What is a WPS?
WPS: A WPS or a welding procedure specification is a written document that gives direction to the welder (or welding operator) to produce a sound weld that shall meet the minimum required mechanical properties.
Qualification is generally made to ASME IX, Qualification Standard for Welding & Brazing Procedures, Welders, Brazers, and Welding and Brazing Operators or AWS D1.1, Structural welding Code: Steel.
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Contents of a WPS
A properly written WPS contains at minimum the specific essential, supplementary, and non-essential parameters for the applicable welding process (It is required as per ASME Section IX to include all these parameters in the WPS, even they are non-essential but applicable to the required welding process. When the referencing Code, standard, or specification requires toughness qualification of the welded joint, then applicable supplementary essential variables shall also be provided in the WPS.
As outlined in QW-200.1(b), unlike PQRs, a WPS must address essential, nonessential, and when required supplementary essential variables. And again a great resource for determining how to address a variable can be found here:
The variables are listed in ASME Section IX QW-250 for each process.
A WPS must also reference supporting Procedure Qualification Record(s). Multiple PQRs can be used to create a new WPS.
A WPS can be revised if nonessential variables change without having to requalify a new PQR. An example of this would be P-No. 8, GTAW process, is qualified with Argon as a backing gas. The WPS can be revised to use a Argon / Helium / nitrogen mix as the backing gas without requalifying a new PQR
Welding Variables in a WPS
Welding variables such as joints, base metals, filler metals, welding positions, preheat, PWHT, gas, etc. (including ranges, if any) under which the welding process must be performed are the required parameters for a qualified WPS. These parameters are known as variables as per ASME Section IX and listed as mainly three types as:
- Essential Variable
- Non-essential Variable
- Supplementary Essential variable
What is a PQR?
PQR: A PQR list the welding variables which actually took place during the welding of the test coupon. For example, the actual base metal used, current & voltage used, and actual preheat and interpass temperature. Apart from the above, a WPS doesn’t contain a range in parameters as they are used in WPS. Click here and learn about PQR before you go further in this article.
So, what a WPS? is it in practical?
A welding procedure specification shall contain, as a minimum, the specific essential and nonessential variables that are applicable to the welding process. When the referencing Code, standard, or specification requires toughness qualification of the welded joint, then applicable supplementary essential variables shall also be provided in the WPS.
All these variables are listed in tables from QW-252 to QW-257. For each welding process, there is a separate table containing the list of all variables in ASME Section IX. E.g., a list of variables (essential, nonessential, and supplementary essential) for Shielded Metal Arc Welding (SMAW) are specified in table QW-253. Example of variables for SMAW as given in ASME Section IX is shown in below picture.
How to prepare & qualify a WPS (Steps):
- Identify the welding process to be used. Either a single process or a combination of welding processes for example it’s a SMAW or GMAW or GTAW or any other welding process. Also, if it’s a combination of two or more welding processes. Once the welding process is decided then we need to see the corresponding table (QW-252 to QW-257) for the complete list of welding variables that need to be addressed. With the help of the table, we can list out all the necessary variables to be used in our WPS. It is advised to use this form to make a pWPS.
- Check the thickness, material grouping applicable for production weld joint and choose a thickness for PQR. Refer to Table No. QW-451 for the thickness range qualified based on the material thickness used on PQR. Below is Table QW451 for reference. pWPS is called preliminary WPS or proposed WPS.
Steps for writing a WPS (Continues)
- Select the filler wire based on the mechanical properties of the base metal. Filler wire strength is always higher than the base metal. If you are new in this field, use consumable manufacturer handbooks to choose filler wire. These handbooks list the base metals for each welding electrode or wire type, they can be used.
- You can take for welding consumable manufacturers handbook for reference welding parameters such as current & voltage etc. for the welding consumable you are planned to use for your PQR qualification.
- After successful welding, the test coupon (or coupons) is subjected to the destructive test, and if the test coupon (or coupons) meets the minimum code requirement then the same pWPS is finalized and approved for further job.
- Tensile test and bend test are the minimum tests required as per ASME Section IX for PQR qualification.
- It is to be noted that NDT such as Radiographic testing is not a mandatory requirement for WPS qualification. Although, we do RT to skip any area which may have internal welding defects. These defects can make the test sample such as bend or tensile fail early and our PQR qualification will fail eventually. So, it is better to perform RT before mechanical testing. All the real-time data during welding of test coupon and destructive test report data is compiled into one document known as Procedure qualification record (PQR).
