4S2 Professor H. C. H. Carpenter [March 7, 



the best known of these materials — namely, what is called the pure 

 carbon steel turning tool. 



Let us begin by considering the manufacture of the tool itself, 

 and afterwards the conditions under which it is put to work and what 

 follows then. 



Although even the purest iron containing only the merest trace of 

 carbon can be hardened to some extent by quenching from a high 

 temperature, a certain minimum percentage of carbon is necessary 

 before the hardening is sufficiently marked to confer what may be 

 called practical hardening properties on tbe steel. This minimum is 

 about U • 7 per cent, and the cutting tools used in the arts range from 

 this figure up to 1 ' 5 per cent. For reasons which will be subsequently 

 apparent, it is simplest to consider the case of a steel tool containing 

 • 9 per cent carbon. The operation of manufacture of such a tool is 

 briefly as follows : In the first place, a charge designed to give the 

 correct composition is melted down in a crucible in a furnace whose 

 temperature ranges from 1500°-1600° C. The steel is cast from the 

 correct temperature into a metallic mould, giving what is called an 

 ingot, e.g. a rectangular bar about 2J inches square and 2 feet long. 

 Tbe ingot is subsequently forged down to a bar, say l^J-l inch in 

 diameter ; it is then cut up into lengths suitable for the dimensions 

 of the tool itself, and next it passes to tbe hands of tbe smith, who 

 forges it by hand and fashions the tool to shape. During this 

 operation it is of the greatest importance that the composition of the 

 steel should be altered as little as possible by oxidation of the carbon, 

 otberwise the tool will not harden properly or evenly. Then follows 

 the actual hardening operation, in which the nose of the tool, as it is 

 called, is carefully heated to a given temperature in the neighbour- 

 hood of 800° C, withdrawn either from tbe smith's fire or, better 

 still, the hardening bath, and quenched outright in a bath of cold 

 water at the ordinary temperature. 



This is an operation requiring the utmost skill. If the surface of 

 the metal has been decarbonized in heating and scale has formed on 

 the surface, the rapid transference of heat from the metal to the bath 

 will not take place, imperfect hardening will result, and very likely 

 cracks will be formed. If, however, the tool, as in the best modern 

 practice, is heated up in a bath of fused salts, no loss of carbon takes 

 place, and when it is removed from the bath it is covered with a thin 

 film of fused salt ; this dissolves almost instantaneously in the water, 

 and tbe necessary rapid transference of heat from the metal to the 

 water takes place. The best quenchings always have a peculiar 

 " bite," accompanied by a dull, albeit sharp, sound as the large 

 bubbles of steam generated by the heat are absoi'bed in the surround- 

 ing water. 



The steel thus hardened!^ although possessing the necessary 

 hardness, is too brittle to l)e used as a tool ; accordingly the temper- 

 ing process follows, in which it is heated to a moderate temperature 



