Effect of Ni Addition on Bainite Transformation and Properties in a 2000 MPa Grade Ultrahigh Strength Bainitic Steel

Abstract

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.

Effects of Nb on Bainite Transformation Behavior and Mechanical Properties of Low‐Carbon Bainitic Steels

“…The morphology of ductile tearing was observed in Figure a, whereas cleavage facets existed in Figure b. It is well known that brittle fracture yields cleavage facets and leads to an inferior tensile toughness . In contrast, ductile fracture causes ductile tearing and provides a high tensile toughness.…”

“…The morphology of microstructure and tensile fracture and energy dispersive spectroscopy (EDS) analyses were conducted on a Nova 400 Nano field emission scanning electron microscope (FE‐SEM). The Image‐Pro Plus software (Media Cybernetics, Rockville, MD, USA) was utilized to measure the sizes of prior austenite grains in different specimens. In order to determine the volume fractions and carbon contents of retained austenite (RA) in different samples, the X‐ray diffraction (XRD) analysis was carried out on an Empyrean diffractometer (equipped with an unfiltered Co‐Ka radiation source) at an operating voltage of 35 kV and a current of 50 mA.…”

Transformation Behavior and Properties of Carbide‐Free Bainite Steels with Different Si Contents

“…It can be observed that both the rate and the final bainite transformation amount decreased with increasing Si content when the specimens were austempered at 380 and 430 °C. Figure represents the curves of time–temperature‐transformation (TTT) for three tested steels, which were calculated by the JMatPro7.0 software . The increase of Si content makes the “ C ” curves of bainite starting transformation move to right, indicating that increasing Si content prolongs the incubation period for bainitic transformation.…”

“…It is known that brittle fracture mode leads to cleavage facet, meaning the inferior tensile toughness. Oppositely, ductile fracture results in ductile tearing, implying the high tensile toughness . The fracture surface of Si‐1 steel consisted of more amount of cleavage facets, while more amount of ductile tearing morphology appeared in the specimens of Si‐2 and Si‐3 steels.…”

“…After the corresponding heat treatments, a Nova 400 Nano field emission scanning electron microscope (FE‐SEM) was used to determine the morphologies of microstructure and fracture. The Image‐Pro Plus software (Media Cybernetics, Rockville, MD, USA) was utilized to count the volume fractions of bainite according to bainitic morphology and grayscale. To distinctly distinguish different transformation products, the micrographs with 5000× and 10 000× magnifications were taken.…”