What is carbon equivalent and why does it matter?

Carbon equivalent (CE) rolls a steel's alloy content into a single number representing hardenability. Higher CE means a greater tendency to form hard, crack-prone martensite in the heat-affected zone, which is why higher-CE steels need more preheat.

Which carbon equivalent formula does this use?

It uses the IIW formula adopted by AWS and EN 1011: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. All inputs are weight percentages taken from the mill certificate.

How is the preheat temperature determined?

Preheat rises with carbon equivalent, thickness, and diffusible hydrogen. This tool maps CE bands to recommended minimum preheat consistent with the AWS D1.1 hardness-control method; for code work always confirm against the governing table and procedure.

What is interpass temperature?

Interpass temperature is the metal temperature just before the next weld pass is deposited. Too low risks cracking; too high degrades toughness, especially in quenched-and-tempered steels, so a maximum is specified by group.

Can I trust this for code-critical welds?

Use it for planning and estimating. For ASME or AWS D1.1 production work the welding procedure specification (WPS) and the governing code table are the authority. Preheat from a qualified WPS always takes precedence.

Weld Preheat & Interpass Temperature Calculator

Preheating steel before welding slows the cooling rate in the heat-affected zone, giving hydrogen time to diffuse out and preventing hard, crack-prone microstructures. 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)/15

The result is banded against thickness and electrode hydrogen level to give a recommended minimum preheat. As a guide: CE below 0.40 often needs little or no preheat on thin sections, CE of 0.40 to 0.45 typically needs 100 C, 0.45 to 0.60 needs 150 to 200 C, and above 0.60 needs 200 C or more with low-hydrogen consumables.

Notes and example

A steel with 0.18 C, 1.20 Mn, 0.20 Cr, 0.10 Mo, 0.15 Ni, 0.20 Cu gives CE = 0.18

0.20 + 0.06 + 0.023 = about 0.46, placing it in the 150 C preheat band for thicker sections. Interpass maximums are commonly 230 to 315 C for C-Mn and low-alloy steels but are kept lower (often 230 C or per the supplier) for quenched-and-tempered grades to protect toughness. Always defer to a qualified WPS for production work.