488 ENGINEERING MATERIALS. . CHAP. XV. 



CAST IRON. The product of the blast furnace, where the iron ore is reduced in the presence 

 of a flux, is called pig iron. The term cast iron 'is commonly applied to pig iron after it has been 

 again melted and cast into finished form. Cast iron contains carbon, silicon, sulphur, phosphorus, 

 and manganese in addition to pure iron, and occasionally very small quantities of other elements. 

 The amount of carbon depends largely upon the presence of other elements. 



Carbon. The percentage of carbon ordinarily varies between if and 4 per cent, but in the 

 presence of manganese the carbon may be much higher. Carbon may occur in the form of com- 

 bined carbon, giving a white brittle cast iron, or in the form of graphite, giving a gray cast iron, 

 which is the form used in structural castings. The proper amount of carbon in cast iron depends 

 upon the amount of other impurities and upon the use that is to be made of the finished product. 



Silicon. The carbon is controlled by varying the amount of silicon and sulphur. Silicon 

 acts as a precipitant of carbon, changing it from the combined form to the graphite form. The 

 silicon in gray cast iron is usually between f and 3 per cent. 



Sulphur. Sulphur has the opposite effect of silicon and its presence is considered objection- 

 able. Sulphur produces " red-shortness " (brittleness when the iron is heated). The amount of 

 sulphur in gray-iron castings should not exceed 0.12 per cent. 



Manganese. Manganese and sulphur both tend to increase the amount of combined carbon, 

 but they tend to neutralize each other. Manganese gives closeness of grain and prevents the 

 absorption of sulphur on remelting. The amount of manganese in gray-iron castings is usually 

 less than ^ per cent; more than 2 per cent makes cast iron brittle. 



Phosphorus. Phosphorus increases the fusibility and fluidity of cast iron but at the same 

 time makes it brittle. A high phosphorus content is necessary in cast iron for light ornamental 

 castings where strength is not required. The phosphorus in gray-iron castings varies from 5. to 

 1 5 per cent. 



Malleable Castings. Small thin castings made of white cast iron may be decarbonized by 

 heating the castings in annealing pots containing hematite ore or forge iron scale. The castings 

 are kept at a cherry red heat for three to four days, and are then allowed to cool slowly. The metal 

 in malleable castings should not exceed J in. in thickness in small castings, nor | in. in large 

 castings, and should be of uniform thickness. 



Strength of Cast Iron. The strengths of gray-iron castings are given in Table I and in the 

 Specifications for Gray-iron Castings of the American Society for Testing Materials. 



STANDARD SPECIFICATIONS FOR GRAY-IRON CASTINGS 



OF THE 



AMERICAN SOCIETY FOR TESTING MATERIALS. 

 ADOPTED SEPTEMBER i, 1905. 



1. Process of Manufacture. Unless furnace iron is specified, all gray castings are understood 

 to be made by the cupola process. 



2. Chemical Properties. The sulphur contents to be as follows: 



Light castings not over 0.08 per cent 



Medium castings o.io 



Heavy castings 0.12 



3. Classification. In dividing castings into light, medium and heavy classes, the following 

 standards have been adopted: 



Castings having any section less than J in. thick shall be known as light castings. 

 Castings in which no section is less than 2 in. thick shall be known as heavy castings. 

 Medium castings are those not included in the above classification. 



4. Physical Properties. Transverse Test. The minimum breaking strength of the " Arbi- 

 tration Bar " under transverse load shall be not under: 



Light castings 2,500 Ib. 



Medium castings 2,900 



Heavy castings ; 3,3 



In no case shall the deflection be under o.io in. 



