Anderson, T.D., M. J. Perricone, J. N. DuPont and A. R. Marder, “The Influence of Molybdenum on Stainless Steel Weld Microstructures”, Welding Journal, Vol. 86(9), pp.281-292, 2007.
Superaustenitic stainless steel alloys can often pose difficulties during fusion welding due to the unavoidable microsegregation of Mo and tramp elements, which lead to the loss of corrosion resistance and solidification cracking, respectively. A method of producing austenitic welds is proposed that can potentially circumvent these issues by designing fusion zone compositions that exhibit a primary ferrite solidification mode and subsequent solid-state transformation of ferrite to austenite. The ferritic solidification mode will minimize microsegregation during solidification due to elevated diffusion rates, while a subsequent solidstate transformation of ferrite into austenite will create the austenitic matrix that is desired for good toughness. Thermodynamic calculations were used to isolate the range of compositions over which this phase transformation sequence can occur in Fe-Ni-Cr-Mo alloys. Experimental stainless steel alloys with a wide range in Ni, Cr, and Mo concentrations were then prepared with an arc button melting technique to observe the microstructures and validate the thermodynamic diagrams. Four solidification modes (A, AF, FA, F) and three solid-state transformations (δ → γ, δ → (σ+γ), and γ → martensite) were observed in this alloy system that produced a wide variety of microstructures. Good agreement was shown between experiment and thermodynamic calculations in the prediction of solidification mode. The amount of ferrite was also determined in each alloy via magnetic measurements. Empirical relations were assessed that relate the ferrite content to alloy composition, and the data were used for comparison with several weld constitution diagrams. Several alloys were identified that exhibited the desired transformation sequence. Electron probe microanalysis measurements on these alloys confirmed that Mo was more uniformly distributed compared to alloys that solidified as austenite. Laser beam welds were also deposited on the surfaces of the button melts in order to observe the influence of higher cooling rates. While no solidification mode shifts occurred (with the possible exception of one alloy), the high cooling rate inherent to laser welding caused a δ massive transformation. This massive structure exhibited an entirely austenitic microstructure with a uniform distribution of Mo at the nominal concentration.