Overall reaction kinetics and morphology of austenite decomposition between the upper nose and the M s of a hypoeutectoi
- PDF / 5,381,687 Bytes
- 14 Pages / 594 x 774 pts Page_size
- 64 Downloads / 158 Views
I.
INTRODUCTION
Two
of the preceding papers in this journal report the results of detailed studies on the transformation kinetics and morphology of austenite decomposition products at and somewhat above the bay temperature m and at and somewhat below the bay temperature [2] in Fe-C-Mo alloys. The present investigation was undertaken to determine the overall reaction kinetics and the morphology of transformation at counterpart temperatures both above and below the bay in an Fe-C-Cr alloy. The primary purpose of this work was to ascertain which of the principal results secured on Fe-C-Mo appears to be generic to austenite decomposition in Fe-C-X alloys, where X is a strong carbide-forming substitutional alloying element and is present at a concentration high enough at the carbon concentration employed to produce a bay in the timetemperature-transformation (TTT) diagram and also the "incomplete transformation = transformation stasis" [31 phenomenon. The latter process has been defined as the cessation of ferrite or bainite formation prior to the appearance of the Lever Rule proportion of ferrite. [4] Among the findings reported in Fe-C-Mo alloys, incomplete transformation is absent above the bay temperature, Tb (defined as the temperaaLre in the intermediate transformation temperature region at which the isothermal reaction time required to initiate transformation passes through a maximum); [1'3[ in this temperature region, isoH. GOLDENSTEIN, formerly Visiting Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is Assistant Professor with the Department of Metallurgical Engineering, Escola Politecnica da Universidade de Sho Paulo, Sho Paulo, Brazil 05508. H.I. AARONSON, R.F. Mehl Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
thermal reaction kinetics are invariably of the simple sigmoidal type. tn Below the bay, however, up to four different patterns of overall reaction kinetics are observed. I21 At those temperatures where transformation stasis occurs, carbide precipitation at a: 3' boundaries in association with the ferrite initially formed does not commence until the end of the stasis interval.t2] Ferrite morphology in Fe-C-Mo alloys exhibiting stasis displays marked differences [1,51 with respect to the usual pattern found in the proeutectoid ferrite and bainite regions of hypoeutectoid Fe-C, plain carbon steels, and many alloy steels. In the latter steels, grain boundary ferrite allotriomorphs predominate at high temperatures; with successive reductions in reaction temperature, Widmanst~tten sideplates, and then intragranular plates, develo
Data Loading...
