How to identify a stainless steel alloy before you weld it

Start with a magnet and finish with a handheld XRF gun, and the family shows itself in one afternoon. A hard pull means ferritic 400-series stock, usually 409 at 10.5-11.75% chromium, or zinc- or aluminum-coated carbon steel at well under 1%; a weak pull means austenitic 304 or 316, which run 18.0-20.0% chromium with 8-10.5% nickel, and the molybdenum reading — trace on 304, 2.0-3.0% on 316 — splits the two. Those percentages pick the TIG filler on the spot: ER308LSi for the 300-series, ER316L when molybdenum shows, low heat and no preheat for ferritic 409. The gun cannot read carbon, so the split between 304L at 0.03% maximum carbon and 304 at 0.08% needs optical emission spectroscopy. Work the sequence below on filed coupons, weld one test piece per group, and keep mystery metal out of load-bearing or pressure work — for brackets, guards and stands it is exactly right.

What the magnet actually tells you

The magnet sorts families, not grades. Ferritic 409 and 430 pull hard, the way plain steel does. Austenitic 304 and 316 barely pull at all in the annealed state, and cold work changes that: bends, rolled edges and drawn tube from a 300-series heat pull harder than the flat stock they came from. Weld beads read differently again, because ER308L weld metal carries a little magnetic ferrite the parent metal never had.

Sweep one magnet across flat stock, bends, cut ends and any existing welds, and write the pull down at each spot. A uniform weak pull puts you in the 300-series. A hard pull on everything puts you in ferritic 409 or 430, or in zinc- or aluminum-coated carbon steel — the coating behavior that also drives stainless and galvanized contact corrosion when the two metals touch.

What you need on the bench

One strong magnet, ceramic or neodymium, used the same way on every piece. One new file for bright bare metal, a cutoff wheel or bandsaw for coupons, tags and a paint pen, and a damp rag. Add a known 304 or 316 offcut if you have one, for the spark comparison. The XRF gun belongs to the machine shop, scrap yard or inspection service — you bring them filed coupons.

The identification sequence, step by step

1. Record what is already on the metal. Photograph stamps, stencils and stickers before cutting anything. Mill markings run along tube and sheet edges; a line reading ASTM A240 usually carries the grade next to it, so read all of it.
2. Split the stash into groups and tag them. Group by form, thickness and surface finish, and give each group a letter. Two pieces that look alike can come from two heats, so plan on at least one coupon per group.
3. Sweep the magnet over every group. Test flat stock, bends, cut ends and existing welds with the same magnet. A uniform weak pull points to the 300-series; a hard pull points to ferritic or coated stock.
4. File a cut edge on a coupon to bright bare metal. Look at the cross-section in good light. A distinct silvery skin over darker steel is an aluminum or zinc coating; one uniform grey metal is solid stainless. Label the filed spot, because that is where the gun reads.
5. Wipe the filed spot with a damp rag and come back in an hour. Heavy orange rust in the filed spot points at a carbon-steel substrate; stainless shows nothing or faint staining. This hour-long check separates aluminized stock from 409 before any instrument is involved.
6. Run the spark comparison if you have a known sample. Carbon steel throws long, bushy, forked sparks; austenitic stainless throws few, short, straight red streaks. Treat it as a sorting aid, not a grade call.
7. Take the filed coupons to an XRF gun. Machine shops, scrap yards and inspection services carry handheld analyzers, and a shot takes a few seconds in alloy mode. Let the trained operator run it; the beam stays on the metal and away from people under their radiation-safety procedure. Ask for the element percentages on the printout, not only the instrument's grade label.
8. Match the numbers to the grade windows and weld a coupon. Work the percentages through the table below, then burn one test weld per group with the filler the chemistry points to. Nick-break or bend it before the stash goes into real parts.

Reading the chemistry against the grade windows

Handheld XRF fires X-rays at the metal and reads what comes back, reporting each alloying element as a percentage. It sees titanium, chromium, manganese, nickel, molybdenum and copper without trouble. Carbon is too light an element for any field XRF, so the L split — 304L holds carbon at 0.03% maximum against 0.08% for 304 — needs optical emission spectroscopy on a prepared spot, mobile or lab-based.

