Investigation of Steels for Improved Weldability in Ship Construction.

Abstract : The purpose of Phase III of this contract is to further develop ship plate steels following on the work done in Phase II. As the initial task of Phase III, the microstructures and properties produced in the heat affected zone (HAZ) of Phase II steels were re-evaluated. Addition of titanium to the base steel improved toughness of the HAZ by refining the austenite grain size and by changing transformation products from Widmanstatten ferrite, pearlite, and bainite to more equiaxed ferrite and pearlite microstructure. Additions of residuals (Cu + Ni + Cr + Mo) to the base steel degraded the HAZ toughness, whereas the addition of titanium with the residuals restored the toughness to the values similar to those in the base steel. The addition of residuals produced Widmanstatten ferrite, whereas residuals plus titanium again produced a more equiaxed ferrite. A steel containing 0.08%V exhibited structure and properties similar to the base steel. It was not clear from these results just what microstructural feature(s) controlled the HAZ toughness. Based on this assessment, 17 new heats were melted and processed into plate for testing of the HAZ properties and microstructures produced by weld simulation and by actual welding.