Weld Inspection and Quality Control

## The Welding Quality System Weld quality is not achieved by inspection alone—it is built into the process through qualified procedures, qualified welders, and systematic in-process controls. The three pillars are: 1. **Welding Procedure Specification (WPS)**: A written document specifying all essential variables—process, filler metal, base metal group, thickness range, joint design, preheat, interpass temperature, shielding gas, electrical parameters, and heat treatment. The WPS is qualified by welding and testing a Procedure Qualification Record (PQR) coupon per the governing code. 2. **Welder Performance Qualification (WPQ)**: Each welder or welding operator is tested on the specific joint types, positions, and processes they will perform. Qualification tests per AWS D1.1, ASME Section IX, or EN ISO 9606 typically require bending or radiographic examination of test coupons. 3. **Inspection and Testing**: Visual, dimensional, and NDT methods verify that production welds meet acceptance criteria. ## Visual Inspection (VT) Visual inspection is the first and most important inspection method—it catches the majority of weld defects at the lowest cost. AWS D1.1 and ASME Section IX specify acceptance criteria for: - **Weld size**: fillet weld legs and throat must meet drawing dimensions. Undersized welds reduce load capacity. - **Weld profile**: excessive convexity, concavity, overlap, or undercut outside code limits are rejectable. - **Surface discontinuities**: cracks (always rejectable), porosity, slag inclusions, and incomplete fusion visible on the surface. - **Dimensional conformance**: misalignment, angular distortion, and root opening within tolerances. VT should be performed before, during, and after welding: - **Before**: joint fit-up, cleanliness, preheat verification - **During**: interpass cleaning, interpass temperature, proper technique - **After**: final weld profile, size, and surface quality AWS Certified Welding Inspector (CWI) certification is the standard credential for personnel performing visual weld inspection in the US. ## Acceptance Criteria by Code Different codes specify different defect acceptance limits based on the criticality of the application: | Code | Application | Porosity Limit | Undercut Limit | Cracks | |------|-------------|---------------|----------------|--------| | AWS D1.1 | Structural steel | Size/frequency tables | 0.8 mm depth (1 mm for stress < 0.5Fy) | None permitted | | ASME VIII | Pressure vessels | Per RT/UT acceptance | 0.4 mm depth | None permitted | | AWS D1.6 | Structural stainless | Similar to D1.1 | 0.8 mm depth | None permitted | | EN ISO 5817 | General, levels B/C/D | Level-dependent | Level-dependent | Level B: none | Level B (EN ISO 5817) is the most stringent quality level and is required for fatigue-loaded structures. Level D is the most permissive and applies to static structures with low consequences of failure. ## Non-Destructive Testing of Welds NDT methods are applied after visual inspection passes. The required method depends on the joint criticality, material, and code: **Radiographic Testing (RT)**: The historical standard for full-penetration butt welds. Provides a permanent film record. Required by ASME VIII Division 1 for Category A and B pressure vessel joints. Detects volumetric defects (porosity, slag) but may miss tight planar cracks parallel to the beam. **Ultrasonic Testing (UT/PAUT)**: Increasingly replacing RT for weld inspection. Phased Array UT provides real-time imaging with better detection of planar defects. ASME VIII now accepts PAUT as an alternative to RT. AWS D1.1 allows UT for CJP groove welds in plate thicker than 8 mm. **Magnetic Particle Testing (MT)**: Applied to the weld surface and HAZ of ferromagnetic steels to detect surface and near-surface cracks. Standard requirement for fillet welds and partial-penetration welds that cannot be radiographed. Wet fluorescent MT (WFMT) under UV light is the most sensitive variant. **Liquid Penetrant Testing (PT)**: Used for surface crack detection on non-ferromagnetic materials (stainless steel, aluminum, nickel alloys) where MT is not applicable. ## Destructive Testing for Qualification PQR and WPQ coupons undergo destructive tests to verify joint properties: - **Bend tests**: guided bends (root, face, side) detect lack of fusion, porosity, and cracking. A specimen bent 180° around a mandrel with no open defect exceeding 3 mm passes per AWS D1.1. - **Tensile tests**: full-section or reduced-section tensile specimens must meet or exceed the minimum specified base metal UTS. - **Charpy impact tests**: required for low-temperature service (ASME VIII, AWS D1.5 bridge welding). Weld metal and HAZ are tested separately. - **Macro/micro examination**: cross-sections are polished and etched to reveal weld profile, penetration, HAZ extent, and microstructural anomalies. ## Documentation and Traceability A complete weld quality record includes: - WPS and supporting PQR for each joint type - Welder qualification records (WPQ) current and valid - Filler metal certificates (mill test reports per AWS A5.01) - Preheat and interpass temperature logs - NDT reports with inspector certification numbers - Heat treatment charts (time-temperature records) - Weld maps showing joint numbers, WPS numbers, and welder identification This documentation package is a contractual deliverable on most fabrication projects and must be retained for the life of the equipment in pressure vessel and nuclear applications.