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A Welding Procedure Specification (WPS) is the job’s instruction sheet for making a weld that meets a defined standard. It tells you more than whether to use MIG, TIG, or stick. A usable WPS specifies the base metals, joint design, filler metal, preheat, amperage, voltage, travel speed, shielding gas, weld position, and inspection requirements.
Read it before fitting parts together or loading wire into the machine. A WPS is not a general suggestion. Changing a listed variable—such as filler classification, shielding gas, or preheat—can produce a weld that looks acceptable but is not qualified for the job.
Identify the WPS and Supporting Documents
Start by checking the WPS number, revision, date, and applicable code. Use the latest revision, not a photocopy from a previous project. The WPS may refer to a Procedure Qualification Record (PQR), which documents the test weld used to prove that the procedure works. It may also refer to drawings, inspection plans, or a welder qualification record.
Make sure your own qualification covers the process and position. A person qualified for flat-position MIG on carbon steel may not be qualified to weld overhead with stick, stainless steel TIG, or a different joint type. The WPS controls the procedure; your qualification controls whether you are authorized to use it.
Check the Base Metal and Joint Design
Find the base-metal specification and thickness range first. “Mild steel” is not always enough information. A WPS might require ASTM A36, A572 Grade 50, or a particular tubing specification. These materials can have different strength, chemistry, and preheat requirements.
Compare the actual parts with the WPS limits. Check material thickness, outside diameter for pipe, and whether the joint uses one material or two different materials. If the parts fall outside the qualified range, stop and ask the welding engineer or supervisor before welding.
Then inspect the joint details:
- Joint type, such as butt, lap, tee, corner, or socket joint
- Groove angle and root opening
- Root face or land
- Backing strip, backing gas, or open-root setup
- Fillet weld size, groove depth, and required weld length
- Number of passes and required contour
A root opening of 1/8 inch is not interchangeable with 1/16 inch just because both can be welded. A narrow root can cause lack of fusion or incomplete penetration. A wide root can create burn-through, excessive reinforcement, or an undersized root pass.
Read the Process and Consumable Details
The process may be listed as GMAW for MIG, GTAW for TIG, or SMAW for stick. Some WPS documents separate short-circuit, spray, pulsed spray, or flux-cored modes. Use the specified mode; changing from short-circuit transfer to spray transfer changes heat input and may require a different qualification.
Verify the filler-metal classification on the spool, box, or rod container. For example, ER70S-6 MIG wire, ER308L TIG rod, and E7018 stick electrodes are not substitutes for one another. Filler selection affects strength, corrosion resistance, hydrogen control, and compatibility with the base metal.
For shielded metal arc welding, check the electrode diameter and storage condition. Low-hydrogen electrodes such as E7018 may require a holding oven after opening, depending on the manufacturer’s instructions and the governing code. Damp electrodes can contribute to hydrogen cracking, especially in thick or restrained steel.
For MIG or TIG, verify the gas type, flow rate, and delivery method. Argon is common for TIG and aluminum MIG. A common steel MIG mixture may be 75 percent argon and 25 percent carbon dioxide, but the WPS takes priority. Too little gas can cause porosity; too much can create turbulence and draw room air into the arc.
Understand the Welding Variables
Separate essential variables from items that may be adjusted within a stated range. A change in process, base-metal group, filler classification, or shielding gas may require a new qualification. Minor changes to technique may be allowed, but only within the WPS and applicable code.
| Variable | What to check | Common failure when ignored |
|---|---|---|
| Amperage and voltage | Machine settings or permitted ranges | Undercut, lack of fusion, excessive spatter, or burn-through |
| Travel speed | Specified inches per minute or controlled technique | Insufficient penetration or excessive heat input |
| Preheat and interpass temperature | Minimum and maximum temperatures | Cracking, distortion, or altered material properties |
| Weld position | Flat, horizontal, vertical, overhead, or pipe position | Unsupported molten metal and poor fusion |
| Shielding gas | Gas composition and flow rate | Porosity, oxidation, or unstable arc |
Do not treat voltage and amperage as independent numbers. With MIG, wire feed speed primarily controls amperage, while voltage affects arc length and bead profile. With TIG, amperage is more directly controlled, while travel speed and torch angle strongly affect penetration. If the WPS gives a heat-input limit, record amperage, voltage, and travel speed rather than relying on machine display alone.
Check Preheat, Interpass, and Cleaning Requirements
Measure preheat on the actual joint, not several inches away on a convenient surface. A temperature-indicating crayon, contact thermometer, or infrared thermometer can work; infrared readings are affected by shiny metal and emissivity. Preheat is often specified as a minimum, such as 150°F, and interpass temperature may have a maximum, such as 450°F.
Preheat slows cooling and reduces cracking risk in thicker or higher-carbon steel. It does not replace clean metal or correct poor fit-up. Remove paint, mill scale, oil, moisture, galvanized coating, and rust from the weld zone. Grinding galvanized steel creates hazardous zinc fumes, so use suitable ventilation and respiratory protection where required.
Between passes, remove slag and inspect the bead. A welding chipping hammer and wire brush are inexpensive tools, but they do not replace grinding when the WPS or inspector requires complete removal of defects.
Prepare the Machine and Safety Gear
Set up the machine for the specified process and polarity. DC electrode negative and DC electrode positive are not interchangeable for every electrode or process. Confirm the work clamp has clean contact and that leads, liners, contact tips, and hoses are in good condition.
Use a properly rated auto-darkening welding helmet with the shade recommended for the process and amperage. Add flame-resistant clothing, gloves suited to the process, safety glasses under the helmet, hearing protection, and ventilation. TIG work may need more precise gloves and a stable torch setup; stick and flux-cored work generally produce more fumes and slag.
Before striking the arc, dry-run the joint. Confirm access, torch or electrode angle, cable reach, purge connections, tack locations, and the required weld sequence. If the joint cannot physically be welded within the WPS limits, do not compensate by turning up the machine. Get the procedure clarified or revised first.