Alloy 825, more widely known as Incoloy 825, is a nickel-based alloy, not a stainless steel: its largest single element is nickel at 38-46% of the total. It also contains iron and chromium, with lesser additions of molybdenum, copper and titanium.
Background
Incoloy 825 was brought to market in 1952, building on Incoloy 800 to better resist sulphuric acid, and for many years it was the default choice for handling sulphuric acid. Its high nickel content gives resistance to stress corrosion cracking, and its mechanical properties stay relatively constant from cryogenic temperatures (around -150degC) up to 540degC (1004degF) - above this, undesirable microstructural phase changes can significantly reduce impact toughness and elongation. Nickel is an expensive addition, but Alloy 825 has a lower nickel content than most other nickel-based alloys, so it has been widely used where lower-cost stainless steels are not suitable.
Langley Alloys stocks Alloy 825 as round bars from 5/8" (15.875mm) to 10" (254mm) diameter, with pipes and hollow bar also available.
Is Alloy 825 stainless steel?
No. A stainless steel is a steel that resists corrosion. Its composition is based on iron, and it needs at least 11.5% chromium to resist general corrosion. More chromium increases corrosion resistance, while additions of nickel, molybdenum and other elements further improve mechanical properties and corrosion resistance.
Although Alloy 825 and stainless steels both contain iron, chromium and nickel, they do so in quite different proportions. Nickel is an expensive addition, so Alloy 825 costs more than most stainless grades, but its higher nickel content means it resists oxidation at higher temperatures and resists many acids. Compared with other nickel-based alloys such as Inconel and Monel grades, its nickel level and price are not as high.
Is Inconel 825 the same as Incoloy 825?
There is no such thing as Inconel 825 - the grade is Incoloy 825. Inconel and Incoloy are two separate families of nickel-based alloys, both originally developed by Special Metals Incorporated, which owns both trademarks. Incoloy is applied to a family of nickel-based alloys primarily alloyed with chromium, and more than ten grades sit within the family, one of the best known being Incoloy 825. Inconel grades are also nickel-based and primarily alloyed with chromium, but carry a generally much higher nickel content, which improves performance at higher temperatures where they retain more of their strength.
Corrosion resistance
The high nickel content of Alloy 825 gives resistance to stress corrosion cracking. It was developed primarily to resist sulphuric acid; copper additions are well known to improve resistance to reducing environments such as sulphuric and phosphoric acids. The same effect is used in Ferralium® 255 (2% copper) and in the far more highly alloyed Alloy 20 (3-4% copper). A controlled titanium addition inhibits the intergranular corrosion that could otherwise occur at elevated temperatures.
Alloy 825 vs super duplex
Alloy 825 is often the entry point when moving from stainless steels up to more expensive nickel alloys. With more than 40% nickel and more than 20% chromium, it is a clear step up in price from super duplex alloys. Duplex and super duplex stainless steels give a lot in return for their price, in both corrosion resistance and strength, but their operating temperature range is limited, which is where Alloy 825 comes in. A standard super duplex alloy achieves a yield strength above 80ksi, against above 36ksi for Alloy 825, and its PREN is above 40, against 31 for Alloy 825. What Alloy 825 adds is temperature range:
- Alloy 825 can be used at operating temperatures up to 540degC, whereas super duplex stainless steels are limited to 250degC.
- Alloy 825 retains much of its impact strength at cryogenic temperatures, whereas duplex and super duplex stainless steel properties tend to drop off at very low temperatures.
Where is Alloy 825 used?
Common applications include valve and pump components for the chemical process industries, thermowells and sensors, fasteners, and down-hole tooling for corrosive service that makes use of its non-magnetic properties.
Alloy 825 and Alloy 925
Alloy 925 is nearly identical to Alloy 825 in composition. However, small additions of titanium and aluminium, together with an extra heat treatment, produce precipitation strengthening: very small precipitates form and help to 'pin' the microstructure under load, increasing strength and toughness. Alloy 925 can therefore provide 3-4 times the tensile strength of the equivalent component in Alloy 825.