Engineering steels 243by modern steelmaking practices, such as arc-AOD, which produce low levels
of sulphur, e.g. often less than 0.01%. To overcome this particular problem, the
steels can be resulphurized to a level just below the maximum generally permitted
in standard specifications, e.g. 0.03% max.
Like carbon and low-alloy steels, large additions of sulphur are also made
to stainless steels in order to provide free cutting grades. Thus AISI 303 is an
18% Cr 9% Ni austenitic stainless steel containing 0.15% S min. However, the
large volume fraction of MnS inclusions in this grade impairs the corrosion
properties such that the performance is significantly worse than that of the stan-
dard 18% Cr 9% Ni (Type 304) stainless steel. Where this presents a significant
problem, free cutting properties coupled with improved corrosion resistance can
be obtained in Type 303Se, the 18% Cr 9% Ni grade containing 0.15% Se rain.
Steels for gas containers
High-pressure gas cylinders have been in use since the 1870s when they were first
introduced for the transportation of liquefied carbon dioxide for the aerated drinks
industry. Since that time, their use has extended enormously to deal with the
conveyance of a variety of permanent gases, such as air, argon, helium, hydrogen,
nitrogen and oxygen, and other liquefiable gases, such as butane, propane, nitrous
oxide and sulphur dioxide. Given the explosive, flammable or toxic nature of
some of these gases, it is not surprising that the manufacture and utilization of gas
containers are the subject of major scrutiny by the Home Office and also by inter-
national bodies. However, whilst still maintaining very high standards of safety,
the use of higher strength steels has been permitted which has greatly increased
the carrying efficiency of gas cylinders in terms of their gas capacity per weight.
In the UK, the relevant British Standard is BS 5045 Transportable gas
containers, of which Parts 1 and 2 are the main sections for steel containers:
9 Part 1" Specification for seamless gas containers above 0.5 litre water capacity
9 Part 2: Steel containers up to 130 litre water capacity with welded seams
This topic is the subject of reviews by Irani Is and Naylor. 19
Steel compositions
BS 5045: Part 1:1982 specifies four grades of steel for seamless cylinders,
covering C, C-Mn, Cr-Mo and Ni-Cr-Mo compositions, and details are shown
in Table 3.10. The most popular compositions are the C-Mn and Cr-Mo grades,
which are hardened and tempered to provide a minimum yield strength value of
755 N/mm 2. However, as indicated later, the yield strength is restricted to lower
levels for certain types of hydrogen containers.
In relation to welded containers, BS 5045: Part 2:1989 permits the use of C
or C-Mn steels but, as illustrated in Table 3.11, these grades provide minimum
yield strength values in the range 215-350 N/ram 2, i.e. significantly lower than
that attained in seamless containers.