Workshop Calculation and Science
What this domain tests
Numeric work applied to real welding jobs: heat input, weld-metal weight, stress and strain, thermal expansion, and simple electrical relationships.
Core rules
- Heat input. H = (60 × V × I) / (1000 × v) kJ/mm, with V in volts, I in amperes, v in mm/min. Heat input governs cooling rate, HAZ hardness and distortion; doubling travel speed halves it.
- Stress and strain. σ = F/A, ε = ΔL/L, and E = σ/ε (about 200 GPa for steel).
- Thermal expansion. ΔL = L·α·ΔT, with α ≈ 11–12 × 10⁻⁶ per K for mild steel.
- Weld metal weight. Fillet cross-section = 0.5 z², so volume = 0.5 z² × length; mass = volume × density (steel 7.85 g/cm³, aluminium 2.7, copper 8.9).
- Electrical. Ohm's law V = IR, power P = VI; arc power in kW is simply V × I / 1000.
Worked example
A butt weld runs at 24 V, 180 A, 150 mm/min. Find heat input, then at 300 mm/min. H = (60 × 24 × 180) / (1000 × 150) = 259 200 / 150 000 = 1.73 kJ/mm. At 300 mm/min: H = 259 200 / 300 000 = 0.86 kJ/mm. The second case cools roughly twice as fast, so the HAZ hardness rises and the risk of cold cracking increases - which is why a slower speed is used on hardenable steel but avoided on thin sheet where it causes burn-through.
Weld-metal check: an 8 mm fillet, 1.5 m long, steel. Area = 0.5 × 8² = 32 mm²; volume = 32 × 1500 = 48 000 mm³ = 48 cm³; mass = 48 × 7.85 ≈ 377 g, plus about 20% for reinforcement and spatter.
Traps
Using travel speed in m/min without converting; forgetting the 60 s factor in heat input; measuring fillet area as leg × leg instead of 0.5 × leg²; mixing mm and cm in the mass calculation.