Welding, Brazing, and Thermal Processes
What this domain tests
Process selection, consumable control, parameter setting, heat input, preheat, and the difference between welding, brazing, and soldering. Expect WPS compliance and electrode classification questions.
Processes and consumables
- SMAW: E7018 is a low-hydrogen, all-position electrode with 70 ksi tensile strength; it must be held in a heated oven at about 120 C and has a limited exposure time out of the oven.
- GMAW: ER70S-6 for steel; short-circuit, spray, or pulsed transfer; shielding gas choice changes transfer mode and penetration.
- FCAW: E71T-1 is gas-shielded and all-position; self-shielded types suit field work.
- GTAW: DCEN for steel, AC for aluminum, high-frequency start, separate filler; purge stainless with argon.
- SAW: high deposition, flat position, granular flux.
Procedure and heat control
Welding follows a qualified WPS supported by a PQR; essential variables include process, filler, preheat, position, and current type. Heat input is (volts x amps x 60) divided by (1000 x travel speed in mm/min), giving kJ/mm. High heat input can reduce toughness; too little can cause hardness and cracking. Carbon equivalent, CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, indicates preheat need; above about 0.4 preheat is generally required. Interpass temperature must stay within the WPS maximum.
Brazing, soldering, and thermal work
Brazing occurs above 450 C and soldering below it; brazing relies on capillary action with a flux and a filler that melts below the base metal. Silver-based fillers join dissimilar metals and thin sections at lower temperature than welding. Thermal processes also include preheat, post-weld heat treatment (stress relief around 595-675 C for carbon steel), flame straightening, and thermal cutting.
Worked example
A WPS allows 1.5 kJ/mm maximum. The welder runs 25 V at 250 A with a travel speed of 250 mm/min. Heat input = (25 x 250 x 60) / (1000 x 250) = 375000 / 250000 = 1.5 kJ/mm. That is exactly at the limit, so any slowing of travel exceeds the WPS. Correct action: increase travel speed or reduce amperage, then record the change.
Traps
- E7018 left in the open all shift.
- Changing gas mix or electrode class without a WPS revision.
- Using DCEN for aluminum GTAW.
- Excess heat input reducing toughness in quenched and tempered steel.
- Confusing brazing with welding in joint design.