Role of gaseous environment and secondary precipitation in microstructural degradation of Cr-Mo steel weldments at high
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I. INTRODUCTION
CHROMIUM-MOLYBDENUM ferritic steels (generally referred as Cr-Mo steels) possess a good combination of mechanical properties, formability, weldability, and resistance to stress corrosion cracking and other forms of corrosion.[1,2] Owing to these attributes at moderately high temperatures, Cr-Mo steels are an extensively used family of engineering materials for applications such as steam generation/handling, petroleum processing/refining, thermal reforming/polymerization/cracking, etc.[1–6] However, the strength of the weldments of Cr-Mo steels is generally inferior, and most of the in-service failures are reported to take place in the weld region.[7,8] In fact, the creep-rupture of the welds is often the life-limiting factor of Cr-Mo steel components.[7,8,9] Given that the weldments are an indispensable part of most component fabrication, the frequent occurrence of weld failures has ensured ongoing research interest in the past few decades. However, most of the research effort has been directed to the correlation of the in-service failure of these steels to the microstructural degradation caused during welding. In recent years, first-ever systematic investigations[10,11,12] have been initiated to study the influence of variation in alloy microstructure on gaseous corrosion of Cr-Mo steel weldments, with a view to developing a rather complete understanding of the complex interplay of the mechanical stress, microstructure, and environment which result in weld failure. R.K. SINGH RAMAN, Senior Research Fellow, is with the Key Centre for Advanced Materials Technology, Department of Materials Engineering, Monash University, Clayton-3168, Australia. Manuscript submitted October 16, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
A. Cr-Mo Steel Weldments and Their Corrosion Microstructures of Cr-Mo steel are very susceptible to thermomechanical treatments.[2,3,13] This microstructural susceptibility is often exploited to develop carbide precipitates of the required chemistry, morphology, and distribution to effect precipitation hardening. However, due to the metastable nature of the composition and the morphology of the strengthening precipitates, the secondary precipitates could undergo undesirable transformations during elevated-temperature service[2,3] and/or during thermomechanical treatments experienced during fabrication, viz., welding, forging, hot rolling, etc.[2,3,8] Microstructural changes due to welding include variations in the grain size in the area adjoining the weld metal (i.e., the heat-affected zone (HAZ)) and enrichment of Cr in the existing secondary precipitates and/or additional Cr-rich precipitate formation.[2,3,10–15] Trapping of “free” chromium (from the matrix) through Cr-rich precipitate formation is reported to alter the oxidation resistance of low-Cr alloys,[11,12,16–18] whereas the increase in grain size is found to facilitate internal oxidation.[19] The surfacescaling-rate data are generally of secondary importance to design engineers (primarily because surface scaling of ste
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