A stress relaxation method for following carbonitride precipitation in austenite at hot working temperatures
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I.
INTRODUCTION
THE kinetics
of precipitation of the transition metal carbonitrides in austenite have been of considerable interest to metallurgists during the last two decades or so. This pursuit arises from both the usefulness of the information for the rational design of controlled rolling schedules and from its fundamental importance in physical metallurgy. However, the goal of a clear understanding of this topic has not yet been realized, in part because of the notorious difficulties involved in the accurate measurement of precipitation rates at high temperatures. The existing methods for monitoring the progress of precipitation are the following: (i) the electron microscopy of thin foils; m21 (ii) electron microscopy applied to extraction replicas; t3'41(iii) analysis of high temperature flow c u r v e s ; [5'6l (iv) decreases in secondary hardening; IT-~~(v) measurement of electrical resistivity; tm and (vi) chemical and electrochemical extraction. I12,~3,141Chemical and electrochemical extraction is the only technique which can give complete information without large sampling errors. Unfortunately, its sensitivity is too low for it to be able to detect the small precipitates that appear in the early stages of precipitation. Complete kinetic information can also be obtained by using a macroscopic (mechanical) technique in conjunction with TEM on thin foils or extraction replicas. The latter methods lead to the particle size distribution, composition, and structure of the precipitates. The use of TEM methods alone for the purpose of studying precipitation kinetics is limited by their significant sampling errors and their time consuming nature. For employment in conjunction with the TEM technique, the analysis of high temperature flow curves is useful because of its relatively high sensitivity, small sampling error, and experimental ease, especially when it is recalled that it is the only technique which is carried out at temperature, directly on the austenite. However, 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 August 31, 1987. METALLURGICAL TRANSACTIONS A
the effectiveness of this method is restricted by the difficulties involved in peak strain measurement on the flow curves, and by the relatively lengthy experimental times involved. There is thus a demand for a better technique at the present time. For this reason, a new mechanical method for determining precipitation-time-temperature (PTT) relationships in microalloyed steels was developed in the present investigation. The method is based on the analysis of stress relaxation data. Stress relaxation measurements have been used extensively for studying dislocation movement in metal and alloys, tlSj However, conventional relaxation tests are usually made at contant temperature and are therefore not suitable for the investigation of precipitation phenomena, which generally involve cooling. The co
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