Aging of 16-15-6 Type Alloys.

On the function of nitrogen as an alloying element in heat-resisting materials-II

Abstract

Effect of N, Ni and C content on aging characteristics of the low Ni Timken 16 Cr-15 Ni-6 Mo (-7.5Mn) type alloys was studied by measurements of differential dilatation, hardness, and microstructure. The results were summerized as follows: (1) Maximum amount of N addition was limited at about 0.3%, above which sound specimens of the alloys couldaot be produced by melting and casting in air. (2) Two stages of contraction in differential dilatation due to the precipitation were observed at 500-600°C and above 800°C during tempering of the solntion-treated alloys irrespective of both the composition and the hot-cold working. It seems that the later contraction was due to the precipitation of nitrides and carbides, especially a large contraction being observed at above 750°C in the carbon containing alloys. (3) In all of the solution-treated (1200°C 1h, water-quenched) alloys, hot-cold worked ones, and subsequently aged ones at 700°C, the hardness, increased with increasing amount of nitrogen addition. Effect of Ni ranging from 13 to 17% on the aging characteristics was not significant, and addition of carbon up to 0.15% promoted the age-hardening but resulted in rapid softening in prolonged heating. (4) X-ray analysis of precipitants in very slowly cooled alloys showed only the formation of the sigma phase, no evidence of other precipitants being found out. Considering both the dilatometric behavior and the microscopic appearance, the rapid hardening in early period of the aging may be attributed to the precipitation of carbides, and the comparatively slow hardening may be due to both the precipitation of the sigma phase and nitrides.