Chemical Composition & Weldability (Carbon Equivalent)
IS 1786 limits carbon, sulphur, phosphorus (and S+P) and, critically, the carbon equivalent (CE). A controlled, low CE is what keeps the bar weldable (IS 9417) and underpins the bend/re-bend ductility — high-carbon-equivalent bar cracks on bending and welds brittle. The chemistry is the root of the mechanical and bend behaviour the structure relies on.
Key Requirements
•C, S, P and S+P limited; carbon equivalent (CE) capped per the grade (D grades typically tighter)
•Low CE → weldable per IS 9417 with controlled procedure; high CE → brittle welds / bend cracking
•Chemistry verified by ladle/product analysis on a representative sample (IS 10790 Part 2 sampling)
•TMT bars: chemistry + thermo-mechanical treatment together give the grade — not chemistry alone
•Don't field-weld bars whose CE / weldability is unknown
Formulas
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
Carbon equivalent — governs weldability and bend behaviour (lower is better)
CE = carbon equivalent (%)C/Mn/Cr/Mo/V/Ni/Cu = element % from chemical analysis
Practical Notes
✓Most rebar weld failures and bend cracking trace to carbon equivalent, not the welder — check CE before welding (IS 9417).
✓TMT bars get strength from the surface-hardened rim + soft core; over-bending or re-bending can expose this — chemistry and CE still bound the behaviour.
Common Mistakes
⚠Welding bars of unknown carbon equivalent / weldability.
⚠Accepting chemistry on a mill tag without representative sampling/testing.
⚠Assuming all 'TMT Fe 500' is equally weldable regardless of CE.