A Welding Procedure Specification (WPS) is the document that tells you exactly how to make a specific weld: what process, filler metal, preheat, amperage range, shielding gas, and joint details to use. Every code-governed weld has a WPS behind it. You don’t have to memorize the whole document, but you need to find and understand the essential variables that affect your work at the joint.
The WPS is a legal document. Welding outside its parameters means the weld doesn’t meet code, even if it looks perfect. Inspectors verify that the welder is following the WPS, and deviations can result in cut-outs and re-welds.
WPS Structure
Most WPS documents follow a standard format (AWS or ASME). The information falls into several sections:
Header Information
- WPS number: Unique identifier (e.g., WPS-001, AWS-2024-01)
- Supporting PQR number: Links to the test record that qualifies this procedure
- Revision number: Tracks changes to the WPS
- Date: When the WPS was written or last revised
- Applicable code: AWS D1.1, ASME Section IX, API 1104, etc.
Joint Design
- Joint type: Butt, tee, corner, lap
- Groove type: V, bevel, J, U, square
- Groove angle: Specified in degrees (e.g., 60-degree included)
- Root opening: Gap dimension (e.g., 1/8 inch)
- Root face: Land dimension (e.g., 1/16 inch)
- Backing: Yes/no, material type if used
- Base metal specification: ASTM or other designation (e.g., A36, A572 Gr50)
- Thickness range: What thickness range this WPS covers
This section matches the joint details to the welding symbol on the drawing. If the drawing shows a V-groove with 1/8 inch root opening, this section of the WPS should match.
Filler Metal
- AWS classification: E7018, ER70S-6, ER70S-2, E6010, etc.
- Specification: AWS A5.1, A5.18, A5.20, etc.
- Diameter: 3/32, 1/8, 5/32 inch
- F-number and A-number: Grouping systems used for qualification ranges
The filler metal classification is an essential variable. Changing from E7018 to E6010 or from ER70S-6 to ER70S-2 may require a different WPS.
Welding Process
- Process designation: SMAW (stick), GMAW (MIG), GTAW (TIG), FCAW (flux-core)
- Transfer mode (GMAW): Short circuit, spray, pulse, globular
- Multiple processes: Some WPSs cover a combination (e.g., GTAW root + SMAW fill/cap)
Changing the welding process is an essential variable change. You can’t substitute MIG for stick without a different WPS.
Electrical Parameters
| Parameter | What It Specifies | Typical Range |
|---|---|---|
| Current type | DCEP, DCEN, or AC | Depends on process and electrode |
| Amperage range | Min and max amps per pass type | Root: 70-95A, Fill: 90-130A, Cap: 90-120A |
| Voltage range | Min and max volts (GMAW/FCAW) | 18-26V typical |
| Wire feed speed (GMAW/FCAW) | Min and max WFS | 150-350 IPM typical |
| Travel speed range | Min and max travel speed | 4-12 IPM depending on process |
| Heat input | Maximum joules per inch | Calculated from amps x volts / travel speed |
These ranges are where the welder has flexibility. You can adjust within the range to suit position and conditions. Going outside the range violates the WPS.
Shielding
- Gas type: 100% argon, 75/25 Ar/CO2, 100% CO2, etc.
- Flow rate: CFH (cubic feet per hour)
- Flux type (SAW, FCAW): Classification and brand
Gas type is an essential variable. Switching from 75/25 to 100% CO2, or from argon to helium blends, requires a different WPS or WPS revision.
Preheat and Interpass Temperature
- Minimum preheat temperature: Temperature the base metal must reach before welding begins
- Maximum interpass temperature: Maximum temperature the joint can reach between passes
- Method of measurement: Contact pyrometer, temp stick, thermocouple
Preheat minimum is an essential variable. If the WPS says 200F minimum, the joint must be at 200F before you strike an arc. No exceptions. See the preheat and post-weld treatment section for temperature details.
Post-Weld Heat Treatment (PWHT)
- Required: Yes or no
- Temperature range: Typically 1100-1200F for carbon steel stress relief
- Hold time: Usually 1 hour per inch of thickness
- Heating and cooling rates: Maximum degrees per hour
If PWHT is specified, it’s an essential variable. Skipping it or running the wrong temperature invalidates the procedure.
Position
- Qualified positions: 1G, 2G, 3G, 4G, 5G, 6G, or “all positions”
- Welding progression: Uphill or downhill for vertical positions
You can’t use a WPS qualified for 1G (flat) to weld in 3G (vertical). Position is an essential variable.
Essential Variables vs. Non-Essential Variables
This is the critical distinction for every welder.
