Mill Test Report (MTR) — How to Read One for D1.1 Welding Compliance
A mill test report certifies the chemical composition and mechanical properties of a specific heat of steel. For D1.1:2025 compliance, the MTR connects your steel to Table 5.6 base metal groups, which determine preheat requirements, filler metal selection, and whether your WPS can be prequalified.
The D1.1 connection: Your MTR shows the ASTM specification. Table 5.6 maps that specification to a base metal group (I through V). The group determines your preheat per Table 5.11 and your filler metal per Table 5.7. Without the MTR, you cannot look up any of these requirements.
What an MTR Contains
A mill test report is the steel producer’s certified record for a specific heat (batch) of steel. Every structural steel delivery should be accompanied by an MTR that traces back to the producing mill. The document typically contains the following information:
- Heat number
- A unique identifier assigned by the mill to a specific batch of molten steel. The heat number is the primary traceability link between the physical steel and its certified properties. Every plate, beam, or shape from the same heat shares the same chemistry.
- ASTM specification and grade
- The standard the steel was produced to meet, such as ASTM A992, ASTM A572 Gr.50, or ASTM A36. This is the field that connects directly to D1.1 Table 5.6. If the specification and grade are listed in Table 5.6, the steel is approved for prequalified WPSs.
- Chemical analysis
- The actual chemical composition of the heat, reported as weight percentages. Key elements include carbon (C), manganese (Mn), silicon (Si), phosphorus (P), sulfur (S), and for some grades, chromium, nickel, molybdenum, vanadium, and copper. These values are used to calculate carbon equivalent for alternative preheat determination per Annex B.
- Mechanical properties
- Test results from specimens pulled from the heat, including minimum yield strength (ksi or MPa), tensile strength (ksi or MPa), elongation (percent), and in some cases Charpy V-notch impact values. These must meet the requirements of the ASTM specification listed on the MTR.
- Product dimensions and quantity
- The size, shape, and quantity of material covered by the report. A single MTR may cover multiple pieces from the same heat.
From MTR to D1.1 Compliance: Three Steps
The entire purpose of reading an MTR for D1.1 compliance is to answer three questions: what group is my steel in, what preheat do I need, and which filler metals match? Here is the workflow:
Take the ASTM specification and grade from your MTR and look it up in D1.1 Table 5.6. The table lists approved base metals organized by group (I through V). For example, A992 appears in Group II. A36 appears in both Group I (for thicknesses up to 3/4 in) and Group II (all thicknesses). If your steel is not listed in Table 5.6, it is not approved for prequalified WPSs and must be qualified by testing per Clause 6.2.1 with a PQR.
The base metal group from Table 5.6 determines which preheat category applies in Table 5.11. The preheat temperature depends on three factors: the steel group, the welding process category (which reflects hydrogen level), and the material thickness. Use the preheat calculator to look up the exact value for your combination.
Table 5.7 specifies which filler metals provide matching strength for each base metal group. Group I and II steels use E60XX or E70XX electrodes for SMAW, and F6XX or F7XX fluxes for SAW. Higher groups require higher-strength filler metals. The filler metal must match or exceed the base metal strength to satisfy D1.1 requirements.
The Five Base Metal Groups
D1.1:2025 Table 5.6 organizes all approved base metals into five groups based on yield strength and alloy content. Understanding which group your steel belongs to is fundamental to every D1.1 decision that follows.
Group I — Mild Steels (30–50 ksi yield)
The most common structural steels for light-duty applications. Includes A36 (up to 3/4 in), A53, A500, A501, A1011 SS, and API 5L. These steels have the lowest preheat requirements and the widest range of approved filler metals. Most small fabrication shops work primarily with Group I steels.
Group II — Structural Steels (36–55 ksi yield)
The workhorse group for structural fabrication. Includes A992 (the standard wide-flange steel), A572 Gr.50, A588 weathering steel, A913 Gr.50, and A36 at all thicknesses. If you are fabricating a steel building frame in the United States, most of your steel is Group II. Preheat requirements are moderate and increase with thickness.
Group III — Higher-Strength Steels (55–65 ksi yield)
Includes A572 Gr.60 and Gr.65, A633 Grade E, and A913 Gr.60 and Gr.65. These steels require higher preheat temperatures and more careful heat input control. They are used in applications where higher strength reduces member sizes and overall structural weight.
