Strengthening Mechanisms in Low Carbon Lath Martensite as Influenced by Austenite Conditioning

Abstract

Low carbon lath martensitic microstructures are used in various steel products requiring high strength and toughness. These microstructures are conventionally produced through re-austenitizing and quenching followed by low or high temperature tempering. It is also possible to produce lath martensite through direct quenching immediately following thermomechanical processing. In this study, deformation below the austenite recrystallization temperature before quenching to form martensite was simulated through laboratory scale Gleeble processing of a 0.2 weight percent carbon ASTM A514 steel microalloyed with up to 0.21 weight percent niobium. Thermomechanical processing generally increases the dislocation density of the as-quenched martensite, which is sensitive to the austenite grain size before thermomechanical processing. The hardness of the thermomechanically-processed steels is generally greater than steels austenitized at comparable temperatures without deformation; this hardness difference is attributed to the increase in dislocation density and increased lath misorientation in the thermomechanically-processed conditions. The hardness is generally independent of prior austenite grain size for the thermomechanically processed conditions in contrast to conventionally austenitized and quenched conditions, which have a Hall-Petch correlation with austenite grain size. The strength increase of the thermomechanically processed conditions compared to the conventionally austenitized and quenched conditions is maintained after tempering. However, there is a larger drop in strength for small prior austenite grain sizes for both conventionally austenitized and quenched and thermomechanically processed steels. Overall, the strength of these lath martensitic steels can be directly related to dislocation density through a Taylor hardening model.

Slurry erosion of thermo-mechanically processed 13Cr4Ni stainless steel

“…Reduced prior austenite grain size of TMP specimens is responsible for refinement in its block size, which is reflected in finer lath martensite. Deformation resulted in higher dislocation density, which enhanced the nucleation kinetics for martensite formation [35] and thereby suppressed the austenite retention. Instead, some amount of ferrite is observed in all the TMP specimens possibly because of less severe air quenching employed in the present work.…”

Strain hardening behaviour of as-quenched and tempered martensite

“…This is a relatively simple method for line profile analysis and may not reflect the detailed evolution of dislocations during straining. More advanced line profile methods such as the modified Williams-Hall and Warren-Averbach methods [21,54,55] and the convolutional multiple whole profile method [38,39,56] should be used if the evolution of dislocation density and arrangement is the main interest.…”