Variation in Morphology and Kinetics of Granular Bainite with Welding Thermal Cycles in High-Nb Fire-Resistant Steel: Experiments and Theoretical Calculations

Abstract

The microstructure and kinetics of granular bainite (GB) were investigated in novel fire-resistant steel containing high niobium and subjected to simulated welding thermal cycles. The dilatometric data were analyzed to determine the activation energy and Avrami exponents. It was observed that austenite transformed to GB over a wide range cooling rates of 0.5-100 K/s. Theoretical calculations indicated that Avrami exponent (n) decreased with increased transformed volume fraction ($f_{b}$) and cooling rate. At a moderate cooling rate of 20-6 K/s, bainitic ferrite as sheaves formed from austenite and the corresponding Avrami exponent was 2 < n < 3. Martensite in martensite/austenite constituent formed with Avrami exponent n ~ 1. At the fastest cooling rate of 50 K/s, lath bainitic ferrite and film-like retained austenite (RA) were obtained with the corresponding Avrami exponent n ~ 1. At lower cooling of 1 and 0.5 K/s, plate-like bainitic ferrite and block-like microphase composed of lamellar RA and ferrite were obtained with Avrami exponent n > 3. The local activation energy for austenite transformation to bainitic ferrite decreased from ~ 198 to ~ 182 kJ/mol with increasing $f_{b}$ from 0.05 to 0.95, while the local activation energy for austenite transformation to martensite decreased from ~ 80 to ~ 71 kJ/mol with increasing $f_{b}$ from 0.05 to 0.95.