How To Write A WPS

This article discusses a few things that a welding engineer must keep in mind while writing a WPS, in accordance with Section IX of ASME BPVC. WPS is short for Welding Procedure Specification.

Being a welding engineer myself, I have touched upon a few things that do not find an obvious mention in the Section IX. Some things can only be learnt through experience.

The information contained herein may be useful to not just a welding engineer, but also the inspector who has to review the WPS. The inspector needs to verify its’ compliance to Section IX.

Occasionally, apprentices/trainees/interns come to my workplace. I task them with reviewing random WPSs. And ascertain whether the WPSs in my shops and the PQR(s) supporting them are Section IX compliant or not. This article may be useful to such apprentices as well.

Note that this entire discussion is pertinent only in reference to Section IX of ASME Boiler and Pressure Vessel Code.

how to write a WPS

What Must A WPS Contain?

Per QG 101, a procedure specification must address “…the essential and non-essential variables that are applicable…” to the process used in the production. This gets further reiterated in QW-101 and QW-200.2 (b) as well.

Of course, these clauses also go on to say that, when applicable – supplementary essential variables shall also be addressed on a WPS.

A notable thing here is that both these clauses mention the word ‘minimum’. That is, the WPS must address, as a minimum, the essential and non-essential variables. This means that the welding engineer is free to add any additional information that he thinks is pertinent to make a code compliant joint.

This additional information may be general, or pertaining to a specific production weldment.

For example, sometimes, he may mention the drag angle or push angle that welder needs to use. Or, sometimes, he may include multiple electrode sizes, and mention that any one or any combination of those sizes may be used while making production weld.

The number of things that are not mandated in the QW 482 format, but a welding engineer is free to include while writing the WPS are many.

Keep in mind is that the WPS is the mode of communication from the welding engineer to the welder. The WPS is meant to give direction to the welder to make a good weld, and a code compliant one.

Is It Necessary That Welder Have A Copy Of WPS?

It is true that the primary objective of the WPS is to give direction to the welder. However, it is not necessary that the welder must have a copy at all times. If his immediate supervisor gives necessary directions to him verbally, that is acceptable too.

It is equally acceptable if a copy is given to the welder too.

In my shops, a copy of the WPS is given to the welder before starting a weld.

However, as QG-101, QG-102, and QG-104 require, the WPS, PQR and WPQR respectively shall be available for review whenever required.

Freedom to Welding Engineer

As noted above, Section IX is a minimum set of requirements. It tells a few things that must be done, a few things that must not be done, and some non-mandatory guidance.

Section IX does not cover by any means all aspects related to writing of WPS and qualifications of procedures and performances. There is a wide spectrum of things that the welding engineer must decide.

Keeping the requirements of the application in mind, the welding engineer must exercise engineering judgment while writing a WPS and qualifying procedures and performances.

As long as all essential, non-essential and supplementary essential variables are addressed, he can include any information that he thinks would be helpful to the welder; and would fulfil requirements of the application.

However, engineering judgment should be consistent with the code’s philosophy and must not be used to override the mandatory requirements laid down by the code.

For example, preheat is a non-essential variable for welder qualifications. In fact, QW-301.2 specifically states that preheat may be omitted when using a WPS requiring preheat for performance qualification.

In my shop, almost all production jobs are with preheat. I decided that welding with preheat is a skill, and therefore all performance qualifications in my shop are done with preheat.

Another example is back gouging. The back gouging is a not a variable for welder qualifications. In my shop, many joints require back gouging (after which weld metal is deposited from the back side), which we do using un-copper coated carbon gouging rods.

I decided that the welder who does the back gouging must be sufficiently familiar with it beforehand.

Performance qualifications in my shop therefore require the welder to do back gouging, and deposit weld metal from the back side.

Engineering Judgment

Although there can be many examples of engineering judgement, here is another instance.

Section IX does not address under what circumstances we must do PWHT of a coupon. That is, it does not say what kind of qualifications must undergo PWHT, and what kind can be exempted from it. This kind of thing is generally addressed by the construction code.

The welding engineer therefore must decide, based on experience and guidance from construction code, whether or not to include PWHT for a procedure qualification.

Now, say, a PQR has been qualified with 1 ¼ inches (32 mm) base metal thickness, without PWHT. Per QW-451, this PQR can be used to support production welds till 2 ½ inches (64 mm) BM thickness. Would it be alright to leave a weld on a 2 ½ inch thick BM without PWHT? No, it would not.