- The results of the PQR are reviewed against the minimum required mechanical, hardness, toughness or any additional testing which is required. Generally, a WPS is deemed to pass if it passes the bend test and meets the below requirements for the tensile test :
In order to pass the tension test, the specimen shall have a tensile strength that is not less than :
(a) the minimum specified tensile strength of the base metal; or
(b) the minimum specified tensile strength of the weaker of the two, if base metals of different minimum tensile strengths are used; or
(c) the minimum specified tensile strength of the weld metal when the applicable Section provides for the use of weld metal having lower room temperature strength than the base metal;
(d) if the specimen breaks in the base metal outside of the weld or weld interface, the test shall be accepted as meeting the requirements, provided the strength is not more than 5% below the minimum specified tensile strength of the base metal.
(e) the specified minimum tensile strength is for full-thickness specimens including cladding for Aluminum Alclad materials (P‐No. 21 through P‐No. 23) less than 1 /2 in. (13 mm). For Aluminum Alclad materials 1 /2 in. (13 mm) and greater, the specified minimum tensile strength is for both full-thickness specimens that include cladding and specimens taken from the core.
- All these results are recorded in the PQR and PQR is considered as qualified. Now, WPS is made based on the PQR variables, and the range of variables are given as permitted in BPVC ASME Section IX for each variable.
Important Articles in ASME Section IX related to WPS qualification.
Article I Welding General Requirements
QW-150 Tension Tests
QW-153 Acceptance Criteria-Tension Tests
QW-160 Guided-Bend Tests
QW-163 Acceptance Criteria-Guided Bend Tests
QW-171 Toughness Tests
QW-172 Toughness Tests-Drop Weight
QW-180 Fillet-Weld Tests
QW-182 Fracture Tests
QW-183 Macro Examination-Procedure Specimens
Article II Welding Procedure Qualifications
QW-210 Preparation of Test Coupon
QW-250 Welding Variables (Refer Table-1)
Article IV Welding Data
QW-401.1 Supplementary Essential Variable (Procedure)
QW-403 Base Metal
QW-404 Filler Metal
QW-407 Post Weld Heat Treatment
QW-409 Electrical Characteristics
QW-420 Base Metal Groupings
Table QW/QB-422 Grouping of Base Metals for Qualification
Table QW-432 Grouping of Electrodes and Welding Rods for Qualification
QW-440 Weld Metal Chemical Composition
Table QW-442 A-Numbers Classification of Ferrous Weld Metal Analysis for Procedure Qualification
Table QW-451.1 Groove-Weld Tension Tests and Transverse-Bend Tests
Table QW-451.2 Groove-Weld Tension Tests and Longitudinal-Bend Tests
Table QW-451.3 Fillet-Weld Tests
Table QW-451.4 Fillet Welds Qualified by Groove-Weld Tests
QW-461.3 Groove Welds in Plate-Test Positions
QW-461.4 Groove Welds in Pipe-Test Positions
QW-461.5 Fillet Welds in Plate-Test Positions
QW-461.6 Fillet Welds in Pipe-Test Positions
QW-462 Test Specimen Table QW-462.1(a) to QW-462.5(e) Fig QW-463.1(a) to QW-469.2
QW-470 Etching — Processes and Reagents
Article V Standard Welding Procedure Specifications (SWPSs)
How to combine two WPS/ PQR?
As outlined in QW-200.4, more than one WPS maybe used to complete a single production joint. For the most part combining WPSs is straightforward, if a WPS is qualified for 1/16” to ¾” base metal and weld metal thickness, it can be combined with another WPS is qualified for the same thickness ranges and be used with those ranges. If you have a WPS that is qualified for 1/16” to ½” and you combine it with a WPS that is qualified from 3/16” to 1”, you would be limited to only using this combination of WPSs for material from 3/16” to ½” thick. This is also specified in interpretation IX-83-80. To view this interpretation go to: https://cstools.asme.org/Interpretation/InterpretationDetail.cfm?TrackingNumber=5495
The Code does permit a special case as outlined in QW-200.4(b), when one PQR has a test coupon of at least ½” thick, it can be combined with one or more PQRs with base metal thicknesses greater than ½”. When these requirements are met, you can use for example a GTAW process with base metal thickness range on the WPS of 3/16” to 1” (PQR ½” base metal thickness used), combine it with a SAW WPS with base metal thickness of 3/16” to 8” (PQR 1.5” base metal thickness used). A new WPS can be created with a base metal thickness range of 3/16” to 8” with GTAW being used up to 1” thick and the remainder with SAW.
Qualifying Dissimilar Base Metals
One Common issue that is faced in welding of groove welds between two base metals that have different thicknesses. ASME Section IX, QW-202.4 address this issue as:
- The thickness of the thinner member shall be within the range permitted by QW-451.
