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Technical Information for
302
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Alloy
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UNS
Number
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SAE
Number
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| 302 |
S30200 |
30302 |
GENERAL PROPERTIES
Types 302, 304, 304L, and 305 stainless steels are variations of the 18 percent chromium – 8 percent nickel austenitic alloy, the most familiar and most frequently used alloy in the stainless steel family. These alloys may be considered for a wide variety of applications where one or more of the following properties are important:
- Resistance to corrosion
- Prevention of product contamination
- Resistance to oxidation
- East of fabrication
- Excellent formability
- Beauty of appearance
- Ease of cleaning
- High strength with low weight
- Good strength and toughness at cryogenic temperatures
- Ready availability of a wide range of product forms
Each alloy represents an excellent combination of corrosion resistance and fabricability. This combination of properties is the reason for the extensive use of these alloys which represent nearly one half of the total U.S. stainless steel production. Type 304 represents the largest volume followed by Type 304L. Types 302 and 305 are used in smaller quantities. These alloys are covered by a variety of construction or use of equipment manufactured from these alloys for specific conditions. Food and beverage, sanitary, cryogenic, and pressure-containing applications are examples. Past users of Type 302 are generally now using Type 304 since AOD technology has made lower carbon levels more easily attainable and economical. There are instances, such as in temper rolled products, when Type 302 is preferred over Type 304 since the higher carbon permits meeting of yield and tensile strength requirements while maintaining a higher level of ductility (elongation) versus that of the lower carbon Type 304. Type 304L is used for welded products which might be exposed to conditions which could cause intergranular corrosion in service. Type 305 is used for applications requiring a low rate of work hardening during severe cold forming operations such as deep drawing. |
RESISTANCE TO CORROSION
General Corrosion
The Types 302, 304, 304L and 305 austenitic stainless steels provide useful resistance to corrosion on a wide range of moderately oxidizing to moderately reducing environments. The alloys are used widely in equipment and utensils for processing and handling of food, beverages and dairy products. Heat exchangers, piping, tanks and other process equipment in contact with fresh water also utilize these alloys. Building facades and other architectural and structural applications exposed to non-marine atmospheres also heavily utilize the 18-8 alloys. In addition, a large variety of applications involve household and industrial chemicals. The 18 to 19 percent of chromium which these alloys contain provides resistance to oxidizing environments such as dilute nitric acid. These alloys are also resistant to moderately aggressive organic acids such as acetic, and reducing acids such as phosphoric. The 9 to 11 percent of nickel contained by these 18-8 alloys assists in providing resistance to moderately reducing environments. The more highly reducing environments such as boiling dilute hydrochloric and sulfuric acids are shown to be too aggressive for these materials. Boiling 50 percent caustic is likewise too aggressive.
In some cases, the low carbon Type 304L alloy may show a lower corrosion rate than the higher carbon Type 304 alloy. The data for formic acid, sulfuric acid and sodium hydroxide illustrate this. Otherwise, the Types 302, 304, 304L and 305 alloys may be considered to perform equally in most corrosive environments. A notable exception is in environments sufficiently corrosive to cause intergranular corrosion of welds and heat-affected zones on susceptible alloys. The Type 304L alloy is preferred for use in such media in the welded condition since the lower carbon level enhances resistance to intergranular corrosion.
Intergranular Corrosion
Exposure of the 18-8 austenitic stainless steels to temperatures in the 800°F to 1500°F (427° to 816°C) range may cause precipitation of chromium carbides in grain boundaries. Such steels are “sensitized” and subject to intergranular corrosion when exposed to aggressive environments. The carbon content of Types 302, 304 and 305 may allow sensitization to occur from thermal conditions experienced by autogenous welds are heat-affected zones of welds. For this reason, the low carbon Type 304L alloy is preferred for applications in which the material is put into service in the as-welded condition. Low carbon content extends the time necessary to precipitate a harmful level of chromium carbides, but does not eliminate the precipitation reaction for material held for long times in the precipitation temperature range.
Stress Corrosion Cracking
The Type 302, 304, 304L and 305 alloys are the most susceptible of the austenitic stainless steels to stress corrosion cracking in halides because of their relatively low nickel content. Conditions which cause stress corrosion cracking are: (1) presence of halide ions (generally chloride), (2) residual tensile stresses, and (3) temperatures in excess of about 120°F (49°C). Stresses may result from cold deformation of the alloy during forming, or by roller expanding tubes into tubesheets, or by welding operations which produce stresses from the thermal cycles used. Stress levels may be reduced by annealing or stress relieving heat treatments following deformation, thereby reducing sensitivity to halide stress corrosion cracking. The low carbon Type 304L material is the better choice for service in the stress relieved condition in environments which might cause intergranular corrosion.
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PHYSICAL PROPERTIES
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Melting
Point
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Density
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Specific
Gravity
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Modulus
of Elasticity
in Tension
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2550-2590° F
1399-1421° C
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.285 lb/in³
7.90 g/cm³
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7.90
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29 X 106 psi
200 Gpa
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MECHANICAL PROPERTIES
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Alloy
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Temper
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Tensile
Strength
Minimum
(psi)
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Yield
Strength
Minimum 0.2% offset
(psi)
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%
Elongation
in 2" Minimum
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Notes
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| 302 | Annealed | 75,000 | 30,000 | 40 % | - | | 302 | 1/4 Hard | 125,000 | 75,000 | 10 % | - | | 302 | 1/2 Hard | 150,000 | 110,000 | 9 % | - | | 302 | Full Hard | 185,000 | 140,000 | 3 % | - |
All values specified are approximate
minimums unless otherwise specified. Values are derived from the applicable
AMS and ASTM specifications.
CHEMICAL PROPERTIES
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Alloy
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C
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Mn
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P
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S
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Si
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Cr
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Ni
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Mo
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Cu
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N
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Other
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| 302 | .15 | 2.00 | .045 | .030 | 1.00 | 17.00-19.00 | 8.00-10.00 | .75 | .75 | .10 | - |
All values are maximum values unless otherwise
specified. Values are derived from applicable AMS and ASTM specifications.
WELDING
| The 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. The Types 302, 304, 304L and 305 alloys are typical of the austenitic stainless steels.
Two important considerations in producing weld joints in the austenitic stainless steels are: (1) preservation of corrosion resistance, and (2) avoidance of cracking.
A temperature gradient is produced in the material being welded which ranges from above the melting temperature in the molten pool to ambient temperature at some distance from the weld. The higher the carbon level of the material being welded, the greater the likelihood that the welding thermal cycle will result in the chromium carbide precipitation which is detrimental to corrosion resistance. To provide material at the best level of corrosion resistance, low carbon material (Type 304L) should be used for material put in service in the welded condition. Alternately, full annealing dissolves the chromium carbide and restores a high level of corrosion resistance to the standard carbon content materials.
Weld metal with a fully austenitic structure is more susceptible to cracking during the welding operation. For this reason, Types 302, 304, and 304L alloys are designed to resolidify with a small amount of ferrite to minimize cracking susceptibility. Type 305, however, contains virtually no ferrite on solidification and is more sensitive to hot cracking upon welding than the other alloys.
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HEAT TREATMENT
| The austenitic stainless steels are heat treated to remove the effects of cold forming or to dissolve precipitated chromium carbides. The surest heat treatment to accomplish both requirements is the solution anneal which is conducted in the 1850°F to 2050°F range (1010°C to °C). Cooling from the anneal temperature should be at sufficiently high rates through 1500-800°F (816°C – 427°C) to avoid precipitation of chromium carbides.
These materials cannot be hardened by heat treatment. |
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