Introduction
The WPS (Welding Procedure Specification) is a technical document that sets out all the important items that must be considered when joining parts by welding.
The WPS contains limits or ranges of parameters such as current type, base metal thickness, base metal type, etc.
A welding procedure is only valid within the limits specified therein.
The WPS must be prepared based on the specifications of the designer, and/or manufacturer of the equipment or structure, welding consumables, base materials and according to the specification of the performer, observing the requirements established by the applicable standards or codes.< br />
When is a new WPS required?
If a procedure cannot produce good quality welds without exceeding the established limits, then a new welding procedure must be used.This is a simple statement and is valid for all types of welding variables, see more details below.
Qualification of the welding procedure
Most qualification standards require the welding procedure to be qualified before it is put to use in the production of welds. This is necessary to ensure that the welded joint reaches the required quality.The materials used in the manufacture of equipment have known mechanical properties. Equipment design is based on these properties.
When equipment is manufactured by welding two or more materials, it is necessary to ensure that this union resists in the same way as the materials used. For that, the designer needs to know which properties the welded joint will have.
In welding, due to the effects of temperature, it is not enough to know only the properties of the base metal and the weld metal individually.
It is also necessary to know the mechanical properties of the entire welded joint (The base metal, the heat-affected zone and the weld metal). This is done through the qualification of the welding procedure.
Welding procedure qualification is the method by which a particular procedure is proven to be adequate to produce welded joints of satisfactory quality.
Qualification is carried out by welding test pieces, according to the previously established procedure, and by evaluating the results of tests on specimens extracted from the test piece.
The results are evaluated against the acceptance criteria established by the applicable qualification standard.
After welding the test piece, the welding parameter ranges must be reviewed to bring the values actually used into line with the tolerances allowed in the applicable qualification standards.
Prequalified Welding Procedures
These are welding procedures that can be used when experience and familiarity with certain base metals and welding consumables have proven the suitability of a specific procedure through service performed over a long period of time.The use of pre-qualified procedure is only done when allowed by the procedure qualification standard, specifications, manufacturing standards, etc.
For pre-qualified procedures, it is not necessary to perform the qualification tests.
When the use of a pre-qualified procedure is not permitted (or, for example, when the procedure does not meet the requirements to qualify as a pre-qualified procedure), the welding procedure must be proven adequate, through tests and/or examinations. And evaluation of results, as required by the standard or specification adopted.
The requirements for the qualification of welding procedures vary widely from one standard to another.
A qualification made according to one standard is generally not valid for another standard.
It is therefore necessary that the welding inspector is aware that the requirements of the applicable standard are being followed, in the qualification of procedures.
Prequalified or already qualified?
It is quite common for people to confuse a "pre-qualified" procedure with an "already qualified". From a technical point of view there are differences.The “already qualified” procedure is the one that has been tested/submitted to tests in accordance with the qualification standards by the company performing the services. The document generated by this process is an WPQR and then an WPS is generated for the fabrication/work.
The “prequalified” procedure and all its criteria are taken directly from the standard. The document generated by this process is an WPS and the WPQR is not required.
An “already qualified” procedure is normally accepted in new works, as this reuse is foreseen in the qualification standards themselves.
However, its use cannot be indiscriminate and a professional with technical knowledge is required to analyze the applicability of an “already qualified” procedure in a new work.
The use of pre-qualified procedures, on the other hand, is already much less accepted. Note that pre-qualified procedures are not accepted by some companies.
My opinion is that this decision is extremely conservative and even retrograde. It's amazing how we waste money having to (re)do a qualification of a welding procedure under the conditions below:
- Material with excellent weldability such as low carbon steel.
- Uses such as stairs.
- Very detailed conditions and parameters, such as those provided in AWS D1.1.
Preliminary Welding Procedure Specification (pWPS)
The preliminary WPS is a document prepared based on knowledge, past experience, or trial and error “technique”. It is the description of how you want to weld your test piece.More often than not, the welding parameters suggested in the pWPS will have minor adjustments on the part of the welder performing the qualification. We have to ensure that these actual changes or parameters will be incorporated into the WPQR.
After qualification welding, the test pieces are tested and if they pass the test, the qualification is successful. The WPQR can be elaborated and will be the basis for the construction of the welding procedure specification (definitive WPS).
Note that preliminary WPS is not mandatory in all standards, but it is still good engineering practice. ASME IX, for example, does not require preliminary WPS, see interpretation IX-10-22.
Qualification Standards
The performance of improper welds during the fabrication of certain types of structures or equipment, such as ships, bridges, pipelines, automotive components and pressure vessels, can result in serious accidents with material and, eventually, human losses and damage to the environment.To minimize the chance of these occurrences and ensure greater uniformity, control and traceability of the process, welding operations for different applications and various other aspects related to welding are regulated by different codes and specifications.
