Metallurgical Progress
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influencing
Commercial-motor Production
SURVEYING current metallurgical progress discloses many features of general interest to the commercialmotor industry. Magnesium die castings, for example, are beginning to compete with those in aluminium. The reasons are to be found in their lightness and the readiness with which they can be machined. Furthermore, they possess satisfactory physical properties, and it is claimed that the change to magnesium castings enables a saving equal to the cost of the dies to he made, whilst superior protective films and finishes for magnesium are also reasons for its adoption. Typical uses for these castings are in portable tools, hand tools, body-finishing equipment and similar parts where lightness
is desirable.
Powder metallurgy is, perhaps, the most striking cleVelopment of recent years, and its application is constantly increasing. It is being employed, for example, in the 'production of a new type of bearing of the copper-lead type, which is manufactured by powdermetallurgy processes.
Reinforcement for Bearings
Another new bearing comprises a powdered metal sintered on to a steel back, to serve as a mechanical reinforcement to a relatively thin layer of babbitt metal. It is stated that trimetallic bearings of this type resist fatigue to a greater extent than the ordinary babbitt-metal bearing.
Another development in bearing's is the introduction of the element tellurium into copper alloys, in order to produce a bearing metal that can be readily machined, forged and hardened.
An interesting advance during the past 12 months is the successful brazing of alumimnm alloys. By means of newly discovered fluxes and aluminiumbase 14azing materials of low melting point, it has been found possible to achieve this important development, which is bound to increase the applicability of aluminium alloys and to lower the cost of their jointing to other metals.
The case-hardening of steels in an atmosphere that will not produce scale has long been a dream of the steel manufacturer and, equally, of the steel user. This dream has been brought near to reality by progress in casehardening and surface hardening methods, as well as in the design of furnaces for the closer control of atmosphere and temperature.
A recent invention is that in whiih steel is treated in an atmosphere consisting of partly burned fuel gas processed to almost pure nitrogen (99.8 NO. The cost of producing an atmosphere of this kind is high, so that use of the method is limited to those parts that warrant it.
Induction hardening, in which local hardening is carried out by the induction heating of the steel part, has been increasingly used in the commercialmotor industry during the past 12
months. Bearing surfaces of crankshafts, the local hardening and strengthening of axle shafts, push-rod seats for tappets, camshafts, rotor :shafts, studs, as well as tools, such as pliers, drills, taps and dies, have all seen the application of induction hardening.
The crankshaft bearing surfaces are hardened up to the fillets. The axle shafts have an annular area hardened so that the shaft can act as the race for the bearing. The push-rod seats are hardened on the surface, and the other parts mentioned are all given a hardness treatment confined to a specific, defined surface area, the degree and depth of hardness being controlled to requirements.
Additional applications of induction hardening include tempering a small section of an already hardened part to a state in which it cab easily be machined. Then there is surface hardening a steel part while, at the same time, copper brazing it to another.
New Metal-protection Discovery Another sensational discovery is the protection of metals from corrosion by silicon impregnation. This comprises placing the part to be protected in a furnace of the rotary case-hardening type, where it is in contact with fine amorphous silicon carbide. When the parts and carbide attain approximately 980 degrees C., a little chlorine is admitted into the chamber and the reaction is allowed to proceed for from 30 minutes to three flours, according to the depth of case desired. There results from this a case of from 0.01-0,05 in. thick, which contains about 14 per cent, of silicon.
The silicon alloy preserves the under
lying basis metal from corrosion, but is excellent in physical properties, is firmly attached to the core, and will neither split nor splinter if properly designed, i.e., without sharp corners. The cost of the process, from the commercial point of view, is about the same as that of ordinary case-hardening. Thousa.nds of automobile parts have been d-ait with in this way and have been substituted for costly alloysteel parts.
One of the most valuable mechanical inventions is" superfinishing." This is a finishing process which takes away surface flaws caused by previous methods of metal finishing, while eliminating tool marks to a degree of fineness measured in millionths of an inch. Brake drums, bearings, etc., have been dealt with by this method, which provides surfaces of such smoothness that the lubricating. oil film is never sheared by angularities. As a result, life is increased and noise lessened, Another advance is the application of shot blasting to such parts as clutch discs and coil springs in order to raise their endurance limit. These parts have to withstand severe stresses, and their peppering with a blast of steel shot not only eliminates surface excoriations and grooves liable to promote fatigue, but also compresses the surface of the metal and, thereby, enables it to offer a higher resistance.
Shot Blasting Reduces. Breakages
It was also found that by shot blasting the area of highest torsional stress in rear-axle shafts intended for heavy duty, breakages could be considerably cut down.
In tool steels, progress has not been outstanding Perhaps the most interesting development has been the introduction of a high-speed steel containing 8 per cent, molybdenum, without tungsten, but including approximately 4 per cent. vanadium and 1.25 per cent. carbon. This steel is said to show in use a remarkable ability to maintain its cutting edge.
A second new high-speed steel contains equal amounts of tungsten and molybdenum in the region of 4-6 per cent., together with the normal percentages of chromium and vanadium.
Tungsten carbide and other carbide tools have not advanced to any marked degree. Chiefly, research and experiment are concentrating on improved grades designed for the machining of, steel, a problem not yet satisfactorily solved. Present practice seems to turn towards adding percentages of titanium carbide to the basis tungsten carbide and cobalt analyses. Tantalum carbide has also been similarly intriduced. It seems definitely established that these additions improve the ability of the tools to machine steels without snipping at the tool nose.
Pre-cracked dies are another novelty. These are dies made in sections and parted at the line of greatest stress or weakness, then built up into a rigid block. This enables the die-maker to employ hot work steels with greater resistance to heat embrittlement and abrasion than could he obtained from a solid die, whilst the dies themselves are harder than has ever previously been regarded as feasible.
The electric heating of dies to prevent their growing cold between shifts, thus cutting out the heat-checking so often causing fracture and failure, is another development in technique worthy of note. In the steels for dies themselves, current practice appears to be to use 9 per cent, tungsten steel for hot work, for such tools as punches, forging dies, etc. For gripper dies on automatics. chrome vanadium low molybdenum steel is being used on account of its toughness am', resistance.
Progress in Other Directions A few additional developments may be mentioned. One is the adoption of steering wheels with stainless-steel spokes. Another is the introduction of 0.12-0.25 per cent. of lead into the composition of stainless steels of the austenitic nickel chromium 18/8 variety. This is designed to increase macbineability, and the lead addition has no harmful effect on the ability of the steel to withstand corrosive influences.
A third is the invention of a new type of stainless-steel covering for food-carrying lorries with sanitary linings. This consists of thin sheets of stainless 18/8 steel (0.0095 in. thick) cemented to a mineral backing material made up of asbestos, volcanic clay, calcined magnesite, and cement. It is proof against fire and the weather, and can be obtained in thicknesses ranging from in. to I in. It can be cut and worked with ordinary hand tools. In the thinner sizes it is flexible enough to he easily bent, whilst in the thicker sections it is rigid and capable of carrying a substantial load.
Finally, there is flame-hardening, by which such parts as valve stems, piston rods, cams, crankshaft bearings, tractor rollers, link pins and the like are surface hardened by the local application of the oxy-acetylene flame.