Study Of C+N Enhancing Alloying Austenitic Steel And Its Mechanical Behaviours

The mechanical and fatigue behaviours of two new C+N enhancing alloyingaustenitic steels (CNEASs) for railway crossings under high speeds and heavy loads wereinvestigated by tensile, Charpy impact, sliding wear tests and low and high cycle fatigue,in comparison with the traditional Hadfield austenitic steel.Mechanical testing results showed that the main plastic deformation mechanism ofthe CNEASs was deformation twinning due to its low stacking fault energy (SFE). Theenhanced strength and plasticity resulted from the large amounts of ultra-finenano-twinning that occurred during plastic deformation, while the oversaturation effect ofnitrogen on CNEASs further increased the hardness and work hardening capacity.Contrary to the increased ductile to brittle transition temperature (DBTT) caused bynitrogen in austenitic steel, the new studied steel exhibited a lower DBTT as compared tothe Hadfield steel, because the combined alloying with C+N enhanced the metalliccharacter of the interatomic bonds, increasing the fracture-resistance under cryogenictemperatures. Sliding wear tests showed that abrasive wear dominated the wear behaviourof the CNEASs. The remarkable improvement of wear resistance in the steels enhancingalloyed with C+N, particularly at high temperatures, was attributed to the formation ofthick tribo-oxides on the worn surface under the effect of Cr and nitrogen.Low cycle fatigue results showed that at the lowest amplitude immediate cyclicsoftening occurred while with the increase of amplitudes initial cyclic hardening in thefirst ten cycles followed by softening to a saturate stress platform in CNEASs. Hadfieldsteel displayed a long period of cyclic hardening, then softening to fracture at all the totalamplitudes. It can be concluded from the analysis of hysteresis loop that the decreasedeffective stress of CNEASs with the prolonged cycles was due to the broken short rangeorder between nitrogen and Cr and decreased stacking fault energy. The results of highcycle three-bending fatigue which is dominated by elastic strain manifested that C0.6N0.3steel possessed a higher fatigue strength compared to Hadfield steel which were817and660MPa, respectively. Therefore, these results demonstrated that the experimental steels possessed a bettermechanical and fatigue properties as compared to Hadfield steel. This makes the new steelan ideal material for railway crossings under high speeds and heavy loads.