MICROSTRUCTURE AND CLEAVAGE RESISTANCE OF HEAT-AFFECTED ZONES IN HIGH-STRENGTH, MICROALLOYED STEEL WELDS

The toughness of high strength microalloyed structural steels is known to be reduced whenever grain coarsening takes place in the austenite during submerged arc welding. In the present work, the behaviour of a modern steel containing little carbon and a combined microaddition of Ti and N has been investigated by direct measurement of the cleavage f racture stress in the different heat affected zones of a welded joint. Depending on the particular chemical composition of the material the microstructure of such zones is either mixed (polygonal ferrite-low ca rbon bainite) or consists entirely of polygonal ferrite. In both cases the cleavage fracture stress depends on the size of the ferrite grain s according to a theoretical equation developed in previous work. The reduction in the cleavage fracture stress brought about by the welding process amounts to only 10% with respect to the base material in the most damaged region, namely the coarse grained zone close to the fusio n line. This can be attributed to the favourable action of TiN particl es in preventing excessive austenite grain growth during exposure to t he highest process temperatures.