The Microstructure Characterization and Mechanical Properties of Quench & Partitioning High Si Steels
Date Issued
2016
Date
2016
Author(s)
Tsai, Yu-Ting
Abstract
The core idea of Q&P processing was proposed by Matas and Hehemann in 1960: during tempering, carbon can diffuse from supersaturated martensite to adjacent austenite. However, carbon partitioning on industrial applications was not proposed until 2003, when Edmonds, Speer and Matas indicated that stabilization of austenite can be achieved by carbon partitioning. They also proposed Q&P methodology, which determine the optimal quenching temperature for this process. However, the measurements on retained austenite volume fraction significantly deviate from predicted fractions, indicating insufficient understanding on Q&P processing. The most important cause is that microstructure is not static after partial martensite formation, and instead, austenite decompositions still occurs. Therefore, this thesis is focused on the phase transformation after partial martensite formation. In this research, the microstructures and the mechanical properties of high Si steels after quench and partitioning (Q&P) processing were studied. Low carbon grades and a high carbon steel were used, and one-step and two-step Q&P heat treatments were respectively conducted for investigation on the resulting microstructures. It was found that for low carbon alloys, during one-step Q&P processing, significant austenite decomposition occurs, and the decomposition rate is faster than bainite transformation. the microstructures after one-stage Q&P are composed of tempered coalesced martensite, lower bainite, upper bainite and interface migrations. Therefore in previous literature, the Q&P optimization methodology does not take isothermal transformation into account, leading to deviation in the predicted austenite amount. For high carbon alloy, during partitioning stage, bainite transformation occurred. The presence of martensite drastically accelerated the formation of bainite by an order of magnitude, and changed the bainite morphology. Conducting Q&P heat treatment is currently the only available method to maintained the strength of nanostructure while accelerate bainite transformation. To study the microstructural evolution during Charpy impact test which develpes tri-axial stress in nanostructured bainite, TEM observation and synchrotron radiation XRD were conducted. It was found that in the fracture surface, only martensitic structure remained and austenite disappeared. The results indicated that crack propagation lead to martensite formation, and brittleness results; in contrast, during split Hopkinson pressure bar compression, twinning deformation is dominant. For Q&P specimens, Charpy results showed no improvement in impact toughness, but split Hopkinson pressure bar shows that ductile fracture was possible even at very high strain rate.
Subjects
Steel
Transmission electron microscopy
Quench and Partitioning (Q&P)
TRIP steels
Type
thesis
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