Preheating steel before welding slows the cooling rate in the heat-affected zone (HAZ), giving hydrogen time to diffuse out and preventing the hard, crack-prone microstructures — primarily martensite — that form when high-CE steels cool too fast. The amount of preheat needed rises with the steel’s carbon equivalent, its thickness, and the hydrogen content of the electrode. This tool computes the carbon equivalent and suggests a minimum preheat and interpass band.
How it works
The carbon equivalent uses the IIW formula adopted by AWS D1.1 and EN 1011:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15All inputs are weight percentages from the steel’s mill certificate. The result is banded against thickness and electrode hydrogen level to give a recommended minimum preheat.
Understanding each term in the CE formula
The IIW carbon equivalent rolls several alloying elements into a single hardenability number, with each element weighted by its relative contribution to HAZ hardness:
- Carbon (C) — the dominant hardenability element; has a coefficient of 1 because it is the reference.
- Manganese (C/6) — strong hardenability agent but weaker than carbon; the divide by 6 reflects this.
- (Cr + Mo + V) / 5 — carbide-forming elements that increase hardenability; divided by 5.
- (Ni + Cu) / 15 — austenite stabilisers with mild hardenability effect; divided by 15 because their contribution is weaker.
A higher CE means the HAZ will be harder after welding, increasing the risk of cold (hydrogen-induced) cracking in the absence of adequate preheat.
Preheat bands by CE and thickness
These are general guidance bands consistent with the AWS D1.1 hardness-control approach:
| CE | Thin sections (up to ~19mm) | Thicker sections (>19mm) |
|---|---|---|
| Below 0.40 | No preheat usually needed | 50–75 °C |
| 0.40–0.45 | 50–100 °C | 100–150 °C |
| 0.45–0.60 | 100–150 °C | 150–200 °C |
| Above 0.60 | 200 °C minimum | 200–250 °C or higher |
Electrode hydrogen level adds to the requirement: high-hydrogen electrodes (cellulosic, E6010 type) demand higher preheat than low-hydrogen electrodes (E7018, wire with flux-cored or gas-shielded process). Always use low-hydrogen consumables on higher-CE steels.
Worked example
A structural steel with the following chemistry from the mill certificate:
- C = 0.18%, Mn = 1.20%, Cr = 0.20%, Mo = 0.10%, Ni = 0.15%, Cu = 0.20%
CE = 0.18 + (1.20/6) + (0.20 + 0.10 + 0)/5 + (0.15 + 0.20)/15
CE = 0.18 + 0.20 + 0.06 + 0.023
CE ≈ 0.46At CE 0.46, a plate section over 19 mm would typically require 150 °C minimum preheat with low-hydrogen electrodes, or higher with cellulosic. For thinner welds (under 10 mm), 100 °C may be adequate.
Interpass temperature
Interpass temperature is the metal temperature measured immediately before the next weld pass is deposited. Both a minimum and a maximum apply:
- Minimum = preheat temperature. The metal must not cool below preheat between passes, or hydrogen cracking risk reappears.
- Maximum = depends on material. For standard C-Mn and low-alloy steels, interpass limits are commonly 230–315 °C. For quenched-and-tempered (Q&T) steels, the maximum is tighter — often 230 °C or as specified by the steel supplier — because excessive heat input between passes degrades the quenched microstructure and reduces toughness.
Monitor interpass temperature with a contact thermometer or temperature-indicating crayon at a representative point on the weld joint before each pass.
Code compliance note
For ASME, AWS D1.1, or EN 1011 production work, the welding procedure specification (WPS) governs. A preheat from a qualified WPS always takes precedence over this tool’s estimates. Use this tool for planning, estimating, and checking whether a preheat requirement is likely before writing the WPS.