Data on Machinability and Wear of Cast Iron 280022
THE hardness or chemical composition of an iron is, by itself, no indication of the wearing property and machinability of the iron. Irons containing a large amount of free ferrite have been found to wear rapidly, whereas others having considerable pearlite or sorbite in their structure show good wearing properties. The presence in cylinder-blocks of excess-carbide spots or of phosphides of high phosphorus-content is deleterious, because such spots wear in relief and the material ultimately breaks out, acting as an abrasive that scores the surfaces.
Causes of wear in cylinder-blocks are discussed, and nickel, or nickel and chromium, intelligently added to the iron is suggested as a means of obtaining the correct microstructure for a combination of good wearing properties and machinability. Since greater hardness is the result of a harder matrix rather than of an increase in the number of carbide spots, it has been found to be a good index of the improved resistance to wear, and to overcome the difficulty due to the hammering of the valves into their seats.
Analyses of cylinder-blocks, pistons, clutch plates, brake drums, cams, and forming-dies, in which nickel and chromium have been used, are given, and the improvements secured in the performance of these parts are described.
The discussion treats largely of the relationship between hardness, lubrication, finish and wear. Wear is stated to be much more dependent upon lubrication than upon quality of the iron. The use of nickel in iron is stated to improve the tightness and homogeneity of the metal. Various methods of testing iron for machinability are outlined.