Group IV — High-Strength Steels (70 ksi yield)
Includes A709 HPS70W (high-performance steel for bridges), A913 Gr.70, and A1066 Gr.70. These steels require the highest preheat temperatures among the commonly used structural grades. Filler metal matching requires E80XX or higher electrodes.
Group V — Very High-Strength Steel (80 ksi yield)
Currently limited to A913 Gr.80. This group has the strictest preheat and heat input requirements. The heat input limitations of Clause 7.7 do not apply to A913 grades per a footnote in Table 5.6, because A913 is produced by a controlled process (quenching and self-tempering) that gives it different thermal response characteristics than conventional quenched and tempered steels.
What to Check on an MTR Before Welding
Before welding begins, the fabricator or QC personnel should verify the following items on the MTR against the contract document requirements:
- Specification matches contract documents
- The ASTM specification and grade on the MTR must match what the contract documents specify. If the contract calls for A992 and the MTR shows A36, the steel does not meet the specification regardless of whether its mechanical properties happen to be adequate.
- Yield and tensile strength meet minimums
- The actual test values on the MTR must meet or exceed the minimum requirements for the specified grade. For A992, minimum yield is 50 ksi and tensile range is 65–100 ksi. Values outside these ranges indicate the steel does not conform.
- Chemistry is within limits
- Each ASTM specification defines maximum (and sometimes minimum) values for key elements. Excessive carbon or manganese can increase hardness and cracking susceptibility. The chemistry values also feed the carbon equivalent calculation used in D1.1 Annex B for alternative preheat determination.
- Heat number traceability
- Every piece of steel should be traceable to a heat number on an MTR. If steel arrives without traceability, the fabricator cannot verify its specification, and D1.1 compliance cannot be demonstrated. Missing traceability is a red flag that should stop fabrication until resolved.
Carbon Equivalent from MTR Chemistry
The chemical analysis on an MTR provides all the values needed to calculate carbon equivalent using the D1.1 Annex B formula. Carbon equivalent is a single number that represents the overall alloy content of the steel and its susceptibility to hydrogen-induced cracking in the heat-affected zone.
D1.1 Annex B uses the CE(IIW) formula: CE = C + (Mn+Si)/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. A higher CE means the steel is more hardenable and may require higher preheat than the standard Table 5.11 values. Annex B provides an alternative method for determining preheat based on CE, hydrogen level, and restraint — useful when the standard table values seem overly conservative or when working with steels near group boundaries.
Use the carbon equivalent calculator to compute CE(IIW) and Pcm directly from your MTR chemistry values.
Frequently Asked Questions
In practice, yes. A mill test report (MTR), mill certificate (mill cert), and certified material test report (CMTR) all refer to the same document: the steel producer's certified record of chemical analysis and mechanical test results for a specific heat of steel. The terms are used interchangeably in the structural steel industry. EN 10204 uses the term inspection certificate, but the content is equivalent.
D1.1:2025 Clause 7.2.1 requires that contract documents designate the specification and classification of base metal. The code requires you to know what steel you are welding so you can apply the correct preheat, filler metal, and procedure. The MTR is the standard industry document that proves which specification applies. While D1.1 does not prescribe the document format, most contract documents and building codes require MTRs for structural steel traceability.
Table 5.6 organizes approved base metals into five groups based on strength and chemistry. Group I includes common mild steels like A36 and A500 with yield strengths of 30 to 50 ksi. Group II includes structural steels like A992, A572 Gr.50, and A588 with yield strengths of 36 to 55 ksi. Group III covers higher-strength steels like A572 Gr.60 and A913 Gr.60 at 55 to 65 ksi. Group IV includes A709 HPS70W and A913 Gr.70 at 70 ksi. Group V is A913 Gr.80 at 80 ksi yield.
Three steps. First, find the ASTM specification and grade on your MTR. Second, look up that specification in D1.1 Table 5.6 to find the base metal group (I through V). Third, use the group to look up the minimum preheat temperature in Table 5.11 based on your welding process category and material thickness. For example, A992 steel is Group II. For SMAW with low-hydrogen electrodes on 1 in thick plate, Table 5.11 Category B requires 50 degrees F minimum preheat.