A New Nonmetallic Inclusion Rating Method by Positive Use of Hydrogen Embrittlement Phenomenon
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I.
INTRODUCTION
THE quality of high strength steels such as bearing steels, tool steels and spring steels are evaluated not only by the microstructure but also the cleanliness. Large nonmetallic inclusions degrade the fatigue properties of high strength steels.[1–3] Among several inclusion rating methods, the statistics of extremes rating method of inclusion sizes[1,4–9] has been used as one of the most reliable and quantitative methods by which we can predict the maximum inclusion size contained in any volume of material and the relative comparison of quality among various steels. Hydrogen degrades the mechanical properties, such as loss of ductility[10,11] and reduction in fatigue properties[12,13] of metals. This phenomenon is expressed as the term hydrogen embrittlement ‘HE’. It is well known that HE occurs more markedly in high strength steels with a Vickers hardness higher than HV 400.[14] The objective of this paper is to propose a new inclusion rating method using a HE tensile test. The basics of the method is nonmetallic inclusion-originated tensile fractures due SHINJI FUJITA, Assistant Manager, is with the Basic Technology Research Center, NSK Ltd., Fujisawa, 251-8501, Japan. YUKITAKA MURAKAMI, Vice Director, is with the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395 Japan and also Director, with the Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS), National Institute of Advanced Industrial Science and Technology (AIST), 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395 Japan. Contact e-mail: murakami.yukitaka.600@m. kyushu-u.ac.jp Manuscript submitted February 22, 2012. Article published online September 11, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
to hydrogen. Cleanness of steels can be inspected more conveniently and reliably by the proposed method than other existing inclusion rating methods, i.e., the methods by fatigue testing or optical microscopy. This method is much faster and less costly than fatigue testing and evaluates a much greater volume than optical microscopy. The proposed method is applied to specimens with a Vickers hardness of higher than HV 447. High cycle fatigue failure in machine components of high strength steels having a Vickers hardness higher than HV 400 occurs from a nonmetallic inclusion.[1] Since this phenomenon was reported by Frith[2] in the 1950s, improvements in steelmaking technologies have resulted in a continual reduction in both size and number of nonmetallic inclusions in steels, and the cleanliness of current commercial steels has been improved remarkably.[1] However, the fact that fatigue failure in machine components with a Vickers hardness of higher than HV 400 originate from a nonmetallic inclusion has not been changed. Therefore, more exact and convenient inclusion rating method has been desired by both steel producing and fatigue design engineers. Various inclusion rating methods have been proposed and standardized in several countries, for example th
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