Fig. 1. (a) and (b) dilation-temperature curves during cooling and isothermal holding for above- and below-Ms austempering, respectively; (c) and (d) dilation-time curves during isothermal holding for above- and below-Ms austempering, respectively; (e) temperature-time-transformation (TTT) curves for bainite transformation measured from the dilation curves; (f) a comparison of the dilatation curves of the specimens isothermally held at Tiso =420 °C and 350 °C, with the curve of the 350 °C specimen shifted towards right and down to overlap the curve of the 420 °C specimen at the end of PM formation. PM: pre-formed martensite, B: bainite, Ms: martensite start temperature, Tiso: isothermal temperature.

Fig. 2. Optical micrographs of the specimens isothermally held at (a, c) 420 °C and (b, d) 365 °C for different times. PAGB: parent austenite grain boundary, PM: pre-formed martensite, Bs: bainite sheaf.

Fig. 3. Typical SEM and TEM images of the specimens austempered at (a, c) 420 °C and (b, d) 365 °C. Bs: bainitic sheaf, M/A: martensite/austenite block, PM: pre-formed martensite, Bf: bainitic ferrite, Af: film-like retained austenite.

Fig. 4. (a) Volume fractions of various constituting phases austempered at different temperatures. PM: pre-formed martensite, Bf: bainite ferrite, FM: fresh martensite, RA: retained austenite. (b) Statistic distributions of bainitic lath thickness in the specimens austempered at 420 °C and 365 °C.

Fig. 5. (a) SEM image of the specimen austempered at 420 °C; (b) EBSD IQ map, (c) phase map and (d) IPF map of the same area outlined in (a). White and black lines indicate the boundaries of bainite packets and blocks, respectively. In (c) green color indicates the bcc phase. (e) <001> bcc pole figure obtained from the EBSD data of bainitic laths in a PAG shown at the left of the pole figure, and (f) theoretical <001> pole figure of bainite variants inside an austenite grain satisfying the K-S orientation. M/A: block of fresh martensite and retained austenite. Yellow arrows in each figure point to large bainite blocks.

Fig. 6. (a) SEM image of the specimen austempered at 365 °C; (b) IQ map, (c) phase map and (d) IPF map of the same area of (a). White and black lines indicate the boundaries of bainite packets and blocks, respectively. In (c), green and red colors indicate the bcc and fcc phases, respectively. (e) <001> bcc pole figure obtained from the EBSD data of bainitic laths in a PAG shown at the left of the pole figure, and (f) theoretical <001> pole figure of bainite variants inside an austenite grain satisfying the K-S orientation. (g) Extracted SEM image, IQ map and IPF map highlighting the area of a single PM band and its adjacent bainite, and (h) the distribution of the misorientation angle along the dotted yellow arrow line in (g). PM: pre-formed martensite, Bs: bainite sheaf.

Fig. 7. (a) Bright-field and (b) dark-field TEM images of the specimen austempered at 365 !!!!!!°C; (c) and (d) Diffraction patterns corresponding to regions 1 and 2 in (a), respectively. PM: pre-formed martensite, Bf: bainite ferrite, Af: austenite film.

Fig. 8. (a) and (b) Vickers indentation morphology; (c) and (d) statistic distribution of Vickers hardness values of the specimens austempered at (a, c) 365 °C and (b, d) 420 °C. (e) Kernel average misorientation (KAM) map of bcc phase in the same area as in Fig. 6(a) for the specimen austempered at 365 °C. Bs: bainite sheaf, PM: pre-formed martensite, M/A: block of fresh martensite and retained austenite.

Fig. 9. (a) Tensile stress-strain curves; (b) yield strength (σs), tensile strength (σb), uniform elongation (δu) and total elongation to fracture (δt) of the specimens austempered at various temperatures above and below Ms.

Fig. 10. (a) Impact load-displacement curves at typical above- and below-Ms austempering temperatures; (b) impact absorption energies and toughness of the specimens austempered at various temperatures above and below Ms.

Fig. 11. Equivalent strain distribution in the specimens austempered at (a, c, e) 420 °C and (b, d, f) 365 °C at different global strains: (a) and (b) 0.015 (c) and (d) 0.04; (e) and (f) 0.08. M/A: block of fresh martensite and retained austenite, Bs: bainite sheaf, PM: pre-formed martensite.

Fig. 12. (a) and (b) Variation of local equivalent strain versus global strain at different locations shown in Fig. 11(a) and (b) for the specimens austempered at temperatures of 420 °C and 365 °C, respectively. (c) and (d) Local equivalent strain versus global strain, along the dotted arrow lines shown in Fig. 11(a) and (b) for the specimens austempered at 420 °C and 365 °C, respectively.

Fig. 13. (a) Schematic of the evolutions of bainitic structure isothermally held at above- and below-Ms temperatures. (b) Variation of the volume fraction of retained austenite with engineering strain of the specimens austempered at various temperatures above and below Ms. Tiso: isothermal temperature, GB: parent austenite grain boundary, PM: pre-formed martensite, UA: un-transformed austenite, Bf: bainitic ferrite, M/A: martensite/austenite block; RA: retained austenite.

Fig. 14. (a) and (b) SEM cross-sectional images showing microvoids formed below but adjacent to the fracture surface of impacted specimens subjected to austempering at 420 °C and 365 °C, respectively. M/A: block of fresh martensite and retained austenite, PM: pre-formed martensite, Bs: bainite sheaf.