CARBON.
Carbon is the controlling ele ment; in fact, iron could not be cast iron with out carbon. Iron as it comes from the blast furnace, in the form known as pig iron, con tains from 3 to per cent carbon. Cast iron as it comes from the cupola generally retains from 3 to 4 per cent carbon; special grades sometimes contain as low as 234 per cent and as high as 4% per cent.
Condition of Carbon in Cast Iron.— Chemically, all the carbon which does not dis solve when the iron is placed in a certain strength of nitric acid is classed as Free Carbon, while the carbon which dissolves in this acid is designated as Combined Carbon.
Free Free carbon occurs as pure carbon interspersed in appreciable-sized particles between the crystals •of iron. While the iron is in a molten condition it will hold in com bination up to about 4.6 per cent of carbon, which is the saturation point. As the tempera ture lowers the saturation point drops, and the excess of carbon separates out. Chromium and manganese in the iron raise its saturation point for carbon; silicon reduces it. Free carbon is subdivided according to its structure and mode of formation into Graphitic Carbon and An nealing Carbon.
Graphitic Graphitic carbon is the crystalline or flaky form of carbon existing in all soft pig and cast iron which has been al lowed to cool slowly from a molten condition.
Annealing carbon is an amorphous modification of graphite which is developed in white iron by continued annealing at a high temperature.
The difference between Graphitic and An nealing Carbon is a physical one and is due to the mode of formation.
Combined Combined carbon ex ists in cast iron in two forms, Hardening Car bon and Carbide Carbon. Neither is visible to the naked eye, but each gives to iron a definite physical structure which is discernible under the microscope. Chemically, they may be separated by treating the iron for a long period with a weak non-oxidizing acid in a non-oxidizing atmosphere.
Hardening Carbon is held in the hardening condition by rapidly cooling iron from a high temperature. The condition in which it remains is considered that of solution. (See ALLOYS ) . This alloy of iron and carbon is known as Austenite. 'The limit, of carbon which will remain in perfect alloy is 0.89 per cent.
Carbide Carbide carbon is the carbon which exists in the carbide of iron FesC, also known as Cementite.
Factors Controlling the Condition of the are three factors which con trol the condition of carbon in cast iron. First, the percentage of carbon in the iron; second, the rate at which the iron cools; third, the percentage of other elements present with the carbon in the iron. Thus: (1) The more car bon there is present in molten cast iron when it begins to cool, the greater will be the per centage of carbon which will separate out in the free condition during cooling. For example
— a 4 per cent carbon iron would contain a greater percentage of free carbon than a 3 per cent carbon iron, provided both were cooled under similar conditions and carried the same amount of other elements. (2) The slower cast iron cools the greater will be the percent age of carbon present in the free condition, and conversely, the faster an iron is cooled, the greater will be the proportion of the car bon present in the combined condition. If a portion of molten cast iron of a correct com position is poured into cold water the carbon will be retained in the combined condition; if another portion of the same iron is cooled very slowly, all the carbon will separate out into the free condition. (3) The condition of the carbon in cast iron is controlled by the per centage of carbon present and the rate of cool ina, consequently any elements which affect these two reactions will affect the relation of the free to the combined carbon. Silicon, sul phur, phosphorus and manganese take a promi nent part in one or both of these reactions and consequently exert an important influence upon the condition of the carbon. (This is discussed later under the individual elements).
Effect of Carbon on the Physical Proper ties of Cast Free carbon makes iron soft and is instrumental in prolonging the fluidity, decreasing shrinkage and strength. Hardening carbon and carbide carbon increase the hardness and strength of cast iron; they prevent the decrease in shrinkage which would occur if their carbon was present in the free condition; they lower the melting point of cast iron.
Action of Carbon in Affecting the Physical Properties of Cast Iron.—As cast iron cools from a molten condition the carbon ssparates out as free carbon, unites with the iron as carbide carbon, and is retained in the form of hardening carbon. Cast iron in the molten state is a solution of carbon in iron; as the tempera ture decreases the ability of the iron to hold carbon in solution decreases, and the carbon begins to separate out as graphitic carbon. Heat rendered latent in the solution of the carbon in the iron will be given up when the carbon separates out from the iron, and this evolved heat will prolong fluidity and conse quently will give a longer time for the separa tion of free carbon, thus increasing the softness and decreasing the shrinkage of the iron.
When cast iron cools, the carbon which did not separate out as free carbon tends to unite with the iron as carbide carbon in the com pound FeaC. This occurs while the iron is passing through the zone of temperature in the neighborhood of 1300° F. The remainder of the carbon, which neither separates as free carbon nor unites with the iron as carbide carbon, re mains present as hardening carbon.