Stress corrosion threshold

Effect of molybdenum content on stress-corrosion threshold stress intensity of austenitic stainless steels. Source Ref 166... 

Stress Corrosion Resistance. The stress corrosion threshold has been reported to be at least 80% of jfiTic except when it is stressed in the short transverse direction, where it is 70% oiKic. 

Clearly the variables that may influence or the threshold stress for initially plain specimens, and hence the susceptibility to stress-corrosion cracking, are P and rj, i.e. 

The implications of a significant role for strain rate are wider than the obvious one that stress corrosion should only occur over a restricted range of strain rates. Thus, in constant load tests, since cracks will continue to propagate only if their rate of advancement is sufficient to maintain the crack-tip strain rate above the minimum rate for cracking, it is to be expected that cracks will sometimes stop propagating, particularly below the threshold stress. Such non-propagating cracks are indeed observed below the thres-hold . Moreover, in constant-load or constant-strain tests, the strain rate diminishes with time after loading, by creep exhaustion if the stress remains sensibly constant, and it is found that the stress-corrosion results are sensitive to the relative times at which the stress and electrochemical... 

Fig. 8.2S Long-time constant-load tests demonstrating a distinct stress-corrosion cracking threshold stress in the case of a straight l3Cr martensitic SS as opposed to a nickel-bearing SS...

Fig. 8.93 Observed and calculated values of the threshold net section stresses for stress corrosion cracking of a C-Mn steel after various prior deformations and ageing treatments...

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Where, for example, chloride stress-corrosion cracking is a risk the process temperature becomes a critical variable. Thus it may be more economic to lower the process temperature to below 70°C, a practical threshold for chloride stress-corrosion cracking, than to incur the extra expense of using stress-corrosion cracking-resistant materials of construction. 

This behavior is characterized by a plateau region, which prevails above a definite threshold K. It is often referred to as stress-corrosion fatigue because SCC systems usually exhibit this behavior, and the most common theory assumes that the crack growth rate results from the addition of SCC, and pure fatigue crack advance. This is a type of... 

Stress-Corrosion Cracking. The conditions for SCC to occur are (i) a crack-promoting environment (ii) the susceptibility of material to SCC (iii) tensile stresses must exceed the threshold value. SCC is distinguished by the fact that the stress corrosion faces suffer very low corrosion, even in solutions that cause some damage to the free surfaces. As an example is the SCC of stainless steel at 200°C in a caustic solution or in aerated chloride solution where almost no traces of dissolution are visible on the crack faces (Figure 6.53).84,95 For example, SCC of metals has been by far the most prevalent... 

---