Every serious buyer of pipes, flanges and fittings checks one thing before anything else: the heat number. If the number on the material does not match the number on the certificate, the material is treated as unknown — no matter how good it looks. Understanding heat numbers and traceability is basic hygiene for any metal supplier who wants export or project business.
What is a heat number?
Steel is made in batches called heats — one melt from an electric arc furnace, induction furnace or converter. Each heat has its own chemical composition, so the mill gives it a unique identification code: the heat number (sometimes called melt number or cast number).
All products made from that heat — billets, bars, plates, pipes — carry the same heat number. The mill tests samples from the heat and records the chemistry and mechanical properties on a mill test certificate (MTC). The heat number is the link between the physical product and those test results.
Heat number vs lot number
- Heat number: identifies the melt and its chemistry.
- Lot number: identifies a production or heat-treatment batch within a heat. Mechanical tests may be done per lot because heat treatment affects properties.
Many certificates show both. For forged fittings and flanges, manufacturers may also add their own job or batch number.
Where the heat number appears
- On the material: stencilled, stamped, etched or printed. For pipes it is usually stencilled along the length; for flanges and fittings it is hard-stamped or laser-marked.
- On the mill certificate: listed against each item or size.
- On the packing list and invoice: good exporters list heat numbers per bundle or crate.
Marking of valves, fittings and flanges commonly follows MSS SP-25, which typically requires the manufacturer's name or trademark, material grade, size, rating or schedule and the heat number or code.
Why traceability matters
- Safety: in pressure systems, a wrong material can fail catastrophically. Traceability proves the correct grade was used.
- Codes and regulations: ASME, PED and most project specifications require material traceability.
- Quality investigations: if a failure occurs, the heat number lets everyone trace back to the original test data.
- Buyer confidence: importers and EPC contractors audit traceability during inspection. Missing or mismatched numbers lead to rejection.
Keeping traceability when you cut or process
Traders and fabricators often cut pipes, machine flanges or make fittings from stock. Traceability must survive each step:
- Transfer the heat number before cutting, so every piece carries it.
- Use low-stress stamps or permanent markers suitable for the material — chloride-free markers on stainless steel.
- Maintain a traceability register linking incoming heats, cut pieces, work orders and outgoing invoices.
- When you issue your own certificate for processing, reference the original mill certificate and heat number.
- Keep copies of all mill certificates for the period your quality system or customer requires.
ISO 9001-certified companies are expected to have a documented procedure for identification and traceability.
Common traceability problems
- Unreadable stencils after shot blasting or coating — re-mark before dispatch.
- Certificates with "various" heats but no item-wise breakdown.
- Typing errors when traders retype mill data into their own format.
- Mixed bundles with more than one heat and no tagging.
- Re-certified material where the original mill certificate is missing.
Each of these can stop a shipment at inspection. Our third-party inspection guide explains what inspectors check.
Digital traceability
Many manufacturers still prepare certificates by copy-pasting into Word or Excel, which is where errors creep in. Dedicated software such as MTCMitra stores heat data once and generates EN 10204 3.1 certificates with correct heat numbers, chemistry and mechanical values every time. Combine it with QR codes on tags and buyers can verify certificates instantly.
For a field-by-field list, see our MTC format checklist. For the difference between certificate types, read EN 10204 3.1 vs 3.2.
Checklist for exporters
Before dispatch, confirm:
- every item is marked with a legible heat number,
- the heat numbers on material match the certificates,
- the packing list shows heat numbers per crate or bundle,
- certificates show grade, standard, size and test results for each heat,
- PMI has been done where the buyer requires it (see PMI testing explained).
Frequently asked questions
Can two different products have the same heat number?
Yes. A plate and a bar can come from the same heat and carry the same heat number, because they share the same chemistry.
What happens if the heat number is missing?
Most buyers will treat the material as non-traceable and reject it, or ask for full retesting at the supplier's cost.
Is the heat number the same as the certificate number?
No. One certificate can cover several heats, and each heat number is listed separately on it.
Can a trader re-mark material?
A trader may transfer the original heat number when cutting or re-bundling, but must never change or invent heat numbers.
Key takeaways
- A heat number identifies one melt of steel and links product to test data.
- It must appear on the material, the certificate and the packing list.
- Transfer heat numbers before cutting and keep a traceability register.
- Clean, accurate certificates win inspections — and repeat orders.
Traceability is invisible when it works and very expensive when it fails. Build it into your process from day one.
