The Mill Test Certificate (MTC) is the core document for verifying fastener quality and ensuring supply chain traceability in international procurement. Yet many buyers only skim an MTC, missing the key information that actually reflects product quality. This article breaks down 7 metrics you must check, and teaches you how to spot a suspicious document.
1. Chemical Composition
This is the baseline data for verifying whether the steel grade meets the standard, typically listing carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), nickel (Ni), molybdenum (Mo) and other element percentages.
- Check against the procurement standard (e.g. ASTM A193 B7 corresponds to AISI 4140/4142) to verify composition is within range;
- Lower phosphorus (P) and sulfur (S) content means better toughness and weldability — an important indicator of material purity;
- For stainless steel, verify chromium (Cr≥16%) and nickel (Ni≥8%) content to ensure adequate corrosion resistance.
2. Mechanical Properties
| Metric | Meaning | What to Check |
|---|---|---|
| Tensile Strength | Maximum tensile stress the material can withstand before fracture | Verify against the strength grade’s minimum standard |
| Yield Strength | Stress at which the material begins plastic deformation | Key input for structural design |
| Elongation | Plastic deformation capacity before fracture | Reflects toughness — too low means brittle failure risk |
| Hardness | Surface resistance to indentation, typically HRC/HB | Excessive hardness may indicate hydrogen embrittlement risk |
Buyer’s tip: If a high-strength bolt (grade 10.9 or above) shows hardness above the standard upper limit, the risk of hydrogen embrittlement fracture increases significantly — hardness re-verification is especially important for grade 12.9 bolts.
3. Heat Treatment
The MTC should note the heat treatment method (e.g. quench & temper, solution annealing) and furnace temperature records. Heat treatment directly determines the material’s final mechanical properties — a report missing this information is questionable.
4. Heat Number / Lot Number
This is the core field for quality traceability. Each heat number corresponds to an independent steel-making run — in theory, material from the same heat should have identical chemical composition. Buyers should require:
- Clear heat/lot number marking on the physical parts (usually laser-etched or paint-marked);
- The heat number on the MTC matches the physical marking and the packing list;
- Retained heat number records for traceability if quality issues arise later.
5. Surface Treatment Verification
If the product requires hot-dip galvanizing, Dacromet, or other surface treatment, the MTC or a separate inspection report should include:
- Coating thickness (μm) — hot-dip galvanizing generally requires ≥45μm (per ISO 1461 or ASTM A153);
- Salt spray test duration (hours) — Dacromet coating generally requires ≥720 hours with no red rust;
- Adhesion test results.
6. Dimensional Inspection Report
This is usually a separate document from the MTC, verifying thread precision (6g/6H tolerance class), head dimensions, length tolerance, and other specs against DIN/ISO/ASTM standards. We recommend requesting 100% critical dimension inspection records, not just sample data.
7. Testing Lab Accreditation
An MTC’s credibility largely depends on the accreditation of the lab that issued it:
- Factory self-inspection reports: lowest cost, but factory lab equipment and personnel qualifications should be carefully verified;
- Third-party inspection: reports from internationally accredited agencies like SGS, Bureau Veritas (BV), or TÜV carry more credibility;
- For high-value or safety-critical projects, we recommend requiring third-party on-site witness inspection.
How to Spot a Suspicious or Fake MTC
- Test data sits “conveniently” right at the standard’s threshold, lacking the normal statistical scatter;
- MTC data is identical across different batches from the same supplier (nearly impossible in actual production);
- The testing agency’s name or stamp is blurry or cannot be verified on their official website;
- The heat number doesn’t match the physical marking, or the packing list is missing the corresponding heat number.
Need a Fastener Supplier With Complete Quality Documentation?
Rongrun Fasteners provides complete MTC and dimensional inspection reports for every batch, and supports SGS/BV/TÜV third-party on-site inspection
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