Environmentally Induced Cracking of High-Strength Stainless Steel under Dynamic Loading Conditions and Hydrogen Evolution Behavior

摘要

The influences of dynamic loading conditions on the environmentally induced cracking have been investigated in a newly developed ultra-high-strength martensitic stainless steel, HSL180. The corrosive environment used was a 3 percent NaCl+ 3g/L NH_4SCN solution at 25 deg C. The tests were conducted at the free corrosion potential and the cathodically charged condition with a constant cathodic current of 1 mA/cm~2. The SSRT (slow-strain rate testing) strength at the free corrosion potential was slightly lower than the tensile strength obtained in laboratory air. However, under the cathodically charged condition, the SSRT strength decreased to about a half of the tensile strength. This decrease is larger in the notched specimens than in the smooth specimens. The K_(ISCC) value obtained at the free corrosion potential was about 10 MPa centre dot m~(1/2). The dynamic SC crack growth rates were accelerated at lower stress intensity, indicating that the small vibratory stresses superimposed at a high R promote the HE crack growth. The fatigue crack growth rate at a stress ratio R of 0.1 and a stress cycle frequency of 0.1 Hz was enhanced at the free corrosion potential compared with that in dry air. This crack growth acceleration was conspicuous when tested under a cathodically charged condition, or under a negative pulse waveform: the crack growth rate under the negative pulse wave form was larger than those under the sinusoidal waveform, in particular at low stress intensity factor range. The hydrogen evolution rate obtained by thermally desorption analyses was increased by the cathodic charging, with a lower temperature peak. The fracture surfaces were closely examined with a field emission type, high-resolution scanning electron microscope, and the fracture mechanisms are discussed.