Research Produces New Metals
Page 49
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CONTINUOUS research work in the %/metallurgical industries is bearing fruit in the development of new materials and processes of special importance and interest to the commercial-motor world, For•example, one of the most recent developments is a new alloy designed for valve inserts.
This comprises in its composition the elements silicon, chrome and molybdenum, and is suitable for every kind of internal-combustion engine. Its • coefficient of expansion under heat is identical with that of cast iron, and it will maintain its hardness at all engine temperatures. .
Furthermore, machining of the alloy presents no difficulties. The actual Brinell number of the material is 600. The insert is pressed into 'the cylinder block, the metal of which is subse qUently spun over. Locking contrivances; such as loCking pins, are not required.
Again, a special material for pistons has been placed on the market, its origin being the United States. A characteristic analysis of this alloy is as per cent, carbon,
per cent manganese, 1.1 per cent. silicon, 0.08 maximum sulphur, 0.1 maximum phosphorus, 2/ per cent. copper, 0.18 'per cent, chromium, the remainder being iron.
easily Machined and Cast.
The advantages of this alloy are that it can be machined without difficulty, is easily cast to the required form and offers great resistance to wear. Furthermore, it has a perfectly satisfactory coefficient of expansion.
In general, it can be said to combine the advantages of cast iron and aluminium pistons. As it possesses a good degree of mechanical strength, the designer is enabled to use a piston with thinner walls.
The shortage of raw materials at present existing has led to an intensive concentration on means for salvaging worn parts. A process which is steadily gaining ground is electroforming, which consists of building up worn or undersized parts by means of heavy electro deposits of nickel.
The principal features of the method are the firm adherence of the nickel to the steel, a strong union being created equal to the strength of the materials. The process itself is carried out at a relatively low temperature (25-27 degrees C.) and, therefore, it is not likely that any stress will be created leading to the distortion of the' part concerned.
The nickel plating can be made so bard that it is not necessary to give any late( heat-treatment. Parts electroformed on these lines can be kept bright and clean, even when brought into contact with various corrosive bodies, whilst their wear resistance is enhanced. The coefficients of expansion of the steel and the nickel are sufficiently near to one another for there to be scarcely any danger of peeling of the plating, even at temperatures so high as 480 degrees C.
Carbon and alloy steels can both be electroformed, as can the rustless steels. The plating deposited is usually. a few thousandths of an inch thick, but, when necessary, fin, thickness of nickel can be deposited.
High-duty Nickel Cast Iron.
Some valuable work is being done with a high-duty nickel cast iron. This has great strength, uniform char
acteristics and adaptability. It has been used to replace steel, or in the reduction of sections, thereby saving Weight. Characteristic uses are for clutch plates in multiple-disc hydraulic clutches, oil-engine liners, machine-cut gearwheels, cylinders, heads, pistons, frames, cams, couplings, valve bodies and the like.
Typical analyses are 3.2 per cent. carbon, 2.25 per cent. silicon, 0.75 per cent, nickel; or 3.0 per cent. carbon, 1.65 per cent. silicon, 1.5 per cent. nickel, 0.3 per cent, chromium. The first analysis is suitable for clutch plates, the second for oil-engine liners.
A process of considerable importance is differential hardening by induction, specifically designed for the surface hardening of crankshaft journals. Known as the Tocco process, the method takes advantage of the fact that a high-frequency current, under certain conditions, sets up eddy currents in a steel, and generates heat in the steel itself. This factor was made use of in the manufacture of highgrade tool steel, which is nowadays melted by the eddy currents generated in this way.
Hardening by Induction.
Without going into highly technical details, it can be said that the circulating current induced in the surface of the journal to be heated generates the heat which hardens the surface to be treated. After hardening, the entire shaft is tempered at a low temperature in order to remove all stresses, and is given a finish grinding to remove the 0.020-0.030 in. left on the diameter before hardening.
The result is a crankshaft of maximum ductility and high bearing hardness, resistant to wear and abrasion, and able to withstand rigorous usage
One of the most important items o: research work has been the productior of a better type of cold-rolled shee1 for automobile work, not susceptible tc deteriaration due to "ageing." Whils1 it is not claimed that perfection has yet been attained, a cold-rolled stee' is on the market which can be deepstamped, is guaranteed not to " age," has all stretcher strain eradicated permanently and, supplied in the tempered condition, will maintain all the characteristics of a temper-rolled steel.
Thus, it is not essential to give it a heat-treatment in advance of manufacture, however long a period the steel may have been kept in stock. Whereas normal mild steels show a sharp yield point when in the temper-rolled state, this new steel is claimed to withstand the cracking encountered in certain arduous drawing processes, and to withstand it indebaitely, i.e., its resistance is not weakened by the steel's having been kept in stock a long time.
For the guard rails, grab rails and grab handles of buses, a nickel-copper allay containing 30 per cent, nickel and 70 per cent, copper has been produced. This material withstands atmospheric corrosion and tarnish, and possesses an attractive silver-white appearance.
New Rustless Steel.
Some interesting advances have been made in rustless steels. Among new metals of this type recently introduced is one with an approximate analysis as follows :-8-10 per cent. chromium. 0.50 maximum manganese, 1.25-1.75 per cent. molybdenum, 0.50 maximum per cent. silicon, 0.15 maximum per cent. carbon.'
This material is of the intermediate type, and is specially suitable for resistance to rather more severe corrosion than is encountered under ordinary atmospheric conditions. It also has a degree of heat resistance and relatively good mechanical properties.
Another development which shows great promise, although British metallurgists are somewhat sceptical as to its general adaptability, is the substitution of manganese for nickel in the rustless steels. It is said that quite satisfactory results have been obtained from a steel of this kind containing 18 per cent. chromium, 8 per cent, manganese, 1 per cent. copper and 0.1 per cent. carbon.
Lower in price than the ordinary IS / 8 stainless steel, it is easier to roll and draw, has excellent mechanical properties and is not difficult to machine, whilst it is more readily welded than the usual 18/8 steels.