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Lecture

Investigation into the thermal mechanical properties, hydrogen embrittlement and fracture toughness of quenching and partitioning (Q&P) sheet steels

Tuesday (27.09.2016)
11:30 - 11:45
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The effect of temperature on tensile behavior of Q&P steels was investigated due to the importance of safety security and formability of Q&P high strength steels. Two different sample statuses of cold rolling (CR) and galvanizing (GI), were studied in microstructures, tensile tests, hydrogen embrittlement and fracture toughness. The average grain size of Q&P-CR steel was smaller than that of Q&P-GI steel and the volume fraction of retained austenite (RA) of the former was larger than the latter. At lower temperature (-60 oC~0 oC), the Q&P-CR and Q&P-GI exhibited similar tensile strengths which decreased with increasing temperature but the total elongations of both steels kept nearly unchanged. When temperature changed between 0~300 oC, the elongation of Q&P-CR and Q&P-GI had a minimum at 200 oC. The strength and elongation of both steels reached maximum values at 300 oC, while at higher temperature, the mechanical properties were dramatically deteriorated. The difference between the two steels resulted from the microstructure and carbon partitioning. By measuring the volume fraction of austenite, it was indicated that the mechanical properties of the Q&P steels were related to the stability of austenite, which had an optimal stability at the deform temperature range, about 300 oC. At this deformation temperature, the strength and ductility of the Q&P steel reached an optimal condition. Hydrogen embrittlement (HE) susceptibility was investigated in respect of the austenite serving as a type of effective hydrogen traps with optimized stability. The HE susceptibility of Q&P-GI steel was better than Q&P-steel. Meanwhile, the fracture toughness KIC was measured by double edge-notched tension (DENT) specimens with fatigue pre-cracks on the Q&P steels. Crack extension force, GIC, evaluated from the measured KIC, was used to analyze the role of retained austenite in different fracture behavior. Stability of retained austenite was found to determine the toughness of Q&P steels containing appreciable amount of retained austenite, which demonstrated martensite transformation under deformation and eventually changed the fracture behavior. The results of the current work can be used to optimize the microstructure from effective heat treatments of Q&P-treated sheet steels for toughness improvement.

Speaker:
Ph.D. Lianbo Luo
Shanghai Jiao Tong University
Additional Authors:
  • Dr. Wei Li
    Shanghai Jiao Tong University
  • Prof. Xuejun Jin
    Shanghai Jiao Tong University