Effect of Intercritical Temperature on the Structure Property Correlation of Multiphase High-C Spheroidized Steel

Abstract

The present investigation deals with the development of multiphase steels combining spheroidal carbides and bainite in a ductile ferrite matrix. An attempt is made to get a promising combination of high strength and ductility through changes of microstructure by heat treatment. A high-carbon (0.61 wt.%) and high-silicon (1.71 wt.%) spring steel (EN45) was annealed to obtain an initial ferrite pearlite microstructure. The samples were given 10% cold working followed by holding at a temperature just below Ac1 for 180 min. Then the samples were held at intercritical temperatures of 770 and 800 °C for different durations varying from 10 to 30 min for partial re-austenitization followed by quenching in a salt bath kept at 350 °C and holding there for 10 min for bainite transformation. The samples were finally water quenched. The heat-treated samples were characterized by optical microscopy, x-ray diffraction, and scanning electron microscopy. The effects of intercritical temperature and holding time on the microstructure and mechanical properties were studied. With more bainitic transformation, the strength values went up considerably with a compromised elongation. The best combination of tensile strength (~805 MPa) with high elongation (~28%) was obtained. Finally, structure property correlation was established.