Development of Ultrafine Bainitic Structure in AISI 431 Stainless Steel

Document Type : Research Paper


1 Department of Materials Engineering, Malek- Ashtar University of Technology (MUT), Shahin-Shahr, Isfahan, Iran.

2 Maleke-ashtar University of Technology


The development of ultrafine bainitic structure in AISI 431 stainless steel was the goal of this study. For this purpose, the AISI 431 specimens were austenitized at 1100 °C for 60 min followed by low-temperature austempering treatment at different temperatures and times. Austempered samples were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and tension test. According to achieved results, the microstructure of AISI 431 steel after austempering treatment consists of ultrafine bainitic ferrite plates and retained austenite with two morphologies of micrometer-block and fine film. At austempering temperature up to 300 oC, the micrometer-blocky morphology of austenite was vanished completely from the microstructure. The maximum values of strength and ductility of about 1664 MPa and 9.5 % were achieved in austempered sample at 350°C for 24 h. By enhancement the austempering temperature and time, the strength and elongation percent were reduced simultaneously as a result of change in shape and size of bainite phase.


P. Olund, S. Larsson, T. Lund, ASM Proceedings: Heat Treating, ASM International, USA, 1998.
C. Garcia-Mateo, F. Caballero, H. Bhadeshia, Development of hard bainite, ISIJ Int. 43 (2003) 1238–1243.
C. Garcia-Mateo, F. Caballero, H. Bhadeshia, Acceleration of low-temperature bainite, ISIJ Int. 43 (2003) 1821–1825.
H.K.D.H. Bhadeshia, Bainite in Steels Transformations, Microstructure and Properties, Second ed., Institute of Materials, Minerals and Mining, London, 2001.
J. Cornide, C. Garcia-Mateo, C. Capdevila, F.G. Caballero, An assessment of the contributing factors to the nanoscale structural refinement of advanced bainitic steels, J. Alloys Compd. 577 (2013) S43-S47.
F.G. Caballero, M.K. Miller, S.S. Babu, C. Garcia-Mateo, Atomic scale observations of bainite transformation in a high carbon high silicon steel, Acta Mater. 55 (2007) 381-390.
X.Y. Long, J. Kang, B. Lv, F.C. Zhang, Carbide-free bainite in medium carbon steel, Mater. Des. 64 (2014) 237-245.
M. Nourbakhsh, M. Tavoosi, S.R. Hosseini, Preparation and mechanical properties of nano/ultrafine bainitic structure in AISI 52100 steel, International Journal of ISSI. 13 (2016) 31-39.
F.G. Caballero, M.K. Miller, C. Garcia-Mateo, Carbon supersaturation of ferrite in a nanocrystalline bainitic steel, Acta Mater. 58 (2010) 2338-2343.
F.G. Caballero,M.K. Miller, C. Garcia-Mateo, J. Cornide, M.J. Santofimia, Temperature dependence of carbon supersaturation of ferrite in bainitic steels, Scr. Mater. 67 (2012) 846-849.
J.H. Jang, H.K.D.H. Bhadeshia, D.W. Suh, Solubility of carbon in tetragonal ferrite in equilibrium with austenite, Scr. Mater. 68 (2013) 195-198.
S.S. Babu, S. Vogel, C. Garcia-Mateo, B. Clausen, L. Morales-Rivas, F.G. Caballero, Microstructure evolution during tensile deformation of a nanostructured bainitic steel, Scr. Mater. 69 (2013) 777-780.
C. Garcia-Mateo, F.G. Caballero, The role of retained austenite on tensile properties of steels with bainitic microstructures, Mater. Trans., JIM 46 (2005) 1839-1846.
C.W. Wegst, Stahlschlussel (Key to Steel), Verlag Stahlschliissel Wegst GmbH, 1992.
C. Garcia-Mateo, F.G. Caballero, C. Capdevila, C. Garcia de Andres, Estimation of dislocation density in bainitic microstructures using high-resolution dilatometry, Scr. Mater. 61 (2009) 855-858.