Sensitization of austenitic stainless steels

A slightly more complicated situation can exist during sensitization of austenitic stainless steels discussed above. Here, carbon activity is assumed to be at equiKbrium because of fast diffusion of interstitial carbon at all relevant temperatures. Moreover, the bulk carbon content is negKgibly... 

In chloride-containing environments, the sensitivity of austenitic stainless steels varies with their chromium, nickel and molybdenum content. Figure 11.35 shows the time to failure as a function of the applied load [15]. The measurements were carried out in a boiling solution of 42% MgCl2. Type 304 (18-20 Cr, 8-12 Ni) resists less well than type 316, which in addition contains 2-3% molybdenum. The types 310 and... 

As mentioned before, austenitic stainless steels are susceptible to IGC due to sensitization caused by exposure to high temperatures (450-850 C). The IGC of austenitic stainless steel can also be characterized by normalized classical tests ASTM G28, ASTM A262-86, SEP 1877, AFNOR A05-159 and AFNOR A05-160, currently known as the Strauss, Huey and Streicher tests [54-57]. These methods however are destructive, difficult to perform on site and require sampling that can be harmful to the integrity of materials during service. For this reason, the electrochemical, non-destructive tests commonly known as EPR (electrochemical potentiokinetic reactivation) and DL-EPR (double loop electrochemical potentiokinetic reactivation) were developed to measure the sensitivity of austenitic stainless steels to IGC [58-66]. However, EPR and DL-EPR are based on measurements of characteristic potentials and currents of passive/active zones on potentiody-namic curves in an aqueous solution (linear voltammetry curve from oxygen to hydrogen evolution in the... 

V. Kain, K. Chandra, K.. N. Adhe, P.K. De, Effect of Cold Work on Low-Temperature Sensitization Behavior of Austenitic Stainless Steels , Journal of Nuclear Materials, 334 (2004), 115-132. 

NOTE Overlays or hard surfaces that contain more than 0,10% carbon can sensitise both low-carbon and stabilised grades of austenitic stainless steel unless a buffer layer that is not sensitive to intergranular corrosion is applied. 

The grain boundaries of austenitic stainless steels may be made susceptible to accelerated intergranular corrosion (sensitized) by exposure of the steels to temperatures in the range of 800-1600°F (426-871°C. ). Depending on the compo-... 

S. Ahmad, M.L. Mehta, S.K. Saraf, and I. Saraswat, Stress Corrosion Cracking of Sensitized 304 Austenitic Stainless Steel in Petroleum Refinery Environment, Corrosion, Vol 38, 1982, p 347-353... 

The phenomenon of knife-line attack within weld HAZs describes susceptibility to ICC and IGSCC in stabilized grades of austenitic stainless steels [61, 68]. Stabilization is a term used to describe depletion of solid solution carbon due to niobium and titanium alloying. These elements produce carbides in the temperature range from 870 to 1150 °C in austenitic stainless steels such as AISI 347 [61]. Little carbon remains in solid solution to be precipitated as (Fe,Cr)23C6. Normally, the initial get-tering of carbon above 870 C eliminates sensitization by Cr-carbide formation that normally occurs over the range from 425 to 815 °C in austenitic stainless steels. 

Ferritic stainless steel has the reputation of being less sensitive to intergranular corrosion than austenitic stainless steel. This type of corrosion can nevertheless take place under certain conditions of thermal treatment [20]. The diffusion coefficients of both carbon and chromium in ferrite are larger than in austenite. Grain boundary precipitation of carbides and nitrides of chromium can therefore occur at temperatures of 540-600 °C already. The behavior differs from that of austenitic stainless steel, which becomes sensitized at higher temperatures only. Because of the larger diffusion... 

Pipe cracking has occurred in the heat affected zones of welds in primary system piping in BWRs since mid-1960. These cracks have occurred mainly in Type 304 stainless steel which is the type used in most operating BWRs. The major problem is recognized to be IGSCC of austenitic stainless steel components that have been made susceptible to this failure by being "sensitized", either by post-weld heat treatment or by sensitization of a narrow heat affected zone near welds. 

Table 4.7 contains an example of CF values adapted from textbook information describing the sensitivity of NDE techniques to SCC defects as a function of material composition. These CF values do not take into account defect size, position, and morphology. However, such values can be used as initial default values during the activation of the 00 module. Subsequent information can then be used to improve and refine the pertinence of the CF values to the specific context and expertise. The following example illustrates how the rule propagation would be made with even a limited information base such as that contained in Table 4.7. In this example, an operator would ask the system if there would be any advantage in combining two techniques for the inspection of a component made of austenitic stainless steel. According to the data in Table 4.7, one would always obtain an increased confidence if two techniques with positive CFs were used. Table 4.8 illustrates some of the combinations envisaged in this example and the estimated gain in probability of detection from using two techniques instead of the better of the two techniques considered.

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