How to Verify a Mill Test Certificate

Quick Answer: To read a mill test certificate (MTC), verify the material grade, check that chemical composition and mechanical properties fall within specification limits, confirm the heat number matches the physical product marking, and ensure the certificate type meets the purchase order requirement (typically EN 10204 Type 3.1 or 3.2).

Key Sections of a Mill Test Certificate

Understanding the layout of an MTC is the first step to reading it correctly. Most MTCs follow a standard format regardless of the issuing mill.

MTC Section	What to Check	Why It Matters
Manufacturer	Mill name on approved manufacturer list (AML)	Confirms authorized source
Product description	Pipe type, process, size, schedule	Must match purchase order
Material standard and grade	ASTM/API/EN specification	Must match PO line item exactly
Heat number	Unique steel melt identifier	Links certificate to physical product
Chemical composition	C, Mn, P, S, Si, Cr, Mo, Ni percentages	Each element must be within spec limits
Mechanical properties	Tensile, yield, elongation, hardness	Must meet or exceed minimums
Impact test (Charpy)	Energy absorbed at specified temperature	Required for low-temperature or sour service
Heat treatment	Normalized, Q&T, annealed, etc.	Must match standard requirement
NDE results	Hydrotest, UT, RT, eddy current	Pass/fail per applicable code
Dimensions	OD, wall thickness, length	Must fall within tolerance
EN 10204 type	Certificate classification (3.1 or 3.2)	Must match PO specification
Signature	QC rep stamp and signature	Validates the document

Step-by-Step MTC Verification

1. Confirm the material grade: Check that the standard and grade (e.g., ASTM A106 Gr. B) match the purchase order exactly. A common error is receiving Gr. A when Gr. B was specified.

2. Check chemical composition: Compare each element against the maximum and minimum limits in the relevant ASTM or EN standard. Pay close attention to carbon (C), sulfur (S), and phosphorus (P), as exceeding these limits affects weldability and corrosion resistance.

3. Verify mechanical properties: Tensile strength, yield strength, and elongation must meet or exceed the minimum values. For sour service, check hardness values against NACE MR0175 limits (typically max 22 HRC).

4. Cross-reference the heat number: The heat number on the MTC must match the marking on the physical material. This is the single most important verification step.

5. Check heat treatment condition: Confirm the treatment matches the standard requirement (e.g., ASTM A106 requires normalizing above certain wall thicknesses).

6. Verify certificate type: The EN 10204 type must match the PO requirement. A 3.1 certificate is not acceptable when 3.2 is specified.

7. Review supplementary requirements: Check for any additional tests (impact testing, PMI, NACE compliance) specified in the PO.

Common Mistake

Reviewing only the mechanical properties without checking the chemical composition. A pipe can pass tensile and yield requirements while having out-of-spec carbon or sulfur content. Always verify both chemical and mechanical data against the full specification limits.

Key Points

• Always compare MTC values against the actual specification limits, not approximate memory.
• Keep a copy of the applicable ASTM/EN standard on hand during MTC review.
• Discrepancies between MTC values and specification limits are cause for material rejection or hold for engineering disposition.
• MTCs must be archived for the operational life of the plant as part of the quality documentation package.

Read the full guide to mill test certificates