Project
Welding Procedure Qualification Heat Input and HAZ Hardness Project
Materials engineering project for qualifying a welding procedure with heat input, travel speed window, preheat, interpass temperature, HAZ hardness, delayed NDE, coupon evidence, and release gates.
This project qualifies a welding procedure for a steel fabrication where heat input and heat-affected-zone hardness control the release decision. The deliverable is a procedure qualification package: a controlled welding parameter range, coupon evidence, hardness map, non-destructive examination plan, acceptance criteria, and requalification triggers.
The project is not a welding heat-input formula sheet. It uses the formula, but the engineering task is broader: prove that a procedure can produce the required joint geometry, metallurgical condition, inspection result, and repeatable production control.
The central project question is:
Can this weld procedure be released for production with controlled heat input, controlled HAZ hardness, credible inspection timing, and clear limits on future parameter changes?
Project Objective
Prepare a welding procedure qualification package for a structural steel bracket welded to a base frame. The final deliverable must include:
- base material and joint scope;
- heat-input range and travel-speed window;
- preheat and interpass temperature requirements;
- carbon-equivalent weldability screen;
- coupon inspection plan;
- HAZ hardness map and acceptance decision;
- delayed non-destructive examination requirement;
- distortion and workmanship checks;
- release, repair, or requalification decision.
The project is written for materials engineering students and early-career engineers. It is simplified and does not replace any welding code, project specification, qualified welding engineer, certified inspector, or procedure standard. It shows how engineering evidence should be assembled before a weld procedure is released.
Baseline Scenario
A fabrication shop needs to qualify a gas metal arc welding procedure for a 22\ \text{mm} thick low-alloy steel bracket. The weld is a multi-pass fillet weld on a restrained joint. The design team is concerned about excessive HAZ hardness, delayed hydrogen cracking, fatigue-sensitive toe defects, and distortion.
Use this simplified qualification basis.
| Parameter | Target or requirement |
|---|---|
| base plate thickness | 22\ \text{mm} |
| process | gas metal arc welding |
| shielding gas and wire | fixed for this qualification |
| target heat input range | 0.70 to 1.10\ \text{kJ/mm} |
| required preheat range | 120 to 180^\circ\text{C} |
| maximum interpass temperature | 230^\circ\text{C} |
| HAZ hardness action limit | 350\ \text{HV} |
| delayed surface and volumetric NDE | at least 48\ \text{h} after welding |
| maximum angular distortion over gauge length | 2.0\ \text{mm} |
| production release basis | macro, hardness, NDE, visual, distortion and record completeness |
Deliverable Structure
The qualification file should contain:
| Section | Evidence required |
|---|---|
| procedure scope | material, thickness range, joint type, process, consumable, position |
| parameter range | voltage, current, travel speed, heat input, preheat, interpass |
| coupon record | welder, equipment, material lot, wire lot, gas, passes, ambient condition |
| inspection | visual, macro, hardness traverse, delayed NDE, dimensional distortion |
| acceptance | heat input, hardness, defect limits, distortion, record completeness |
| controls | storage, preheat measurement, travel-speed control, interpass checks |
| requalification triggers | material change, thickness change, consumable change, heat-input excursion |
The package should be practical enough for production: every critical limit must have a measured value, an owner, and a reaction rule.
Step 1: Carbon Equivalent Screen
Use a common carbon-equivalent screen for low-alloy steels:
Use the material certificate:
| Element | Mass percent |
|---|---|
| C | 0.18 |
| Mn | 1.20 |
| Cr | 0.25 |
| Mo | 0.10 |
| V | 0.02 |
| Ni | 0.15 |
| Cu | 0.20 |
Calculate:
Engineering Comment
A CEV near 0.48 is a warning that preheat, hydrogen control, heat input, cooling rate, and delayed inspection matter. The number does not by itself qualify or reject the material, but it justifies treating HAZ hardness and hydrogen cracking as controlled risks.
