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Effects of prior microstructure on quenched and partitioned steels
Smith, Douglas
Smith, Douglas
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2023
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Abstract
Quenching and partitioning (Q&P) produces microstructures of metastable retained austenite (RA) and tempered martensite through a process consisting of austenitization or intercritical (IC) annealing, quenching to produce a calculated fraction of martensite, and partitioning C from martensite to stabilize the remaining austenite. Prior microstructure effects on Q&P heat treatment were investigated in terms of microstructural length scale, chemical banding, and the presence of carbides in ferrite/pearlite, martensitic, and finely spheroidized prior microstructures. Because the austenitization or IC annealing step at the beginning of the Q&P process is the only opportunity to reduce chemical banding and dissolve carbides, prior microstructure effects on Q&P were primarily investigated in terms of microstructural development during this step and on-cooling to the initial quench temperature (QT). Three experiments were developed to understand:
1) The effects of prior microstructures and Cr/Nb additions on austenitization rate and cementite dissolution during IC annealing and full austenitization prior to Q&P.
2) High-temperature microstructure conditions after IC annealing and full austenitization and how they affect ferrite formation on-cooling at rates of 12, 35, and 100 °C∙s-1 to the QT.
3) How high-temperature microstructure conditions lead to local differences in microstructural development during Q&P processing as a function of QT.
Through these experiments it was found that fine microstructural length scales and fast carbide dissolution led to rapid austenite formation and less variation in the high temperature microstructures before Q&P processing. In terms of bulk alloy composition, Cr additions dramatically slowed cementite dissolution and solute redistribution, providing control over chemical heterogeneity in the high temperature microstructures through choice of prior microstructure and annealing conditions before Q&P. Understanding these chemical heterogeneities through modeling and experimental observations, it was found that ferrite formation on-cooling was most rapid in regions with low C, Mn, and Cr contents. IC ferrite retained from the prior microstructures led to extensive ferrite growth on-cooling due to a reduced requirement to nucleate fresh ferrite.
In terms of Q&P processing, ferrite formation on-cooling and chemical heterogeneity in the high-temperature microstructures led to local differences in martensite start temperature (Ms). These differences affected microstructural development during the quenching and partitioning steps of Q&P heat treatment. In regions of the microstructure with low Ms temperatures, blocky RA and martensite/austenite constituent (MA) were identified after Q&P due to limited martensite formation during the initial quench. In regions with high Ms temperatures, however, extensive martensite formation during the initial quench led to microstructures of tempered martensite and film-like RA after Q&P. These results illustrated that annealing conditions could be designed to create predictable distributions of austenite stability prior to Q&P.
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