Steel Plate Marking Guide: Heat Numbers, Methods and Machines

Steel plate marking writes the plate’s ID on the steel: grade, heat number (the batch it came from), size and maker. Mills stamp or paint it. Shops that cut the plate copy it onto each part, often with a dot peen or laser marking machine.

A note on bias: I work for HeatSign. We build dot peen and fiber laser marking machines, not the stamping lines inside steel mills. Standards and figures come from the public sources listed at the end.

What information is marked on a steel plate?

Every structural steel plate carries four things: the steel grade, the heat number, the size, and the mill’s name or logo (ASTM A6). Plate for pressure vessels, such as tanks and boilers, also carries a plate number (ASTM A20). The heat number links the plate to its mill test report.

Fields: ASTM A6 clause 18.1 and ASTM A20 clause 13.1. Examples are made up for this guide, not copied from a real plate.

How to read a plate marking, piece by piece

Here is a made-up structural plate marking, laid out the way many mills paint it:

[mill logo]  ASTM A572 GR 50  HT 2B4567  0.500 X 96 X 240  PL 0012

Why ask for a 3.1 certificate? A 2.2 “test report” can show results from the same grade of steel, but not from your batch. A 3.1 or 3.2 certificate shows tests on the steel you actually received. Even then, the paper only counts if the same heat number is on the plate.

An engineer on an engineering forum put it bluntly: “Anybody can supply a piece of paper (MTR) for a material, that doesn’t mean it is the actual material used.”

How do steel mills mark plates?

Mills use three methods: steel stamps, paint, or tough labels and tags (ASTM A6). Pressure-vessel plate over 6 mm (1/4 in) must be stamped. Thinner plate gets paint or gentle “low-stress” stamps that will not start cracks (ASTM A20). Plates over 1,800 mm get two marks.

Clause numbers come from older published editions of ASTM A6 and A20. They can move between editions, so check the current edition named on your order.

What mills actually use

The same SCI note reports that a large UK plate mill dot-matrix stamps the mill, heat, grade and plate number. Then it paints the size, weight and order number. A German heavy-plate mill stamps the grade, which large plate each piece was cut from, the plate number, the direction the steel was rolled, and the inspector’s mark.

So the mill mark you receive is often already dots. Whether a buyer accepts a dot peen or laser mark in place of a die stamp is a contract question. Agree it with your customer or inspector before the order, not after.

Have a marking clause in your order you are unsure about? Send us the clause and your plate sizes, and we will tell you which of our machines can meet it.

What happens to the heat number when you cut a plate?

When a shop cuts a plate into parts, most parts lose the mill marks. So the shop copies the heat number onto each part, before or right after cutting. Pressure-vessel work requires this (ASME UG-77). For building and bridge steel, the contract decides how strict the tracking must be.

This is where the paper trail usually gets lost: at the cutting table, not at the mill. A 2,400 × 6,000 mm plate can become 60 brackets, and only one or two of them still carry the mill stamp. Being able to trace each part back to its batch is called traceability.

1. Check the plate in. Match the heat number on the plate to the heat number on the test report.
2. Mark before you cut. Write or mark the heat number on every piece that will come out of the plate.
3. Cut. Each part now leaves the table with its own mark.
4. Record it at assembly. When parts are fitted together for welding, note which heat went into which part.
5. Show it at inspection. The inspector checks the marks against the test reports.

Which rules ask for heat number transfer?

UG-77 is summed up as most shops apply it; the piping point comes from ASME interpretation 8-19.

How strict can tracking get? AISC describes three levels. Lot level links a batch of parts to a set of heats. Piece-mark level links each part number to a heat, and is common on main bridge beams. Full piece level links every single piece to its heat, on work such as nuclear plants. Each step up costs more.

Paint pen, dot peen or laser for heat number transfer?

Long heat numbers on small parts are a real problem. One engineer on a piping forum called marking a 12-character heat number on half-inch fittings “nearly impossible”.

The fix is the short ID number that ASTM allows, or a Data Matrix code, a small square barcode a bit like a QR code. See how to mark a Data Matrix code on metal.

Need heat numbers on cut parts every day? Send us your plate thickness and a photo of the surface, and we will suggest a marker.

Which marking method works best on steel plate?

Use a dot peen marking machine when the plate still has mill scale or primer, or will be galvanized. Use a fiber laser marking machine for clean or stainless plate, fine text and Data Matrix codes. Paint, inkjet and labels are fast, but they do not survive blasting or coating.

Sources: SCI guidance note GN 5.05 (stamps), New Steel Construction (inkjet), The Fabricator (plasma), our own depth tests (dot peen and laser).

Where a fiber laser marking machine wins

Where a dot peen marking machine wins

A dot peen marker drives a hard pin into the steel and leaves a line of dents. The pin goes through mill scale, rust and thin primer. A deep dent still reads after the plate is painted or galvanized.

A handheld or magnetic-base model goes to the plate, so you do not have to lift a 2-tonne sheet onto a machine. For a full side-by-side, see permanent metal marking methods compared.

How deep should a mark be under scale, primer, paint or zinc?

Make the mark several times deeper than whatever will cover it. Mill scale and shop primer are only about 6–25 µm thick. Zinc from galvanizing is 85 µm or more, and bridge paint about 300 µm. For galvanized plate, mark about 0.8 mm deep.

Sources: NZ Steel (scale), paint maker data sheets (primer), ISO 1461 and the American Galvanizers Association (zinc), SCI GN 5.05 (bridge paint and hand stamps). 1,000 µm = 1 mm.

The SCI note makes the point for bridges. Hand stamps are 300–500 µm deep, and they are “likely to be substantially obscured” (mostly hidden) by modern bridge coatings. Its advice is to cover (mask) the stamp area during painting.

In daily work, a dot peen mark over 0.5 mm is already deep. Our pneumatic markers reach about 1 mm at most in mild steel. So 0.8 mm for galvanizing sits near the top of the range: use slower settings or extra passes, and test a sample first.

The plate below was marked about 0.8 mm deep. Mill scale wears pins faster, so plan to change pins every 3–6 months on heavy use.

The other route is to mark after coating. A fiber laser marking machine can mark zinc or burn through paint once the plate is finished. See whether a laser mark is readable after painting or galvanizing.

Which machine fits your plate marking job?

For cut parts in a fabrication shop, use a pneumatic (air-powered) dot peen marker; it marks through scale and primer. For yards and sites with no compressed air, use a battery handheld dot peen marker. For nameplates, use a tag marker. For barcodes on clean plate, use a fiber laser marking machine.

Steel Plate Marking FAQ

Steel plate marking has one job: to prove which batch each piece of steel came from, from the mill to the finished part. So decide two things first: who copies the heat number after cutting, and what coating goes on next. The right method and depth follow from those two answers.