On Chinese pump quotes, “SS304” is often a paint colour, a nameplate, or a 304 impeller dropped into an iron volute. The cast equivalent of 304 is CF8 / ASTM A351 Grade CF8 (GB/T 2100 ZG06Cr19Ni10). If the foundry cannot say which of those they poured, they have not offered 304. This note is a buyer protocol, not a laboratory accreditation. Use it to fail a dishonest heat before it reaches a chloride line.
Name the alloy as a casting, not as a slogan
Wrought 304 (UNS S30400, GB 06Cr19Ni10) is bar, plate and pipe. Pump casings and most closed impellers are cast. Asking for “304 casing” and receiving a mill cert for 304 bar is a classic swap: the cert is real, the volute is not from that heat. Write the purchase order as casing and cover: ASTM A351 CF8 / GB/T 2100 ZG06Cr19Ni10, impeller either the same or a named wrought grade if it is machined from bar, shaft as 304/304L or 316/316L bar with a specified diameter. “不锈钢水泵” in a Chinese catalogue almost always means an iron or 201 body and a brighter impeller.
What the substitution actually is
Three swaps show up constantly. First, 201 / 202 (high manganese, nickel often 3.5–5.5 %) sold as 304. Second, ferritic 430 or a mixed heat that is strongly magnetic, sold as “stainless”. Third, HT200 / HT250 iron with a stainless-coloured coating, a 304 nameplate and sometimes a 304 wear ring at the suction eye so the first PMI spot looks honest. A milder fraud is real CF8 with nickel pushed down toward 8 % or carbon left high so the casting weld-repairs and then knife-line corrodes.
| Check | Honest CF8 / 304 | Typical fake |
|---|---|---|
| Ni (wt %) | CF8 typically 8–11; wrought 304 8.0–10.5 | 201 often 3.5–5.5; “thin 304” 6–7.5 |
| Mn (wt %) | 304 / CF8 usually ≤ 2 | 201 often 5.5–7.5 |
| Mo (wt %) | ~0 in 304/CF8; 2.0–3.0 in CF8M/316 | 316 quoted, XRF shows no molybdenum |
| Magnet | Cast CF8 weakly magnetic (δ-ferrite). Fully annealed 304 almost none; cold-worked bar a little. | 430 and iron strongly magnetic; 201 often more magnetic than CF8 |
| C (needs OES / combustion, not XRF) | 304L / CF3 ≤ 0.03; CF8 / 304 ≤ 0.08 | High-C heat that looks 304 on XRF and fails after weld repair |
Shop-floor tests that are cheap and not sufficient
Magnet. Carry a rare-earth magnet, not a fridge magnet. A volute that slams onto the magnet is not CF8. A volute that does not, still might be 201 or a low-nickel austenitic. Use the magnet to fail iron and 430 in ten seconds. Do not use it to pass 304.
File and grind. Iron throws dull red sparks and a dark grind; 304 a thinner straw spark. This is a foundry-gate screen, not evidence. Paint and hardfacing lie.
Copper sulfate on a fresh grind (acid-copper, used with gloves and disposal rules): a copper film on the grind is iron. Stainless should stay clean. Coatings and contamination false-positive. Grind through paint first.
Pickling colour. Honest CF8 after pickle/passivate is matte silver-white, not chrome-mirror and not yellow. A rainbow heat tint that was never pickled, or a bright nickel-paint, is a warning, not a grade.
PMI: what XRF can and cannot do
A handheld XRF on a ground spot (paint off, 15–20 mm bright metal) will read Cr, Ni, Mn, Mo well enough to catch 201 and fake 316. It will not read carbon. It will not certify intergranular corrosion resistance. It will read the 2 mm of metal you ground, so a 304 weld overlay on iron will pass if you are lazy. Specify: grind to base metal, three spots on the casing (suction, volute cutwater region, discharge), two on the impeller, one on the shaft, photograph the spark with the serial number in frame, and keep the instrument report with those photos.
If the order is chloride, sour, or food, add OES or combustion carbon on a coupon or a gated riser from the same heat, plus a ferrite number on the casting (CF8 is allowed some ferrite; a fully ferritic reading is the wrong alloy family).
Paper that does not belong to the pump
- A 304 mill cert for φ80 bar dated last year, offered as the casing cert.
- Heat number on the cert that cannot be found as a raised or stamped number on the volute.
- One cert covering casing, cover and impeller when the impeller is obviously a different colour and grain.
- GB/T 1220 bar certificate used for a sand casting.
- EN 10204 3.1 with a supplier name that is not the foundry, and no heat trace to the pump serial.
Bind the heat number to the serial number on the nameplate and the packing list. If the foundry pours once a week and machines 40 casings, you still want the heat on each casing, not “batch 304”. Repair welding on CF8 without a low-carbon grade and a pickle is how fake-looking 304 fails six months later at the weld toe.
The 304 impeller / iron body trick
Open the suction cover. If the impeller is pale and the casing interior is darker, painted, or rusty at a grind, you do not have a 304 pump. Quote lines that say “stainless steel impeller, cast iron casing” are at least honest; treat them as iron pumps with a consumable impeller. Quote lines that say “SS304 pump” and then list HT200 in the small print are the ones this guide exists to kill.
What to put in the RFQ and the inspector’s bag
- Part-by-part grade: casing, cover, impeller, wear rings, shaft, sleeve, fasteners in the wetted zone.
- Casting spec (A351 CF8 or named GB equivalent), not “304”.
- PMI method, grind requirement, number of spots, and that XRF is for alloy family, not carbon.
- Heat number on the casting, 3.1 cert from the foundry, serial-to-heat map.
- Hold point: no paint until PMI. Painted stainless is a PMI refusal.
- For chloride service: reject 304/CF8 unless you have explicitly accepted pitting risk; do not let a fake-304 fight distract you from needing CF8M / 316L / duplex.
KanBeng model tags that list “304” mean a catalogue option, not a verified heat. If a factory cannot walk you through CF8 versus 304 bar versus 201, they are not ready to pour your wet end.