- A magnet separates 400-series (magnetic) from 300/200-series (non-magnetic) steel, but cannot tell 304 from 202 or 316.
- A molybdenum spot test kit tells 316 from 304 in a minute.
- PMI (handheld XRF) identifies the grade reliably but cannot measure carbon, so it cannot separate 304 from 304L.
- The mill test certificate plus a matching heat number on the material is the simplest proof of grade.
Stainless steel grades look identical. A sheet of SS 202 costs much less than SS 304, and SS 304 much less than SS 316, so it is worth knowing how to check what you have been supplied. This guide covers the tests in order of effort, from a fridge magnet to a laboratory, and explains what each result actually proves.
1. The magnet test
Hold a strong magnet against the material.
- Strongly magnetic: a 400-series grade such as 430 (ferritic) or 410 (martensitic), or plain mild steel. Duplex grades are also noticeably magnetic.
- Not magnetic, or only very weakly: an austenitic grade: 304, 316, 321, 310, or a 200-series grade such as 202 or 201.
The magnet test separates 400-series from 300/200-series. It cannot tell 304 from 202, or 304 from 316, because all of them are austenitic and non-magnetic in the annealed state. Note also that cold working (bending, deep drawing, heavy rolling) makes 304 slightly magnetic at the worked area, so a weak pull at a bend does not mean the material is 202.
2. Looks and surface
Unreliable, but worth a glance. 200-series sheets often have a slightly duller, yellowish tone compared with the blue-grey of 304, and 430 looks brighter and whiter. Finishes, PVC film and lighting change all of this, so never buy on appearance alone.
3. The spark test
Touch the edge of the material lightly to a bench grinder in a dim area and watch the sparks.
| Material | Spark pattern |
|---|---|
| Mild steel | Long, bright yellow-white streams with many bursts |
| SS 430 / 410 | Shorter streams, few bursts, reddish-orange tint |
| SS 304 / 316 | Short, dull orange streams with very few bursts (nickel suppresses them) |
| SS 202 / 201 | Similar to 304 but often more bursts (higher carbon and manganese) |
Experienced inspectors can tell 304 from 202 this way, but it takes practice and a known sample to compare against. Treat it as a hint, not a verdict.
4. Copper sulphate test (stainless or not)
Clean a small area and put a drop of copper sulphate solution on it. On mild steel the drop quickly plates out as a copper-coloured spot. On stainless steel nothing happens. This test only confirms the material is stainless; it does not identify the grade.
5. Molybdenum spot test (316 vs 304)
Commercial test kits detect molybdenum, which 316 contains (2–3%) and 304 does not. A drop of the reagent turns dark or coloured on 316 and stays clear on 304. This is the quickest field test for the most expensive confusion in the trade, 304 sold as 316, and costs very little per test. Follow the kit instructions and test on a cleaned, unpolished spot.
6. Manganese spot test (202 vs 304)
200-series grades contain 5–10% manganese against 2% maximum in 304. Spot-test kits for manganese exist, and some suppliers use a nitric-acid based test, but these need care and proper safety equipment. For a decisive answer between 202 and 304, PMI is the sensible route.
7. PMI: positive material identification
A handheld X-ray fluorescence (XRF) analyser reads the chemistry of the surface in a few seconds and reports the grade: chromium, nickel, manganese, molybdenum and the rest. It is non-destructive and accurate to a fraction of a percent for the main elements. Most inspection agencies and many larger fabricators own one, and PMI testing services are available in every industrial area, including Vadodara, for a modest per-piece fee.
Limits: XRF reads the surface, so clean off paint, scale and PVC film first, and it does not measure carbon, so it cannot distinguish 304 from 304L. For that you need a laboratory test (optical emission spectroscopy, OES).
8. Read the mill test certificate
Every lot we supply comes with the mill test certificate (MTC), which lists the heat number, the actual chemistry and the mechanical test results. The heat number is also hard-stamped or stencilled on plates and printed along coils and pipes. Match the number on the material to the number on the certificate, and you have traceability without a single test. Our guide to reading an MTC explains each column.
Which test for which question
| Question | Test | Confidence |
|---|---|---|
| Is it stainless at all? | Copper sulphate drop, or magnet plus appearance | High |
| Is it 400-series or 300/200-series? | Magnet | High |
| Is it 316 or 304? | Molybdenum spot kit; PMI to confirm | High |
| Is it 304 or 202? | PMI (manganese and nickel readings) | High with PMI; low with spark test alone |
| Is it 304 or 304L? | Laboratory OES (carbon); or trust the MTC | High |
| Does it match the certificate? | Heat-number check plus PMI on a sample | High |
Practical advice for buyers
- Buy from a stockist who supplies the original mill test certificate and can show the heat number on the material.
- For large or critical orders, ask for PMI on a sample before dispatch, or have your third-party inspector witness it.
- Keep a molybdenum test kit in the stores if you buy both 304 and 316; it pays for itself the first time it catches a mix-up.
- If a quoted price for "304" is far below the market, it is probably 202. Compare rates on our price list page.
Every sheet, coil, pipe and bar from Sagar Sales Corporation is supplied with its MTC, and we can arrange PMI or third-party inspection on request. See our quality and certificates page or get in touch.