Developments in
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Surface Hardening
How Efforts are Being Made to Increase the Strength and Hardness of Standard Metals to Offset the Costliness of Some of the New Steels and Alloys
WHILST new steels and alloys conVV tinue to be introduced, many of them are costly from the point of view of the commercial-vehicle manufacturer or repairer. A good deal of genuine importance attaches, therefore, to methods of increasing the strength and hardness, at least superficially, of less-costly standard materials.
If a standard moderately priced material can be given a surface which is more resistant to corrosion, erosion, and wear than the same steel untreated, much of value has been accomplished. Much progress has actually been made in this direction. Ferrous alloys have been cemented with costly alloys of the zirconium type. Surface cementation of cast-iron and steel have also been carried out by means of beryllium. Cementation in this sense means, of course, surface or skin-hardening.
A further development is the Follsain process. In this, an alumino-thermic reaction provides asurface finish of aluminium and chromium character, chromic oxide being carried in the mixture. As a rule, a cementation process is a long-drawn-out affair, but efforts to speed it up have been very successful, as in the process described
above. In another .process, metal is sprayed by compressed air on to whitehot material,-and this again is a speedy and adaptable method.
Metallic Cementation Progress.
Metallic cementation with the aid of powdered aluminium has been carried out by the Japanese, commercial aluminium powder being cemented into iron, copper and nickel at temperatures within 600 and 1,200 deg. C. Aluminium diffuses into these materials at a temperature over 650 deg. C. —at which temperature aluminium melts—and the speed of absorption becomes greater as the temperature increases.
A new case-hardening process has been developed to increase resistance to oxidation, abrasion and corrosion. It can be applied to certain cast irons, malleable iron, rolled, cast and forged steel, and to the majority of ferrous alloys in which no free copper appears. The process has been applied to brake. drums, engine valves, crankshafts, etc. Few details have been published concerning this new Allen process, but it is said to be superior to standard casehardening, or nitriding.
A new case-hardening steel of American origin is worthy of mention.
It is a rapid machining, open-hearth steel, which case-hardens in a third less time than the normal case-hardening steels. After case-hardening, the steel does not distort or bend, has a tough core, an extremely hard case, and no
brittleness. To produce a case in. deep takes approximately 61 hours as against 8 to 10 hours:
n42 The case itself has a Rockwell hardness of 66C, and its grain structure is of a highly refined type. This material is a 0.2 carbon steel, high in sulphur, with a percentage of manganese in its
composition. Its tensilestrength in the hot-rolled condition is 32 tons per sq. inch. In the cold-drawn state its
tensile is 38 tons. Machinability is greater than in standard steels, and it is claimed that surface speeds of 200 ft. a minute can readily be achieved. Small gears have proved a highly advantageous application for this new steel.
Advance in Case Carbonizing.
The Ca.rbonal case-hardening process is another interesting development. In this, the part to be case-carbonized is first heated thoroughly in a retort. Oil is then allowed to enter the retort in such a way that it impinges on the heated walls; this is done by means of a swiftly rotating electric fan. Volatilization of the oil takes place, and the gas thus formed constitutes a highly efficient case-hardening medium. Uniformity is obtained by means of close control of the heat in the retort and the volume of oil admitted.
The retort is heated up in an electric vertical carburizer. Technically described, this is a standard-type pit furnace having heating elements of nickel-chromium alloy, of the round rod, return-bend design. The retort is gas-tight, and the parts to be casehardened are contained within it in a perforated basket, through which the oil-gas enters. The oil used is a vegetable compound, carefully blended.
A new surface-hardening process, competitive with nitrIding, has been devised, and is known as the sodiumcyanide case-hardening process. In this, a number of basic salts is fused with sodium-cyanide at the time of manufacture. These all contribute towards the setting up of certain chemical reactions when the mixture is used for case-hardening. The molten bath is full of tiny gas bubbles.
When the part to be dealt with is plunged into the bath, it is enveloped in a thin film of carbon-monoxide, con taining also a little nitrogen. This is
the gas that serves to carburize the steel. By raising the temperature of
the bath the gas-film can be thickened.
but this will have no effect on the speed of case-hardening. What accelerates
the case-hardening when f-c temperature is raised is the greater velocity of the gas molecules.
In Germany some valuable research work has been carried out in connection with the successful nitriding of cast iron. Soviet Russia has also succeeded in reducing the time required for nitriding by a modification of the standard practice in the direction of a three-stage heat, a low temperature being followed by a high temperature and then a low temperature again.
Chrome-vanadium alloy steel has always been of great importance in the automobile industry. Experim.ents have proved that a steel of this type can be given a surface hardness equivalent to that of the case-hardened steels by means of a nitriding treatment. Highly alloyed tool steels and– highspeed steels have also been shown to respond to nitriding, while the surfacehardening of austenitic nickel-chromium-iron alloys, into which aluminium also enters, has been found quite practicable.
Increasing Core Toughness. .
Atter. ts have also been made to increase the core toughness of the chrome-molybdenum steels containing aluminium after nitriding. This is done by adding a small nickel content, or, alternatively, by raising the percentage of molybdenum, adding tungsten and vanadium, and slightly increasing the carbon percentage.
Hitherto, the presence of aluminium in steels for nitriding has been curtsidered indispensable, but later progress suggests that this opinion has been a trifle hasty. A range of steels containing chromium, molybdenum, and vanadium, but entirely without aluminium, has been found to respond more effectively to nitriding than the standard aluminium-chromium-molybdenum nitriding steels, so far, at all events, as depth of hardened case is concerned.
It has also been discovered that these steels have a more ductile surface skin, are tough, and maintain their hardness and tensile strength extremely well. A typical analysis is 2.65 per cent. chromium, 0.14 per cent. carbon, 0.44 per cent. manganese, 0.24 per cent. silicon, 0.56 per cent, molybdenum, and 0.27 per cent. vanadium. Steels of this type have a close grain structure, both in the case and in the core.