Effect of Rare Earth Additions on Stress Corrosion Cracking of 4340Steel.

摘要

The addition of rare earth elements was investigated as a method of improving the stress corrosion cracking resistance of high strength steels. The addition of cerium at levels of 0.20 and 0.30 weight percent had only a small effect on the stress corrosion cracking resistance of AISI 4340 steel heat treated to a yield strength of approximately 215 ksi (1480 MPa). The stress corrosion cracking threshold (K(Iscc)) in 3.5 percent sodium chloride solution at room temperature was about the same for the two cerium-bearing steels as it was for 4340 steel without cerium, ranging from 15 to 17 ksi sq. root in. The higher cerium (0.30%) material had longer failure times and lower average crack growth rates than the lower cerium (0.20%) material. The failure times and average crack growth rates for the steel without cerium could not be directly compared with those for the two cerium-bearing steels because of crack branching, which occurred only in the material without cerium. However, it was estimated that, in the absence of branching, the failure times for the non-cerium steel would be shorter and the average crack growth rates higher than those for the lower cerium steel. The cerium additions had no effect on the fractographic morphology of stress corrosion cracking, which was intergranular at low stress intensity levels, with an increasing proportion of dimpled rupture as the stress intensity level increased.