Step 2: Heat Input for the Qualification Coupon
Heat input per unit length is:
where:
- \eta is process efficiency;
- V is arc voltage;
- I is current;
- v is travel speed.
The proposed qualification coupon uses:
Calculate:
Convert:
Engineering Comment
The heat input is inside the target range of 0.70 to 1.10\ \text{kJ/mm}. That does not qualify the procedure by itself. It only shows that the coupon was welded inside the planned thermal process window.
Step 3: Travel-Speed Window
For the same voltage and current, calculate the travel-speed range that keeps heat input inside the target.
First compute effective arc power:
For the maximum heat input:
Minimum travel speed:
For the minimum heat input:
Maximum travel speed:
Therefore the qualified travel-speed window for this voltage-current combination is approximately:
Engineering Comment
Travel speed is now a production control, not only a welder technique preference. If a production weld is made at 3.5\ \text{mm/s} or 7.0\ \text{mm/s} with the same voltage and current, the heat input is outside the qualified basis.
Step 4: Preheat and Interpass Control
The procedure requires:
and:
The coupon record shows:
| Measurement | Value | Decision |
|---|---|---|
| preheat before first pass | 145^\circ\text{C} | pass |
| interpass after pass 2 | 188^\circ\text{C} | pass |
| interpass after pass 4 | 224^\circ\text{C} | pass |
| interpass after pass 5 | 238^\circ\text{C} | fail |
Engineering Comment
The coupon cannot be accepted as-is because one interpass temperature exceeded the qualified maximum. A high interpass temperature may slow cooling and reduce hardness, but it can also change microstructure, bead shape, toughness, residual stress, and distortion. The qualification basis must match the production basis.
The correct response is not to ignore the excursion because the weld looks good. The team must either rerun the coupon inside the window or intentionally revise and justify the procedure window.
Step 5: HAZ Hardness Map
A first coupon is examined with a hardness traverse across the weld metal, HAZ, and base metal. The HAZ readings are:
| Location | HAZ hardness |
|---|---|
| toe left | 318\ \text{HV} |
| coarse-grain HAZ left | 342\ \text{HV} |
| root-side HAZ left | 365\ \text{HV} |
| toe right | 336\ \text{HV} |
| coarse-grain HAZ right | 352\ \text{HV} |
| root-side HAZ right | 329\ \text{HV} |
Maximum HAZ hardness:
The action limit is:
Exceedance:
Engineering Comment
The coupon fails the HAZ hardness gate. The failure is consistent with a weldability concern: the material has meaningful hardenability, and at least one local region cooled into a hard HAZ condition. This is not the same as a detected crack, but it is a release blocker because hardness is a proxy for hydrogen-cracking susceptibility and local brittleness.
Step 6: Corrected Coupon
The procedure is adjusted:
- preheat target increased to 160^\circ\text{C};
- interpass control tightened with a mandatory wait above 220^\circ\text{C};
- travel speed held between 4.6 and 5.6\ \text{mm/s};
- low-hydrogen consumable storage and exposure time recorded;
- delayed NDE held until at least 48\ \text{h}.
The corrected coupon records:
| Location | HAZ hardness |
|---|---|
| toe left | 306\ \text{HV} |
| coarse-grain HAZ left | 331\ \text{HV} |
| root-side HAZ left | 338\ \text{HV} |
| toe right | 314\ \text{HV} |
| coarse-grain HAZ right | 327\ \text{HV} |
| root-side HAZ right | 336\ \text{HV} |
Maximum corrected HAZ hardness:
Margin to action limit:
Engineering Comment
The corrected coupon passes the simplified hardness gate. The margin is real but not large. Production should keep preheat, interpass, travel speed, consumable handling, and inspection timing under control because small changes can consume the hardness margin.
Step 7: Delayed NDE Timing
Hydrogen cracking can appear after the weld cools and hydrogen diffuses under residual stress. Immediate inspection is useful for workmanship, but delayed inspection is needed for cracking risk.
