Stainless steel is an attractive material for a variety of applications, usually due to the corrosion resistance and strength but sometimes also for aesthetics. However, designers that are new to stainless steel can find the choice of grades confusing. Alistair Rae provides an overview of the different types, typical applications and recent developments.
Virtually everyone comes into contact with stainless steel daily, if only in the form of cutlery. For engineering applications, however, stainless steel is considered to be more of a specialist material, and only used where essential. This is because it can be difficult to machine compared with carbon steels and aluminium, and more costly to purchase. On the other hand, for some applications - such as those in the food, pharmaceutical and chemical industries - the use of stainless steel is all but essential because of its corrosion resistance. Those design engineers not familiar with stainless steel can soon become confused by the different types and grades, and it is important to select the correct grade in order to create a satisfactory design that can be manufactured at a reasonable cost.Stainless steel differs from carbon steels primarily in its chromium content of at least 10.5 per cent by weight, though adding up to 26 per cent can increase corrosion resistance in harsh environments. The chromium forms a thin passive layer of chromium oxide on the surface of the steel that prevents surface corrosion from progressing into the metal. Should the oxide layer become damaged, it repairs itself quickly by virtue of fresh chromium becoming exposed and oxidised.
One of the advantages of this corrosion resistance is that stainless steel is readily recyclable. Unlike carbon steel, which might be badly corroded or contaminated with finishes such as paint or plating, stainless steel is 100 per cent recyclable. Indeed, it is estimated that an average stainless steel object today contains 60 per cent recycled material.
As with most metals, stainless steel can be processed into a variety of different forms of supply to suit diverse applications, including strip, sheet, plate, bars, wire and seamless tubing (Fig.1). It can also be cast or processed into a powder for use in selective laser sintering of rapid prototypes and rapid-manufactured components. Other specialised forms of supply include high-precision strip (Fig.2).
Stainless steels are often classified in terms of their crystal structure, as outlined below.
- Austenitic. Known as the SAE 300 series (chromium-nickel) and 200 series (chromium-manganese) stainless steels, austenitic grades are the most commonly used. They contain a maximum of 0.15 per cent carbon and a minimum of 16 per cent chromium, plus small amounts of nickel and/or manganese to ensure that the austenitic structure is maintained from cryogenic temperatures up to the melting point. The addition of 6 per cent or more molybdenum, plus nitrogen, results in superaustenitic stainless steels (in the 600 series), which exhibit improved resistance to pitting and crevice corrosion, and a higher nickel content gives better resistance to stress-corrosion cracking.
Note that austenitic stainless steels have good formability - and cold working can increase the strength of some grades - as well as toughness and weldability. Typical applications for austenitic stainless steels include cooking utensils, containers and pipework in the food industry, and specialised grades - such as 316LVM - are suitable for surgical implants.
- Ferritic. This type of stainless steel has from 10.5 to 27 per cent chromium by weight and little or no nickel. Other alloying elements can include molybdenum, lead, aluminium and titanium. While ferritic grades generally have better engineering properties than austenitic grades - such as good ductility - the corrosion resistance and weldability is less good. Applications for ferritic grades include cooking utensils, electrical enclosures, domestic appliances, automotive exhausts and highly polished automotive trim. Although stainless steels are generally considered to be non-magnetic, ferritic grades are an exception to this rule. Note that both ferritic and martensitic grades can be found within the 400 series.
- Martensitic. With lower corrosion resistance than austenitic or ferritic stainless steels, martensitic grades nevertheless benefit by being strong, tough, readily machinable and can be hardened by heat treatment. Chromium content is typically 12 to 14 per cent, with other alloying elements being molybdenum, nickel and carbon. Cutlery is made of martensitic stainless steel due to its hardness and the ability to produce a polished surface and an edge that stays sharp. Martensitic stainless steel is popular for blades and similar components used in food processing, as well as fasteners, shafts, valves and tools.
- Precipitation-hardening martensitic. Compared with conventional martensitic grades, precipitation-hardening martensitic stainless steels can achieve higher strengths, with corrosion resistance that is similar to that of austenitic grades. Typically precipitation-hardening martensitic grades (which are designated the 600 series) have 17 per cent chromium and 4 per cent nickel content. Applications for these grades include equipment used in the paper industry, turbine blades and aerospace components.
