. Official proceedings . is amechanical mixture of this same constituent and of iron freefrom carbon, in the proportion of 12% of the former to 88%of the latter, called by the metallographist IVarlite, so actuallythe structure consists of but two elemental substances, the hardcarbide-of-iron, and soft iron free from an)7 carbon whatever. We have seen under what conditions soft iron and very soft■carbon can unite together to form a chemical compound as hardas quartz, but fortunately it has been found that this compoundis not stable when exposed to a high temperature, and if it beheated long- en
. Official proceedings . is amechanical mixture of this same constituent and of iron freefrom carbon, in the proportion of 12% of the former to 88%of the latter, called by the metallographist IVarlite, so actuallythe structure consists of but two elemental substances, the hardcarbide-of-iron, and soft iron free from an)7 carbon whatever. We have seen under what conditions soft iron and very soft■carbon can unite together to form a chemical compound as hardas quartz, but fortunately it has been found that this compoundis not stable when exposed to a high temperature, and if it beheated long- enough at such temperature it will split up into itstwo original soft constituents, which when chemically unitedcaused it to be so bard, with the result that we obtain due to thistreatment a structure consisting of a ground mass of very softand ductile iron throughout which is uniformly distributed smallnodules of free carbon, as can be plainly seen by reference toMicrograph No. 2. By comparing Micrograph No. 1, with Photograph 2 the profound change that takes place through heat treatmentwill be immediately seen. While this seems easy enough to accomplish in practice,certain precautions must be exercised in connection with thecomposition of the white iron, for while it has been stated thatthe hard carbide is unstable under high temperature, the state-ment must be modified somewhat by stipulating that the degreeof stability depends largely upon the composition of the whiteiron. Either low silicon or low carbon seem to stabilize thecarbide and prevent its being broken up into its two soft con-stituents by heat treatment, while high silicon and high carbonfacilitate its decomposition, so we see that by a proper adjust-ment of these percentages, and attention to a few other detailsto which I will refer later, the manufacturer can control hisproduct with intelligence. Commercially, the malleable iron process is conducted in twosteps, the first consisting in making a mixtur
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