THE TEMPERING OF STEEL 71 



directly and indirectly laid the foundation for our 

 knowledge of thermal treatment. Among these 

 developments the modern gun, armour, and armour- 

 piercing shell have already been referred to. 

 Equally important is that very interesting class 

 of materials known as "high speed" tool steels. 



Mention has already been made of the fact 

 that tool steel can only be hardened by rapidly 

 cooling or "quenching" it from a temperature 

 above its critical or transformation point. Equally, 

 if hardened steel be heated, then as the critical 

 temperature is approached the hardness gradually 

 diminishes and disappears entirely when the critical 

 point is passed. Advantage is, of course, taken of 

 these facts in the ordinary "tempering" of hardened 

 steel, when a tool or blade is gently heated so 

 as to diminish its hardness somewhat in order to 

 gain increased toughness. When, however, a 

 hardened tool is used in a powerful machine, such 

 as a lathe, the point of the tool becomes heated 

 by the violent friction of the metal which it is 

 cutting, and the speed at which the cutting process 

 can be carried on depends entirely upon the fact 

 that the temperature of the point of the tool must 

 not be allowed to rise high enough to bring about 

 the softening of the hardened steel. If this does 

 occur, then the cutting edge immediately breaks 

 down and the tool is spoiled. Now in ordinary 

 carbon tool steels the critical point at which the 

 complete transformation or softening occurs lies 



