Alloy 20Cb-3 Forgings (UNS N08020)

Alloy 20Cb-3 was developed in 1951 for dilute sulfuric acid service, where 304 and 316L stainless steels corrode actively. Nominal composition is 35 Ni, 35 Fe and 20 Cr, with copper, molybdenum and niobium additions. Nickel content of 32 to 38 % is above the range of standard austenitic stainless steels, so ASTM covers the grade under its B-series nickel alloy specifications and the trade treats it as a nickel alloy. The microstructure is austenitic, and several datasheets describe it as a super-austenitic or highly alloyed iron-base stainless steel. Both descriptions are in common use.

Function of the copper addition

Copper at 3 to 4 % is the addition that gives this alloy its resistance to sulfuric acid. In reducing acids such as sulfuric and phosphoric, copper holds the corrosion potential in the passive range, which chromium alone cannot do. Molybdenum at 2 to 3 % adds pitting and crevice corrosion resistance and gives a PREN of about 28, calculated as Cr + 3.3 Mo. The nickel content resists chloride stress corrosion cracking, which is a common cause of failure in 304 and 316L in chloride-bearing streams.

Function of the niobium addition

Niobium and tantalum are added at eight times the carbon content, to a maximum of 1.00 %. They combine with carbon to form stable niobium carbides, which prevents chromium carbide precipitation at grain boundaries during welding. Welded assemblies therefore go into service without post-weld heat treatment in most cases. The Cb in the designation stands for columbium, the older American name for niobium.

Alternative designations

The same material is sold as Alloy 20, Alloy 20Cb-3, Carpenter 20, Carpenter 20Cb-3, Incoloy alloy 020, Incoloy 20, 20Cb3 and NiCr20CuMo. The cast equivalent is ACI CN-7M under ASTM A351 and A743. Quoting UNS N08020 on an enquiry avoids confusion between these names.

Limits below are the ASTM B462 and B473 requirements for UNS N08020, in weight percent.

Nb+Ta is specified as eight times the actual carbon content as a minimum, capped at 1.00 %. At the 0.07 % carbon maximum this gives a 0.56 % Nb+Ta minimum.

Room-temperature minimum values for forged and solution-annealed material under ASTM B462.

Delivered tensile values are typically 551 to 689 MPa (80 to 100 ksi), with yield strength of 241 to 276 MPa (35 to 40 ksi). Heavy sections may require testing at a specified location. State the required test position on the enquiry.

Physical values are typical for the annealed condition and are given for design reference. They are not acceptance criteria. Certified values for a specific heat are available on request.

Forgings are normally supplied to ASTM B462 or B473. Where the project references an ASME code case, a client specification or NACE MR0175 / ISO 15156, state it on the enquiry so the route can be planned before melting.

Alloy 20 has a narrow hot working window. Forging below 980 °C risks cracking, and omitting the water quench after annealing reduces corrosion resistance. The route below is used on all Alloy 20Cb-3 orders.

1. Melt and refineAOD refining or vacuum induction melting. Vacuum arc or electroslag remelting is used where the drawing requires a cleaner ingot.
2. Heat to 1150 to 1230 °C (2100 to 2250 °F)Controlled soak so the full section reaches temperature before the first blow.
3. Open-die forge above 980 °C (1800 °F)Upset and draw to break down the cast structure. A cooling piece is reheated rather than forged.
4. Ring roll or shape to formRings are punched and rolled. Other parts are forged to shaft, disc, sleeve, tube sheet or flange blank with machining allowance.
5. Solution anneal at 940 to 1010 °C (1725 to 1850 °F)Minimum 30 minutes per 25 mm of section thickness to dissolve carbides.
6. Water quenchRapid cooling through the sensitisation range so chromium carbides cannot form at grain boundaries.
7. Machine and testRough or finish machining, then ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388, with tensile, hardness and chemical analysis.
8. Certify and shipEN 10204 3.1 works certificate, or 3.2 with third-party witness by TUV, SGS, BV or Lloyd's Register.

All parts are produced to drawing or to supplied dimensions. Common shapes:

Sizes outside this envelope are often possible. Send the drawing and feasibility will be confirmed before quotation.

Alloy 20 costs more than 316L and less than the nickel-molybdenum alloys. It is normally selected for sulfuric acid service where 316L will not last and where C-276 is not justified.

PREN is calculated as Cr + 3.3 Mo using mid-range composition. Relative cost applies to forged bar and moves with the nickel and molybdenum markets. Corrosion data should be confirmed against the actual medium, concentration, temperature and aeration.

Heat numbers are stamped on each piece and are traceable to the melt record. Third-party inspection can be arranged at any nominated hold point.

Where Alloy 20Cb-3 does not suit the medium, these grades are the usual alternatives: