01. Carbon is the Switch
Add carbon to iron and you change the game. 0.05% C: soft, ductile, forgiving. 0.8% C: hard, brittle, hungry for preheat.
The HAZ doesn’t care about your intentions. It responds to chemistry.
02. The Three Steels
A36 Mild Steel
Carbon ≤0.26%
Yield Strength 250 MPa
Conductivity 50 W/(m·K)
Melting Point 1370–1510°C
Yield Strength 250 MPa
Conductivity 50 W/(m·K)
Melting Point 1370–1510°C
Behavior: Forgiving. Absorbs thermal shock. Ideal for field work in variable conditions. Preheat optional below −10°C.
1045 Medium Carbon
Carbon 0.43–0.50%
Yield Strength 565 MPa
Conductivity 46 W/(m·K)
Hardness (quenched) HRC 55+
Yield Strength 565 MPa
Conductivity 46 W/(m·K)
Hardness (quenched) HRC 55+
Behavior: Demanding. Requires controlled cool rate. Preheat ≥150°C mandatory outdoors. Post-weld bake to relieve stress.
304 Stainless
Chromium 18–20%
Nickel 8–10.5%
Conductivity 16 W/(m·K)
Expansion Coeff. 17.3 μm/(m·K)
Nickel 8–10.5%
Conductivity 16 W/(m·K)
Expansion Coeff. 17.3 μm/(m·K)
Behavior: Traps heat locally. Warps less but burns easier. Use pulsed GMAW or TIG. Purge argon or risk chromium carbides.
Rule: Above 0.4% carbon, you are no longer welding. You are heat-treating. Respect the phase diagram.
03. The Fillers
ER70S-6 forgives sulfur. ER308L demands purity. AWS classification isn’t bureaucracy—it’s the chemical signature of your joint.
I keep three rods on the bench. Nothing more. Precision over variety.