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: TABLE 15.--CARBON CONTENT OBTAINED AT VARIOUS TEMPERATURES At 1,500 deg.F., the surface carbon content will be 0.90 per cent At 1,600 deg.F., the surface carbon content will be 1.00 per cent At 1,650 deg.F., the surface carbon content will be 1.10 per cent At 1,700 deg.F., the surface carbon content will be 1.25 per cent At 1,750 deg.F., the surface carbon content will be 1.40 per cent At 1,800 deg.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: --------------------------------------------------------- | Temperature Penetration |----------------------------- | 1,550 | 1,650 | 1,800 ---------------------------|---------|---------|--------- Penetration after 1/2 hr. | 0.008 | 0.012 | 0.030 Penetration after 1 hr. | 0.018 | 0.026 | 0.045 Penetration after 2 hr. | 0.035 | 0.048 | 0.060 Penetration after 3 hr. | 0.045 | 0.055 | 0.075 Penetration after 4 hr. | 0.052 | 0.061 | 0.092 Penetration after 6 hr. | 0.056 | 0.075 | 0.110 Penetration after 8 hr. | 0.062 | 0.083 | 0.130 --------------------------------------------------------- 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. CARBURIZING MATERIAL