Fracture toughness improvement of lower bainite in ultra high-strength low-alloy steels

This study investigated the toughening mechanisms of lower bainite and established a relationship between microstructure and fracture toughness in an ultra high strength low alloy steel. Firstly, toughness was studied as a function of heat treatment parameters. It was then related to the microstructural parameters that controlled fracture toughness of the steel.;With low transformation temperatures, the steel showed a strong intergranular fracture tendency in room temperature tests. As the transformation temperature was increased from 220 to 320;Increasing austenitizing temperature and bainite transformation temperature simultaneously imposed a stronger effect on fracture toughness. With austenitizing temperatures above 900;Microstructural analysis showed that toughness increased with prior austenite grain diameter until grain growth temperature was reached. Toughness improvement from the increase of bainite transformation temperature, on the other hand, arose from the reduction of carbon supersaturation in bainitic ferrite due to the increase in carbide volume fraction.;A fracture mechanics model was developed to account for the effect of prior austenite grain size and bainite lath width on toughness. The energy needed to cleave a bainite lath and to tear a bainite lath boundary as well as the energy required to rupture an austenite grain boundary were also embraced.;For a comprehensive understanding of the relationship among phase transformation, microstructure, and fracture toughness, this work also studied overall bainite transformation kinetics. It applied a modified Johnson-Mehl-Avrami equation to the collected isothermal transformation data and revealed a relationship between the curve shape constant and the transformation temperature. Using this relationship, the temperature ranges for lower bainite, upper bainite, and pearlite transformations were determined. The validity and accuracy of the determinations were subsequently confirmed by microscopic analysis.;The coupling of the microstructure characterization and the fracture toughness comparisons rationalized the toughness ranking of the microstructures. Lower bainite had higher toughness than tempered martensite. Duplex martensite/bainite structures with bainite matrix were inferior to bainite, while those with martensite matrix were less tough than quenched and tempered martensite, especially when the bainite was transformed at temperatures below 280...