A WPS for a multi-pass weld is not a stacking of single-pass parameters. The order in which beads are deposited, the temperature of the metal between passes, and the cleaning method between layers all affect the mechanical properties of the finished joint. AWS D1.1 requires the WPS to address these controls, and a procedure that lists only process parameters without specifying sequence and interpass requirements is incomplete for production use.
Why Pass Sequence Affects Weld Quality
Each pass in a multi-pass joint acts as a small heat-treatment cycle for the metal beneath it. The first bead deposited onto the base metal creates a coarse-grained HAZ. The second pass, laid alongside or over the first, partially reheats and refines that grain structure. A subsequent pass does the same. Done correctly, multi-pass welding produces a final joint with refined microstructure throughout most of its volume — better toughness than a single-pass weld of the same size.
Done incorrectly — out of sequence, with excessive interpass temperature, or with the final cap pass left coarse and unrecrystallized — the same joint can have pockets of degraded toughness at the surface where visual inspection finds nothing wrong.
The variables that control the outcome:
- Number of passes and bead sequence — how many beads per layer, their order, and whether backstep or block welding applies
- Minimum preheat and interpass temperature — the floor that prevents hydrogen cracking
- Maximum interpass temperature — the ceiling that prevents grain coarsening and toughness loss
- Interpass cleaning method — what removes slag, spatter, and surface discontinuities before the next bead
What the WPS Must Specify
AWS D1.1 requires the WPS to document the welding sequence for the joint. For multi-pass welds, this means:
Pass arrangement by layer. How many beads per layer? Are fill passes stringer beads or weave passes? The distinction matters — wide weave passes put more heat into the joint and produce a wider HAZ than narrow stringer beads. If the PQR was qualified with stringers, using a wide weave in production without requalification is not a minor variation.
Root pass requirements. The root pass is the most critical bead. It bridges the gap across the joint opening. The WPS should specify root opening tolerance, whether back-gouging is required before welding the back side, and how to handle arc starts and stops within the root pass.
Travel direction and sequencing strategy. For joints where distortion control is critical — thin flanges, long seam welds, joints near finished dimensions — the WPS may specify backstep welding or balanced welding on alternate sides of the neutral axis. These decisions belong in the written procedure, not in the welder's judgment on a given shift.
Weld pass table for critical work. For CVN-qualified procedures, seismic work, or high-strength base metals, a pass table documents each bead: pass number, approximate size, electrode or wire classification, current, voltage, and travel speed. This table is the baseline against which production records are audited.
Interpass Temperature: Both Limits Matter
Most quality programs check the minimum preheat and interpass temperature. The maximum interpass limit gets less attention in non-CVN work, but it matters equally for grain structure control.
The minimum prevents hydrogen cracking by keeping the weld area above the temperature where hydrogen can accumulate in the HAZ. The maximum prevents grain coarsening by limiting how hot the adjacent weld metal gets before the next bead solidifies over it. For a detailed breakdown of how these limits work together, see preheat and interpass temperature on a WPS.
For CVN-qualified procedures, the maximum interpass temperature documented in the PQR becomes a supplementary essential variable under AWS D1.1:2025 Table 6.8. If production welds run at interpass temperatures above the qualified limit, the CVN data from the PQR no longer applies and the procedure is unsupported.
Temperature measurement has to be practical: use a contact pyrometer or temperature-indicating crayon applied to the weld area within 1 inch of the joint centerline. Measuring at the far edge of the base plate where access is easier produces readings that understate the actual joint temperature. Document each interpass temperature check in the inspection record by pass number.