Add the alloying elements in an order that keeps each from being burned off or contaminated: melt the base steel, blow the carbon down, skim the slag, then add dry ferrochrome last so its chromium is not oxidised, with nickel charged early where it is not at risk. Weigh every addition and calculate the target, because a village has no analyser, and expect to add extra ferrochrome to cover chromium lost to the slag.
Part of Stainless Steel Production
The order, quantities and checks for adding ferrochrome, nickel and any trim to a stainless melt.
The Order Protects Each Element From The Blow
The carrier’s sequence is charge, melt, decarburize, add ferrochrome, add nickel, adjust, tap. The logic: once chromium is in the bath, blowing it for carbon oxidises chromium into the slag, and the carrier notes chromium inhibits further carbon removal. So everything that must be burned off is burned off before chromium goes in. My own refinement, as I recall and cannot source, is that nickel is not readily oxidised by a blow and can go in with the charge to dissolve while the bath heats; the carrier adds it after ferrochrome, which also works. Whichever you choose, do it consistently so batch records are comparable.
What You Need
- A weighed charge with each lot recorded.
- Dry, crushed ferrochrome (Ferrochrome Production) and nickel (Nickel Sources), preheated.
- A skimming rod and slag pit, dry.
- A sample spoon and a mould for a test button, dry.
Method: One Heat
Step 1: Charge and melt the base steel (and nickel if added early) per EAF Operation for Stainless.
Step 2: Blow and skim. Lower carbon, then skim the iron-oxide-rich slag so the next addition is not reacting with it.
Step 3: Add ferrochrome in dry lumps in portions, letting each dissolve. Cold lumps chill the bath, and damp ones can throw metal.
Step 4: Stir and sample with a dry spoon. Pour a test button for the checks in Achievable Grades and Passivation Treatment.
Step 5: Trim. If the sample says the chromium is short, add a small further dose and re-sample. Record every gram.
Worked Example
Target. 100 kg of austenitic melt at 18% chromium and 8% nickel (the low ends of the carrier’s 18-20% and 8-10%). Ferrochrome 65% chromium.
Chromium without loss. 18 / 0.65 = 27.7 kg of ferrochrome. Nickel 8 kg. Base steel 100 - 27.7 - 8 = 64.3 kg.
With loss. Suppose 90% of the added chromium is recovered (an assumed example; the corpus gives no recovery). Ferrochrome needed = 27.7 / 0.90 = 30.8 kg. Base steel = 100 - 30.8 - 8 = 61.2 kg.
Carbon check. 30.8 kg of ferrochrome at the recalled 6% carbon brings 1.85 kg of carbon into about 100 kg, or 1.8%. This is the problem treated in Carbon Control: no addition sequence repairs it. The sequence only avoids losing chromium; it cannot lower the carbon the alloy itself carries.
Common Mistakes
- Adding ferrochrome before the blow. The chromium oxidises into the slag.
- Adding cold lumps in one dump. The bath chills, the lumps sink undissolved, and the first sample reads low and the second high.
- Trimming without a record. Without analysis the arithmetic is the control.
Safety
Additions into molten steel
Any wet or oil-contaminated addition, or a damp tool, turns to steam on contact with a 1,600 °C bath and ejects metal. Dry and preheat everything, add in small portions, and stand outside the splash line. Fume from the bath carries chromium compounds; keep upwind.
What’s Next
- Learn next: Achievable Grades - what composition to aim for
- Combine with: Carbon Control - the limit on the whole route