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Technical Information for 321

Alloy
UNS Number
SAE Number
 321
 S32100
 30321


GENERAL PROPERTIES

Type 321 is a stabilized stainless steel which offers as its main advantage an excellent resistance to intergranular corrosion following exposure to temperatures in the chromium carbide precipitation range from 800 to 1500 F (427 to 816 C). Type 321 is stabilized against chromium carbide formation by the addition of titanium.

While Type 321 continues to be employed for prolonged service in th4e 800 to 1500 F (427 to 816 C) temperature range, Type 304L has supplanted this stabilized grade for applications involving only welding or short time heating.

Type 321 stainless steel s also advantageous for high temperature service because of its good mechanical properties. Type 321 stainless steel offers higher creep and stress rupture properties than Type 304 and, particularly, Type 304L which might also be considered for exposures where sensitization and intergranular corrosion are concerns. This results in higher elevated temperature allowable stresses for this stabilized alloy for ASME Boiler and pressure Vessel Code applications. The Type 321 alloy has a maximum use temperature of 1500F (816 C) for code applications like Type 304, whereas Type 304L is limited to 800 F (426 C).



RESISTANCE TO CORROSION

General Corrosion The Type 321 alloy offers similar resistance to general, overall corrosion as the unstabilized chromium nickel Type 304. Heating for long periods of time in the chromium carbide precipitation range may affect the general resistance of Type 321 in corrosive media.

Intergranular Corrosion
Type 321 has been developed for applications where the unstabilized chromium-nickel steels, such as Type 304 would be susceptible to intergranular corrosion.

When the unstabilized chromium-nickel steels are held in or slowly cooled through the range of 800 to 1500 F (427 to 816 C), chromium carbide is precipitated at the grain boundaries. In the presence of certain strongly corrosive media, these grain boundaries are preferentially attached, a general weakening of the metal results, and a complete disintegration may occur.

Organic media or weakly corrosive aqueous agents, mil and other dairy products, or atmospheric conditions rarely produce intergranular corrosion even when large amounts of precipitated carbides are present. When thin gauge material is welded the time in the temperature range of 800 to 1500 F (427 to 816 C) is so short that with most corroding media the unstabilized type material is generally satisfactory. The extent to which carbide precipitation may be harmful depends upon the length of time the alloy was exposed to 800 to 1500 F (427 to 816 C) and upon the corrosive environment. Even the longer heating times involved in welding heavy gauges are not harmful to the unstabilized L grade alloys where the carbon content is kept to low amounts of 0.03% or less.

In general, Type 321 is used for heavy welded equipment which is operated between 800 to 1500 F (427 to 816 C) or slowly cooled through this range. Experience gained in a wide range of service conditions has provided sufficient data to generally predict the possibility of intergranular attach in most applications. Please review the comments under the HEAT TREATMENT section.

Stress Corrosion Cracking
The Type 321 austenitic stainless steel is susceptible to stress corrosion cracking (SCC) in halides similar to Type 304 stainless steel. This results because of their similarity in nickel content. Conditions which cause SCC are: (1) presence of halide ion (generally chloride), (2) residual tensile stresses, and (3) environmental temperatures in excess of about 120 F (49 C). Stresses may result from cold deformation during forming operations, or from thermal cycles encountered during welding operations. Stress levels may be reduced by annealing or stress-relieving heat treatments following cold deformation. Type 321 is a good choice for service in the stress-relieved condition in environments which might otherwise cause intergranular corrosion for unstabilized alloys.

Type 321 is particularly useful under conditions which cause polythionic acid stress corrosion of non-stabilized austenitic stainless steels such as Type 304. Exposure of non-stabilized austenitic stainless steel to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. On cooling to room temperature in a sulfide-containing environment, the sulfide (often hydrogen sulfide) reacts with moisture and oxygen to form polythionic acids which attach the sensitized grain boundaries. Under conditions of stress, intergranular cracks form. Polythionic acid SCC has occurred n oil refinery environments where sulfides are common. The stabilized Type 321 alloy offers a solution to polythionic acids SCC by resisting sensitization during elevated temperature service. For optimum resistance, these alloys should be used in the thermally stabilized condition if service related conditions may result in sensitization.

