Direct Answer
A mill certification (or Material Test Report / MTR) is a document from the steel producer certifying the chemical composition and mechanical properties (yield strength, tensile strength, elongation) of each heat of steel. Structural steel used on a project must have an MTR on file for every heat number, traceable to the member it came from. MTRs are the primary evidence that the material meets the specified ASTM grade.
Full Explanation
MTRs report: ASTM designation (A992, A572, etc.), grade, heat number, section/plate size, chemical analysis results, and mechanical test results. The heat number is typically stenciled or stamped on the steel. To trace a specific member back to its MTR, you need the heat number from the piece. AISC's quality certification program (AISC Certification for Steel Buildings) requires that all fabricators maintain MTRs in their quality system. Special inspectors routinely request MTRs during inspection.
What This Means for Your Shop
Maintain MTRs organized by heat number and project. If material arrives without an MTR, it cannot be used on a project with inspection requirements until the mill produces the certification. Buying steel from distributors who provide verified MTRs prevents this problem.
Common Mistakes
Losing track of which MTR corresponds to which material after material is cut or mixed with other orders. Implement a heat-number tracking system that follows material from receiving to the shop floor.
Recommended Process
At receiving, record the heat number of each bundle or piece, match to the MTR, and file. When material is cut and staged for a work order, tag each cut piece with the heat number. Maintain a heat number log as part of your project QC record.