The decomposition of austenite in a high purity iron-chromium-nickel alloy
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15 mm long, with a wall thickness of 0.5 mm. Each specimen was austenitised for 20 min at 1150 ~ followed by a helium-gas quench to the required preset isothermal transformation temperature. Both temperature and length of specimen were continuously monitored on a double-pen chart recorder, from which reaction start and finish times could be obtained. Specimens for metallographic examination were austenitised, again at 1150 ~ for 20 rain under flowing argon prior to isothermal transformation in a deoxidized tin bath at the required temperature. For examination by light microscopy, heat-treated specimens were mechanically polished and electrolytically etched using a 10 pct chloral solution at 10 V followed by a chemical etching treatment using an acidic supersaturated picric acid solution containing 5 wt pct cuprous chloride. This procedure was found to clearly distinguish the high temperature reaction product from those formed on quenching. Thin foils for transmission electron microscopy were prepared from discs slit from the optical specimens using standard jet polishing techniques and an electrolyte consisting of 10 pct perchloric acid, 20 pct glycerol, 70 pct ethanol. 3. REACTION KINETICS The dilatometrically determined TTT curve for this alloy is shown in Figure 1. The Ac~ and Ac 3 temperatures were determined by heating at a rate of 0.5 ~ and the following values were obtained: Ac~ 730 ~ Ac3 760 ~ Despite these temperatures, no isothermal ferrite reaction was detected until approximately 660 ~ Between 660 and ~600 ~ a ferrite reaction was observed to nucleate although growth was sluggish. A complete reaction was never recorded in this temperature regime, even after holding for 24 h after which time the test was discontinued. Conversely, at temperatures below 600 ~ a much faster reaction was detected. This low temperature reaction was found to nucleate too rapidly for the start times to be accurately recorded and hence in Fig. 1 the reaction start curve is shown dashed and the experimental points as crosses; this reaction went to completion after approximately 1000 s. These two separate reaction kinetic characteristics
ISSN 0360-2133 t 8110911-1587500.75 ! 0 METALLURGICAL TRANSACTIONS A 9 1981 AMERICAN SOCIETY FOR METALS AND VOLUME 12A, SEPTEMBER 1981--1587 THE METALLURGICAL SOCIETY OF AIME
Fig. l--Dilatometrically determinedTIT curve for the Fe-7 pct Cr-2 pct Ni alloy used in this investigation.The data points indicated by crosses are of limited accuracy due to the inability of the dilatometer to quench fast enough to the transformation temperature before the reaction started. There is an obvious discontinuity in the reaction start
(a)
curve at ~600 *C. imply that two different ferrite reactions are occurring, which produce a discontinuity in the T T T curve located at ~ 6 0 0 ~ This will be discussed in Section 5. 4. T R A N S F O R M A T I O N M I C R O S T R U C T U R E S 4.1 As Quenched Typical examples of the microstructures produced by direct quenching from the austenitizing temperature (1150 ~ into
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