Chromium-nickel alloys oxidation, elevated temperatures

This service test method covers the determination of the resistance to oxidation of nickel-chromium-iron electrical heating alloys at elevated temperatures under intermittent heating using a constant temperature cycle test. This test is used for internal comparative purposes only. 

To fully understand the formation of the N13S2 scale under certain gas conditions, a brief description needs to be given on the chemical aspects of the protective (chromium oxide) Ci 203/(nickel oxide) NiO scales that form at elevated temperatures. Under ideal oxidizing conditions, the alloy Waspaloy preferentially forms a protective oxide layer of NiO and Ci 203 The partial pressure of oxygen is such that these scales are thermodynamically stable and a condition of equilibrium is observed between the oxidizing atmosphere and the scale. Even if the scale surface is damaged or removed, the oxidizing condition of the atmosphere would preferentially reform the oxide scales. 

For resistance against fatigue, Nimonic 75 has been used with Nimonic 80 and Nimonic 90. Nimonic 75 is an 80-20 nickel-chromium alloy stiffened with a small amount of titanium carbide. Nimonic 75 has excellent oxidation and corrosion resistance at elevated temperatures, a reasonable creep strength, and good fatigue resistance. In addition, it is easy to press, draw, and mold. As firing temperatures have increased in the newer gas turbine models, HA-188, a Cr, Ni-based alloy, has recently been employed in the latter section of some combustion liners for improved creep rupture strength. 

The main characteristic of attack by halogens at elevated temperatures is that most reaction products are volatile compared with the solid products that form in all cases considered hitherto in this chapter. Thus, in cases where metals are exposed to pure halogen gases large mass losses are usually reported with very little external scale formation. Li and Rapp " showed that internal chloridation occurred when nickel-chromium alloys were exposed to Ni + NiClj powders at 700-900°C. However, where oxide scales can also form, as in combustion gases, the oxide layer was usually highly... 

Inconel (typically 76% Ni, 7% Fe, 15% Cr) is used primarily for acid resistance at high temperatures. It maintains its strength at elevated temperature and is resistant to furnace gases, if sulfur free. It is not suitable for use in sulfidizing environments. Nickel alloys with higher chromium content such as Incoloy 800 (21% Cr) and RA-33 (25% Cr) have better oxidation resistance at higher temperatures. 

Nickel and high-nickel alloys tend to oxidize along grain boundaries when subject to alternate oxidation and reduction. Alloying with chromium reduces this tendency. Also, in contact with sulfur or sulfur atmospheres at elevated temperatures, nickel and high-nickel alloys are subject to intergranular attack. Consequently, nickel is not usefully resistant to such atmospheres above about 315 °C... 

In a similar time frame, another superaustenitic alloy was introduced based on the wrought version of the heat-resistant cast alloy, HT. This alloy, identified as RA330 stainless, contains about 35% nickel and 20% chromium with an addition of silicon. This superaustenitic stainless also was assigned a nickel-based UNS number (N08330). N08330 offers excellent oxidation and carburization resistance in combination with good elevated temperature mechanical properties. 

This is a nickel-chromium-iron alloy with the addition of silicon. Refer to Table 11.1 for its chemical composition. Type 330 stainless has good strength at elevated temperatures, good thermal stability, and excellent resistance to carburizing and oxidizing atmospheres. It is weldable and machinable. This alloy has been used in low-stress applications to temperatures as high as 2250°F (1230°C) and has moderate creep to 1600°F (870°C). 

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