The Effect of CWTSAW on the Toughness of Heavy-Gauge X70 — Part II: Correlating Microstructure with Charpy Toughness

The effect of martensite-austenite (MA) constituent morphology, prior austenite grain size, fracture length, and notch placement on the Charpy toughness and micro-hardness in the heat-affected zone (HAZ) of heavy-gauge (19.1 mm thick) X70 microalloyed steel welded by cold-wire tandem submerged-arc welding (CWTSAW) was studied. In Part I, a series of single-pass CWTSAW samples were made using cold wire feed rates (effective heat input) of 0 mm/s (2.9 kJ/mm), 16.9 mm/s (2.6 kJ/mm), and 33.9 mm/s (2.3 kJ/mm). In Part II, the absorbed energy values at –10°C and –30°C ranged widely from 240 J (max) to 8 J (min) for effective heat inputs of 2.9 kJ/mm and 2.3 kJ/mm. The higher Charpy energies for the 2.9 kJ/mm and 2.3 kJ/mm samples tended to occur when the notch center was located in the fine-grained HAZ (FGHAZ), and much of the crack propagated through the FGHAZ. The lower Charpy energies for the same heat inputs occurred when the notch center was positioned in the coarse-grained HAZ (CGHAZ), which had a significant amount of coarse-stringer MA constituents. The 2.6 kJ/mm samples had consistently high absorbed energy results (> 190 J), whether the notch center was positioned in the coarse-grained HAZ or the FGHAZ. The number of coarse-stringer MA constituents in the CGHAZ of 2.6 kJ/mm sample was the lowest. A severity parameter (SP) was developed to correlate the severity of coarse-stringer MA and crack length in the CGHAZ/FGHAZ. The SP value increased as the number of coarse-stringer MA and fracture length in the CGHAZ increased, which correlated to reduced Charpy toughness.