- The thickness of the thicker member shall be:
- For P‐No. 8, P‐No. 41, P‐No. 42, P‐No. 43, P‐No. 44, P‐No. 45, P‐No. 46, P‐No. 49, P‐No. 51, P‐No. 52, P‐No. 53, P‐No. 61, and P‐No. 62 metal, there shall be no limitation on the maximum thickness of the thicker production member in joints of similar P‐Number materials provided qualification was made on base metal having a thickness of 1/4 in. (6 mm) or greater.
- For all other metal, the thickness of the thicker member shall be within the range permitted by QW-451, except there need be no limitation on the maximum thickness of the thicker production member provided qualification was made on base metal having a thickness of 11/2 in. (38 mm) or more.
An example of what this means:
- You have a WPS qualified for base metal thickness of 1/16” to ½” for P-No. 8 material. The pressure vessel being fabricated has a 1” nozzle with wall thickness ¼” and it is welded to a 16” 300# B16.5 flange. Is the WPS qualified to be used? The answer is yes.
- You have a WPS qualified for base metal thickness of 1/16” to ½” for P-No. 1 material. The pressure vessel being fabricated has a 1” nozzle with wall thickness ¼” and it is welded to a 16” 300# B16.5 flange. Is the WPS qualified to be used? The answer is no.
- You have a WPS qualified for base metal thickness of 1/16” to 8” for P-No. 1 material. The pressure vessel being fabricated has a 1” nozzle with wall thickness ¼” and it is welded to a 16” 300# B16.5 flange. Is the WPS qualified to be used? The answer is yes.
When requalification of a WPS/ PQR is required?
The question of “When do I need to requalify my WPS?” is not asked that often, but it arise many times. A welding procedure specification (WPS) as we know is a set of instructions that if followed, will insure a sound weld. A WPS will typically provide ranges rather than single values for some variables such as current, voltage, and travel speed. Nonetheless, if the values used for making a weld are outside of the parameters of the WPS, then it has been violated by the WPS user.
So, when should you requalify a WPS? The answer is always, when the code or specification require it. When doing code work, you should always reference the code you are working to for guidance. Most codes have similar requirements as to when requalification of a WPS is necessary. In general, any change beyond the limits of the procedure qualification record (PQR) will require requalification. If any of these variables change consult your code or specification to see if you need to requalify a WPS based on the changes happened outside the qualified range.
Example for a WPS requalification requirements:
Following is an example of reviewing whether or not a requalification is necessary. XYZ Ltd. is doing work governed by the AWS D1.1, Structural Welding Code – Steel. For a cost reduction and efficiency improvement measure the company decides to make the following changes to the existing qualified WPS:
- Switch to .052” ER70S-6 MIG wire from .045” ER70S-6 wire for higher deposition rates in some areas.
- Increase Welding amperage from 250A to 265A with .045” ER70S-6 wire to increase deposition rates in remaining areas.
- Change from tapered nozzles to ¾” ID coupled with increase from 40cfh to 45cfh to reduce issues with porosity.
- Switch their shielding gas mixture from 75% argon/25% CO2 to 90% argon/10%CO2 to reduce spatter and decrease grinding.
- Some welds will be made vertical up to improve penetration and reduce leaks (using same parameters).
If we consider each of these changes individually here is how things would conclude. We reference AWS D1.1, Structural Welding Code – Steel – Table 4.5 – PQR Essential Variable Changes Requiring WPS Requalification for SMAW, GMAW, FCAW, and GTAW and review each change.
- Switch from .045” to .052” – Table 4.5(10) indicates that “any increase” in filler metal diameter shall require requalification.
- Amperage increase from 250 to 265 staying with .045” wire – Table 4.5(12) requires requalification if the change in amperage exceeds 10%; therefore, we do not need to requalify in this case since a 15 amp increase only represents a 6% change.
- Change in nozzle size and gas flow rate – Table 4.5 does not mention changes in nozzle size as an essential variable. However, when in doubt go to the engineer or to the customer for clarification. The change is gas flow rate is does not require requalification per Table 4.5(20). A change is only necessary if the flow rate goes up by more than 50% or is reduced by more than 20%.
It is important to consider the effects of changes here as well. A change in nozzle may not be an essential variable, but if the larger nozzle hampers access and requires the welder to run a longer contact tip to work distance this could decrease the amperage to the point which require requalification.
- Switch from 75/25 to 90/10 argon/CO2 shielding gas – Table 4.5 (19) requires a change when the gas composition is changed exceeding code limits. In this case a change of more than 10% of the minor component requires requalification.
- Addition of vertical up welds – requires qualification as well per Table 4.5 (30) which states that requalification is needed “for any pass from uphill to downhill or vice versa.”
Sound knowledge of the welding process and understanding of the code or specification involved is critical for right decision.