Match the percentages against the composition windows, and most stashes collapse to one line. Ask the operator for the raw numbers, because a library label reading 304 on a nickel result of 0.12% is the instrument guessing, not the metal. For which grade a job actually needs once the metal is known, the 304 versus 316L comparison covers salt air, chlorides and washdown.

Windows per ASTM A240 composition ranges for sheet and plate; a certified mill test report from the supplier wins over any table when one exists.

The stash of magnetic thin-wall tubing: 409 or aluminized steel

This is the classic bench case: exhaust-style thin-wall tube and matching sheet, all one type, a hard magnet pull everywhere, and two magnetic candidates — ferritic 409 and aluminized carbon steel.

Inputs, all from the bench work above:

• Magnet: hard pull on the flat tube, at the bends and at the cut ends.
• Filed cross-section at a cut end: one uniform grey metal, no bright skin over darker steel.
• Damp-rag check after one hour: faint staining only, no heavy orange rust.
• XRF on the filed spot, alloy mode: chromium 10.8%, nickel 0.12%, molybdenum 0.03%.

The rule in words: match every element against the ASTM A240 windows, and let nickel eliminate first, because its window is the widest separator between the candidate families.

• 304 holds nickel between 8.0% and 10.5%. The reading of 0.12% sits 8.0 - 0.12 = 7.88 points below the floor. Out.
• 430 holds chromium between 16.0% and 18.0%. The reading of 10.8% sits 16.0 - 10.8 = 5.2 points below the floor. Out.
• 409 holds chromium between 10.5% and 11.75% and nickel at 0.5% maximum. The reading of 10.8% chromium and 0.12% nickel sits inside both windows. Match.

Result: 409-family ferritic stainless. Decision: call it 409 whenever the hard pull comes with chromium 10.5-11.75% and nickel under 0.5%. Weld it with ER409Nb, or with ER308L for non-critical work — short arc, single pass, no preheat, because heavy heat input grows ferritic grains and the weld loses toughness. Had the filed spot read chromium under 1% beneath a bright coating skin, the call flips to aluminized carbon steel, welded with ER70S-2 as ordinary carbon steel. Guess 304 and weld hot, and the penalty is a cracked bead under exhaust vibration.

Picking the TIG filler for each family

Decide straight off the readings: nickel 8-10.5% with molybdenum trace means 304 and ER308LSi; nickel 10-14% with molybdenum 2-3% means 316 and ER316L; chromium 10.5-11.75% with nickel under 0.5% means 409 and the low-heat routine; chromium under 1% under a coating means aluminized steel and ER70S-2.

When the weld shows — a visible bracket, a seam on display — the finish counts as much as the rod, and enclosure weld finish specifications cover dressing and passivation. Stainless enclosure gauge, finish and weld care covers the rest of the fabrication.

Checks that prove the call

Weld one coupon per group with the chosen filler, then bend it cold in a vise through about 90 degrees over a rod close to the stock's own thickness. A sound bead holds with no cracks at the toes; a nick-break through the weld shows complete fusion. Re-shoot one piece per group after you cut into it, because that catches a mixed stash the first pass missed. Paint-pen each group with the grade, the filler and the date, and keep the coupons with the record — they are the proof if anyone asks later.

Mistakes that cost you the answer

Reading the coating instead of the metal is the number one error. The gun reports whatever sits in front of it, so give it the filed substrate, and hand the lab the filed coupon. Extending one result across an untagged pile is the second — one reading per group, minimum, and a re-shoot after cutting. Trusting the instrument's library label over the percentages is the third; 0.12% nickel can never be 304, whatever the screen says. And keep unidentified stock out of anything that carries load, pressure or a safety function. Buy traceable metal for those jobs, and match the grade to the site — what 304 tolerates in coastal air is worth reading before you build brackets for the shore.

When a job needs known metal rather than a mystery, order it that way: our stainless enclosures come in 304 or 316L, cut to your drawing, and are delivered in 10 business days counted from your approval of the drawing. The grade arrives written on the print, which is the one identification method that never fails.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

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