When you release material from stock, what unit of traceability does your system track? If the answer is "the lot" or "the heat" — do you know what the difference is, and whether your choice matches what your customers actually require?
Most service centers pick one model by default — typically whichever one the ERP system was configured to support at implementation — without evaluating whether it satisfies their customer mix. The mismatch gets discovered when a customer rejects a shipment for insufficient traceability, or when an audit finding identifies that the service center's documentation system doesn't match the spec requirements of its work.
Heat-Based Traceability: How It Works
Heat-based traceability uses the mill heat number as the primary traceability identifier. Every piece of material released from stock references its heat number, which in turn ties directly to the mill's test report for that specific melt.
The heat number is the metallurgical identifier — it's the number assigned to the specific charge of molten steel from which the material was produced. The MTC for that heat contains the actual chemistry and mechanical test results for that specific batch of steel.
When it's required: Any application where the customer needs to verify that specific, tested material was used. Common in pressure vessel fabrication, structural steel construction governed by AISC or AWS D1.1, power generation components, and defense procurement. When a customer's PO says "provide MTCs," they mean documents with heat numbers that can be traced to the mill's test results.
The operational challenge: A single heat number can span multiple production runs, multiple coils, and multiple plate lengths — all carrying the same heat number because they came from the same melt. The heat number doesn't identify a unique physical piece; it identifies the batch. When multiple pieces from the same heat are in stock at different locations, the system must track heat number plus physical location to identify specific pieces.
Lot-Based Traceability: How It Works
Lot-based traceability uses a lot number as the primary identifier. A lot is an administrative grouping — it may contain material from a single heat or from multiple heats produced under similar conditions.
Lot numbers are the standard traceability model for high-volume manufactured items: fasteners, fittings, bar stock, pipe fittings, flanges. A bolt lot cert tells you that the bolts in this lot were tested and met specification requirements — but it doesn't tell you which specific melt they came from.
When it's acceptable: When the customer's specification allows lot certification and does not require heat-specific test data. Common in commercial construction hardware, non-critical structural applications, and applications where ASTM standards permit lot certification rather than heat certification.
When it's not acceptable: When the customer's specification requires heat number traceability, a lot number is not a substitute. The lot cert doesn't contain the heat-specific chemistry and mechanical test results that the customer needs to verify specification compliance.
Why Mixing the Models Causes Problems
A service center that tracks inventory by lot but has customers requiring heat-specific traceability will fail receiving inspection. The lot cert doesn't answer the question the customer is asking. The customer is asking: which heat was this? The lot cert says: which production batch was this? Those are different questions with different answers.
This mismatch is operationally invisible until receiving inspection catches it. At that point, the service center either needs to reconstruct the heat number from lot-to-heat mapping records (if they exist), or issue a corrected shipment with proper heat number documentation.
The corrective action is straightforward. The pattern of repeated occurrences — the same mismatch on multiple shipments to the same customer — is what generates an approved supplier audit or a corrective action request.
How to Match the Model to Your Customer Base
Review the traceability requirements on your active customer purchase orders. Specifically:
- Does the PO reference a material specification that requires heat number identification (ASTM A6, ASTM A480, ASTM A999)?
- Does the PO require MTCs with specific data fields that include heat number?
- Does the customer's quality plan or approved supplier requirements specify heat-based traceability?
Count the split. If the majority of your revenue comes from customers requiring heat number traceability, your inventory and documentation system must support heat-based traceability end to end.
If you have a mixed customer base, your system needs to support both models — or you need to physically segment stock by traceability level, maintaining heat-tagged stock for critical customers separately from lot-tracked stock for commercial customers.
The Operational Implication of Heat-Based Traceability
Operating a heat-based traceability system means: when a coil is received, the heat number is recorded and linked to the cert. When the coil is slit, each strip carries the heat number. When a strip is shipped, the delivery document and the cert reference the heat number from the original mill cert.
This is more operationally intensive than lot tracking. The heat number must flow through every transaction — receipt, processing, storage, and shipping — without breaking the chain. That requires process discipline at each step, and a system that doesn't allow the heat number to fall off at processing operations.
It's the standard that critical-application customers require. If your customer base includes those customers, it's the standard your operations must meet.