Shear-mode Crack Initiation Behavior in the Martensitic and Bainitic Microstructures

¹ Guraduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
² AIST-Kyushu University Hydrogen Materials Laboratory (HydroMate),744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
³ Japan Casting & Forging Co., 46-59 Sakinohama Nakabaru Tobata-ku, Kitakyushu 804-8555, Japan
⁴ Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁵ Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁶ International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University,744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

^* Corresponding author: k.wada.007@s.kyushu-u.ac.jp

Abstract

Fully reversed torsional fatigue tests were conducted to elucidate the behaviour of shear-mode crack initiation and propagation in one martensitic and two bainitic steels. The relationship between the crack initiation site and microstructure was investigated by means of an electron backscatter diffraction (EBSD) technique. From the S-N diagram, two notable results were obtained: (i) the shear-mode crack was initiated on the prior austenitic grain boundary in martensitic steel, while in bainitic steels, the crack was initiated along the {110} plane; one of the slip planes of bcc metals, and (ii) the torsional fatigue limit of lower bainitic steel with finer grains was 60 MPa higher than that of upper bainitic steel with coarser grains even though the hardnesses were nearly equivalent. The mechanism determining the torsional fatigue strength in these steels is discussed from the viewpoint of microstructure morphology.