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Stainless Steel - Grade 321 (UNS S32100)

Grades 321 and 347 are the basic austenitic 18/8 steel (Grade 304) stabilized by Titanium (321) or Niobium (347) additions. These grades are used because they are not sensitive to intergranular corrosion after heating within the carbide precipitation range of 425-850 °C. Grade 321 is the grade of choice for applications in the temperature range of up to about 900 °C, combining high strength, resistance to scaling and phase stability with resistance to subsequent aqueous corrosion.

Grade 321H is a modification of 321 with higher carbon content, to provide improved high-temperature strength.

A limitation with 321 is that titanium does not transfer well across a high-temperature arc, so it is not recommended as a welding consumable. In this case grade 347 is preferred - the niobium performs the same carbide stabilization task but can be transferred across a welding arc. Grade 347 is, therefore, the standard consumable for welding 321. Grade 347 is only occasionally used as parent plate material.

Like other austenitic grades, 321 and 347 have excellent forming and welding characteristics, are readily brake or roll-formed and have outstanding welding characteristics. Post-weld annealing is not required. They also have excellent toughness, even down to cryogenic temperatures. Grade 321 does not polish well, so is not recommended for decorative applications.

Grade 304L is more readily available in most product forms, and so is generally used in preference to 321 if the requirement is simply for resistance to intergranular corrosion after welding. However, 304L has lower hot strength than 321 and so is not the best choice if the requirement is resistance to an operating environment over about 500 °C.

Key Properties

These properties are specified for the flat-rolled products (plate, sheet, and coil) in ASTM A240/A240M. Similar but not necessarily identical properties are specified for other products such as pipe and bar in their respective specifications.


Typical compositional ranges for grade 321 stainless sheets of steel are given in table 1.

Table 1. Composition ranges for 321-grade stainless steel

Grade   C Mn Si P S Cr Mo Ni N Other
321 min.
2.00 0.75 0.045 0.030 17.0
- 9.0
0.10 Ti=5(C+N)
321H min.
2.00 0.75 0.045 0.030 17.0
- 9.0
- Ti=4(C+N)
347 min.
0.08 2.00 0.75 0.045 0.030 17.0
- 9.0
- Nb=10(C+N)


Mechanical Properties

Typical mechanical properties for grade 321 stainless sheets of steel are given in table 2.

Table 2. Mechanical properties of 321-grade stainless steel

Grade Tensile Strength (MPa) min Yield Strength 0.2% Proof (MPa) min Elongation (% in 50 mm) min Hardness
Rockwell B (HR B) max Brinell (HB) max
321 515 205 40 95 217
321H 515 205 40 95 217
347 515 205 40 92 201


Physical Properties

Typical physical properties for annealed grade 321 stainless sheets of steel are given in table 3.

Table 3. Physical properties of 321-grade stainless steel in the annealed condition

Grade Density (kg/m3) Elastic Modulus (GPa) Mean Coefficient of Thermal Expansion (μm/m/°C) Thermal Conductivity (W/m.K) Specific Heat 0-100 °C (J/kg.K) Electrical Resistivity (nΩ.m)
0-100 °C 0-315 °C 0-538 °C at 100 °C at 500 °C
321 8027 193 16.6 17.2 18.6 16.1 22.2 500 720


Grade Specification Comparison

Approximate grade comparisons for 321 stainless sheets of steel are given in table 4.

Table 4. Grade specifications for 321-grade stainless steel

Grade UNS No Old British Euronorm Swedish SS Japanese JIS
BS En No Name
321 S32100 321S31 58B, 58C 1.4541 X6CrNiTi18-10 2337 SUS 321
321H S32109 321S51 - 1.4878 X10CrNiTi18-10 - SUS 321H
347 S34700 347S31 58G 1.4550 X6CrNiNb18-10 2338 SUS 347


Possible Alternative Grades

Possible alternative grades to grade 321 stainless sheets of steel are given in table 5.

Table 5. Possible alternative grades to 321-grade stainless steel

Grade Why it might be chosen instead of 321
304L The requirement is for resistance to intergranular corrosion, not for high-temperature strength
3CR12 Only a mild "high temperature" environment is present… up to about 600 °C.
304H Only a mild "high temperature" environment is present… up to about 800 °C.
310 The temperature of the operating environment is up to about 1100 °C - too high for 321 or 321H.
The temperature of the operating environment is up to about 1150 °C- too high for 321 or 321H.

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Corrosion Resistance

Equivalent to Grade 304 in the annealed condition, and superior if a weldment in these grades has not been post-weld annealed or if the application involves service in the 425-900 °C range. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60 °C. Considered resistant to potable water with up to about 200 mg/L chlorides at ambient temperatures, reducing to about 150 mg/L at 60 °C.

Heat Resistance

Good oxidation resistance in intermittent service to 900 °C and in continuous service to 925 °C. These grades perform well in the 425-900 °C range, and particularly where subsequent aqueous corrosive conditions are present. 321H has higher hot strength and is particularly suitable for high-temperature structural applications.

Heat Treatment

Solution Treatment (Annealing) - heat to 950-1120 °C and cool rapidly for maximum corrosion resistance.

Stabilizing - heat to 870-900 °C for 1 hour per 25mm of thickness and air cool. Stabilization is recommended for most severe service conditions (above 425 °C) and particularly for material annealed at the upper side of the annealing temperature range.

Stress Relief - Heat to 700 °C for 1 to 2 hours and air cool.

These grades cannot be hardened by thermal treatment.


Excellent weldability by all standard fusion methods, both with and without filler metals. AS 1554.6 pre-qualifies welding of 321 and 347 with Grade 347 rods or electrodes; a high silicon version of 347 is also pre-qualified for welding of 321.


Typical applications include:

  • Aircraft exhaust manifolds
  • Expansion joints
  • Bellows
  • Furnace parts
  • Heating element tubing
  • Heat Exchangers
  • Woven or welded screens for high-temperature mineral processing
  • Spiral Welded tube for burner pipes and flues


  1. R Durda R Durda United States says:

    Have you ever encountered TiC inclusions in 321 thin sheet and tubing?  also, are large TiC phases precip'ed from GTA welding temperatures?

  2. أحمد الخويلدي أحمد الخويلدي Libya says:

    What happens to the properties of stainless steel 321 increase in the current method of welding tig.


    What is the minimum HRC for C 95 SS MATERIAL.?

  4. John Leeds John Leeds Canada says:

    Why should a graphite pencil not be used to mark 321ss that will be used for an exhaust system.

    • Ibrahim Imran Butt Ibrahim Imran Butt U.A.E. says:

      It will increase the carbon content in SS321 if welding is carried out where graphite markings are present. This is undesirable for welds and may contribute to cracking on heat cycles.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoM.com.

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