Mill Test Certificate MTC — EN 10204 Types, Verification & Procurement

The Mill Test Certificate is the most important document in steel pipe procurement. Every specification requirement, every material property, and every quality control outcome that matters for the pipe's performance in service is — or should be — recorded on it. Yet MTC review is frequently treated as an administrative step rather than a technical verification, and the gaps between what the MTC says, what it proves, and what the well or pipeline actually needs are where procurement failures occur.

ZC Steel Pipe issues EN 10204 3.1 as standard and EN 10204 3.2 with named third-party inspection bodies including Bureau Veritas, SGS, and Intertek for all sour service, HPHT, and subsea pipe supply. We supply OCTG and line pipe to operators and EPC contractors across Africa, the Middle East, South America, and Southeast Asia. This guide explains the EN 10204 certificate types, when each is appropriate, what to verify on an MTC before accepting pipe, and the procurement traps that cause costly mistakes.

EN 10204:2004 — The Framework

EN 10204:2004, Metallic Products — Types of Inspection Documents, is the European standard that defines what constitutes a valid material inspection document. It is the global industry reference for pipe MTC classification and is explicitly referenced in API 5CT, API 5L, and most EPC project specifications worldwide when they specify a level of inspection documentation.

EN 10204 defines four types of inspection documents:

The critical distinction is inspector independence: 2.x documents are self-certified by the manufacturer; 3.x documents require validation by an authorized inspection representative. Within the 3.x types, the 3.2 requires an additional independent party. The document type tells you nothing about what was tested — only who validated the test results.

What Each Certificate Type Proves

2.1 — Declaration of Compliance

A 2.1 is a manufacturer's written statement that the product complies with the order requirements. No test data is provided. Appropriate only for commodity pipe purchases where the application is non-critical and the purchaser is willing to rely on the manufacturer's quality assurance system without independent verification. Not acceptable for OCTG, sour service, or any safety-critical application.

2.2 — Test Report

A 2.2 provides test results, but those results are based on non-specific inspection — meaning the tests are performed according to the manufacturer's own quality plan rather than witnessed or validated by an independent party. The data may or may not represent the specific lot delivered. Appropriate for general commercial pipe where some data is preferred but independent inspection is not required. Not acceptable for sour service or HPHT applications.

3.1 — Inspection Certificate (Mill Authorized Inspector)

A 3.1 is validated by the manufacturer's authorized inspection representative: a person who is organizationally independent from the production department but may be a mill employee, a contracted inspector, or an employee of the mill's inspection department. The inspector validates that specific tests have been performed on the specific heat or lot, and that the results meet the ordered specification.

For the certificate to qualify as 3.1, the authorized representative's identity and credentials must appear on the certificate alongside the test results. A 3.1 provides a material-specific, lot-specific record of compliance — it is substantively more reliable than 2.x documents. It is appropriate for standard OCTG in sweet service, intermediate casing strings, general-purpose line pipe, and commercial applications where the purchaser is not specifying independent TPI.

3.2 — Inspection Certificate (Independent Third-Party Inspector)

A 3.2 requires validation by both the manufacturer's authorized representative AND an inspection representative designated by the purchaser OR an officially recognized independent inspection body. In oil and gas pipe procurement, this means a named TPI body — Bureau Veritas, SGS, Intertek, TÜV SÜD, DNV, or equivalent — has attended the mill, witnessed key tests, and countersigned the certificate.

The TPI witness at a minimum covers mechanical testing (tensile, Charpy if specified, hardness), dimensional inspection, and stencil verification. For sour service pipe, TPI scope typically extends to witnessing HIC test preparation, hardness survey completion, and NDE. For HPHT OCTG, TPI may include witnessing hydrostatic testing and drift testing.

EN 10204 3.2 is the mandatory documentation standard for:

• Sour service pipe (any grade with NACE MR0175 qualification)
• HPHT OCTG (P110, Q125, C110)
• Subsea and offshore line pipe
• Any application where the project specification, insurance requirement, or operator standard mandates independent inspection

For the complete grade ladder with tensile, hardness, and chemistry limits, see the API 5CT specification tables →

To match a grade to your well conditions, use the AI Pipe Grade Selector →

What to Verify on an MTC

Before accepting any pipe consignment, the following checks must be performed against the MTC:

1. Heat/lot traceability

Every pipe joint has a stencil including the heat number or lot number. Verify that the stencil numbers match the heat or lot numbers listed on the MTC. Any joint whose stencil does not appear on an MTC in the consignment must be segregated until traceability is resolved. For sour service or HPHT pipe, perform this check on 100% of joints — not a sample.

2. Grade marking matches order

Confirm the grade stencil (e.g., API 5CT T95 or API 5L X70 PSL2) matches the ordered grade exactly. Verify the PSL level, delivery condition suffix (for API 5L), and any supplementary requirements referenced on the MTC match the purchase order.

