| name | weld-process |
| description | Welding process selection — SMAW, GMAW, GTAW, FCAW, SAW, resistance welding, laser. Heat input, distortion control, preheat, PWHT, inspection methods, AWS D1.1 qualification. |
| metadata | {"priority":5,"promptSignals":{"phrases":["welding process","GMAW","MIG","TIG","GTAW","stick welding","SMAW","weld distortion","heat input","weld inspection"],"minScore":4}} |
Welding Process Selection — Complete Skill
Process Comparison
| Process | Acronym | Current | Position | Deposition Rate | Notes |
|---|
| Shielded Metal Arc | SMAW (Stick) | DCEP/DCEN/AC | All | 1-5 kg/hr | Most versatile, simple, slow |
| Gas Metal Arc | GMAW (MIG) | DCEP | All (except vert/OH with spray) | 2-8 kg/hr | Fast, continuous, semi-auto |
| Gas Tungsten Arc | GTAW (TIG) | DCEP/DCEN/AC | All | 0.5-2 kg/hr | Highest quality, slowest, thin material |
| Flux-Cored Arc | FCAW | DCEP | All | 3-12 kg/hr | Good for outdoors (FCAW-S), high deposition |
| Submerged Arc | SAW | DC/AC | Flat only | 8-40 kg/hr | Highest deposition, structural fab |
| Resistance Spot | RSW | — | Fixed | Very fast/spot | Sheet metal, automotive |
| Laser Beam | LBW | — | Automated | Fast, narrow bead | Precision, thin to thick |
| Electron Beam | EBW | — | Vacuum | Deep penetration | Aerospace, exotic |
Heat Input
HI = E×I×60/(1000×v) [kJ/mm]
E = voltage [V], I = current [A], v = travel speed [mm/min]
Low HI (<1 kJ/mm): GTAW, thin materials, low distortion
Medium HI (1-3 kJ/mm): SMAW, GMAW — general structural
High HI (>3 kJ/mm): SAW — thick sections, slower cooling rate
Effect of heat input:
Higher HI → larger HAZ → more distortion → coarser grain → lower toughness
Lower HI → smaller HAZ → faster cooling → harder HAZ → risk of cold cracking
Preheat Requirements (AWS D1.1 Table 3.2 / D1.1-2020)
Preheat slows cooling → reduces hardness → prevents cold cracking (hydrogen cracking)
Carbon equivalent: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
| CE | Prequalified Preheat (ASTM A36, t>20mm) |
|---|
| < 0.40 | 10°C min (50°F) |
| 0.40-0.60 | 65-150°C |
| > 0.60 | 150-230°C |
Interpass temperature: T_min (same as preheat), T_max = 315°C (prevents softening)
Post-Weld Heat Treatment (PWHT)
Purpose: stress relief, improve toughness, reduce hardness
Typical: 595-650°C for 1 hour per 25mm thickness
Required (ASME PV): t > 38mm carbon steel, certain Cr-Mo alloys at any thickness
Required (AWS Bridges): specific cases per D1.5
Distortion Control
Causes: non-uniform heating/cooling → shrinkage → angular, transverse, longitudinal distortion
Control methods:
- Backstep welding (reduces longitudinal shrinkage)
- Balanced welding (symmetric about neutral axis)
- Preset/precamber (allows for expected distortion)
- Clamps and fixtures (resist shrinkage during welding — releases stress after)
- Minimal heat input (small beads, low current)
- Welding sequence (short beads from center outward)
- Peening (reduces tensile residual stress — light peening only for root/fill passes)
Distortion correction: Flame straightening (localized heating), mechanical straightening
AWS Qualification (D1.1)
Prequalified joint details: no WPS qualification needed if follows D1.1 Table 4.12
WPS = Welding Procedure Specification: documents how to make a weld
PQR = Procedure Qualification Record: test results proving WPS works
WPQ = Welder Performance Qualification: individual welder qualification test
Essential variables: change requires re-qualification
Base metal, filler metal class, position, preheat, PWHT, electrical characteristics, joint design
Non-Destructive Inspection (AWS D1.1)
Visual Testing (VT): always required, per D1.1 Sec. 6.9
Liquid Penetrant (PT): surface/near-surface cracks, non-magnetic materials
Magnetic Particle (MT): surface/subsurface, ferromagnetic only
Radiographic (RT): volumetric — internal voids, slag, porosity. Detects: ≥2% of thickness
Ultrasonic (UT): volumetric, better than RT for planar flaws (cracks). Phased array UT (PAUT): detailed imaging
Eddy Current (ET): surface, conductive materials
Common Weld Defects and Causes
| Defect | Cause | Prevention |
|---|
| Porosity | Moisture, contamination, too long arc | Clean base metal, dry electrodes |
| Undercut | Excessive current, wrong angle | Reduce current, correct technique |
| Lack of fusion | Low heat, contamination, wrong bead | Increase HI, clean, correct angle |
| Crack (hot) | High sulfur, low Mn/S ratio | Use low sulfur base metal |
| Crack (cold/hydrogen) | Hydrogen, high restraint, hard HAZ | Preheat, low-H electrodes, PWHT |
| Slag inclusion | Poor cleaning between passes | Chip/grind between passes |
Filler Metal Selection
AWS electrode classification:
E7018 (SMAW): E=electrode, 70=70ksi Sut, 1=all position, 8=low-H iron powder
ER70S-6 (GMAW): ER=electrode-rod, 70=70ksi, S=solid, 6=composition
E71T-1C (FCAW): 1=all position, T=tubular, 1=usage designation, C=CO₂ shielding
Output
Recommend: process, electrode/wire, preheat, HI range, inspection method, distortion control plan.
