Nitrogen desorption by high-nitrogen steel weld metal during CO 2 laser welding
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I. INTRODUCTION
THE gradual recognition during the second half of the last century of the beneficial effects of nitrogen on the properties of high-alloy steel, such as strength, corrosion resistance, austenite stability, etc., led to the widespread development of high-nitrogen steels (HNSs), as demonstrated in numerous applications.[1–4] However, an exchange of the intentionally raised nitrogen with the atmosphere due to alloying or to pressure and powder metallurgy is quite likely to occur, resulting in porosity and detrimental changes in properties during fusion welding. Although pressure welding processes such as flashbutt welding are not considered to be problematic with respect to nitrogen, the limitations of these welding methods and the usual precautions necessitated by the use of high-nitrogen alloy materials must be taken into account.[5] To establish a suitable welding method for HNSs, it is essential to investigate the nitrogen absorption and desorption of the weld metal during the welding of HNSs. The behavior of nitrogen in weld metals of iron and stainless steels with low-nitrogen contents during arc welding has been extensively investigated.[6–21] The results of such investigations show that the nitrogen absorption by the weld metal is enhanced above the equilibrium solubility of nitrogen predicted by Sieverts’ Law and that the existence of monatomic nitrogen dissociated from molecular nitrogen in the plasma is responsible for the enhancement of the nitrogen absorption in the low-nitrogen steel weld pool during arc welding. Laser welding has recently received increasing attention due to its high energy density and low heat input as compared with conventional fusion techniques. Laser welding is expected to have a great impact on fabrication and manufacturing industries in the welding of steel structures within the next decade. Previous studies have shown that the nitrogen WEI DONG, Research Fellow, HIROYUKI KOKAWA, Professor, and YUTAKA S. SATO, Research Associate, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. SUSUMU TSUKAMOTO, Subgroup Leader, is with the Welding Metallurgy Group, Steel Research Center, National Institute for Materials Science, Tsukuba 035-0047, Japan. Manuscript submitted February 18, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
content in iron and stainless steels with a low-nitrogen-content weld metal during CO2 laser welding is higher than during the neodymium:yttrium aluminum garnet (Nd:YAG) solidstate laser welding, but lower than during gas tungsten-arc (GTA) welding.[22–25] The smaller reaction area of the molten pool with monatomic nitrogen, which is confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen,[23,25] is considered to lead to less nitrogen absorption during CO2 laser welding than during arc welding. The difference in nitrogen absorption between CO2 laser welding and YAG laser welding is attributed to the low level of monatomic nitrogen during Y
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