The effect of gradient order on impact toughness of carbon S38C axle steels with induction hardening

•Instrumented Charpy impact test was performed for a gradient steel structure of railway axles.•Gradient orientation in the S38C steel plays a crucial role in the impact behavior.•The gradient material with coarse grains to fine grains shows an excellent damage tolerance.•Hardening treatment at both inner and external surface are suggested to create a new structure.