Stainless Steel 321, 321H,

321 is generally the preferred grade for use in what can be described as the carbide precipitation range - 425oC to 850oC. It is, essentially, the standard austenitic 18/8 (18% chromium, 8% nickel) Grade 304 to which titanium has been added to stabilise it.
It offers good performance up to temperatures of around 900oC - high strength, resistance to scaling and embrittlement.

Properties

Stainless steel 321 is an austenitic steel with excellent welding and forming characteristics.

It can be brake or roll formed. The addition of titanium means results in post-weld annealing being unnecessary. However, it should be noted that titanium does not transfer well across the welding arc so it is generally recommended that Stainless 347 is used for welding rods and electrodes. (347 employs niobium and tantalum in preference to titanium)

Grade 321 contains between 0.3% and 0.7% Titanium. Titanium helps to control the grain size and readily forms carbides thereby improving strength, toughness and corrosion resistance.

It also maintains its strength down to cryogenic temperatures

t is the second most commonly used stainless steel and represents around 20% of the entire world production of stainless steel.

The key element that differentiates it from most other stainless steels is the inclusion of molybdenum, between 2% and 3%, which enhances its resistance corrosion in general and pitting corrosion in particular.

It is malleable and ductile and has good weldability. Its austenitic structure allows it to be deep drawn without intermediate annealing. It is also unnecessary to anneal it following welding thin sections.

However, the inclusion of molybdenum may have some adverse effects on its formability.

It is widely used in:

  • Chemical processing and storage equipment.
  • Refinery equipment
  • Medical devices
  • Marine environments, especially those with chlorides present

In its annealed state it is virtually non-magnetic even when cold worked, unlike 304 which can be significantly attracted to a magnet after cold working. This may make it more suitable in some applications.

What are the main uses of stainless 321?

Stainless 321 is similar in its properties to 304 but its resistance to the precipitation of chromium carbide in the temperature range 425 - 850oC results in it being the grade of choice for any 304 type application where those temperatures are likely to be encountered.

Typical applications include:

  • Exhaust Systems
  • Manifolds
  • Chemical Plant
  • Heat Exchangers
  • Piping
  • Furnace Parts
  • Chimney and Stack Liners
  • Storage Tanks
  • Bellows

What are its main benefits of 321?

Corrosion Resistance

The major benefit of 321 is its resistance to intergranular corrosion.

Austenitic steels are prone to intergranular corrosion; when they are cooled the chromium combines with the carbon to form chromium carbide, depleting the passive chromium layer and form wider grain boundaries which are then attacked by the corrosive materials. The formation of chromium carbide often occurs during welding. Provided the parts to be welded are thin, the duration of the heat application should be short enough to avoid the problem but when thicker parts have to be welded the higher temperature persists and chromium carbide precipitation is likely. 

If the materials are to used at operating temperatures of between 425 and 850oC, 321 will resist the formation of chromium carbide and the danger of intergranular corrosion

Grade 321 stainless steel has higher creep and stress rupture properties than 304 and particularly 304H. This makes it particularly suitable for boiler and pressure vessels.

Heat resistance

321 resists oxidation up to 900oC in intermittent use and to 925oC in continuous use. It also avoids intergranular corrosion in the range 425 and 850oC.

Stabilised 321

321 can be further stabilised by heating to between 870 and 890.C for 2 hours per 25mm thickness. This process is recommended for use in the more severe environments and the higher temperature ranges.

What are the limitations 321?

While 321 has excellent performance at temperatures up to 925oC this should be regarded as the limit of its operating range. If operational temperatures are at the top end of this range consideration should be given to using the 321H variant - the extra carbon producing better high-temperature strength.

Stress Corrosion Cracking

Austenitic stainless steels can be subject to stress corrosion cracking (SCC) and 321 is no exception. 321 is susceptible to SCC in the presence of halides and as a result of cold deformation. 

Stabilising as described in the previous section and Stress Relieving  by heating to 700oC for 2 hours will reduce the danger of Stress Corrosion Cracking.

Marine Environments - Pitting and Crevice Corrosion

321 is not advised for marine environments - rapid corrosion and severe discolouration can be expected.

It should be emphasised the production of a good surface finish and good welding techniques do much to reduce the vulnerability of all forms of corrosion. However, 321 does not polish well and it is not a good choice if a decorative finish is required

Chemical composition of 321

It is the addition of Titanium (Ti) that confers the resistance to carbide precipitation and intergranular corrosion to 321. The quantity of titanium is defined by reference to the quantity of carbon and nitrogen present in the steel. 321 must have 5 times the quantity of carbon and nitrogen combined to a maximum of 0.7%. In the case of 321H, it must contain 4 times.

In addition, a number of other chemicals may be present but these are expressed as permitted levels with the exception of the increased quantity of carbon required in 321H - i.e. a minimum of .04% and a maximum of 0.10%

GradeCMnSiPSCrTiNiN
321 min.
max.		 -
0.08		 -
2.0		 -
0.75		 -
0.045		 -
0.030		 9.0
12.0		 5(C+N)
0.7		 9.0
12.0		 -
0.10
321H min.
max.		 0.04
0.10		 -
2.0		 -
0.75		 -
0.045		 -
0.030		 19.0
12.0		 4(C+N)
0.7		 9.0
12.0		 -

Due to the fact that there is an overlap in the constituents of the two specifications, it is not uncommon to find 321 as dual certified. However, if you are seeking the best high-temperature performance the selection of 321H certified as containing the higher carbon content will give you greater strength at elevated temperatures. 

Detailed properties

Physical Properties

 

Metric

English

Density

7.9g/cc

0.2854 lb/in³

               

Mechanical Properties

Hardness, Brinell

217

217

Converted from Rockwell B hardness.

Hardness, Rockwell B

95

95

  

Tensile Strength, Ultimate

515 MPa

74694 psi

  

Tensile Strength, Yield

205 MPa

29732 psi

at 0.2% offset

Elongation at Break

40 %

40 %

in 50 mm

Modulus of Elasticity

193  GPa

27992 ksi

  

Electrical Properties

Electrical Resistivity

7.2e-005 ohm-cm

7.2e-005 ohm-cm

at 20°C (68°F); 1.16E-04 at 650°C (1200°F)

Thermal Properties

CTE, linear 20°C

16.6 µm/m-°C

from 0-100°C

CTE, linear 250°C

17.2 µm/m-°C

at 0-315°C (32-600°F)

CTE, linear 500°C

18.6 µm/m-°C

at 0-650°C

Specific Heat Capacity

0.5 J/g-°C

from 0-100°C (32-212°F)

Thermal Conductivity

16.1 W/m-K

at 0-100°C, 21.5 W/m°C at 500°C

Melting Point

1398-1446° C

 2550-2635° F

  

Variants - 321H - and also 347

The main variant of 321 is 321H which contains a higher quantity of carbon. 321 may contain up to 0.08% while 321H must contain between 0.04% and 0.08% carbon.

The higher quantity of carbon confers greater strength at elevated temperatures.

347 is mentioned here since it is broadly similar to 321 but uses Niobium in place of Titanium. Niobium easily forms very hard, extremely small grained carbides giving it even greater resistance to intergranular corrosion.

Mention should also be made of 316Ti in which the titanium has broadly the same effect on 316 making it suitable where the performance required is similar to that of 321 but in a marine environment.

Forms available

Forms Available

  • Tube
  • Pipe
  • Fittings
  • Flanges
  • Special Sections
  • Sheet
  • Plate
  • Flat Bar
  • Round Bar
  • Hollow Bar
  • Angles
  • i Beam
  • U Channel

Austenitic