Integrated Metallurgy Case Study: Welding Procedure Review and Defect Prevention Plan

Integrated Metallurgy Case Study: Welding Procedure Review and Defect Prevention Plan

By the end of a structural or pipe welding program, you should be able to look at a Welding Procedure Specification (WPS) and immediately recognize whether it will produce a sound weld — or set you up for failure. This lesson ties together every metallurgy concept from the course into one practical exercise: reviewing a real-style WPS package and building a written plan to prevent defects before the first arc is struck.

What a WPS Package Contains

A complete WPS package for structural work includes the WPS itself (listing base metal, filler metal, process, position, preheat, interpass temperature, heat input limits, and post-weld heat treatment if required), the Procedure Qualification Record (PQR) that proves the procedure was tested, and the Mill Test Report (MTR) for the base metal. The MTR gives you the actual chemical composition and mechanical properties of the heat of steel you are welding — not just the specification minimums. You use the MTR chemistry to calculate the Carbon Equivalent (CE), which predicts susceptibility to hydrogen-induced cracking (HIC).

The IIW Carbon Equivalent Formula

The International Institute of Welding (IIW) formula most commonly used for structural steels is:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

All values are in weight percent (wt%), taken from the MTR. As a general guideline, steels with CE above approximately 0.40 are considered higher hardenability and typically require preheat to suppress martensite formation in the heat-affected zone (HAZ) and reduce hydrogen cracking risk. AWS D1.1 provides preheat tables based on base metal category, thickness, and process; always use those tables in conjunction with the CE result — CE alone is a screening tool, not a substitute for the code table.

Verifying Preheat Adequacy for A572 Grade 50

A572-50 is a high-strength low-alloy (HSLA) structural steel widely used in bridge girders and building frames. It has a specified minimum yield strength of 50 ksi. For a cyclically loaded bridge girder operating at a minimum design temperature of −20°F, the consequences of hydrogen cracking or brittle fracture are severe. Check three things against AWS D1.1: (1) the preheat temperature listed on the WPS matches or exceeds the code requirement for the actual plate thickness and process; (2) the minimum interpass temperature is at least equal to the preheat; and (3) the maximum interpass temperature is specified to protect toughness — excessive interpass temperature can coarsen the HAZ grain structure and reduce Charpy impact values.

Heat Input Control

Heat input (HI) is calculated as:

HI (kJ/in) = (Amps × Volts × 60) / (Travel Speed in in/min × 1000)

High heat input grows the HAZ, reduces cooling rate, and can degrade notch toughness — critical for a fatigue-loaded, low-temperature application. Low heat input raises cooling rate, can increase hardness, and increases hydrogen cracking risk in higher-CE steels. The WPS must specify a range, and you must verify that range is achievable with the listed parameters.

Five Most Likely Metallurgical Risks for This Joint

1. Hydrogen-induced cracking (HIC): Root cause is diffusible hydrogen from moisture or cellulosic electrodes, combined with a hard HAZ and tensile stress. Control: low-hydrogen filler (E7018, stored in a rod oven per manufacturer requirements), verified preheat, and hydrogen bake-out if required.

2. Lamellar tearing: Risk exists when through-thickness shrinkage stress acts on plate with low reduction-of-area (RA%) in the short-transverse direction. Check the MTR for through-thickness ductility. Joint design that minimizes Z-direction stress and using low-yield-strength butter passes can reduce risk.

3. Undercut: Caused by excessive amperage, incorrect travel angle, or too-fast travel speed. Undercut is a stress concentration especially harmful under cyclic loading. Control by staying within WPS parameter ranges. AWS D1.1 limits undercut depth for tension members.

4. Slag inclusion: Associated with incomplete interpass cleaning, especially in multi-pass groove welds. Control: wire brush and grind each pass before depositing the next; maintain correct travel angle.

5. Distortion: Differential thermal expansion and contraction create angular and longitudinal distortion. Control by sequencing passes, back-stepping, balanced welding, and clamping or strongbacks per the WPS.

Post-Weld Heat Treatment (PWHT) Decision

For A572-50 structural plate groove welds covered by AWS D1.1, PWHT (stress relief) is not routinely required by the structural code unless specifically required by the Engineer of Record or contract documents. However, you must document this decision in your defect-prevention plan with a code reference, not just state "not required" without justification.

Inspection Hold Points

A hold point is a defined stage where work stops until an inspection is completed and documented. Typical hold points for a structural groove weld: (1) fit-up and joint dimensions before welding; (2) root pass — visual and, if specified, MT or PT; (3) interpass — visual for slag, cracks, and profile; (4) final pass — visual inspection (VT), then nondestructive examination (RT or UT) per the contract. Acceptance criteria come from AWS D1.1 tables for the applicable weld category.

Safety in This Lab

Although this lesson is a document-review and planning exercise, you are working with welding references that will govern live weld procedures. When you move to verification weld tests, always: wear appropriate PPE (welding helmet with correct shade, leather gloves, FR clothing); follow lockout/tagout (LOTO) procedures on any equipment before adjusting it; ensure ventilation is adequate for the filler metal being used; and store low-hydrogen electrodes in a heated rod oven at the temperature specified by the manufacturer to prevent moisture pickup, which is the primary source of diffusible hydrogen.