Influence of Quenching Temperature and Mn Content on the Microstructure Development of New Q&P Processed Martensitic Stainless Steels

Master thesis (2021)

Authors

B. de Bakker Mechanical Engineering

Contributors

Maria Jesus Santofimia Team Maria Santofimia Navarro - Mechanical, Maritime and Materials Engineering (supervisor 1)
G. Li Team Maria Santofimia Navarro - Mechanical, Maritime and Materials Engineering (supervisor 2)
Y. Gonzalez Garcia Team Yaiza Gonzalez Garcia - Mechanical, Maritime and Materials Engineering (supervisor 2)
C. Kwakernaak Team Maria Santofimia Navarro - Mechanical, Maritime and Materials Engineering (supervisor 2)
A. Smith Rina Consulting - Centro Sviluppo Materiali (supervisor 2)
Faculty
Mechanical Engineering
Copyright: © 2021 Benne de Bakker
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Published Date

19-03-2021

Language

English

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Faculty

Mechanical Engineering

Abstract

Quenching and Partitioning
(Q&P) is a novel steel heat treatment to create microstructures containing
martensite and retained austenite. In recent years, there has been a growing
interest in applying Q&P treatments to martensitic stainless steels with
the aim to create stainless steels with the mechanical properties of Advanced
High-Strength Steels (AHSS). The development of Q&P stainless steels for
lightweight structural applications could be a game changer for the automotive
industry. The application of Q&P stainless steels can increase the service
life of a car, reduce maintenance cost and contribute to lowering CO2
emissions. In this thesis, Q&P treatments with varying quenching
temperatures were applied to two novel stainless steel alloys with composition
0.2C-0.35Si-0.7Mn-12.5Cr and 0.2C-0.35Si-3.0Mn-12.5Cr. The effect of quenching
temperature and Mn content on the microstructure development was investigated
using dilatometry, X-ray diffraction (XRD), optical microscopy, electron
backscatter diffraction (EBSD), electron probe micro analysis (EPMA) and phase
field simulations. It was found that Mn addition lowers the optimal quenching
temperature and increases the maximum retained austenite fraction that can be obtained
by Q&P treatment. Austenite phase fractions of approximately 0.22 and 0.3
were stabilized in the microstructures of the low-Mn and the high-Mn alloy,
respectively. The carbon concentration of retained austenite increases with
decreasing quenching temperature but is much lower than expected from the full
partitioning assumption. The non-uniform distribution of primary martensite and
untransformed austenite as well as carbide precipitation was seen as a means by
which austenite enrichment and retention are reduced. Additionally, results
from this study indicate that segregation of Mn and Cr affects the local Ms
temperature and causes microstructural banding of primary martensite which
leads to a non-uniform retained austenite distribution in the final microstructure.