Phase recrystallization of steels alloyed with molybdenum
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The initial crystallographically ordered structure produced after martensite or bainite transformation is retained during subsequent heating up to the temperature at which there is formation of grains with high-angle boundaries, detected in the fracture.
Numerous intragranular interfaces obtained after initial martensite quenching, on which the dense carbide chains are predominantly found, prevent further disorientation of the austenite forming through advancement of the diffusion front deep into the ferrite plate absorbed by it. Further disorientation of the low-angle austenite plates, produced until the formation of independent grains with high-angle boundaries, is delayed until a higher heating temperature at which there is dissolution of the bulk of the carbide particles deposited on the ferrite (former martensite) plate boundaries.
As the amount of molybdenum in the steel is increased, given the same amount of carbon, there is sharp increase in the resistance of carbide particles to dissolution in austenite, on account of the increased molybdenum in them. There is a corresponding increase in the temperature difference between Ac3 and the temperature at which the carbide particles dissolve and grains detectable in the fracture are formed.
In steel with an initial crystallographically ordered structure, the heating rate has a substantial effect on the temperature at which there is absorption of the ferrite plates and carbide particle chains creating suitable conditions for the formation of polyhedral grains. The heating rate affects the composition of the ferrite and carbides as well as their size, shape, arrangement, and conditions for dissolving carbides, and, therefore, also influences the effectiveness of phase recrystallization through nonuniform grain growth during slow heating.
Dissolution of carbides and formation of austenite grains in initially quenched steels is completed at higher temperatures than in annealed steels through slowing down of the diffusion processes under the influence of intragranular interfaces.
The arbitrary process of austenite grain formation occurring in initially quenched steel after the carbide particles have dissolved at temperatures considerably above Ac3 leads to a reduction in the free surface energy of the system.
KeywordsCarbide Ferrite Austenite Martensite Bainite
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