Impact Test Failure on a Qualified Procedure
A PQR for SMAW on 20 mm carbon steel specifies impact testing at minus 30 C. The test results show three specimens with absorbed energies of 38 J, 41 J, and 22 J. The specification requires a minimum average of 34 J with no single value below 27 J. The welding engineer notes that the 22 J specimen had a visible slag inclusion and wants to discard that result and average the remaining two specimens. The PQR is otherwise complete and the shop needs the procedure for a current job.
1. Evaluate whether the impact test results pass and whether the 22 J specimen can be discarded.
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The results do not pass as reported. The average of 38, 41, and 22 J is 33.7 J, below the 34 J minimum average. The 22 J specimen is also below the 27 J single-value minimum. Both criteria fail.
Discarding the specimen: A specimen with a visible slag inclusion may be considered invalid if the inclusion is a testing artifact rather than a material property. However, the decision to discard must follow the code's rules for invalid specimens - typically the specimen must be re-tested, not simply omitted. The engineer cannot unilaterally drop the lowest value to achieve a passing average.
Correct approach:
- Document the slag inclusion and determine whether it invalidates the specimen under the applicable code.
- If invalid, retest with a replacement specimen set as required.
- If valid, the PQR fails and the procedure must be revised (heat input, preheat, consumable, etc.) and requalified.
Conclusion: The results cannot be accepted by discarding the 22 J value. Either a valid retest is performed or the procedure is revised.
2. If a retest is permitted, describe the conditions that must be met and the risk of repeating until passing.
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A retest is permitted only under the code's rules for invalid or failed tests, and the conditions must be documented:
- Valid reason: The specimen must be shown to be invalid (e.g., machining defect, inclusion unrelated to weld quality). A visible slag inclusion may or may not qualify; the code's definition controls.
- Retest scope: The retest should use a replacement set of specimens from the same coupon or a new coupon, tested under the same conditions.
- Documentation: Record the original results, the reason for invalidation, and the retest results on the PQR.
Risk of repeat-until-pass: Repeatedly retesting until a passing set is obtained undermines the qualification's validity. If the first set failed because of a real material or process issue, retesting will likely reproduce the failure. The engineer should investigate the root cause - heat input, interpass temperature, consumable condition, or specimen preparation - before retesting.
Recommendation: Investigate the slag inclusion source, verify specimen preparation, and retest once with documented justification. If the retest fails, revise the procedure rather than retesting again.