Welding and Joining Processes
Process selection on the job
- SMAW with E7018 remains the workhorse: 70 = 70 ksi minimum tensile, 1 = all positions, 8 = low-hydrogen iron-powder coating, usable on AC or DCEP.
- FCAW and GMAW give higher deposition for long seams; GTAW for stainless and small-bore roots; SAW for straight longitudinal and circumferential seams; stud welding for waterwall and shear studs.
- Polarity and amperage come from the WPS. Never substitute a rod because it happens to be in the oven.
Qualification chain
WPS → PQR → welder performance qualification, under ASME Section IX, CSA W47.1 or CSA W59 as the contract requires. Welder qualification is specific to process, position and often joint type and thickness range. A welder qualified for flat GMAW is not qualified for vertical-up SMAW.
Preheat, interpass and heat input
- Preheat slows cooling so hydrogen escapes and hard martensite does not form; it is set by material, thickness and code.
- Interpass temperature has an upper limit as well as a lower one: excess heat input coarsens the heat-affected zone and drops toughness.
- Heat input: HI (kJ/mm) = (V × A × 60) ÷ (1000 × travel speed in mm/min).
Worked example: heat input
25 V, 200 A, 250 mm/min travel. HI = (25 × 200 × 60) ÷ (1000 × 250) = 300 000 ÷ 250 000 = 1.2 kJ/mm. If the welder slows to 150 mm/min, HI rises to 2.0 kJ/mm - a 67% increase from identical settings. That alone can push the work outside its qualified range.
Traps that cost marks
- Reading E7018 as a position code rather than a tensile and coating designation.
- Leaving low-hydrogen electrodes in an open can or skipping the rebake the manufacturer requires.
- Arc strikes and careless tacks outside the joint: stress raisers that must be ground out and inspected.
- Assuming a qualified welder is automatically qualified for the procedure - each is qualified separately.