Materials & Processes
4 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.
Stainless steel types, applications and new developments
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
V
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
Fig. 1. Stainless steel can be delivered in a variety of forms to suit diverse applications, including strip, sheet, plate, bars, wire and seamless tubing.
Photo: Sandvik.. 30
www.engineerlive.com
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. Stainless steels are often classified in terms of their crystal structure, as outlined below.
l 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
irtually 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
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.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36