Stainless steel

Author: Stainless steel in structural applications

Stainless steel

There are many different types and grades of stainless steel. These have been formulated over the last 80 years or so to optimise certain characteristics such as corrosion resistance in specific environments, weldability and mechanical properties. Stainless steels can be classified into five groups, according to their chemical composition and thermomechanical treatment. Each group has different properties, particularly in respect of strength, corrosion resistance and ease of fabrication.

The five groups can be summarised thus:

Austenitic stainless steels

These are the most commonly used stainless steels.

Ferritic stainless steels

The ferritic stainless steels contain relatively little nickel and have a ferritic microstructure.

Martensitic stainless steels

These steels can be hardened by heat treatment and are not normally used in welded fabrication.

Duplex stainless steels

These steels have a mixed microstructure and combine the best of the properties of the austenitic and ferritic groups.

Precipitation hardening steels

These offer the highest strengths, obtained by suitable heat treatments. They are not normally used in welded fabrications.

Most structural applications use austenitic grades 1.4301, 1.4401 or their low carbon variants 1.4307 and 1.4404. A wide range of product forms is available in these grades. (Note that in Germany, the low carbon version of 1.4301 widely used is grade 1.4306, a slightly higher alloyed version of 1.4307.) The duplex grade 1.4462 is also widely available and experience of this grade has been gathered in the offshore industry. Duplex grades offers advantages in mechanical strength and corrosion resistance, especially where stress corrosion cracking may be a problem.

Cast forms generally have equivalent corrosion resistance to that of the wrought forms but several differences exist. One of the more important of these is that the microstructure of cast austenitic stainless steels contains a greater amount of ferrite. This not only facilitates weld repair of castings but also increases the resistance to stress corrosion cracking. Cast steels also differ in mechanical properties, physical properties and chemical composition. Because of the formation of larger grain sizes and other differences in microstructure, mechanical properties of cast steels exhibit a wider range and are generally inferior to wrought steels.

For example, when joining stainless steel to carbon-manganese steel, a highly alloyed electrode must be used to avoid cracking in the weld metal. Stainless steels are modern materials. Ever since they became available to industry their use has constantly extended into new applications. This process continues even today. To make successful use of the stainless steels in building applications, it is necessary to know their properties, their capabilities regarding corrosion resistance, the availability of product forms and surface finishes. It is also of interest to be aware of reference applications that prove the success of the selection of stainless steels many years ago. Stainless steels are friendly to the environment; they actively and passively help to keep it clean and they are recycled to a high degree (50 - 70 %). If a minimum of 11% of chromium is added to such a steel a "stainless steel" is obtained.

Austenitic stainless steels have a long history of successful applications in the building industry. Their excellent corrosion resistance is the prime reason for low maintenance costs and excellent durability. These properties are becoming increasingly important in any building project. The surface of stainless steels can be obtained in many different qualities such as mirror polished, ground with different grit sizes, brushed, cold rolled, sand blasted, roll textured and coloured. Stainless steel combines aesthetically with any other material without dominating it yet maintaining its timeless elegance especially if slender (but strong) elements are used.

The stainless steels are heat resisting materials. They outperform any other conventional structural material in fire or high temperature applications. Their use is indicated for fire escape systems, fire doors, enclosures, cable trays, chimneys. Stainless steels have very interesting mechanical properties that can be varied within wide limits due to their response to cold work. The austenitic stainless steels are tough and ductile resulting in an exceptionally high plastic deformation before they fail. These properties may be important in safety barriers, blast walls and for aseismic building frames.

The austenitic stainless steels can be welded with all known processes but for the other metallurgical groups restrictions apply. Whatever the grade of stainless steel which has to be welded it is necessary to select the most cost effective procedure. Stainless steels for structural applications are selected primarily for their corrosion resisting properties. It must be assured that the weld matches the properties of the base material. Austenitic steels can be welded to structural carbon steel by observing proper precautions.

Design of stainless steel structures is covered in Eurocode 3