Chemical Formula
Fe, <0.08% C, 17-19% Cr, 9-12% Ni, <2% Mn, <1% Si, 0.3-0.7% Ti, <0.045% P, <0.03% S
Background
Grade 321 and 347 include the basic austenitic 18/8 steel (Grade 304) stabilised by Titanium (321) or Niobium (347) additions. These grades are widely-used because they are not responsive to intergranular corrosion after heating inside the carbide precipitation selection of 425-850°C. Grade 321 is the products selection for applications from the temperature range up to about 900°C, combining high strength, resistance to scaling and phase stability with effectiveness against subsequent aqueous corrosion.
Grade 321H is a modification of 321 which has a higher carbon content, to supply improved temperature strength.
A limitation with 321 is that titanium won't transfer well across a high temperature arc, same goes with a bad idea like a welding consumable. In this case grade 347 is preferred - the niobium performs exactly the same carbide stabilisation task but tend to be transferred across a welding arc. Grade 347 is therefore the standard consumable for welding 321.
Grade 347 is merely occasionally utilized as parent plate material.
Like other austenitic grades, 321 and 347 have excellent forming and welding characteristics, are readily brake or roll formed and still have outstanding welding characteristics. Post-weld annealing is not required. They likewise have excellent toughness, even down to cryogenic temperatures. Grade 321 does not polish well, same goes with not recommended for decorative applications.
Like other austenitic grades, 321 and 347 have excellent forming and welding characteristics, are readily brake or roll formed and still have outstanding welding characteristics. Post-weld annealing is not required. They likewise have excellent toughness, even down to cryogenic temperatures. Grade 321 does not polish well, same goes with not recommended for decorative applications.
Grade 304L is much more easily obtainable generally in most product forms, and so is usually used in preference to 321 in the event the requirement is just for effectiveness against intergranular corrosion after welding. However 304L has lower hot strength than 321 and so isn't the best choice if the requirement is effectiveness against an operating environment over about 500°C.
Key Properties
These properties are specified for flat rolled product (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.
Composition
Typical compositional ranges for grade 321 stainless steel is 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.
max
|
-
0.08
|
2.00
|
0.75
|
0.045
|
0.030
|
17.0
19.0
|
-
|
9.0
12.0
|
0.10
|
Ti=5(C+N)
0.70
|
321H
|
min.
max
|
0.04
0.10
|
2.00
|
0.75
|
0.045
|
0.030
|
17.0
19.0
|
-
|
9.0
12.0
|
-
|
Ti=4(C+N)
0.70
|
347
|
min.
max
|
0.08
|
2.00
|
0.75
|
0.045
|
0.030
|
17.0
19.0
|
-
|
9.0
13.0
|
-
|
Nb=10(C+N)
1.0
|
Mechanical Properties
Typical mechanical properties for grade 321 stainless steels 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 50mm) 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
|
321H also has a requirement for a grain size of ASTM No 7 or coarser.
| |||||
Physical Properties
Typical physical properties for annealed grade 321 stainless steels 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 steels 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
|
These comparisons are approximate only. The list is intended as a comparison of functionally similar materials not as a schedule of contractual equivalents. If exact equivalents are needed original specifications must be consulted.
| |||||||
Possible Alternative Grades
Possible alternative grades to grade 321 stainless steels 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 mild "high temperature" environment is present… up to about 600°C.
|
304H
|
Only 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.
|
S30815
(253MA)
|
The temperature of the operating environment is up to about 1150°C- too high for 321 or 321H.
|
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 200mg/L chlorides at ambient temperatures, reducing to about 150mg/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.
Stabilising - heat to 870-900°C for 1 hour per 25mm of thickness and air cool. Stabilisation 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.
Welding
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; high silicon version of 347 is also pre-qualified for welding of 321.
Applications
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
• Stainless steel 321 blind flange for Natural gas and Oil pipeline
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