Non-technical chats on iron and steel, and their application to modern industry

by La Verne W. Spring

Malleable Cast Iron Swivels of Which Parts No. 2 Are Cast Tightly Around No. 1 and Loosened Only upon Annealing.

Comparison of photomicrographs No. 35 and No. 30 given on page [181], will show at once one of the reasons for the much greater malleability of malleable cast iron. While the total carbon present is very nearly the same in the two irons, the difference in physical form causes great difference in the malleability of the two. In the gray cast iron, No. 30, the carbon is crystalline and in the form of long brittle flakes which cut through and separate the grains of iron. Thus “planes of cleavage” are formed which make the alloy unable to resist severe shock and cause it to be anything but malleable. It is not so with annealed malleable cast iron. Here the carbon is in the form of small pellets which are imbedded among the grains of pure iron, the malleability of which is not seriously impaired largely because of the continuity of the “pure iron” structure. A second reason for the ability of malleable cast iron to withstand shock is that in the burning out of the carbon of the outer portions of the casting very small cavities are left. These allow the surface to become considerably deformed and battered under successive shocks without great strain on the casting itself.

Nothing has been said so far concerning one trait of all of the irons and indeed of most metals and alloys which are used for casting purposes. This is the tendency to “shrink” during the solidification and cooling of the metal of the casting. On account of the freezing of the outer portions of the casting before the metal of the inside, there must result certain hollow places or cavities after the inside metal has cooled unless some channel is kept open through which fluid metal can pass inside to keep cavities from forming. We will not here go into the matter of shrinkage with its great worry to the molder nor the ingenuity and strategy through which he produces castings without shrinkage cavities. One of the methods taken to overcome the trouble will be explained in the chapter on Cast Steel which is to follow.

There is, however, another type of shrinkage—that exhibited by the contraction of the entire piece of metal as it gradually cools after solidification. This presents a rather curious and interesting case.

It is well known among founders and pattern makers, that gray cast iron shrinks during cooling about ⅛ inch per foot, white iron ¼ inch per foot and cast steel ⁵⁄₁₆ inch per foot. That is, a bar cast exactly one foot long will be found when cold to be ⅛ inch short if of gray cast iron, ¼ inch short if of white cast iron and ⁵⁄₁₆ inch short if of cast steel. The patterns have to be made larger than the castings desired to allow for this shrinkage.

But, during annealing, white cast iron loses one-half of its ¼ inch per foot shrinkage and the resulting malleable cast iron is found to have a net shrinkage of but ⅛ inch per foot which is the same as that of gray iron.

It appears that the precipitation of the temper carbon expands the bar throughout to practically the same dimensions which it would have had if flake graphite had been allowed to precipitate through slow cooling, as is the case with gray cast iron.

This is cleverly taken advantage of by manufacturers of swivels of malleable iron, such as those shown. The inner portions are separately cast first and thoroughly cleaned after which they are imbedded in another mold. The outer portions are then cast around them, shrinking so tightly upon the inner portions that they cannot be turned at all. However, upon annealing they loosen enough that they can readily be turned yet remain tight enough that they cannot be separated.

Malleable iron from which the carbon has not been removed can be hardened and given a steely fracture by sudden cooling from a red heat even if it has previously been annealed. Decarbonized malleable iron, also, can readily be recarbonized by the cementation process. These characteristics are often taken advantage of for the manufacture of tools from malleable iron. Hammers, wood working chisels, gears, etc., are quite largely made. Where they are sold at a cheaper price than the better steel tools and without misrepresentation, there can be little objection, but sometimes they pass for steel.