Microstructural Basis of Alloy Design for High-Strength, Tough Structural Steels

摘要

Optimum mechanical properties may be obtained in medium carbon steels with microstructures consisting of dislocated lath martensites in which laths are surrounded by films of retained austenite. Direct studies of the chemical composition of the retained austenite phase, using the experimental technique of atom probe microanalysis, show that the austenite is substantially enriched in carbon and a large peak in carbon concentration is found at the austenite/martensite interface even in the as-quenched condition. Carbon partitioning depleting the martensite matrix must occur during transformation. Although the enrichment of austenite by carbon provides some chemical stability, mechanical stabilization must also be a major factor. The austenite may be destabilized by tempering or by plastic deformation. The relationship between these results and the strength and fracture toughness properties of a range of Fe-Cr-0.3C steels containing Mn or Ni are presented, together with recent results of in-situ studies of fracture in a high voltage electron microscope. 6 figures, 3 tables. (ERA citation 08:010234)