THE WELDING PROCEDURE

THE WELDING PROCEDURE

A welding procedure is a way of controlling the welding operation.

Purpose of procedure:

1) To prove a joint can meet design procedure - consistency

2) Instruction for welder

3) Ensure repeatability

Weld procedures are approved to ensure they are functional and fulfil the physical and mechanical properties necessary to reach the required standard (to establish the essential variables for contractual obligations).

Welders are approved to ensure a particular welder is capable of welding to a procedure and obtaining a result that meets specification.

The task of collecting the data and drafting the documentation is often referred to as ‘writing’ a weld procedure. In many ways this is an unfortunate term as the writing of documents is the last in a sequence of tasks.

Producing a weld procedure involves;

Planning the tasks

Collecting the data

Writing a procedure for use or for trial

Making test welds

Evaluating the results of the tests

Approving the procedure of the relevant code

Preparing the documentation

In each code reference is made to how the procedures are to be devised and whether approval of these procedures is required. In most codes approval is mandatory and tests to confirm the skill of the welder are specified. Details are also given of acceptance criteria for the finished joint.

The approach used depends on the code, for example:

BS 2633 (Class 1 arc welding of ferritic steel pipe work for carrying fluids) provides general comments on various aspects of a suitable weld procedure.

AWS D.1.1 (Structural welding code - steel) favours more specific instructions for different joints and processes that are, in effect, pre-qualified procedures.

Other codes do not deal specifically with the details of the weld procedure but refer to published documentation, e.g. BS 5135 ‘process of arc welding carbon and carbon manganese steels’.

COMPONENTS OF A WELD PROCEDURE

Items to be included in the procedure can be some of the following:

Parent Metal

a. Type

b. Thickness (for pipe this includes outside diameter)

c. Surface condition

d. Identifying marks

Welding Process

a. Type of process (MMA, TIG, SAW etc.)

b. Equipment

c. Make, brand, type of welding consumables

d. When appropriate, the temperature and time adopted for drying and baking of electrodes

and / or consumables

Joint Design

a. Welding position

b. Edge preparation

c. Method of cleaning, degreasing etc.

d. Fit up of joint

e. Jigging or tacking procedure

f. Type of backing

Welding Position

a. Whether shop or site weld

b. Arrangement of runs and weld sequence

c. Filler material, composition and size (diameter)

d. Welding variables - voltage, current, travel speed

e. Weld size

f. Back gouging

g. Any specific features, e.g. heat input control, run-out length

Thermal Treatment

a. Preheat and interpass temperatures including method and control

b. Post weld treatment including method and control

ESSENTIAL VARIABLES

An essential variable is a variable that will influence or change the mechanical or metallurgical properties of the welded joint - changes affecting the procedure approval. Any change in an essential variable requires a new welding procedure specification (WPS).

Essential variables include: wall thickness, joint design, process, materials, consumables, welding position, direction, heat input (voltage, amperage, travel speed), heat treatment.

APPROVING THE PROCEDURE

When the data has been collected, the procedure must be validated by producing and testing a trail weld.

If the procedure is to be used on a fabrication, which has been designed to meet the requirements of a code, the test weld is done under the supervision of an independent witness. The detailed arrangements for the test are subject to agreement between the contracting parties.

A number of British Standards make cross-reference to another standard which covers approval testing. Other codes of practice include their own weld procedure / welder approval information. In general they include a standard format, which can be used to report the results of an approval test.

Range of approval. (extent of approval, scope of approval)

Provides a working range over which certain variables may alter without requiring a new welding procedure or welder approval.

Variables include thickness (e.g. 1/2 down to 2x above), diameter (e.g. 1/2 down to 1/2 above) materials (different materials can be covered), position, process, parent plate group, and consumables.

Re-approval of a welding procedure is necessary if there is a change of any of the essential variables or considerable defect re-occurrence.

ABBREVIATIONS

WPS welding procedure specification - an approved and accepted welding procedure; an

authorised document.

PQR procedure qualification records - proof the procedure works - record of tests undertaken to qualify procedure.

WAC welder approval certificate - required to ensure a particular welder is capable of welding to a procedure.

WATC welder approval test certificate.

WAR welder approval record.

WPAR welding procedure approval record.

WQT welder qualification test.

pWPS preliminary WPS - unauthorised (contains all essential variables and, after welding the test piece and all NDT and destructive tests have been accepted, then the WPS can be approved).

pWPS and WPAR give final WPS (various WPS can derive from one pWPS).

DOCUMENTATION

The objectives of a procedure or welder approval test are:

a. to prove the procedure meets the necessary requirements with reference to feasibility, mechanical strength etc.

b. to prove the welders are competent to work on a particular job.

If a customer queries it, evidence can and would be supplied to prove validity.

Approval Test Specifications call for a paper record, which can be known as:

Procedure / welder approval certificate

Procedure / welder approval record

Procedure / welder approval report

The following records should also be kept:

NDT reports

Records of visual examination or mechanical testing

Test pieces from destructive testing

Other records that are equally important are proof of regular employment on a job for scheduling re-tests to avoid duplication on procedure approval.

