A Welding Procedure Specification (WPS) is a formal written document describing welding procedures, which provides direction to the welder or welding operators for making a sound and quality production welds as per the code requirements .
It is essential that all the welded joints are sound and have suitable properties for their application when fabricating structures and pressurized items by welding. WPSs control the welding by providing detailed written instructions about the welding conditions that must be used to ensure that welded joints have the required properties.
Although WPSs are shop-floor documents designed to instruct welders, welding inspectors need to be familiar with them because they will refer to them when checking that welders are working within the specified requirements.
Welders need to be able to understand WPSs, make non-defective welds and demonstrate these abilities before being allowed to make production welds.
Qualified Welding Procedure Specifications
It is the standard practice for the to use qualified WPSs for most applications. A welding procedure is usually qualified by making a test weld to demonstrate that the properties of the joint satisfy the requirements specified by the applicable standard and the client or the end user.
Demonstrating the mechanical properties of the joint is the principal purpose of qualification tests, but showing that a defect-free weld can be produced is also very important.
Production welds made in accordance with welding conditions similar to those used for a test weld should have the same properties and therefore suitable for their intended purpose.
A Typical Sequence for Welding Procedure Qualification By Means of A Test Weld.
- The welding engineer writes a preliminary Welding Procedure Specification (pWPS) for each test coupon to be welded.
- A welder makes the test coupon in accordance with the pWPS and a welding inspector records all the welding conditions used to make the test coupon (the as-run conditions). An independent examiner/examining body/third party inspector may be requested to monitor the procedure qualification.
- The test coupon is subjected to NDT in accordance with the methods specified by the Standard – visual inspection, MT or PT and RT or UT.
- The test coupon is destructively tested (tensile, bend, macro tests). The code/application standard client may require additional tests such as hardness, impact or corrosion tests –depending on the material and application.
- A WPQR is prepared by the welding engineer
giving details of:
- As run welding conditions.
- Results of the NDT.
- Results of the destructive tests.
- Welding conditions allowed for production welding.
If a third-party inspector is involved, he will be requested to sign the WPQR as a true record of the test.
Welding Standards for Procedure Qualification
The European and American Standards have been developed to give comprehensive details about:
- How a welded test piece must be made to demonstrate joint properties.
- How the test piece must be tested.
- Which welding details need to be included in a WPS.
- The range of production welding allowed by a particular qualification test weld.
The principal European Standards that specify these requirements are:
- EN ISO 15614- Specification and qualification of welding procedures for metallic materials, welding procedure test.
Part 1- Arc and gas welding of steels and arc welding of nickel and nickel alloys.
Part 2 – Arc welding of aluminum and its alloys.
The principal American Standards for procedure qualification are:
- ASME Section IX – Pressurized systems (vessels and pipework).
- AWS D1.1 – Structural welding of steels.
- AWS D1.2 – Structural welding of aluminum.
The Qualification Process for Welding Procedures
Although qualified WPSs are usually based on test welds made to demonstrate weld joint properties; welding standards also allow qualified WPSs for some applications to be written based on other data.
Some alternative ways that can be used for writing qualified WPSs for some applications are:
- Qualification by the adoption of a standard welding procedure – test welds previously qualified and documented by other manufacturers.
- Qualification based on previous welding experience – weld joints that have been repeatedly made and proved to have suitable properties by their service record.
Procedure qualification to European Standards by a test weld (similar in ASME Section IX and AWS) requires a sequence of actions typified by those shown below;
The Relationship Between a WPQR and a WPS
Once a WPQR has been produced, the welding engineer can write qualified WPSs for the various production weld joints that need to be made.
The welding conditions that are allowed to be written on a qualified WPS are referred to as the qualification range and depend on the welding conditions used for the test piece (as-run details) and form part of the WPQR.
Welding conditions are referred to as welding variables by European and American Welding Standards and are classified as either essential or nonessential variables and can be defined as:
- Essential variable – A variable that has an effect on the mechanical properties of the weldment and if changed beyond the limits specified by the standard will require the WPS to be re-qualified.
- Non-essential variable – A variable that must be specified on a WPS but does not have a significant effect on the mechanical properties of the weldment and can be changed without the need for re-qualification but will require a new WPS to be written.
Because essential variables can have a significant effect on mechanical properties they are the controlling variables that govern the qualification range and determine what can be written in a WPS.
If a welder makes a production weld using conditions outside the range is given on a particular WPS, there is a danger that the welded joint will not have the required properties and there are two options:
- Make another test weld using similar welding conditions to those used for the affected weld and subject this to the same tests used for the relevant WPQR to demonstrate that the properties still satisfy specified requirements.
- Remove the affected weld and re-weld the joint strictly in accordance with the designated WPS.
Most of the welding variables classed as essential are the same in both the European and American Welding Standards but their qualification ranges may differ.
Some application standards specify their own essential variables and it is necessary to ensure these are considered when procedures are qualified and WPSs wrote.
Examples of essential variables (according to European Welding Standards) are;
- AWS D1.1/D1.1M: “Structural welding code – Steel”
- ASME Boiler and Pressure Vessel Code section IX: “Qualification Standard for welding and brazing procedures, welders, brazers and welding and brazing operators”, Article II, Section QW-200.
- EN ISO 15607:2005 “Specification and qualification of welding procedures for metallic materials – General rules (ISO 15607:2003)”
- API 1104: “Welding of pipelines and related facilities”, parts 3 (Definition of Terms)
- CSWIP 3.1 Course note