Welding Processes and Weld Design
Arc processes, heat input, carbon equivalent and preheat, fillet weld sizing, distortion and inspection.
Welding processes
| Process | How it works | Good for | Notes |
|---|---|---|---|
| MIG / MAG (GMAW) | a continuous wire electrode and a shielding gas | production, structural, sheet; steel, aluminum | fast, easy to learn; wind blows the shield away; lack of fusion on cold starts |
| TIG (GTAW) | a non-consumable tungsten arc, a separate filler rod and inert gas | thin material, stainless, aluminum, titanium, root passes | high quality and control, slow, needs skill |
| Stick (SMAW) | a flux-coated electrode | site and repair work, outdoor, thick plate | simple equipment; slag to chip off; low-hydrogen rods need drying |
| Flux-cored (FCAW) | a tubular wire with a flux core | heavy fabrication, outdoors (self-shielded) | high deposition; fume and slag |
| Submerged arc (SAW) | the arc is buried under a granular flux | thick plate, long straight welds, pipes, pressure vessels | very high deposition and quality; flat position only |
| Resistance (spot, seam) | current through the stack-up melts a nugget | sheet metal assemblies, car bodies | fast, no filler; access to both sides |
| Laser and electron beam | a focused beam melts the joint | precision, deep narrow welds, little distortion | tight fit-up; expensive |
| Friction stir (FSW) | a rotating tool stirs the joint in the solid state | aluminum plates, tanks, rail | no melting, no cracking; the joint stays strong |
| Oxy-fuel brazing, soldering | a filler melts below the parent metal | thin sections, copper tubing, dissimilar metals | not true welding; see Soldering for electronics |
Heat input
Heat input (kJ/mm) = η × V × I × 60 ÷ (1 000 × travel speed in mm/min)
Worked example: heat input of three processes
- MIG at 24 V, 200 A and 240 mm/min (4 mm/s): 0.8 × 24 × 200 × 60 ÷ (1 000 × 240) = 0.96 kJ/mm.
- TIG at 12 V, 150 A and 100 mm/min: 0.65 kJ/mm (lower: thin material).
- Submerged arc at 30 V, 500 A and 400 mm/min: 2.25 kJ/mm (high: deep penetration in thick plate).
- Low heat input cools the weld quickly (hard, brittle HAZ in hardenable steel; fine grains and high strength in stainless); high heat input cools it slowly (soft, coarse grain, more distortion). Procedure qualifications specify the range.
Weldability, carbon equivalent and preheat
Steel that is hardenable can form martensite in the heat-affected zone, which is hard, brittle and prone to hydrogen cracking. The tendency is measured by the carbon equivalent, CE = C + Mn ÷ 6 + (Cr + Mo + V) ÷ 5 + (Ni + Cu) ÷ 15. Below about 0.40 steels weld readily without preheat in thin sections; between 0.40 and 0.60 they need preheat and low-hydrogen consumables, and above that, special procedures.
Worked example: S355 frame against 4140 rod
- S355J2 (C 0.18, Mn 1.40, Cu 0.03, Cr 0.03, Ni 0.03): CE = 0.18 + 1.40 ÷ 6 + 0.03 ÷ 5 + 0.06 ÷ 15 = 0.42: weldable; preheat only for thick sections (over about 30 mm) or in the cold.
- 4140 (C 0.40, Mn 0.85, Cr 0.95, Mo 0.20): CE = 0.77: it hardens in the HAZ to 50 HRC or more. Weld only in the annealed state with a preheat of about 240 °C or more (the simple rule T = 350 √(CE − 0.30) gives 240 °C), a low-hydrogen process and slow cooling, and then temper. Do not weld the hardened piston rod.
- A36 (CE 0.39) and 1018 (0.31) are the easy cases.
Preheat (typically 100 to 250 °C) slows the cooling of the weld, so hydrogen escapes and the HAZ stays tougher. The rules (EN 1011-2, AWS D1.1) depend on the carbon equivalent, the plate thickness, the hydrogen level of the consumable and the restraint. Keep the interpass temperature below the specified maximum, and use dry low-hydrogen electrodes (baked and kept in a heated quiver).
Sizing a fillet weld
A fillet weld is a triangular bead in the corner of a joint. Its size is the leg length w; the throat a = 0.707 w is the shortest distance through the weld to the root, and it carries the load. A simplified design strength per unit length of weld uses the shear strength of the weld metal: fvw,d = (fu ÷ √3) ÷ (βw γM2) with fu the tensile strength of the weaker part, βw a correlation factor (0.9 for S355) and γM2 = 1.25 (EN 1993-1-8).
Worked example: the press cylinder foot
Worked example: welding the cylinder foot plate to the frame
- The cylinder pushes with 80.4 kN (Fluid Power Basics) into a foot plate welded to the S355 frame with two fillet welds each 100 mm long.
