The dark net work shown in Fig. 1 marks the junction lines between adjacent grains. It is made apparent by suitable treatment of the polished surface, generally by immersing it in some acid, the acid corroding the metal more deeply between the grains than over their sur face.
These grains of which pure metals are com posed are not single-crystals. If the polished sample be etched more deeply and examined under higher power, it will be found that each grain is made up of a great number of small crystals, frequently cubic. It will be found, moreover, that all these small cubes which make up a single grain are oriented in the same di rection in the same grain, but that their direc tion changes as we pass from one grain to an other.
The size of the grains varies with the nature of the metal, some metals being made up of much larger grains than others, even when cast and solidified under exactly the same con ditions.
The presence of impurities, sometimes of an extremely small amount, frequently exert a powerful influence on the size of the crystal line grains, some impurities increasing it, others reducing it.
The size of the grain in the same metal is also greatly affected by the temperature to which the metal is heated and from which it is allowed to cool, and by the rate of cooling. Generally speaking, it may be said that the higher the temperature, the larger the grain, and also that the slower the cooling the larger the grain. These results might have been an ticipated, if it be considered that slow cooling from a high temperature are conditions favor able to the formation and growth of crystals.
Undisturbed cooling is a necessary condi tion to the free development of crystals. If the metal be agitated while solidifying, and worked—that is, subjected to powerful mechan ical pressure — while cooling from a high tem perature, the formation of the crystalline grains will be greatly hindered, if not altogether pre vented, the metal assuming an amorphous-like structure, hence the important influence of work upon the structure of metals.
The Microstructure of Pure Pure iron is not a commercial product. It can only be obtained in small quantities by carefully con ducted laboratory manipulations, and even with the most refined care it is quite impossible to produce it absolutely pure. The purest com mercial iron is of Swedish origin and may con tain as much as 99.8 per cent of iron.
When a sample of this iron is properly pre pared and examined under the microscope, some regions may be found which are absolutely free from slag and from carbon, and which exhibit, therefore, the crystalline structure of pure iron, or, at least, of carbonless iron. (See Fig. 2). It will be noted that this structure is very similar to that of gold shown in Fig. 1. Like gold, it is made up of polyhedric grains, generally hexagonal. Iron like gold and like many other metals crystallizes in the cubic sys tem.
Ferrite.— Pure iron, or rather carbonless iron, considered as a microscopical constituent, has been called ferrite, a name which suggests its nature and which was proposed by Prof. Henry M. Howe. This constituent necessarily makes up the whole mass of carbonless iron, while in low carbon steel and in gray cast iron ferrite occurs in decreasing amounts as the per centage of combined carbon increases.
The physical properties of ferrite are evi dently those of carbonless iron. It is, there fore, soft, being easily scratched by a needle, a test which will occasionally be found useful by the metallographist. It is moderately strong, having a tensile strength of some 50,000 pounds per square inch. It is very ductile, its elonga tion amounting to 20 or more per cent. Ferrite does not possess any hardening power since carbonless iron cannot be hardened by sudden cooling from a high temperature.
The Microstructure of Commercial Wrought Iron. Longitudinal Section.—The microstructure of the longitudinal section of a wrought iron bar is shown in the accompany ing illustration (Fig. 3). The ground mass or matrix of the metal will be found to be made up of polyhedric crystalline grains similar in every respect to the crystalline grains of pure iron and of pure metals in general. Many irregular black lines, varying much in thickness and length, but all running in the same direction, will also be noted in Fig. 3. These lines indi cate the location of the slag which is always present in commercial wrought iron, and which assume the shape of streaks or fibres running in the direction of the rolling or forging, thus imparting a fibrous appearance to the metal. Wrought iron then is made up of a mass of ferrite, as might have been anticipated, assum ing its characteristic crystalline structure, and of numerous elongated particles of slag.