GTAW Welding of Carbon Steel and Stainless Steel (Plate and Pipe)
This domain tests technique and control for carbon steel and stainless steel plate and pipe, including root passes, fill passes, caps, and heat control.
Core rules
- For carbon steel, use DCEN, clean joint faces, and ER70S-2 or ER70S-6 filler as specified. Establish a root keyhole, add filler to control the pool, and tie in tacks without creating arc strikes. Preheat only when the WPS requires it for thickness or grade.
- For stainless steel, use low heat input, stringer beads where specified, and ER308L or ER316L filler matching the base metal. Avoid weaving that overheats the pool. Control interpass temperature and clean between passes.
- Stainless pipe roots normally need an inert back purge to prevent sugaring and oxidation. Monitor purge quality per the WPS. Do not purge with argon into an occupied space without ventilation.
- For pipe in 5G or 6G position, maintain root gap and alignment, control keyhole size, and feed filler consistently. For plate, sequence passes to balance heat and prevent distortion.
- Prevent contamination: stainless-only brushes, clean gloves, clean filler, and no carbon steel contact.
Worked example
A 6G stainless pipe root is being welded. The welder confirms the purge is established, uses DCEN with a small tungsten and ER308L filler, keeps a short arc, and makes a narrow root with a controlled keyhole. Travel is steady and the weld is not woven. After the root, the welder checks for excessive heat tint and cleans before the hot pass. A wide, slow weave would overheat the pipe, widen the heat-affected zone, and increase oxidation and distortion.
Traps that cost marks
- Welding stainless with a carbon steel brush or contaminated gloves.
- Skipping purge and producing sugared root surfaces.
- Excessive heat input or interpass temperature on stainless.
- Losing the keyhole or leaving incomplete penetration in the root.
- Striking the arc outside the joint and leaving arc strikes.