01. The Law of the Puddle
A weld isn’t a joint. It’s a transient thermal event. The arc deposits energy. The base metal steals it away. The race between input and bleed determines whether the bead fuses or cracks.
We call it the Heat Input Equation. It’s not theory. It’s the reason your root pass collapses in the wind.
Q = Heat Input [J/mm]
E = Voltage [V]
I = Current [A]
η = Efficiency (0.8–0.95 for GMAW)
v = Travel Speed [mm/s]
In the shop, I don’t calculate this with a calculator. I feel it in the hum. Slow travel? More heat. Fast drag? Less. But the math is the same. And when the wind hits, η drops because convective loss spikes.
02. Conductivity as Destiny
Material choice is thermal destiny. Mild steel conducts at ~50 W/(m·K). Aluminum screams at ~200+. Cast iron crawls at ~40. Change the alloy, change the budget.
| Material | Conductivity [W/(m·K)] | Specific Heat [J/(kg·K)] | Density [kg/m³] |
|---|---|---|---|
| Mild Steel (A36) | 50 | 490 | 7850 |
| Carbon Steel (1045) | 46 | 500 | 7850 |
| Stainless 304 | 16 | 500 | 7900 |
| Aluminum 6061 | 200 | 897 | 2700 |
| Cast Iron (Gray) | 40 | 460 | 7200 |
Notice stainless? Low conductivity means heat stays local. That’s why it warps less—but also why it burns through if you don’t feather the arc.
03. Diffusion Geometry
Heat doesn’t spread evenly. It follows Fourier’s law: flux proportional to the negative gradient.
q = Heat Flux [W/m²]
k = Thermal Conductivity
∇T = Temperature Gradient
In practice: preheat reduces the gradient. No preheat? The gradient is a cliff. The metal fractures trying to climb it.