Циклическая трещиностойкость высокопрочных сталей, склонных к водородному охрупчиванию, в коррозионной среде

Abstract

High-strength steels (yield point 500 MPa and above) have long been used in deepwater technology. Nowadays they are becoming increasingly popular in shipbuilding and offshore engineering for certain highly-loaded structural members. Stronger steel means higher absolute stresses in structure, so it becomes necessary to analytically estimate fatigue strength, not only high-cycle, but low-cycle, too, where fatigue crack growth stage comprises a considerable share of service life. Presently, literature offers only scarce data on fatigue cracking toughness of these steels because until quite recently these materials were required only for certain specific applications. Meanwhile, this kind of materials features a number of peculiarities mostly related to the effect of corrosive environment upon their behavior: specifically, hydrogen embrittlement that might be caused only by static tension load but by cyclic loading as well. This paper presents the experimental data on how high-strength steel proneness to hydrogen embrittlement (the main mechanism of corrosive cracking under stress) affects fatigue crack growth rate in corrosive environment. The studies were performed on the specimens of Cr-Ni-Mo-Va steel alloy (rated yield point at least 780 MPa). It is shown that steels with greater susceptibility to corrosive cracking under stress feature 2–2,5 higher growth rate in corrosive environment. In extreme cases, fatigue crack toughness curves not taking into account potential susceptibility of material to hydrogen embrittlement and actual loading rate might lead to considerably overstated strength estimates. The study also yielded limit coefficients for fatigue cracking toughness curves that enable sufficiently conservative assessments.