Bainite

About: Bainite is a research topic. Over the lifetime, 9520 publications have been published within this topic receiving 145305 citations.

A Novel Observation on Cementite Formed During Intercritical Annealing of Medium Mn Steel

Abstract: A medium Mn steel with the martensitic microstructure was rapidly heated at a high rate of 300 K/s to the intercritical temperature of 923 K (650 °C) and then isothermally hold for 5 minutes. Although cementite should dissolve above the Ae1 temperature due to the constraint of thermodynamics, it is surprising to find cementite particles after such intercritical annealing (IA), whose Mn contents and sizes are even up to 30 wt pct and 60 nm, respectively. Numerical simulations have been performed to reveal the mechanism responsible for this new observation. They indicate that a small nucleus of cementite in martensite could rapidly grow to the observed size before austenitization takes place during IA. Such a rapid growth is driven by the rapid partition of C from martensite into cementite. It is then concluded that the precipitation of cementite during the IA of martensitic Mn-alloyed steel appears inevitable no matter how high the heating rate is. Moreover, the growth kinetics of cementite depend on the composition and size of its neighboring martensitic phase, rather than those of cementite nucleus.

Tempering characteristics of a vanadium containing dual phase steel

Abstract: Dual phase steels are characterized by a microstructure consisting of ferrite, martensite, retained austenite, and/or lower bainite. This microstructure can be altered by tempering with accompanying changes in mechanical properties. This paper examines such changes produced in a vanadium bearing dual phase steel upon tempering below 500 °C. The steel mechanical properties were minimally affected on tempering below 200 °C; however, a simultaneous reduction in uniform elongation and tensile strength occurred upon tempering above 400 °C. The large amount of retained austenite (≅10 vol pct) observed in the as-received steel was found to be essentially stable to tempering below 300 °C. On tempering above 400 °C, most of the retained austenite decomposed to either upper bainite (at 400 °C) or a mixture of upper bainite and ferrite-carbide aggregate formed by an interphase precipitation mechanism (at 500 °C). In addition, tempering at 400 °C led to fine precipitation in the retained ferrite. The observed mechanical properties were correlated with these microstructural changes. It was concluded that the observed decrease in uniform elongation upon tempering above 400 °C is primarily the consequence of the decomposition of retained austenite and the resulting loss of transformation induced plasticity (TRIP) as a contributing mechanism to the strain hardening of the steel.

Effects of carbon equivalent and cooling rate on tensile and Charpy impact properties of high-strength bainitic steels

Abstract: The effects of carbon equivalent and cooling rateon tensile and Charpy impact properties of high-strength bainitic steels were investigated. Eight steel plates were fabricated with varying C, Cr, and Nb additions under two different cooling rates, and their microstructures, tensile, and Charpy impact properties were evaluated. Volume fractions of microstructural components present in the steels increased in the order of granular bainite, acicular ferrite, bainitic ferrite, and martensite as the carbon equivalent or cooling rate increased, which resulted in decreased ductility and upper shelf energy and increased energy transition temperature in spite of increased strength. In the steels containing about 50 vol.% of bainitic ferrite and martensite, the tensile strength was about 900 MPa, while the elongation and upper shelf energy were about 20% and 200 J, respectively. In order to achieve the best combination of tensile strength, ductility, and upper shelf energy, e.g., 860–900 MPa, 20%, and 200 J, respectively, granular bainite, and acicular ferrite were produced by controlling the carbon equivalent and cooling rate, while about 50 vol.% of bainitic ferrite and martensite were maintained to keep the high strength.