Austenitization kinetics of pearlite and ferrite aggregates in a low carbon steel containing 0.15 wt pct C
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THE microstructure of a slowly cooled low carbon steel consists of pearlite patches distributed in a ferrite matrix. Austenitization of such a steel occurs in two stages. In the first stage, the pearlite transforms to austenite. At temperatures higher than 800 ~ the first stage is complete in less than one second? The second stage of the process consists of ferrite to austenite transformation. At temperatures lower than 910 ~ the second stage does not involve any further nucleation of austenite and the process occurs by the growth of existing austenite particles in the ferrite matrix. The volume fraction of austenite increases with time and its average carbon decreases. The process continues until the equilibrium volume fraction of austenite is attained. At temperatures between 800 ~ and 910 ~ the overall transformation kinetics are controlled by the second stage of the process. Karlson 2 has studied the austenitization of pearlite and ferrite aggregates in a steel containing 0.18 wt pct C at 855 ~ The process was monitored by estimating the volume fraction of austenite at various times and it was concluded that the rate controlling step is the diffusion of carbon in austenite. The present work is a quantitative microstructural analysis of the austenitization kinetics of pearlite and ferrite aggregates in a low carbon steel containing 0.15 wt pct C. The results demonstrate that it is necessary to monitor at least two independent microstructural parameters (for example, volume fraction and surface area) to understand the kinetics of the process from the information contained in the global microstructural properties. EXPERIMENTAL PROCEDURE I. Materials The alloy was prepared by melting appropriate amounts of 99.99 pct pure iron and high purity graphite in an induction furnace in an argon atmosphere. In D. P. DATTA is Graduate Engineer, Durgapur Steel Plant, Durgapur, India and A. M. GOKHALE is Departmental Manager, Research & Development Centre, Hindustan Brown Boveri Ltd., P.O. Box 284, Baroda 390 001, Gujarat State, India. Manuscript submitted October 11, 1979. METALLURGICAL TRANSACTIONSA
order to ensure chemical homogeneity, the alloy button was melted twice and hot forged at 1100 ~ A layer of about 1.5 m m thickness was removed from all the external surfaces of the forged rod and then it was cold swaged to 7.5 m m diam. The chemical analysis showed 0.15 wt pct C, 0.1 wt pct Si, 0.01 wt pct S and 0.02 wt pct P. Samples of 3 m m thickness were used for the experimental work. II. Preliminary Heat Treatment The samples were vacuum sealed in vycor tubes. These specimens were kept in a furnace at 900 ~ for 1 h, and then furnace cooled. The heat treatment produced a suitable starting microstructure consisting of ferrite and pearlite aggregates, required for the austenitization studies. The samples were removed from the vycor tubes and then they were used for the subsequent austenitization experiments. III. Austenitization Experiments To monitor the kinetics, the samples were kept in a lead bath at temperatures ranging
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