A high magnification is required for the resolution of pearlite, generally not less than 200 diameters. The samples should also be very carefully etched The Microstructure of Medium High Carbon The microstructure of steel containing 0.38 per cent of carbon is illustrated in Fig. 7 under a magnification of 100 diameters. It will be noted in comparing this structure to that of low carbon steels (Fig. 5.), that the introduction of more carbon in the iron has re sulted, as should be expected, in the presence of a greater amount of pearlite and of a cor respondingly smaller proportion of ferrite. The pearlite occupies now roughly about one-third of the total area. Under sufficiently high power the pearlite areas exhibit the characteristic lamellar structure illustrated in Fig. 6.
On further addition of carbon, the .amount of pearlite, which is evidently proportional to the percentage of carbon, increases correspond ingly, as shown in Fig. 8, which illustrates the microstructure of steel containinir about 0.59 internal structure of these small masses of fer rite, however, is still made up of small cubic crystals as previously described. A high power photomicrograph of this steel would re veal the laminations of the pearlite shown in Fig. 6.
The Microstructure of High Carbon Steel. — It has been shown that the introduction of an increasing amount of carbon in steel results in the formation of a correspondingly increasing proportion of pearlite and decreasing amount of ferrite. A degree of carburization, therefore, must necessarily be reached, when the whole mass will be made up of pearlite, the ferrite having finally disappeared. This critical point in the structure of steel is attained when the metal contains about 0.85 per cent carbon. In exceptionally pure steel a little more carbon may be required to cause the complete disappear ance of ferrite, while in the presence of much impurity a. smaller percentage may be sufficient.
Steel made up exclusively of pearlite is sometimes said to be saturated, it is also called °eutectic? or “eutectoid," steel, the latter term having been suggested by Professor Howe. If it contains less than 0.85 per cent carbon, and, therefore, an excess of ferrite, it is called under-saturated or ahypo-eutectoid° steel, while if it contains more than this amount of carbon (and therefore, as will be seen, an excess of cementite) it is called over-saturated or °hyper eutectoicP steel. The use of the terms eutectic and eutectoid implies that steel is considered as an alloy of two constituents, ferrite and ce mentite, which, upon cooling, gives rise to the formation of a third constituent, pearlite, made up of small particles of both components. Many
alloys contain a constituent possessing the same characteristics and which is called °eutectic alloy? The structure of steel made up exclusively of pearlite is shown in Fig. 9. The magnifi cation is not sufficiently high to show clearly the laminations of pearlite.
Let us now consider what effect a further addition of carbon will have upon the structure of the metal. Fig. 10 is the reproduction of a drawing showing the structure of a steel con taining 1.20 per cent carbon, or much more carbon than the amount required to convert the whole mass into pearlite. It will be noted that while the bulk of the metal is still made up of pearlite, it now contains also another constituent, which in Fig. 10 occurs chiefly as a light net work surrounding the meshes of pearlite. This structure recalls that of medium hard steel and an inexperienced eye might be led to infer that this light constituent is fer rite. This net work, however, consists of ce mentite which is now present in excess over the amount required to form pearlite, just as in the case of low carbon steel, ferrite was in excess.
Cementite has a more metallic lustre the:, ferrite and remains bright and structurele" even after prolonged etching, while ferrite is colored and resolved into 'grains after such treatment. Cementite is extremely hard and stands in relief in the structure, while ferrite is soft and is depressed by the polishing Fer rite is readily scratched by a needle, while ce mentite cannot be marked. The excess of ce mentite, however, does not always assume the shape of a fine net work, its mode of occur rence depending upon the treatment to which the steel is subjected.
With further increase of carbon. the amour.
of cementite will necessarily increase and the proportion of pearlite decrease correspondingly The structure of unhardened carbon steel just described may, for our purpose, be ac counted for as follows: The carbon present in the steel unites with a portion of the iron to form the carbide FesC or cementite (which con tains 6.67 per cent carbon). The remaining iron, or ferrite, and this cementite then unite structurally in definite proportion to form pearlite, leaving, as the case may be, an excess either of ferrite or of cementite, the former in low carbon steel, the latter in highly carburized steel.