Local properties of undermatched steel weld metal

Samples from two undermatched, multipass welds on 50.8-mm-thick HY-100 steel were tested using a novel microtensile test machine and the local material properties were investigated using a chemical analysis, metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microtensile test technique allowed samples from individual weld beads and weld-metal heat-affected zones to be tested in three orthogonal directions. Relationships between local microhardness and tensile properties were established. The filler metals for the two welds were MIL-70S and MIL-100S. The MIL-70S weld formed ferritic microstructures; the weld-metal heat-affected sites were predominantly polygonal ferrite, while the as-deposited regions were a mixture of lath and polygonal ferrite. This weld showed a large variation in properties from the central weld bead to the outer ones. The outermost site exhibited significant anisotropy in strength that was not revealed by microhardness measurements. The yield strength specification was 483 MPa, while the average at the center of the weld was 675 MPa and the outer sites had an average of 445 MPa. Elongation for the samples from the center was significantly lower as well, 5 pet as compared to 18 pet for the outer sites. The yield strength showed a strong correlation with the size of inclusions measured by TEM. Microprobe analysis found no dilution of the base metal alloying additions into the weld metal. The MIL-100S filler formed predominantly fine acicular ferrite throughout the weld. The strength was much more uniform; the yield strength specification was 690 MPa, while the center of the weld was 756 MPa and the outer sites had an average of 616 MPa. The inclusion size did not play an important role in the variation in mechanical properties.