Stainless steel

The discovery of stainless steel was made in 1913 by Harry Brearley of Sheffield,while he was experimenting with alloy steels. Among the samples which he threw aside as unsuitable was one containing about 14 per cent chromium. Some months later he saw the pile of scrap test pieces and noticed that most of the steels had rusted butthe chromium steel was still bright. This led to the development of stainless steels. The classic Rolls-Royce radiator was one of the first examples of the use of stainless steel.
The designer,engineer or fabricator of a particular component may think that stainless steel is going to be both difficult to work and expensive. This is quite wrong,and perhaps stems from the fact that many people tend to fall into the trap of the generic term ‘stainless steels’. In fact,this is the title for a wide range of alloys. Therefore if such materials are to be used effectively and maxi- mum advantage is to be taken of the many benefits they have to offer,there should be very close collaboration and consultation over which grade of stainless steel is best for the particular job in hand.

There are over 25 standard grades of stainless steel specified by BS 1449:Part 2. Each provides a particular combination of properties,some being designed for corrosion resistance,some for heat resistance and others for high-temperature creep resistance. Many,of course,are multipurpose alloys and can be considered for more than one of these functions. In terms of composition,there is one element common to all the different grades of stainless steel. This is chromium,which is present to at least 10 per cent. It is this element which provides the basis of the resistance to corrosion by forming what is known as a ‘passive film’on the surface of the metal. This film is thin,tenacious and invisible and is essentially a layer of chromium oxide formed by the chromium in the steel combining with the oxygen in the atmosphere. The strength of the passive film,in terms of resistance to corrosion,increases within limits with the chromium content and with the addition of other elements such as nitrogen and molybdenum. The formation of the passive film, therefore,is a natural characteristic of this family of steels and requires no artificial aid. Consequently,if stainless steels are scratched or cut or drilled,the passive film is automatically and instantaneously repaired by the oxygen in the atmosphere.

Stainless steels can be conveniently divided into the following three main groups:
Austenitic Generally containing 16.5–26 per cent chromium and 4–22 per cent nickel. Ferritic Usually containing 12–18 per cent chromium. Austenitic/ferritic duplex Usually containing 22 per cent chromium,5.5 per cent nickel,3 per cent molybdenum,and 0.15 per cent nitrogen. Martensitic Based on a chromium content of 11–14 per cent,although some grades may have a small amount of nickel.
Of the above groups,the austenitic steels are by far the most widely used because of the excellent combination of forming,welding and corrosionresisting properties that they offer. Providing that the correct grade is selected as appropriate to the service environment,and that the design and production engineering aspects are understood and intelligently applied,long lives with low maintenance costs can be achieved with these steels.
HyResist 22/5 duplex is a highly alloyed austenitic/ferritic stainless steel. It has more than twice the proof strength of normal austenitic stainless steels whilst providing improved resistance to stress corrosion cracking and to pitting attacks. It possesses good weldability and can be welded by
conventional methods for stainless steel. The high joint integrity achievable combined with good strength and toughness permit fabrications to be made to a high standard. It is being increasingly used in offshore and energy applications.
Table 4.4 shows typical stainless steels used in motor vehicles.