Effect of cooling after welding on microstructure and mechanical properties of 12 Pct Cr steel weld metals
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INTRODUCTION
THE main purpose of developing 12 pct Cr steels is to provide a substitute for low Cr steels and to permit manufacture of equipment that could be used at temperatures at or over 600 7C in power generation plants. During the development, the microstructure of 12 pct Cr steels has been studied extensively.[1–13] These steels contain nominally 12 pct chromium, molybdenum, and tungsten, and some vanadium, nickel, and other alloying elements. Molybdenum and tungsten are the major elements in increasing thermal strength of the steels. The ordinary 12 pct Cr steels contain 1 pct molybdenum. In the latest developed steels, the content of molybdenum is reduced to about 0.5 pct and the tungsten content is increased to about 2 pct, by which creep strength of the steels is increased further.[14,15,16] The carbon content in 12 pct Cr steels is generally below 0.2 pct. In composition design of the 12 pct steels, it is a principle to get a balance between the ferrite-stabilizing elements (Cr, V, etc.) and the austenite-stabilizing elements (C, Ni, etc.) to suppress the formation of d-ferrite, which is believed to be detrimental to the properties of the material. In recent years, nitrogen has also been used as an austenite-stabilizing element in the steel.[17,18,19] Because of the high alloy content, 12 pct Cr steels have a high hardenability. Therefore, after hot working and quenching, tempering has to be carried out to render the material usable. During tempering of the material, the alloying elements react with carbon GUANG-JUN CAI, formerly Graduate Student, Department of Physics, Chalmers University of Technology, is Associate Professor, Department of Materials Engineering, Southwest Jiaotong University, Chengdu 61 00 31, People’s Republic of China. HANS-OLOF ANDRE´N, Professor, is with the Department of Physics, Chalmers University of Technology, S-412 96, Go¨teborg, Sweden. LARS-ERIK SVENSSON, Laboratory Manager, is with the Central Laboratories, The Esab Group, S-402 77 Go¨teborg, Sweden. Manuscript submitted September 29, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
and nitrogen and form precipitates, such as M23C6, M2C, and MC. It is of great importance to investigate the microstructure development in the steel in order to optimize its properties and composition. Most of the studies on the microstructure of the 12 pct Cr steels deal with the microstructure development during tempering. Austenite decomposition during isothermal transformation and continuous cooling has also been investigated.[1,20–23] Since the 12 pct Cr steels are structural materials, weldability has been an important consideration in their development. Welding and weldability of the 12 pct Cr steels, as well as the microstructure and properties of weld metals, have been studied.[24–37] In a weldment, because of different transformation conditions, the microstructure of the weld metal is different from the common heat-treated parent metals. To weld 12 pct Cr steels, preheat and postweld heat treatments are necessary.[25,32,38] It i
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