Effects of short-time tempering on mechanical properties and fracture of 4340 steel

Abstract

Steel is commonly processed via quenching the high temperature austenite phase to form martensite, a microstructure that exhibits high strength with limited ductility and toughness. As-quenched martensite is typically tempered to achieve varying combinations of strength and ductility/toughness, where strength decreases and ductility/toughness increases with higher tempering temperatures and longer tempering times. However, a phenomenon known as tempered martensite embrittlement (TME) produces a decrease in impact toughness at room temperature in the tempering range of 200 to 400 C for typical tempering times such as 1 hour (3600 s). Tempered martensite embrittlement is also manifested through an increase in the ductile to brittle transition temperature (DBTT). Rapid heating and tempering via induction heating has the potential to improve toughness for tempering temperatures ranging from approximately 500 – 700 C; however, limited efforts have been focused on applying short-time tempering for high-hardness applications at lower tempering temperatures within the regime of TME. The present work focuses on the effect of rapid tempering on mechanical properties within the TME tempering regime. Microstructural and mechanical property effects of short-time (1, 10, 100 s) tempering compared to conventional (3600 s) tempering at equivalent tempering parameters are investigated in the present study. Toughness results indicate an improvement in both ductile-to-brittle transition temperature and room temperature toughness with decreasing tempering time at an equivalent tempering parameter. In addition to overall improved toughness, the TME “trough” is observed to diminish with short time tempering when compared to longer tempering times within an equivalent tempering parameter regime. Tensile tests reveal decreased yield and tensile strengths, and increased reductions of area for shorter tempering times within an equivalent tempering parameter regime. When comparing the same strength level, the trends of increasing room temperature impact toughness and increasing reduction of area with shorter tempering times remain applicable. The observed changes in properties suggest that short-time tempering may offer the potential for desirable strength‑toughness combinations. Some time-temperature combinations utilized in the present study fall within regions where tempering stages overlap, or regions where the classical tempering stages (i.e. stage I, II, and II) may operate out of sequential order when iso-tempering curves are considered in the context of characteristic diffusion distance calculations. Greater diffusion distances are shown to be associated with longer tempering times at a given tempering parameter (for mechanisms associated with stage I and II tempering) when tempering parameter and characteristic diffusion distance are compared. Hardness is not a complete indicator of time-temperature equivalence, and additional fundamental understanding of time‑temperature relationships is needed to describe microstructures and properties produced by quenching and tempering (Q&T).