IRON AND STEEL 237 



curves for this case, and determine whether or not an elastic limit is indicated. 

 Also compute the strength of the column by Rankine's formula and Johnson's 

 straight-line formula, and compare the results with those obtained from the test. 



188. Malleable castings. The castings with combined carbon are 

 hard and brittle. These are heated with some oxide, so that the 

 carbon near the surface is burned out, leaving the outer surface 

 tough and strong, like wrought iron. The interior of the casting 

 is somewhat annealed, but the finished product consists of a hard 

 interior portion with a ductile outer portion. This structure insures 

 strength both statically and as regards impact. 



189. Specifications for cast iron.* The following specifications 

 are for special hard cast iron (close-grained). They are taken from 

 the J. I. Case Threshing Machine Company's specifications, and may 

 be considered as typical 



CHEMICAL COMPOSITION 



Silicon must be between 1.20 and 1.60 per cent. (Below 1.20 the 

 metal will be too hard to machine; above 1.60 it is likely to be 

 porous unless much scrap is used.) 



Sulphur must not exceed 0.095 per cent, and any casting showing 

 on analysis 0.115 per cent or more of sulphur will cause the rejec- 

 tion of the entire mixture. (Above 0.115 per cent sulphur produces 

 much shrinkage, shortness, and " brittle hard " iron.) 



Phosphorus should be kept below .70 per cent unless specified for 

 special thin castings. (High phosphorus gives castings brittle under 

 impact.) 



Manganese should not be above .70 per cent except in special chilled 

 work. 



PHYSICAL TESTS 



Transverse breaking strength. The test bars should be 1 in. square 

 ami 1 3J in. long, and should be tested with a load of 2400 Ib. applied 

 at the center of a 12-in. span. 



These specifications, as well as all others quoted, are given so that the student may 

 '_'>-t ;m idea of the composition and properties required of commerial cast iron or other 

 i;il. Specifications issued by different companies vary, and those issued by the 

 same company are frequently changed on account of the requirements of service. 



