Carbon 0.15 to 0.25 per cent Silicon Not over 0.20 per cent Manganese 0.30 to 0.60 per cent Sulphur Not over 0.04 per cent Phosphorus Not over 0.04 per cent A tension test should register at least 60,000 lb. per square inch. TABLE 17.--CASE-HARDENING 3-1/2 PER CENT NICKEL STEEL Carbon 0.12 to 0.20 per cent Manganese 0.65 per cent Sulphur Not over 0.045 per cent Phosphorus Not over 0.04 per cent Nickel 3.25 to 3.75 per cent TABLE 18.--CASE-HARDENING NICKEL CHROMIUM STEEL Carbon 0.15 to 0.25 per cent Manganese 0.50 to 0.80 per cent Sulphur Not over 0.045 per cent Phosphorus Not over 0.04 per cent Nickel 1 to 1.5 per cent Chromium 0.45 to 0.75 per cent TABLE 19.--CASE-HARDENING CHROMIUM VANADIUM STEEL Carbon Not over 0.25 per cent Manganese 0.50 to 0.85 per cent Sulphur Not over 0.04 per cent Phosphorus Not over 0.04 per cent Chromium 0.80 to 1.10 per cent Vanadium Not less than 0.15 per cent Having determined what is required we now proceed to inquire into the question of carburizing, which is of vital importance. USING ILLUMINATING GAS The choice of a carburizing furnace depends greatly on the facilities available in the locality where the shop is situated and the nature and quantity of the work to be done. The furnaces can be heated with producer gas in most cases, but when space is of value illuminating gas from a separate source of supply has some compensations. When the latter is used it is well to install a governor if the pressure is likely to fluctuate, particularly where the shop is at a high altitude or at a long distance from the gas supply. Many furnaces are coal-fired, and although greater care is required in maintaining a uniform temperature good results have been obtained. The use of electricity as a means of reaching the requisite temperature is receiving some attention, and no doubt it would make the control of temperature comparatively simple. However, the cost when applied to large quantities of work will, for the present at least, prevent this method from becoming popular. It is believed that the results obtainable \with the electric furnace would surpass any others; but the apparatus is expensive, and unless handled with intelligence would not last long. The most elementary medium of carburization is pure carbon, but the rate of carburization induced by this material is very low, and other components are necessary to accelerate the process. Many mixtures have been marketed, each possessing its individual merits, and as the prices vary considerably it is difficult to decide which is the most advantageous. Absorption from actual contact with solid carbon is decidedly slow, and it is necessary to employ a compound from which gases are liberated, and the steel will absorb the carbon from the gases much more readily.