# Alloy Melting & Casting

> Vacuum induction melting (VIM) for precise thermocouple alloy composition control. Every melt receives a unique furnace number — the foundation of our batch traceability system.


## Vacuum Induction Melting

### Process

The melt runs in a fixed sequence — the order matters, because chemistry can only be corrected while the bath is still liquid:

1. **Charge** — raw metals (nickel, chromium, silicon, manganese, aluminium, iron, copper) weighed to ±0.01 kg.
2. **Evacuate and back-fill** — chamber pumped below 10⁻² mbar, then back-filled with high-purity argon.
3. **Melt** — induction coils heat the charge to 1500–1650 °C; electromagnetic stirring keeps the bath homogeneous.
4. **Sample** — a mid-melt sample is drawn for spectrometer analysis.
5. **Adjust** — composition is corrected iteratively until every element is within ±0.1% of the target specification.

### Equipment

- Leybold VIM furnace — 500 kg capacity
- Optical emission spectrometer (OES) for real-time chemistry
- Carbon/sulphur analyser (LECO)
- Oxygen/nitrogen/hydrogen analyser
- Every melt: unique furnace number, full chemistry record

## Why VIM matters for thermocouple accuracy

The Seebeck coefficient of a thermocouple depends directly on its alloy composition. A **0.1% shift in chromium content** in **Type K** Chromel changes the EMF output by approximately **2–3 µV at 1000 °C** — equivalent to a 0.5–0.8 °C measurement error. Air-melted alloys typically have ±0.5% composition tolerance. VIM holds this to ±0.1%.

For batch-to-batch consistency — critical for aerospace (AMS 2750) and power generation applications where sensors from different batches are used side-by-side — VIM melting with spectrometer verification is the gold standard.