The microstructural development of several Ni-Cr-Mo and Fe-Ni-Cr-Mo alloys over a range of conditions has been examined. A commercial alloy, AL-6XN, was chosen for analysis along with three experimental compositions to isolate the contribution of individual alloying elements to the overall microstructural development. Detailed microstructural characterization on each alloy demonstrated that the observed solidification reaction sequences were primarily dependent on the segregation behavior of molybdenum (Mo), which was unaffected by the large difference in cooling rate between differential thermal analysis (DTA) samples and welded specimens. This explains the invariance of the amount of eutectic constituent observed in the microstructure in the welded and DTA conditions. Multicomponent liquidus projections developed using the CALPHAD approach were combined with solidification path calculations as a first step to understanding the observed solidification reaction sequences. Discrepancies between the calculations and observed reaction sequences were resolved by proposing slight modifications to the calculated multicomponent liquidus projections.

The good corrosion resistance of superaustenitic stainless steel (SASS) alloys has been shown to be a direct consequence of high concentrations of Mo, which can have a significant effect on the microstructural development of welds in these alloys. In this research, the microstructural development of welds in the Fe-Ni-Cr-Mo system was analyzed over a wide variety of Cr/Ni ratios and Mo contents. The system was first simulated by construction of multicomponent phase diagrams using the CALPHAD technique. Data from vertical sections of these diagrams are presented over a wide compositional range to produce diagrams that can be used as a guide to understand the influence of composition on microstructural development. A large number of experimental alloys were then prepared via arc-button melting for comparison with the diagrams. Each alloy was characterized using various microscopy techniques. The expected d-ferrite and c-austenite phases were accompanied by martensite at low Cr/Ni ratios and by r phase at high Mo contents. A total of 20 possible phase transformation sequences are proposed, resulting in various amounts and morphologies of the c, d, r, and martensite phases. The results were used to construct a map of expected phase transformation sequence and resultant microstructure as a function of composition. The results of this work provide a working guideline for future base metal and filler metal development of this class of materials.

HASTELLOY C-22 and C-276 are engineering nickel-based alloys that served as the focus of a comprehensive examination of the microstructural development of this class of materials presented previously by Cieslak et al. The work presented here provides a re-examination of this study. The use of computational thermodynamic algorithms combined with solidification-path calculations based on experimentally determined solute-partition data eliminates the need for many assumptions and serves as the basis for the solute-redistribution model that is presented here. A new series of solidification sequences for HASTELLOY C-22 and C-276 is proposed based on this model, and these conclusions are supported by the microstructural characterization presented in the previous work.

A series of 64 Mo-bearing stainless steel compositions ranging from 0 to 10 wt pct Mo and over a broad range of Ni and Cr contents were analyzed over a variety of cooling rates. Alloys were created using the arc button melting process, and laser welds were prepared on each alloy at constant power and travel speeds ranging from 4.2 to 42 mm/s. The presence of the r-sigma intermetallic was observed in several primary c-austenite alloys with Mo contents ‡2.5 wt pct Mo and in several primary d-ferrite alloys with Mo contents ‡6 wt pct Mo. However, its formation cannot be explained by the eutectic solidification reaction previously explored in this class of materials by the present authors. Instead, r-sigma was determined to form by the eutectoid decomposition of d-ferrite (d fi c + r) in the as-solidified arc melt buttons in both primary c-austenite and primary d-ferrite alloys. The high cooling rates in the laser welds (estimated to range from 10 4°C /s to 10 5°C /s) largely prevented this transformation from occurring, resulting in the retention of metastable d-ferrite to room temperature. A correlation was observed between the composition of the d-ferrite in the microstructure and the calculated onset temperature of sigma stability. By combining multicomponent liquidus projections and isothermal sections, a good correlation of d-ferrite compositions that decompose to c-austenite + r-sigma upon cooling is presented. It is suggested that the absence of the chi (v) phase, often observed in a similar composition range at high Mo contents, could be attributed to the extremely low carbon content in these alloys.