8.3 Welding-Related Calculations
Key Takeaways
- Heat input formula: HI (kJ/in) = (V × A × 60) / (Travel Speed in/min × 1,000)
- Fillet weld theoretical throat = 0.707 × leg size (for equal-leg fillet welds)
- Deposition efficiency: SMAW ~60%, GMAW ~93%, FCAW ~84%, SAW ~97%, GTAW ~97%
- Fillet weld cross-section area ≈ L²/2 × 1.2 (with reinforcement factor)
- Steel density = 0.283 lb/in³ for weld metal weight calculations
- Dilution = percentage of base metal melted into the total weld metal
8.3 Welding-Related Calculations
Welding-related calculations account for approximately 6% of Part A (~9 questions). These are practical math problems that a CWI needs in the field.
Heat Input Calculation
Heat Input (HI) = (V × A × 60) / (Travel Speed × 1,000) (kJ/in when travel speed is in in/min)
Example: V = 28V, A = 250A, Travel Speed = 12 in/min
HI = (28 × 250 × 60) / (12 × 1,000) = 420,000 / 12,000 = 35 kJ/in
Fillet Weld Throat Calculation
Theoretical throat = 0.707 × leg size (for equal-leg fillet welds)
Example: 1/2" fillet weld leg size Throat = 0.707 × 0.500 = 0.354"
Fillet Weld Strength (Shear)
Allowable shear load = 0.707 × leg size × weld length × allowable shear stress
Example: 3/8" fillet weld, 10" long, allowable shear stress = 21 ksi (AISC E70XX) Load = 0.707 × 0.375 × 10 × 21,000 = 55,678 lbs ≈ 55.7 kips
Electrode Consumption Estimation
Electrode needed = (Weight of weld metal deposited) / (Deposition efficiency)
| Process | Typical Deposition Efficiency |
|---|---|
| SMAW | 60–65% |
| GMAW | 90–95% |
| FCAW | 80–88% |
| SAW | 95–99% |
| GTAW | 95–99% (filler rod only) |
Weld Metal Volume — Fillet Weld
Cross-section area of a fillet weld = (leg × leg) / 2 × reinforcement factor
For a standard equal-leg fillet: Area ≈ L²/2 × 1.2 (reinforcement factor for fillet)
Weight of deposited weld metal = Area × Length × Steel density Steel density = 0.283 lb/in³
Example: 5/16" fillet weld, 24" long
- Area = (0.3125)² / 2 × 1.2 = 0.0586 in²
- Weight = 0.0586 × 24 × 0.283 = 0.398 lbs
Weld Metal Volume — Groove Weld
For a single-V groove weld:
- Area = (root opening × plate thickness) + (tan(groove angle/2) × plate thickness² / 2) + reinforcement area
Unit Conversions Common on the Exam
| Conversion | Value |
|---|---|
| 1 inch | 25.4 mm |
| 1 foot | 304.8 mm |
| 1 ksi | 1,000 psi |
| 1 ksi | 6.895 MPa |
| 1 ft-lb | 1.356 J |
| °F to °C | (°F − 32) × 5/9 |
| °C to °F | (°C × 9/5) + 32 |
| 1 kg | 2.205 lbs |
Dilution
Dilution is the percentage of base metal mixed into the weld metal:
Dilution (%) = (Area of base metal melted / Total weld area) × 100
Higher dilution means more base metal composition influences the weld metal properties.
For the Exam: Practice the heat input formula until it is second nature. Also know the fillet weld throat calculation (0.707 × leg) and the deposition efficiency values for each process. Bring a basic calculator to the exam if permitted.
Calculate the heat input: Voltage = 30V, Amperage = 300A, Travel Speed = 15 in/min.
A welder deposits 10 lbs of weld metal using SMAW. Approximately how many pounds of electrodes were consumed?
What is the theoretical throat of a 3/8" equal-leg fillet weld?