A new steel heat treatment process has been developed to achieve strength comparable to Advanced High Strength class of steels. This process is unique for its very short treatment time (<10s) and is hence termed Flash Process. The strength of the steel obtained from this process (ultimate tensile strength (UTS): 1694 MPa, elongation: 7.1%) showed at least 7% higher UTS and 30% greater elongation than published results of martensitic advanced high strength steel (UTS: 1585 MPa, elongation: 5.1%). Arc welding of flash processed steel resulted in softening in the heat affected zone, which could be a potential site for crack initiation. In order to devise better welding procedures to join this steel, the microstructure evolution during flash processing has to be understood.
The main goal of this thesis is to explain the formation of the flash process microstructure, which governs the strength of the steel. In order to achieve this goal the task was divided into two parts. In the first part, the thermal profile of the flash process is quantified. The second part focuses on the characterization of the microstructure in the final steel. Based on the results obtained from the two analyses, the microstructure evolution is explained. This work forms a basis for future research on the weldability of the steel produced from this process.
Initial confirmatory tests were carried out to verify the properties of the flash processed steels. The steels showed high strength (UTS > 1500 MPa) and good ductility (elongation > 8.7%). Thermocouples were attached to the steel plates to measure the thermal profile of the process. The total time of the process was recorded to be around 6s with a peak temperature of over 1000°C. Analysis of the heating curve indicated complete austenitization with Ac1 and Ac3 temperatures lying in the ranges 925- 930°C and 1045-1052°C respectively and a very short dwell time (< 2s) in the austenite region. Analysis of the thermal profile provides an insight into the microstructure evolution during the flash process.
The initial microstructure of the steel consisted of ferrite with carbides. Optical images of the flash processed steel showed very fine features that could not be clearly identified. Scanning electron microscopy of the final steel also indicated the presence of these carbides. This pointed to incomplete carbide dissolution during the process. On further analysis, transmission electron microscope images showed the presence of bainite in the steel, along with martensite.
The presence of carbides shown by microstructural characterization and the short austenite dwell time from thermal analysis, indicate a non homogenous carbon distribution in the austenite phase. This inhomogeneity is related to the formation of a mixed microstructure of bainite and martensite.
Finally, the high strength observed in the flash process steel is rationalized by modifying published theoretical models on the strength of steel. It is shown that a mixed microstructure of bainite and martensite is much more stronger than completely martensitic steel.