How to Read a Welding Procedure Specification (WPS)

Every code-compliant weld starts with a sheet of paper that many welders glance at once and then ignore. Knowing how to read a WPS, short for welding procedure specification, turns that sheet into a working tool. It tells you which filler to pick up, what current to dial in, how hot the joint may get between passes, and when you must stop and ask before going on.

A WPS looks intimidating the first time. There are dense boxes, abbreviations such as P-No., F-No. and PWHT, and ranges where you expected single numbers. The good news is that almost every WPS, whether it follows AWS, ASME or ISO rules, answers the same handful of questions in roughly the same order.

This guide covers what a WPS is, how it connects to the PQR and to welder qualification, what each section tells you, how to check a heat input limit yourself, and the mistakes that most often turn a sound weld into a nonconformance.

What a WPS Actually Is

A welding procedure specification is a written set of instructions for making one type of weld. It is not a suggestion and it is not a training aid. Once a WPS is issued for a job, every practice written on it has to be followed in production, including conditions the governing code does not strictly require the document to list.

The WPS sits inside a small family of documents that are easy to confuse:

The qualification record and the WPS even look different. A PQR records the actual values used on the test coupon, not the limits written on the WPS that guided the test. A WPS then turns those proven values into the ranges a welder is allowed to work within.

The third document is the one you carry as a welder. A performance test is made to a written procedure, and test facilities evaluate welders against standard procedures, industry specifications or company-provided procedures. If you are working toward your first test, the certified welder path explains how that fits together.

Where a WPS Gets Its Authority

A WPS is only worth following if something proves it works. There are two common routes.

The test-based route (ISO and ASME)

Under the ISO system, a welding engineer drafts a preliminary WPS, a test weld is made and examined, and if it passes, a welding procedure qualification record (WPQR) is issued. Several production WPSs can then be written from that one record, because it approves a range of variables rather than a single set of values.

The rule that matters on the shop floor: a WPS is valid for production only while every parameter stays inside the limits recorded in the supporting WPQR. ISO 15609-1 sets out what an arc welding WPS must contain.

ASME Section IX works in the same spirit with different names. The WPS has to be consistent with the data recorded on its supporting PQRs.

The prequalified route (AWS D1.1)

AWS D1.1 adds a shortcut for structural steel. Its Clause 5 describes the conditions under which a WPS is exempt from qualification testing. If the joint, base metal, filler, process and parameters all stay within the code's prescribed limits, no PQR is needed.

The shortcut is narrower than many people assume. In the 2025 edition the prequalified WPS tables cover SMAW, SAW, solid-wire GMAW, and FCAW with metal-cored GMAW. GTAW appears in the qualification tables of Clause 6 instead, so a TIG procedure under D1.1 normally has to be proven by a PQR.

Practical takeaway: read the top of any WPS for a reference to its authority, either a PQR/WPQR number or a statement that it is prequalified. A procedure that cites nothing has no visible proof behind it, and that is a question for the welding engineer.

How to Read a WPS, Section by Section

Layouts differ between companies and codes. ASME Section IX, for instance, accepts any format as long as every required variable is included or referenced. The content still falls into the same blocks, so read it in this order.

1. Header: process, code and revision

Start with the housekeeping. You are checking:

• the WPS number and revision
• the governing code and edition
• the supporting PQR or WPQR number
• the process (here, GTAW) and whether it is manual or machine welding

Revision control is the cheapest mistake to avoid and one of the most expensive to make. A superseded WPS can look identical to the current one.

2. Base metal

On an ASME-style WPS the base metal is grouped by P-Number, a classification of metals with similar welding behavior. P-1 covers common carbon steels, for example, and P-8 covers the austenitic stainless steels. You will also see a thickness range and, for pipe, a diameter range.

All three must fit your job. A matching grade does not help if the plate is thicker than the range the procedure was qualified for. Grouping exists because composition drives cracking sensitivity, preheat needs and heat-affected zone behavior, which the welding metallurgy basics guide covers in detail.

3. Filler metal

This block names the filler specification and classification, the diameter, and, on ASME forms, the F-Number (a grouping by usability characteristics) and A-Number (a grouping by weld metal chemistry). The classification itself carries information. In ER70S-2, for instance, the S identifies a solid wire.

Check the filler in your hand against this block, including the diameter. Wrong or mixed-up filler is easy to do and hard to spot afterward.

4. Joint design and backing

Here you find the groove type (single-V, J-groove and so on), included angle, root face, root opening, and whether backing is required, optional or prohibited. Many WPSs include a sketch. Others point you to the construction drawing, in which case the joint detail on that drawing becomes part of your procedure.

If the fit-up on the bench does not match the sketch, that is a problem to raise before welding, not after.

5. Position and progression

This block lists the positions the procedure covers (flat, horizontal, vertical, overhead) and, for vertical welds, whether you progress uphill or downhill. A procedure limited to the flat position says nothing about progression because it does not apply.

6. Preheat, interpass temperature and PWHT

These entries control how much heat the joint carries:

1. Minimum preheat sets the temperature before welding starts.
2. Maximum interpass temperature is a ceiling, not a target.
3. PWHT (postweld heat treatment) states whether heat treatment after welding is required, permitted or prohibited.

Read the PWHT line carefully. Under ASME guidance, a blank or an "N/A" does not prohibit heat treatment, and it does not document that none was applied. A well-written WPS says plainly something like "PWHT: Not permitted."

