The Effect of Test Temperature on Deformation Microstructure and Fracture Mechanisms in CrMn High-Nitrogen Steels Alloyed (0-3 wt.%) with Vanadium

Abstract

A temperature dependence of the tensile mechanical properties, microstructure and fracture mechanism of high-nitrogen Fe-(19-23)Cr-(17-21)Mn-(0-3)V-(0.1-0.3)C-(0.5-0.9)N vanadium-free and vanadium-containing steels was investigated. For all steels, the 0.2% offset yield strength and strain-hardening drastically increase with a decrease in test temperature. This is associated with high interstitial solid solution strengthening of the steels and more pronounced twinning and stacking-fault formation during straining below room temperature. For the vanadium-free steel, a ductile-to-brittle transition was evaluated: at 77K specimens destroy by cleavage mechanism while at room temperature steels show ductile fracture. Vanadium-alloying provides a particle strengthening of the steels and, at the same time, reduce solid-solution strengthening. Increase of vanadium concentration fully or partially suppress brittle fracture of the steels at 77K. Particle strengthening changes interstitial solid-solution effect, dislocation arrangement and slip/twinning relation in vanadium-containing high-nitrogen steels compared to vanadium-free one.

On Temperature Dependence of Microstructure, Deformation Mechanisms and Tensile Properties in Austenitic Cr-Mn Steel with Ultrahigh Interstitial Content C + N = 1.9 Mass.%

“…This mechanism was proposed for low-temperature deformation regime of vanadium-alloyed Fe-Cr-Mn-N,C and Fe-Cr-Mn-Ni-N,C steels in our recent research [35]. Alloying of HNSs with 1.5% vanadium increases strength and ductility of these steels due to formation of fine-grained structure and precipitation hardening with VN and V(C,N) phases [35][36][37][38]. Moreover, vanadium increases the solubility of nitrogen in steels [9] and ultrahigh-interstitial CrMn steels with the concentrations of C + N = 1.2 mass.% and C + N = 1.9 mass.% have been previously designed [39].…”

“…Therefore, this C/N-ratio was assumed for design of the steel. Vanadium content was chosen on the basis of our previous study [36]. The initial 5-kg steel billet was produced by induction melting and casting in air atmosphere using standard laboratory equipment.…”

On the Superplastic Deformation in Vanadium-Alloyed High-Nitrogen Steel

“…The average grain size was 94 µ m. Steel microstructure also contained large particles of vanadium-based nitrides/carbonitrides, which were homogeneously distributed both in grain bodies and along grain boundaries. These coarse particles are peculiar for V-HNSs, and they do not dissolve during solid-solution treatment [25][26][27]. The details on the elemental composition of these V-based nitrides are described previously in [27].…”

“…During high-temperature tensile deformation in a temperature interval from 850 to 1000 • C and strain-rate range from 4 × 10 −4 s −1 to 6 × 10 −3 s −1 , this microstructure demonstrated the characteristics of superplastic flow: elongation in the interval 400-900%, strain-rate sensitivity exponent m = 0.40-0.49, grain boundary sliding mechanism. The maximum elongation to failure (900%) was reached at deformation temperature 950 • C and strain rate 4 × 10 −4 s −1 .precipitation hardening [24][25][26][27]. Despite the visible advantages of the high-nitrogen austenitic steels as a construction material, there is limited data related to the possibility of realization of superplastic flow in them.…”

“…precipitation hardening [24][25][26][27]. Despite the visible advantages of the high-nitrogen austenitic steels as a construction material, there is limited data related to the possibility of realization of superplastic flow in them.…”