Essential Variables (Cannot Change Without Requalification)
| Variable | What It Means |
|---|---|
| Welding process | Can't switch SMAW to GMAW |
| Filler metal classification | Can't switch E7018 to E6010 |
| Filler metal F-number | Can't change filler grouping |
| Preheat minimum | Can't go below the specified temp |
| PWHT | Can't add or eliminate if qualified without/with |
| Base metal group | Can't switch material types beyond qualified range |
| Shielding gas type | Can't change gas composition |
| Position | Can't weld in an unqualified position |
| Thickness range | Can't exceed the qualified thickness limits |
| Electrical characteristics | Can't change current type (DCEP to DCEN) |
Non-Essential Variables (Welder Can Adjust)
- Travel speed (within range)
- Arc length
- Weave width and pattern
- Number of passes (as long as heat input stays in range)
- Electrode diameter within the WPS range
- Cleaning method between passes
What Welders Need to Know vs. What Engineers Specify
A welder doesn’t need to write a WPS. But you need to know where to find these things on the document:
Before you start:
- What process am I using?
- What filler metal (classification and size)?
- What’s the preheat requirement?
- What’s the interpass max temp?
- What joint prep dimensions are specified?
- What position is qualified?
- What shielding gas and flow rate?
During welding:
- Am I within the amperage range for this pass type?
- Am I maintaining preheat?
- Am I staying under the interpass max?
- Am I welding in the qualified progression (uphill vs. downhill)?
After welding:
- Is PWHT required?
- What NDE is specified? (Usually referenced on the drawing, not the WPS itself)
Prequalified WPS (AWS D1.1)
AWS D1.1 allows certain joint configurations and processes to skip the PQR testing step. These are called prequalified WPS documents. They use standard joint details from AWS D1.1 Tables 3.1 through 3.4 and standard conditions:
- Processes: SMAW, GMAW (except short circuit), FCAW, SAW
- Standard groove preparations listed in D1.1
- Materials from the approved base metal list
- Standard preheat per Table 3.2
A prequalified WPS still has to be written and documented. “Prequalified” doesn’t mean “no paperwork.” It means no destructive testing is required to validate the procedure.
The PQR Connection
Every WPS (except prequalified) must be backed by a Procedure Qualification Record (PQR). The PQR documents:
- The actual parameters used to make the test weld (not ranges, but actual values)
- The test results: tensile strength, bend test results, macro examination, impact testing if required
- Pass/fail determination
- The signature of the responsible engineer
The WPS is then written to cover a range around the PQR’s actual values. The PQR proves the procedure works. The WPS tells you how to replicate it.
WPS Availability at the Workstation
AWS D1.1 Section 5.4 requires that the WPS be available to the welder at the point of welding. “Available” means the welder can reference it during the job. It can be a paper copy posted at the workstation, a laminated card in the welder’s pocket, or accessible digitally on a tablet.
The purpose isn’t to make welders memorize the document. It’s to give them a reference when questions arise. “What’s my preheat on this joint?” “What’s the maximum interpass temperature?” “Can I use 5/32 rod on the fill passes?” These questions have specific answers on the WPS.
On a job site with multiple WPSs (different joints, different materials, different positions), make sure the correct WPS is matched to each weld joint. Using the wrong WPS for a joint is a nonconformance, even if the settings happen to overlap.
Revisions and Amendments
WPS documents get revised as conditions change or errors are found. Always use the latest revision. Check the revision number and date before starting work. Old revisions should be removed from the workstation to prevent accidental use.
When a WPS is revised, any essential variable change requires requalification (new PQR). Changes to non-essential variables can be documented as a revision to the existing WPS without retesting.
Common Mistakes
Not reading the WPS before welding. On a new job with a new WPS, read it before you light up. Don’t assume it’s the same as the last job.
Exceeding the amperage range. If the WPS says 90-130 amps for fill passes and you’re running at 140 because the puddle looks better, you’re out of compliance. The inspector can reject the weld.
Ignoring preheat. “It’s 50 degrees outside, close enough” doesn’t work when the WPS says 150F minimum. Verify with a contact pyrometer or temp stick.
Using the wrong electrode size. If the WPS lists 3/32 and 1/8 inch E7018, you can’t use 5/32 inch. It changes the amperage range, heat input, and deposition rate, all essential variables.
Confusing WPS number with PQR number. The drawing references the WPS. The WPS references the PQR. Don’t mix them up when filling out weld logs or traveler sheets.
For welding symbol interpretation, see the welding symbol chart. For inspection methods used to verify WPS compliance, visit the weld inspection category page. Return to weld symbols or the welding techniques pillar for more topics.