The Effect of the Liquid Fe-C Phase on the Kinetics in the Carburization of Iron by CO at 1523K.

Abstract

In order to evaluate the effect of liquid Fe-C alloy produced during the carburization on the kinetics of iron carburization, the rates of carburization of iron by CO-Ar gas mixtures have been measured gravimetrically at 1 523 K and the growing rates of the liquid Fe-C layer thickness are also measured. The measurements were made under conditions for which the gas phase mass transfer was negligible. It is found that the weight gains due to the carburization reaction are almost proportional to t 2/3 within experimental scatters. For all runs except that of 1% CO, the liquid phase is observed after the carburization time of 300 s. The liquid phase thickness is found to be almost constant for the first 3.6 ks. After 3.6 ks, the liquid phase thickness starts to increase almost linearly with time. The mechanism to maintain the constant thickness is briefly discussed. The carburization reaction rate is calculated based on the well established decarburization rates and is reasonably agreed with the experimental carburization rate. Compared the experimental results with the calculated liquid phase diffusion rates and the chemical reaction rates, it is confirmed that the ratecontrolling step of the carburization reaction of iron with CO gas at 1 523 K is the chemical reaction COϩO ad →CO 2 at the surface after the liquid Fe-C phase is produced.KEY WORDS: carburization; liquid-solid interface; liquid Fe-C phase, melting point; ironmaking. ExperimentalA schematic diagram of the experimental apparatus is shown in Fig. 1. An iron plate (20ϫ20ϫ5 mm) was buried in Al 2 O 3 powder contained in an alumina crucible (30ϫ40 mm) and heated by a SiC furnace. The alumina reaction tube (ID 60 mm) was fitted with water-cooled O-ring sealed brass caps. The reacting CO-Ar gas mixture was jetted onto the surface of the metal. A 4-mm bore alumina tube held 10 mm above the metal surface was used to introduce the reacting gas. The end position of the gas inlet tube was adjusted to keep the center of reacting tube by using the Al 2 O 3 adjusting ring. The Ar-CO gas mixtures were prepared by mixing the high purity gases, after drying, by passing through columns of magnesium percolate and, for CO, by passing ascarite. Flow rates were controlled by mass flow controllers. All the experiments were carried out at 1 523 K. High purity iron was used for specimen preparation. The typical impurity content of this material (in wt pct) is: O, 0.04, C, 0.02, Al, 0.007, S, 0.002. The specimen was prepared by surface grinding and metallographically polishing the upper surface.The pre-weighted (about 20ϫ10 Ϫ3 kg) sample was slowly brought to temperature under flowing purified Ar. The inlet tube was adjusted to the appropriate height and then the reaction gas, which had been preset, was introduced over the sample. Experiments were terminated by switching back to argon and the sample was pulled down to the lower part of the furnace and allowing to cool to room temperature. The weight gain of carbon during an experiment was determined by measuring the total weig...

Carburization of Iron by Ar-CO-H2 at 1523 K.

“…Unfortunately, most of these results [15][16][17] were found to be under the effect of mass transfer process. In the previous work, 6) based on the established chemical reaction rate constant of CO 2 dissociation on the liquid iron, it was confirmed that the rate of the carburization reaction after the formation of the liquid Fe-C phase was controlled by the reaction of: As already mentioned, no work of the carburization of molten iron with CO-H 2 has been reported. Also the rate of CO dissociation on the liquid iron has not yet been established.…”

“…Relatively high S content compared with the previous study 6) was intended to decrease the chemical reaction rate by blocking the available reaction sites. The specimen was prepared by surface grinding and metallographically polishing the upper surface.…”

“…The general experimental technique was essentially the same as that used in the previous study 6) except for S content and H 2 addition. An iron plate (20ϫ20ϫ5 mm) was buried in Al 2 O 3 powder contained in an alumina crucible (i.d.…”

“…Several works 6,[15][16][17] for the carburization of molten iron with CO have been carried out. Unfortunately, most of these results [15][16][17] were found to be under the effect of mass transfer process.…”

“…Recently the carburization reaction rate measurement was carried out to investigate the role of the liquid phase on the carburization process. 6) It was concluded that the rate of the carburization reaction after the formation of the liquid Fe-C phase was controlled by the surface chemical reaction of: being sufficiently fast to be at virtual equilibrium. The subscript "ad" indicates the species that is adsorbed on the surface.…”