PQR Impact Toughness Failure at -30 C
A fabricator qualifies a GMAW-P procedure on 25 mm ASTM A572 Gr.50 plate using ER70S-6 filler and a 75% Ar/25% CO2 blend. The PQR shows tensile tests passing and a 180-degree bend passing, but Charpy V-notch testing at -30 C returns 20 J, 22 J, and 19 J average 20 J. The applicable code requires 27 J minimum average at -30 C. The engineer notes the heat input was 1.2 kJ/mm, interpass temperature reached 250 C, and the root was ground flush before subsequent passes. Production is scheduled to start in five days.
1. Diagnose the likely causes of the low toughness and decide whether the PQR can be salvaged.
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The Charpy results are close to the minimum, which suggests the procedure is marginal rather than fundamentally wrong. Several factors can depress toughness: a high interpass temperature of 250 C can promote coarse grain growth in the heat-affected zone and reduce toughness in the reheated region; the 1.2 kJ/mm heat input may be low enough to produce fast cooling and hard, untempered microstructures in the HAZ of a 25 mm section; and ER70S-6 with 25% CO2 produces a relatively high oxygen content in the deposit, increasing inclusions. Root grinding is not the issue. The PQR is not acceptable as written because it fails the required 27 J. It can be salvaged only by re-qualifying with changes that are essential variables: reduce interpass temperature to 200 C or below, increase heat input modestly, or switch to a filler with better toughness such as ER70S-3 or a low-hydrogen basic flux-cored wire. Any of these requires a new PQR, not a retest of the same coupon.
2. What testing and documentation steps are required to demonstrate the revised procedure meets the code?
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Prepare a new PQR using the revised essential variables: record actual heat input, interpass temperature, gas mix, and filler lot. Weld a coupon of the same thickness and joint configuration, then perform the full mechanical test set: two tensile specimens, two root and two face bend specimens, and a Charpy set of three specimens at -30 C, with the option to test a second set of three if the first is marginal. Test the Charpy specimens from the weld metal and, if the code requires, the HAZ. Document specimen location, orientation, notch position, and test temperature. If the results meet 27 J average with no individual value below the code minimum, the PQR can support a revised WPS. Record all variables in the PQR and cross-reference them in the WPS so the welder can follow the qualified ranges. Do not begin production until the revised PQR is signed and the WPS is issued.