Mandatory requirements regarding qualification of welding procedures, welders and welding operators are included or referenced in these documents.
It is the responsibility of the welding engineer to determine, through the design specifications or purchase documents, the standard that regulates welding and what qualification requirements are determined in these documents.
ASME Section IX – Welding and Brazing Qualifications
It is a standard applied to equipment such as boilers, pipelines, pressure vessels, nuclear components. All sections of the “ASME Boiler and Pressure Vessels” standard refer to “Section IX” for qualifications.API Std. 1104 – Standard for Welding Pipelines and Related Facilities
It is a unique qualification standard for pipeline welding. Generally limited to terrestrial pipelines here in Brazil.AWS D1.1 – AWS Structural Welding Code – Steel
This standard is dedicated to the welding of metallic structures in steel. There are other “D”s that deal with the welding of aluminum, stainless steel etc…This document contains its own requirements for qualifications, which are mandatory when welding work must be in accordance with AWS D1. 1.
The table below shows the most common design and manufacturing standards for equipment with the specified qualification standards.
Once the qualification standard has been determined, specific requirements for each case can be established for the qualification of procedures and of welders and welding operators.
As mentioned before, qualifications are made by evaluating the results of tests carried out on specimens extracted from test pieces, welded according to a previously established procedure.
Preparation of test pieces
The preparation of test pieces is based on the requirements of the applicable qualification standard and the information of the welding procedure to be qualified.Items related to test piece preparation are linked together.
Thus, the analysis must be joint, keeping in mind that the smallest number of test pieces is always desirable, within the limitations of the applicable standard.
Types of test coupons
The test piece can be a tube, a sheet or a combination of these. The type is not always considered an essential variable, that is, a variable for which requalification is necessary in case of changes.In general the test piece should be representative of the work to be performed. If the qualification is for tube welding, possibly the test piece should be a tube, in order to reduce the number of test pieces needed.
Material of the test coupon
In general, the test pieces must be of the same material as the equipment. To reduce the number of qualifications, the standards define exceptions that, whenever possible, should be adopted.The standards group materials in order to facilitate the use of materials similar to the equipment to be welded. For example, according to ASME Sec. IX, the test piece must have the same “P number” as the equipment (materials with the same weldability are grouped under the same number).
Also, according to ASME Sec. IX, for the qualification of welders, the material may be carbon steel (number P = 1), even if the equipment is of another material (see ASME Sec. IX QW-310.4).
The choice of material must always be based on the requirements of the standards and, within the allowed exceptions, on the availability and cost of the material.
Dimensions of the test coupon
The test piece must have dimensions that allow the removal of the predicted specimens. In order to determine the length of a test plate, for example, it is necessary to know the quantity and width of the specimens. The width of the test piece depends on the length of the specimens.The AWS D1 Standard. 1 directly establishes the dimensions, leaving only the inspector to identify the specific design to be used.
The ASME Sec. IX shows the distribution and dimensions of the specimens separately, leaving it to the inspector to establish the dimensions to be used.
NOTE: Dimensions are indicated in mm and represent minimum dimensions
Thickness of the test coupon
One of the most important factors to determine before test piece preparation is the thickness of the piece. This is because the thickness of the test piece determines the thickness limits qualified by the test.In general, the thickness of the test piece should be at least half of the maximum thickness established in the procedure. In any case, the thickness of the test piece is representative of a certain range of thicknesses. The API Std standard. 1104, for example, limits the qualification to certain thickness groups.
Proper choice of thickness will reduce the number of test pieces.
Diameter of the test coupon
When the test piece is a tube, the influence of the diameter on the validity of the qualification must be analyzed.Qualification standards differ in this regard. According to ASME Sec. IX, for example, the diameter only influences the qualification of welders (see table below). When the diameter of the tube is an essential variable, it will represent a range of diameters, and the choice must be made in such a way as to cover the range foreseen by the procedure.
Welding Position
The position has an influence on the welding energy and the difficulty of performing welds, and therefore directly affects the qualification of procedures and welders/welding operators.In view of this, the standards define the fundamental positions and the domain range of each position, which makes the position in which the test piece must be welded can be established.
Whenever the welding position is considered an essential variable, the qualification must be limited to the position in which the part is welded or, according to the applicable standard, to two or more positions, for example, we have the AWS D1 Standard .1.
Faced with a specific case, it is verified which welding positions are foreseen in the procedure, if the welding position is an essential variable for the welding procedure in the qualification standard and, if so, what are the limits of the positions.
Welding Consumable
The correct specification of the consumable to be used is linked to the integrity of the welded joint, in order to guarantee the mechanical property required by the project.The grouping of consumables is done differently, the API 1104 standard specifies 11 groups, the AWS D1.1 standard the consumables are correlated with the type of base material, in the ASME Standard Section IX the consumables are specified through No. F, as indicated in paragraph QW 432.
