Ti(CN) precipitation in microalloyed austenite during stress relaxation
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I.
INTRODUCTION
THE precipitation
of carbonitrides in microalloyed austenite containing one or more of the transition metals Nb, Ti, and V is of primary importance in controlling the properties of HSLA steels. During the last two decades, most of the investigations in this subject area have been concerned with Nb bearing steels. It has been clearly demonstrated that the precipitation of Nb(CN) in deformed austenite exhibits the usual C-curve kinetics, t~-~~ and that such precipitation takes place mainly on dislocations lH-141 and/or dislocation substructures. [~5'~6'~71By contrast, relatively few studies have been carried out on Ti and V bearing grades. It was only recently that the kinetics of Ti(CN) precipitation in deformed austenite were determined by Akben et al. [181 and by the present authors I~9'2~ using different mechanical methods. These results showed that Ccurve kinetics are also valid for a series of Ti bearing steels. However, the detailed progress of Ti(CN) precipitation was still not clear because of the inherent limitations involved in the mechanical techniques. The primary aim of the present study was therefore to investigate the growth and coarsening behaviors of Ti(CN) formation in deformed austenite. A secondary aim was to confirm the validity of the stress relaxation technique for following carbonitride precipitation which has been described in a recent publication. I2~
II.
EXPERIMENTAL P R O C E D U R E
A. Experimental Material and Stress Relaxation Testing The experimental material was one of a series of four Ti bearing steels employed in the previous investigation, r2~ It was prepared at the Physical Metallurgy Research Laboratories of the Department of Energy, Mines and Resources, Ottawa. Its detailed chemical composition follows: 0.05 pct C, 0.25 pct Ti, 0.007 pct N, 1.43 pct Mn, 0.27 pct Si, w.J. LIU, Research Associate, and J.J. JONAS, Professor, are with the Department of Metallurgical Engineering, McGill University, 3450 University Street, Montreal, PQ, Canada, H3A 2A7. Manuscript submitted September 28, 1987. METALLURGICALTRANSACTIONSA
0.01 pct A1, 0.01 pct S, and 0.005 pct P. Compression cylinders l1.5 mm high and 7.6 mm in diameter were machined from the as-received 13 mm thick hot rolled plate. The specimens were given a prior heat treatment at 1000 ~ for two hours followed by air cooling to eliminate the rolling texture in the plate. Stress relaxation tests were carded out on a computerized MTS machine provided with a high temperature, high vacuum furnace (17 kVA CENTORR model M60). A helium quenching device, which consists of a sample pusher and a gas quenching cup, was designed and installed inside the furnace. Each sample was solution treated at 1260 ~ for half an hour immediately prior to testing, and then cooled to the test temperature at a rate of about 2 ~ When the temperature had become stabilized, a 5 pct prestrain was applied at a true strain rate of 0.1 s -~. Stress relaxation was begun immediately following the prestrain. Further details of the stress relaxatio
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