NITRIDING ALLOY STEEL PARTS IN A FUSED SALT BATH

摘要

1340917 Nitriding alloy steel JOSEPH LUCAS (INDUSTRIES) Ltd 31 March 1971 8362/71 Headings C7U and C7A High-chromium steel is nitrided in a fused salt mixture including cyanides and cyanates of sodium and potassium, the composition being so controlled that the ratio K: Na is between 2:1 and 1:1, while cyanide radicals are present up to 20% of, and cyanate radicals not less than 10% of, the active components of the mixture. The process is applied particularly to steels containing in percentage by weight:- which may be initially hardened by heating to 1040 - 1060‹ C in a vacuum of 10SP-3/SP to 10SP-5/SP Torr, an inert gas or an endothermic atmosphere, cooling in oil or inert gas, reheating to 575 - 585‹ C, cooling to -60 to -80‹ C and tempering at 575 - 585‹ C. It is then treated in the salt bath for at least 30 hours. The bath may be sampled periodically e.g. at 4 hour intervals and appropriate additions made 1 hour later according to the analysis. The latter may be effected by dissolving part of the sample in water, precipitating carbonates with calcium nitrate, and titrating the residual solution against silver nitrate solution to two end points using meta cresol purple and potassium chromate to give cyanide and cyanate contents respectively. Furthermore a second sample is taken and converted to an aqueous chloride solution; additional potassium chloride is added to produce a solution containing about 100 p.p. m. of KCl and 1 to 4 p.p.m. of Na Cl which is then atomized in an air-acetylene flame and the sodium content obtained by comparing the absorption with that of a standard solution, the potassium content of the original sample being obtained by difference. Carbonate content is determined by precipitation with calcium nitrate followed by titration with acid. The carborizing process may be applied to the treatment of gear blanks.