3. Chemistry results within standard limits

Compare the heat chemistry results on the MTC against the applicable standard's table. For API 5CT, check against the 11th Edition tables for the ordered grade. For API 5L, check against the 46th Edition tables for the grade and delivery condition. For sour service pipe, verify that sulphur, phosphorus, and any other tightened elements meet the project-specific limits (which are typically tighter than the standard API limits).

4. Mechanical properties within limits

Verify yield strength falls within the grade's minimum and maximum, tensile strength meets the minimum, and elongation is acceptable. For sour service grades with hardness limits (L80, C90, T95), confirm hardness survey values on the MTC do not exceed the applicable NACE limit. For Q125, confirm hardness variation is within limits.

5. Supplementary requirements documented

For each supplementary requirement specified on the purchase order (SR2 Charpy, SR15C HIC, SR13 seamless verification), confirm the results and acceptance statement appear on the MTC or as an attachment. A 3.2 certificate that does not include HIC test results when SR15C was ordered is incomplete.

6. Inspector identity and signature

For 3.1 certificates: confirm the authorized inspector's identity, signature, and authorization reference appear on the document. For 3.2 certificates: confirm both the mill's authorized representative and the TPI body representative have signed, with the TPI stamp present.

MTC for Sour Service and HPHT Pipe

Sour service pipe requires the following on the MTC beyond standard content:

Read each row of this table as a line-item checklist, not a general list. If any row has no corresponding entry on the MTC, the pipe is not documented as sour-qualified for that requirement — regardless of what the certificate type says.

HPHT OCTG (P110, Q125) requires:

MTC Hardness Verification: Converting HRB to HRC

A T95 casing MTC may report hardness in HRB rather than HRC. The NACE MR0175 / ISO 15156 limit for T95 pipe body is HRC 25.4. The receiving engineer must convert each HRB value to HRC using ASTM E140 Table 3 (Approximate Hardness Conversion Numbers for Nonaustenitic Steels) and compare each individual converted value against the NACE limit. The conversion is not optional — accepting "HRB values look acceptable" without converting is the mechanism for one of the most common sour-service pipe failures.

Here is the conversion process for 5 sample readings from a T95 MTC:

Step 1: Locate the HRB value in ASTM E140 Table 3.

Step 2: Read the corresponding HRC value from the same row.

Step 3: Compare each individual HRC value (not the average) to the NACE MR0175 limit for the ordered grade. T95 pipe body: HRC 25.4 max. Any single reading above this limit requires rejection of the affected heat.

Step 4: If the MTC reports hardness only in HRB without conversion, perform the conversion yourself using ASTM E140 — do not accept a statement that "hardness meets NACE" without verifying the conversion. Reading 5 (HRB 106 → HRC 26.1) fails and triggers rejection of the heat.

The table above makes clear why individual readings matter: readings 1 through 4 would support acceptance, but reading 5 alone is sufficient grounds for heat rejection. An inspector who averages HRB values before converting, or who visually estimates the HRC equivalent without looking it up in ASTM E140, will miss this failure in the real case.

When NOT to Accept EN 10204 3.1 Alone

EN 10204 3.1 is a technically valid and widely used certificate type for many pipe applications. It is not adequate for the following:

The last row in this table is the one most often overlooked. A 3.2 certificate with a well-known TPI gives procurement teams confidence — but that confidence is misplaced if physical heat-number verification was not performed either at the mill by the TPI or at the receiving yard by the inspection team. The certificate and the physical verification are complementary controls, not alternatives.

Procurement Traps

Trap 1 — NACE supplementary requirements not on the PO

Wrong PO: "T95 Type 2 casing, API 5CT 11th Ed., EN 10204 3.2 with Bureau Veritas."

What ships: T95 Type 2 casing with a genuine BV 3.2 certificate — but the purchase order did not specify: SR2 Charpy impact testing at −46°C; SR15C HIC/SSCC testing per NACE TM0284/TM0177; hardness survey with individual readings reported to the NACE MR0175 limit (HRC 25.4 for T95). BV witnessed the standard API 5CT mechanical tests only — tensile, yield, and the standard hardness spot-check. The MTC is correctly issued and independently witnessed. It does not qualify the pipe for sour service because the sour-specific tests were never ordered.

Correct PO: "T95 Type 2 casing, API 5CT 11th Ed., Grade T95 Type 2. EN 10204 3.2 with [named TPI]. Supplementary requirements: SR2 Charpy impact 40J average / 30J minimum individual at −46°C, transverse orientation. SR15C HIC testing per NACE TM0284, Solution A: CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%. Hardness survey per API 5CT Annex B: individual readings to be reported on MTC; none to exceed HRC 25.4 (T95 body). Product analysis on each heat. [Named TPI] to witness mechanical tests, hardness survey, and stencil verification at mill."

