The relative influence of dynamic and static precipitation on the hot ductility of microalloyed steels
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I.
INTRODUCTION
NUMEROUSinvestigations ~'2'3 into
the hot ductility of microalloyed steels have confirmed that the precipitation of nitrides and/or carbides plays an important role in the fracture behavior of such steels after certain thermal cycles. At intermediate strain rates, the precipitate size and volume fraction are particularly important parameters. During hot working, these precipitates may be present prior to deformation, or be formed during deformation. It is well known that the precipitation of A1N, NbCN, and VCN is accelerated by deformation, compared to the precipitation rates measured during isothermal transformations in undeformed structures at the same temperature. 4-7 It is believed that this acceleration of precipitation is due to the introduction of favorable nucleation sites by the deformation process. The exact nature of these nucleation sites cannot be determined directly for transformable steels, but it seems likely that these sites are dislocation networks and vacancy clusters. This is supported by the observation of NbC precipitated on dislocations in austenitic stainless steel following high temperature testing, s The proportion of static and dynamic precipitates present at a particular temperature after deformation is dependent on the cooling rate from the solution temperature, and also on the deformation rate. Slow cooling rates favor static precipitation. For the particular case of AIN, cooling rates of less than 1 ~ are believed to be required for the precipitation of A1N during continuous cooling, a particularly sluggish reaction. 9 Dynamic precipitation may not occur at very high strain rates, since deformation usually ceases before the incubation time for dynamic precipitation has been reached. However, static precipitation may subsequently occur in the deformed structure. This static precipi-
tation in a deformed structure occurs at rates intermediate between that of dynamic precipitation and static precipitation in undeformed metal. 6 The investigation of Weiss and Jonas 6 showed that for the dynamic precipitation of NbCN, strain rates of less than 10-J s-~ were required, the exact value being temperature dependent. A later study by Akben et al. 7 showed that a similar strain rate was required for the dynamic precipitation of VN. Weiss and Jonas ~~ have also shown that dynamically precipitated NbCN is finer than the equivalent static precipitates formed at the same temperature. Thus it can be seen from the foregoing that precipitate volume fraction and precipitate size are dependent on whether the precipitate is formed in a dynamic or static situation. Therefore, since both precipitate volume fraction and size play an important role in determining the hot ductility of microalloyed steels, it would be expected that dynamic and static precipitates would each have a distinct influence on hot ductility. Investigations into the influence of the static and dynamic precipitation of AIN have led to the conclusion that precipitates present before deformation are more detrimental to h
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