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Austenitic SS 321

Austenitic SS 321

ALLOY 321 / ALLOY 321H | 1.4878 | X8CRNITI18-10 | S32100 / S32109

Austenitic SS 321 is a highly stabilized chromium-titanium or niobium-nitrogen alloy with exceptional oxidation resistance properties and creep strength.

The grade has properties like grade 304 with an exception of titanium stabilization which gives the grade better resistance to carbide precipitation during welding as well as heat treatment.

321 is widely used as a replacement of grade 304 where inter-granular corrosion is a problem. The un-stabilized grades when cooled slowly between 430-820°C tend to form chromium-carbide precipitation at the grain boundary, thus weakening the metal.

Titanium is a strong carbide former and makes up for an exceptional stabilizing element in the grade. An addition of titanium by five times the carbon content (0.70% max) ensures the stabilization of carbides at the heat affected zones.

This improves the intergranular corrosion resistance of the grade, especially after the processes followed by 430-820°C, which includes the intermetallic sigma phase. For this purpose, the grade can be used with continuous as well as intermittent service in the range of 430-820°C. The grade also has exceptional mechanical properties even at elevated temperatures. The grade is readily selected for applications below 950°C since it efficiently retains its strength, phase stability, and resists scaling. It has a high melting point of about 1450°C. The grades' refurbished version - 321H, which has higher carbon content can be suitably used at elevated temperatures for better strength.

Heating for prolonged periods should be avoided since it might affect the general resistance properties of the metal against different media. The grade 321 has got adequate creep resistance to sustain elevated temperatures and cyclic thermal loads. The grade is one of the most weldable alloys because of its titanium stabilization at the heat affected zone, preservation of corrosion resistance, and resistance to cracking. The grade is generally non-magnetic in annealed condition due to its microstructure. The same cannot be said after welding and cold working.

  • Chemical composition
  • Physical properties
  • General data
  • Standards Specifications
  • Forms of Supply
  • Corrosion Resistance
  • Heat Treatment
  • Weldability
  • Machining
  • Applications
  • Possible grade alternatives
Grade   Ni C Cr Mn Si P Ti S N Fe
321 Min. 9.0 - 17.0 - - 0 - - - 65.045
Max. 12.0 0.08 19.0 2.0 1.0 0.045 5(C+N) -0.70 0.030 0.10 -
321H Min. 9.0 0.04 17.0 - - - - - - 65.25
Max. 12.0 0.010 19.0 2.00 1.00 0.045 4(C+N) -0.70 0.030 0.10  
Density 0.286 lbs / in3
7.920 g /cm3
Melting Point 2550 – 2635°F
1398 – 1446°C
Specific Heat 0.12 BTU/lb-°F (32 – 212°F)
500 J/kg-°K (0 – 100°C)
Thermal Conductivity @ 212°F 9.3 BTU/hr/ ft2/ft /°F
16.0 W/m-°K
Elastic Modulus 28.0 x 106 psi
193 GPa
Electrical Resistivity 72 Microhm-cm at 20°C
Annealing 950-1120°C
Standard 321 321H
UNS S32100 S32109
WERKSTOFF NR. X6CrNiTi18-10 X10CrNiTi18-10
EN 1.4541 1.4878
BS 321S31 321S51
Swedish 2337 -
JIS SUS 321 SUS 321H
Pipes & Tubes
Summary Standards
Standard Specification for Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and Heat-Exchanger Tubes ASTM A213
Standard Specification for Welded Austenitic Steel Boiler, Superheater, Heat-Exchanger, and Condenser Tubes ASTM A249
Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Service ASTM A269
Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes ASTM A312
Standard Specification for Electric-Fusion-Welded Austenitic Chromium-Nickel Stainless Steel Pipe for High-Temperature Service and General Applications ASTM A358
Standard Specification For Seamless Stainless Steel Mechanical Tubing ASTM A511
Standard Specification for Welded Large Diameter Austenitic Steel Pipe for Corrosive or High-Temperature Service ASTM A409
Fittings & Flanges
Summary Standards
Standard Specification for Wrought Austenitic Stainless Steel Piping Fittings ASTM A403
Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service ASTM A182

Pipe and tubing, fittings and flanges, washers.

The grade has excellent resistance to oxidation as well as general corrosion with exceptional creep resistance. It can take thermal cyclic loads efficiently at elevated temperatures. The grade performs exceptionally well in the presence of organic and inorganic acids. The grade does not fare very well in the presence of chloride environments, and might be subject to stress corrosion cracking. Apart from this, the grade has good localized corrosion resistance like pitting and crevice corrosion resistance. With proper service, the grade could perform better than grade 304 at around 430-820°C. The inter-granular corrosion is easily resisted due to its titanium stabilization of carbides. It shows resistance to most oxidizing acids and agents, combustion gases, nitric acid and even sulphuric acid to some extent. The surface needs to be passivated to ensure maximum corrosion resistance. Also, it needs to be free of foreign particles and impurities.

The grade needs to be suitably annealed and stress relieved after various working processes. It is solution annealed at a temperature of about 930-1100°C and then cooled rapidly for maximum corrosion resistance. The process is normally done to impart the required softness and ductility. Stress can be suitably removed from the grains with proper heat treatment. A treatment of about 700°C will efficiently relieve stresses, without doing any damage to intergranular corrosion. However, more than required heating of the grade might result in lowering its corrosion resistance properties. Stabilizing anneal is sometimes required for better corrosion resistance. Stabilizing anneal is done around 900°C, above the general carbide precipitation range. At this range, the carbides that are previously formed dissociate, and the chromium is restored to its solid form, while the titanium is converted into a harmless titanium carbide. Hence, stabilization takes place. Also, the grade is non-hardenable by heat treatment.

The grade has great welding properties and can be welded by conventional methods like common fusion and electric resistance welding. The titanium stabilization gives the heat affected zone a relief from carbide precipitation. The grade needs to avoid hot cracking at the weld to manage its ferritic phase by using convenient weld fillers.

The grade work hardens very quickly, and hence proper machining techniques are required. Slow speeds, rigid tooling and powerful machinery is recommended. The grade can be formed and drawn as required.

Annealing covers, tempering equipment, firewalls, stack liners, exhaust systems, pressure vessels, superheaters, refinery equipment, bellows, expansion joints, aircraft equipment, bellows, furnace parts, chemical processing, pharmaceutical production, waste treatment.

Grades - 304
Grade   Ni C Cr Mn Si P Fe S N
304 Min. 8.00 - 18.00 - - - - - -
Max. 10.50 0.08 20.00 2.00 0.75 0.045 BAL. 0.030 0.10
Grade - 304L
Grade   Ni C Cr Mn Si P Fe S N
304L Min. 8.00 - 18.00 - - - - - -
Max. 10.50 0.08 20.00 2.00 0.75 0.045 BAL. 0.030 0.10
Grade - 310
Grade   Ni C Cr Mn Si P S N
310 Min. 19.0 - 24.0 - - - - -
Max. 22.0 0.25 26.0 2.00 1.50 0.045 0.030 0.10

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