THE MAKING AND WORKING OF STEEL 411 



affected; but for the process of annealing to be satisfactory, 

 the metal must be dry and warm. 



As steel is sensitive to heat, it is necessary to exercise 

 extreme care when annealing or hardening it. If it is heated 

 above a certain temperature, the grain of the metal is opened 

 and made coarse. If the piece is broken, the appearance of 

 the fracture is granular. Such steel is weak and for this 

 reason steel should not be overheated. 



471. An Alternative Method. Steel that is kept red hot 

 for a long period also is materially weakened. On the other 

 hand, its temperature must not be lowered so fast that the 

 metal chills and hardens. The work should be left in the 

 annealing box until nearly, or quite, cold. 



Another method of annealing steel consists in packing 

 the pieces in some material in an iron box, placing the box 

 in a furnace, and heating it until the contents are red hot 

 throughout. The pieces may then be removed from the 

 furnace and allowed to cool while in the box. When removed 

 from the furnace the box should be placed where no current 

 of air or any moisture will strike it. 



472. Annealing Tool Steel. Often articles of tool steel 

 which are to be threaded give trouble when machined, if 

 annealed by either of the methods mentioned. When an- 

 nealing steel for taps, extreme care must be exercised, for 

 if the steel is hard, the threading tool will not work 

 properly; if overannealed the steel is not strong enough 

 to withstand the cutting action of the tool, and rough 

 threads result. Either condition is fatal to the production 

 of a good tap. 



A satisfactory method of annealing for work of this nature 



