1) The inspection should verify the mechanical and chemical properties of the material from the laboratory reports, material selection
2) The test result should be compared with the values given, the material specification should conform that the material to be for fabrication shall meet the quality requirements
3) Use of suitable filler materials with alloying content which shall increase the toughness. Welding parameters in which higher heat input and travel speed so that there will be certain increase in the toughness values. The material shall be tested for NDT to ensure the material is defect free
The welding procedure approval test is carried out by a competent welder and the quality of the weld is assessed using non-destructive and mechanical testing techniques. The intension is to demonstrate that the proposed welding procedure will produce a welded joint which will satisfy the specified requirements of weld quality and mechanical properties.
Procedure provides guidelines to the welder in order to produce the sound weld for the given joint
Welder approval test examines a welder skill and ability in producing a satisfactory test weld. The test may be performed with or with out a qualified procedure, (note, without an approved welding procedure the welding parameter should be recorded). Welder approval must be done prior to start the welding on the production site. Welder should be qualified to do the task
A change in welding parameters which effect the mechanical properties of a weld are called essential variables. E.g. process, type of material, electrode, flux, shielding gas, preheating, PWHT, current and voltage etc.,
2) Non – essential variables
A change in welding parameters which will not affect the mechanical properties of the weld metal, are called non essential variables e.g. Groove angle, method of cleaning etc.
3) Supplementary variables.
The welding procedure shall be attached with PQR to show the evidence that the procedure meets the mechanical properties described by the code specification.
Wrought steel gain much refinement during the hot/cold working and many defects are also removed. This improvement is marked in the rolling direction but it usually results in a loss of strength through the thickness
Set aside the piece, take one more test piece and repeat the test. Assess the failure, whether the failure is within the weld metal, weld junction or in the HAZ. A retest is very much needed in cause of failure
The advantage of using austenitic stainless steel electrodes for repair is that hydrogen entering the weld metal during welding is held in the weld metal and so will not diffuse in to the hardened HAZ; hence hydrogen induced cracking in the HAZ is unlikely to happen.
Weld decay – steels with high carbide forming characteristic such as these will react if the temperature is allowed to dwell about 550degC. If this occurs then the chromium is no longer available for combination with oxygen for reformation of the protective oxide film and corrosion may result.
Weld decay – depletion of chromium carbide in stainless steel.
Lamellar tearing is a defect in the parent metal of a weld metal due to high through thickness residual stresses and a low through thickness strength and ductility arising from the bands within the steel
In the HAZ the tensile residual stresses are across the weld, so the hydrogen cracks are along the length of the weld. In weld metal the tensile residual stresses are along the weld, so the hydrogen cracks are across the weld.
Hydrogen cracking is typically formed at right angles to the stress and is positively identified by its Trans granular appearance when viewed at X – 100 magnifications. In ferritic steels hydrogen which enters the weld metal during welding moves into the HAZ and due to gas forming characteristics and the residual stress, cracking may result.
When the weld metal has been deposited and its contracts during solidification it is vital that the contraction can be fed by the depression of the outer surface
Contraction fed by the weld metal surface
As the chromium content is increased, increased quantities of sigma phase is produced which embrittles the steel. Solidification cracking is a problem. Single phase alloying suffer grain growth and with heating and the loss of strength and toughness
Extent of approval is the range over which certain variables may alert with out requiring new procedure i.e. when there is a limitation in the welding qualification i.e.
1) The minimum and maximum diameter of the pipe that the test sample covers e.g. two inch test piece would allow pipe down to 1” and up to 4”
2) The test may only allow welding of consumables in the same grouping, any other consumables would requires re-test
3) The sue of set electrical characteristics would not allow change with out retest
4) The direction of welding, if changed may required re test
5) Change of material to be tested would require re – testing
After inspection the results has to be recorded in a format. If the sample is rejected then the type of defects and its location has to be incorporated in a sketch and the report has to be given for further remedial action.