In terms of chemical composition, Inconel 690 has a higher chromium content, typically around 27 - 30%, while Inconel 625 usually contains 20 - 23% chromium. Additionally, Inconel 690 has a lower iron content (around 8 - 10%) compared to Inconel 625 (up to 5%). Inconel 625 is notable for its significant molybdenum (8 - 10%) and niobium (3.15 - 4.15%) additions, which are present in much smaller amounts or absent in Inconel 690.
Regarding mechanical properties, Inconel 625 offers excellent high - temperature strength and fatigue resistance due to the solid solution strengthening from molybdenum and niobium. It maintains good ductility and toughness across a wide temperature range. Inconel 690 has good strength at elevated temperatures but is generally not as strong as Inconel 625 in high - stress applications. Its mechanical properties are more focused on corrosion resistance rather than extreme strength.
In terms of corrosion resistance, Inconel 690 is renowned for its outstanding resistance to stress corrosion cracking in chloride environments and excellent corrosion resistance in nuclear - related applications, such as steam generator tubing in pressurized water reactors. Inconel 625 also has excellent corrosion resistance, including to pitting, crevice corrosion, and various aggressive media like seawater and acidic solutions. The combination of molybdenum and chromium in Inconel 625 provides broad - spectrum corrosion protection, making it suitable for applications where both corrosion and high - temperature strength are required.
In terms of applications, Inconel 690 is mainly used in nuclear industry components, such as steam generator tubes, due to its superior corrosion resistance in high - purity water and resistance to chloride - induced stress corrosion cracking. Inconel 625 is widely used in aerospace components, marine engineering, chemical processing equipment, and oil and gas industry parts, such as turbine blades, fasteners, and subsea pipelines, where it needs to withstand high temperatures, high pressures, and corrosive environments simultaneously.
In summary, the differences between the two alloys lie in their chemical compositions, which lead to variations in mechanical properties and corrosion resistance, and thus determine their distinct application scenarios.
