Effect of solidification conditions on the solidification mode in austenitic stainless steels
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TRODUCTION
THE composition,
i.e., the balance between austenite and ferrite-forming elements, basically determines how an austenitic stainless steel will solidify. 1-5 Four different solidification modes, austenitic, austenitic-ferritic, ferriticaustenitic, and ferritic, can be recognized as a function of the ratio of the chromium and nickel equivalents. 3'5 The effect of solidification conditions on the solidification mode is less well known, however. Sometimes no changes have been found with the compositions and cooling conditions used, while sometimes they are associated with changes in composition, especially in the nitrogen content. 6 Fredriksson7 uses gradient experiments to suggest that more rapid cooling favors ferrite as the primary phase. Comparison of the results obtained from welds with those from the slowly cooled thermal analysis samples suggests the opposite conclusion, however, that more rapid cooling favors austenite as the first solidifying phase. 8 The cooling rate is nevertheless only of secondary importance. The aim of this work is to evaluate the effects of solidification conditions on the solidification mode in the composition range in which primary austenitic and primary ferritic solidification compete and to discuss those basic parameters which suitably describe these effects. For this purpose the effects of various welding parameters on the solidification mode are studied in autogeneous gas tungsten arc (GTA) welds. The results are compared with those obtained from other laboratory experiments. II.
autogenous GTA welding on the plate. The materials were the same as used in previous studies on weld metal solidification. 9'1~ The "rewelding" was carded out at the same welding speed (8 cm per minute), but with the welding current halved. This doubled the cooling rate at solidification temperatures from 300 ~ per second to 600 ~ per second, as estimated by simple calculations (see Appendix 1). No marked changes were found in the solidification mode, however, and the changes in the Ferrite Number (FN) were also small: IAFN[ < 2. Secondly, the speed of the autogeneous GTA welding was varied on sheets of thickness from 1.5 mm to 3 mm. Two welding speeds, 10 cm per minute and 30 cm per minute, were used, and the current was selected so that the fullpenetration welds were produced. The welds were sectioned and the solidification mode analyzed metallographically. The results are presented in terms of two different chromium and nickel equivalents in Figure 1. As shown in this figt/re, the equivalents developed by Hammar and Svensson4 give between the composition and solidification mode a better correlation than DeLong's equivalents. There is a narrow transition zone close to the value Cr~q/Nieq = 1.5" where * ( C r ~ --- pet Cr + 1.37 x pet Mo + 1.5 • pet Si + 2 x pet Nb Ti J Nioq = +pet3 Ni• pet + 0.31 x pet Mn + 22 x pet C + 14.2 • pet N + pet Cu These equivalents have been planned for the solidification mode. 4 They are used in Figure 1 and throughout this paper, since an excellent correlation is
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