Trap 2: "Complies with API 5CT" does not mean NACE-qualified. API 5CT compliance covers dimensional, mechanical, and chemistry requirements as specified in the standard. For grades with NACE hardness traps (T95 Type 1 in particular), full API compliance does not guarantee the hardness result is below the NACE limit. The NACE qualification must be explicitly verified from the hardness survey on the MTC.

Trap 3: Heat analysis is not the same as product analysis. Ladle chemistry (heat analysis) represents the steel as cast, before rolling. Rolling can segregate certain elements, particularly sulphur and phosphorus. For sour service pipe with very tight sulphur limits, product analysis (sampled from the rolled pipe) provides better assurance than ladle analysis alone. Specify product analysis in the purchase order when sulphur ≤ 0.002% is required.

Trap 4: Accepting MTCs without physical pipe marking verification. An MTC without corresponding pipe body stencil verification is a paper exercise. Always confirm heat numbers at receiving inspection — a 3.2 MTC for one heat applied to unstencilled or incorrectly stencilled pipe from a different heat is worthless. If TPI was not present at the mill during stencilling, this verification at receiving inspection is the last reliable checkpoint.

MTC Verification Failure Modes to Specify Against

Procurement specifications and receiving inspection procedures that name specific failure modes are more effective than generic MTC acceptance requirements. The three failure modes below account for the majority of post-installation MTC-related failures on sour service and HPHT wells.

Failure Mode 1 — Heat number mismatch between MTC and physical pipe

Mechanism: The mill ships a consignment of T95 casing to site. The MTC covers heat #A24B17 — a compliant heat with NACE-acceptable hardness and chemistry. The pipe joints in the shipment are stencilled with #A24B17. However, 8 joints were substituted from heat #A29C03 — a heat that was produced in the same run but not tested for sour service — and re-stencilled with #A24B17 using a portable paint stencil. The substitution is invisible to an inspector who looks at the stencil without checking it against the original mill production records.

Diagnostic: Physical inspection: photograph the stencil on each joint and compare to MTC heat numbers. Stencil quality on substituted joints is often visibly different (paint adhesion, font consistency) from the mill-applied stencil. PMI (XRF) spot-check on a sample of joints to confirm chemistry matches the MTC heat analysis — a substituted heat with different chemistry will show variance outside the MTC values.

Fix: Require TPI presence at the mill during stencilling and joint-to-heat allocation — this is the most reliable prevention. At receiving inspection, verify heat numbers on 100% of joints for sour service or HPHT pipe. For large consignments where 100% check is impractical, verify all joints in any lot that shows a suspicious pattern (stencil quality variation, sequential heat numbers from non-adjacent heats on the MTC, etc.).

Failure Mode 2 — NACE supplementary requirements absent from MTC despite 3.2 certificate

Mechanism: A project specification requires T95 casing with "EN 10204 3.2, NACE qualified" — but the purchase order does not list the specific supplementary requirements (SR2, SR15C, hardness survey per API 5CT Annex B). The mill issues a 3.2 certificate covering standard API 5CT tests only — tensile, yield, elongation, and a spot hardness check. Bureau Veritas witnesses and signs the 3.2. The MTC is technically correct and fully compliant with everything ordered. The pipe is not NACE-qualified because the NACE-specific tests were never ordered. The failure occurs at the point of use — the well is completed, and SSC initiates at 9 months.

Diagnostic: MTC review post-failure confirms absence of hardness survey results, no HIC/SSCC test results, and no SR2 Charpy data. The MTC is complete for what was ordered; the purchase order is deficient. Root cause is a specification gap between "NACE qualified" in the project specification and the absence of explicit supplementary requirements on the PO.

Fix: The MTC checklist for sour service pipe must explicitly list each required supplementary requirement as a line item to verify: SR2 present/values meet minimum; SR15C present/CLR-CTR-CSR within limits; hardness survey individual values all below NACE grade limit. If any line item is missing from the MTC, the pipe is not NACE-qualified regardless of certificate type.

Failure Mode 3 — Hardness values reported in HRB not converted to HRC before NACE comparison

Mechanism: A T95 MTC reports pipe body hardness survey results as HRB 99 to HRB 106. The receiving engineer reviews the MTC, sees all values are "approximately HRC 22–25" from memory, and accepts the consignment. HRB 106 converts to HRC 26.1 per ASTM E140 — above the NACE MR0175 T95 limit of HRC 25.4. The non-compliant joints are installed in the sour well. SSC initiates at the highest-hardness joints within 8 months.

Diagnostic: Post-failure MTC review with ASTM E140 conversion confirms that HRB 106 reported on the MTC corresponds to HRC 26.1, above the T95 limit. The original receiving inspector did not perform the conversion.

Fix: Add to the receiving inspection procedure: "For any sour service pipe with hardness reported in HRB on the MTC, perform ASTM E140 Table 3 conversion to HRC for each individual reading. Compare each converted HRC value to the NACE MR0175 grade-specific limit. Do not accept an average — a single reading above the limit requires heat rejection." This is a 5-minute calculation per MTC that prevents this failure mode entirely.