- Design shear strength: fvw,d = (510 ÷ √3) ÷ (0.9 × 1.25) = 262 MPa (N/mm²).
- Required throat: a = F ÷ (n L fvw,d) = 80 425 ÷ (2 × 100 × 262) = 1.54 mm, a leg of 2.2 mm. This is tiny, so the design is governed by the practical minimum, not strength: a 4 mm throat (6 mm leg, 5.7 mm exactly) carries 209 kN, 2.6 times the load.
- Minimum leg sizes follow the thicker plate (5 to 6 mm on 20 mm plate, to avoid a fast-cooling, cracked weld). The press load is also cyclic (a pressure cycle every 10 s): fatigue category of a fillet weld (EN 1993-1-9 detail category 36 to 80) rather than yield is likely to govern; see Fatigue and Failure.
- Weld metal for a 6 mm fillet: area ½ × 6² = 18 mm², so 0.14 kg per meter of weld (0.15 kg of wire at 95 percent deposition efficiency): the cost grows with the square of the leg, so never over-weld.
Distortion and residual stress
The weld metal shrinks as it cools, pulling the parts together; the result is distortion (the plate bends or buckles) and residual stress (stresses up to the yield strength remain in the weld area). A transverse butt weld shrinks about 0.2 mm for a typical pass; a longitudinal weld shortens about 1 mm per meter of weld. Reduce them by: the smallest weld that meets the design; symmetrical joints; a balanced sequence (back-step, alternating sides); fixtures and tack welds; low heat input; pre-setting the parts to allow for shrinkage; and stress relieving when needed (550 to 650 °C). Residual stress adds to the load and lowers fatigue life, and tensile residual stress can drive cracks in brittle conditions.
Defects and inspection
| Defect | Cause | Detection |
|---|---|---|
| Porosity | moisture, dirty metal, lost shielding gas | visual, radiography, ultrasonics |
| Lack of fusion / penetration | low current, wrong angle, poor joint preparation | ultrasonics, radiography, macro-etch |
| Cracks (hot, cold, hydrogen) | wrong filler, high restraint, hydrogen, fast cooling | visual, dye penetrant, magnetic particle, ultrasonics |
| Undercut, overlap | too fast, too much current, poor technique | visual with gauges |
| Slag inclusions | poor cleaning between passes | radiography, ultrasonics |
| Distortion | unbalanced shrinkage | measurement |
Welds are inspected visually and, depending on the consequences of failure, by dye penetrant (PT), magnetic particle (MT), ultrasonic (UT) or radiographic (RT) testing, and by destructive tests on procedure qualification samples (bend, tensile, impact and hardness). Pressure equipment, lifting gear and structures are welded to a qualified procedure (WPS/PQR) by qualified welders (EN ISO 15614, ASME IX, AWS D1.1).
Safety
- Arc eye and burns: an auto-darkening helmet of the right shade (10 to 13), flame-resistant clothing and gloves; UV burns skin and eyes in seconds. Screen other people.
- Fume: welding fume (and hexavalent chromium from stainless, zinc fume from galvanized steel, manganese) is hazardous: use local extraction and respiratory protection (Workshop Safety).
- Fire and explosion: sparks travel 10 m or more; clear flammables, never weld on closed containers or tanks that held fuel, and keep an extinguisher and a fire watch.
- Electric shock from the electrode and the machine (Electrical Safety); gas cylinders must be secured.
Related
- Previous: Aluminum, Copper, Titanium and Other Alloys. Next: Corrosion and Protection.
- Weld Size Calculator.
- Joints, Fasteners and Welds.
- Materials and Processes: the whole guide.
FAQ
What is the difference between leg and throat?
The leg is the side length of the triangle; the throat is the height from the root to the face, 0.707 times the leg, and is the load-carrying dimension.
Do I need preheat for mild steel?
Not for thin sections of low-carbon steel in a warm shop. Preheat for thick sections, in the cold, for high restraint or for any steel with a carbon equivalent over about 0.4.
Why does stainless steel warp so much?
It expands about 50 percent more than carbon steel and conducts heat poorly, so the heat stays near the weld: use low heat input, tack closely and balance the sequence.
Can I weld galvanized steel?
Yes, but grind the zinc off the joint and weld in a well-ventilated place or with extraction: zinc fume causes metal fume fever.
References
- Lincoln Electric, The Procedure Handbook of Arc Welding; Kou, S., Welding Metallurgy, Wiley; Cary, H. B., Helzer, S. C., Modern Welding Technology, Pearson.
- EN 1993-1-8 (design of joints), EN 1011-2 (arc welding of ferritic steels), AWS D1.1 (structural welding code, steel), ASME BPVC Section IX and EN ISO 15614 (procedure qualification).
- American Welding Society, Welding Handbook; The Welding Institute (TWI) knowledge base.
Last reviewed: October 2026. Efficiencies and design factors are typical; use the standard and a qualified procedure.