Austenitic SS Alloy 20

Austenitic SS Alloy 20

Alloy 20, popularly known as Carpenter 20, was developed as an austenitic alloy with nickel-iron-chromium to make it resistant to acid attack, especially sulfuric acid. The nickel content in alloy 20 makes it resistant to chloride-ion stress-corrosion cracking. 

The additional copper and molybdenum make it resistant to reducing environments. Molybdenum makes it resistant to pitting and crevice corrosion. The chromium content makes it resistant in oxidized environments just like nitric acid. Carpenter 20 steel is best resistant with superior mechanical properties and relatively easy fabrication. 

Resistance to Corrosion

When chloride stress corrosion cracking is the issue, alloy 20 stainless steel is the best option as it is resistant to pitting and crevice corrosion especially in piping applications. The niobium, when added to alloy 20, makes alloy 20 resistant to the effect of carbide precipitation during welding, hence making it durable and resistant to intergranular corrosion.

Alloy 20 stainless steel can fall under both the stainless and nickel categories as it has characteristics of both. Incoloy 20 was designed to make it maximum resistant to acid attack and demonstrates superior resistance to cracking due to stress-corrosion in boiling 20% to 40% sulfuric acid and also has superior general corrosion resistance to chloride stress and sulfuric acid corrosion cracking. 

Grade Density
Elastic Modulus
Co-eff of Thermal Expansion(μm/m/°C) Thermal Conductivity (W/m.K) Specific Heat
Electrical Resistivity
NICKEL ALLOY 20 8.05 193 25-100°C at 100°C 0-100°C 1.08
14.7 12.3 500
Fabrication and Heat Treatment

Both hot-working and cold-working can be used to form alloy 20 stainless steel using traditional methods. The Hot-work method is useful in temperatures between 2100°F and 2250°F with extreme temperature control ensuring that the material does not fall below 1800°F before forging. After hot working, the material needs to be annealed by heating to 1725°F and 1850°F for a minimum of 30 minutes per inch of thickness followed by a water quench.

Cold working is needed on annealed material of alloy 20 stainless steel. Carpenter 20 steel has a work-hardening rate as good as austenitic stainless steel and the forming equipment must be adapted accordingly. When cold working is implemented, interstage annealing cannot be avoided. After cold reductions of more than 15%, final annealing to make it stable is required before use.

GRADE Ni Cr Si C Mn P S Fe Cu Mo Nb
NICKEL ALLOY 20 MIN % 32 19 3 2
MAX % 38 21 ≤1 ≤0.07 ≤2 ≤0.045 ≤0.035 35 4 3 ≤1

Incoloy Alloy 20 or UNS N08020 is an austenitic superalloy with nickel, iron, chromium with additions of Copper and Molybdenum which make it resistant to hostile environments, pitting, and crevice corrosion. Alloy 20 can also be stabilized with Columbium to lessen carbide precipitation during welding.

This superalloy is excellently resistant to general corrosion, pitting, and crevice corrosion in chemicals with contents of chlorides and sulfuric, nitric and phosphoric acids. It also contains niobium to make it stable against sensitization and resultant intergranular corrosion. 

How to weld alloy 20?

Methods like TIG & MIG can be used to weld alloy 20 or UNSN08020 along with the submerged arc welding (SAW) process except for the oxyacetylene welding process. Alloy 20 can also be hot formed using the same forces needed to hot work austenitic stainless steels. In terms of machinability, extra-ordinary finishes are possible using the same set-up and process speeds used for austenitic stainless steels 316 or 317.

Industries and Application
  • Synthetic rubber manufacturing equipment
  • Process industries like pharmaceuticals, plastics and organic and heavy chemicals
  • Tanks, heat exchanger, piping, pumps, valves, and other process equipment
  • Acid cleaning and pickling equipment
  • Chemical process piping, reactor vessels
  • Bubble caps
  • Petrochemical process equipment
  • Food and dye production

Also, this superalloy is used in mixing tanks, heat exchangers, metal cleaning and pickling equipment, and piping.

  • ASME SB-462 – SB-464
  • ASME SB-468
  • ASTM B 462 – B 464
  • ASTM B 468
  • ASTM B 472 – B 474
  • ASTM B 751

Other common names: Carpenter 20, 20Cb-3, INCOLOY alloy 20, UNS N08020, W.Nr. 2.4660