RATE OF ABSORPTION
According to Guillet, the absorption of carbon is favored by those special elements which exist as double carbides in steel. For example, manganese exists as manganese carbide in combination with the iron carbide. The elements that favor the absorption of carbon are: manganese, tungsten, chromium and molybdenum those opposing it, nickel, silicon, and aluminum. Guillet has worked out the effect of the different elements on the rate of penetration in comparison with steel that absorbed carbon at a given temperature, at an average rate of 0.035 in. per hour.
His tables show that the following elements require an increased time of exposure to the carburizing material in order to obtain the same depth of penetration as with simple steel:
| When steel contains | Increased time of exposure |
|---|---|
| 2.0 per cent nickel | 28 per cent |
| 7.0 per cent nickel | 30 per cent |
| 1.0 per cent titanium | 12 per cent |
| 2.0 per cent titanium | 28 per cent |
| 0.5 per cent silicon | 50 per cent |
| 1.0 per cent silicon | 80 per cent |
| 2.0 per cent silicon | 122 per cent |
| 5.0 per cent silicon | No penetration |
| 1.0 per cent aluminum | 122 per cent |
| 2.0 per cent aluminum | 350 per cent |
The following elements seem to assist the rate of penetration of carbon, and the carburizing time may therefore be reduced as follows:
The temperature at which carburization is accomplished is a very important factor. Hence the necessity for a reliable pyrometer, located so as to give the temperature just below the tops of the pots. It must be remembered, however, that the pyrometer gives the temperature of only one spot, and is therefore only an aid to the operator, who must use his eyes for successful results.
The carbon content of the case generally is governed by the temperature of the carburization. It generally proves advisable to have the case contain between 0.90 per cent and 1.10 carbon; more carbon than this gives rise to excess free cementite or carbide of iron, which is detrimental, causing the case to be brittle and apt to chip.
T. G. Selleck gives a very useful table of temperatures and the relative carbon contents of the case of steels carburized between 4 and 6 hrs. using a good charcoal carburizer. This data is as follows:
| At 1,500°F., the surface carbon content will be 0.90 per cent |
| At 1,600°F., the surface carbon content will be 1.00 per cent |
| At 1,650°F., the surface carbon content will be 1.10 per cent |
| At 1,700°F., the surface carbon content will be 1.25 per cent |
| At 1,750°F., the surface carbon content will be 1.40 per cent |
| At 1,800°F., the surface carbon content will be 1.75 per cent |
To this very valuable table, it seems best to add the following data, which we have used for a number of years. We do not know the name of its author, but it has proved very valuable, and seems to complete the above information. The table is self-explanatory, giving depth of penetration of the carbon of the case at different temperatures for different lengths of time:
From the tables given, we may calculate with a fair degree of certainty the amount of carbon in the case, and its penetration. These figures vary widely with different carburizers, and as pointed out immediately above, with different alloy steels.