2.08.2.2.1 Chemical compositions, physical properties, and mechanical properties

The chemical compositions of typical nickel — chromium-iron and nickel-chromium-iron — molybdenum alloys are shown in Table 3, together with those of other nickel-based alloys.

As described earlier, nickel is a very versatile corrosion-resistant metal. The addition of chromium confers resistance to sulfur compounds and also pro­vides resistance to oxidizing conditions at high tem­peratures or in corrosive solutions, with the exceptions of nitric acid and chloride solutions. In addition, chromium confers resistance to oxidation and sulfidation at high temperatures.

Alloy 600 consists of about 76% nickel, 15% chromium, and 8% iron. The alloy is not precipitation- hardenable and can only be hardened and strengthened by cold-working. It has excellent resistance to hot halogen gases and has been used in processes involv­ing chlorination. It has excellent resistance to oxida­tion and chloride SCC. It is widely applied as a structural material in many industrial fields owing to its strength and corrosion resistance.10

The thermal expansion coefficient of Alloy 600 is smaller than those of austenitic stainless steels and somewhat larger than those of ferritic steels, as shown in Table 7. It is also highly resistant to sensitization in heat-affected zones during welding. The alloy and its weld metals such as Alloys 82, 132, and 182 have there­fore been widely used for dissimilar metal weld joints to reduce residual stresses and strains after welding.

Alloy 601 has a higher chromium content (about 23%) than Alloy 600 and about 1.4% aluminum. The alloy is resistant to high-temperature oxidation and has good resistance to aqueous corrosion. Oxidation resistance is further enhanced by its aluminum con­tent. The alloy has been applied to the muffles of heat-treatment furnaces and in catalytic convertors for exhaust gases in automobiles.11

Alloy X-750 contains titanium, aluminum, and niobium, and is hardened by precipitation of the g0 phase as Ni3(Ti, Al, Nb).12 Alloy 718, on the other hand, contains niobium, molybdenum, titanium, and aluminum, and is hardened by the precipitation of both the g0 phase as Ni3(Ti, Al, Nb) and the g00 phase as Ni3Nb.13 These alloys were developed as high creep-strength and high creep-rupture-strength materials for jet-engine blades and vanes in the 1940s. These precipitation-hardened materials have also been used in industrial gas-turbine materials. In addition, Alloy X-750 has been used as a bolting material and Alloy 718 has been applied to bellows, springs, etc. for industrial products.

Alloy 690 (UNS N06690) was developed in the late 1960s and has a higher chromium content (about 30%) than Alloys 600 and 601. It exhibits excellent resistance to many corrosive aqueous media and high-temperature atmospheres. The properties of Alloy 690 are useful in a range of applications involv­ing nitric or nitric/hydrofluoric acid production, and as heating coils and tanks for nitric/hydrofluoric solutions used in the pickling of stainless steels, for

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example.

Alloy 800 (UNS N08800) is an iron-based nickel — chromium alloy. This alloy has been compared to Alloys 600 and 690 from the view point of its corrosion resistance in many environments. It was introduced for industrial use in the 1950s as an oxidation-resistant alloy and for high-temperature applications requiring optimum creep and creep — rupture properties. Alloy 800 has been widely used as an oxidation-resistant material and is suitable for high-temperature applications due to its high resis­tance to а-phase embrittlement after heating in the range of 650-870 °C.15

Alloy 825 (UNS N08825) was developed from alloy 800 by the addition of molybdenum (about 3%), copper (about 2%), and titanium (about 0.9%) for improved aqueous corrosion resistance in a wide vari­ety of corrosive media. In this alloy, the nickel content confers resistance to chloride-ion SCC. Nickel in con­junction with molybdenum and copper gives outstand­ing resistance to reducing environments such as those containing sulfuric and phosphoric acids. Molybde­num also enhances its resistance to pitting and crevice corrosion. In both reducing and oxidizing environ­ments, the alloy resists general corrosion, pitting, crev­ice corrosion, intergranular (IG) corrosion, and SCC. Some typical applications include various components used in sulfuric acid pickling of steel and copper, com­ponents in petroleum refineries and petrochemical

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plants (tanks, valves, pumps, agitators), equipment used in the production of ammonium sulfate, pollution con­trol equipment, oil and gas recovery, and acid production.

Alloy A-286 (UNS S66286) is an iron-based nickel-chromium alloy with added molybdenum

and titanium. The alloy is age-hardenable to achieve superior mechanical properties. It maintains good strength and oxidation resistance at temperatures up to about 700 °C.16

The mechanical and physical properties of typical nickel-chromium-iron and nickel-chromium-iron — molybdenum alloys are shown in Tables 4 and 5, respectively, together with those of other nickel — based alloys.