Steel and Iron 447 



,60, .71, 1, or 1.48; this means that a steel has .27 per cent 

 carbon, and so on. A very soft steel, similar to wrought 

 iron, may have one tenth of one per cent carbon. High 

 carbon steel varies from one half to one and one half per cent 

 of carbon. 



217. Tempering steel. One important property of steel 

 must be considered. How this property is utilized may best 

 be observed in a blacksmith shop where such tools as drills and 

 picks are sharpened. Steel that has been heated red hot and 

 then cooled slowly is very soft and useless for working hard 

 rock or similar materials. If cooled very quickly, as by immer- 

 sion in water, it becomes very hard, brittle, and likewise useless 

 for such work. An intermediate degree of hardness or temper 

 is necessary. Practical tool sharpeners know how to produce 

 the right temper without the aid of thermometer or other 

 apparatus. They have learned to interpret the colors which 

 appear in the heated steel. For example, a sharpened edge 

 of a drill or pick is immersed in water until it becomes black. 

 One side of it is brightened by rubbing it on a stone or brick at 

 the forge, when the play of colors is observed until the right 

 color appears. Then the tool is plunged into water to cool 

 and prevent further change. Thus a pick or drill is tempered 

 or made with the right degree of hardness. 



The colors observed by the tool sharpener indicate fairly well- 

 defined ranges of temperature. (See page 89.) Heat passes from 

 the main body or shank of the tool to the point, and the rise 

 in temperature is shown by the successive colors and shades. 

 The process is stopped by sudden cooling. The first color to 

 appear is lemon, light at first but gradually becoming darker. 

 It indicates a temperature of 392 F. to 435 F. or 440 F. If the 

 steel was cooled when this color reaches the very edge, it would 

 be brittle and hard, not tempered suitably for rock cutting. 

 An orange color follows, then a pink or " pigeon- wing," which 

 indicates the temperature suited for rock and stone-cutting 

 tools. A blue gives a temper suitable for mining bituminous 



