Abstract
There has been an on-going discussion on the difference in formation mechanisms of upper and lower bainite. Various suggestions have been supported by reference to observed morphologies and illustrated with idealized sketches of morphologies. In order to obtain a better basis for discussions about the difference in mechanism, the morphology of bainite in an Fe-C alloy with 0.7 mass pct carbon was now studied in some detail from 823 K to 548 K (550 °C to 275 °C) at temperature intervals of 50 K or less. The work focused on bainite seen to start from a grain boundary in the plane of polish and showing an advancing tip in the remaining austenite. The results indicate that there is no essential difference with temperature regarding the ferritic skeleton of feathery bainite. The second stage of bainite formation, which involves the formation of both ferrite and cementite, was regarded as a eutectoid transformation and the resulting morphologies were analyzed in terms of two modes, degenerate and cooperative eutectoid transformation. There was no sharp difference between upper and lower bainite. Ways to define the difference were discussed.
Highlights
MEHL[1] introduced the terms upper and lower bainite
Hultgren[2] presented a sketch of the start of upper bainite where the plates were nucleated on a grain boundary and somewhat elongated cementite particles formed in the interspaces
The same alloys with 0.3 mass pct carbon were used in the second part of the recent study,[17] which focused on the second stage of upper bainite formation, in which the thin interspaces between the ferrite plates transform, triggered by the occurrence of cementite
Summary
MEHL[1] introduced the terms upper and lower bainite. He illustrated upper bainite with micrographs of feathery bainite, which originates from grain boundaries. It was concluded that the lengthening of bainitic ferrite does not depend on repeated nucleation and rapid growth of subunits to a limited length as suggested by Goodenow and Hehemann[11] and still widely accepted.[16] The same alloys with 0.3 mass pct carbon were used in the second part of the recent study,[17] which focused on the second stage of upper bainite formation, in which the thin interspaces between the ferrite plates transform, triggered by the occurrence of cementite. The elongated cementite particles, typical of upper bainite, were predominant in the whole experimental range of temperature but small colonies, typical of lower bainite, appeared in the lower part of the range This confirmed the observation by Hillert,[4] who concluded that the eutectoid transformation in the second stage of bainite formation can occur in two modes, a cooperative or degenerate eutectoid reaction. Due to the importance of studying units sectioned along their full length, the main attention was again paid to objects starting from grain boundaries in the plane of polish
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