Morphology and aging of the martensite induced by cathodic hydrogen charging of high-carbon austenitic steels
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I.
INTRODUCTION
H Y D R O G E N embrittlement of stainless steels is mostly attributed to phase instability of the austenitic phase with respect to cathodic hydrogen charging.[~-5] For instance, in Fe-Ni-C t6'71 or Fe-Ni-Cr-C [7"8'91 alloys, martensitic transformation proceeds to form the a ' bct and e hcp phases. *tl~ Thus, several models have been proposed *Whether the e phase is hcp, i.e., corresponds to an ABA . . . stacking sequence, is open to discussion, since it has been shown recently that the high-pressure hydride phase e-Felix is dhcp, i.e., corresponds to an ABCBA . . . stacking sequence, tIll
to evaluate in which respect and through which mechanism the mechanical properties can be influenced by the induced phase transformation. However, since different phases are obtained at the same time, it is obviously difficult to separate their respective roles. Therefore, it is interesting to investigate the behavior under hydrogen charging of the austenite out of thermodynamical equilibrium which has never been reported and to compare it to that of the austenite in equilibrium conditions. In this study, X-ray diffraction and M6ssbauer spectroscopy are used to analyze the phases present after hydrogen charging in high-carbon iron alloys which are initially completely austenitic after quenching to room temperature. Phase analysis is carried out to investigate the phase transformation. The major result is that no e phase whatsoever is detected. The morphologies of the a' martensite which forms by hydrogen charging and that which is obtained by direct quench of the austenite to liquid nitrogen temperature for the same Fe-C alloy are compared as well as their aging and tempering behaviors. O.N.C. UWAKWEH, former Graduate Student, and J.-M.R. GI~NIN, Professor, are with the Laboratoire du Centre National de la Recherche Scientifique Maurice Letort and Department of Materials Science, the University of Nancy 1 (ESSTIN), Parc R. Bentz, F54500, Wandoeuvre, France. Manuscript submitted August 1, 1990. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
Samples are made out of 80-/~m-thick ARMCO* iron *ARMCO is a trademark of Armco, Inc., Middletown, OH.
sheet carburized at 1140 ~ in a C H 4 - H 2 gas mixture in the 5:1 ratio according to the procedure explained in References 12 and 13 in order to obtain a 1.95 wt pct C steel which is entirely austenitic after quenching at room temperature. The thickness is reduced to 50 /~m after electrolytic cleansing. Cathodic charging is performed at a constant temperature ranging from 18 ~ to 50 ~ under different current densities in a bath containing 0.1 M H 2 S O 4 impregnated with few drops of a 250 mg/1 NaAsO3 solution. The anode is platinum. Time of charging treatment is also varied to follow the effect on phase transformation up to 120 hours which is the time over which the transformation ceases to proceed. In order to correlate the charging conditions with respect to the decomposition of the austenitic phase and to characterize the induced phases, transmission electr
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