An Introduction to High Performance Stainless Steels
High performance stainless steels supplied across Europe by NeoNickel offer superior corrosion resistance compared to standard stainless steel grades. They’re better able to fight pitting and stress corrosion in aggressive environments, especially ones containing the chloride ion.
This highly corrosion resistant steel is manufactured by using enhanced levels of chromium, nickel, molybdenum and nitrogen alloying. Because these grades are produced with low carbon contents they preserve their resistance and allow hot fabrication and welding.
Development of High Performance Stainless Steels
High performance stainless steel production became a commercial reality in the 1970s with the dawn of steel melting and refining technologies that meant compositions with low carbon content and tight composition control could be produced in a cost-effective way.
Technologies such as vacuum melting, electron beam remelting, electroslag remelting led the way, with vacuum oxygen decarburisation (VOD) and argon oxygen decarburisation (AOD) being the most significant today from a commercial point of view.
The financial importance of these steels has increased ever since and more varieties have been produced.
There are three main kinds of high-performance steel – austenitic, ferritic and duplex (austenitic-ferritic). Each kind has a lot in common, but the wide variations between them in terms of corrosion resistance and other characteristics means one kind – or an individual grade – can be best for a particular job.
What Makes a Stainless Steel High Performance?
High performance stainless steels are often compared using the pitting resistance equivalent number (PREn). PREn is a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels based on their chemical compositions. It specifically suggests how resistant an alloy is to pitting in a chloride-containing environment, and is only meaningful within the same stainless steel family (austenitic, ferritic or duplex).
Molybdenum and nitrogen levels are typically weighted to take into account their strong influence on pitting corrosion resistance. Engineers often consider a PREn of 32–35 as the minimum suitable for seawater service for austenitic and duplex stainless steels, while duplex stainless steels with a PREn over 40 are considered acceptable for hydrogen sulphide environments.
Find out more in our guide to PREn: Pitting Resistance Equivalent Number.
Austenitic
The higher nickel content of austenitic stainless steel grades means they’re often chosen for severe acid service and for their resistance to chloride pitting and stress corrosion cracking. Typical uses include flue gas cleaning equipment that handles acid condensates, or where materials are exposed to acidic solutions containing strong oxidants, as in paper bleaching.
Austenitic grades are relatively easy to weld so they’re often chosen for situations such as offshore and nuclear piping where weld quality is extremely important and much of the construction must be done in the field.
Super Austenitic Stainless Steel: AL-6XN®
AL-6XN® is a 6% molybdenum, low carbon, nitrogen-bearing super-austenitic stainless steel. With many years of proven seawater service and 50% higher strength than standard stainless steel, it is the alloy of choice in high chloride, seawater and brine applications. It has also been used successfully in very corrosive environments, including biopharm markets. Its strength and corrosion resistance make it a better choice than conventional duplex stainless steels, and a cost-effective alternative to more expensive nickel-base alloys, where formability, weldability, strength and corrosion resistance are essential.
Ferritic
Thin sheet stainless steel is usually most cost-effective when produced from ferritic grades, so they are, for instance, used as the tube material in many kinds of heat exchangers.
Duplex
Duplex stainless steel grades are often used in situations where strength is a major consideration. They are found in pressure vessels for the chemical process industry and in heat exchangers, for example.
Each type of high performance stainless steel will offer superior resistance to chloride pitting and stress corrosion cracking to standard grades such as Type 304L (which contains only 18% chromium and 8% nickel) and Type 316L (which contains similar amounts of chromium and nickel and 2% molybdenum). Fabrication considerations are often the biggest factor in deciding which type to select.
Duplex Stainless Steel: UNS S31803/2205
UNS S31803/2205 is a 22% chromium duplex stainless steel, developed to combine the strength of ferritic grades with the corrosion resistance of austenitic grades. It is often used as a practical solution to chloride-induced stress corrosion cracking, and is resistant to sulphide stress corrosion cracking in sour-gas environments. 2205 plate is commonly used for pressure vessels and tank fabrication, while 2205 pipe and tube are widely specified for corrosive fluid handling.
Super Duplex Stainless Steel: ZERON® 100 and UNS S32760
ZERON® 100 is a highly alloyed super duplex stainless steel for use in aggressive environments, made to UNS S32760. With a PREn above 40, it has superior crevice corrosion resistance in seawater. It is also highly resistant to corrosion in a wide range of organic and inorganic acids, including most concentrations of sulphuric acid. With NORSOK approval, ZERON® 100 has been used successfully for pumps, tooling and piping systems in the North Sea.
High Strength Stainless Steels
Many high performance stainless steels also offer higher strength than standard grades. Duplex grades provide twice the yield strength of austenitic grades, and AL-6XN® offers 50% higher strength than standard stainless steel.
Where even higher strength is needed, precipitation hardening grades like Alloy 13-8 and Alloy 17-4 combine high strength with corrosion resistance, and are used in demanding applications such as aircraft landing gear.
Where Are High Performance Stainless Steels Used?
In the oil and gas industry, duplex stainless steels, nickel alloys and super austenitic grades are used in place of more commercial austenitic grades, which are prone to general pitting, stress cracking and have limited strength. ZERON® 100 super duplex and AL-6XN® are used in offshore oil rigs, pressure vessels and cyclones, and ZERON® 100 has been used in seawater fire protection systems by major operators.
AL-6XN® has also been used successfully in the biopharm market, while duplex grades are found in pressure vessels and heat exchangers for chemical processing.
See more examples in our guide to the top applications of stainless steel across industries.
Working with High Performance Stainless Steels
High performance steels are a sophisticated product for demanding applications. So they are more technically demanding than Types 304 and 316 in regard to metallurgy and fabrication requirements. It’s vital to have a full understanding of these stainless steels to use them well.
There are many complex applications for high performance stainless steels and a large number of grades to select from. NeoNickel’s technical staff will be delighted to discuss the advantages and limitations of individual types of high performance steel, along with their specific individual requirements, to help you make the optimal choice.
Contact us today and we’ll be able to help.
