Effect of nitrogen on the structure and mechanical properties of steels in the system Fe-Mn-Cr-V-C
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Alloying steels of the system Fe-Mn-Cr-V-C with nitrogen promotes austenite grain refinement after austenitizing, an increase in strength, and a reduction in ductility properties. With ∼0.6% C the relative elongation and reduction of area are at a maximum, which is due to occurrence of deformation twinning.
During aging carbon-nitrogen steels are strengthened at a slower rate than for carbon steel, which is due to preparation of increased σ5 and ψ with quite a low level of strength. Steels of the optimum composition type 60G14Kh9AF2 (0.2% N) after aging have the following mechanical properties: σf =1350 N/mm2, σ0.2,= 980 N/mm2,σ5 =40% ψ=40%.
Aging of unstable carbon-nitrogen steels increases the stability of austenite towards strain-induced martensitic γ→α-transformation.
Partial substitution of carbon with nitrogen causes a slow down in the austenite grain growth rate and an increased stability of the steel structure towards overheating. Nitrogen promotes an increase in solubility of substitution alloying elements in solid solution during austenitizing which during subsequent aging prevents formation of coarse excess phases along grain boundaries. This provides preparation of high ductility and toughness properties.
Carbon-nitrogen steel compared with carbon steel after austenitizing with the same ductility is stronger, but after aging with the strength it is more ductile. The optimum austenitizing temperature for steel 60G14Kh9AF2 is 1200°C.
KeywordsNitrogen Growth Rate Mechanical Property Austenite Solid Solution
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