Selecting the right stainless steel or duplex steel grade is essential to achieving reliable performance, regulatory compliance, and long-term value in demanding engineering applications. Understanding the differences between grades such as 304, 304L, 316L, duplex and super duplex stainless steels helps engineers make informed decisions based on corrosion resistance, strength, fabrication requirements, and operating conditions.

In this FAQ hub, Brown McFarlane provides practical guidance on stainless and duplex steel specifications, material selection, certification requirements, and industry standards. Drawing on extensive experience supplying specialist steel plate solutions, including precision engineering and waterjet cutting, our experts answer common questions to help engineers and procurement teams select the right materials for their projects.

Explore key FAQs covering stainless steel grades, duplex performance, super duplex applications, and the considerations involved in specifying high-performance materials for critical environments.

Stainless steel basics

What does the “L” stand for in 316L stainless steel?

The “L” in 316L stainless steel stands for low carbon. It indicates a maximum carbon content of 0.03%, compared to the standard 316 grade, which allows up to 0.08%. This reduced carbon content helps prevent weld decay (or intergranular corrosion) caused by the welding process. This makes 316L particularly suitable for fabricated components and demanding applications where corrosion resistance, weldability, and long-term performance are critical, such as marine environments and industrial applications.

What is the difference between 304 and 304L stainless steel?

The main difference is that 304L has a lower carbon content than 304. Standard 304 stainless steel has a maximum carbon content of 0.08%, while 304L has a maximum of 0.03%. This difference may seem small, but it helps prevent 304L stainless steel from losing its corrosion resistance after welding. In contrast, 304 can suffer from sensitisation or carbide precipitation during the welding process, which depletes the protective layer that surrounds the weld.

What is the difference between 304L and 316L stainless steel?

The main difference between 304L and 316L stainless steel is corrosion resistance. Both are low-carbon austenitic stainless steels, but 316L contains molybdenum, which provides improved resistance to pitting and chloride corrosion. This makes 316L better suited for marine, offshore, and chemical environments, while 304L is a cost-effective choice for general corrosion-resistant applications.

Which is better, 304 or 316?

Neither 304 or 316 stainless steel is universally better than the other; their performance depends on the environment. The addition of 2% molybdenum in the composition of 316 stainless steel gives it superior resistance to pitting and chloride (salt) corrosion. This makes 316 a better choice for offshore structures and marine environments. However, 316 is more expensive than 304 and typically harder to fabricate. So, for standard applications, 304 can be a more cost-effective and practical choice.

Duplex stainless steel

Is 316L duplex?

No, 316L is not duplex. It is an austenitic stainless steel, whereas duplex stainless steel has a mixed microstructure, with a roughly 50:50 split of austenite and ferrite. Their different compositions and properties make them suitable for different applications. For instance, while duplex steel generally has a higher yield strength than 316L, it can be more challenging to weld. This can make 316L a better choice where fabrication simplicity outweighs the need for maximum strength.

What are the benefits of duplex stainless steel?

Duplex stainless steel combines the advantages of austenitic and ferritic stainless steels through its approximately 50:50 microstructure. It offers high strength, excellent corrosion resistance, good resistance to stress corrosion cracking, and improved cost efficiency compared with many traditional stainless steel grades. Its strength also allows reduced material thickness, making duplex ideal for demanding applications such as offshore, chemical processing, and marine environments.

What is the difference between duplex and super duplex stainless steel?

The primary difference between duplex and super duplex stainless steel is their chemical composition. Super duplex stainless steel grades, such as UNS S32750 (1.4410) and UNS S32760 (1.4501), contain higher amounts of chromium, molybdenum, and nickel. In contrast with standard stainless steel grades, super duplex offers:

• Superior corrosion resistance
• Higher resistance to stress corrosion cracking
• Higher yield and tensile strength

These characteristics make super duplex stainless steel the ideal choice for extremely aggressive environments, and it’s typically used in offshore structures and the petrochemical sector.

Corrosion and performance

What is PREN in stainless steel?

PREN (Pitting Resistance Equivalent Number) is a calculated value used to compare the resistance of stainless steel grades to pitting corrosion. It is based on the alloy’s chemical composition, including chromium, molybdenum, and nitrogen content. Higher PREN values indicate greater resistance to chloride-induced corrosion, helping engineers select suitable materials for demanding environments.

Which stainless steel grade is best for marine environments?

316L stainless steel is commonly specified for marine environments due to its molybdenum content, which offers improved resistance to chloride corrosion compared with 304L. For more aggressive offshore and seawater applications, duplex and super duplex stainless steels offer higher strength and superior corrosion resistance, making them suitable for demanding marine conditions.

Specification and standards

What stainless steel standards should engineers specify?

Engineers should specify the stainless steel grade, product standard, testing requirements, and certification needed for the application. Common standards include ASTM (such as ASTM A240 for stainless steel plate), EN standards, UNS grade designations, and EN 10204 material certification. Brown McFarlane supplies stainless, duplex and super duplex steel plate to recognised industry standards and certification requirements.

What is EN 10204 3.1 certification?

EN 10204 3.1 certification is a material inspection document that confirms stainless steel meets the specified standard and requirements. It includes details such as chemical composition, mechanical properties, heat number, traceability, test results, and compliance information. This certification provides assurance of material quality, consistency, and traceability for stainless, duplex, and super duplex steel applications.