Microstructure of welded and weld-simulated 3Cr-1.5Mo-0.1V ferritic steel
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INTRODUCTION
FERRITIC Cr-Mo steels are commonly used in pressure vessel applications. The most common alloy is a 2.25Cr1Mo steel. In order to improve operational efficiency, it is desirable to extend the temperature range over which this alloy can be utilized to higher temperatures. To achieve this goal, several investigators have made microalloying additions to the basic 2.25Cr-lMo alloy to improve the alloy's elevated-temperature strength and also its resistance to hydrogen embrittlement. [~-~] Additions to this alloy have included vanadium and niobium for carbide stabilization and nickel for improved hardenability. Furthermore, chromium levels have been increased to improve the alloy's resistance to hydrogen attack. One such modified alloy is the 3Cr-l.5Mo-0.1V steel developed by Wada and Cox. I~'51 For typical applications of these Cr-Mo steels, the materials are used in the form of thick sections. Welding of these thick-section plates and ring forgings is an integral part of the fabrication process. Compared to conventional welding processes, such as shielded metal arc welding (SMAW) and submerged arc welding (SAW), electron-beam (EB) welding is an attractive alternative method for welding such thick sections because of several potential advantages. Electron-beam welding is potentially less expensive, faster, and does not require the use of filler metals. However, some limitations do exist. For example, joint preparation is critical, joint fit-up is less flexible, and the welds must be made under vacuum.
A study was recently undertaken to examine the feasibility of EB welding thick sections of three ferritic steels, including 3Cr-I.5Mo-0.1V. t6] It was found that EB welding is an attractive alternative to arc welding for heavy-section ferritic steels. [61 Electron-beam welds of high integrity could be produced in 102-mm- (4-inch-) thick sections of 3Cr-1.5Mo-0.1V in a single pass. The tensile strengths of the fusion and heat-affected zones after postweld heat treating were comparable to those of the base material after a similar heat treatment. Tensile tests indicated that failures occurred outside the weld metal and the heat-affected zone (HAZ). The impact properties of the weld metal were comparable to the base material and were, in fact, better than the impact properties of weld metal produced by conventional arc methods. The weldments also passed code specifications for bend testing. Thus, the feasibility of making acceptable, singlepass EB welds was demonstrated. The object of the present study was to examine the microstructure of the EB-welded 3Cr-I.5Mo-0.1V ferritic steels. The fusion and heat-affected zones were examined in both the as-welded and the postweld heat-treated conditions. In addition, simulated weld heat-affected zones in which structures similar to that of an actual EB-weld HAZ can be reproduced over a wider distance were also examined to better define the different structural regions. Although only one ferritic steel was evaluated, it is expected that many of the structural findings
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