Nitrogen absorption by iron and stainless steels during CO 2 laser welding
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I. INTRODUCTION
RECENTLY, nitrogen has been recognized as an element that improves many properties of steel such as corrosion resistance, strength, creep strength, toughness, etc., although it sometimes has negative influences.[1–5] The development of steel with nitrogen content far in excess of thermodynamic equilibrium is one example of the successful application of nitrogen. In any case, the exchange of nitrogen between the atmosphere and the molten pool during arc welding can be expected to occur, resulting in a great change in the properties of weld metals. The behavior of nitrogen during arc welding has been extensively investigated.[6–14] The results of such investigation show that the weld metal contains more nitrogen than the equilibrium solubility predicated by Sieverts’ law during arc welding of low alloy steels. This is caused by the dissociation of molecular nitrogen into monatomic nitrogen in the arc plasma and the welding thermal cycle.[12,15] Welding processes, however, do not always allow sufficient time for complete nitrogen absorption by molten metals,[15] such as in high chromium steels,[16,17] at pressurized nitrogen atmospheres.[18,19] The nitrogen content of arc-welded steel depends on the equilibrium solubility of the molten metal and the atmosphere.[15] Laser welding has recently received increasing attention due to its high energy density and low heat input 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. The interaction of a laser beam with the material is a complex phenomenon. When a high energy density laser beam impinges on the surface of material, the irradiated surface rapidly melts and subsequently vaporizes, resulting in formation of a cavity known as a keyhole.[20] The keyhole
and metal vaporization, which characterize laser welding, may affect the nitrogen absorption during laser welding. In order to control the amount of nitrogen absorbed from the atmosphere by the weld metal, fundamental knowledge of the behavior of nitrogen absorption during laser welding is necessary. The authors have previously studied the nitrogen absorption of iron and steel during YAG laser welding.[21] The results showed that nitrogen contents of weld metals absorbed from the atmosphere during YAG laser welding are considerably less than those absorbed during arc welding. This may be the result of the active evaporation of metal, the lower temperature of atmosphere, and the shorter thermal cycle as compared with arc welding. These factors may result in insufficient nitrogen absorption by the weld metal during YAG laser welding, even in comparison with the equilibrium solubility predicted by Sieverts’ law. The CO2 laser is another heat source that can be applied in practical welding. One of the main differences between the CO2 laser and the YAG laser is the wavelength of the impinging laser. The wavelengths of the CO2 laser and the YAG laser are 10.6 and 1.06
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