Competitive mechanisms occurring during quenching and partitioning of three silicon variants of 0.4 wt.% carbon steels

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurring during partitioning either in parallel or in succession. Three 0.4 wt.% carbon steels with varying Si contents were quenched to 150 degrees C for similar to 20% untransformed austenite, and partitioned for 10-1000 s in the temperature range 200-300 degrees C. The steel with low Si content (0.25 wt.%) exhibited substantial bainitic transformation during partitioning at 300 degrees C and only 4% retained austenite (RA) at room temperature (RT) even after 1000 s hold. In contrast, a high Si fraction (1.5 wt.%) enabled retention of similar to 18% austenite under similar conditions. While h-carbides precipitated within the martensite laths in the high-Si steel, cementite precipitated in the low-Si variant. Furthermore, carbide precipitation and growth were strongly suppressed by high Si content. Secondary martensite formation occurred from carbon-enriched austenite during final cooling, irrespective of Si-content. Results illustrate that Si retards austenite decomposition at higher partitioning temperatures but does not improve carbon partitioning at lower temperatures. (c) 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

Traditional quenching and partitioning (Q&P) treated steels are usually alloyed with silicon (Si) but the exact role of Si in microstructural mechanisms during partitioning process is not fully understood. This study combines dilatometric analysis with detailed microstructural characterization to reveal competing mechanisms during partitioning. Three 0.4 wt.% carbon steels with varying Si contents were quenched and partitioned at different temperatures, showing that Si content affects bainitic transformation, austenite retention, and carbide precipitation. Silicon retards austenite decomposition at higher temperatures but does not improve carbon partitioning at lower temperatures.