7. Shielding and backing gas

The WPS gives the gas type, the composition if it is a mixture, and a flow rate range. For TIG, argon is the common choice, with helium or argon/helium mixtures used to add heat on thicker material. Where the root needs protection, such as stainless pipe, there is usually a separate backing (purge) gas entry.

8. Electrical characteristics and tungsten

This is the block TIG welders look at first:

• Current type and polarity. DC electrode negative is typical for carbon steel and stainless steel, while AC is common for aluminum and magnesium because it breaks up the oxide layer.
• Amperage and voltage ranges, often listed separately for root and fill passes.
• Tungsten type and diameter.
• Pulsing. For GTAW, ASME treats the pulsed power source as a variable the WPS must address. A shop with no pulse capability writes "Not permitted," but if your machine has pulse and the WPS prohibits it, leave it off.

9. Technique

Technique entries cover stringer versus weave beads, the number of passes, travel speed, interpass cleaning and, sometimes, torch angle. Some of these are not code variables at all. That makes no difference to you: if the engineer wrote them on the WPS, you have to follow them.

Essential Variables: What You Can and Cannot Change

Not every number on a WPS is equally rigid. ASME Section IX sorts variables into three groups:

• Essential variables affect the mechanical properties of the weld. Moving outside the qualified range requires a new qualification.
• Supplementary essential variables apply in addition when the construction code demands impact (toughness) testing.
• Nonessential variables do not affect those properties. The WPS can be revised without requalification, but it still has to address them.

The ISO approach reaches a similar place through qualification ranges. Some changes always call for a fresh preliminary WPS and a new procedure test: a different welding process, a different consumable designation, or a different type of welding current, such as switching from DC to AC.

What this means in practice: inside the ranges, you adjust to suit conditions. Outside them, you do not experiment. You tell your supervisor, who takes it to the welding engineer.

Worked Example: Reading a TIG WPS Excerpt

The excerpt below is hypothetical and for learning only. Do not use these values for real production work.

Checking the heat input limit yourself

Heat input ties current, voltage and travel speed together. ASME audit guidance gives the check in one line: take the maximum voltage times the maximum amperage times 60, divide by the minimum travel speed, and compare the result with the maximum heat input recorded on the PQR.

In SI units that is volts × amps × 60 ÷ travel speed in mm/min, divided by 1,000 to give kJ/mm. Try a mid-range fill pass first:

• 12 V × 130 A = 1,560 W
• 1,560 × 60 = 93,600 J/min
• 93,600 ÷ 100 mm/min = 936 J/mm, or 0.94 kJ/mm

That passes. Now check the corner of the ranges, with the highest voltage and current at the lowest speed:

• 14 V × 130 A = 1,820 W
• 1,820 × 60 = 109,200 J/min
• 109,200 ÷ 80 mm/min = 1,365 J/mm, or about 1.4 kJ/mm

That exceeds the 1.0 kJ/mm limit even though every individual number sits inside its own range. The lesson is that ranges listed independently can combine to break a limit.

ASME guidance cautions that it is usually inadequate to state a heat input ceiling without giving the welder a practical way to stay under it, which is why better WPSs tie minimum travel speed to current. If you meet this situation in a real WPS, ask the welding engineer to clarify rather than guessing.

Codes also differ slightly in how they define heat input, and some apply a thermal efficiency factor. Use the formula your WPS names.

Common Mistakes When Reading a WPS

Most WPS problems are not technical failures. They are reading failures:

• Using the wrong revision. Confirm the number and revision against the job paperwork before you start.
• Reading the PQR as if it were the WPS. The PQR shows what was done on one test piece. The WPS shows what you are allowed to do.
• Treating a blank or "N/A" as permission. An omitted variable does not prohibit anything and does not prove it was never used.
• Ignoring a variable because your machine seems to make it irrelevant. A pulsing power source is the classic case.
• Checking ranges one at a time. As the example showed, heat input depends on the combination.
• Assuming a matching grade is enough. Thickness, diameter and P-Number ranges apply as well.
• Improvising when a condition cannot be met. Guidance for ASME audits is direct: a welder who cannot follow the WPS should notify the supervisor, who contacts the welding engineer. A welder found welding outside the WPS generates a nonconformance.

A Quick Check Before You Strike an Arc

Before the first pass, run through this list:

1. Does the WPS number and revision match the job paperwork?
2. Does it cite a PQR/WPQR or prequalified status, and is the code edition correct?
3. Do your material grade, thickness and pipe diameter fall inside the base metal ranges?
4. Do the filler classification and diameter match what you are holding?
5. Does the joint preparation match the sketch or drawing, including backing?
6. Are polarity, tungsten, gas type, flow rate and any purge set as written?
7. Have you measured preheat and interpass temperature rather than estimating them?
8. Does your own welder qualification cover this process, position and thickness range? It is a separate record from the WPS.

Who Writes the WPS, and Where Your Job Starts

Reviewing WPQRs and preparing production WPSs is a complex task that should always be done by a qualified welding engineer. For formal procedure tests, an examiner from an independent body usually witnesses the work and countersigns the record.

Your job starts when the paper reaches the booth: read it in full, follow it exactly, and speak up when something does not fit. Also check that the welding equipment is calibrated and that your qualifications are current and relevant to the procedure.

How to Read a WPS: The Bottom Line

Read a WPS in a fixed order: its authority first, then the material, consumables and joint, then the conditions around the weld, and last the parameters and limits. That sequence matches how the document was built, and it makes unusual entries stand out.

Treat every written entry as binding, remember that ranges can interact, and never read a blank as permission. When something is unclear or cannot be met, stop and ask. A one-minute conversation with the welding engineer costs far less than a rejected weld.