MECHANICS AND USEFUL ARTS. 69 



ON THE HARDENING OF STEEL. 



" There are few things of which it is more difficult to understand the 

 rationale than hardening steel ; or why the same operation, of heating red 

 hot and plunging into a cold fluid, which hardens steel, should soften 

 copper. 



" Some persons will explain everything, whether they understand it or not, 

 and for this also have they found, in their own imaginations, a perfectly 

 satisfactory answer, and cut the difficulty by saying steel is condensed by 

 the operation ; but, unfortunately for their theory, the reverse is the fact, and 

 instead of being condensed, it is expanded by hardening, as any one may 

 soon satisfy himself by taking a piece of steel as it leaves the forge or anvil, 

 and fitting it exactly into a gauge, or between two fixed points, and then 

 hardening it ; it will then be found that the steel will not now go into the 

 gauge or between the fixed points. Or let him rivet together a piece of steel 

 to a piece of iron, filing the ends of both even, so that they may be exactly 

 the same length, then heat them to a proper heat to harden the steel, and 

 plunge them into water, he will find the expansive force of the steel has 

 nearly torn the rivets out, and that it extends beyond the iron at both ends. 

 Any article may be taken with steel on one surface and iron on the other 

 such as a joiner's plane-iron in the forged state flat on both surfaces, and 

 hardened ; and the expansion of the steel will cause that side to be convex, 

 and the iron side concave. 



" All steel expands in hardening, but that the most which is most highly 

 converted, and in direct proportion to the amount of carbon it received in 

 that process. No other general rule can be given for the heating of steel for 

 hardening than this that it should in all cases be heated as regular as pos- 

 sible to the lowest temperature at which that particular kind of steel will 

 harden, and as little as possible beyond it, remembering that the more highly 

 converted the steel is, the lower the temperature at which it will harden ; 

 and that a small article, such as a pen-knife blade, will harden at a lower 

 temperature than a more bulky one made of the same steel, because the 

 small article is more suddenly cooled. The hardening of very bulky arti- 

 cles, such as the face of an anvil, cannot be effected in the same way as 

 smaller articles, by plunging them into water ; for the length of time re- 

 quired in cooling will be almost certain to leave the middle of the face soft, 

 w r here it is of the most consequence that it should be hard. Where the anvil 

 forge is worked by water-power, they possess the best means of hardening 

 them, which is this : The anvil properly heated, should be placed in a 

 water tank, face upwards, under a shuttle connected with the mill-dam ; the 

 shuttle drawn, and a heavy and continuous stream of water let fall from a 

 height of ten or twelve feet upon the anvil face, which effectually hardens 

 the surface. 



" A red hot anvil plunged into water, would, for a time, be surrounded by 

 an atmosphere of steam, which would prevent its direct contact with the 

 cold water, whereby its cooling would be retarded too much to harden the 

 face; and hence the advantage of a continuous stream of cold water. 



