The Effects of Nickel and Nitrogen on the Stress Corrosion Cracking Susceptibility of Austenitic Stainless Steels in H₂SO₄-NaCl Solution

Abstract

Stress corrosion cracking (SCC) susceptibility was investigated by stress relaxation tests in a H₂SO₄-NaCl solution for a series of vacuum melted 18%Cr-xNi stainless steels (x=8∼30%Ni) and SUS 316 steels with varying nitrogen contents.
As the nickel content of 18%Cr-xNi steels was increased, the time to failure increased adruptly and SCC was not observed in the steels containing 20∼30%Ni for 2500 h. The time to failure of SUS 316 steels containing 0.03∼0.18%N slightly decreased with increasing nitrogen content.
Among the deformation structures only a strain induced α′-martensite reveals a magnetic effect. Magnetic measurements were, therefore, performed to examine the existence of α′-martensite. The permeability of the steels containing 8∼11%Ni became higher with an increase in the degree of prestraining. The same behaviour was not observed on other steels. The time to failure of SUS 316 steels, which did not show any change in permeability, depended on the degree of prestraining. Transmission electron microscopic observations showed the existence of ε-martensite in SUS 316 steels prestrained 10% in elongation. With the increased nickel content, the deformation structures of 10% prestrained specimens changed successively in the order of α′ and ε-martensites (8%Ni, 11%Ni), ε-martensite (13.87%Ni), band structure (14.06%Ni), bundle structure (20%Ni) and cellular structure (30%Ni).
It is thought that the addition of nickel in austenitic stainless steel increases the stacking fault energy and makes the formation of strain induced martensite difficult. This should decrease the transgranular SCC susceptibility of the high Ni steels in a H₂SO₄-NaCl solution.