Comprehensive Guide to Welding Procedure and Performance Qualifications

Welding Procedure and Performance Qualifications

1.0 WELDING PROCEDURE QUALIFICATION

 

A Total Quality Assurance Scheme would have certainly made all attempts to ensure the qualifications of all the resources and the environment meant for production, raw materials manufacturing and inspection procedures, overall facilities and systems, qualified manpower—all these factors are listed and approvals are obtained before work is begun.

 

Depending on the type of construction, broad welding procedure specifications are usually drawn. A list of General Welding parameters are given below:

 

1.      Scope of Work and Code by which the work is covered etc.

2.      Welding process or processes employed.

3.      Base metals and applicable specifications.

4.      Type, classification, and composition of filler rods and weldment.

5.      Type of current and current range.

6.      Welder qualification requirements.

7.      Joint designs and tolerances

8.      Joint preparation and cleaning of surfaces for welding.

9.      Tack welding.

10.  Joint welding details

11.  Positions of welding involved at factory and at site.

12.  Preheat, interpass, and post-heat temperatures.

13.  Hot peening.

14.  Heat input - Electrode run length, carriage speed, etc.

15.  Post weld heat treatment.

16.  Repair of welds.

17.  Inspection quantum and stages - acceptance levels.

18.  Records

The purpose of the Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR) is to determine that the weldment proposed for construction is capable of having the required properties for its intended application. It is pre-supposed that the welder, or the welding machine operator performing the welding procedure qualification, is a skilled artisan. The Welding Procedure Qualification is, therefore, strictly to establish the properties of the weldment and not the skill of the welder or the welding operator. In addition to the basic mechanical property requirements, such as engineering tension test results and bend test results, a weldment is sometimes prescribed for severe service conditions where the required degree of the ductile to brittle transition temperature of the weldment has to be low. Procedural tests in those cases include impact tests as well, to assess the notch toughness property of the weld.

 

In Performance Qualifications, the basic attempts are to establish the ability of the welder to deposit sound metal. In the case of welding operator qualifications, it is his mechanical ability to operate the welding machine that is tested and acknowledged.

 

Depending on their influence on obtaining a desired weldment, the welding parameters are classified and listed. Some of them are listed as "essential variables", some of them as "supplementary essential variables", and others as "non-essential variables."

 

Essential variables, when changed beyond the allowable limits while welding, alter the prescribed weld properties. Hence, production welds with such altered parameters should not be continued without requalification and certification.

 

More numbers of variables from the list are declared as essential in cases where a procedure qualification is applied for a weldment going for high notch toughness applications.

 

Those welding variables which, when changed during welding (within logical limits), do not cause alteration of the desired weld properties are just entered in the Procedure Qualification Record (PQR). Hence, production welds with such altered parameters could be continued without any need for requalification of the welding procedure.

 

During welder qualifications, only the essential variables (as applicable to welder skills) are considered, and production welds with such altered parameters should not be continued without requalifying the welders. A welding machine operator usually gets qualified along with the procedure test. The requalification's are to be suitably enforced as and when certain parameters are declared as essential variables by inspection agencies.

 

2.0 WELDING PERFORMANCE QUALIFICATIONS

 

A large number of industrial codes nowadays recognize prequalified welding procedures as the manufacturing firms are able to produce documentary evidence for such procedures as in vogue in their organizations. The selection, training, and qualification of welders are the next few steps. Some of the popular codes to look for at this juncture are:

 

1.      Indian Boiler Regulations - IBR Chapter XIII.

2.      ASME Boiler and Pressure Vessel Code Section IX (Welding, Brazing, and Soldering Qualifications).

3.      DIN 8560 - (Testing of Welders).

4.      BS 4872 - (Approval Testing of Welders when welding procedural approval is not required).

There are also a number of other specifications for welder testing, pertaining to individual codes of construction.

 

A welder's plate test coupon can be a maximum of 300 mm x 400 mm with appropriate thickness to cover the allowed specified thickness ranges. Similarly, 2 pipes (or tubes) having a maximum of 125 mm length each are joined together for a pipe weld test coupon.

 

The test coupons are then subjected to radiographic testing. Welders are outrightly failed for the following indications in the radiographic film. All other defects are acceptable within limits:

 

·         Severe root porosities/blow holes.

·         Cluster porosities.

·         Cracks.

·         Lack of fusion/lack of penetration.

After necessary post weld heat treatment, the plates or pipes are marked, stamped, cut, and machined into mechanical test specimens. Though the requirements of the codes are slightly varied, 2 face bend tests and 2 root bend tests alone are conducted for performance qualifications. In lieu of the above, a radiographic evaluation is done.

 

An opening of around 3 mm on the outer bent surface of the weld causes rejection. Premature cracks at the corners of the bend specimen are linearly admitted.

 

Some of the essential variables for the welders' performance qualifications are listed below:

 

1.      Deletion of backing in arc welding processes; and also change from double-side welding to single-side welding in butt welds.

2.      Addition of backing in gas welding processes.

3.      Exceeding the allowable thickness limits which are qualified.

4.      Violating the allowable diameter limits which are qualified in case of pipe welding.

5.      Change from one type of electrode coating specified to the other (which is more difficult for the welder), e.g., from rutile to basic covering.

6.      Change from one welding position to a more difficult position.

7.      Change from vertical up welding to vertical down welding and also vice versa.

8.      Changing the material classification within limits.

Having prescribed in the first place the desired weldment, two major agencies are to act upon the joint to produce the weldment. They are:

 

Energy carrier (electric discharge energy in the form of an arc with or without additional filler wire).

Application mode (e.g., manual welding through welder).

Both the above media are subjected to their inherent random variations, but within logical limits, an attempt is made to establish the repeatability of the selected joining processes to achieve calculated weld properties through procedure and performance qualifications. This can be represented as below:

 

P=C⋅A

 

Where: P is the desired weld property constituted of:

·         Metallurgical compatibility.

·         Mechanical soundness.

C is the energy carrier factor (including the effect of filler wire) which decides the capability of the selected process to control chemical composition and heat input factors, contributing together to the metallurgical compatibility of the weld.

A is the application factor, deciding the ability of the welder (or any other torch manipulator) to deposit mechanically sound weld metal.

The energy carrier effect, C, is chiefly verified through a procedure test. The application factor, A, on the other hand, influences the incidence of the deposit's mechanical discontinuities and the same can be revealed in most cases through radiography. American codes and also the current German specification for welders testing (DIN 8560-1982) have accepted this fact and have incorporated this in the code requirements (for non-impact service), doing away with engineering bend tests (except as a nick-break test in DIN) over and above the radiographic tests. The welder's performance test coupon of the prescribed weld length has thus to clear only a radiographic evaluation. Mechanical bend tests have been suggested at the same time as an alternative to radiographic tests in the American codes, mostly in the absence of radiographic inspection facility.

 

CONCLUSIONS

 Procedure and performance qualifications are to be rightly understood and resorted to while undertaking code constructions.

Monitoring of approved welding procedures and also procedure and welder requalification's are to be conducted as and when warranted.

The welder's performance test has to be limited only to verify his ability to deposit mechanically sound weld metal and the requirements of metallurgical compatibility are to be attributed to the welding process characteristics.

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