Crack initiation steel

Moderate cathodic protection can improve a corrosion fatigue performance of low strength steels. In the case of high strength alloy, cathodic protection may accelerate fatigue crack initiation. Steel specimens, AISI 4040 Rc 20), in 3% NaCl were stressed 10% above and below the fatigue limit. The specimens were maintained at... 

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

Times to failure for various stainless steels tested in MgClj have been shown to increase with increasing proportions of martensite present Perhaps the role of martensite under anodic dissolution conditions is comparable to that of ferrite in duplex stainless steels where the enhanced dissolution of one phase prevents crack initiation in the other. There is, of course, another aspect of martensitic transformation that should be mentioned, i.e. the transformation of austenite to martensite either in the bulk material or at a growing crack tip that can give increased susceptibility to... 

It has also been noticed that thicker corrosion films form on the martensite phase in cold worked steels than on the untransformed matrix, and thicker films can be more brittle and aid crack initiation . ... 

While a new tire may have excellent resistance to crack initiation and propagation between the steel belts, an aged tire of the exact same construction can exhibit dramatically reduced crack growth resistance, which in some cases may contribute to tire failure. To further underscore the point, the United States National Highway Traffic Safety Administration (NHTSA) determined that tire aging was a main contributor to the Firestone Wilderness AT tire recall [1]. Figure 34.1 shows the failure rate of Wilderness AT tires as a function of tire age (determined from date of tire manufacture). 

P. A. Klein and R. A. Hays, The Hydrogen Cracking Initiation Susceptibility of HY-I30 Steel Base Plates and Weldments in Marine Enviroments, (1989), David Taylor Research Center, Annapolis, MD. 

Figure 20. Improvement in lower critical stress as well as delayed cracking initiation introduced by REM additions even when the steel is down to 0.003% sulfur...

Figure 6.44 Section showing fretting damage and fatigue crack initiation in 0.2% C steel (Waterhouse)5...

Mechanisms of SCC. Crack initiation of EAC is complex and not well understood till now. Most of the SCC systems exhibit short initiation times ranging from minutes to weeks and cracking often occurs due to the change in the environment rather than to a very long initiation time. Stress-corrosion crack growth rates are usually 10 11 and 10-6 m s In systems such as stainless steels in chloride solutions, localized corrosion may create the local conditions prone to crack development, but it is still difficult to explain the initiation of the crack in the absence of localized corrosion in environmental conditions different from that of the crack propagation.95 It should be mentioned that dealloyed surface layers such as certain copper alloys in ammonia-containing solutions are believed to cause SCC.54... 

Due to the large thermal expansion mismatch between barium chromate and the sealing glass or ferritic stainless steel (e.g., AlSl 446),"° the extensive formation of barium chromate resulted in crack initiation and growth between the sealing glass and alloy coupons, as shown in figure 11.5. 

In this section, we review the literature with respect to the effect of fluid flow on the initiation and growth of cracks in steels, mostly in high-temperature aqueous systems. Our purpose is to identify the important factors that lead to flow-induced effects, as deduced from laboratory experiments and theory, for systems under well-controlled experimental conditions. 

Fig. 28. Micrographs of a fractured specimen, showing numerous small cracks on the gauge length (enlarged in (b)), and intergranular fracture (c) enlargement of fracture surface [57]. Copyright 1991. Electric Power Research Institute/EPRl ER-7247. Intergranular Corrosion of Stainless Steel. Vol. 1 Mechanism of Crack Initiation. Reprinted with permission.

Fig. 52. Crack initiation time as a function of flow velocity for sensitized Type 304 stainless steel and ASTM A508 C1.2 steel in oxygenated water ([O2] = 8 ppm) at 250 °C [102], Reproduced from Corrosion J. 38, 76 (1982) by permission of the Editor.

Intergranular Corrosion of Stainless Steel, Vol. 1 Mechanism of Crack Initiation, EPRI ER-7247, Electric Power Research Institute, Palo Alto, CA (March 1991). 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

Crack initiation is promoted by corrosion and, particularly, by pitting or crevice attack, even of low depth, which can cause local conditions of acidity and thus the development of hydrogen. The duration of this stage depends both on the characteristics of the steel and the environment (e. g. the surface finishing of the steel bars, the pH and chemical composition of the environment, etc.) and the time required to initiate the preliminary pitting or crevice attack. However, it does not depend on the stress applied to the steel. 

Figure 10.7 Stress-corrosion cracking initially propagates perpendicularly to the crystalline grains of the steel subsequently it propagates longitudinally [4]...

---