Effects of tempering on the carbon activity and hydrogen attack kinetics of 2.25 Cr-1 Mo steel
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I.
INTRODUCTION
STEELS exposed to hot high pressure hydrogen suffer a loss in mechanical properties after a long incubation period, during which no changes in the mechanical properties are noticed. This phenomenon called Hydrogen Attack (HA) is due to the formation of methane bubbles which grow with time and may eventually coalesce to form fissures. The principal means of controlling HA has been the addition of strong carbide formers as alloying elements to the steel. Of the steels thus developed 2.25 Cr-lMo steel is the leanest alloy with good HA resistance. This steel has performed successfully in industry up to temperatures of 450 ~ as seen from the Nelson Curves of the American Petroleum Institute. ~ However, laboratory tests on this steel 2'3'4 have shown that appreciable bubble growth occurs at hydrogen pressures of 20 MPa and temperatures above 500 ~ It is therefore desirable to analyze the possible means of controlling the HA of this steel. To control hydrogen attack, it is necessary to understand and control processes occurring during the incubation stage, as was pointed out by Shewmon. 5 It is now well known that the incubation stage is characterized by the growth of isolated methane bubbles driven by the pressure of methane within the bubbles. Thus, controlling the pressure of methane is a direct means of controlling hydrogen attack. Recent work by Lopez 6 and by Wang v indicates that the methane pressure also influences the rate of bubble nucleation, and thus the number of bubbles growing in any given volume. At hydrogen pressures up to at least 25 MPa the density of bubbles in Q&T 2.25 Cr-1 Mo steel does not change with time of exposure while in carbon steels a hydrogen pressure of even a few MPa is enough to nucleate a much higher density of bubbles, and thus make a substantial contribution to the much higher rate of HA found in carbon steels.
T.A. PARTHASARATHY, Postdoctoral Researcher, and P.G. SHEWMON, Professor, are with The Ohio State University, Department of Metallurgical Engineering, 116 West 19th Avenue, Columbus, OH 43210. Manuscript submitted April 11, 1984.
METALLURGICAL TRANSACTIONS A
The methane pressure that develops within a steel is dependent on the activity of carbon maintained in the steel. The carbon activity depends on the stability of the least stable carbide. Geiger and Angeles 8 have analyzed the thermodynamics of methane formation from the various carbides in 2.25 Cr-1 Mo steel. They assumed that: 1. the carbides were in equilibrium with a ferrite matrix whose composition is the same as that of the bulk material, 2. the M3C carbide had a metal content of (0.2Cr, 0.8Fe), and 3. the thermodynamics of Cr23C 6 and Cr7C 3 represented those of the alloy carbides M23C 6 and MTC 3 (due to lack of thermodynamic data for alloy carbides). They express their results in the form of an equation relating the hydrogen pressure (p) to the methane fugacity ( f ) through a temperature dependent constant K(T), as follows:
f = K(T) 9pZ
[1]
The value of K (T) varies with the stability of th
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