Influence of microstructural variations in the weldment on the high-temperature corrosion of 2.25Cr-1Mo steel
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I.
INTRODUCTION
ANY welded component can be broadly divided into three different regions, m namely, the weld metal, the heat-affected zone (HAZ), and the base metal. Microstructural differences in these regions are caused by the different peak temperatures and cooling rates. In fact, as one moves away from the fusion line toward the base metal, one comes across sharp changes in the microstructure even within the HAZ, due to the different heating cycles experienced by the HAZ, at different distances from the fusion line. The HAZ of the 2.25Cr-lMo steel consists of at least three distinct microstmctures, namely, the coarse- and fine-grain bainitic regions and the intercritical region (ICR). 12m The terms coarse-grain bainite (CGB) and fine-grain bainite (FGB) describe the size of the prior-austenite grains (i.e., the size of the packets of bainite) wherein the bainitic laths are confined, and are not to be mistaken for the grain size of the bainite phase. There are also reports on the variation in the secondary precipitation in different regions of the weldment of this steel. 14 7] Roy and Lauritzen [5] describe a predominant precipitation of Cr-based carbides in the HAZ of the weldment. Lundin et al.[7] suggest Fe- and Mo-based carbides as the secondary phases occurring predominantly in the as-welded microstructure. Singh Raman R.K. SINGH RAMAN, formerly with the Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkan, India, is Research Associate, School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia. Manuscript submitted August 20, 1993. METALLURGICALAND MATERIALSTRANSACTIONSA
and co-workers [8,9,1~ have reported that the heat treatments which cause depletion of Cr (by forming Cr-rich carbides) impair the oxidation resistance of 2.25Cr-lMo steel. Hence, the predominant precipitation of Cr-based carbides in the HAZ of the weldment of this steel, as described by Roy and Lauritzen, I5] is thought to be deleterious for the oxidation resistance of this region. Though there are passing remarks in the literature suggesting enhanced grain boundary oxidation in the HAZ of this steel, and localized Cr depletion as the cause for the same, [l~] little effort seems to have been directed toward a systematic understanding of the high-temperature corrosion behavior of the microstructurally different regions of the weldments of ferritic steels, and its bearing on the mechanical properties of the weldment. The performance of the HAZ of the weldment of these alloys, including 2.25Cr-1Mo steel, has been a matter of concern, [3'7'12] as the creep rupture strength of the HAZ is reported to be inferior to that of the weld metal and the base metal. 13'7'12'13] Hence, it was deemed worthwhile to carry out a systematic investigation of the oxidation behavior of the weldments of 2.25Cr-lMo steel. Results of such a study may also help in understanding the possible contribution of the oxidation process to the inferior mechanical properties of the welds. Accelerated oxidation of
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