The common designations of austenitic grades of stainless steel, such as grades 304 and 316, include sub-grades – L and H variants used for specific applications.
The low carbon “L” grades are used for applications involving welding treatments or high temperature exposure, e.g. welding of heavy or medium sections. The low carbon content of steels prevents carbide precipitation at grain boundaries, which can cause inter-granular corrosion under corrosive operation conditions. Carbide precipitation occurs at temperatures ranging from about 450 to 850°C.
Based on their applications, “L” grade steels are available in the form of pipe, plate and, most commonly, in round bars. The corrosion resistances of standard austenitic and “L” grades are identical in the absence of high temperature exposure or welding treatments.
"H" grades are high carbon stainless steel grades, having improved strength at high temperatures - generally above 500°C. These high carbon grades have high short-term and long-term creep strength. They are usually produced in the form of pipe and plate. Most commonly employed grades are grade 304H and 316H, but ASTM A240/A240M also covers high carbon versions of grade 309, 310, 321, 347 and 348. When used at temperatures of about 450-850°C, these grades are subjected to sensitization which, in turn, causes aqueous corrosion, and reduction in toughness and ductility at ambient temperatures.
Grades 304 and 304L have the same composition limits for all components except carbon. However, neither grade 304 nor grade 304L is specified to have minimum carbon content.
Grade 304H is a high carbon version of grade 304, and it has the same composition specification as that of standard grade 304. Grade 304H does not have the 0.10% nitrogen maximum limit which is applicable to both "L" and standard grades. All austenitic "H" grades should have a grain size of ASTM No 7.
The three grades 316, 316L and 316H are the major counterparts of grade 304. Only the percentage of carbon contents differentiates these grades. The following table provides the carbon content of the alternatives covered under ASTM A240/A240M:
NOTE: Specifications for some other products, particularly tube and pipe, have a limit of 0.035% or 0.040% maximum for 304L and 316L, but are otherwise the same.
In practice, steel mills generally ensure that the "L" grade heats meet the strength requirements of the standard grades.
Pressure vessel codes, such as AS1210, and pressure piping codes, such as AS4041, provide allowable working pressures for each of the stainless steel grades at high temperatures. These codes provide high-pressure ratings for standard stainless steel grades when compared to that of "L" grades at any temperature. The codes avoid utilization of "L" grade steels at temperature of 425 or 525°C.
The codes consist of a clause stating that standard grades, which contain at least 0.04% carbon, must be used at temperatures above 550°C. Therefore, grades 304 or 316, having 0.02% carbon, are not allowed to be used at high temperatures. The pressure vessel codes provide the same allowable pressure rating for "H" grades.
It is possible to use a product labelled as a standard grade instead of "L" or "H" grade, or vice versa, owing to the availability issues. This substitution can be made under the following conditions:
"L" grades can be used as standard grades as long as high temperature strength is not required, and the mechanical properties conform to the standard grade requirements. "L" grades nearly always comply with the standard grade requirements, but this must be investigated on each occasion.
Standard grades can be used as "L" grades if their carbon content meets the "L" grade limits of 0.030 or 0.035% maximum.
Standard grades are often used instead of "H" grades as their carbon contents meet the "H" limits of 0.04 to 0.10%. Additional testing may be required to satisfy the grain size requirement. "H" grades having a maximum 0.10% of nitrogen and 0.08% of carbon are used as standard grades.
It is very common for steel mills to supply "L" heats when standard grades have been ordered. In fact, some of the product and test certificates are marked "304/304L". In some cases the product is marked only as standard, or "L". The details provided on the mill test certificate will show if the alternative grades comply with the standard.
The requirement of an "H" grade for any high temperature application must be specified during the time of order. The required high carbon steel will be supplied, based on availability. The product and its test certificate will confirm grade compliance.
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