The table below illustrates the grouping of welding consumables according to API1104 Standard.
Preparation of the Joint to be Welded
Joint preparation must be representative of the actual conditions of the equipment to be welded. Thus, the type and dimensions of the chamfer, the mounting aids and the cleaning of the joint must be equivalent to the real conditions.Some standards (eg AWS) define the chamfer type as an essential variable. In view of this, depending on the types of chamfer provided for in the procedure, the number of test pieces can be influenced.
Removal of specimens
From the definition of the test piece that was made according to the information of the welding procedure and the variables of the qualification standard, the requirements regarding the removal and preparation of the specimens can be determined.All requirements now depend on the test piece and the qualification standard. From the test piece, because they depend on the type, thickness and diameter (for tubes) of the test piece and the qualification standard because the standards, also in this factor, have exclusive requirements for the procedure qualification and for the qualification of welders and welding operators.
Evidence Withdrawal Position
In general, the specimen withdrawal position depends on the qualification standard, the type of test piece, the diameter of the test tube and the thickness of the test piece.The 3 figures below illustrate the position of removal of the bodies according to the requirements of the most usual qualification standards.
Preparation of Specimens
This item is of great importance in the qualification because it depends on the validity and repeatability of the test results.It is the responsibility of the welding inspector to ensure that the requirements regarding the dimensions and finish of the specimens are met.
Validity of qualifications
Both in the qualification of welding procedures and in the qualification of welders and welding operators, the standards differ from each other in the validity of qualifications, that is, when a procedure becomes unfit for use or when a welder or welding operator must be requalified.It is the job of the welding inspector to ensure that only qualified procedures are used (see note below), that welders and welding operators only work on the services for which they are qualified and that they are requalified when necessary.
It is worth remembering again that the fact that a procedure is qualified does not guarantee its applicability to any work. Each work has its own criteria and requirements and technical judgment is required to apply a welding procedure.
Qualification limits are first established through welding variables.
Variables are welding conditions that, if changed, affect the quality of the welds.
For the coated electrode welding process, for example, switching from a rectifier to a generator power source normally has no influence on the probability of altering the quality of the welds and therefore is not considered as a variable for this process. .
Anyway, it can be a variable for another welding process.
Some qualification standards give all variables the same importance. Others give different levels of importance to them, classifying them into essential variables and non-essential variables.
Each standard determines specific variables, applicable to each of the welding processes.
With knowledge of the applicable variables, the welding inspector can determine if welding is performed properly.
Essential variables
These are variables that, if changed, require requalification.Supplementary Essential Variables
These are variables that, if changed, require requalification, however their analysis is only necessary when there is a requirement for impact on the welded joint.Non-essential variables
These are variables that, if changed, do not require requalification. By using a qualified procedure and within the limits of the variables, they should result in acceptable welds, if the welder does not introduce defects. However, a change beyond the limits established in the procedure can affect the mechanical properties of the welds, even if the welder performs the weld without defects.A change to a non-essential variable still requires a review of the WPS but not the WPQR.
Example of welding variables
Type of welded joint
The type of joint is determined taking into account the thickness of the material to be welded. In general, the type of joint should be used so that the amount of metal deposited is small, thus avoiding waste of weld metal, machine and welder time, as well as avoiding the introduction of a large amount of heat into the part.< br />Chamfer angle
The bevel angle is what allows the electrode access to regions close to the weld root and facilitates the welding of multiple passes, avoiding an overheating of the part. It must have a variable dimension within a certain limit, because if too “closed” they make the root pass operation difficult and if too “open” high and unnecessary amounts of addition material are deposited.Root opening
The opening of the root allows the filler metal to penetrate the root of the weld. The opening of the root must also be controlled, because if it is too large, there may be excessive penetration of the weld or complete fusion of the nose faces, and thus overflowing from the entire root and too small may not allow any penetration.Root face
It also has an important role in penetration as it serves as a solid support for molten metal. Its dimension must also be controlled so that there is a perfect fusion of its edges, and consequently an adequate penetration.Use of “backing”
In butt joints in the flat position, in order to have a controlled penetration independent of the factors mentioned above, the backing is chosen, which serve as a support of molten metal obtaining desired shapes. This type of device is often used in joints where access to the other side of the joint is not available.WPS form (example)
The WPS form must contain the information provided for in the respective applicable qualification standards, for example:- Identification of the qualification standard.
- Sketch of the gasket and chamfer.
- Sketch of the pass sequence (usually ignored for practical reasons).
- Specification, classification and trademark of consumables.
- WPS and WPQR number.
- Control of heat input when applicable.
- Special requirements for the consumable used (such as using the G suffix).
- Preheat range for qualified thicknesses.
- Date of preparation and approval.
- Full name of the responsible engineer.
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