Carbide-free tempered martensite in Fe[sbnd]C steels

Quenched martensite is very strong but brittle. Thus, tempering is necessary in order to improve the ductility. However, carbides are formed during tempering even with the addition of silicon and aluminum. These carbides can serve as the nucleation sites for voids and cracks, which are detrimental to mechanical performance. This work provides a strategy to completely prevent carbide precipitation in martensite during tempering through Mn heterogeneous distribution between Mn-enriched austenite and Mn-depleted martensite. Probably due to the strong attraction between carbon and Mn, carbon atoms diffuse from Mn-depleted martensite into its neighboring Mn-enriched austenite and, in turn, carbide formation is inhibited. This Mn-heterogeneity can be produced by fast heating Mn-enriched carbides or Mn-enriched retained austenite. Additionally, the present work, for the first time, successfully applies this strategy to the quenching and partitioning process even without the addition of silicon and aluminum. The carbide formation is completely inhibited in the martensitic matrix, carbon atoms diffuse into austenite and the austenite is stabilized at room temperature. For these carbide-free Fe[sbnd]C martensitic steels, the product of strength and ductility can be improved due to the absence of carbide precipitation and the existence of retained austenite. The strategy for completely avoiding carbide precipitation during tempering in the martensitic matrix will promote the development of a new series of carbide-free Fe[sbnd]C martensitic steels.