Here it will be noticed that the vivid reaction, which we might have expected would occur as the temperature indicating the first critical range was reached, was materially reduced. This might lead us to suspect that the desired allotropic change had not completely taken place at this point. In fact we noticed that the pyrometer needle did not record a vivid critical point until a very much higher temperature was reached. All of these observations serve as a possible explanation or indication of why it is necessary to employ very much higher temperatures in the hardening of High Speed Steel than it is in the hardening of a piece of simple Carbon Tool Steel. In a later chapter of this little volume we define Carbon Steels as those which do =not= contain enough of any element other than carbon to materially affect the physical properties which the steel will have when hard. High Speed Steels which are one of a very important group of special alloy steels, are those steels to which some element =other= than carbon has been added in sufficient amount to materially effect the physical properties which the steel will have when hard. The element which stands out alone as the most vital and important one as affecting the wonderful and highly desirable features looked for in High Speed Steels is Tungsten. We will discuss the various effects which the different elements give to the different alloy steels in a later chapter, but for the present we will confine ourselves to a brief discussion of the heat treatment of the now famous modern High Speed Steel. [Illustration: High Speed Steel. Carbon .58%. Structure: Very fine pearlitic condition, with particles of free carbide. Mag. 500x] As previously suggested the pressing demand of modern industry for quicker work, greater efficiency and enormously increased out-put of product, gave rise to the necessity of producing far more remarkable tools than was possible with the old fashioned carbon tool steel. Therefore it became necessary to produce a steel which could be rendered sufficiently hard to cut deep furrows in the various metals which have to be machined and, which could be made sufficiently tough to stand the enormous cutting strains and chatter and vibration of the machine, and at the same time maintain all these characteristics when the work done by upsetting the chip of the machined member actually rendered the cutting edge of the tool red hot.