Improvement of the fatigue crack growth resistance in AISI 4140 steel under single- and multi-austempering heat treatments

Abstract

AISI 4140 steel under annealing conditions was subjected to single- and multi-austempering heat treatments. The mechanical performance of the compact tension (CT) specimens was evaluated via axial tensile tests and fatigue crack growth (FCG) tests. The single-austempered steel showed a significant increase in its tensile strength with a gradual reduction in the elongation as the temperature decreased. However, the multi-austempered steel had a higher tensile strength than the single-austempered steel at 362 °C and 412 °C. The FCG performance of the austempered steel at 362 °C had the highest number of cycles compared to those of the austempered steel at 312 °C and 412 °C; additionally, the multi-austempered steel underwent more cycles than those of the steel under single-austempering conditions at 312 °C and 412 °C. The bainitic structure with a larger martensitic austenite (MA) island and greater thickness of the retained austenite (RA) in the microstructure of austempered steel at 362 °C provided the optimum mechanical properties in terms of higher tensile strength and the highest fatigue crack growth resistance. The intergranular fracture in the pyramidal like-morphology of BF, the cracking of the larger MA islands, the more ductile fracture of RA, and the steel under multi-austempering conditions are the main reasons for the highest fatigue crack growth resistance compared to steel austempered at 312 °C and 412 °C.

Keywords

• AISI 4140 steel
• Austempering
• Bainite structure
• Fatigue crack
• Pyramidal surface fracture