- Duplex. As the name suggests, duplex stainless steels have a mixed microstructure containing both austenite and ferrite in roughly equal proportions. Strength is approximately twice as high as austenitic stainless steels, and duplex grades also benefit from improved resistance to pitting, crevice corrosion and stress-corrosion cracking. Duplex grades have 19 to 28 per cent chromium, up to 5 per cent molybdenum, moderate amounts of nickel and small amounts of other alloying elements (including molybdenum, nitrogen, manganese, copper and tungsten, depending on the grade). Compared with super-austenitic grades, similar material properties can be achieved but with a lower overall alloy content, which makes duplex grades more cost-effective where designers have a choice. The most commonly used duplex type is 2205. Applications for duplex stainless steels can be found in the marine, chemical, petrochemical, and pulp and paper industries.
- New developments. Given the diversity of applications for which stainless steels are suitable, it is not surprising that producers are continually developing new grades to meet the needs of particularly demanding applications. For example, in November 2010, ArcelorMittal launched a new grade, designated K44X, for a specific automotive application (note that the stainless steel business of ArcelorMittal has subsequently been spun off as Aperam). Grade K44X is designed for high-temperature sections of automotive exhaust systems, typically from the manifold to the catalytic converter, where 1000-1050° C can be encountered. This resistance to higher temperatures has become necessary because Euro 6 emissions targets for automotive manufactures are resulting in smaller engines being used, but these are having to work harder.
K44X (AISI 444, EN 1.4521) has a chromium content of 19 per cent (by weight), with 2 per cent molybdenum and 0.6 per cent niobium. It is resistant to high-temperature oxidation, creep up to 1050° C, and offers good durability and thermal fatigue strength. It also benefits from good weldability and formability that is similar to ferritic grades.
For applications requiring a combination of formability, high strength, toughness, hardenability and corrosion resistance, Sandvik has developed Sandvik Nanoflex, which is a precipitation-hardenable austenitic stainless steel that utilises nanotechnology (Fig.3).
As delivered, Nanoflex is easily formed, then the mechanical strength can be increased significantly by heat treatment of the final product at relatively low temperatures and without causing distortion.
Normally the material is delivered in the cold rolled condition, but the material can alternatively be supplied in the annealed condition and then heat-treated after forming to increase the strength.
Another leading producer of stainless steels is ThyssenKrupp Nirosta. This company has recently been involved in a project to build stainless steel metro trains for use in Hamburg. The DT5 multiple unit, developed by the consortium Alstom/Bombardier Transportation, is described as being low on pollution, resource-friendly and quiet, while at the same time offering high safety standards and ride comfort. The body of the DT5 is made completely from Nirosta 4318 stainless steel and is based on a new lightweight design that delivers ecological benefits.
Wolfgang Gebel, from ThyssenKrupp Nirosta technical customer support, states: "With its high yield strength and good corrosion resistance, 1.4318 is ideal for lightweight construction. Even thin panels display high strength and rigidity."
Andreas Knitter, Alstom's chief executive, adds: "We decided to do without external painting altogether. Stainless steel surfaces are easier to clean, make expensive painting superfluous, and the high stainless steel content translates into another benefit for the environment: the new vehicles are made of 95 per cent recyclable material."
Versatile material
From rail vehicles to cutlery, and from surgical implants to turbine blades, stainless steel (see panel page 30) is an exceptionally versatile material. In some respects that is one of its problems, in that there are so many different grades that it is misleading to think of it as 'one material type'.
As with any material, designers investigating the use of stainless steel should look at all aspects of the design if they are to make the most of this material; for example, careful consideration of forming and joining methods is required if the optimum design is to be produced. Nevertheless, it seems certain that we will see more use of stainless steel in the future, not less.
Stainless steel statistics
Statistics released by the International Stainless Steel Forum (ISSF) in January 2011 indicate that total global production for the first nine months of 2010 was 23 million metric tons, which represents an increase of 29 per cent over the same period in 2009 - though this has to be seen in the context of a 5.3 per cent fall in production in 2009.
Total production in 2009 was 24.5million metric tonnes, compared with 25.9million metric tonnes in 2008.
Looking at production in terms of different grades, it can be seen that the increased market shares of chrome/manganese (200 series) and chrome (400 series) stainless steels has been at the expense of chrome/nickel (300 series) stainless steels.
The increase in demand for ferritic stainless steels is mainly due to the excellent performance of the global car industry, while the increased market share for chrome-manganese steels can be accounted for by the very high growth achieved by privately owned stainless steel producers in China.
In the first nine months of 2010, chrome/nickel (300 series) stainless steels accounted for 57.1 per cent of production, chrome (400 series) stainless steels accounted for 31.1 per cent and chrome/manganese (200 series) accounted for 11.8 per cent.
