Stainless steels fatigue

Prompted by the success, TOFD measurements were conducted on a fatigue crack in a stainless steel compact tension specimen. Test and system parameters were optimised following the same procedure used for carbon steel specimens. A clear diffracted signal was observed with relatively good SNR and its depth as measured from the time-of-flight measurements matched exactly with the actual depth. 

Many initiators attack steels of the AISI 4300 series and the barrels of the intensifiers, which are usually of compound constmction to resist fatigue, have an inner liner of AISI 410 or austenitic stainless steel. The associated small bore pipework and fittings used to transfer the initiator to the sparger are usually made of cold worked austenitic stainless steel. The required pumping capacity varies considerably from one process to another, but an initiator flow rate 0.5 L / min is more than sufficient to supply a single injection point in a reactor nominally rated for 40 t/d of polyethylene. 

In the case of the fibrous laminate not much work has been done, but it has been observed that a significant loss of stiffness in boron—aluminum laminate occurs when cycled in tension—tension (43,44). Also, in a manner similar to that in the laminated PMCs, the ply stacking sequence affects the fatigue behavior. For example, 90° surface pHes in a 90°/0° sequence develop damage more rapidly than 0° pHes. In the case of laminates made out of metallic sheets, eg, stainless steel and aluminum, further enhanced resistance against fatigue crack propagation than either one of the components in isolation has been observed (45). 

When a fast-breeder reactor is shut down quickly, the temperature of the surface of a number of components drops from 600°C to 400°C in less than a second. These components are made of a stainless steel, and have a thick section, the bulk of which remains at the higher temperature for several seconds. The low-cycle fatigue life of the steel is described by... 

Majumdar, S. et al., Interium Fatigue Design Curves for Carbon, Low Alloy, and Austensic Stainless Steels in LWR Environments, 1993. 

Valves must be made of fatigue-resistant carbon or alloy steel or 18-8 stainless steel, depending upon the service. The 18-8 stainless and 12-14 chrome steel is often used for corrosive and/or high temperature service. Any springs, as in the plate-type valves, are either carbon or nickel steel. Valve passages must be smooth, streamlined, and as large as possi-... 

D ye penetration inspection. This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator, it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks. 

Stainless steels are subject to fatigue failure under dry conditions as are all metallic materials, having distinct fatigue limits where level is dependent on steel type and heat treatment. The limits can be depressed by the simultaneous action of a corrodent, the degree depending upon the nature of the corrodent. Under severe conditions the limit can be displaced to very low values and it is customary to describe resistance by an endurance limit, that is the cyclic stress to give rupture at a specific number of cycles when in contact with a specific corrodent. Some comparative data are in Table 3.25. 

Table 3.25 Rotating bend fatigue test results for a number of stainless steels...

Fig. 8.72 Effect of applied potential on corrosion fatigue behaviour of a ferritic stainless steel in 3% NaCl (after Amzallag el al )...

The substitution of a more corrosion-resistant material, e.g. Monel metal or stainless steel, is often advocated but this is not necessarily a solution a 15% chromium steel, for instance, is prone to failure in corrosion fatigue because of the disruption of the normally protective surface film although as noted earlier some other materials are effective, e.g. Ti-6A1-4V alloys... 

James, L. A., Fatigue crack propagation in Austenitic stainless steel . Atomic Energy Review 14, 37-86 (1976)... 

Amzallag, C., Rabbe, P. and Desestret, A., Corrosion fatigue behaviour of some special stainless steels, ASTM STP 642, pp. 117-32 (1977)... 

The stress intensification factors in Appendix D of ASME B31.3 have been developed from fatigue tests of representative piping components and assemblies manufactured from ductile ferrous materials. The allowable displacement stress range is based on tests of carbon and austenitic stainless steels. Caution should be exercised when using eqs. (la) and (lb) (para. IP-2.2.10) for allowable displacement stress range for some nonferrous materials (e.g., certain copper and aluminum alloys) for other than low-cycle applications. 

Fig. 12.8. Short incipient fatigue cracks in stainless steel (a) negative defocus, z = —20 (tm (b) positive defocus, z = +32 m 0.37 GHz (Rowe et al. 1986).

When selecting a bellows valve, it is important to pay some special attention that the material selection is in accordance with the process conditions. Some SRV manufacturers use as standard bellow material INCONEL alloy 625LCF-UNS N06625 (ASME SB0443). This material is not perfect either but, compared to simple stainless steel, has an enhanced resistance to mechanical fatigue and sour gases it is commonly used in refinery FCC systems for expansion joints. 

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