Carbon Content. Upper Critical. Lower Critical. 0.00 1,706 deg.F. 1,330 deg.F. 0.20 1,600 deg.F. 1,330 deg.F. 0.40 1,480 deg.F. 1,330 deg.F. 0.60 1,400 deg.F. 1,330 deg.F. 0.80 1,350 deg.F. 1,330 deg.F. 0.90 1,330 deg.F. 1,330 deg.F. 1.00 1,470 deg.F. 1,330 deg.F. 1.20 1,650 deg.F. 1,330 deg.F. 1.40 1,830 deg.F. 1,330 deg.F. 1.60 2,000 deg.F. 1,330 deg.F. It is immediately noted that the critical range narrows with increasing carbon content until all the heat seems to be liberated at one temperature in a steel of 0.90 per cent carbon. Beyond that composition the critical range widens rapidly. Note also that the lower critical is constant in plain carbon steels containing no alloying elements. [Illustration: FIG. 46.--Microphotograph of steel used in S. K. F. bearings, polished and etched with nitric acid and magnified 1,000 times. Made by H. O. Walp.] This steel of 0.90 carbon content is an important one. It is called "eutectoid" steel. Under the microscope a properly polished and etched sample shows the structure to consist of thin sheets of two different substances (Fig. 46). One of these is pure iron, and the other is pure cementite. This structure of thin sheets has received the name "pearlite," because of its pearly appearance under sunlight. Pearlite is a constituent found in all annealed carbon steels. Pure iron, having no carbon, naturally would show no pearlite when examined under a microscope; only abutting granules of iron are delicately traced. The metallographist calls this pure iron "ferrite." As soon as a little carbon enters the alloy and a soft steel is formed, small angular areas of pearlite appear at the boundaries of the ferrite crystals (Fig. 47). With increasing carbon in the steel the volume of iron crystals becomes less and less, and the relative amount of pearlite increases, until arriving at 0.90 per cent carbon, the large ferrite crystals have been suppressed and the structure is all pearlite. Higher carbon steels show films of cementite outlining grains of pearlite (Fig. 48). This represents the structure of annealed, slowly cooled steels. It is possible to change the relative sizes of the ferrite and cementite crystals by heat treatment. Large grains are associated with brittleness. Consequently one must avoid heat treatments which produce coarse grains. [Illustration: FIG. 47.--Structure of low carbon steel, polished, etched and viewed under 100 magnifications. Tiny white granules of pure iron (ferrite) have small accumulations of dark-etching pearlite interspersed between them. Photograph by H. S. Rawdon.]