Effect of Silicon Content on the Decomposition of Austenite in 0.4C Steel during Quenching and Partitioning Treatment

Abstract:

Although quenched and partitioned (Q&P) steels are traditionally alloyed with Si, its precise role on microstructural mechanisms occurring during the partitioning process is not thoroughly investigated. In this study, a systematic investigation has been carried out to reveal the influence of Si on austenite decomposition, phase transformation and carbide precipitation during Q&P treatment. Using a Gleeble thermomechanical simulator, three medium carbon steels with varying Si contents (0.25, 0.70 and 1.5 wt.%) were hot-rolled, reaustenitized, quenched into the M_s -M_f range, retaining about 20% austenite at the quench-stop temperature (T_Q), and held for 1000 seconds above T_Q in the temperature range of 200-300°C in order to better understand the mechanisms operating during partitioning. Dilatometric measurements combined with microstructural characterization using SEM-EBSD, TEM, and XRD clearly revealed the occurrence of various mechanisms. The effect of partitioning temperature/time on the hardness of the Q&P samples was correlated with the microstructural features. Steel containing low Si content (0.25%) was incapable of promoting carbon enrichment of austenite during partitioning, leading to its continuous decomposition into isothermal martensite and/or bainite without any detectable austenite retained even holding at 300°C. In comparison, 1.5% Si content promoted retention of about 19% austenite under similar Q&P conditions. Small fractions of bainite and high-carbon martensite formed during final cooling in both steels after partitioning at 200°C. Moreover, carbide precipitation was strongly retarded by high Si content.

• Periodical: Materials Science Forum (Volume 1016)
• Pages: 1361-1367
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.1016.1361
• Online since: January 2021
• Authors: Ilkka Herman Miettunen*, Sumit Ghosh, Mahesh Chandra Somani, Sakari Pallaspuro, David A. Porter, Jukka I. Kömi
• Keywords: Dilatometric Analysis, Quenching & Partitioning, Retained Austenite, Steel

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