| 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.
CARBURIZING MATERIAL
The simplest carburizing substance is charcoal. It is also the slowest, but is often used mixed with something that will evolve large volumes of carbon monoxide or hydrocarbon gas on being heated. A great variety of materials is used, a few of them being charcoal (both wood and bone), charred leather, crushed bone, horn, mixtures of charcoal and barium carbonate, coke and heavy oils, coke treated with alkaline carbonates, peat, charcoal mixed with common salt, saltpeter, resin, flour, potassium bichromate, vegetable fibre, limestone, various seed husks, etc. In general, it is well to avoid complex mixtures.
H. L. Heathcote, on analyzing seventeen different carburizers, found that they contained the following ingredients:
| Per cent | |||
|---|---|---|---|
| Moisture | 2.68 | to | 26.17 |
| Oil | 0.17 | to | 20.76 |
| Carbon (organic) | 6.70 | to | 54.19 |
| Calcium phosphate | 0.32 | to | 74.75 |
| Calcium carbonate | 1.20 | to | 11.57 |
| Barium carbonate | nil | to | 42.00 |
| Zinc oxide | nil | to | 14.50 |
| Silica | nil | to | 8.14 |
| Sulphates (SO3) | trace | to | 3.45 |
| Sodium chloride | nil | to | 7.88 |
| Sodium carbonate | nil | to | 40.00 |
| Sulphides (S) | nil | to | 2.80 |
Carburizing mixtures, though bought by weight, are used by volume, and the weight per cubic foot is a big factor in making a selection. A good mixture should be porous, so that the evolved gases, which should be generated at the proper temperature, may move freely around the steel objects being carburized; should be a good conductor of heat; should possess minimum shrinkage when used; and should be capable of being tamped down.
Many "secret mixtures" are sold, falsely claimed to be able to convert inferior metal into crucible tool steel grade. They are generally nothing more than mixtures of carbonaceous and cyanogen compounds possessing the well-known carburizing properties of those substances.