Page:Steamlocomotivec00ahrorich.djvu/93

Rh mentioned. “Case-hardening” is a carbonizing process which produces a hard steel surface on the parts, without which they would be subject to undue wear. All link motion parts and pins, and also slide bars undergo this process. The parts are placed in cases, and packed round with pieces of charcoal, leather, and bones, and carefully sealed up. The cases are then heated in a furnace for 12 to 18 hrs. after which the parts are removed and cooled in water. Case-hardening has been described as a somewhat brutal process, though nothing better has yet been found. It has the disadvantage that the parts so treated warp somewhat in the process, and have therefore to be returned to the fitters and machine shop to be corrected. The case-hardened surfaces are so hard that they cannot be touched with a file, hence the holes are ground out to limit gauges by very small emery wheels or, alternatively, they are “lapped” out by revolving spindles made of lead and covered with emery powder and oil. Due allowance for the warping is made in the previous machining and fitting, a very slight amount of extra material being left for the final grinding to finished size after case-hardening.