Stress nickel-based alloys

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Flange face areas experience stagnant conditions. Additionally, some gasket materials, such as asbestos fiber, contain leachable chloride ions. This creates crevice and stress corrosion cracking problems on sealing surfaces. Where necessary, flange faces that are at risk can be overlaid with nickel-based alloys. Alternatively, compressed asbestos fiber gaskets shrouded in PTFE may be used. 

With regard to stress-corrosion cracking in the Ni-Cr-Fe system, including both nickel-base alloys and stainless steels, a vast number of papers has been published. A detailed review of work published before 1969 is available and the authors have since published additional data . 

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

Replacing some of the nickel with iron produces a family of alltws with intermediate corrosion resistance between stainless steels and the Ni-Cr-Mo alloys. Alloys such as Incoloy 825 and Hastelloy G-3 and G-30 are in this family. Incoloy 825 has 40 percent Ni, 21 percent Cr, 3 percent Mo, and 2.25 percent Cu. Hastelloy G-3 contains 44 percent Ni, 22 percent Cr, 6.5 percent Mo, and 0.05 percent C maximum. These alloys have extensive applications in sulfuric acid systems. Because of their increased nickel and molybdenum contents they are more tolerant of chloride-ion contamination than are standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. Many of the nickel-based alloys are proprietary and are coverecf by the following specifications ... 

Some practical cases are determination of residual stress in steel springs, the effect of mechanical loading on stress relaxation of machined and shot-peened nickel-base alloys,65 determination of residual stress level in turbine engine disks as they accumulate engine cycles,65 66 effect of manufacturing processes on residual stress, measurement of stress gradients in mechanical, electronic and structural components, effect of heat treatment on residual stress in steel coil springs, effect of variable heat treatment temperature on residual stress in iron alloys, measurement of stress in multiphase materials and composites and stress measurements at locations of stress concentrations. 

Until recently, there were no tool materials that would stand up to the high stresses and temperatures necessary for FSW of materials with higher melting points, such as steels, stainless steels, and nickel-base alloys. In 1998, tungsten alloys and polycrystalline cubic boron nitride (PCBN) were developed to create FSW tools for use in steel, stainless steel, titanium alloys, and nickel-base alloys. Properties of the resultant welds have been shown to be outstanding. Although some issues remain (primarily limited tool life with tungsten-base tools), FSW has been demonstrated as a technically and eco-... 

An example of a nickel-based alloy that is resistant to stress-corrosion cracking is inconel. Inconel is composed of 72% nickel, 14-17% chromium, 6-10% iron, and small amounts of manganese, carbon, and copper. 

Nickel and nickel-based alloys play a major role in the chemical process industry because of their excellent corrosion resistance and their low susceptibihty to stress corrosion cracking. 

Fig. 11.4 Cyclic oxidation resistance of nickel-base alloys and stainless steels in air at 900 °C [18]. Reprinted, with permission, from STP165 Symposium on Effect of Cyclic Heating and Stressing on Metals at Elevated Temperatures, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA.

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