It is recommended that for forging these steels it be heated slowly and uniformly to a bright red, but not in a direct flame or blast. Harden at a dull red heat, about 1,300 deg.F. A clean coal or coke fire, or a good muffle-gas furnace will give best results. Fish oil is good for quenching although in some cases warm water will give excellent results. The steel should be kept moving in the bath until perfectly cold. Heated and cooled in this way the steel is very tough, takes a good cutting edge and has very little expansion or contraction which makes it desirable for long taps where the accuracy of lead is important. The composition of these steels is as follows: Per cent Manganese 1.40 to 1.60 Carbon 0.80 to 0.90 Vanadium 0.20 to 0.25 [Illustration: FIG. 13.--Effect of copper in steel.] EFFECT OF A SMALL AMOUNT OF COPPER IN MEDIUM-CARBON STEEL This shows the result of tests by C. R. Hayward and A. B. Johnston on two types of steel: one containing 0.30 per cent carbon, 0.012 per cent phosphorus, and 0.860 per cent copper, and the other 0.365 per cent carbon, 0.053 per cent phosphorus, and 0.030 per cent copper. The accompanying chart in Fig. 13 shows that high-copper steel has decided superiority in tensile strength, yield point and ultimate strength, while the ductility is practically the same. Hardness tests by both methods show high-copper steel to be harder than low-copper, and the Charpy shock tests show high-copper steel also superior to low-copper. The tests confirm those made by Stead, showing that the behavior of copper steel resembles that of nickel steel. The high-copper steels show finer grain than the low-copper. The quenched and drawn specimens of high-copper steel were found to be slightly more martensitic. HIGH-CHROMIUM OR RUST-PROOF STEEL High-chromium, or what is called stainless steel containing from 11 to 14 per cent chromium, was originally developed for cutlery purposes, but has in the past few years been used to a considerable extent for exhaust valves in airplane engines because of its resistance to scaling at high temperatures. Percentage Carbon 0.20 to 0.40 Manganese, not to exceed 0.50 Phosphorus, not to exceed 0.035 Sulphur, not to exceed 0.035 Chromium 11.50 to 14.00 Silicon, not to exceed 0.30 The steel should be heated slowly and forged at a temperature above 1,750 deg.F. preferably between 1,800 and 2,200 deg.F. If forged at temperatures between 1,650 and 1,750 deg.F. there is considerable danger of rupturing the steel because of its hardness at red heat. Owing to the air-hardening property of the steel, the drop-forgings should be trimmed while hot. Thin forgings should be reheated to redness before trimming, as otherwise they are liable to crack.