Background

      Alloy 304 (UNS S30400) stainless steel PIPE is a variation of the 18% chromium – 8% nickel austenitic alloy, the most familiar and most frequently used alloy in the stainless steel family. This alloy may be considered for a wide variety of applications and exhibits good corrosion resistance, ease of fabrication, excellent formability, and high strength with low weight.
      Grade 304L, the low carbon version of 304, does not require post-weld annealing and so is extensively used in heavy gauge components (over about 6mm).
      Grade 304H with its higher carbon content finds application at elevated temperatures. Alloy 304H is a modification of Alloy 304 in which the carbon content is controlled to a range of 0.04-0.10 to provide improved high temperature strength to parts exposed to temperatures above 800°F. ?The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.

Key Properties

Composition (ASTM A312/A312M)
      Typical compositional ranges for grade 304 stainless steels are given in table 1.
Table 1. Composition ranges for 304 grade stainless steel

Mechanical Properties (ASTM A312/A312M)
      Typical mechanical properties for grade 304 stainless steels are given in table 2.
Table 2. Mechanical properties of 304 grade stainless steel

Physical Properties
      Typical physical properties for annealed grade 304 stainless steels are given in table 3.
Table 3. Physical properties of 304 grade stainless steel in the annealed condition

Grade Specification Comparison
      Approximate grade comparisons for 304 stainless steels are given in table 4.
Table 4. Grade specifications for 304 grade stainless steel

Possible Alternative Grades
      Possible alternative grades to grade 304 stainless steels are given in table 5.
Table 5. Possible alternative grades to 304 grade stainless steel

Corrosion Resistance
1.General Corrosion
      The Alloys 304, 304L, and 304H 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.
      The 18 to 20 percent of chromium which these alloys contain provides resistance to oxidizing environments such as dilute nitric acid, as illustrated by data for Alloy 304 below.

      Alloys 304, 304L, and 304H are also resistant to moderately aggressive organic acids such as acetic and reducing acids such as phosphoric. The 8 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 Alloy 304L may show a lower corrosion rate than the higher carbon Alloy 304. The data for formic acid, sulfamic acid, and sodium hydroxide illustrate this. Otherwise, the Alloys 304, 304L, and 304H 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 Alloy 304L is preferred for use in such media in the welded condition since the low carbon level enhances resistance to intergranular corrosion.
      Excellent in a wide range of atmospheric environments and many corrosive media. 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.
2.Intergranular Corrosion
      Exposure of the 18-8 austenitic stainless steels to temperatures in the 800°F to 1500°F (427°C 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 Alloy 304 may allow sensitization to occur from thermal conditions experienced by autogenous welds and heat-affected zones of welds. For this reason, the low carbon Alloy 304L 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.

3.Stress Corrosion Cracking
      The Alloys 304, 304L, and 304H are the most susceptible of the austenitic stainless steels to stress corrosion cracking (SCC) in halides because of their relatively low nickel content. Conditions which cause SCC 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 tube sheets 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 cold deformation, thereby reducing sensitivity to halide SCC. The low carbon Alloy 304L material is the better choice for service in the stress-relieved condition in environments which might cause intergranular corrosion.

4.Pitting/Crevice Corrosion
      The 18-8 alloys have been used very successfully in fresh waters containing low levels of chloride ion. Generally, 100 ppm chloride is considered to be the limit for the 18-8 alloys, particularly if crevices are present. Higher levels of chloride might cause crevice corrosion and pitting. For the more severe conditions of higher chloride levels, lower pH, and/or higher temperatures, alloys with higher molybdenum content such as Alloy 316 should be considered. The 18-8 alloys are not recommended for exposure to marine environments.
Heat Resistance
      Good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C. Continuous use of 304 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important. Grade 304L is more resistant to carbide precipitation and can be heated into the above temperature range.
      Grade 304H has higher strength at elevated temperatures so is often used for structural and pressure-containing applications at temperatures above about 500°C and up to about 800°C. 304H will become sensitised in the temperature range of 425-860°C; this is not a problem for high temperature applications, but will result in reduced aqueous corrosion resistance.

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 1121°C). Cooling from the anneal temperature should be at sufficiently high rates through 1500-800°F (816°C - 427°C) to avoid reprecipitation of chromium carbides.
These materials cannot be hardened by heat treatment.
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 Alloys 304 and 304L 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 (Alloy 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, Alloys 304 and 304L are designed to resolidify with a small amount of ferrite to minimize cracking susceptibility.
Dual Certification
      It is common for 304 and 304L to be stocked in "Dual Certified" form, particularly in plate and pipe. These items have chemical and mechanical properties complying with both 304 and 304L specifications. Such dual certified product does not meet 304H specifications and may be unacceptable for high temperature applications.

Applications
          Typical applications include:
          Food processing equipment, particularly in beer brewing, milk processing & wine making.
          Kitchen benches, sinks, troughs, equipment and appliances
          Architectural panelling, railings & trim
          Chemical containers, including for transport
          Heat Exchangers
          Woven or welded screens for mining, quarrying & water filtration
          Threaded fasteners
          Springs