Effect of Composition on the Formation of Sigma during Single-Pass Welding of Mo-Bearing Stainless Steels

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 σ-sigma intermetallic was observed in several primary γ-austenite alloys with Mo contents ≥2.5 wt pct Mo and in several primary δ-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, σ-sigma was determined to form by the eutectoid decomposition of δ-ferrite (δ → γ + σ) in the as-solidified arc melt buttons in both primary γ-austenite and primary δ-ferrite alloys. The high cooling rates in the laser welds (estimated to range from 10⁴ °C/s to 10⁵ °C/s) largely prevented this transformation from occurring, resulting in the retention of metastable δ-ferrite to room temperature. A correlation was observed between the composition of the δ-ferrite in the microstructure and the calculated onset temperature of sigma stability. By combining multicomponent liquidus projections and isothermal sections, a good correlation of δ-ferrite compositions that decompose to γ-austenite + σ-sigma upon cooling is presented. It is suggested that the absence of the chi (χ) phase, often observed in a similar composition range at high Mo contents, could be attributed to the extremely low carbon content in these alloys.