Dual Certification in Stainless Steel: What It Means and Why It Matters

Dual certification is when a particular material is certified to meet the technical specifications of two particular alloys. For engineers and fabricators, this certification comes in handy when determining which material to use for a particular project.

Typically, dual certifications refer to stainless steel that meets both the standard grade and low-carbon (L) variant. For example, 304 stainless steel might also meet the requirements for 304L, meaning one piece can work in both situations. 

For procurement professionals, dual-certified steel is much more versatile and cost-effective. This certification helps streamline inventory by reducing the number of variants to have on-hand, and the material comes with advanced technical advantages, such as increased weldability.

The Cornerstone of Material Quality

What Is a Mill Test Report (MTR)?

A Mill Test Report is a document showing a material’s physical and chemical properties. Think of the report as showing the metal’s “DNA.” Without this document, it’s impossible to tell whether a particular piece will meet the standards necessary for your project. Overall, MTRs are essential for traceability and quality assurance.

The core components of a mill test report include: 

• Material heat number
• Material grade
• Chemical composition of the metal
• Mechanical properties
• Material dimensions
• Industry standard compliance (e.g., ASME rating)

The Role of Industry Standards

National and international standards are critical for the manufacturing and fabrication industries. Without these standards, quality control and assurance would be impossible, as there could be differences between materials from different suppliers or batches. 

The two most common standards for stainless steel are the American Society for Testing and Materials (ASTM) and the American Society of Mechanical Engineers (ASME). A mill test report demonstrates how a particular piece of stainless steel meets these standards, enabling commercial dual-certification.

The Core Concept of Dual Certification

A Commercial Solution to a Technical Problem

As recently as the 1970s, low-carbon stainless steel was more expensive to produce, meaning these variants came at a premium price. For manufacturers and engineers, having to buy and stock two types of steel was cost-prohibitive, not just because of the price difference, but because of the added labor and storage needs. 

Thankfully, technical advances in producing low-carbon steel helped reduce its cost and allowed for dual certification. This way, companies could stock a single type of stainless steel, streamlining inventory management and reducing overall costs. 

Carbon’s Critical Role: Preventing “Weld Decay”

For stainless steel to be classified as “low carbon,” it typically needs less than 0.03 percent of carbon. By comparison, standard grades may have up to 0.08 percent, which helps increase their mechanical and tensile strength. 

Unfortunately, there’s a trade-off when working with high-carbon steel, which is that it’s harder to weld efficiently. Weld decay refers to intergranular corrosion, which is what happens when chromium carbides precipitate in the heat-affected zone (HAZ). This precipitation can weaken the bonds within the weld, making it susceptible to corrosion or cracking under pressure. 

Thankfully, low-carbon steel variants don’t have this problem. Because there are fewer carbides in the material, it’s immune to sensitization, creating a stronger weld. This also means you don’t have to rely on costly post-welding annealing to fix the decay. 

Key Dual-Certified Grades in Practice

304/304L: The Everyday Workhorse

In most cases, 304 stainless steel works well. It has high tensile strength, impact resistance, machinability, and corrosion resistance. This versatile material is suitable for many applications, including construction, food service, medical equipment, automotive parts, and chemical processing. 

304L stainless has almost all the same characteristics as its high-carbon counterpart, but it’s much easier to weld because it’s immune to weld decay. Therefore, having a dual-certified version of 304 steel provides the best of both worlds. Here’s a breakdown of the chemical compositions of both materials for reference.

 

304 Stainless Steel	304L Stainless Steel
Carbon – Max 0.08%	Carbon – Max 0.03%
Chromium – 18-20%	Chromium – 18-20%
Manganese – Max 2%	Manganese – Max 2%
Nickel – 8-12%	Nickel – 8-12%
Nitrogen – Max 0.10%	Nitrogen – Max 0.10%
Phosphorous – Max 0.045%	Phosphorous – Max 0.045%
Silicon – Max 0.75%	Silicon – Max 0.75%
Sulfur – Max 0.03%	Sulfur – Max 0.03%

316/316L: The Harsh Environment Specialist

Although 304 stainless steel works in most applications, it’s not suitable for harsh environments, such as high temperature applications. Instead, 316 stainless steel is a better alternative, thanks to the addition of molybdenum. 

As with dual-certified 304 steel, having dual-certified 316 stainless allows you to get both the tensile strength of the standard grade with the weldability and enhanced machinability of the low-carbon variant. Here’s a breakdown of the chemical composition of both of these grades. 

Recommended Read: Real-World 316 Stainless Steel Applications Across Key Industries

316 Stainless Steel	316L Stainless Steel
Carbon – Max 0.08%	Carbon – Max 0.03%
Chromium – 16-18%	Chromium – 16-18%
Nickel – 10-14%	Nickel – 10-14%
Molybdenum – 2-3%	Molybdenum – 2-3%
Manganese – Max 2%	Manganese – Max 2%
Silicon – Max 0.75%	Silicon – Max 0.75%
Nitrogen – Max 0.10%	Nitrogen – Max 0.10%
Phosphorous – Max 0.045%	Phosphorous – Max 0.045%
Sulfur – Max 0.03%	Sulfur – Max 0.03%

The Strategic Value and Verification of Dual Certification

Practical Benefits for the End User

As you can see, buying dual-certified stainless steel offers greater flexibility and cost-effectiveness. Rather than managing two inventory stocks, you can buy one type of steel for more applications. Plus, dual certification ensures better weldability while retaining the strengths and benefits of standard-grade steel. 

Verifying Dual Certification on the MTR

Because dual certification is a commercial practice, not an engineering one, you must follow a checklist when verifying if a particular steel meets these standards. 

• First, visually inspect the piece to ensure the code stamped on it is the same as your MTR. 
• Second, verify that the carbon content is low enough to meet low-carbon stainless steel standards. 
• Next, verify that the mechanical properties match those for the standard grade. 
• Finally, look for a certification stamp and inspector signature. Ideally, there will be an inspector from the supplier and an independent third-party tester. This way, you can be sure the data on the MTR is precise and accurate. 

Here’s a sample MTR table for reference. 

Part	Quantity	Heat #	Physical Properties	Chemical Properties	Country of Origin	Grade
EX1234	1	123	TS, YS, E, Hardness	Carbon/Chromium etc.	USA	Standard or L Grade

 

The part number refers to the particular part you’re inspecting, while the Heat Number refers to the batch. The physical properties will include tensile strength (TS), yield strength (YS), elongation (how much the metal will stretch under pressure), and hardness. The chemical composition will list each chemical and its actual percentage within the steel. 

When verifying that a particular piece is dual certified, pay close attention to the physical and chemical properties. Although the grade (or conforming material) section should indicate which grades it matches, you’ll need to double-check. Also, any MTR without a certification stamp or signature should be returned.

Important Nuances and Limitations

Debunking Misconceptions

As we mentioned, dual certification is a commercial practice—not an engineering one. It’s not recognized by all standardization organizations, which is why it’s imperative to double-check the details yourself. 

Also, just because a particular piece is dual-certified doesn’t mean it will meet each standard requirement listed on the MTR. There may be slight variances that you must account for when using the material. For added quality assurance, discuss tolerances and physical properties with the manufacturing mill. 

The Bottom Line About Dual Certification in Stainless Steel

Dual certification is a complex and sophisticated process that can yield significant cost savings and operational benefits. In most cases, buying dual-certified stainless steel is the best option, as it’s more versatile than buying pieces with single-grade certifications. 

Fortunately, Specialty Steel is committed to quality assurance and traceability. Partnering with us can help streamline the procurement process and improve cost-effectiveness. Let us help you find the steel for your next project.