The Latest Developments in
Page 49
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Metals for the Motor Industry
CONSTRUCTIONAL materials for the motor industry continue to develop. The industry has always made considerable use of rolled sheets and strips for the more important members in structural parts, particularly those highly stressed. They have been employed in the fiat-rolled state or in the desired shapes, usually produced by cold-working,
The manufacture of parts of this kind frequently demands severe bends in any given direction, Such bends call for a steel sheet; the tensile• and bending strength of which will be equal whatever the angle of the axis of. the specimen to the edges of the sheet. To produce strip of the maximum strength, an alloy steel of approximately 0.30 per cent. carbon, 3.75 per cent. nickel, 1.0 per cent. " chromium, with, occasionally, the addition of a percentage • of either vanadium or molybdenum, has been evolved. This is usually coldworked to dimensions, and afterwards subjected to hardening and tempering processes.
Medium4irength Material.
Where strip steel of medium strength is desired, it is customary to employ a milder alloy steel, hardened and ternpered, or a simple carbon steel with ' about 0.5 per cent. carbon; cold-worked. to dimensions and " blued." The alloy sheets of either type may be softannealed and finished by cold-rolling, or simply hot-rolled and annealed or normalized, in order to obtain high tensile strength. "
In the construction of engines, designers and makers have made use, to a certain extent, of non-ferrous materials. Among these may be mentioned the nickel bronzes; which are being employed for bearing bushes, worm gears, etc. Nickel bronze is quite satisfactory for bearings which have to undergo heavy loading, whilst its bearing properties in other respects are excellent.
A Nickel Bronze with Excellent Properties.
A characteristic nickel bronze of this type has 83.5 per cent. copper, 10 per cent. tin, 3 per cent. lead, and 3.5 per cent, nickel. Its tensile strength ranges from approximately 17 to 21 tons per sq. in. and the yield point is 11 tons per sq. in.
The proportion of nickel in these nickel bronzes varies usually from 1 per cent. up to 5 per cent. The effect of the nickel is to speed up solidification .of the Metal, and to dissolve more quickly the lead in molten copper. When 3 per cent. of nickel exists in the metal, liquid copper and lead can be dissolved in all proportions.
Nickel in these bearing bronzes also has the advantage of increasing resistance to compression. Bearing metals of this type will carry a heavy, load and withstand severe shocks better than will an ordinary bearing metal. Furthermore, they will retain a degree of plasticity great enough to enable them to conform without difficulty to irregularities in a shaft or journal.
Another bearing material of recent development is ,the lead-bronze alloy, containing a high percentage of lead, together with 74 per cent, of copper and 5 per cent. of tin. Metal of this type is said to be excellent for service Where lubrication is irregular.
• Nickel Bronze Particularly Suitable for Gears.
• Nickel bronze is highly suitable for worm or spiral gears, for which, also, a non-ferrous metal composed of 11 per cent. tin, 0.25 per cent, phosphorus and the remainder copper, has been invented and has given good results. Both nickel bronze and the alternative metal can be much improved by a chilling process, 'which affords a finer grain and greater hardness. • , A number of other ,alloys has been developed for use where extremely high strength is demanded. One of these is super-manganese bronze, another is special aluminium-nickel bronze, and a third is special nickel bronze.
The first of these has a composition of approximately 2 per cent. manganese, 2.5 per cent. iron, 6.5 per cent. aluminium, 20 per cent, zinc, and 69 per cent. copper. The second has Il per cent. aluminium, 5 per cent. nickel, 5 per cent. iron and 79 per cent.
copper. The third has 10 per cent, nickel, 2 per cent. aluminium, 1 per cent, zinc and 87 per cent. copper. The aluminium-nickel bronze oilers excellent impact-resistance properties, together with great fatigue strength, and is used mainly for spiral and worm gears. The special nickel bronze, with its large nickel percentage, is extremely strong and can be heat-treated.
A further material, of Japanese invention, is an aluminium bronze with a comparatively small percentage of iron. The composition of this metal is approximately 0.35 per cent. iron, 5.3 per cent. nickel, 7.0 per cent. aluminium, the remainder being copper. This has a tensile strength of 38 -tons
per sq. in, ft cannot, however,be used for casting.
tor engine valves, a wide range Of materials has been evolved by the steel metallurgist. Some of these have been in use for a considerable period. Others are of more recent origin and have their special characteristics.
Three Interesting Steels.
Amongthese metals may be mentioned the high-nickel chromium tungsten steels, the -cobalt chromium steels and the silicon chromium steels, -Of these, the cobalt chromium steels have a very low impact value at room temperature after normal heat-treatment, but at high temperatures this value is considerably raised.
The austenitic steels are highly resistant to scaling and corrosion caused by high temperatures. Their one disadvantage in use is that their hardness on the surface, as represented by the Brinell number, is not great, with the result that wear is found to take place where the stems fit in the Valve guides. This drawback has, however, been overcome by careful use of the nitriding process, but it has to be borne in mind that nitrided surfaces are subject to corrosive attack by the products of combustion of tetraethyl lead.
Welding Hard Alloy on to Valve Seatings and Stems.
Another method adopted has been to weld a hard alloy containing cobalt, such as Stellite, on to the valve seatings and stems, and encouraging results have already been obtained from this process. A further range of valve materials comprises the semi-austenitic steels of the chrome-nickel type, which offer good resistance to tetraethyl lead'.
The latest news is of the discovery of a further nickel-iron-aluminium steel for magneto and other permanent magnets, a steel even more valuable than the original nickel-iron-aluminium trnateriai mentioned some time ago in these columns. As yet, no details of composition or performance can given, but experts agree that the new development is of extreme importance,