Fabrication, applications and inspection
This domain tests the production chain from qualified procedure to accepted weld. Expect WPQR and WPS logic, welder qualification, fit-up and distortion control, heat input calculation, and the selection and timing of non-destructive testing methods.
Documents and sequence
A welding procedure qualification record, produced to ISO 15614, supports a welding procedure specification to ISO 15609. Welders are qualified to ISO 9606. The WPS controls essential variables: process, consumable, current, voltage, travel speed, preheat, interpass temperature and heat input. Qualification of the procedure and of the welder are separate requirements.
Distortion control
Use balanced or alternating welding, backstep technique, presetting, minimum necessary heat input and correct sequencing. Restraint reduces angular distortion but raises residual stress and cracking risk.
NDT selection
Visual inspection comes first. Penetrant testing finds surface-breaking defects in any material; magnetic particle testing only works on ferromagnetic steels. Radiography is volumetric and suited to thin and medium sections; ultrasonic testing suits thicker sections but is difficult in coarse-grained austenitic welds unless specially qualified.
Worked example
A WPS permits 1.0–2.0 kJ/mm. The welder uses 250 A, 26 V and 250 mm/min. Heat input = (60 x 26 x 250)/(1000 x 250) = 1.56 kJ/mm, which is inside the range, so the parameter is acceptable. Reasoning: the 60 converts minutes to seconds and the 1000 converts joules to kilojoules; doubling travel speed would halve the heat input to 0.78 kJ/mm and take it outside the qualified range.
Traps that cost marks
- Treating a WPQR as if it were the WPS issued to the welder.
- Selecting magnetic particle testing for an austenitic stainless weld.
- Carrying out radiography before visual inspection has been completed and accepted.
- Confusing arc energy with heat input when a procedure specifies an efficiency factor.