Arc Lights on Metallurgical Progress
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Stainless-clad Steel Now Being Welded, New Method of Sheet Making, Better Sparking-plug Electrode Materials, Avoiding Wear in Piston-ring Grooves
A BIRD'S-EYE view of metallurP-1 gical progress during the past few months shows further advance of direct interest to the commercial-motor industry. The production in this country of stainless-clad steel has been referred to previously. It is of interest to note that a good deal of work has been done on the welding of this valuable new material.
Thin hi-metallic sheets composed of rustless steel about 3 mm. thick, fastened to a cladding of ordinary steel and making up a stainless-clad sheet about lf mm. thick, have been subjected to arc-welding. The edges of the sheet were bent in diverse directions prior to the.welding operation. After a large number of experiments, it was found that stainless-clad material could be efficiently arc-welded, and that the same welding electrodes as are used for welding the austenitic nickel-chromium stainless steels proper could be employed.
Edge Form.
With regard to the form given to the edges, results showed that the best was that in which the sheet is bent back on itself, with the weld carried out on the stainless side. The experiments have greatly increased the commercial applicability of the new materials. • The United States have introduced a novel sheet-making method called galvannealing. This resembles the ordinary galvanizing process as applied to sheets, until the stage is reached at which the sheet is removed from the zinc pot.
In the normal galvanizing operation, the sheet is cooled down, but in the new process it is introduced into a furnace for heat-treatment. Here it is submitted to a temperature of 650 degrees C. or even higher, varying with the sheet thickness, the weight of the coating, the speed of transit, and the • length of the furnace itself. The sheet is then air-cooled.
The result is claimed to be the entire freedom of the sheet from " spangles " and the achievement of a silver-grey mat surface. The coating acquires a greater degree of malleability, particularly where it is normally heavy and brittle, and the structure of the coating itself is to some extent of greater homogeneity.
America has also produced a new chromium-molybdenum case-hardening steel. It is said to have qualities resembling those of the costly casehardening steels containing alloys, but to be much lower in price than these. Its working properties are good.
It is possible to quench it straight from the case-hardening box, without any serious distortion of the part. This means that extremely complex components need only a minimum of final grinding before being fit for use. The case is of great hardness and offers r high degree of resistance to wear. The case depth is uniform, and both case and core show satisfactory mechanical characteristics, whilst the matefial is readily machinable. It has been employed and found entirely suitable for such parts as heavy-duty gear pinions, worms and pins.
Best for Electrodes.
An important inquiry has just been concluded into the best type of material for sparking-plug electrodes. As a result of the examination and test of 18 different materials, it was ascertained that pure nickel was generally the most suitable metal available at the moment.
Its erosion under severe conditions is extremely low and it requires only a low voltage. It is, moreover, adaptable for the manufacture of adjustable sparking-plug electrodes, in the form of inserts in a steel centre and body. Nickel-base alloys have also been found to give quite good results, although not quite so good as those afforded by the pure metal,
One of the points which the designer has to hear in mind in connection with B31 oil engines is that their combustion chambers develop high pressures and temperatures. This means that, as a rule, excessive wear occurs in the grooves of piston rings, especially if the piston be composed of an aluminium alloy.
Manufacturers have for some time been endeavouring to overcome these difficulties, with the result that a type of piston has been introduced with the ring grooves machined in an insert of austenitic nickel-alloy cast iron. The method of fabrication adopted is to make the groove or insert first, and cast the piston around it afterwards. Nickel iron is used for the reason that its expansion coefficient resembles closely that of numerous alloys of aluminium, and it will remain tightly in position after casting.
Monel Metal for Tubes.
A development of potential importance is the employment of monel metal, a nickel-alloy material, for flexible metallic tubing, which is being used for brake tubes and other purposes. Whilst, in the normal way, monel metal would hardly be employed in ordinary automobile construction for flexible tubing, it is extremely useful wherever the conditions are such that severe corrosion is liable to occur. The monel tubing can be made in sizes from 24 ins, down to in. diameter.
Great attention is being paid to the reductiom in weight of petrol tank wagons. This has been achieved, to some extent, by the employment of a
high-tensile nickel-copper steel for the tank body. This method enables the thickness of section to be reduced to well below the point which would represent safety in an ordinary steel plate.
Furthermore, the nickel-copper steel has the additional advantage that it resists corrosion from the petrol carried and from the atmosphere, which makes for greater purity in the spirit.
A new steel-trekting process has aroused great interest and is known as " austempering." It consists of beating the steel to a definite temperature above the structural change points, in order to make it what is called " austenitic " in structure. The metal is then plunged into a hot-quenching bath maintained at a pre-established constant temperature below the structural change paints. The treatment demands thin sections, but it is claimed to result in a great improvement in ductility.
So many cast irons are employed, to-day, in commercial-motor construc, tion that a new discovery demands notice. It has been found that the addition of amorphous graphite to grey cast iron in the molten state reduces " chill," decreases hardness and, therefore, increases the iron's machinability.
In order to reap the full advantage from this process, the graphite is added to the iron emerging from the spout, when it is at a temperature of about 1,455-1,510 degrees C. Time has to be given for an adequate reaction between graphite and metal, but usually a couple of minutes suffices. Generally, the graphite is added in amounts ranging from 2-4 lb. per ton of metal.
New Alloy for Crankshafts.
A new material of alloy type for crankshafts has been introduced. It comprises a nitriding steel containing 0.25-0.35 per cent. carbon, 0.35 per cent. silicon, 1.0 per cent. manganese, 1.0 per cent. nickel, 0.5-1.5 per cent. chromium, and 0.9-1.5 per cent. molybdenum.
A further steel for crankshaft manufacture contains 0.25-0.35 per cent. carbon, 3.0-3.75 per cent, nickel, and 0.5-1,5 per cent. chromium. Nickelalloy iron has also been employed for the same purpose, and has an analysis of 3.1-3.4 per cent. carbon, 2.1-2.4 per cent. silicon, 0.5-0.75 per cent. manganese, 0.75-1.0 per cent, chromium, 0.2-0.4 per cent, nickel, and 0.4-0.6 per cent. molybdenum. Piston rings have also been made from this alloy iron. .