Hydrogen attack behavior of the heat affected zone of a 2.25Cr-1Mo steel weldment
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I.
INTRODUCTION
H Y D R O G E N Attack (HA) is a form of material degradation that occurs in steels exposed to high temperature (over 250 ~ high pressure (over 2 MPa) hydrogen. Reaction between hydrogen and the carbon in the steel results in the formation of methane bubbles, which nucleate and grow preferentially along grain boundaries. After a long incubation time the steel loses its room temperature mechanical properties (in carbon steels) or its creep ductility (in 2.25Cr-lMo steels). Additions of stable carbide formers such as Cr and Mo result in a substantial increase in HA resistance. Of the steels developed 2.25Cr-lMo is the leanest alloy with good HA resistance. It has been widely used in the industry and is a potential structural material for future applications. Dilatometric measurements on the kinetics of HA of carbon and 2.25Cr-lMo steels have been made by several investigators. 2-6 Analysis of these dilatometric data 7 suggests that the high HA resistance of 2.25Cr-lMo steels compared to carbon steels is chiefly due to the lower bubble density that it develops. The analysis shows that HA kinetics depend sensitively on the bubble density. Transmission electron microscope replica work by Lopez and Shewmon 8 has shown that the bubble density in 2.25Cr-lMo steels is determined directly by the density of pre-existing microvoids. This work also indicated that no new bubbles nucleated during HA at 550 ~ and 20.5 MPa. However, Thygeson and Molstad reported that exposure of this steel to much higher hydrogen pressures of 100 MPa (15 ksi) at 500 ~ (for 1700 hours) leads to a high density of bubbles on the grain boundaries. 9 In carbon steels where the carbon activity is 10 times higher, such active nucleation of new bubbles takes place at 3 MPa of hydrogen at 450 ~176 which translates to an equilibrium methane pressure of 380 MPa. If the methane pressure for active bubble nucleation in 2.25Cr-lMo steel was the same as that found in carbon steel, one would expect an increase in
T. A. PARTHASARATHY, formerly with The Ohio State University, is Assistant Professor, Department of Metallurgy and Mineral Engineering, University of Illinois, Urbana, IL 61801. P. G SHEWMON is Professor, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted May 2, 1986. METALLURGICAL TRANSACTIONS A
bubble density on exposure to 25 to 30 MPa of hydrogen at 550 to 580 ~176 It has been shown by many workers that in carbon steels plastic strain prior to HA greatly increases the density of bubbles, especially along grain boundaries." There has been less study of this effect in 2.25Cr-lMo steel, but our understanding of the mechanism in carbon steel, and the enhanced nucleation of voids in creep samples as a result of prior strain, 12'13 strongly suggest that prior strain would also increase the rate of HA in 2.25Cr-lMo steel. Limited work on the HA of 2.25Cr-lMo weld metal shows that it is hydrogen attacked faster than the wrought base metal,~4 primarily as a result of the higher densi
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