Carbon must stay below about 0.08% in most stainless grades because carbon ties up chromium as carbides and leaves the steel beside them unprotected. The carrier’s route of blowing the melt down to low carbon and then adding ferrochrome undoes itself if the ferrochrome was made by carbon reduction, because that alloy carries several percent carbon: a 100 kg melt given 26 kg of it ends at roughly 1.6-2.1% carbon.
Part of Stainless Steel Production
Keeping carbon low enough that chromium stays in solution: the carbon budget, where carbon enters the melt, and what a village can do about it.
Carbon Steals Chromium From The Steel Around It
The carrier states the rule: carbon should stay below 0.08% for most grades, and above 0.12% chromium carbide forms at grain boundaries on cooling. Those carbides take chromium from the adjacent metal, leaving zones that fall below the 10.5% needed for passivity (How Chromium Creates Passivation). The carrier calls this sensitization and names it the most common failure mode of amateur stainless.
The Carbon Budget Is A Mass Balance
Carbon enters the melt from the base steel, the ferrochrome, the electrodes and the lining, and leaves by oxidation. The ferrochrome term dominates once chromium is added, because the alloy is made by reduction with carbon (Ferrochrome Production). I recall carbon-reduced ferrochrome at 6-8% carbon, a figure that is not in the corpus and that I cannot source.
Worked Example
Inputs. Target 17% chromium (inside the carrier’s 16-18% ferritic range). Ferrochrome 65% chromium (midpoint of the carrier’s 60-70%). Ferrochrome carbon 6% or 8% (recalled). Base steel after blowing: 0.03% carbon (an assumed value, not a corpus figure).
Ferrochrome needed per 100 kg melt. 17 / 0.65 = 26 kg, leaving 74 kg of base steel.
Carbon added.
- From ferrochrome at 6%: 26 × 0.06 = 1.56 kg.
- From ferrochrome at 8%: 26 × 0.08 = 2.08 kg.
- From base steel: 74 × 0.0003 = 0.022 kg.
Result. 1.56 + 0.022 = 1.58 kg in 100 kg, or about 1.6% carbon; at 8% ferrochrome, 2.08 + 0.022 = 2.10 kg, or about 2.1%. The carrier’s limit is 0.08%.
How much chromium could that lock up? If all carbon formed Cr₇C₃, the chromium-to-carbon mass ratio is (7 × 52.0) / (3 × 12.0) = 364 / 36 = 10.1 (atomic masses recalled). So each 0.1% carbon could tie up about 1% chromium, and 1.6% carbon could tie up about 16% chromium, nearly all of the 17% added. Real carbides also hold iron, so this is an upper bound, not a prediction. At 0.08% carbon the same bound is 0.08 × 10.1 = 0.8% chromium, which is why the carrier’s limit is a workable number.
The same arithmetic for a martensitic target of 12% chromium: 12 / 0.65 = 18.5 kg of ferrochrome, and 18.5 × 0.06 = 1.1 kg of carbon, or about 1.1%, still far over the carrier’s 0.15% for that grade.
What A Village Can Actually Do
- Use low-carbon scrap as the chromium carrier. Salvaged stainless is already low in carbon; remelting it, with only a small ferrochrome top-up, keeps the budget small.
- Accept the carbon and make a high-carbon martensitic grade. A carbon-rich melt at 12-14% chromium is the carrier’s martensitic family, harder and less corrosion-resistant (Martensitic). It will not match the carrier’s 0.15% maximum.
- Refine after adding chromium. Industrial practice uses oxygen blowing diluted with inert gas or under vacuum to burn carbon without burning chromium, as I recall. The corpus has nothing on it and I cannot source a village version.
- Make low-carbon ferrochrome. That uses a different reduction route I cannot source.
Other Carbon Sources To Watch
- Electrodes and charcoal: carbon electrodes or charcoal in contact with the melt can add carbon, as I recall; I cannot source the amount.
- Crucible linings: graphite-bearing crucibles (Crucibles) are a carbon source for a remelt. Prefer a non-carbon lining for stainless.
Common Mistakes
- Counting carbon only in the scrap. The ferrochrome term is the larger one.
- Reading the spark test as a carbon analysis. Metal Identification uses sparks to sort steels roughly by carbon; it cannot tell 0.08% from 1.5% reliably in an alloy.
- Skipping a sample quench. Heat a small piece of the finished steel and quench it; if a file then skates over it, the carbon is far too high for the austenitic or ferritic targets.
Safety
Blowing a melt
Blowing air or oxygen through a melt throws sparks and slag, and the carbon burns off as carbon monoxide (CO Risk). Use a long lance, stand clear, work outdoors, and keep every tool and lance dry before it touches the bath.
What’s Next
- Learn next: Alloy Addition Sequence - ordering the additions around the carbon budget
- Combine with: Solution Annealing - dissolving carbides after the fact