Abstract
An evaluation method for bainite transformation kinetics was established by theoretical derivation, dilatometric curve analysis, and microstructure observation. The isothermal transformation of ultra-fine bainite under different austenitizing temperatures and contents of prior martensite was studied using a DIL805L dilatometer. The kinetic parameters (activation energy Q*, autocatalytic factor λ, temperature rate constant κ, unit volume transformation rate, and the number density of nucleation sites Ni) of ultra-fine bainite transformation under different austenitizing temperatures and contents of prior martensite were calculated based on the displacement growth bainite dynamics model. It was found that the autocatalytic factor λ is linear with the austenite grain size d, and the number density of nucleation sites Ni is closely related to the average volume of the bainite subunit Vb. Moreover, the formation of prior martensite and its increase can increase the number of nucleation sites and the nucleation rate of the ultra-fine bainite; thus, the ultra-fine bainite transformation can be accelerated.
Highlights
Ultra-high-strength steel is widely used in lightweight automotive parts, armor steel plates, and some wear-resistant materials
The mechanism for accelerating the bainite transformation using this parameter has the following four main parts: (1) the phase transformation strain introduced by the preformed martensite [15]; (2) preferred nucleation sites provided by the increased martensite phase interface [16,17]; (3) reduced interfacial energy introduced by martensite [18]; and (4) dislocations introduced by martensitic transformation promoting bainite transformation [19]
The phase transformation kinetics model based on the bainite displacement growth mechanism was used to calculate the activation energy, nucleation rate, and nucleation density of ultra-fine Bainite transformation, which can provide a theoretical basis for the austenite grain size, primary martensite, and its content in accelerating the mechanism of ultra-fine bainite transformation
Summary
Ultra-high-strength steel is widely used in lightweight automotive parts, armor steel plates, and some wear-resistant materials. Caballero and Bhadeshia et al [1,2,3] developed a high-carbon, silicon-rich, ultra-fine bainitic steel which exhibits excellent strength and tensile ductility. This kind of steel, containing nanoscale lath-like carbide-free bainite, has become a focus for many researchers. The phase transformation kinetics model based on the bainite displacement growth mechanism was used to calculate the activation energy, nucleation rate, and nucleation density of ultra-fine Bainite transformation, which can provide a theoretical basis for the austenite grain size, primary martensite, and its content in accelerating the mechanism of ultra-fine bainite transformation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
