Role of carbon and alloying elements in the formation of bainitic ferrite
- PDF / 113,693 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 56 Downloads / 222 Views
. INTRODUCTION
ONE of the controversies about the formation of bainite concerns the role of carbon. Hultgren was probably expressing the general opinion at his time when he in 1926[1] proposed that “needles of troostite” first formed as martensite needles, which were then self-tempered. Another opinion was expressed by Robertson in 1929[2] who proposed that the growth of the ferritic constituent of bainite is controlled by carbon diffusion. Wever and Lange in 1932[3] illustrated his idea with an Fe-C phase diagram where the / phase boundary was extrapolated to progressively higher carbon contents at lower temperatures. Evidently, the idea was that bainitic ferrite grows with a carbon content determined by local equilibrium with austenite. In 1946, Zener[4] put the first alternative in scientific terms by proposing that bainite could form only below the T0 line, where thermodynamics would allow ferrite to inherit all the carbon of the parent austenite by partitionless growth. Kinetically, this would be possible only at high growth rates, where there is no time for carbon to escape the migrating interface. On the other hand, after extensive microscopic studies, Hultgren in 1947[5] emphasized observations made on upper bainite and proposed that the bainitic transformation starts with growth of Widmanstätten ferrite, which is diffusion controlled because carbon has to partition between ferrite and austenite. The transformation is then completed by sidewise growth of ferrite together with carbide. In 1952, Ko and Cottrell[6] reported two observations on bainite, which have been used as arguments for the two opinions, one for each. Their observation of a surface relief supports the similarity with martensite, but their observation of slow growth supports control by carbon diffusion. Since then, some have tried to make the martensitic picture self-consistent by maintaining that the growth is rapid and all the carbon is inherited even though most of it will be lost quickly. They have thus been forced to propose that growth occurs in small but quick steps.[7] On the other hand, they have accepted that M. HILLERT, Professor Emeritus, L. HÖGLUND, Research Associate, and J. ÅGREN, Professor of Physical Metallurgy, are with the Department of Materials Science and Engineering, KTH, SE-10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted June 26, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
Widmanstätten ferrite grows by carbon-diffusion control, although that structure also shows a surface relief. Others have tried to make the other picture self-consistent by arguing that Widmanstätten ferrite, upper bainite, and lower bainite form a continuous series of acicular ferrite.[8] The difference between Widmanstätten ferrite and bainitic ferrite is denied by reference to a smooth variation of the growth rate and of the critical carbon content with temperature.[9] It should be mentioned that by alloying with Al or Si, it is possible to retard the formation of cementite in bainite, which will then simp
Data Loading...