There are no checks in Section IX to prevent this. Here, the welding engineer must exercise engineering judgement to prevent this from happening.

Another example is that of filler metals. The Section IX does not address what filler metals should be used with what base metals. So can one use E347-15 rods to weld carbon steel metals?

No, surely not. However, there are no checks in Section IX to prevent such things from happening.

The filler metal must have a comparable chemical analysis and tensile strength to base metal. This judgment must be made by welding engineer, with guidance from construction code and design of the application.

The moral of the lesson is that Section IX is not a how-to guide. It is not a handbook. It is only a set of minimum rules that must be followed in WPS writing and qualification of procedures and performances.

Section IX relies upon the welding engineer to take into account the requirements of the application and regulate welding activities with respect to those requirements.

Any Format is Fine

There is a format for WPS given at Appendix B of Section IX.

Another thing to note here is that the format QW-482 is only a suggestive one. The format QW-482 given at the Appendix B says as much on the top of the page. Section IX gives us liberty to write a WPS in any format we want as long as all essential and non-essential variables are addressed. This has been communicated in QW 200.1 (d) as well.

Common Mistakes In Writing WPS

Below are a few errors that welding engineers tend to make while writing WPSs.

Non-essential Variables Need Not be Addressed on a WPS

Sometimes, beginners tend to think that non-essential variables need not be addressed on the WPS. The reasoning goes that since a change in these variables does not have any effect on the mechanical properties, these are trivial. And therefore, these need not be addressed on the WPS.

Also, it appears sometimes that some of the non-essential variables are not pertinent to a joint, and so, need not be addressed. For example, a WPS has been written showing welding a groove joint welded from a single side. There would not be any back gouging. One might think that the ‘method of back-gouging’ field can be left blank, or unaddressed in this WPS.

It must be noted that it is the intent of Section IX that each non-essential variable relative to the welding process, whether or not it is pertinent to the particular WPS, must be addressed on the WPS in some way. This is clearly specified in QG-101 and QW-101. This was further reinforced through Interpretations IX-89-03 (qs 1) and IX-17-46 (qs1).

This was until the 2019 edition of the Section IX. The 2019 edition brought in a change (under QW-101) that gave a long overdue relaxation from this stipulation. A line has been introduced under QW-101 that basically conveys that the variables that are not pertinent to a WPS or the variables that are indirectly addressed through other variables need not be addressed on a WPS.

Let us see a few examples of this.

Some variables are indirectly addressed through other variables.

For example, QW 404.23 – which requires that a change from one product form to another (for example from solid wire to flux cored wire, etc.), is an essential variable. This means that the product form of the filler given on the WPS must be specified at the appropriate place on WPS.

Now, say the filler metal ER70S-A1 has been specified on the WPS. The ‘S’ in the filler designation indicates that it is a solid rod (as against a composite rod, or metal cored rod).

In other words, information regarding the product form of the filler is contained in the filler metal designation itself. For such cases, it is not needed to address the product form separately on the WPS.

Another example is QW 410.64, which says that when P-No. 11A and 11B are involved, the groove preparation for the qualification coupon shall be done using thermal processes when such processes are required to be employed in the fabrication.

Now, when a WPS has been written for P-No. 1 or P-No. 3 metal (for example), it precludes the possibility that P-No. 11A or 11B is involved. For this  case too, QW 410.64 need not be specifically addressed on the WPS.

Another example is QW 406.2 which says that maintenance of preheat upon completion of welding prior to any PWHT is a non-essential variable. Now, if a WPS has been written without PWHT (that is, in as –welded condition), this variable becomes redundant.

It is understood, without having to specify it separately on WPS. So in this case too, it is not required to specifically address this variable on the WPS under such conditions.

Another example is QW 404.34 which is applicable only for P-No. 1 metals. So if a WPS is written for any P-No. other than P-No. 1, this variable need not be specifically addressed on the WPS.

The examples given above are those that are either intrinsically addressed through other variables, or outside the scope of the WPS. Section IX now intends that such variables need not be specifically addressed on the WPS.

Wording to this effect has been introduced in the 2019 edition of the Section IX under paragraph QW-101. This somewhat modifies the old understanding established through the interpretations IX-89-03 (qs 1) and IX-17-46 (qs1).