WELDER APPROVAL

Welder approval tests are used to determine the ability of a welder to produce welds of an acceptable quality with the processes, materials and welding positions that are to be used in production. Dependant on the requirements and administration the manufacturer or contractor may choose to qualify their own welders or they may employ outside personnel who can meet the requirements. These requirements usually specify verification of the tests by an authorised inspector or independent body. The requirements for the qualification of welders are usually laid down in the governing code or specification or the contract specification.

Approval tests improve the probability of obtaining satisfactory welds in production. However it is true to say that approval test welds are made with special attention and effort and so cannot show whether or not the welder can do so under every production condition. For these reasons complete reliance should not be placed on these qualifications - production welds should be inspected regularly to ensure that the standard is being kept up.

Types Of Approval Tests.

Tests that are prescribed by most codes and standards are in the main similar. Common tests are:

a. Plate and structural members.

b. Pipe welding.

c. Positions of welding.

d. Testing of approval testpieces.

e. Re-tests.

a. Plate and structural members.

The requirements for welders of plate and structural parts (including pressure vessels) usually require the welder to make one or more test welds on plate or pipe assemblies with the qualified welding procedure. Each weld is tested in a specific manner, often both destructively and non-destructively. The requirements normally state the applicability of material thickness and welding positions that will qualify for production work. Other details will cover joint type and direction of welding when depositing vertical welds (vertically up or vertically down).

b. Pipe welding.

The requirements for the approval of welders for pipe welding differs from those for welding plate and structural members chiefly in the type of test assemblies and test positions. As a rule the welds must be made on pipe and not plate. In some cases the space within which the test piece must be welded may be restricted if the production work involves welding in cramped conditions. As a general rule welders who qualify for certain joints on pipe need not qualify for plate work, but qualifications on plate do not apply to pipe work.

c. Position of welding.

Approval tests are normally expected to be made in the most difficult positions that will be encountered in production welding. For example qualification in the vertical, horizontal and overhead positions usually qualifies for welding in the flat position.

d. Testing of welder approval test pieces.

All codes and specifications will have definite rules for the testing of approval welds to determine compliance. Most frequently this involves the removal of specimens for mechanical tests, such as bend tests, and specimens for macro examination from specific locations in the test pieces. Non-destructive testing may be required in conjunction with the mechanical tests.

Other properties required of the procedure qualification welds such as tensile strength, hardness, etc. are not generally specified in welder approval tests since these properties depend primarily on the parent and filler materials used on procedure details that are beyond the individual welder's control.

Welders whose test welds meet the requirements are qualified to use the process and to weld with the filler metals and procedures similar to those used in testing. It should be mentioned that a welder who has successfully welded a procedure test specimen is not required to undergo an approval test, unless the requirements of production welding are different from those of the procedure in which he has qualified.

e. Re-tests.

The circumstances for the re-testing of a welder include the following:

1. Failure of the initial test welds.

2. A significant change in welding procedure.

3. A welder has not been engaged in welding for an extended period. (Usually three months.)

4. There is reason to question the welder's ability.

5. Change of employment without the transfer of his approval certificates.

CHECK LIST FOR WELDER AND PROCEDURE APPROVAL

1. The test being carried out is the correct one required.

2. Welders are in possession of all relevant information concerning the test.

3. Test materials confirm in all respects to requirements.

4. Joint configuration and tolerances are correct.

5. Welding plant and consumables.

6. The welder's identification is clearly marked on the test piece.

7. Where it is specified for a root run to be stopped and restarted in a certain position, that this

position is clearly marked.

8. In the case of joints welded in fixed positions the test piece is so fixed that it cannot be moved.

9. All ancillary tools such as chipping hammers, wire brushes, grinders, etc. are available.

The tests should be carried out without interruption but with sufficient supervision to ensure that the requirements are being complied with. Where welder approval is carried out in accordance with ASME section IX it states that the person supervising the test may, if in his opinion he considers that the welder will not meet the required standard, terminate the test at any time. If it is necessary to apply this ruling, it is suggested that full reasons for termination be recorded. It is further recommended that the test piece should also be kept for a short period as a means of backing up written statement.

If the test is to be supervised by a representative of an independent authority he should be given all the relevant details of the testing required.

Where British standards are involved, they generally state that if the welder is of the opinion that his first attempt may not pass any subsequent testing, he may withhold it and weld a second. In this case it is the second test piece that is submitted for examination and the first one must be scrapped.

TEST CERTIFICATE

Should state clearly that it is a welder approval and not a procedure approval, and, depending on the particular standard, should contain the following:

a. welder’s name and identity number

b. date of test

c. standard of code in full, e.g. British Standard 4872 PT 1: 1982

d. test piece details including material specification

e. equipment and consumable details

f. extent of approval

g. sketch of run sequence, preparation and dimensions

h. other factors, operating parameters etc.

i. the test results (visual, NDT, DT etc.)

j. remarks

k. witnessed by

l. test supervisor

m. location

Most standards give an example of a test certificate.

Signatures on certificates must be endorsed by company stamp.