Quality engineers coming from pharmaceutical, electronics, or plastics manufacturing encounter steel traceability with a reasonable question: is the heat number the same as the lot number? What about the melt number? The answer is no — they are not the same, and the difference matters when an auditor asks for the specific tested batch.
Using a lot number when the auditor needs a heat number is one of the most common documentation errors in steel receiving inspection. Understanding why requires knowing what each identifier actually represents.
Melt Number (or Cast Number)
The melt number — sometimes called the cast number — is the identifier assigned to a specific charge of molten steel in the steelmaking furnace. One melt represents one batch of liquid metal, produced in a single heat cycle.
All chemistry testing (heat analysis) is performed on the melt. The chemistry results on a mill test report — carbon content, manganese, phosphorus, sulfur, and alloying elements — are the results for this specific melt. No other batch of steel has these exact results.
Some steel mills use melt number and heat number interchangeably. Others maintain a distinction: the melt number identifies the steelmaking batch, while the heat number is assigned during the rolling or forming operation and may represent a subset of the melt or a combined production run. In practice, when an MTC uses one term, it means the same thing regardless of which label is printed.
Heat Number
The heat number is the primary traceability identifier on a mill test report. It's the number stamped or stenciled on the physical material, printed on the MTC, and referenced in every traceability chain for steel construction.
In North American practice, heat number is the standard term. European practice often uses cast number or melt number — same concept, different label. For any steel application governed by ASTM, ASME, AWS, or AISC standards, the identifier that matters is the one printed on the MTC alongside the chemistry and mechanical test results.
This is the identifier auditors want. When an ASME inspector asks "what heat is this component?", they want the heat number from the MTC — because that number is the key to retrieving the specific test data that proves the material meets specification.
A heat number is not an administrative grouping. It is not a shipping unit. It is not a batch of convenience. It is the identity number for a specific, tested batch of molten steel.
Lot Number
The lot number is an administrative grouping identifier. A lot is defined by the producer or the service center for operational purposes — it may contain material from a single heat or from multiple heats that were produced under similar conditions.
Lot numbers are the standard model for tracing high-volume, relatively uniform items: bolts, nuts, washers, pipe fittings, bar stock cut into standard lengths. A lot cert for fasteners tells you that the fasteners in this lot were tested and met the specification requirements for that lot. It does not tell you which specific melt they came from.
Lot-based certification is valid and sufficient for many applications. ASTM A325 and A490 bolts are certified by lot. ASTM A36 bar stock from a service center may be certified by lot. If the customer's specification accepts lot certification, a lot cert is appropriate.
Lot-based certification is not acceptable when heat-specific traceability is required. A customer who specifies ASME SA-516 Gr. 70 plate with heat traceability to the MTC is asking for a heat number. A lot number is a different kind of identifier — it does not tell the customer which specific heat was tested, what the chemistry results were, or whether those results satisfy the material specification.
Why the Distinction Matters
An auditor asking for a heat number will not accept a lot number as a substitute. This is not a bureaucratic rule — it reflects the actual information content of the two identifiers.
A lot number tells you: this material was grouped into this batch for production or shipping purposes.
A heat number tells you: this material came from this specific melt, which was tested and produced these specific chemistry and mechanical results.
When a specification requires traceable compliance to tested properties, only the heat number provides the link between the physical material and the documented test results. The lot number simply doesn't contain that information.
The Practical Rule
If your MTC shows a heat number, use the heat number as your traceability anchor throughout the supply chain. It is the identifier that connects the physical material to the test results, and it must appear on every document in the cert chain — receiving record, material tag, traveler, weld map, shipping document.
If your cert only shows a lot number — no heat number — understand the implication: you may not be able to satisfy customers who require heat-specific traceability. Before accepting lot-only certified material for an application requiring heat traceability, verify whether your customer's specification permits lot certification or requires heat identification.
The distinction is established at the mill. By the time the material reaches a service center or fabricator, the heat number is either on the cert or it isn't. If it isn't, the only path to heat-level traceability is to contact the mill and request documentation that links the lot to the specific heat(s) included. Mills can sometimes provide this; not all can, and not all will without a formal request.