Duplex Stainless Steels Introduction, Applicable Standards, Chemical Compositions, PREN

Introduction

Duplex stainless steels form the latest family of stainless steels, tested as early     as in the 1930’s and mass produced from the 1970’s onwards. Their excellent mechanical properties and their corrosion resistance have made them a first choice material for high-load applications in corrosive environments. The development of the modern duplex stainless steel family began with a few grades, the most representative being UNS S31803, later S32205, often referred  to as 2205 (EN 1.4462).

Then it went on in two opposite directions:

ƒMore corrosion-resistant “Superduplex” and “Hyperduplex” stainless steel grades to meet the demands of very aggressive environments.

ƒ“Lean Duplex” stainless steel grades for demands of higher strength but for less corrosive environments, typically for structural applications.

They offer low maintenance and low Life Cycle Costs, just like the other stainless steel families. Their high strength, high corrosion resistance, good processing properties – once their specifics are taken into account - have begun to be valued in a wide range of new applications such as long-lasting structures (bridges, floodgates...) and equipment (desalination plants, water and energy utilities…).

Duplex stainless steels complement other stainless steel families, particularly austenitics. In some cases, duplex stainless steels should be the first choice, while in other cases, duplex stainless steels solve unexpected corrosion problems.

Applicable Standards

Duplex stainless steel grades are covered by major standards, such as EN, ISO, ASTM for a number of products and/or applications . Although there is a fairly long list of UNS grades, only a few of them make the largest part of today’s production.

Chemical compositions

Duplex stainless steels have a structure of about 50 % ferrite (a body centered cubic crystallographic structure) and 50 % austenite (a face centered cubic crystallographic structure). To achieve this, elements that stabilise the ferrite phase (Cr, Mo, Si, W …) are balanced by elements that stabilise the austenite phase (Ni, N, Mn …). The ferrite/austenite ratio depends not only on the alloying elements but also on heat treatments.

The classification of duplex stainless steels is done according to their corrosion resistance which depends on the alloying elements. It is customary to distinguish them according to their Pitting Resistance Equivalent Number (PREN). The next section provides

More information on PREN.

–Lean duplex grades (typefied by UNS S32304 -EN1.4362),

PREN 22-27, with a lower Ni content, without or with some Mo, are best for less severe environments.

–Standard Duplex (typified by S32205 (EN 1.4462), PREN 28–38, with 22%Cr and 3%Mo, which is mid-range in terms of corrosion resistance.

–Super duplex (typically S32520 (EN 1.4507)), PREN 39–45, with 25%Cr, 3.5% Mo and 0.22-0.3%N.

–Hyper duplex with PREN >45 for very severe environments, usually in the oil and gas industry.

Note: PREN stands for Pitting Resistance Equivalent Number: PREN = Cr+3.3Mo + 16N where Cr, Mo and N are the contents expressed in weight percent of the three elements chromium, molybdenum and nitrogen respectively.

Many duplex stainless steel grades are offered today, partly as a result of patents, partly because new grades are needed to meet new requirements, particularly for use in the oil and gas industry.