Tempering of Lath Martensite in Low and Medium Carbon Steels: Assessment and Challenges

This paper is a directed review of the microstructures and the mechanisms, by which they are formed by tempering of lath martensite in low and medium carbon steels. As-quenched martensitic structure is first reviewed, then low-temperature tempering (LTT) that produces ultrahigh strength and good toughness, and high-temperature-tempering (HTT), which can produce excellent combinations of strength, ductility, toughness, and good resistance to hydrogen embrittlement in sour gas and oil environments, are covered. It is well known that hardness decreases continuously with increasing tempering temperature, but the superimposed multiple mechanisms of microstructural changes by which the softening and its retardation occur during various stages of tempering have received little attention on the integrated scales of microstructure, substructure, and nanostructure. Thus, earlier descriptions of the reasons for softening need deeper examination. In particular, the reasons for the changes in the very fine crystal size and high dislocation density of as-quenched lath martensite produced by high temperature tempering, sometimes producing a low dislocation density, un-recrystallized, fine lath-morphology ferritic grain size, sometimes producing an equiaxed, recrystallized ferritic grain size, are not clear. Deformation behavior and mechanical properties as related to the residual components of martensitic microstructure that survive tempering are discussed. Questions remain and the use of newer techniques, such as Electron Back Scatter Diffraction and atom probe tomography, in addition to light microscopy, transmission electron microscopy, Mossbauer effect spectroscopy, and Mechanical Spectroscopy used in characterizing quench and tempered structures are noted.