The author determined the rate of diffusion and the heat of diffusion of carbon in solid iron and steel.
As the material, the ordinary commercial steels and high purity steels especially prepared were received in the form of forged bars about 25mm in diameter. These bars were cut into 60mm lengths, carefully machined to 19mm in diameter within an accuracy of ±0·005mm over the entire length of each, and were ground with the 0000 emery paper in a precision lathe. Iron was electroplated onto these bars to a depth of about 0·7mm, and then the bars were heated in vacuum at about 650°C for 1 hour to remove any occuluded hydrogen. Thus degassed ironplated steel bars were machined to a uniform diameter of 20mm, keeping as concentric as possible to the electroplated interface. The diffusion annealing treatments were carried out in a nichrome resistance furnace equipped with automatic temperature controller for the prescribed times at the desired temperatures.
An outer layer usually 0·1mm thick was cut out from the each bar after the diffusion annealing and analyzed for carbon.
The diffusion coeffiicent D was calculated by the following equation in which Cc is the concentration of the carbon after diffusion at distance x from the outer surface after the time interval t, Cco the concentration of carbon in the core steel, h the thickness of eleetroplated iron layer, and Ψ the probability integral.
The results obtained were summarized as follows.
(1) The rate of diffusion of carbon increases with increasing concentration of carbon in steel, at first slowly, then with increasing rapidity in a concentration range of 0·3 to 1·1 weight per cent carbon.
(2) Impurities normally present in commercial steels do not greatly affect the rate of diffusion of carbon.
(3) The rate of diffusion of carbon is retarded by the presence of chromium appreciably.
(4) The heat of diffusion, which was determined graphically from the variation of coefficient of diffusion with temperature, is approximately in accordance with the Dushman-Langmuir equation.