2.2 Mill Test Reports (MTRs) & Heat Traceability

2.2 Mill Test Reports (MTRs) & Heat Traceability

Quick Answer: Certified Mill Test Reports (CMTRs) are the official documents confirming that structural steel meets the required ASTM specifications. They provide chemical composition (including carbon equivalent for weldability) and physical test results (yield, tensile, elongation). The heat number ties the physical steel on site to the CMTR. For heavy shapes and fracture-critical members, CMTRs must also report Charpy V-Notch (CVN) impact toughness data to prevent brittle fracture.

Before structural steel is fabricated or erected, its quality and origin must be verified. The primary tool for this verification is the Certified Mill Test Report (CMTR), often simply called a Mill Test Report (MTR). As a structural steel special inspector, reviewing CMTRs and ensuring they correspond to the physical material on the job site is a fundamental, daily responsibility.

The Certified Mill Test Report (CMTR)

A CMTR is an affidavit issued by the steel producing mill certifying that the material conforms to a specific ASTM standard. It acts as the steel's "birth certificate." A complete CMTR must contain three critical categories of information: identifying information, chemical composition, and mechanical (physical) test results.

Heat Numbers and Identification

The most important piece of identifying information on a CMTR is the heat number. When a steel mill melts a batch of scrap and raw materials in an electric arc furnace, that specific batch of molten steel is called a "heat." The mill assigns a unique alphanumeric heat number to this batch. Every structural shape rolled from that batch will share the same metallurgical properties and the same heat number.

In addition to the heat number, the CMTR will list the shape or plate size, the quantity produced, and sometimes a slab number or melt identification code, which further traces the material through the continuous casting and rolling process.

Chemical Composition and Weldability

The CMTR details the exact chemical makeup of the steel heat, usually expressed as weight percentages of various elements. The governing ASTM specification dictates maximum (and sometimes minimum) limits for these elements.

• Carbon (C): Increases strength but decreases ductility and weldability.
• Manganese (Mn): Acts as a deoxidizer and increases strength and toughness.
• Silicon (Si): Another deoxidizer; its content affects the galvanizing process.
• Sulfur (S) and Phosphorus (P): Generally considered impurities; high levels can lead to cracking and brittleness, so their maximum limits are strictly controlled.

For welding applications, the most critical chemical metric is the Carbon Equivalent (CE). The CE is a calculated value combining the effects of carbon and other alloying elements to evaluate the steel's overall weldability and its susceptibility to hydrogen-induced cold cracking. A higher CE indicates lower weldability and may necessitate preheating before welding. AWS D1.1 (the Structural Welding Code) includes a CE-based guideline as an alternative method for determining preheat, and the welding procedure must account for the steel actually supplied.

Mechanical Test Data Verification

The mill physically tests coupons taken from the heat to verify the steel's mechanical properties. The CMTR will report:

• Yield Point / Yield Strength (Fy): Must meet or exceed the ASTM minimum.
• Tensile Strength (Fu): Must fall within the specified ASTM range.
• Elongation: The percentage the coupon stretched before breaking, which proves the material has sufficient ductility.

When reviewing a CMTR, the inspector must cross-reference these reported values against the requirements of the specified ASTM standard (e.g., verifying that a piece of A992 steel shows a yield point between 50 ksi and 65 ksi).

Charpy V-Notch (CVN) Impact Toughness

While standard tensile testing is done at room temperature with a slow application of force, structural steel can behave very differently under rapid loading or at cold temperatures. To assess the material's resistance to brittle fracture under these conditions, the mill performs Charpy V-Notch (CVN) testing.

In a CVN test, a standardized, notched steel specimen is struck by a swinging pendulum at a specified temperature (often 40°F or 70°F). The energy absorbed by the specimen as it breaks is measured in foot-pounds (ft-lbs). Higher absorbed energy indicates higher toughness and better resistance to brittle fracture.

CVN testing is not required for all structural steel. It is required in these cases:

1. Fracture critical members (FCMs): a bridge-design term for tension members whose failure would cause partial or total collapse; building projects may impose similar toughness requirements through the project specifications.
2. Heavy shapes (AISC 360 Sections A3.1c and A3.1d): rolled shapes with flanges thicker than 2 in. and built-up members with plates thicker than 2 in., when they are used as members subject to primary tension (from tension or flexure) and are spliced or connected with complete-joint-penetration groove welds that fuse through the thickness. The design documents must require CVN results with a minimum average of 20 ft-lb at a maximum temperature of +70°F. The requirement does not apply when those splices and connections are bolted. Thick sections cool more slowly during rolling, which can coarsen the grain and lower toughness at the core.

When CVN testing is required by the project specifications, the inspector must confirm that the CVN energy values and the testing temperature reported on the CMTR meet the engineer's requirements.

Material Traceability in the Field

A CMTR is only useful if it can be definitively linked to the physical steel on the job site. This is achieved through material traceability.

When steel arrives at the fabrication shop or the erection site, the inspector must visually verify the markings on the steel. Steel mills apply physical markings to the members, which can include:

• Stenciling: Painted alphanumeric codes indicating the heat number, grade, and mill.
• Die Stamping: Characters physically pressed into the metal (less common on highly stressed tension areas due to stress concentration concerns).
• Color Coding: Painted ends (though this is typically a fabricator's internal system, not a mill standard).

The inspector must verify that the heat number stenciled or stamped on the steel beam or plate exactly matches the heat number on the submitted CMTR. How much traceability is required depends on the material. AISC 303 Section 6.1.1 requires the fabricator to demonstrate, by written procedure and actual practice, a method of material identification that remains visible up to the point of assembling members:

In each case, the fabricator furnishes material test reports if the owner's designated representative for design requests them before mill material is ordered. Under Section 6.1.2, each piece of material ordered to special requirements must carry a fabricator's or original supplier's identification mark until assembly, following a material identification system that is on record for the inspector before fabrication starts, and Section 6.1.3 forbids giving such members the same erection mark as otherwise identical members of ordinary material. AISC 303-10 lists the shop-standard grades as A992 for W and WT shapes; A36 for M, S, MT, ST, HP, L, C and MC shapes and for plates and bars; A500 Grade B for HSS; and A53 Grade B for pipe. AISC 303-16 changed the shop-standard grades for HP shapes and HSS, so check the edition in use.

Material that cannot be identified to the required level is treated as "unidentified steel." AISC 360 Section A3.1b permits unidentified steel, free of injurious defects, only for members or details whose failure will not reduce the strength of the structure, locally or overall, and only with the engineer of record's approval; its User Note gives curb plates and shims as typical uses. Using such steel in a primary member would require the engineer of record to accept laboratory testing that re-establishes its properties.