Quantitative prediction of crack growth depends on deriving data for the current density and the rate of refilming on the bared surface at the crack tip neither of which are directly measurable. The hydrostatic tensile stress field at a crack tip promotes hydride formation under conditions where they would normally be thermodynamically unstable as they have an increased volume per metal atom. This transition is particularly significant in the mechanism of impurity-atom embrittlement. The environment affects the rate of surface diffusion through enhancement of the rate of consumption of adatoms, but more importantly in this model through the effect of surface compounds on the surface diffusivity of the metal or alloy. The process has been referred to as hydrogen-enhanced local plasticity and the mechanism of failure involves shear localization. The 3D fracture area is larger than projected from 2D systems.
Thus, the fatigue crack propagation is primarily caused by the macroscopic coalescence of cracks, a phenomenon that depends on crystallographic characteristics , as well as the microscopic coalescence of the cracks initiated repeatedly in the vicinity of the tip of the main crack, as mentioned above. Crack initiation and propagation accompany fracture. The extent of sensitization of grain boundaries can be important. These processes are of course relevant to most mechanisms of stress corrosion cracking. Under conditions of localized strain, sufficient to rupture protective or partially protective films, oxidation of the bared metal surface will occur and the crack will advance because of the localized metal loss. Cracking then occurs when the local tensile stress is greater than the reduced cohesive force. For this reason, a post-weld heat treatment is needed before reinstalling stainless steel parts that have been welded.
Turnbull, in , 2001 3 Mechanisms of Stress Corrosion Cracking There have been numerous mechanisms and models proposed to explain and predict stress corrosion cracking Turnbull 1993. These ligaments are assumed to be formed between the cleavage fractured areas. The intergranular crack propagation did not stop, while the transgranular crack propagation was retarded by crack closure and strain aging. Corresponding to the reduced chromium content, the rate of refilming following a mechanical depassivation event at the crack-tip is decreased and so more dissolution occurs. They react with the carbon to form the corresponding carbides, thereby preventing chromium depletion. Cutting fluids are thus inhibited where necessary. The key feature is the very localized nature of the deformation process so that the macroscopic deformation remains small.
It is an attractive model in many respects, not least the correlation between the percolation threshold for sustained dissolution of the more active alloying element associated with a continuous connecting network and stress corrosion crack growth rates. Demonstrated in experiments by Newman et al. Prevention of Intergranular Corrosion How to prevent intergranular corrosion? The latter is considered to be induced by vacancy production generated by crack-tip dissolution. The fracture surface shown in consists of three parts, which are denoted as A, B, and C. However, the fatigue cracks branched to the grain boundaries, resulting in the further propagation of the main crack.
. Based on these concepts, Sugimoto et al. Grain-boundary diffusion kinetics are dependent on the presence of impurities on the boundary and so creep behavior of high temperature alloys like superalloys can be related to the segregation phenomenon. Stress applied at elevated temperatures creep , grain boundary precipitates, thermal treatment causing segregation at grain boundaries, and environmentally assisted weakening of grain boundaries can lead to intergranular fatigue. In ceramics, interganular fractures propagate through grain boundaries, producing smooth bumpy surfaces where grains can be easily identified.
Relationship between pH and potential conditions for severe cracking susceptibility of carbon steel in various environments, and stability regions for solid and dissolved species. The second phase accelerates corrosion of the adjacent matrix by micro-galvanic corrosion; the applied stress opens the crack and allows propagation through the alloy. The results shows that relatively large amounts of energy are consumed in this process and that the energy consumption is increasing rapidly with increasing crack tip speed. Of greater practical importance is the fact that precipitates which may form during exposure of metals to elevated temperatures, for example, during production, fabrication, and welding, frequently nucleate and grow preferentially at grain boundaries. The adsorbed hydrogen atoms can enter the metal and will diffuse and localize in regions of hydrostatic stress and at microstructural sites hydrogen trapping such as grain boundaries, dislocations, matrix-particle interfaces. An example of high-resolution analytical electron microscope observations of all these effects is shown in Fig.
It was not possible, based on fractographic studies, to associate the creep mechanism at 982 C with either of those observed at the intermediate temperatures. The collapse process of these ligaments is numerically examined. In order to retain crack geometry the rate of the anodic reaction on the adjacent walls of the crack must be sufficiently low relative to the rate of growth of the crack. As a result, intergranular cracking was the dominant cause of fatigue damage in the steel. The toughness of ferrite is decreased with decreasing temperatures but, being present only as a constituent of a two-phase alloy, this not does not produce as sharp a ductile-to-brittle transition as in ferritic stainless steels.
Therefore, carbon-related phenomena such as carbide formation and carbon segregation are the important factors in fatigue of steels. The thermodynamic stability of the films can give an insight into the domains of solution chemistry and potentials for which cracking might be expected as shown Jones 1987 in the potential—pH diagram of Fig. These processes are of course relevant to most mechanisms of stress corrosion cracking. Dislocations emitted from crack tips normally blunt the crack and inhibit cleavage, inducing ductile behavior. The fracture process proceeds by hydride formation in the stress concentrated region at the crack tip, cleavage cracking of the hydride, crack arrest at the hydride—matrix interface, and a cyclic repetition of the above events. There are many examples of hydrogen embrittlement in which high resolution microscopy reveals evidence of localized plasticity, for example shallow dimples on the fracture surface. For instance, in terms of the microstructure, we found that the existence of a cementite morphology has a marked effect on the small fatigue crack propagation behavior of ferritic steels as well as on their fatigue strength.
Austenitic and ferritic steels are affected and in all cases a large proportion of intergranular cracking is observed. Namely, the fatigue limit is determined by threshold remote stress for the fatigue crack propagation rather than that for the fatigue crack initiation. The suppression of crack initiation by strain age hardening even at higher stresses highlights an important aspect of the intergranular crack initiation mechanism, namely, that intergranular fatigue cracks are initiated by the localization of plastic strain in the vicinity of grain boundaries and not by stress concentration at grain boundaries. For stainless steels, the intergranular attack is associated with highly localized deformation, and for the nickel alloys, selective oxidation of chromium and formation of a Ni-rich layer by dealloying. Mechanisms of Intergranular Corrosion What causes intergranular corrosion? The higher the ferrite content, the sharper the ductile-to-brittle transition. When the emitted dislocations stay near the crack tip sessile dislocations , they do blunt the crack but brittle cleavage can occur after the emission of a sufficient number of dislocations.
This usually occurs when the phase in the grain boundary is weak and brittle such as cementite in iron's grain boundaries. Derlet, Intergranular Fracture in Nanocrystalline Metals. Static tests were carried out for periods extending to 111 days. Solutes like hydrogen are hypothesized to stabilize and increase the density of strain-induced vacancies, leading to microcracks and microvoids at grain boundaries. The thermodynamic stability of the films can give insight into the domains of solution chemistry and potentials for which cracking might be expected as shown Jones, 1987 in the potential-pH diagram in Figure 6. Both mechanisms have their protagonists but both as yet lack a predictive capability.