The most expensive sentence on a fabrication floor is “it’s stainless, she’ll be right.” The old Alltrade counter at least named 304, 316 and 430. That is the identification habit this essay is defending.
You cannot see molybdenum. You cannot taste nickel. You can ask for paper, you can use a magnet as a first sort, and you can refuse to guess. ASSDA notes and Atlas Steels grade data exist so a buyer can match a number to a chemistry. They do not exist so a site supervisor can invent a grade from a brushed face.
This journal will not run a laboratory. It will tell you when you are bluffing. The grades and alloys hub is the family tree; this page is the refusal to treat the tree as decoration.
Start with the paperwork
A mill certificate (MTCs, 3.1 documents, whatever the merchant calls the PDF) should name the grade, the heat, and a chemistry. Match the heat number to a stamp on the sheet or bundle where it still exists. If the offcut on your rack has no story, treat it as unknown metal — useful for a bracket test, dangerous as a coastal post.
Purchase orders should already say 304, 304L, 316, 316L or 430. If they say marine grade or food grade, send the buyer back to what marine-grade means and food-grade stainless in Australia. Adjectives are not UNS numbers.
Historic Melbourne custom-cut dockets named the three stocked grades because a blank without a name becomes a callback. Keep the PDF in the job file. Keep a photocopy or a clear photo of the bundle stamp before the film comes off and the remnant is mixed into the rack.
The magnet, used honestly
- Strongly magnetic, indoor, cheap sheet: often 430.
- Non-magnetic, kitchen or architectural sheet: often 304 or 316.
- Slightly magnetic after a heavy fold: still could be 304 or 316 after cold work.
- Magnetic and specified as duplex: possible; do not “correct” it to 430.
The magnet never separates 304 from 316. That is the whole scam of the pocket test. 304 versus 316 is a chemistry and environment decision, not a fridge-magnet decision. Duplex alloys sit in duplex versus 316 and will often attract a magnet because ferrite is half the microstructure.
| Observation | Honest reading | Dishonest reading |
|---|---|---|
| Magnet sticks hard | Could be 430, duplex, or cold-worked austenitic | “Fake stainless” |
| Magnet does not stick | Could be 304 or 316 (annealed) | “Must be 316” |
| Slight pull after folding | Cold work on an austenitic | “It turned into 430” |
| Fastener unmarked | Unknown — do not install as A4 | “It’s stainless, she’ll be right” |
| Certificate matches stamp | Identified metal | Optional paperwork |
Shop-floor clues that are not proof
Finish is not a grade. No. 4 brushed 430 and No. 4 304 can look related in a dim rack. Thickness is not a grade. A spark test can hint carbon steel versus stainless; it will not print a UNS number. Colour after a weld is a heat-tint story, covered in welding 304 and 316, not an alloy assay. Finishes tell you how the face was rolled or polished, not what is underneath.
Portable XRF and optical emission tools exist. They are how a plant confirms incoming goods. They are not a substitute for buying identified metal in the first place. A hire-gun reading on a dirty, coated, or thin-plated surface can lie. Clean metal, a known standard, and an operator who has done this before — or send a coupon to a lab.
Molybdenum spot tests exist in some shops. They can separate a 316 from a 304 when used correctly. They are still a check on incoming goods, not a replacement for the heat number on a coastal balustrade.
Australian duty: why the guess is expensive
Coast
A 304 post sold as 316 on an Australian coastal balcony is a tea-stain machine with a lawyer attached. Chloride films do not care that the salesperson said marine. Coastal care cannot invent molybdenum. Identify before the posts go in. The marine and balustrade topic is full of jobs that fail at the stamp, not at the polish.
Food plant
A welded wet bench needs a named austenitic and a hygienic finish. Calling it food grade without a certificate is how a mystery ferritic lands in a washdown room. Inspectors look at coving and cleanability; engineers look at the alloy. Both are right. Neither is a magnet.
Indoor
Indoor 430 is honest if everyone knows it is 430. Indoor 304 is the default bench. Mixing remnants because “it is all stainless” is how a kickplate alloy becomes a splashback that pits under bleach.
When identification has already failed
If a coastal rail is staining in stripes, you may have mixed metals, residual swarf, or genuine 304 sold as 316. Tea staining explains the surface. A laboratory analysis explains the bar. Do not polish first and test later — you may remove the evidence and the passive film together. Passivation comes after you know what you are resetting.
Unknown bar used as a fastener is how A2 versus A4 jobs go brown at the screw first. If the bag is unlabelled, it is not A4. Head marks exist for a reason; if you cannot read them, do not guess the atmosphere.
A 316 balustrade with one magnetic picket in a run of non-magnetic tube is not a curiosity. It is a mixed bundle. Isolate it before you weld the rest.
Specification language: identify at purchase
Write the grade, the L suffix if welding, the product standard if the engineer named one, and “mill certificate to be supplied and retained.” Write the heat number onto the remnant when you break a sheet. Label the rack with a marker that survives a month of gloves, not a hope.
Refuse these as identification:
- “Marine grade”
- “Food grade”
- “Same as last time”
- “The shiny one”
- “Non-magnetic so it must be 316”
Accept these:
- 304 / 304L / 316 / 316L / 430 / 2205, plus product form
- A2-70 or A4-70 on fasteners
- A matching certificate
How to order so you do not have to guess later
Order named metal. Ask the merchant to print the grade on the docket the way the old Melbourne counter did. Photograph the stamp. File the PDF. If you are sending blanks to a fabricator, send the certificate with them. If you are receiving a prefabricated rail, ask where the heat numbers went.
On site, keep stainless away from carbon grinding dust so a later stain is not mistaken for the wrong alloy. Identification and cleanliness are the same discipline in two directions.
A working rule
Identify at purchase. Label on the rack. Certificate in the job file. Guessing on the day of install is how a custom-cut blank becomes a callback. If you cannot name the grade, you cannot name the environment it is allowed to enter.
Related pages for the alloys you are trying to tell apart: what 430 is, 304 versus 316, duplex versus 316, and food-grade language. Use them as specifications, not as vibes.