Austenite Formation from Fe-13Ni-1.1C Martensite
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PREVIOUSpapers 1-3have described changes which occur in iron-15 pct nickel-1 pct carbon martensite during tempering up to temperatures of 450 ~ The present paper describes for a similar alloy the reformation of austenite and its change in carbon level as annealing is continued to still higher temperatures. It presents an overall view of the reaustenitizing process and does not contribute substantially to the detailed information on martensitic reversal4-8 but it does contact thermodynamic data on carbon solubilities9-1~and by comparison with them points out the form of the carbon-bearing precipitates. EXPERIMENTAL PROCEDURES An Fe-13 wt pct Ni-0.25 wt pct AI-I.1 wt pct C alloy was prepared by melting the metallic components (purity of each 99.9 pct or better) under a 50 vol pct CO-50 vol pct gettered Ar atmosphere (total pressure 1/3 atm) in a 99.8 pct alumina crucible by induction heating at 450 kHz, and adding carbon to bring the carbon level to 1.10 wt pct C. The melt was held for 25 min under this atmosphere and chill cast by sucking into a fused silica tube to form a polycrystalline rod 12 mm in diameter and 90 cm long) 2 The rod was sealed in an evacuated fused silica capsule and annealed for 10 days at 1165 ~ for homogenization. Discs were then cut for experimental work. The discs were resealed in fused silica capsules and cycled three times between 196 ~ (15 min) and 1030 ~ (30 min) then heated to 1100 ~ for 1 h and water quenched. The resulting austenite intercept grain size was about 64/zm. Samples were quenched into liquid nitrogen for 15 rain and returned to room temperature and reheated to temperatures between 100 ~ and 1200 ~ for 1 h. Samples reheated to below 700 ~ were immersed in liquid salt or lead. Samples heated above 700 ~ and all X-ray samples were sealed in evacuated silica capsules and heated in a muffle furnace. On some of the samples -
H. S. KIM, formerly Visiting Professor at Purdue University, is currently Professor of Materials Engineering, Jeonbug National University, Seoul, Korea. H. R. HARRISON is Materials Research Scientist, Purdue University, W. Lafayette, IN 47904. P. G. WINCHELL (deceased) was Professor of Materials Engineering, Purdue University, W. Lafayette, IN 47907. Manuscript submitted July 21, 1980. METALLURGICAL TRANSACTIONS A
used for metallography and hardness, secondary lowtemperature tempering treatments were imposed after the primary reheating to improve the contrast developed during etching. Samples were mechanically polished and etched with nital. The austenite fraction was determined by systematic point counting using 900 total points at 1280 times magnification of carefully etched samples. The lattice parameters were measured on a Siemens powder diffractometer with cobalt radiation ~K,, - 1.788965~,.13 The diffractometer was controlled by a Canberra computer system and was equipped with a Li-drifted silicon detection system whose single channel analyser was set to receive CoKe. The K~, and K~2 lines were sufficiently well resolved that the K~, wavelength was u
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