| The microstructure, crystallography, mechanical properties and fractography of martensite and isothermally transformed lower bainite in a 0.3%C-3%Cr-2%Mn steel have been studied and compared in order to (i) design a strong, tough martensitic steel, (ii) characterize the lower bainitic structure and (iii) examine its applicability for heavy-gauge, high strength uses.; In the as-quenched condition, martensite of the present steel consists of heavily dislocated laths containing fine auto-tempered carbides and bounded by thin films of retained austenite. Similarly, lower bainite after 10 minutes of isothermal holding at 360(DEGREES)C consists of dislocated bainitic ferrite laths, unidirectional intralath carbides and interlath films of retained austenite. However, when the isothermal holding is prolonged to one hour, retained austenite has decomposed into interlath carbides in a similar manner to that observed in martensite temperated at 240-400(DEGREES)C. Crystallographic studies have revealed that bainitic ferrite laths are related to adjacent laths as well as to parent austenite through the same orientation relationships as those found for martensite laths, which strongly confirms the shear aspect in the bainite transformation. Bainitic carbides, identified as cementite, are found to have the Isaichev orientation relationship with ferrite, which supports the idea that cementite has formed directly from austenite. The lower bainitic structure is more similar to that of martensite tempered in the tempered martensite embrittlement (TME) range rather than as-quenched martensite.; These microstructural characteristics have been reflected in the mechanical properties. The as-quenched martensite exhibits a high strength and toughness, the latter being greatly enhanced by tempering at 200(DEGREES)C. The strength and toughness of lower bainite are substantially lower than those of as-quenched martensite, and they remain relatively unchanged upon tempering up to 400(DEGREES)C. Fractographic examinations indicate the similarities in the fracture mode between TME and lower bainite; in both cases, the absence of retained austenite films and the presence of coarse carbides appear to cause brittle fracture.; These results show that lower bainite is not favorable for high strength-toughness applications as compared to martensite which has achieved a high strength-toughness combination. |
