72 MODERN SCIENCE OF METALS 



in the neighbourhood of 700 C. a dull red heat ; 

 several hundred degrees before this temperature 

 is reached, however, the steel softens so much as 

 to become rapidly useless. Here again we see a 

 series of well-known facts of every-day practice 

 for which the new science finds a theoretical 

 explanation of great scientific interest ; and again 

 we find a practical application of the highest im- 

 portance as the next step. If the existence of the 

 critical point near 700 C. serves to limit the 

 utility of steel cutting tools, cannot that limit of 

 utility be raised by finding some form of steel in 

 which the critical point occurs at a much higher 

 temperature? Fortunately it has been found 

 possible to do this ; the addition of either tungsten 

 or molybdenum to steel, together with chromium, 

 produces a whole class of alloy-steels in which the 

 critical temperature is very much higher, and 

 although it is much more difficult to harden these 

 steels it is necessary to raise them to a white 

 heat (i2OOC.) first yet when once hardened it 

 is not at all easy to soften them again, and tools 

 made of these materials maintain their cutting- 

 power even when worked at such high speeds and 

 pressures that the point of the tool is red-hot and 

 the turnings come away from the lathe visibly 

 "blue" with oxidation owing to the heat generated. 

 The importance of this advance in high-speed 

 machine cutting tools can hardly be over-rated, 

 and there can be no doubt that it is the direct 



