Incoloy® Alloy 925, Nickel-Iron-Chromium Superalloy

Excellent corrosion resistance combined with high strength

Incoloy 925 Equivalents

Incoloy 925 Specifications

Incoloy 925 Applications

Incoloy 925 Basecoats & Top Coats

Typical performance for Incoloy 925

General Corrosion

Incoloy 925 is highly resistant to corrosion in a wide range of organic and inorganic acids. The copper content gives excellent resistance to corrosion in many oxidising acids. Commercial acid applications often contain chlorides and other impurities which can cause corrosion of some stainless steels. Incoloy 925 offers much improved corrosion performance in these environments.

Figure 2 : Iso-corrosion curves 0.004 ipy (0.1mm/y)
for some stainless steels in Sulfuric Acid

Figure 3 : Iso-corrosion curves 0.004 ipy (0.1mm/y)
for some stainless steels in Hydrochloric Acid

Incoloy 925 is also highly resistant to strong alkalis. The production of caustic soda results in hot, strong solutions and even in 60 wt% caustic soda, Incoloy 925 has very low corrosion rates (<0.004 ipy). Caustic soda is often found with chlorides in extraction processes and even with 10g/L chloride, Incoloy 925 has excellent corrosion resistance. Three years service experience of fabricated Incoloy 925 pipework in 2M caustic soda with chlorides at 446°F (230°C) has been excellent.

Pitting Corrosion

Exposure to 6% FeCl3 for 24 hours in accordance with ASTM G48 method A to determine the maximum temperature at which no pitting occurs (the critical pitting temperature, CPT), has given the following results:
Solution annealed wrought and cast Incoloy 925: 158 - 176°F (70-80°C) depending on product form and manufacturing route.
Incoloy 925 welded with Zeron 100X filler metal: 95-140°F (35 - 60°C), depending on the welding variables, i.e. process, joint geometry, procedure etc.
These values are for single exposure testing; testing a single specimen at a series of increasing temperatures gives a higher CPT value.

Crevice Corrosion

The resistance to localized corrosion is often assessed by use of the PREN number (%Cr + 3.3 x %Mo + 16 x %N). Incoloy 925 is made to a minimum PREN of 40, ensuring a guaranteed and high resistance to pitting and crevice corrosion. Incoloy 925 has been in service in seawater since 1986 as castings, and since 1989 as wrought pipes and fittings giving satisfactory performance.
At sea water temperatures above ambient, 68°F (20°C), the risk of crevice corrosion increases. Incoloy 925 resists crevice corrosion up to 131°F (55°C) but is limited by the pitting resistance of the welds to about 104°F (40°C). With the application of post weld surface treatments, seawater temperatures up to 149°F (65°C) have been handled successfully.

Repassivation

Short term elevated temperatures upsets are not uncommon in cooling water circuits. Laboratory tests and field experience have shown that Incoloy 925 does not suffer crevice corrosion easily during short upsets and, when corrosion does initiate, repassivation occurs readily on cooling, and at temperatures higher than the normal operating temperature of 104°F (40°C). Super austenitic steels and super duplex steels that do not contain copper and tungsten have lower repassivation temperatures than normal operating temperatures, so any corrosion that is initiated during the upset will continue to propagate when the system reaches normal operating conditions once more.

Stress Corrosion Cracking

Incoloy 925 has excellent resistance to stress corrosion cracking (SCC) in both chloride environments, and process environments containing H₂S and CO₂. Incoloy 925 is listed in ISO 15156 / NACE MR0175 part 3 as being suitable in sour service at 0.2 bar partial pressure of H₂S. In brines with lower chloride contents Incoloy 925 can tolerate much higher pressures of H2S, as shown in Figure 4. As the pH, at temperature and pressure, increases, so does the resistance to sulfide SCC (Figure 4).

Figure 4 - Limits of use for Incoloy 925 in brines containing H2S

Hydrogen Embrittlement

In common with all high strength steels, duplex and super duplex stainless steels can be susceptible to hydrogen embrittlement if stressed above the specified minimum yield strength in the presence of hydrogen. Hydrogen embrittlement therefore becomes an area for consideration when these steels are used subsea with conventional cathodic protection. However, the proper application of normal design stress criteria and coating technology has allowed many subsea projects to utilize duplex and super duplex stainless steels successfully for a number of years.

Incoloy 925 Manufacturing

Heat Treatment

Incoloy 925 should be solution annealed in the temperature range 2012-2084°F (1100-1140°C) followed by water quenching.

Hot Forming

Hot forming of Incoloy 925 should be carried out in the temperature range 2012 - 2336°F (1100°C - 1280°C). It is recommended that this is followed by solution annealing and water quenching. Components should subsequently be pickled or fully machined.

Cold Forming

Incoloy 925 can be adequately cold formed by various processes but the high mechanical properties should be taken into account. It is recommended that any cold work in excess of 10% -15% is removed by solution annealing and water quenching.
It should be noted that cold working above these limits can result in hardness levels above those specified in standards such as ISO 15156 / NACE MR0175 for cold worked superduplex.

Welding

Where a solution anneal and quench as a post-weld heat treatment is to be carried out, Incoloy 925 is usually welded with matching composition consumables (Zeron 100M). With overalloyed consumables (Zeron 100X), no post-weld heat treatment is necessary. Corrosion and mechanical properties similar to the parent metal can be obtained following recommended procedures. A separate brochure on the optimum parameters for a range of welding operations is available from our Sales Department as bulletin number 105.

Machining

Incoloy 925 requires a little more care in machining than the 300 series austenitics. If heavy or uneven machining to tight tolerances is to be carried out on wrought Incoloy 925 components, machining should be carried out in stages. As a last resort, a stress relieving heat treatment can be applied but this can result in a reduction of toughness and corrosion resistance.

Incoloy 925 Testing Standards

ASTM A923 - Test Method A - Sodium Hydroxide Etch Test for Classification of Etch Structures of Duplex Stainless Steels
ASTM E562 - Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count
ASTM G48 - Test Method A - Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels
ASTM A923 - Test Method C - Ferric Chloride Corrosion Test for Classification of Structures of Duplex Stainless Steels

Incoloy 925 Product Range

Based on the application & Usage

Bars / Rods

Forgings

Hex Nuts

Heavy Hex Nuts

Bolts

Hex Bolts

Heavy Hex Bolt

Socket Head Cap Screws

Push Bolts / Rods

Hex Cap Screws

Set Screws / Grub Screws

Washers

Countersunk Screws

U Bolt

Threaded Rods

Studs

Stud Bolts

12 point screws