A Novel Approach for Analytical Description of the Isothermal Bainite Transformation in Alloyed Steels

Abstract

A novel approach for modeling the bainite transformation in alloyed steels during isothermal quenching is presented. The approach is based on the application of the logistic function with the logarithmic argument of the isothermal holding time. The proposed model involves two time-independent fitting parameters, a and b, where are specified for the given isothermal quenching temperature and other conditions (e.g., steel grade, austenite grain size, etc.). The adequacy of the modeling results is estimated using the sum of squared differences between the experimental and calculated bainite fraction formed at every time step Δτ during isothermal quenching. The logistic function was found to provide a fit between the experimental and calculated bainite transformation kinetics that was up to ten times better than that of the conventionally applied Kolmogorov–Johnson–Mehl–Avrami equation. An excellent agreement is achieved between the modeling results and the experimental data for commercially produced high-strength alloyed steels 300M, HY-TUF, and D6AC isothermally quenched in the temperature range of the bainite transformation. The obtained temperature dependences of the model parameters a and b reveal the potential for further theoretical investigation and verification of their physical meaning.

Microstructure and Mechanical Properties of the Heat Treated Hy-TUF Steel

“…The decrease of the austempering temperature (to 370 °C) led to the increase of the impact strength up to 0.75 J/cm 2 due to the presence of the lower bainite. However the tensile strength was reduced from 1400 MPa (at 430 °C) to 1180 MPa (at 370 °C) due to more complete bainite transformation at 370 °C (75 % of bainite) compared to the bainite transformation at 430 °C (65 % of bainite) [17]. Thus the amount of the "fresh" martensite formed during the final oil cooling from the austempering temperature was lower for the lower austempering temperature.…”

Влияние Термической Обработки На Механические Свойства И Микроструктуру Перспективной Высокопрочной Стали

“…Àíàëèç ïðåâðàùåíèé ïåðåîõëàaeäåííîãî àóñòåíèòà â óñëîâèÿõ íåïðåðûâíîãî îõëàaeäåíèÿ èëè ïðè èçîòåðìè÷åñêîé âûäåðaeêå îñóùåñòâëÿëè íà îñíîâàíèè ïîëó÷åííûõ äèëàòîìåòðè÷åñêèõ äàííûõ ñîãëàñíî ìåòîäèêå [13,14].…”

Microstructure of the Heat Treated Advanced Low Carbon Steel

“…The analysis of the experimental isothermal bainite transformation kinetics (Fig. 4) was performed using the logistical equation ( 2) [19,20]:…”