Heat Number
Also known as: Heat lot, Melt number, Cast number
A heat number is the identifier a steel mill assigns to one batch of metal poured from a single melt. Every piece in that batch shares the same chemistry and is covered by the same mill test report, which makes the heat — not the bar, not the purchase order — the unit that material properties are guaranteed against.
A heat number is the identifier a steel mill assigns to one batch of metal poured from a single melt. Every piece of material in that batch shares the same chemistry and is covered by the same mill test report.
That is what makes it the important number. Material properties — chemistry, tensile strength, hardenability — are certified per heat. They are not certified per bar, per bundle, or per purchase order. So the heat number is the smallest unit you can make a defensible statement about.
Why it is the unit that matters
When a customer asks a forging shop to prove what a part was made from, the answer that satisfies them is a heat number plus the certificate that goes with it. A purchase order tells them who you bought from. A bar tag tells them what diameter it was. Neither tells them the chemistry, because chemistry is a property of the melt.
The same logic runs in the other direction. If a mill later reports a problem with a heat, or a customer finds a part out of specification, the heat number is what makes containment possible. A shop that has traced it can name exactly which parts came from that melt. A shop that has not must treat everything shipped in the surrounding period as suspect.
How it is recorded
A heat number arrives stamped, stencilled or tagged on the material and printed on the mill test report. Both need to be captured at receiving, and they need to match. A bar whose stamp corresponds to no certificate in the paperwork is untraceable material regardless of what the packing list claims.
Because mills assign heat numbers from their own sequences, the numbers are not globally unique. Two suppliers can legitimately ship you material bearing the same heat number. Recording the heat without recording which mill issued it creates a collision that no downstream record can resolve.
Where the link usually breaks
Almost always at the same place: pooling. The moment material from two heats is added into one on-hand quantity, the connection between a physical piece and its certificate is gone. Nothing downstream can recover it, because the information was never wrong — it was discarded.
This is why traceability is a property of how inventory is structured rather than a report you run at the end. Quantities have to be held per heat from receiving onward, and every operation that transforms material has to carry the heat forward to what it produced.
What a heat number does not tell you
It does not tell you the material is good. It identifies a batch and points at a certificate; reading and accepting that certificate is a separate step. It also carries no information about processing — two parts from the same heat can have very different mechanical properties after different heat treatment, which is why heat identity and process history are tracked separately and both are needed.
Common questions
Is a heat number the same as a lot number?
Not necessarily. A heat number identifies material as it left the mill. A lot number is assigned by whoever is processing it, and one heat can be split across many production lots. The heat is upstream of the lot, and a lot record that does not name its heat has broken the chain.
Where do I find the heat number on incoming material?
It is stamped, stencilled or tagged on the material itself and printed on the accompanying mill test report. The two must agree — a bar whose stamp does not match any certificate in the paperwork cannot be traced, whatever the packing list says.
Can two different mills issue the same heat number?
Yes. Heat numbers are assigned by each mill from its own sequence and are not globally unique. That is why a heat number only identifies material unambiguously when it is recorded together with the mill that issued it.
Related terms
- Mill Test Report — The certificate a steel mill issues for a heat, recording its chemical composition and mechanical properties, and what a forging shop is expected to do with it.
- Lot Traceability — The ability to follow a defined batch of material forward to the parts made from it and backward from a part to the material it came from.
- Country of Melt and Pour — Where raw steel was first melted and poured into its first solid shape, why US importers must report it, and what that means for a forging shop's records.