The Rapid Development of
Page 35
If you've noticed an error in this article please click here to report it so we can fix it.
STAINLESS STEELS
The Concluding Instalment of a Series of Three Articles on the Progress in the Production of Steel
FEW steels have shown so many changes and developments since their introduction as have the stainless or rust-resisting steels, and no account of steel progress would be complete without a reference to them. The part played by these steels is, at present, less extensive in the commercial-vehicle field than in that of the private automobile, but their inherent possibilities of application are so numerous that they must be borne in mind.
Perhaps it will be as well to begin first with the malleable stainless, or what are commonly known as the "eighteen-eight," steels, that is, steels containing chrome and nickel in percentages represented by these figures. They consist of six grades.
The first, or standard quality, has a small copper content, is manufactured as a rule in an electric arc furnace, is non-magnetic and has high corrosion resistance. It is used for radiator pressings, lampshells, windscreen fittings and the like. Similarly, it is of great value for storage tanks for acids.
Some Qualities of Malleable Stainless Steel.
The second quality has no copper and is primarily employed for tubes. The third contains a little tungsten (between per cent. and 1 per cent.), whilst the copper addition is diminished to about per cent. There are reasons for this. There are occasions when it is necessary to fabricate parts of stainless steel by welding sheets together. After the welding has been carried out there is a zone on each side of the weld which, having been subjected to intense heat during welding, has become less able to resist corrosion.
This effect is considerably reduced by using the quality containing tungsten. As a general rule it is always recommended to normalize (that is, put it back into its original condition) the material after welding. Wherever this cannot be done, however, either because there are no facilities or because the part, when finished, is larger than can be accommodated, this type of steel enables the operation to be dispensed with.
The fourth quality contains a small percentage of titanium, and its purpose is similar to that of the third quality, except that it carries out the same work more efficiently and does not need normalizing.
The fifth quality contains 3-4 per cent. of molybdenum, and the purpose of this is to provide a ferrous metal which will withstand to a greater extent than any other the corrosive action of sulphuric acid. The sixth quality is of special importance to the manufacturer of commercial vehicles because it has a lower chromium and higher nickel percentage.
It is, therefore, specially suitable for deep pressing and stamping operations, or other processes incorporating heavy deformation in the cold state. It is also amply resistant to corrosion. This feature makes it ideal for radiator pressings.
All these types of steel can be had in bars, plates, sheets, forgings, billets or slabs, and all, save the sixth quality, as castings.
Possibly no advance has been more striking than the increase in the production of stainless-steel castings, and this can be ascribed entirely to a remarkable improvement in quality. In the past there was continual difficulty in making stainless-steel castings that would meet the requirements of customers, especially for vehicle fittings. Roughness of surface and pitting seemed to be inevitable,
Overcoming Difficulties.
Special methods adopted in the foundries that undertake this work have, however, resulted in overcoming most of these troubles, and the parts can now be obtained with a perfectly smooth finish. In consequence, numerous fittings previously not available in this material can now be bought at more reasonable prices and quicker delivery given.
Thus far the " malleable " stainless steels have been dealt with. There are, in addition, four standard qualities of what are known as the high-chrome stayess steels. These include the specially hard cutlery stainless steel; the material to British Standard Specification 5.80, a steel specially designed to comply with official requirements; the engineering qUality, which is extremely hard and resistant, and is suitable for numerous purposes where a non-malleable steel with good mechanical properties is required ; and, finally, the rustlese iron, which is, to some extent, in
tended to meet the same needs as the malleable steels, but has not so high a resistance to corrosion and is lower in price.
These 10 qualities sum up thE standard lines of development, but there are other materials whieh deserve brief mention on account of the possibilities that they open up.
Among these may be mentioned the two-ply stainless steel, which consists of thin sheets of stainless steel allied to mild steel by a. process ensuring that they do not separate in use. This is an American patent, but the original notion was German. It is believed that this material will shortly be manufactured in this country.
Then there is the selenium-containing steel, a recent discovery. One of the great disadvantages with regard to stainless steels in the past has been the difficulty of readily machining them. To reduce this difficulty where great mechanical strength, in addition to corrosion-resistance, was not the prime need, manufacturers used to increase the sulphur content, which made the steel easier to machine.
The Addition of Selenium Increases Machinability.
It has now been discovered that the addition of selenium to the base analysis has the same effect in increasing machinability, but does not reduce the mechanical properties to the same extent and preserves more of the corrosion resistance.
• Stainless steel veneered is another recent development, in which ordinary steel sheets are veneered on both sides with stainless material in layers of 0.015-in. thickness. Coils and straight lengths treated in the same way can be obtained.
It is important to remember that all these stainless steels, apart from the two-ply and veneered qualities mentioned, have, in addition to their non-corroding qualities, great heat
resistance. They will re t ai ii mechanical properties to a much greater extent than ordinary steels or alloy steels at high temperatures, and will scale or oxidize much less.
Another interesting development is the manufacture of alloy steels containing 4-6 per cent. of chromium. These are for use wherever low price and a fair degree of corrosion-resistance are needed. They are not so resistant to rust, etc., as the standard stainless steels for this purpose, but, on the other hand, they last much longer than ordinary, carbon steels.