The effect of niobium on the hardenability of microalloyed austenite
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I.
INTRODUCTION
S E V E R A L models exist for the calculation of the microstmcture in the heat-affected zone of steel welds, t~,2,31 There is also research that enables the dissolution kinetics of microalloying elements such as niobium to be estimated. 14,5j The results have been used previously to improve the austenite-grain-size calculations for the heataffected zone on the basis that precipitates pin the grain boundaries and hence influence grain size. However, there is evidence that the precipitation or dissolution of carbides in microalloyed steels have effects beyond those expected just from the pinning effect. 16-~~ The kinetics of ferrite formation appear to be retarded even though the concentration of niobium might be very small indeed. The purpose of the present work was to verify that a microalloyed steel containing niobium undergoes detectable changes in hardenability as a function of heat treatments designed to alter the distribution of niobium between the austenite and carbides. II.
MATERIALS
The steel used for most of the experimental work was a low-carbon normalizing steel with a microalloying addition of niobium. An additional alloy with a similar chemistry but without niobium was also studied for comparison purposes. The chemical compositions of the two steels (labeled steel 1 and steel 2, respectively) are given in Table I. Whereas it was anticipated that niobium would form carbonitride compounds in steel 1, it was assumed here that precipitates would be predominantly carbides, and the term niobium carbide is used throughout this work.
A. Dilatomet~ Simulation experiments were performed on a Gleeble 1500 (Duffers Instruments) or a Thermechmastor thermomechanical (Fuji Electric) simulator with computer control and data acquisition. Both are capable of C. FOSSAERT, Engineering Student, Universit6 de Technologie de Compi~gne, and T. MAURICKX, Research Engineer, are with SOLLAC, Dunkerque, France. G.I. REES, Research Associate, and H.K.D.H. BHADESHIA, Reader in Physical Metallurgy, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB 1 3Q2, United Kingdom. Manuscript submitted November 24, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
monitoring the transformations as they happen via the accompanying dimensional changes, using laser (Thermechmastor) or mechanical (Gleeble) transducers. Both simulators used cylindrical samples, of dimensions 5 • 70 mm for the Gleeble and 8 x 12 mm for the Thermechmastor. A diagram illustrating the typical form of the heat treatments used for the experiments is given in Figure 1. The thermal cycle parameters of interest are the austenitization time (t~) and temperature (T~), the cooling rate (V~nt) through the intermediate temperature range 980 ~ to 750 ~ and the cooling rate from 750 ~ to room temperature (Vfl. For a given series of thermal cycles, as the austenitization temperature was varied, the duration of the hold at the peak temperature was adjusted so that the total time spent above 980 ~ was the same in a
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