High Strength Low Alloy Steel Weldments with Accommodated Qualities to the Base Metal

Abstract

In the development of high-strength and ultra-high-strength low alloy steels (HSLA), boron alloyed quenched and tempered steels, dual phase and multi phase steels, all metallurgical strengthening mechanisms like grain refinement, precipitation hardening, solid solution hardening and transformation hardening are applied to achieve the required steel properties regarding strength level and toughness. Microalloying elements like Nb, Ti, V, and B play thereby also an important role. The extent of activation of these strengthening mechanisms in the steel is dominated by the alloying concept and the milling capability of the steel plant which results in different chemical steel composition for similar strength levels and plate thicknesses depending on the steel manufacturer. If high-strength and ultra-high-strength low alloy steels are welded, changes within the optimized strip microstructure caused by the weld heat input take place in the vicinity of the weld. This area is called heat affected zone (HAZ). Close to the weld the heating of the base metal caused by the weld pool is strong: temperatures clearly above Ac3 are achieved. In this high temperature region often an undesired grain coarsening and an increase in hardness can be found. Somewhat away from the fusion line the temperatures are lower. If they are beyond Ac1 soft-zones within the microstructure can be found. Furthermore, the weld heat input can cause carbide and/or carbonitride precipitation which can embrittle the HAZ but also reheated weld areas. The selection of an appropriate filler metal is often difficult due to different alloying concepts of the steel producers. In general, a dilution with the base metal of about 25 % (depending on welding process and technology) has to be taken into account. Therefore the filler metal has to be chemically adjusted regarding the alloying concept of the steel strip to establish optimized welds. For this reason Voestalpine has developed own filler metals (solid wires, cored wires, submerged arc wire and powders) which take into account the chemical composition of its high strength and ultra high strength hot-rolled steel strips (alform® strip, alform® strip x-treme). In combination with various welding parameters, welding procedures and strip thicknesses "alform®-welding-system" supports customers with a huge set of mechanical test data of alform®-joints to give the best support.