The effect of intercritical annealing temperature on the structure of niobium microalloyed dualphase steel |
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Authors: | Mark D Geib David K Matlock George Krauss |
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Affiliation: | (1) Department of Metallurgical Engineering, Colorado School of Mines, 80401 Golden, CO |
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Abstract: | The structures produced in a Nb-microalloyed steel by oil quenching after intercritical anneals at 760 and 810 °C have been
examined by light and transmission electron microscopy. After both anneals, the periphery of the austenite pool transforms
on cooling to ferrite in the same orientation as the ferrite retained during intercritical annealing. Thus the ferrite forms
by an epitaxial growth mechanism without the formation of a new interface or grain boundary. The new ferrite is precipitate-free
in contrast to the retained ferrite which develops a very dense precipitate dispersion during intercritical annealing. In
the carbonenriched interior of the austenite pool beyond the epitaxial ferrite only martensite forms in specimens annealed
at 760 °C but various mixtures of ferrite and cementite form in specimens annealed at 810 °C. The latter structures include
lamellar pearlite, a degenerate pearlite, and cementite interphase precipitation. All Nb is in solution in the austenite formed
at 810 °C, and therefore the low hardenability of the specimens annealed at that temperature is best explained by the effect
of low austenite carbon content. |
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