Fig. 1 Curve of the temperature as a function of time during thermal simulation using a Gleeble 3800 apparatus

Fig. 2 SEM image showing the scratch lines of approximately 20 μm in length across GB, marked by a FEI NOVA 200 Nano Lab SEM/FIB

Fig. 3 Plot of compressive stress as a function of time at 550 °C showing the stress relaxation

Fig. 4 Schematic drawing of the specimen, 0.5 mm in thickness, for in situ tensile testing

Fig. 5 Initial microstructures of T24 following heat treatment. a Optical micrograph. b TEM micrograph. The MX precipitates at lath boundaries are indicated by arrows

Fig. 6 EBSD image showing the microstructure of T24 following thermal simulation. The ferritic matrix (red color) and retained austenite (γ phase, green color) were primarily distributed along lath bainitic ferrite boundaries and the prior austenite GBs

Fig. 7 TEM micrographs of T24 following thermal simulation. aBright field image showing the M/A islands at GBs. bBright contrasted phases in dark field image are the retained austenite with a typical size of approximately 200 nm

Fig. 8 Microstructures of the specimen following stress relaxation testing at 550 °C for 30 min. aBright field image, showing the ferrite and retained austenite. The white frame schematically indicates the area for electron diffraction pattern. b Enlarged image of the white frame of a, showing the morphology of Fe3C at GB and dislocation slipping. c Corresponding dark field image of a showing the morphology of retained austenite. dDark field image showing a Fe3C particle at GB. e Overlapped electron diffraction pattern originated from the framed area in a. The austenite, ferrite and Fe3C were indexed. The (\(1\bar{1}2\)) reflection from the austenite was selected for taking dark field image c

Fig. 9 Microstructures of specimen following stress relaxation testing at 550 °C for 90 min. a Fe3C located at GB. b Dislocation slipping, blocked by a Fe3C particle

Fig. 10 Values of Vickers hardness and microhardness. a Simulated CGHAZ and M/A islands at GB without annealing. b Simulated specimen following annealing at 550 °C for 400 h

Fig. 11 SEM image showing the straight shaped and continuous scratch lines, as indicated by the white arrows, following stress relief testing at 550 °C for 30 min. This implies that no GB sliding occurred

Fig. 12 SEM images showing the crack initiation a and propagation b, as indicated by the arrows, adjacent to a triple point junction during in situ tensile testing at room temperature. The specimen was annealed at 650 °C for 10 h

Fig. 13 A schematic drawing showing the cracking mechanism. a Morphology of the retained austenite adjacent a triple point junction. b The retained austenite decomposes into ferrite and M3C carbide. M denotes Fe, Cr, Mo, Nb or V. c Dislocations are initially generated within the relatively soft ferrite. Subsequent dislocation slipping is impeded by the M3C adjacent the triple point junction, resulting in dislocation pile-up and stress concentration at the triple point junction