Cold resistance of low-carbon Mn−Ni−Mo−V steels for pressure vessels

Abstract

1. The growth in the managanese content in low-carbon Mn−Ni−Mo−V steels from 0.86 to 2.3% increases their strength and diminishes their ductility and toughness. There is a threshold manganese content above which the impact toughness (cold resistance) drops intensely. For a heattreated steel this threshold corresponds to 1.4% Mn; for a normalized steel it is 1.5% Mn. 2. The growth of the concentration of nickel from 0.46 to 3.0% increases the strength of the steel. The growth of the nickel concentration to 3.0% at 0.6–1.0% Mn does not affect the ductility parameters of the steels and at 1.3–1.4% Mn causes their decrease. 3. As the nickel content increases from 0.46 to 2.9% (at 1.3% Mn), the impact toughness at a negative temperature increases monotonically. At 1.4% Mn, a nickel addition in an amount of up to 2.5% does not affect the impact strength at a negative temperature. At 2.7–2.9% Ni, it decreases. 4. The reduction of the carbon concentration from 0.12 to 0.035% and that of aluminum to 0.016% causes a decrease in the impact toughness, especially at a negative temperature. The introduction of 0.04–0.06% Al into steels with 0.12 and 0.035% C increases the impact toughness at all the test temperatures. The decrease in the impact toughness is connected with the development of the process of nitrogen segregation in the solid solution, and its growth is caused by the formation of aluminum nitrides and the respective reduction of the nitrogen content in the solid solution. 5. The reduction of the carbon content in Mn−Ni−Mo−V steels to 0.08–0.12% and the use of quenching with tempering as the base heat treatment provides a material with enhanced strength, ductility, and cold resistance suitable for pressure vessels.