Metallurgical Research Brings Striking Results
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Recent Developments That Have an Important Bearing on Commercial-vehicle Construction. fit el d in g Cast Iron; Heat-treating Tubes; Graphitic Steel Introduced
EVERY week that passes brings the announcement of some new discovery of economical value in the metallurgical world. Even confining attention to those developments of commercial-motor interest, it is almost impossible to keep pace with discovery. For example, not only are Great Britain,. the United States, and -Germany each adding their quota of invention and research, but now, Italy, Russia, and even Japan and Poland, are themselves contributing to metallurgical progress and knowledge.
Russia, one of the original discoverers of stainless steels in 'which nickel is replaced by manganese, has been examining these new rust-resisting metals more closely, and has proved that they have an additional advantage in the form of a diminution of " creep." It is quite on the cards that these new steels will prove suitable for use at high temperatures, where a fair amount .of mechanical strength is desirable.
Welding Cast Iron.
In this country, investigation has developed a welding process for cast iron, by means of which welds of this material can he machined at a reasonable cost. The heavy charge for grinding, or the complete loss caused by having to reject and scrap a damaged • casting, is thereby eliminated.
The process requires the employment of electrodes composed of monel metal for the final two deposits. Technique of the process has to be carefully studied, but in all other respects the method has no problems.
In the United States, the commercial heat treatment of seamless tubes has been effectively carried out. Originally, it was possible to raise the physical properties of this tubing only by introducing into its composition small amounts of such alloys as manganese. The only other improving procedure was control of the finishing temperature in advance of cooling by air.
Heat Treatment for Tubes.
The new advance consists of continuously normalizing the tubes in lengths of 40 ft. in a furnace fired with natural gas. The burher orifices are adjusted in such a' way as to produce tubing, the exterior of which is almost entirely free from oxide films.
New, also, is graphitic steel, which is likely to prove of extreme importance to the commercial-motor industry. It possesses some of the good points of steel and some of those of cast iron. Containing approximately 1.5 per cent. of carbon, it can be hardened, tempered, worked hot and welded. Cold working is also feasible.
Graphitization of the carbon is controlled by the amount of silicon in the n38 composition of the metal and by the heat-treatment given to it. Machining can be carried out without difficulty, and the wear-resisting properties are equivalent to those of grey cast iron.
Principally, the material is suitable for such parts as cylinder liners, sleeves, bushings, pistons and guides, piston rings and brake drums.. It can also be used for dies, of which the period of service, it is claimed, is raised by double to 10 times.
A slightly different brand of this metal has in its analysis a small molybdenum percentage, and is thereby made suitable for oil hardening, whilst its physical strength is a trifle higher. This particular variety is employed solely for punches and dies, and has proved extremely successful.
In the foundry industry of this country, some valuable work has been done on the shrink-fitting of austenitic nickel-cast-iron cylinder liners. Liquid oxygen used as the cooling medium proved too drastic, and was abandoned in favour of a milder cooling medium, namely, solid carbon dioxide placed in a bath of white spirit.
Liners thus treated soon recovered their normal size and were still in the completely austenitic state. They dropped for a third of their length into the bores before binding, and light pressure from a hand press was all that was necessary to drive them home. A full account of these experiments will be found in the Foundry Trade Journal dated July 1, 1937.
Surface Hardening.
In the manufacture of nickel-alloy.teel crankshafts, local surface hardening of nickel-chromium-molybdenum steel is being employed as a commercial
proposition. For high production work, a heat-treated nickel-chromiummolybdenum steel is also being employed for dies, and a die life in the final stampings of 4,000 crankshafts per sink is claimed for this steel.
For oil-engine gear wheels, the Americans are using with succeSs a casehardened nickel-chromium sled. They are also utilizing chromium-molybdenum case-hardening steels in the construction of hypoid gears, such steels having been found to have a low degree of distortion during the treatment process.
Another improvement that America has begun to introduce is the employment of nickel case-hardening steel for tyre chains. In practical experiments it was conclusively shown that for lorries and buses for heavy duty, nickelsteel chains were likely to last twice as long as ordinary steel products.
The tests were carried out in winter', and it was found that the increased length of life was due not so much to the greater wear-resistance of nickel steel, but to the fact that, being tougher and more resistant to shock, it could wear down to a much greater thinness before a stage was reached where fracture took place.
From America comes, also, the use of a silicon-vanadium steel for helical springs. The increasing loads and speeds of modern times call for a material of much greater elastic limit for these springs, so as to allow the employment of superior maximum fibre stress and stress range, with an increase in deflection and resistance to settling or failure.
Strong and Tough.
The new material has the desired high strength, together with extreme toughness. Heat-treatment does not cause it to incur a fibrous state or an inclination to radial cracking on quenching. The analysis of the material includes 0.88-0.98 per cent. carbon, 0.415-0.65 per cent. manganese, 0.5-0.75 per cent, silicon and a minimum of 0.15 per cent. vanadium.
Manganese-vanadium steel for forged parts of motor vehicles, etc. is another American innovation. This material has high shock and fatigue-resisting properties, and is not sensitive to heat checking of bearing surfaces.
It is as easy to forge as ordinary carbon steel, is free scaling, free from interior and exterior blemishes, and needs only simple heat -heath-tent, a normalizing succeeded by tempering. It machines readily, and its analysis is approximately 0.22-0.30 per cent. carbon, 1.45-1.75 per cent, manganese and minima of 0.15 per cent, of both silicon and vanadium.
For Many Purposes.
The steel can be used for main rods, driving and other axles, crank pins, connecting-rod straps, crank-axle pins, piston rods, eccentric rods, knuckle pins and many other parts. The steel is also suitable for 'castings, but for these purposes the carbon and silicon contents are slightly raised, the vanadium content being from 0.08-0.12 per cent.
Another steel for casting uses, such as brake drums, levers, etc., is nickelvanadium, which is readily machinable and can easily be welded. This material has an analysis of approximately 0.25-0.35 per cent. carbon, 0.85-1.10 per cent, manganese, 0.250.50 per cent. silicon, 1.25-1.75 per cent. nickel and 0.08-0.12 per cent. vanadium.
As has been shown, metallurgical progress continues at an amazing rate and shows at the moment no signs of decrease.