Steel Substrate Identification and Weldability
This domain tests whether you can identify the steel you are joining and match it to a validated weld schedule. Substrate identity drives wire, gas, heat input, and whether the part is repairable at all.
Steel families
Mild steel (CRS, HR) welds easily. HSLA is micro-alloyed for strength. AHSS includes dual-phase (DP), complex-phase (CP), TRIP, martensitic (MS), and boron grades, and each responds differently to heat. Coatings matter too: galvanized and galvannealed zinc causes porosity if not removed, and Al-Si coated boron parts need coating removal per procedure.
Weldability versus strength
Weldability is not the same as strength. DP steels soften in the HAZ and lose strength locally; boron steels harden and can crack; higher carbon equivalent raises preheat and cracking risk. The OEM weld schedule - wire type, diameter, gas, current, weld type, count, and spacing - is validated for that substrate. Nothing in the shop substitutes for it.
Identifying the part
Do not rely on color, a magnet, or guesswork. Use the OEM procedure, VIN/build data, part stampings, and sectioning charts. If the part has no procedure, it has no repair - that is the exam's position.
Worked example
A tech finds an unmarked reinforcement and plans a generic 0.030 in ER70S-6 setup with "a little more current." Reasoning: an unidentified substrate means unknown heat tolerance, so no one can predict HAZ softening, cracking, or coating behavior. The correct step is to identify the part through the OEM procedure and apply that schedule - or stop the repair.
Traps
Using ER80S-D2 thinking a "stronger" wire makes a stronger joint; assuming a magnet identifies boron steel; skipping coating removal on galvanized parts.