Fracture Failure of Zinc-Plated Hub Bolts by Hydrogen Embrittlement
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CASE HISTORY—PEER-REVIEWED
Fracture Failure of Zinc-Plated Hub Bolts by Hydrogen Embrittlement Na Xu . Ning Ding . Junbo Shi . Weimin Guo . Chi-Man Lawrence Wu
Submitted: 25 November 2014 / in revised form: 26 April 2015 / Published online: 10 June 2015 Ó ASM International 2015
Abstract In this work, the failure behavior of a batch of zinc-plated hub bolts with high strength was detected and analyzed to determine the failure mechanism. The fracture failure occurred at the head-to-shank transition. The initial region of the crack was at the external edge of the bolts. Brittle fractures containing intergranular and transgranular features were observed using a scanning electron microscope. Yawning grain boundaries as well as micropores on the grain surfaces were observed at many areas of the fracture surfaces. According to the analysis results, hydrogen embrittlement is considered as the probable mechanism of the failed specimens. Poor baking treatment may be the reason leading to the incomplete removal of hydrogen. Keywords Hydrogen embrittlement Hub bolt Zinc plating Fracture
Introduction Hydrogen embrittlement, which can significantly reduce the mechanical properties of metallic materials, is caused
N. Xu N. Ding (&) J. Shi W. Guo C.-M. L. Wu Research Center of Failure Analysis and Engineering Safety Assessment, Shandong Academy of Sciences, Jinan, People’s Republic of China e-mail: [email protected]; [email protected] N. Ding C.-M. L. Wu (&) Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, People’s Republic of China e-mail: [email protected]
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by the invading of hydrogen along grain boundaries [1, 2]. The diffusion of hydrogen in the crystalline structure can dramatically reduce the ductility of the material, thus, it often causes brittle fracture and reduces the service life of many metallic components. Alloy steels with high strength are susceptible to hydrogen embrittlement when plating or etching processes are involved. Delayed fracture normally occurs when the hydrogen containing parts are applied with the operating load. Only a few ppm. of hydrogen could lead to serious damage in hardened and tempered steels [3, 4]. As delayed fracture after assembly very often occurs with hydrogen embrittlement, the dangerous situation is not immediately realized by users. During the past decades, there has been much research attention on the effect of hydrogen on properties of materials [5–7]. It was reported that the tensile strength of metal could be significantly affected by embrittlement due to the presence of hydrogen along grain boundaries. Although the importance of prevention of hydrogen embrittlement in metallic materials has been realized for many years, failures caused by hydrogen embrittlement are still of common occurrence [8–10]. Steel fasteners play an important role in industry. Due to the requirements on surface quality or certain performance such as corrosion resistance, acid pickling and electroplating are commonly carried
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