Incoloy ® 825

 Incoloy 825 is a nickel-iron-chromium alloy with additions of molybdenum, copper and titanium. This nickel steel alloy’s chemical composition is designed to provide exceptional resistance to many corrosive environments. It is similar to alloy 800 but has improved resistance to aqueous corrosion. It has excellent resistance to both reducing and oxidizing acids, to stress-corrosion cracking, and to localized attack such as pitting and crevice corrosion. Alloy 825 is especially resistant to sulfuric and phosphoric acids. This nickel steel alloy is used for chemical processing, pollution-control equipment, oil and gas well piping, nuclear fuel reprocessing, acid production, and pickling equipment.

 Chemical Composition  Incoloy ® 825

Applications of Incoloy ® 825

Commercial applications for this engineering material include:

• Chemical Processing
• Pollution-control
• Oil and gas well piping
• Nuclear fuel reprocessing
• Components in Pickling equipment like heating coils, tanks, baskets and chains
• Acid production

Physical Properties

The following table discusses the physical properties of INCOLOY alloy 825

Corrosion Resistance Incoloy 825

Alloy 825 is a versatile engineering alloy with resistance to corrosion in acids and alkalis under both oxidizing and reducing conditions.
The high nickel content gives the alloy virtual immunity to stress corrosion cracking. The corrosion resistance in various media like sulfuric, phosphoric, nitric and organic acids is good, as well as the corrosion resistance in alkalis or ammoniac, sea water and caustic chloride.
The versatility of VDM® Alloy 825 is illustrated by its use in nuclear fuel element dissolvers where a variety of corrosive media, e. g. sulfuric and nitric acids and sodium hydroxide, are handled in the same equipment

 Fabrication and heat treatment

Heating: Workpieces must be clean and free of any contaminants before and during heat treatment. Sulfur, phosphorus, lead and other low-melting-point metals can lead to damage when heat treating VDM® Alloy 825. Sources of such contaminants include marking and temperature indicating paints and crayons, lubricating grease and fluids, and fuels. Fuels should contain as little sulfur as possible. Natural gas should contain less than 0.1 wt.-% of sulfur. Heating oil with a sulfur content of a maximum of 0.5 wt.-% is also suitable. Electric furnaces are to be preferred due to precise temperature control and freedom from contamination due to fuel. The furnace atmosphere should be set between neutral and slightly oxidising, and should not change between oxidising and reducing. Direct flame impingement needs to be avoided.

Hot working: Alloy 825 may be hot-worked in the temperature range 1,150 to 900 °C (2,100 to 1,650 °F) with subsequent rapid cooling down in water or by using air. The workpieces should be placed in the furnace heated to hot working temperature in order to heat up. Once the temperature has equalised, a retention time of 60 minutes for each 100 mm (4 in) of workpiece thickness is recommended. After this, the workpieces should be removed immediately and formed during the stated temperature window. If the material temperature falls below the minimum hot working temperature, the workpiece must be reheated. Heat treatment after hot working is recommended in order to achieve optimum properties and corrosion resistance

Cold working: Cold working should be carried out on annealed material. VDM® Alloy 825 has a higher work hardening rate than austenitic stainless steels. This must be taken into account during the design and selection of forming tools and equipment and during the planning of the forming processes. Intermediate annealing may be necessary at high degrees of cold working deformation. After cold working with more than 15 % of deformation, the material should be soft annealed.

Heat treatment: Soft or stabilizing annealing should be carried out at temperatures between 920 and 980 °C (1,690 to 1,800 °F), preferably at 940 ± 10 °C (1,725 ± 15 °F). Water quenching should be carried out rapidly to achieve optimum corrosion characteristics. Workpieces of less than 3 mm (0.12 in) thickness can be cooled down using air nozzles. For strip and wire products, the heat treatment can be performed in a continuous furnace at a speed and temperature that is adapted to the material thickness. The workpiece has to be put into the pre-heated furnace. The furnace should be heated up to the maximum annealing temperature. The retention time during annealing depends on the workpiece thickness and can be calculated as follows:

• For thicknesses d ≤ 10 mm (0.4 in) the retention time is t = d • 3 min/mm
• For thicknesses d = 10 to 20 mm (0.4 to 0.8 in) the retention time is t = 30 min + (d - 10 mm) • 2 min/mm
• For thicknesses d > 20 mm (0.8 in) the retention time is t = 50 min + (d - 20 mm) • 1 min/mm

Note: The cleanliness requirements listed under ‘Heating’ must be complied with

Machining

Alloy 825 should be machined in the annealed temper. As the alloy is prone to work-hardening, low cutting speeds and appropriate feed rates should be used and the tool should be engaged at all times. Sufficient chip depths are important to get below the work-hardened surface layer. The optimum dissipation of heat through the use of large amounts of appropriate, preferably water-containing cooling lubricants is crucial for a stable machining process

 Conclusion

In the oil and gas industry, the two most important nickel alloys are Inconel 625 and Incoloy 825. Incoloy 825 is an austenitic nickel-iron-chromium-molybdenum-copper alloy containing high levels of chromium, nickel, copper, and molybdenum to provide high levels of corrosion resistance to both moderately oxidizing and moderately reducing environments. This balance of alloying elements grants this alloy exceptional resistance to both chloride stress corrosion cracking, as well as crevice corrosion and general corrosion. It is the high level of nickel in combination with the amount of molybdenum and copper in this nickel alloy produces substantially improved corrosion resistance in a corrosive environment such as reducing environments compared to stainless steel. The addition of titanium in this alloy also helps to stabilize the alloy against intergranular corrosion. Chloride stress corrosion cracking is a type of localized intergranular corrosion on materials that are put under tensile strength, in high temperatures and in an environment that includes oxygen and chloride ions such as seawater.
As an austenitic, nickel alloy, the material is ductile over a wide range of temperatures from cryogenic to well more than 1000 °F (538 °C). Fabricability is typical for a nickel alloy, with the material readily formable and weldable by a variety of techniques.