Welding of Ultra-High-Strength Steel and Heat-Affected Zone Management
UHSS and heat input
Ultra-high-strength steel (UHSS) commonly refers to martensitic and boron steels with tensile strength above roughly 1000 MPa. Their strength comes from a martensitic microstructure that tempers and softens when heated. The HAZ is the zone that did not melt but lost strength. Management means reducing heat input, not just making a pretty weld.
Control methods
Use the lowest OEM-approved current and fastest travel speed; use stitch or skip welds where specified; clamp heat sinks; allow cooling between passes; avoid preheating unless the OEM requires it. Do not quench with water or compressed air unless the procedure says so. Grinding can thin the section and add heat-keep it minimal. Many UHSS parts are STRSW-only or rivet-bond-only.
Worked example
A rocker panel near a boron reinforcement needs a plug weld. The OEM says plug welds are allowed only at two locations, with a maximum 6 mm hole and a stitch sequence. The reasoning: follow the locations and sequence, use a heat sink, and do not run a continuous bead along the reinforcement. If the procedure prohibits plug welds, use the specified squeeze welds instead.
Traps
Using a torch to bend UHSS; running a continuous bead to “be safe”; over-grinding the HAZ; welding without checking the OEM restriction; assuming a larger weld is stronger.