The project sets:
The corrected coupon inspection record shows:
| Inspection | Timing | Result |
|---|---|---|
| visual inspection | after cleaning | acceptable profile, no undercut above limit |
| surface crack test | 4\ \text{h} | no relevant indication |
| ultrasonic inspection | 52\ \text{h} | no relevant planar indication |
| repeat surface crack test | 54\ \text{h} | no relevant indication |
Engineering Comment
The delayed NDE evidence is aligned with the failure mode. If the only crack inspection occurred immediately after welding, the project would still have an evidence gap.
Step 8: Distortion Screen
The maximum allowed angular distortion over the gauge length is:
The corrected coupon measures:
Distortion margin:
Utilization of the distortion limit:
Engineering Comment
The distortion screen passes, but production fixtures and weld sequence must match the coupon basis. A qualification coupon welded flat and free can understate distortion for a restrained production assembly.
Step 9: Qualification Risk Review
A simplified risk table identifies the main procedure risks.
| Failure mode | Effect | Initial S | Initial O | Initial D | Initial RPN |
|---|---|---|---|---|---|
| low preheat or uncontrolled interpass | hard HAZ, delayed cracking | 9 | 4 | 4 | 144 |
| travel speed outside heat-input window | lack of fusion or hard HAZ | 8 | 4 | 4 | 128 |
| immediate-only inspection | delayed cracking missed | 9 | 3 | 5 | 135 |
| consumable exposure not controlled | hydrogen-assisted cracking | 9 | 3 | 4 | 108 |
| production joint more restrained than coupon | residual stress and distortion | 8 | 3 | 5 | 120 |
After controls, the first risk is scored:
Controlled RPN:
Engineering Comment
The controls do not make cracking impossible. They make the procedure auditable: production can show preheat, interpass, heat input, consumable control, delayed NDE, and requalification evidence when a future weld is challenged.
Step 10: Release Decision
The recommended decision is:
Release the corrected welding procedure for the defined material, thickness range, joint type, consumable, position, heat-input window, preheat/interpass window, and inspection plan. Do not release the first coupon with interpass and hardness failures.
Release conditions:
- production welds must stay inside the qualified voltage, current, travel-speed, preheat and interpass range;
- HAZ hardness evidence from the corrected coupon is retained with the procedure record;
- delayed NDE is required for the defined high-risk joint class;
- consumable storage and exposure controls are recorded;
- production fixtures, restraint and weld sequence must match the qualified basis or receive engineering review;
- any material grade, thickness, consumable, process, position or heat-input excursion outside the qualified range triggers requalification or documented engineering disposition.
Final Deliverable
The final qualification package should include:
- welding procedure specification and qualification record;
- material certificates and carbon-equivalent calculation;
- voltage, current, travel speed and heat-input calculations;
- preheat and interpass temperature records;
- macro section, bead profile and workmanship evidence;
- HAZ hardness map with acceptance decision;
- delayed NDE records with timing and method;
- distortion measurement and fixture basis;
- risk review and controls;
- release decision and requalification triggers.
Common Mistakes
- Treating heat input as a single calculated value instead of a controlled window.
- Accepting a coupon that passed visual inspection but failed interpass or hardness limits.
- Inspecting immediately after welding and missing delayed cracking risk.
- Qualifying an easy coupon that does not represent production restraint, thickness or access.
- Changing travel speed, consumable, preheat, interpass or material grade without requalification review.
- Reporting hardness averages while ignoring the maximum local HAZ reading.
- Keeping a procedure record without a reaction rule for out-of-window production welds.
Transferable Lesson
Welding procedure qualification is materials engineering under production constraints. A useful procedure does more than describe how to make a bead. It defines the thermal cycle, hydrogen controls, inspection timing, metallurgical evidence, dimensional limits, and change boundaries that make the weld repeatable.
The qualified procedure is credible only when heat input, preheat, interpass temperature, HAZ hardness, inspection timing, and production conditions all remain inside the evidence envelope.