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Developments in Steels
Further News of Interesting Progress in Steels of Various Classes. New Advantages Provided by 'Research
APREVIOUS article published in this journal (April 14 last), summarizing progress in steels, has conveyed a wrong impression if it left readers believing that the changes it described completed the story of steel progress. Actually, one brief article could by no means sum up the amazing developments that have taken and are taking place. The present notes give some further important details.
Perhaps the most novel and interesting advance is the making of steel crankshafts and connecting rods as castings instead of forgings. This revolutionary change naturally met with opposition in some quarters, opposition that has probably even now not quite ceased. Yet it is claimed for this method of manufacturing a vital part that an improved component results.
The essence of the problem is the employment of alloy steels following closely a predetermined analysis, and closeness of the steel to specification is absolutely necessary. The advantage of cast-steel crankshafts, as against forged crankshafts, etc., is the possibility that they afford of their being redesigned in such a way as to give greater strength exactly where weakness generally develops. The weight can also be reduced. ,
Cast-steel Crankshafts.
This redesigning could not be carried out with the old practice of forging in dies. For cast-steel crankshafts, the steel used must have a nickel content of 0.7 to 0.75 per cent., manganese from 1.6 to 1.7 per cent. and carbon about 0.035 per cent. Triple heat-treatment is usually employed in order to ensure superior mechanical properties.
Another striking development of importance to manufacturers of commercial vehicles is the introduction and satisfactory working of a method of making light, intricate steel castings in what might almost be called mass-production quantities. The method itself is a foundry secret that cannot be disclosed, but it has numerous advantages.
In the first place, it enables the steel founder to make do with a minimum machining allowance, which means that the weight of the castings themselves can be diminished. Moreover, there is less machining to be done.
Then, again, jigs can be utilized for production, by reason of the fact that the castings manufactured in this way are admirably true to shape. From the point of view of the user, machining losses can be eliminated, because of the freedom from flaws discovered only during the machining operation.
Light, intricate, sound steel castings can now be obtained in large quantities, where formerly only malleable or non-ferrous castings could be employed. Some of the castings made by this method weigh little more than a few ounces.
New Spheres for Alloy Steels.
Heat-resisting steels and stainless steels have been developed to such an extent that they cannot be summed up in the course of this article, but demand separate treatment. Some reference must be made, however, to the alloy steels, which have begun to enter fields in which they have previously never been commercially empleyed.
As an example, one may mention chisels for hand use. These were, in the past, usually made from a good-quality carbon tool steel, but it has been found after extensive tests that the addition of about 3 per cent, of nickel to a good steel results in a material for chisels which cannot be excelled. Thus, when the cutting edge of a hand chisel has become blunt, it is no longer essential to take it to the grindstone for resharpening, in order to obtain more service from it. All that is necessary is to touch up the cutting edge with a file, Furthermore, the steel is tougher, does not splay out at the chisel head as quickly as with a carbon tool steel, has a longer life, and more than saves its slightly higher cost,
Some Special Steels.
In addition to these alloy steels for hand chisels, there are special alloy chisel steels which harden in air and are self-tempering. These give great length of life, and keep their sharp cutting edges longer than do ordinary carbon-steel chisels.
Turning to another advance, some interesting investigations have been carried out in connection with spring steels, and it has been demonstrated, by experiments, that the state of the surface of these steels has a great deal to do with the length of time that the springs themselves last. It is suggested, as a result of these researches that in many cases it is possible to double the strength of springs by removing the fop layer.
The molybdenum manganese alloy constructional steels call for some mention. These afford the engineer an intermediate choice between expensive steels containing high percentages of alloying elements, and the low-priced carbon steels.
Their advantages are that they lack that dangerous quality of temper-brittleness to be found in many steels, they give consistent Izod impact figures, can be machined fairly readily in the heat-treated state, and offer both higher strength and better mechanical properties than do the plain carbon steels.
While dealing with the subject of alloy constructional steels, some reference must be made to the low alloy steels containing chromium, silicon and manganese, for use where good ductility and strength are needed. These steels are comparatively inexpensive. They contain, as a rule, 0.5 per cent, of chromium, 0.8 per cent, of silicon and about 1.3 per cent. of manganese. The carbon content alternates between 0,3 per cent. and 0.4 per cent.
Low Alloy Steels.
These low alloy steels give a good yield point, are robust, easily machined, ductile, uniform in composition and character, and offer a high elastic limit. They have the additional advantages that they can be .rolled, pierced and drawn', into tubes if desired, with little extra trouble or expense.
Then there is the invention of a semi-mild steel for chains, containing a percentage of nickel. The steel for these chains is cast into separate links, which are then joined together in a complete chain by the casting of intermediate links, two and two, in special moulds. The chain fabricated in this fashion is afterwards heat-treated by annealing. This process is for the purpose of restoring the grain; that is, removing strains and putting the steel into a satisfactory physical condition. Water-hardening and tempering follow.
The recently developed cobalt and other steels for permanent magnets have revolutionized such industries as that concerned with the manufacture of magnetos. Seven different alloy steels are being produced for magnets, the majority containing a percentage of cobalt, although tungsten and chrome magnet steels are also used.