Comprehensive mechanical evaluation of heat-treated AISI 4140 (ETD 150): fatigue behavior and novel indentation-based characterization of fracture toughness and hardness with relevance to rail steels

• Spherical indentation better estimates K IC and hardness in “e.t.d” 150 steel. • “e.t.d” 150 shows comparable or superior strength and fatigue to some rail steels. • Indentation J-integral enables fast, non-destructive fracture toughness testing. • Multiple mechanical properties can be attained in one indentation cycle. This study investigates the fracture toughness (K JC ) and hardness of heat-treated, cold-drawn AISI 4140 alloy (“e.t.d” 150) alongside three representative rail steels (JP, EV, and CZ) to comparatively evaluate their mechanical performance. Importance is placed on “e.t.d” 150 to assess its potentiality as a viable alternative material for rail steel applications based on current insights to rail steel material recycling and substitution. Fracture toughness (K JC ) was assessed using a chamfered cylindrical flat-end and spherical indenter based on a novel virtual J-integral approach that minimizes the plastic J-integral component based on Irwin’s elastic solution, while hardness was obtained using only spherical indentation. A comparison between the K JC outcomes of both indenters are presented and discussed, showing preference to spherical indentation. The virtual J-integral approach with limit load analysis applied for estimating K JC in both indenters, incorporated stress triaxiality to account for pressure sensitivity and the hydrostatic pressure component in indentation testing. Results show that “e.t.d” 150 achieves fracture toughness and Brinell hardness values comparable to rail steels, particularly suitable for curved track sections where wear resistance and durability are critical. Fatigue analysis was also conducted for “e.t.d” 150, which confirms moderate-to-good resistance to rolling contact fatigue. These findings suggest that “e.t.d” 150 offers a reliable alternative for substituting conventional rail steels, with potential benefits for railway performance, safety, and maintenance cost reduction.