Pitting/Crevice Corrosion
The resistance of the stabilized Type 321 alloy to pitting and crevice corrosion in the presence of chloride ion is similar to that of Type 304 or Type 304L stainless steels because of similar chromium content. Generally, 100 ppm chloride in aqueous environments is considered to be the limit for both the unstabilized and the stabilized alloys, particularly if crevices are present. Higher levels of chloride ion might cause crevice corrosion and pitting. For more severe conditions of higher chloride level, lower pH and/or higher temperature, alloys with molybdenum, such as Type 316, should be considered. The stabilized Type 321 alloy passes the 100 hour, 5% neutral salt spray test (ASTM-B-117) with no rusting or staining of samples. However, exposure of these alloys to salt mists from the ocean would be expected to cause pitting and crevice corrosion accompanied by severe discoloration. The Type 321 alloy is not recommended for exposure to marine environments.



PHYSICAL PROPERTIES

Melting Point
Density
Specific Gravity
Modulus of Elasticity
in Tension
 2550-2635 F
1398-1446 C
 .286 lb/in
7.92 g/cm
 7.92
 28 X 106 psi
193 Gpa


MECHANICAL PROPERTIES

Alloy
Temper
Tensile Strength
Minimum
(psi)
Yield Strength
Minimum 0.2% offset
(psi)
% Elongation
in 2" Minimum
Notes
321
Annealed
75,000
30,000
40 %
-
All values specified are approximate minimums unless otherwise specified. Values are derived from the applicable AMS and ASTM specifications.


CHEMICAL PROPERTIES

Alloy
C
Mn
P
S
Si
Cr
Ni
Mo
Cu
N
Other
321
.08
2.00
.045
.030
.75
17.00-19.00
9.00-12.00
.75
.75
.10
Ti=5x(C+N) min to .70 max
All values are maximum values unless otherwise specified. Values are derived from applicable AMS and ASTM specifications.


WELDING

Austenitic stainless steels are considered to be the most weldable of the high-alloy steels and can be welded by all fusion and resistance welding processes.

Two important considerations in producing weld joints in the austenitic stainless steels are: (1) preservation of corrosion resistance and (2) avoidance of cracking.

It is important to maintain the level of stabilizing element present in Type 321 during welding. Type 321 is more prone to loss of titanium. Care needs to be exercised to avoid pickup of carbon from oils and other sources and nitrogen from air. Weld practices which include attention to cleanliness and good inert gas shielding are recommended for these stabilized grades as well as other non-stabilized austenitic alloys.

Weld metal with a fully austenitic structure is more susceptible to cracking during the welding operation. For this reason, Type 321 is designed to resolidify with a small amount of ferrite to minimize cracking susceptibility. Columbium stabilized stainless steels are more prone to hot cracking than titanium stabilized stainless steels.

Matching filler metals are available for welding Type 321 stabilized stainless steel. Stabilized alloys may be joined to other stainless steels or carbon steels.



HEAT TREATMENT

The annealing temperature range for Type 321 is 1800 to 2000 F (928 to 1093 C). While the primary purpose of annealing is to obtain softness and high ductility, this steel may also be stress relief annealed within the carbide precipitation range 800 to 1500 F (427 to 816 C), without any danger of subsequent intergranular corrosion. Relieving strains be annealing for only a few hours in the 800 to 1500F (427 to 816 C) range will not cause any noticeable lowering in the general corrosion resistance, although prolonged heating within this range does tend to lower the general corrosion resistance to some extent. As emphasized, however, annealing in the 800 to 1500 F (427 to 816 C) temperature range does not result in a susceptibility to intergranular attack.

For maximum ductility, the higher annealing range of 1800 to 2000 F (928 to 1093 C) is recommended.

When fabrication chromium-nickel stainless steel into equipment requiring the maximum protection against carbide precipitation obtainable through use of a stabilized grade, it is essential to recognize that there is a difference between the stabilizing ability of columbium and titanium. For these reasons the degree of stabilization and of resulting protection may be less pronounced when Type 321 is employed.

When maximum corrosion resistance is called for, it may be necessary with Type 321 to employ a corrective remedy which is known as a stabilizing anneal. It consists of heating to 1550 to 1650 F (843 to 899 C) for up to 5 hours depending on thickness. This range is above that within which chromium carbides are formed and is sufficiently high to cause dissociation and solution of any that may have been previously developed. Furthermore, it is the temperature at which titanium combines with carbon to form harmless titanium carbides. The result is that the chromium is restored to solid solution and carbon is forced into combination with titanium as harmless carbides.

When heat treatments are done in an oxidizing atmosphere the oxide should be removed after annealing in a descaling solution such as a mixture of nitric and hydrofluoric acids. These acids should be thoroughly rinsed off the surface after cleaning.

This alloy cannot be hardened by heat treatment.