For such variables that are not pertinent to a WPS, writing a N/A should be enough.

The principle that may be kept in mind is that each non-essential variable is required to be addressed, except the variables that are not pertinent to the WPS, or the variables that are indirectly addressed through other variables.

Also, Section IX does not mind how a variable is addressed. It may be addressed directly, indirectly, through a sketch, or even through a reference to another document. Guidance to this effect appears in the Interpretations IX-10-16 and IX-17-46 (qs 2).

Non-essential variables can be violated in production welding

Another mistake that the beginners (or people not intimate with Section IX) tend to make is that the non-essential variables on the WPS can be violated during the production welding.

The reasoning behind this goes that since (in Section IX’s own words) change in non-essential variables does not affect mechanical properties of a weld.

For example, a change in the groove design is a non-essential variable. That is, change in groove design (U-groove, V-groove) etc. does not require re-qualification of procedure. In other words, a procedure qualified with a V-groove can be used to support a production joint with U-groove as well.

Say, a WPS has been written showing a V-groove. It is tempting to think that this WPS can be used for welding a joint with U-groove too.

This is incorrect.

Once a WPS has been issued, the welder must follow it to the last word. If there needs to be a change in non-essential variables in the production welding, the welding engineer must revise the WPS. Doing so will not require re-qualification of the WPS. The revision would only be editorial.

In any case the welder cannot violate the directions given on the WPS.

In the above case, the welding engineer must revise the WPS (or issue a new one) showing a U-groove. This can be done without re-qualifying the procedure.

Related: How to Address Groove Design and Root Spacing on a WPS

It is of course needless to say that if there is a change in any of the essential variables, the PQR must be re-done with the changed variable(s).

Leaving A Blank Is An Acceptable Way Of Addressing A Variable

Another mistake that beginners make is thinking that the fields for variables that are not applicable can be left blank.

For example, a WPS has been written for production joints that will remain as-welded. The WPS has been supported by a PQR that did not involve PWHT of the qualification coupon. In such a WPS, it is a mistake to leave the PWHT field blank.

Also, it is not appropriate to write N/A to address this variable.

Leaving a blank against any variable in a WPS is not acceptable according to the philosophy of Section IX.

Each variable must be suitably addressed, although Section IX does not specify how a variable should be addressed. It is left to the wisdom of the welding engineer to suitably address each of the variables.

This understanding, that leaving a blank against a variable is not acceptable, has been further reinforced through an Interpretation. The interpretation number is IX-83-03 (qs 1).

In the aforementioned case, it must be clearly specified in the WPS in the PWHT field that ‘PWHT is not permitted’, or ‘Not permitted’, or ‘No PWHT’, etc.

Another thing to note here is that only the variables pertaining to the welding process (for which the WPS is being written) need to be addressed.

For example, flux type (QW 404.34) is an essential variable, but only for SAW process. So, it need not be addressed (a ‘N/A’ should suffice) on a WPS being written for SMAW process.

How to Address Non-essential Variables on a WPS?

We have seen in the above discussion that it is compulsory to address the non-essential variables on a WPS. But how does one go about it?

How does one know what root gap should be adopted? What joint design should be adopted, whether or not backing should be used, etc.?

We know that the essential variables on the WPS have to be addressed keeping in mind the PQR. Section IX defines the range qualified for all essential variables vis-a-vis the PQR. So the task of welding engineer is cut out while addressing the essential variables.

But the matter is not so straight forward in case of non-essential variables.

The answer to the questions in the first paragraph above is that these values have to be addressed based on judgment, design requirement and construction code requirement.

The construction code defines sometimes (but not all construction codes) the acceptable/recommended root spacing, reinforcement, joint design, etc.

Usually, the design department (or the engineering department) prepares fabrication drawings to be used in shop. These drawings are made to conform to the construction code. So, some of these values (type of joint, bevel angle, root spacing, etc. can be adopted from these drawings. Or simply, a reference to the drawing can be given on the WPS.

All non-essential variables that cannot be addressed by referring fabrication drawing or the construction code. Such variables have to addressed based on engineering judgment.

So these were a few observations on the subject of writing a WPS in accordance with Section IX of ASME BPVC. Please feel free to share your thoughts in the comments section below.

Thanks for reading.

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