窒素吸収処理による20%Cr-Ni-Fe合金の材質改善について

耐熱材料における合金元素としての窒素の作用について-IX

抄録

Studies have been made on the formation of the nitrogen-bearing austenite in 20% Cr-Ni-Fe alloys containing nickel up to 10% by the authors' nitrogen-absorption method and the thermal behavior of the formed austenite. The corrosive resistivity and the elevated temperature spring property obtained by this treatment have also been investigated. The results obtained are as follows:
1) Austenite containing about 0.4% of nitrogen is formed in 20% Cr-Ni-Fe alloys by nitrogen-absorption treatment at 1250°C. for 4 hours. The depth of the nitrogen-bearing austenite zone from the surface is widened with increase of the nickel content of alloys, while the nitrogen content of this zone is lowered gradually as the nickel content is raised in the alloys containing 4% or more of nickel.
2) When the nickel content of the alloys is 2% or more, the nitrogen-bearing austenite is retained by the quenching in water. In the alloys containing 6% or more of nickel, no martensite is formed even by the subzero-treatment using the liquid oxygen.
3) In the nitrogn-absorbed alloys the lamellar phase is developed at grain boundaries during the aging at 700°C. The fine precipitants are also found within grains after the prolonged aging if the nickel content of alloys is higher than about 6%. The hardness of the aged alloys up to 780 hours is always kept to be higher than that of the alloys before the aging.
4) The hardness of the nitrogen-absorbed alloys in both water-quenched and cold-rolled conditions is appreciably higher than that of alloys before the absorption, and the softening temperature of the cold-rolled alloys on heating is raised markedly by nitrogen-absorption treatment.
5) The spring prbperty at elevated temperatures of the nitrogen-absorbed 20% Cr-Ni-Fe alloys containing 4% nickel is superior to that of 18-8 steel or 17-7 PH steel.
6) The corrosive resistivity of 20% Cr-Ni-Fe alloys against aqueous solutions of 65% HNO₃, 5% H₂SO₄ and 1% HCl in respective boiling conditions is improved remarkably by nitrogen-absorption treatment; the nitrogen-absorbed alloys containing 2-3% nickel is found to have nearly the resistivity equivalent to that of 18-18 steel.
7) The resistivity to intergranular corrosion of the nitrogen-absorbed alloys is superior to that of carbon-free 18-8 steel.