Microstructural modification of austenitic stainless steels by rapid solidification
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INTRODUCTION
AUSTENITIC stainless steels form an important class of engineering materials in several energy systems. A significant problem in the production of fully austenitic stainless steel welds is their tendency toward hot cracking. To minimize this tendency the compositions of weld filler materials are generally modified to produce small amounts of 6-ferrite in the as-welded microstructure. Such compositional changes promote the formation of ferrite as the primary solidification phase, and this in turn reduces the segregation of elements such as sulfur and other low melting constituents. 1.2.3Further cooling of the welds results in the transformation of ferrite to austenite. This sequence of reactions can be easily understood by examining a representative vertical section of the Fe-Cr-Ni equilibrium phase diagram, shown in Figure 1. The section is for 70 wt pct Fe, with the approximate composition of type 308 stainless steel superimposed. The vertical section shows that for a range of compositions, the ferrite formed during solidification is unstable at lower temperatures where single phase austenite is the equilibrium state. Under typical welding conditions, the cooling rates encountered are sufficiently high to retain some ferrite in a metastable form. For example, in type 308 stainless steel, although much of the primary ferrite transforms to austenite during the solidification and subsequent cooling, approximately 5 to 10 pct ferrite remains, forming the duplex structure shown in Figure 2. Much work has been done recently on the characterization and understanding of the various ferrite microstructures found in welds. 4-7 Though ferrite has been found to prevent hot cracking effectively,~'2'3 it may also lead to corrosion susceptibility and embrittlement at elevated temperatures. Hence, it would be highly desirable to produce fully austenitic microstructures in stainless steel welds without any tendency toward hot cracking. Rapid solidification techniques present one means by which the normal solidification structure can J.M. VITEK and A. DASGUPTA, Research Staff Members, and S.A. DAVID, Group Leader, Welding and Brazing Laboratory, are all with Oak Ridge National Laboratory, Oak Ridge, TN 37830. Manuscript submitted December 13, 1982.
METALLURGICAL TRANSACTIONS A
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1400
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6 +~f
1200 o ILl
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1000
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70% Fe 400 Ni% 30
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Cr% 0
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Fig. 1 --Vertical section of the Fe-Cr-Ni phase diagram at 70 wt pct Fe. The dashed line represents approximately the composition of type 308 stainless steel.
be significantly altered. The formation of high temperature and/or equilibrium structures can be suppressed due to the large undercooling and high cooling rates encountered during rapid solidification, leading to a variety of new structures including metastable nonequilibrium phases, s VOLUME 14A, SEPTEMBER 1983-- 1833
reflected in their net chromium and nickel equivalents ~4calculated as: Cr~q = Cr + Mo + 1.
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