Increase in the Alpha to Gamma Transformation Temperature of Pure Iron upon Very Rapid Heating
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INTRODUCTION
OVER 40 years ago, two investigations concerning the increase in the a (bcc) to c (fcc) transformation temperature upon rapid heating for pure iron were reported. At heating rates of about 10,000 K/s, Boedtker[1] reported an increase in the transformation temperature from the equilibrium value of 1185 K (912 °C) into the range 1300 to 1500 K (1027 to 1227 °C) depending upon preheat treatment of the specimens. Haworth and Parr[2] reported an increase from 1185 K (912 °C) to about 1230 K (957 °C) for a similar heating rate. This pioneering work, done in the early 1960s, was largely of academic interest at the time. However, with the advent of laser welding, heating rates of one million K/s can be achieved[3,4] and, coupled with rapid cooling, complex, nonequilibrium microstructures can result.[5] The ability to predict the microstructures resulting from rapid heating and cooling is presently gaining increased importance. These very high heating rates may raise the melting temperature by hundreds of Kelvin to the point where vaporization of some of the more volatile elements may occur.[5] Therefore, an investigation was initiated to study the a (bcc) to c (fcc) transformation temperature in pure iron at rates considerably in excess of 10,000 K/s with the objective of better understanding transformations occurring during rapid heating.
JEREMY LANGNER, formerly Graduate Student, Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg R3C2P4, is Renewable Energy Engineer in Training, Customer Engineering Services, Manitoba Hydro, Winnipeg R3T5V6, Manitoba, Canada. J.R. CAHOON, Senior Scholar, is with the Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg R3T5V6, Manitoba, Canada. Contact E-mail:[email protected] Manuscript submitted June 17, 2009. Article published online February 24, 2010 1276—VOLUME 41A, MAY 2010
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EXPERIMENTAL
A. Rapid Heating Circuit The rapid heating of specimens was achieved with the circuit shown schematically in Figure 1. Four 12-V leadacid automotive batteries (E1 through E4) connected in parallel were used to deliver current to heat the specimen. Each battery was rated at 780 cranking amps, allowing a maximum rated current of 3120 amps. A carbon disc variable resistor (R1) was used to control the current to the specimen. To provide maximum current, the carbon disc resistor was bypassed by moving the single pole double throw knife switch (S1) from the neutral position to the forward position. The selector switch was set in the reverse position to balance the thermocouple circuit, as described subsequently. The specimen was fixed in place and connected to the circuit by clamping each end to an aluminum terminal. The main power switch (S2) was used to activate the circuit. Heating rates lower than 100 K/s were obtained by placing a specimen with thermocouple attached into a resistance furnace and recording the time-temperature curve.
B. Temperature Measurement The temperature measurement system was e
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