Precipitation of Nb(CN) during High Strain Rate Compression Testing of a 0.07 Pct Nb-Bearing Austenite G. FITZSIMONS, K.

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Precipitation Of Nb Cn During High Strain Rate Compression Testing Of A 0 07 Pct Nb Bearing Austenite G Fitzsimons K 1 (260.28 KiB) Viewed 29 times

Precipitation of Nb(CN) during High Strain Rate Compression Testing of a 0.07 Pct Nb-Bearing Austenite G. FITZSIMONS, K. TUTTO, R. FIX, and A. J. DeARDO The recrystallization of austenite in plain carbon steel (C-Mn) following high temperature deformation is a rapid process which may take only fractions of a second to com- plete. - It is also well known that additions of minute amounts of elements such as niobium to a C-Mn austenite strongly delay the recrystallization of austenite, and are responsible for the development of pancaked austenite dur- ing controlled rolling. While this retarding effect of niobium has been recognized for almost 20 years, i 12 there continues to exist some controversy about the nature of the mechanism responsible for the observed retardation. Some authors.1,15 suggest that this retardation results from solute drag effects, while others 16-18.20 propose that it results mainly from precipitation effects. Results of recent work?.indicate that recrystallization is substantially delayed in Nb-bearing austenite decarburized to carbon levels of 0.002 pct. Since precipitation has been prevented by the decarburization treatment, the observed retardation is attributed to solute drag effects alone. How- ever, the question remains as to which mechanism is, in fact, responsible for the observed recrystallization re- tardation, when precipitation is allowed as a competing pro- cess at the normal carbon levels (20.02 pct C) typical of microalloyed (MA) steels. Retardation of recrystallization of the microalloyed aus- tenite unequivocally attributable to precipitation effects has been extensively reported from results of experiments per- formed using different techniques. These include direct studies of precipitation using clectron microscopy of thin foils and replica specimens, gross precipitate extrac- tion:9 and indirect interrupted mechanical testing meth- ods-10.13-65.17-19 However, in most of these studies, the lack of rapid quenching prevented reliable metallographic obser- vations from being made during the first few critical seconds after the completion of the deformation. At the same time, it is well known that C-Mn austenite can undergo significant recrystallization in elapsed times of the order of one seconds-1 when deformed under conditions similar to those employed in the precipitation studies in the MA steels. The inability to observe specimens quenched within a fraction of a second has, therefore, restricted our understanding of the role played by precipitation in the early stages of retardation of recrystallization The purpose of this paper is to demonstrate that when hot deformation is followed by rapid quenching, it is possible to show clearly that well-developed precipitates do form in austenite in less than 0.5 second. The steel investigated has the following chemical com- position, in weight percent: 0.08 C, 1.25 Mn, 0.40 Si, 0.005 P, 0.005 S, 0.07 Nb, and 0.025 N. The steel was vacuum melted as a 22 kg laboratory heat and rolled to 16 mm diameter rod from which cylindrical compression specimens were machined. The specimens were reheated to 1300 °C for 30 minutes in an inert atmosphere, and water quenched to room temperature. TEM studies indicated that all of the Nb was in solution after this treatment. The hot compression tests were run as follows: The speci- mens were first reheated again in the testing furnace for four minutes at 1300 °C and deformed in uniaxial compression in the range of temperatures between 1250 and 800 °C and at constant true strain rates of 2 and 12 s', using a modified MTS machine, True strains up to 0.9 were imposed and uniform deformation was obtained by using glass lubricants and a modified Rastagaev's technique. Following defor- mation, the specimens were quenched in agitated iced brine. In all cases, temperatures below 600°C were reached within 0.75 second of the completion of the deformation. These times were measured by means of a thermocouple embedded in each specimen. In addition, selected specimens were exposed to the two successive reheating treatments and al- lowed to cool down to the deformation temperature but were quenched without being deformed. TEM observations in these reheated but undeformed specimens failed to show evidence of precipitation in the undeformed austenite. The specimens for TEM studies were sectioned parallel to the compression axis and thinned both mechanically and chemically before the final jet electropolishing. The foils were studied in a JEOL 100/120 U electron microscope operated at 120 kV. Precipitates at least as small as 2 nm could be detected in this study. That the precipitates had formed in the austenite was confirmed by using the orien- tation relationships. Details of this technique are given else- The flow curves of the specimens were measured between 850 and 1250 °C at a strain rate of 2 - and are shown in Figure 1. Some measurements at 12 s' were also per- formed. The flow curves shown in Figure 1 indicate that a TRUE STRESS, MPa 280 0.07 Nb Reheated 1300°C 245ė=2 850°C where. 23.2023 7,16-18,20 210 175 140 975°C - 1000°C 1050°C -1100°C 1150°C 1200°C 105 / 70 35 1245°C G. HITZSIMONS, K. TIITTO. R. FIX, and A. J. DeARDO are all with the Department of Metallurgical and Materials Engineering, University of Pittsburgh, 848 Benedum Hall, Pittsburgh, PA 15261 Manuscript submitted June 7, 1983. METALLURGICAL TRANSACTIONS A 0 0.25 0.50 0.75 1.0 1.25 TRUE STRAIN Fig. 1-Flow curves at a strain rate of 2 s. for the 0.07 Nb-0.025 N austenite. Note continuous hardening below 1100 °C. VOLUME 1SA, JANUARY 1984-241