What the Metallurgist Has Done
Page 41
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for the Motor Industry
THERE are few metallic parts of commercial vehicles which have not been heat-treated or finished in one way or another. Furthermore, heat-treating, in the modern sense, implies the manipulation of the surface of a metal to transform it into a desired condition, as well as changes in the structural and physical characteristics of a material, as a result of heat. Metallurgy has, of recent months, brought nearly to perfection a number of heat-treating and finishing processes, which are summarized here.
Bright annealing is among the most important and interesting of theseprocesses. It has been developed to, satisfy the requirements of industry for metals with smooth surfaces, free from stain and discoloration, dry, and bright in appearance. Low carbon steel, mild steel, copper, brass, stainless steel and high carbon steel can all be dealt with by the brightannealing method.
Gases of great purity are used in conjunction with a purification plant in a closed atmospheric cycle. The gas has to be extremely accurately controlled, and the state of the surface of the material when placed in the furnace is exceptionally important.
For example, when the bars have been rolled, they often carry with them a coating of oil. By changing the character of the oil, used during rolling, it has been found possible greatly to facilitate the bright-annealing operation and to produce mirror-bright steel. Stainless steel has, so far, been bright-annealed only experimentally in the laboratory, but there is reason to believe that, before long, it will be possible to deal with it on commercial lines.
Annealing High Carbon Steels.
In modem bright annealing of high carbon steels, the present practice is to employ hydrogen as the gaseous medium, or dissociated ammonia, which contains scarcely a trace of water vapour. In bright annealing copper, there was a tendency for the hydrogen to make the metal brittle, but this handicap has been overcome by using a special type of copper entirely without oxygen in its composition.
Gas carburizing has evolved considerably. Continuous gas carburizers have already been employed in the United States for parts such as steering sectors and worms, free-wheel driving shafts and camshafts. In these appliances, the parts to be carburized are set out on trays and fed gradually through, a miffile, which has air locks at each end.
The gas for carburizing comes in with the cool parts, and they advance together through belts or regions of increasing heat.When the gas enters the furnace, it decomposes, to some extent, into its component elements, and, as a reult, a film of carbon forms on the components. • When both the parts and the carburizing gas have progressed to a belt at the temperature of approximately 000 degrees C., the carbon dioxide serves as an efficient carrier of this carbon, which quickly penetrates the skin of the steel. The parts, when carburized, are entirely free from oxidation and have a bluish-black colour.
Interrupted Hardening.
Graded or interrtipted hardening is a new heat-treatment process applied to certain steels for the motor industry, e.g., nickel chromium steel. By this process, steel is cooled fairly quickly to certain regions of temperature known as sub-critical (a technical phrase connected with the metallography of steel), maintained at these temperatures for various periods and then cooled further or reheated.
The graded heat treatment is varied according to the type of structure, that it is desired to produce in the material. One can anneal, quench, or temper by this method. It is claimed to produce uniform hardness or structure throughout the steel, and is specially adapted for use in connection with low alloy steels for various purposes. It can also be applied to high-speed tool steel, but is not to be recommended for plain carbon tool steels, if a high degree of hardness be required.
Hot quenching is another new method of heat treatment developed recently. In this process, the steel, when heated to the required temperature, is immersed quickly in a quenching bath at tempering temperatures. This treatment has the advantage of economy, being less costly than the method of quenching followed by reheating. Its one disadvantage is that hardness and ductility are a trifle lower than with the ordinary method, where tensile strengths are equal.
It should be made clear that these last two treatments are both, as yet, in the embryonic stage, and have not been applied on any considerable scale
to commercial production. They show, however, the trend of metallurgical development, and it will pay cornmercial-motor manufacturers to watch progress in these directions, and, perhaps, experiment for themselves on these or similar lines.
Removing the oxidation scale from small forgings is the subject of an interesting advance. In a drum which can be rotated is a number of starshaped stainless-steel objects. The forgings are fed into this drum, which is then revolved, and the tumbling action rids them of scale.
By reversing the drum, the parts are automatically ejected, the stainlesssteel stars being, at the samej time, separated and returned to the cone surrounding the screen. A cleaning process of this kind can be carried out dry, or with the use of a slurry of sand and water. In a modification of the process, diluted sulphuric acid is employed, and the parts are tumbled in this, along with the stars.
Heat-treating in a gaseous atmosphere on continuous lines is also being rapidly developed, and, by means of a specially designed furnace, can be applied to many purposes. Any required atmosphere can be maintained in the muffle. The reciprocating hearth is sometimes flat and sometimes channelled to the work. It is itself the conveyer, being within the furnace throughout and in consequence subjected to only one heating.
When it is desired that the furnace should work continuously, the forward movement of the hearth is halted quickly, and the parts progress jerk by jerk through the furnace by their own momentum. When quenching is completed, the components are discharged through a chute, sealed in the quenching liquid.
Heat-treatment of Bolts.
Bolts are being bright heat-treated in an interesting manner, where the requirements as to dimensions and mechanical properties are severe. They are heat-treated to the requisite temperature in an atmosphere which minimizes or completely eliminates oxidation scale, quenched by way of a submerged outlet. into oil, and tempered in a corresponding atmosphere.
The upshot of this process is a high degree of accuracy in the size of the bolts (to 1/1,000 in.) and complete constancy of shape. Carbon is a trifle lower, perhaps, but this disadvantage can be mitigated by quick work (holding at the required heat for a comparatively brief period) on mild or medium carbon contents. Variation in the atmosphere will give a slightly blued finish with a little better resistance to corrosion (a point of sales value).