How to Read a Mill Test Report (MTR) for Stainless Steel: A Simple Guide

How can you be certain that the stainless steel you purchased is actually Grade 304 or 316? The answer lies in a document called the Mill Test Report (MTR), also known as a Mill Test Certificate (MTC). This document is the "birth certificate" for a specific batch of metal. Reading an MTR is a crucial skill for procurement officers, engineers, and quality control (QC) teams to verify that the material meets the required specifications.

What is a Mill Test Report (MTR) and Why is it Important?

An MTR is a document issued by the material manufacturer (the mill) that certifies the properties of a specific batch of metal. It primarily details two key areas: Chemical Composition and Mechanical Properties. This serves as official proof that the metal conforms to the specified international standards.

How to Read Key Information on an MTR

While the format can vary slightly between manufacturers, here are the essential sections you need to check:

1. General Information:
   • Heat No. / Lot No.: This is a unique identification number for that specific batch of molten metal. It's like a serial number used for traceability and must match the number marked on the material itself.
   • Product Description: Describes the product, e.g., STAINLESS STEEL SHEET.
   • Specifications: Lists the standards the material conforms to, such as ASTM A240 (for plates/sheets) or JIS G4304.
2. Chemical Properties / Composition:
   • This section lists the elemental composition by percentage (%). Key elements to check include:
   • C (Carbon): A lower carbon content improves weldability (e.g., in "L" grades like 304L, 316L).
   • Cr (Chromium): The primary element that makes steel "stainless" (must be >10.5%).
   • Ni (Nickel): Contributes to strength and toughness.
   • Mo (Molybdenum): **Must be present in Grade 316**. This is the easiest way to differentiate 316 from 304 on an MTR.
3. Mechanical Properties:
   • These are the results of physical tests on the material's strength:
   • Tensile Strength: The maximum stress a material can withstand before breaking.
   • Yield Strength: The stress at which the material begins to deform permanently.
   • Elongation (%): A measure of the material's ductility or ability to stretch before breaking.

Pro Tip: The MTR will typically show a "Specified" or "Required" value (the standard) and an "Actual" or "Measured" value (the test result). The actual results must fall within the range required by the standard.