High-chromium steels alloyed with nitrogen

Conclusions

• 1. The addition of 0.5% N to high-chromium steel produces an economically alloyed corrosion-resistant high-strength steel suitable for operation at moderately low temperatures. The steel consists of austenite+ferrite or austenite alone.

• 2. The best combination of properties of high-strength corrosion-resistant high-chromium steel alloyed with nitrogen in amounts exceeding its normal solubility is obtained with ≥3–5%N. Both the strength and corrosion resistance increase with rising nitrogen concentrations.

• 3. Hexagonal nitride Cr₂N is precipitated during air cooling of the nitrogen-containing steel after heating at 900–1200°. In nickel-free steel of the Kh25A type, especially in the presence of molybdenum, colonies of eutectoid pearlitelike structure consisting of thin interlayers of Cr₂N and austenite are found along with boundary precipitates of Cr₂N. In steels of the Kh25N3(5)AM3 type a similar structure is formed only with furnace cooling or after tempering at 750–800°.

• 4. The corrosion resistance and strength of these steels depend not on the ratio of ferrite and austenite but largely on the nitrogen concentration and the form in which it exists (solid solution, excess phase). Formation of pearlitelike structure lowers the ductility, fracture toughness, and corrosion resistance to a larger extent than formation and precipitation of nitrides with the usual petallike lamellar shape.