Microstructure Control in High Strength Steel Welds

The modern low-alloyed high-strength steels offer an attractive combination of high strength (yield strength of 100 ksi and higher), toughness, and ductility. These steels are classified by their strength, which implies that variable chemistries and thermo-mechanical processing routes can be utilized in production of the same steel grade. Heat-to-heat variations in base and weld metal chemistry may result in loss of heat-affected zone (HAZ) and/or weld metal mechanical properties, and may require complete redevelopment and prequalification of welding procedures. Narrow operational windows of heat input and difficulties in development of robust welding procedures have delayed the widespread application of these steels by the oil and gas, petrochemical, and shipbuilding industries.

This paper demonstrates an approach in determination of optimal heat input window for welding high strength steels that utilizes continuous cooling transformation (CCT) diagrams. The technique of Single Sensor Differential Thermal Analysis was used to develop in-situ CCT diagrams in the coarse grained HAZ of steel HSLA100 and in weld metal of ER100S-G consumable. The weld and HAZ microstructure was characterized using light optical microscopy, scanning electron microscopy, and hardness mapping.

The developed CCT diagrams show significant differences in the weld metal and HAZ phase transformation behavior that results in inhomogeneous microstructure and hardness distribution. Only a narrow operational window (cooling times t_8/5 from 6 to 8s, heat input from 14.4 to 15.3 kJ/in for 0.25 in. thick plate) was found where both the HAZ and weld metal matched the base metal hardness, Figure 1. Loss of HAZ toughness and loss of HAZ and/or weld metal strength was predicted outside this window.

The demonstrated approach shows that extensive databases of high strength steel and weld metal CCT diagrams can be developed and utilized for selection of base metals and welding consumables and for development of robust welding procedures.