Stainless Steel Actuated Ball Valves
Stainless steels containing about 18 per cent chromium and 8 per cent nickel, 18-8 austenitic stainless steels, are regularly used as body and bonnet materials for services at elevated and subzero temperatures and for , highly corrosive conditions. The addition of molybdenum to the basic type 18-8 steel and a slight increase in nickel materially increases its corrosion resistant properties and valves made of 18-10-3 Mo steel are used extensively in the chemical industry for handling acetic acid, nitric acid, alkalis, bleaching solutions, food produces, fruit juices, sulphurous acid, tanning liquors, and many other industrial chemicals.
For use at elevated temperatures a further modification is made by the addition of niobium and this steel, known as 18-10-Nb, is suitable for temperatures up to 800 °C.
Austenitic stainless steels usually do not suffer embrittlement at extremely low temperatures, so valves in materials such as 18-8 and 18-10-3Mo are very suitable for operating on cryogenic services; instances are the handling of liquefied gases such as natural gas, methane, oxygen, and nitrogen.
Special Stainless Steels
Where conditions are too severe for the standard stainless steels the next group of interest is that of the more highly alloyed stainless steels. Probably the most common of these is '20' alloy, which contains about 29 per cent nickel and 20 per cent chromium with additions of molybdenum and copper. This alloy is extremely resistant to sulphuric acid over a wide range of concentrations and temperatures. In addition it will handle phosphoric and acetic acids under most conditions, especially where chlorides or other impurities are present.
Even more highly alloyed materials are available, such as 'Incoloy 825' and 'Carpenter 20Cb3', which may be required for the more extreme conditions.
There is also a growing use of duplex stainless steels (having a ferritic or austenitic structure) which contain 20 per cent or more of chromium and 5 per cent or so of nickel, with some molybdenum. These alloys are stronger and harder than the standard austenitic stainless steels and have better resistance to selective corrosion in the more severe conditions of sulphuric and phosphoric acids.