Alloy 20 (UNS N08020) Open-Die Forgings

What is Alloy 20?

Alloy 20, designated UNS N08020 and often sold under the trade names Alloy 20Cb-3 and Carpenter 20, is a superaustenitic nickel-iron-chromium alloy. It was developed in the 1950s to solve a single problem that neither stainless steel nor the nickel alloys of the day could solve economically: resisting sulfuric acid across the concentration range where corrosion is worst.

Jiangyin Jiangnan Metal Co., Ltd. puts the trade-off to customers this way. Alloy 20 costs roughly three to four times as much as 316L and roughly a third as much as Hastelloy C-276, and its corrosion performance in sulfuric acid sits in the same proportion. When 316L pits and C-276 is over-specified, Alloy 20 is usually the correct answer.

Is Alloy 20 stainless steel or a nickel alloy?

Both descriptions circulate, and the confusion is understandable. Iron is still the largest single element at roughly 35%, which makes the alloy behave like a stainless steel in the shop. But at 32–38% nickel it carries three times the nickel of 316L, and ASTM classifies it under its nickel alloy standards (B462, B472, B473) rather than its stainless standards. The precise term is superaustenitic nickel-iron-chromium alloy.

Why does Alloy 20 resist sulfuric acid?

Three additions do the work. Understanding them covers most of what a buyer needs to know.

Copper, 3–4%

This is the element that defines Alloy 20. Sulfuric acid is a reducing acid, and chromium oxide films, the mechanism that protects ordinary stainless steel, are not stable in reducing conditions. Copper shifts the alloy's corrosion potential into the passive region, so a protective film forms where chromium alone would fail. Remove the copper and Alloy 20 becomes an ordinary, unremarkable stainless grade.

Molybdenum, 2–3%

Molybdenum handles what copper does not: localised attack. Real process acid is rarely pure, and chloride contamination drives pitting and crevice corrosion at exactly the crevices a forged flange face or a valve seat creates. Molybdenum stabilises the passive film against chloride penetration.

Niobium, at least eight times the carbon content

This addition is what makes Alloy 20 practical as a forged and welded component. Between roughly 425 °C and 815 °C, chromium carbides precipitate at grain boundaries and strip the adjacent metal of chromium. This is sensitisation, and it becomes intergranular corrosion in service. Niobium forms carbides preferentially, leaving chromium in solution. That is why a properly stabilised Alloy 20 weld usually does not require post-weld solution annealing.

Alloy 20 chemical composition

Composition limits below are those of ASTM B473 for UNS N08020, which Jiangyin Jiangnan Metal Co., Ltd. applies to all Alloy 20 forging stock.

Alloy 20 mechanical and physical properties

What Alloy 20 forgings does Jiangyin Jiangnan Metal produce?

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. Open-die working suits Alloy 20 well: the alloy is expensive enough that near-net closed dies rarely pay back, and most demand is for one-off or small-batch pressure-containing parts.

These are the figures Jiangyin Jiangnan Metal Co., Ltd. publishes for its open-die works as a whole. Alloy 20 is a costly nickel-bearing grade and practical sizes are set by billet availability, so the Alloy 20 envelope is confirmed per enquiry rather than quoted as a fixed maximum. Product forms produced in Alloy 20 are forged shafts, discs, seamless rolled rings, sleeves, hollow bars, blocks, flange blanks and tube sheets.

Delivery condition is solution annealed and water quenched as standard, supplied rough machined with an agreed allowance or finish machined to drawing. Heat treatment available in-house covers annealing, normalising, and hardening and tempering.

How is an Alloy 20 forging produced?

The route below is the standard eight-stage sequence used by Jiangyin Jiangnan Metal Co., Ltd. The stages run in this order because each one depends on the last.

1. Billet sourcing and PMI

   Alloy 20 billet to ASTM B472 is received with the mill certificate and independently verified by positive material identification before it is released to the forge. Mixed-grade incidents happen at goods-in, not at the press.

2. Preheat and soak

   The billet is through-heated to 1120–1180 °C and held until core and surface equalise. Alloy 20 conducts heat poorly, roughly 11.7 W/m·K against about 50 for carbon steel, so soak time is generous.

3. Upsetting

   The billet is upset under the press to break down the cast dendritic structure. A forging ratio of at least 3:1 is targeted to guarantee sound, refined grain through the section.

4. Drawing, punching, expanding

   The piece is drawn down to a shaft, or punched and expanded over a saddle mandrel for rings and sleeves. Reductions are kept moderate because Alloy 20 work-hardens quickly and is unforgiving of heavy bites.

5. Controlled reheats

   The forging returns to the furnace as often as needed so deformation always finishes above 925 °C. Chasing one more blow out of a cooling piece is the single most common cause of Alloy 20 forging cracks.

6. Solution annealing and quenching

   The forging is solution annealed at 940–1010 °C and rapidly water quenched. Cooling speed through 815–425 °C determines whether the part has the corrosion resistance the drawing calls for.

7. Rough machining and ultrasonic testing

   The forging is rough machined to a testable surface, then volumetrically inspected by ultrasonic testing to ASTM A388 / ASME SA-388 against the acceptance class stated on the order.

8. Testing and certification

   Heat chemistry, tensile and yield strength, elongation, hardness and intergranular corrosion resistance to ASTM G28 Method A are documented on an EN 10204 3.1 inspection certificate.

Which standards apply to Alloy 20 forgings?

Alloy 20 vs 316L, 904L and Hastelloy C-276

The question Jiangyin Jiangnan Metal Co., Ltd. is asked most often is not "what is Alloy 20" but "is Alloy 20 enough". This table is the short answer.

Read the last two rows together. Alloy 20 buys most of C-276's sulfuric acid performance for roughly a third of the money. Where the medium carries strong oxidisers, hot hydrochloric acid or wet chlorine, that trade stops working and C-276 becomes the correct specification.

Where are Alloy 20 forgings used?

The failures we are asked to investigate are almost never composition failures. They are quench failures. A forging that cooled slowly through 815 to 425 °C carries the right mill certificate chemistry and corrodes anyway.

What to include in an Alloy 20 forging enquiry

Jiangyin Jiangnan Metal Co., Ltd. can quote from a sketch, but a complete enquiry is quoted faster and more accurately. Include:

Frequently asked questions about Alloy 20

Glossary

Open-die forging

Hot forging between flat or simply shaped dies that do not enclose the workpiece. Used for large shafts, discs, rings and blocks where tooling for closed dies would never pay back.

Solution annealing

Heating to dissolve precipitated carbides into solid solution, then quenching fast enough to keep them dissolved. For Alloy 20: 940–1010 °C followed by a water quench.

Sensitisation

Precipitation of chromium carbides at grain boundaries between roughly 425 °C and 815 °C, depleting chromium in the adjacent metal and causing intergranular corrosion.

Niobium stabilisation

Adding niobium at eight times the carbon content or more so that niobium carbides form preferentially and chromium stays in solution.

Forging ratio

The ratio of starting cross-section to finished cross-section. A measure of how thoroughly the cast structure has been broken down.

EN 10204 3.1

An inspection certificate issued by the manufacturer's own independent inspection department, reporting actual test results for the material delivered.

Jiangyin Jiangnan Metal Co., Ltd. (2026). "Alloy 20 (UNS N08020)
Open-Die Forgings." Jiangyin, Jiangsu, China.
https://www.steelforgepieces.com/Nickel-Alloy/Alloy-20.html
Last reviewed 11 August 2026.