|
|
|
|
Stress corrosion cracks propagate over a range of velocities from about 10-3 to 10 mm/h, depending upon the combination of alloy and environment involved. Their geometry is such that if they grow to appropriate lengths they may reach a critical size that results in a transition from the relatively slow crack growth rates associated with stress corrosion to the fast crack propagation rates associated with purely mechanical failure. This transition occurs when the stress intensity, which is a function of the geometry of the component, including the crack size, reaches the fracture toughness value for the material concerned i.e. when: (reference)
where Kc = fracture toughness, σ = stress, c = crack length and ry = length of the plastic zone associated with the crack. In practice, because of the relaxation and the dimensions of the component, stress intensity equations are somewhat more complex, but the principle is not altered. The previous equation leads to the concept of a critical crack length, Ccr, in terms of the stress intensity, Kscc, below which stress corrosion crack propagation does not occur, such that:
where X = a factor dependent upon geometry, and σy = yield strength.
Whether or not a stress corrosion crack will grow to reach the critical size for fast mechanical fracture will depend, among other factors, upon the source of the stress that initiates cracking. If this is a fabrication stress, such as a residual welding stress, or if it derives from misalignment of fastener holes, crack propagation may well result in stress relief and the crack may cease to propagate if the crack tip stress intensity falls below Kscc before Kc is reached. On the other hand, if the source of the stress that drives the slow stress corrosion crack is derived from the operating conditions, the crack tip stress is not likely to relax and catastrophic failure eventually will occur. Fortunately, operating stresses are frequently below the minimum stress required for stress corrosion cracking and most instances of stress corrosion failure arise from the presence of stresses of yield stress magnitude left in structures as the result of fabrication procedures.
The exceptions are usually pressure vessels, such as chemical reactors, high pressure gas transmission lines and steam boilers, and it is probably not insignificant that the incidence of stress corrosion failure has increased considerably over the last two decades as engineering design efficiency has improved, involving higher operating stresses and higher yield strength materials, and as the problems of corrosion spread relatively uniformly over exposed surfaces have been largely overcome, resulting in the possibility of more localized forms of corrosion.
The exact alloy composition, microstructure and heat-treatment can have a marked effect on SCC performance. There are few general rules governing the influence of material strength on SCC susceptibility. For hydrogen embrittlement processes a higher strength normally increases the susceptibility; additionally, higher strength materials generally have a low KIC, and therefore fail by fast fracture with a smaller SCC crack. Processes that rely on plastic strain at the crack tip will be easier for lower strength materials. Hence, many SCC systems, such as caustic cracking of carbon steels, will become more susceptible as the strength decreases.(reference)
