Stress corrosion cracking of high strength steel under cyclic environmental exposure

The effects of cyclic wet (3.5% NaCl aqueous solution) and dry (ambient) environments on the stress corrosion cracking (SCC) characteristics of quenched and tempered AISI 4340 steel were studied. A contoured double cantilever beam (CDCB) specimen was used to maintain a constant stress intensity value during testing. A completely computerized MTS testing system controlled and recorded the applied constant load, the crosshead displacement, the crack mouth opening displacement, and the cyclic exposure as a function of time.; In all cases, the crack growth velocity during the wet exposure cycle was greater than that during the dry exposure cycle or a constant exposure. The crack growth rate was non-uniform in all cyclic environmental exposures. A typical crack growth process in an alternate exposure cycle may be divided into two stages. The wet portion of the cycle is considered as stage I, and the dry cycle is considered as stage II. In the initial couple of minutes of each wet exposure cycle of stage I, the average crack growth exhibited the fastest rates, approximately one order of magnitude higher than that in the later wet exposure. The constant crack growth implies a dynamic equilibrium between the mechanical driving force and the electrochemical driving force. In stage II, the crack growth slowed or stopped and increased oxidization occurred. Then, constant crack growth occurred one order of magnitude slower than that for stage I.; Changes in cyclic periods were estimated and: (i) constant crack growth as a: function of time is not observed, (ii) the crack growth is dependent on stress intensity, (iii) there is a critical number of wet cycles per unit time above which the crack growth rates are faster than constant exposure, and below which crack growth rates are slower than constant exposure.; Chevron patterns appeared on the fracture surface of AISI 4340 tested under a constant wet environment, while striation patterns appeared in the cyclic wet and dry exposure. Scanning electron microscope (SEM) fractographs of the cyclic SCC showed the combination of microcleavage, intergranular, and plastic tear fractures. The application of focused ion beam (FIB) imaging and the transmission electron microscopy (TEM) lift-out techniques to SCC failure analysis extended the understanding of the microfracture on a 3-D scale. TEM and FIB micrographs revealed microcleavage cracking across martensite laths and along the martensite lath boundaries. The fracture mechanism of the SCC observed in this study is believed to be both hydrogen embrittlement and dissolution.