Effects of weld strength undermatch on fracture toughness of HAZ notched weldments in a HSLA steel

In the present work the effects of weld strength undermatch on fracture tou ghness of heat affected zone (HAZ) have been studied. In the investigation a high strength low alloyed steel (HSLA) with 800 MPa strength class was us ed, and the undermatched welded joints were made with two weld strength mis match levels. Three-point bending test specimens with crack depth to specim en width ratio a/W ranging from 0.05 to 0.5 were extracted from the welded joints. The test results show that strength mismatching gives an obvious in fluence on the fracture toughness of coarse grained HAZ for the undermatche d joints. The lower the weld strength mismatching, the higher the fracture toughness of the HAZ. In addition the tendency of fracture toughness change with crack depths is much the same as in previous studies on base metals o r weld metals, that is, fracture toughness of the HAZ is increased with red uction of crack depths. From the measured results it shows that the macrosc opically mechanical heterogeneity of the welds may have more important infl uence on the fracture toughness of the HAZ than the meso-heterogeneity in t he reheated coarse grained HAZ. Furthermore, numerical verification indicat es that the stress triaxiality at crack tip may be the essential reason for the change of fracture toughness of HAZ. It is also shown that the yield s trength of HAZ determined by the Limit load in the three-point bend test re presents the combinative effects of HAZ and its surrounding materials.