Effects of Atomic Hydrogen and Flaking on Mechanical Properties of Wheel Steel
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hairline cracks or shatter cracks, commonly referred to as (snow) flakes, have been a major problem in large steel forgings and parts since the early part of the twentieth century.[1–6] It is safe to say that these defects are associated with hydrogen, and it is recognized that the best method to avoid the hydrogen flaking is to reduce the hydrogen concentration in steel to typically less than 2 ppm.[1–6] However, with the development of ladle metallurgy, secondary refining, and continuous casting, flaking in continuous casting clean steels with low hydrogen, e.g., less than 1.5 ppm, again becomes a serious problem.[7,8] In 1998, delayed failure of a tyre occurred in Britain, and cracks were found in many other tyres, due to flaking.[8] In China, flaking in wheel and bearing steels is still a major problem. For example, about ten tyres experienced delayed fracture after being fitted in recent years. When the concentration of hydrogen in steel exceeds a threshold value, recombination of hydrogen molecules in voids can result in a high pressure of hydrogen molecules, which is sufficient to produce hydrogen blistering or flaking.[22,23] Besides molecular hydrogen, there exists atomic hydrogen in hydrogenated steel.[2] The effects of atomic hydrogen and pre-existing hydrogen damages, e.g., flaking, hydride, and hydrogen-induced martensite, on the mechanical properties are different. For example, the elongation of hydrogenated 304L austenitic stainless steel decreases with the decreasing of the strain rate only X.C. REN, Doctoral Student, W.Y. CHU, Y.J. SU, J.X. LI, and L.J. QIAO, Professors, are with the Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted August 9, 2005. Article published online May 12, 2007. 1004—VOLUME 38A, MAY 2007
when e_ KIH, a hydrogen-induced crack will initiate and propagate. However, if a WOL sample precharged with i = 10 mA/cm2 for 100 hours corresponding to C0 = 5.6 ppm is outgassed at 200 C for 100 hours, and the sample is then loaded to cracking and kept at the constant displacement in air, no HIDC then occurs for the subsequent 100 hours. The results show that HIDC is due to atomic hydrogen and has no relation to flaking because there is no HIDC in the precharged and outgassed sample in which there is no atomic hydrogen but a large amount of flaking (C0 = 5.6 ppm). Although atomic hydrogen and flaking at the levels in this investigation have no effect on fracture toughness, they may affect dramatically the impact toughness and fatigue properties, which are also important for the safety of the wheel and deserve to be investigated further. The experiment involving WOL samples showed that HIDC under constant displacement is due to atomic hydrogen instead of flaking. Therefore, the delayed failure of a wheel or tyre after being fitted is also due to atomic hydrogen rather than flaking. The average diffusible or total hydrogen concentration measured in the samples taken from the inner of a w
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