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Annealing as Applied to Steel Castings.

7th May 1914, Page 5
7th May 1914
Page 5
Page 5, 7th May 1914 — Annealing as Applied to Steel Castings.
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It is well known that there is no steel-casting process which can altogether dispense -a ith the work of annealing. The reason for producing a Casting which shall satisfactorily take the place of a forging, and yet give like results from the point of view of tensile strength, etc., is, of course, on account of the fact that structures which are cast often embody intricate shapes which cannot be carried out in forging practice. It naturally follows that the material used for the casting' must be of quite as good quality as that used in the forging. Up to acertain point, how-ex-ea, both productions have like characteristics. in that, in the case of the forging, the steel is first formed into a bloom, whilst the article made from a pattern is produced by the pouring of metal into a sand 211U aid. In both instances the product is, in the main, of a coarse, crystalline nature. Both the materials are still in a transition stage towards final development, the bloom having to be reheated before it can undergo the forging process under the hammer, press or roller, whereby it is so refined as to embody those qualities which are looked for in a good forging.

In the case of the steel casting, this process of refinement is not practicable, because of its moulded shape. There is only one way to achieve this result, and that is by adequate annealing, and it is in this process that the greatest amount. of caution and care

ii observation of the so-called critical temperature is necessary. This desiredend cannot be attained without prolonged experience. The casting is rapidly heated to that precise temperature which depends upon the chemical composition of the casting, and in steel of the usual tensile strength the temperature varies between SaC degrees and IOW degrees Cent. Moreover, the casting has to be sub. leeted to the proper heat: treatment for the correct ength of time, a factor which also varies according to the design of the construction. To complete the process, the adequate cooling of the reheated casting lias also to be carried out with exactitude.

Thus we have seen that a bloom, although castof excellent raw material, with a view to its becoming a forging, is of coarse crystalline structure of far less tensile strength than it will eventually, have at-tamed when it is properly reheated and forged, and to this second process the finished forging owes its refined quality.

In the case of the casting, the same characteristic holds good in the first instance, in that the coarse crystalline condition prevails, and the unit, up to this degree, is incapable of withstanding sudden shocks. If subjected to any stress, fracture is often liable to he set up ; the only possible way of removing i this danger is in refinement by annealing. There s

no known melting process, electric or otherwise, that can controvert this physical law, and no chemical camposition, however pure, can ensure that the grain of the cooling metal will become fine instead of coarse. _Moreover, there axon° means of obtaining an equal cooling or unequally-thick walls in the same casting. The forces interfering with the free settlement or the molecules in the casting during the period of its solidification are, indeed, in action in all melting processes, electric or otherwise, and there is no process and zio way of refinement except that of annealing. Here it is that long experience and careful study are essential, and it often happens that even forged or rolled material of high-class chemical composition will show a brittle condition in consequence of inadequate treatment in the fire. Overheated boiler plates have been known to fractureupon a slight blow. The same applies to the hest steel castings when th6 annealing is carried out in an indifferent manner.

In castings for commercial-vehicle purposes, which have to sustain continual and severe shocks, it is never advisable to dispense with propel: annealing, because it is the inure desirable to obtain from the material the best structural condition of which it is capable. It is a well-known fact that, notwithstanding the fact of a metal's possessing good physical characteristics, it is likely to fail when submitted to shocks. To ensure the finding of the best qualities of a production, the impact test. is the only reliable one, and the severity of this strain affords the only conclusive evidence of the value of the cast product. Our illustration shows the amount of difference in the stresses to which an annealed and unammaIed steel casting can be subjected, by impact test, before fracture sets up. Both castings treated were from the same pattern, and poured from the same "cast." The unannealed form gave way when subjected to only a2 per cent, of that impact to which the annealed specimen was subjected,

It follows that, where annealing can be properly (tarried out, the section of the structure can be kept -down to a minimum, thus eliminating bulky and weighty castings. Our right-hand illustration .shows a.good, annealed, cast wheel which has been 3/1 service for upwards of two years. It is of the metal-tire type, so that all road shocks to which it has been subjected while in service have been in no way " damped." This is an .instance which offers evidence that, before a, casting can be put on an equal footing with a forging, all other essentials being fulfilled, good annealing is a requirement which cannot possibly be emitted without risk of failure. It is not admitted, however, that steel castings are superior to forgings of simple construction.

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