Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures

Abstract

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.

Experimental and Modelling Research on the Effect of Prior Ferrite on Bainitic Transformation in Medium-Carbon Bainitic Steel

“…The microstructural morphology of the three specimens is shown and compared in Figure 4. The bainite transformation kinetics of the specimen austempered at 300 • C are reported in the literature [37]. When the volume fraction of prior ferrite in the intercritical zone increases from 0 to 9.6%, the incubation time (completion time) of the bainite transformation decreases from 1364 s (5.3 h) to 687 s (1.7 h).…”

Development of Air‐Cooled Carbide‐Free Bainite/Martensite Multiphase Steel: Transformation Kinetics, Microstructure, and Mechanical Properties

“…[7] Hence, lowering transformation temperature is used to refine CFB microstructure and reduce the amount of blocky RA, but it makes the transformation time too long due to extremely slow bainitic formation kinetics at low temperature. [11] In this context, aircooled bainitic steels have been developed to save energy and reduce CO 2 emission during manufacturing process. [12][13][14][15] It is reported that the air-cooled bainitic steels exhibit high strength and toughness when the alloying is appropriately designed.…”

Investigation of the Thermodynamic Transformation and Rare Earth Microalloying of a Medium Carbon-Medium Alloy Steel for Large Ball Mill Liners

“…Extensive studies and diverse processing approaches have been carried out to alter the microstructure of alloy steel and improve its impact resistance, stiffness, wear resistance, and strength [7][8][9][10][11][12]. Abbasi et al analyzed the microstructural features of three different types of medium carbon steel (namely, MnCrB-, NiCrSi-, and NiCrMoV-bearing steels) and found that the addition of small amounts of alloying elements and varying austenitic grain sizes were mainly responsible for the changes in the microstructure and mechanical properties of the steel [13].…”