Metallurgy for Motors Moves Ahead
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AREVIEW of current metallurgical progress reveals many new features which are of special interest to the commercial-motor industry. On the production side, and for repair work that includes machining of parts, an American development is worthy of mention. This comprises a new steel for cutting-tools, which is claimed to be intermediate with the tungsten carbides and the super high-speed steels. The material in question has an approximate analysis as follows:-36 per cent, cobalt, 8 per cent. molybdenum, 6 per cent. chromium, 1 per cent, carbon, and 0.4 per cent. vanadium.
The new steel can be readily machined, cut, welded, forged, and brazed, whilst by a treatment known as precipitation hardening, it can be given a hardness corresponding to Rockwell A88 or C72. Actual shopperformance figures for this material are not yet available, so that the claims made must be considered with care. Results, however, are said to be highly promising.
Advances in nitrogen hardening were stressed in a previous article. America, again, has been experimenting extensively in this direction, and with important results. Previously, only the chromium molybdenum aluminium steels were regarded as the main subjects for a nitrogen-hardening treatment, but this skin-hardening process is now being adapted to a considerable range of alloy steels, including such materials as the aluminium free chromium molybdenum steel, and the carbon vanadium steel, the highchromium steel and the austenitic manganese steel.
Good Surface Hardness.
Whilst nitriding is not regarded as endowing these materials with quite so hard a surface as is possessed by the true nitriding steels, it does give a reasonably good surface-hardness, whilst the core is satisfactorily tough, and the steels as a whole display excellent mechanical properties.
Some of these new nitrided materials have been used with great success for the crankshafts of commercial vehicles.
The steel most popular in the United States, for this work, contains 0.5 per cent, molybdenum and 3 per cent. chromium, because this is found to have the maximum skin hardness after nitriding, so far as the new materials are concerned.
The nitrided high-chromium steels call for a special treatment to eliminate the surface passive film, which would inhibit the skin-hardening effect of the process, but, after this, they are quite suitable for engine valves.
Metallurgists in Italy have also been busy, and have brought out a chromium-molybdenum steel which is employed, in the form of castings, in an anti-skid device for lorries, whilst the si46 same castings are also replacing forgings, in the construction of certain vehicle parts.
The finishing of metallic surfaces has been amplified by the invention of a method of producing a coloured surface on metal by means of an electro-plating process. Almost any colour in the spectrum can be given to metal by this method, not once, but again and again, on a mass-production basis, and in conformity with a specification. The basis of the operation is that alkaline solutions of copper lactate set up cathodic deposits, at low current density, which are highly coloured. The colour varies in rainbow order through a repeat/1g time cycle.
Progress in Annealing.
In Russia, research has been carried out in attempts to reduce the time taken in annealing steel bars, i.e.. softening them for machining. The principle on which metallurgists have been working is that known as isothermal annealing, which means that the temperatures of heating and quenching the steel are slightly altered to give the desired results.
Annealing special alloy steels usually takes about 36 hours, and, as a result of their experiments, the Russians claim that they have cut down this time to one-sixth. The materials affected by this process are a 12 per cent, chromium steel, a nickel-molybdenum steel, a chromium-tungsten steel, and a chromium-molybdenum-vanadium steel.
In this country, a new shearing tool has been put on the market which is claimed to cut flat steel sheets up to 3-32 ins, thick. In shearing sheets of all sizes, distortion is cut down to a minimum, whilst the shearing is uniform and the operation simple. The entire apparatus weighs a mere 8 lb., is self-feeding, and can readily be changed from a bench tool to a portable one with the aid of an adapter.
In the manufacture of commercial vehicles, hammers are used extensively for bodywork. These have, generally, been fashioned from a normal carbon steel and they so often break that their cost becomes a considerable item. Metallurgists have been investigating this problem, and a steel for hammer heads for this work has been designed; it gives a greater length of life combined with an improvement in quality. The steel they have chosen is a casehardening nickel steel, with, as an alternative, a 31 per cent nickel steel. Another small but valuable advance is the discovery that, by adding 11 per cent, of nickel to bronze worm-gears, the strength is increased.
Refuse-collecting vehicles are now . having their bodies made from a special high-tensile nickel-copper alloy steel. The advantage of this material is that it permits a great saving in weight, whilst it offers high resistance to weather conditions and to corrosion effects.
In the manufacture of carburetters, difficulty is experienced, on occasion, because of complaints that the petrol filter becomes corroded by the spirit, or is rusted by, water, or the chemicals sometimes occurring in extremely small proportions in commercial petrol. To overcome this trouble, wire screens of a high-nickel alloy are being used.
Alloy cast-iron is being applied to the production of the wheels of commercial vehicles of the lighter kind, such as trolleys for transporting materials from point to point in a works or factory. These wheels run from 4 ins. to 6 ins. diameter, and weigh from 21 lb. to 6 lb. each. The main requirements are strength and good wearing properties. It has been found that these are best given by a material with the following approximate analysis:-2.8-3.2 per cent. carbon, 0.6-0.8 per cent. manganese, 1.25-1.75 per cent, nickel, 0.3-0.5 per cent. molybdenum. Silicon is an essential constituent in an iron for this purpose: unless the section be less than /-in. thick, it should run from 1.8 to 2.4 per cent. If, however, the section exceed / in. but be less than 2 ins., it should run from 1.5-2.0 per cent.
New Aluminium Alloys.
One of two aluminium alloys, for use in the manufacture and construction of commercial vehicles, may be mentioned. The first is of medium strength, and contains 4-5 per cent. copper, together with small additions of manganese and silicon. Hardness is given by a heat-treatment followed by quenching, and the alloy is, usually, obtained in sheet form. It possesses a tensile strength of approximately 30 tons per sq. in and an elongation of 15 per cent. in 2 ins. Another valuable metal comprises magnesium and silicon in suitable proportions. It is easily fashioned into the desired forms, both in the annealed and the hot worked states, and can later be hardened by an easy heattreatment designed to produce a tensile strength of about 28 tons per sq. in., with an elongation of 9 per cent. in 2 ins.
Another alloy has been found valuable for forged pistons and cylinder heads, This contains 4 per cent. copper, 2 per cent. nickel, and 1.6 per cent. magnesium. It has, at normal temperatures, a tensile strength of about 32 tons per sq. in., rising to 35 tons, with 18 to 25 per cent. elongation.