When a manufacturing process or repair method requires welding, it’s common practice to draft a WPS- Welding Procedure Specification. This document provides direction to welders on how to make quality welds to production standards, as well as to applicable code requirements.
The WPS details accepted procedures to ensure that repeatable and reliable welding techniques are employed. Since there can be a wide array of base materials and welding types used in production processes, the best practice is to develop a WPS for each combination. This document summarizes all of the welding variables and materials used for many welds.
The WPS defines a set of procedures and applicable materials to be used. To verify the validity of these initial procedures, a series of welds are created, which are then mechanically tested to verify they meet all valid requirements. The procedures for creating the sample welds, as well as the final testing results, are documented on a Procedure Qualification Record, or PQR.
The PQR is required to quantify the results during the actual welding tests, subsequent inspections, and mechanical testing data. These test methods can include:
- Proof load
- Specially-fixtured load
- Nick break
- Chemical analysis
Mechanical testing methods are used most frequently in this process. For example, tensile testing is performed to determine the ultimate strength of the welded joint compared to the base materials being joined. In some instances this can also reveal internal weld defects and other fusion anomalies.
Bend testing is used to evaluate the ductility and soundness of the weld joint. This method can exhibit cracks or openings referred to as discontinuities. The maximum or aggregate lengths are compared to the specification or code limits.
Impact testing is the primary method used to determine the toughness of a material, or its resistance to crack propagation. This testing can be performed at all service temperatures as well as in the base metal, at the fusion line, heat affected zone (HAZ), and weld.
Once the test results are determined to be acceptable, the PQR can be approved. It then serves as the template for development of WPS. The PQR document provides evidence that a given WPS can be used to produce an acceptable, repeatable weld.
Following the steps of the WPS will help ensure consistent welds with similar mechanical properties. To put the WPS into practice, welders need to be tested for qualification that they have the ability to successfully follow the steps of the WPS, and produce the weld to specification. The Weld Procedure Qualification (WPQ) provides documentation from an independent third-party testing resource.
Once a welder earns the WPQ, their employer/customer can be reasonably confident that the welds they produce fall within the guidelines defined in the WPS.
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