Test Bars with One Edge Cast against a “Chill”
The composition of the mixture is regulated according to the depth of the chill as well as by appearance of test sprues.
We must not forget that the malleable cast iron as produced in this country is an entirely separate and distinct thing from the European malleable iron, as will be shown later. So our immense industry is our own and not a copied one.
It is only certain members of the cast iron family that can be made malleable by proper heat treatment. Alloys No. 14 and No. 15 represent one of these alloys before and after the annealing process. While No. 14 was given as a typical analysis for white cast iron for malleableizing it must be understood that compositions can vary considerably without detriment from that given.
There is one thing, however, which is absolutely necessary and that is that all or practically all of the carbon of the alloy must be in the combined form previous to the annealing process. This means that the alloy shall be white cast iron and have no free graphite, for any graphite flakes will remain through and after the annealing process and weaken the alloy just as it weakens gray cast iron.
Sketch of a Coal-Fired Air Furnace
For producing this white cast iron two processes are in general use—the “cupola” and the “air furnace.” The latter predominates.
Operation of the cupola for malleable iron requires great skill and very close attention to detail, for, to malleableize easily and with the best results, the composition of the alloy must be regulated within narrow limits, very much narrower than for gray cast iron. However, this is entirely possible and cupolas are operated continuously for malleable cast iron for ten or more hours with very slight fluctuation.
In general, operation is very similar to that described for cast iron except that the composition of the charge is necessarily different, much lower silicon being required, and more coke has to be used for the melting.
Most malleable iron castings are made in sand molds, and, as stated, the iron poured must be of such composition and temperature that the castings so made will be white of fracture. It is possible to get a quick indication of the condition of the iron for pouring by making test pieces, every one in the same way, which, after cooling and breaking, will show by fracture the approximate composition of the metal. According to these test pieces, called “sprues,” which, at times, may be cast as often as every five or ten minutes, the mixture is regulated to produce a uniform product.