Microstructure and Mechanical Properties of Quenched and Nonisothermally Partitioned Low‐Alloy Steels

The current study examines the microstructure and mechanical properties of two low‐alloy steels processed through quenching and nonisothermal partitioning (Q&NP) treatment. Toward this, continuous cooling and Q&NP treatments are initially performed in a dilatometer to understand the phase transformations. The Q&NP experiments reveal an additional transformation in dilatation curves during the initial stages of nonisothermal partitioning, which corresponds to bainite/fresh martensite, followed by fresh martensite up to the end of the partitioning process. In addition, Q&NP treatments are performed on large size samples in muffle and salt bath furnaces to evaluate tensile and Charpy impact toughness properties. The dilatometer samples do not show retained austenite at any of the quench temperature (QT), whereas retained austenite is observed in the larger size samples, which might be due to the formation of bainite during initial quenching to the QT in the salt bath. Despite the formation of bainite, QT dependent retained austenite fraction in large Q&NP samples follows the trend calculated by the optimum QT models. The Q&NP treated samples exhibit increased tensile elongation (≈85% for steel 1 and 400% for steel 2) and Charpy impact toughness (≈390% for steel 1 and ≈250% for steel 2) compared to water quenched samples.