Zeron® 100, 25Cr Super Duplex Stainless Steel

PREN ≥ 40, CPT ≥ 50°C per ASTM G48 Method E, yield ≥ 550 MPa

Supply States, SA, FG, FLT, AFP

Equivalent Designations, UNS, Werkstoff, SAF

Zeron 100 sits in the super duplex tier (PREN ≥ 40) alongside the Cu-bearing variant Ferralium 255 (UNS S32550) and the W-free Super Duplex 2507 (UNS S32750). Where Zeron 100's W contribution offers crevice-resistance margin, Ferralium 255's 2% Cu addition is preferred for reducing-acid service (HCl, H₂SO₄, phosphoric); 2507 is the most-stocked grade with the broadest standards coverage.

Standards & Specifications (ASTM, EN, NORSOK, NACE, API)

Applications, Seawater, Oil & Gas, Desalination, FGD

Anti-Friction Coatings, Xylan, Xylar, SermaGard, Dacromet (AFP supply state)

Corrosion Performance, HCl, H₂SO₄, Seawater, Sour Service

General Corrosion

Zeron 100 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. Zeron 100 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

Strong-alkali resistance is similarly high. The production of caustic soda results in hot, strong solutions and even in 60 wt% caustic soda, Zeron 100 has very low corrosion rates (<0.004 ipy). Caustic soda is often found with chlorides in extraction processes and even with 10g/L chloride, Zeron 100 has excellent corrosion resistance. Three years service experience of fabricated Zeron 100 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 Zeron 100: 158 - 176°F (70-80°C) depending on product form and manufacturing route.
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). The minimum PREN of 40, ensuring a guaranteed and high resistance to pitting and crevice corrosion. Zeron 100 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. Zeron 100 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 Zeron 100 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

Stress corrosion cracking (SCC) resistance is high in chloride and H₂S/CO₂ environments, this is what gets Zeron 100 onto sour-service projects (SCC) in both chloride environments, and process environments containing H₂S and CO₂. Zeron 100 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 Zeron 100 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 Zeron 100 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.

Manufacturing, Heat Treatment, Hot/Cold Forming, Welding, Machining

Heat Treatment

Solution-anneal at 1100-1140°C (2012-2084°F), then water-quench. No precipitation hardening, no aging cycle, the duplex microstructure (50/50 austenite-ferrite) develops the strength on its own.

Hot Forming

Hot forming of Zeron 100 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

Cold forming Zeron 100 works across the usual processes, but plan for the high yield strength, springback and tooling load are noticeably higher than 316L. It is recommended that any cold work in excess of 10% -15% is removed by solution annealing and water quenching.
Cold working beyond these limits raises hardness above the 28 HRC ceiling that ISO 15156 / NACE MR0175 enforces for sour-service super duplex.

Welding

Where a solution anneal and quench as a post-weld heat treatment is to be carried out, Zeron 100 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

Machining wants slightly more care than 300-series austenitics. lower surface speed, sharper tooling, generous chip clearance. If heavy or uneven machining to tight tolerances is to be carried out on wrought Zeron 100 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.

Testing Standards (ASTM A923, E562, G48 Method E)

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

Zeron 100 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