Crack propagation of high strength steels in a gaseous hydrogen atmosphere
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I.
INTRODUCTION
IT is well known that alloying elements and/or impurities such as Mn and P have an effect on the delayed fracture of high strength steels. 1~ Sandoz investigated effects of Cr, Mo, Co, Mn, C, S, and P on the critical stress intensity factor K~scc in a 3.5 pet NaC1 solution and K~ in gaseous hydrogen using AISI 4340 steel. 1 He reported that K1scc decreased monotonically with an increase in Mn content up to 2.8 wt pet and with a rise in C content up to 0.33 wt pet followed by increasing gradually with increasing C content up to 0.55 wt pet. It is also reported in the same paper that Kth in gaseous hydrogen increased with Co content and it was scarcely influenced by C content. A decrease in Kth was much smaller than that in K~sccwith increasing Mn content. It is reported that in gaseous hydrogen no effect of alloying elements or impurities on K~ was obtained except for the case of Mn and Si where a small amount of both the elements decreased K~. 4 An appropriate amount of Si reportedly improves Klscc in a 3.5 pet NaC1 solution. 3 The relationships between the crack propagation rate and the applied stress intensity factor K in hydrogen induced crack growth of high strength steels have been obtained by solving analytically or by approximation a diffusion equation of hydrogen in steels in a hydrostatic stress field. 5-9 The crack propagation rate in stage I is reported to be proportional to the applied g 5'6'7 or g 2 . 8 Especially in hydrogen gas, the crack propagation rate is proportional to the applied K. 7 In this paper the effects of Mn and C on the crack propagation behavior of high strength steels are investigated in gaseous hydrogen. The influence of these alloying elements on K~sccin a NaC1 solution has been reported to differ from that on K~ in hydrogen gas as mentioned above. The applied K dependence of the crack propagation rate in hydrogen is also discussed.
MORIHIKO NAKAMURA, Senior Researcher, and EI-ICHI FURUBAYASHI, Head, are both with High Strength Materials Division, National Research Institute for Metals, Tsukuba Laboratories, SakuraMura, Ibaraki 305, Japan. Manuscript submitted July 7, 1982.
METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL
Eight steels were prepared by a 10 kg vacuum melting, and hot forged and hot rolled to 18 mm thick plates with a width of 65 to 70 mm, from which modified CT specimens (Figure 1) and round bar tensile specimens with a diameter of 4 mm and a gauge length of 22 mm were machined. The loading direction was parallel to the rolling direction. Here, a notch and a fatigue precrack of CT specimens were made after heat treatment, because oil-quenching from elevated temperatures often caused cracking at a notch or fatigue crack. Table I shows the chemical composition of the steels used. The tensile specimens and modified CT specimens without a notch and a fatigue precrack were annealed at 1123 K for 1.5 ks in a salt bath followed by oil-quenching. They were also tempered in the temperature range of 473 to 673 K for 3.6 ks. Both surfaces of CT specimens wer
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