Abstract
The quenching and partitioning (Q&P) process for heat treatment of steel has previously been shown yield to good combinations of strength and ductility owing to the presence of martensite and austenite. Interface mobility has been discussed in previous literature, mostly related to local driving forces. The present work considers the migration of the martensite/austenite interface in two steels (containing CMnSiMo or CNiSiMo). Experimental data show clear evidence of interface migration in the CMnSiMo steel during partitioning treatments at temperatures between 200 and 400°C for times ranging from 30 to 1000 s; conversely, the interface in the CNiSiMo steel was stationary during the same partitioning treatments. The different behaviours observed are considered in the context of differences in interface mobility and driving forces, and it appears that interface crystallography differences could influence the partitioning behaviour of Q&P steels.
Highlights
Quenching and partitioning (Q&P) is a thermal process aimed at stabilising retained austenite.[1,2,3,4] The process involves an initial partial or full austenitisation, quenching to a temperature between the martensite start and finish temperature to create a controlled fraction of martensite, followed by a ‘partitioning step’ to allow carbon to diffuse from martensite into the remaining austenite
The austenite fractions and austenite carbon content estimated from X-ray diffraction (XRD) are shown for both steels in Fig. 1 as a function of partitioning time (Pt)
The higher partitioning temperatures resulted in a greater increase in the austenite fraction for the partitioning times examined
Summary
Quenching and partitioning (Q&P) is a thermal process aimed at stabilising retained austenite.[1,2,3,4] The process involves an initial partial or full austenitisation, quenching to a temperature (typically .100uC) between the martensite start and finish temperature to create a controlled fraction of martensite, followed by a ‘partitioning step’ to allow carbon to diffuse from martensite into the remaining austenite. The present work utilised relatively highly alloyed Q&P chemistries that allowed for direct quenching from austenitisation temperatures to room temperature (instead of some elevated quench temperature) while maintaining significant fractions of austenite. The steels were characterised after this quenching step using a variety of techniques (e.g. SEM/EBSD and XRD), subjected to a partitioning treatment (e.g. reheated to 200–400uC for 30–1000 s) and cooled back to room temperature and characterised again.
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