Effect of threshold stress intensity on fracture mode transitions for hydrogen-assisted cracking in AISI 4340 steel
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I.
INTRODUCTION
HIGH-strength steels are usually fractured in an intergranular (IG) manner, sometimes in a transgranular quasicleavage (QC) mode t~,2,3] or a transgranular microvoid coalescence (MVC) mode, t4,5,6]by the hydrogen-assisted cracking (HAC) process. A fracture mode transition may result, even from IG to MVC, with changes in experimental factors, such as applied stress intensityt4,5] and test temperature, t3] In view of the high susceptibility of high-strength steels to intergranular hydrogen-assisted cracking (IG HAC), the dependence of the threshold stress intensity, Kth, on hydrogen pressure or hydrogen concentration within the materials has been theoretically and experimentally investigated by several authors, tT-~~ However, the mode of fracture has been disregarded in the analyses of the dependence of Kth on the hydrogen pressure. Hydrogen-assisted cracking is influenced by three factors, materials parameters (yield strength and microstructure), environmental factors (hydrogen fugacity and temperature), and stress state (plane stress and plane strain), and is usually characterized in terms of the quantity Kth, which is presumably closely related to the HAC fracture mode. It has been recognized that IG and QC fracture modes are stress-controlled processes, but the MVC fracture mode is a strain-controlled process, ml Thus, it is suggested from observation of the differences in the fracture modes that the corresponding micromechanisms of the respective HAC fracture modes are different. Hence, for a more reliable understanding of the HAC process, one should consider the dependence of Kth on the factors affecting HAC in a specific fracture mode, such as the IG, QC, or MVC mode. In our laboratory, Choi and
SU-IL PYUN, Professor, and HYO-KEUN LEE, Graduate Student, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Chongyangni, Seoul, Korea. Manuscript submitted January 24, 1990. METALLURGICAL TRANSACTIONS A
Pyun I121have proposed a theoretical concept for fracture mode transition from the IG to MVC mode with varying hydrogen pressures in high-strength steels. Therefore, the present study is aimed at experimentally confirming the fracture mode transition from the IG to MVC mode and at obtaining a deeper insight into the concepts underlying HAC fracture mode transitions in high-strength steels from an equtibfium aspect. For these purposes, Kth and the fracture surface morphology were determined from double cantilever beam (DCB) specimens of AISI 4340 steel, and the relation to hydrogen fugacity for various yield strengths is discussed. The known micromechanisms for the IG and MVC modes permit us to reach a concept of the fracture mode transition.
II.
EXPERIMENTAL PROCEDURE
The chemical composition of AISI 4340 steel used in this study is presented in Table I. All specimens were austenitized in vacuum at 850 ~ for 2 hours, followed by oil quenching, and then tempered at various temperatures in order to obtain various yield strength
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