STUDIES ON DUPLEX-GRAIN STRUCTURES OF AUSTENITE (II)

Ca uses of formation of duplex-grain structures in steels (Part-1)

Abstract

By the systematic experiments and some theoretical considerations from the view point of grain-growth characteristics of austenite, causes of formation of duplex-grain structures in steels were made clear, and some suggestions were given in relation to the prevention of such structures.
Results of these investigations are summarized as follows:
(1) "Uniform duplex-grains" appear in the stage of abrupt coarsening of austenite resulting from the dissolution of grain-growth inhibitors into it.
(2) "Non-uniform duplex-grains" are due to the primary segregation of grain-growth inhibitors leading to the non-uniform distribution of initial austenite grains on heating through the critical point.
(3) Grain-growth inhibitors are predominantly AIN in Al-treated steels, and the relation between the amount of AlN and its degree of segregation determines whether these steels have "uniform duplex-grains" or "non-uniform duplex-grains".
(4) For the prevention of "duplex-grains", it is necessary to reduce the primary segregation as little as possible and to preserve the suitable amount of AlN in steels.
(5) In addition, it is indicated that "uniform duplex-grains" appear also in the stage of grain-growth after recrystallization of hot-worked steels and "non-uniform duplex-grains" appear also in hyper-eutectoid steels resulting from the primary segregation of carbides.