Most foundries check casting chemistry with an optical emission spectrometer. A small sample is poured from each melt, ground flat, and sparked in the instrument, which reports the percentage of each element in a minute or two.
The spectrometer lets the foundry correct the melt before pouring. It is the single most important piece of lab equipment in a serious foundry, and you should expect to see one.
How the test works
An electric spark vaporises a tiny amount of metal from the sample surface. Each element gives off light at its own wavelengths, and the instrument measures the intensity. Software converts this into concentrations using calibration curves.
For cast iron, the sample is usually poured into a chill mould so it solidifies white, without free graphite. Graphite in the sample would upset the carbon reading. This is why the sampling step matters as much as the instrument.
What is measured
- Carbon and silicon, which together set the carbon equivalent and how the iron solidifies.
- Manganese, phosphorus and sulphur, which affect strength, brittleness and defects.
- Magnesium for SG iron, because too little gives poor nodules and too much causes other defects.
- Chromium, copper, nickel, molybdenum and tin, which are added for strength or arrive as residuals in scrap.
- For steel castings, carbon, manganese, silicon, the alloying elements, and residuals that affect weldability.
What a spectrometer does not tell you
Chemistry is necessary but not enough. For SG iron, correct magnesium does not prove the graphite formed round nodules. Nodularity and matrix structure are checked by metallography, where a polished sample is examined under a microscope.
Mechanical properties also depend on cooling rate and heat treatment. A melt with perfect chemistry can still give a weak part if a thick section cooled slowly. That is why tensile tests and hardness checks sit alongside chemistry on a proper material test certificate.
Questions to ask the foundry
- Do you have your own spectrometer, and how often is it calibrated against certified reference samples?
- At what stage do you take samples: in the furnace, after treatment, or at pouring?
- Do you keep a record of every melt's chemistry linked to heat numbers?
- For SG iron, how do you check nodularity, and how often?
- Can you send a sample analysis for a recent heat in the grade we need?
Independent checks
For a new supplier or a critical part, have some samples checked at an independent accredited laboratory. Cut a piece from an actual casting, not just from the foundry's test bar, if you want to know what is in the part itself. Comparing the two results tells you whether the foundry's own lab can be trusted. Our guide on reading a lab certificate explains what the report should contain.
Next step
We review a foundry's lab and records during the audit, and we can have castings tested by an independent laboratory before shipment. To plan testing for your parts, book a free sourcing review.