Characterization of the Fine-Scale Weld Deposit Microstructure and its Influence on the Elevated Temperature Properties of 2.25Cr - 1Mo - 0.25V Weldments in Heavy Wall Pressure Vessels

The research herein was conducted to characterize the fine-scale microstructure of 2.25Cr‑1Mo‑0.25V (22V) submerged arc weld deposits and to study the influence of the microstructure on creep behavior and reheat cracking susceptibility. Scanning electron microscopy and transmission electron microscopy examinations concentrated on carbide morphology and evolution as a function of time and temperature, since the majority of properties that are associated with 22V weld deposits are attributable to the carbide type and location throughout the microstructure.

Five distinct carbides were observed in the range of heat treatments studied: MC, M2C, M23C6, M7C3, and M6C. It was shown that each carbide type could be readily identified in terms of the relative EDS spectra peak heights of iron, chromium, molybdenum, and vanadium. The carbide evolution, as a function of heat treatment, revealed that grain matrix and grain boundary carbides coarsened as the postweld heat treatment time increased. MC type matrix carbides evolved into M23C6 carbides while grain boundary carbides (M23C6, M7C3) coalesced and evolved into M6C carbides. As expected, the overall weld deposit chemical composition was shown to have a significant influence on the formation and evolution of carbides and thus influenced the elevated temperature properties.

The potential presence of impurity elements on reheat cracked surfaces and creep-crack surfaces were investigated. Due to the fact that molybdenum, sulfur, and lead each exhibit peaks at approximately 2.4keV when recorded by the EDS method, a definitive quantity of these elements cannot be readily attained. A direct comparison of the spectra from the transgranular cleavage surface vs. the reheat- and creep-cracked surfaces suggests that lead may be present as the peak intensity at 2.4keV increases when the reheat- or creep-crack surface are the only regions excited by the beam.

The research herein is intended to provide industry and researchers with an accurate identification of the carbides present in 22V submerged arc weld deposits. It is felt that the totality of this work that will be of importance to the utilization of the 22V alloy and similar alloy systems employed in the current and future power production and petroleum refining sectors of industry.