Investigation of Precipitate Evolution in Multipass Cr-Mo Steel Welds

Targeted creep strength and low-temperature toughness are required to qualify Cr-Mo steel-welding consumables relevant to the power generation industries. Routine Charpy toughness testing at -40°F conducted of two experimental submerged arc welds showed interesting phenomena.  For the same filler wire, process parameters and post weld heat treatment conditions; large variations in toughness (18 to 65 ft-lbs) were observed with two different fluxes. The lower values are indeed below the required industry standards (20 ft-lbs at -40°F). This was observed over a range of temperatures in the transition region (from -60°F to -20°F). This suggests that the toughness properties may be related to the variation in microstructure (size of inclusions and precipitates). Hence, to rationalize these scatter in properties, recovery of alloying elements including Cr, Mo into the weld metal region was analyzed using computational thermodynamic models.  The methodology involves multi-component equilibrium calculation of a system whose overall composition is set by a mechanical mixture of (50%-50%) flux and wire constitution. The equilibrium between liquid, slag and gas was calculated at 1800K. The calculated weld metal composition was compared with experimentally measured compositions.  The recovery of Cr was found to be very sensitive to the concentration of deoxidizers including aluminum, silicon and manganese in the system. Based on the above results, we hypothesize that the observed scatter in properties could be attributed to spatial variation of alloying element recovery.  Further analyses of weld metal and slag interface are being evaluated with electron microscopy to evaluate the above hypothesis.