486
METALLURGY OF CAST IRON.
etc. In Fig. 107, we see an irregular circle, outside of which we find the deepest close-grained sections at the corners A B. The lower the '' grade '' of the iron and the damper the sand the deeper will these corners chill or close up the grain of an iron. There is a limit to the extent to which combined carbon shown in the closing of the outer grain can cause strength in the test bar, where it is combined with a soft center or graphitic core as seen at D. A test bar can, by a radical difference in the grain of the core and outer body, embody such contraction strains within its own elements as to break with a lighter load compared with the true natural qualities of metal as exhibited by actual working results in castings or from a turned test bar. Degrees in "temper" or dampness of the sand comprising a mould have every influence in changing results in the corners of a test bar. A square bar is an erratic bar at its best; one cannot say what it will do in often showing different grades of iron to be partly the opposite of what a use of the castings would demonstrate. This is especially true where sqtiare test bars are cast flat.
We will.now turn our attention to the round bar, Fig. 108. It surely requires but little observation to impress one with the regularity of its outline comprising the surface.or close-graiiied metal; and it appears like adding insult to injury to discuss the favorable
FIG. IO7.
FIG. I08.rs. There is a great difference in iron in its susceptibility to elements tending to chill. Some iron, if poured into a dry sand mould, would show a gray fracture, but if poured into an iron or green sand mould, would show at the surface a • white or chilled iron, the depth of which depends upon the character of the iron, the thickness of castings, either in a letter discussing the testing of cast iron and attacked the usual formulae for loaded beams asound burs..........................*fi$7 "