Microstructure observation indicates the presence of a severely banded microstructure mainly consisting of untempered martensite and bainite. The average hardness of the failed washer is HRC 59.1. The hardness value is much higher than specified (HBW 260-300) and is in the range of hydrogen embrittlement susceptibility. The significant number of elongated MnS inclusions acting as traps of hydrogen are present in the martensite regions. The delayed fracture associated with the predominance of intergranular fracture micromechanism and the high hardness level and the presence of microstructure susceptible for the hydrogen embrittlement strongly suggest the hydrogen embrittlement being the mostly possible failure mechanism. The likely sources for hydrogen entrapment are the electro-galvanizing process and acid-pickling before galvanizing. The hydrogen was retained in the washer due to absence of baking treatment to remove it or insufficient baking. The incorrect heat treatment process before galvanizing resulting in the high hardness level of the washer is mainly responsible for the occurrence of hydrogen embrittlement on the washer.
X. Xu (&) Z. Yu Department of Material Science and Engineering, Dalian Maritime University, Dalian 116026, People’s Republic of China e-mail: [email protected] H. Yu College of Science, Northeast University, Shenyang 110004, People’s Republic of China
Keywords Washer Hydrogen embrittlement Intergranular fracture Galvanizing Improper heat treatment
Background A washer used in a locomotive impeller fractured after an accumulative service of 4–5 h. The failed washer is made of 42CrMo steel. The hardness value of core material is specified as HBW 260-300 by manufacturer manual. The washer was galvanized for corrosion protection. No other detailed information about the manufacturing process of the washer was obtained. Therefore, it is not clear how the washer was heat treated prior to galvanizing operation nor whether it was baked afterwards. In general, the fabrication steps of washer should be forming ? quenching ? tempering (for the specified hardness) ? galvanizing ? baking to remove hydrogen (at a temperature of around 200 °C for at least 3 h [1]).
Examination Methods The fracture surfaces were observed visually and using scanning electron microscopy (SEM). The microstructure was observed by optical microscopy (OPM). The chemical composition of the failed washer material was determined by spectroscopy chemical analysis. A specimen taken from the failed washer was heated at 200 °C for 4 h
Data Loading...
