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L & H Grades of Stainless Steels
"L", "H" AND STANDARD GRADES OF STAINLESS STEELS
Within the usual designations of the common austenitic grades of stainless steel, such as 304 and 316, there are "sub-grades" - "L" and "H" variants - with particular applications.
The low carbon "L" grades are useful where welding or
other high temperature exposure will occur, particularly
welding of medium or heavy sections. The low carbon is
one way of delaying or preventing grain boundary
chromium carbide precipitation (often referred to as
sensitisation) which can result in intergranular
corrosion in many corrosive service environments. As
shown in the timetemperature- sensitisation curves
below, the precipitation of carbides occurs over time at
WHAT "H" GRADES ARE & WHY THEY ARE USED
"H" grades are the higher carbon versions of each of the
standard grades. The high carbon results in
WHAT THE DIFFERENCES ARE
1. Composition limits for 304 and 304L are identical in all
respects except for carbon content (304L
2. The high carbon version of 304 is 304H. This has the same
composition specification as standard
3. The three grades 316, 316L and 316H are exact counterparts
to the 304 series. Again only the Compositions of
the alternatives are therefore as in the following table
(from ASTM A240/A240M-07; for full compositions refer to the
Compositions of the alternatives are therefore as in the following table (from ASTM A240/A240M-07; for full compositions refer to the standard).
Specifications for some other products, particularly tube and
pipe, have a carbon limit of 0.035% or
4. There are also mechanical property specification
differences (again from ASTM A240/A240M):
5. There are no dimensional or other differences between
standard, "L" and "H" grades.
6. Pressure vessel codes (e.g. AS 1210) and pressure piping
codes (e.g. AS 4041) give allowable
7. The pressure vessel codes give the same allowable
pressure rating for "H" grades as for standard
ALTERNATIVE GRADE USAGE
Because of availability issues it is sometimes desirable
to be able to use a product labelled as a standard grade
when an "L" or "H" grade has been specified, or vice
versa. Such substitution can be made under the following
1. "L" grades can be used as standard grades so long as the
mechanical properties (tensile and yield)
2. Australian pressure codes generally preclude use of “L”
grades at high temperature (over about
3. Standard grades can be used as "L" grades so long as their
carbon content meets the "L" grade
4. Standard grades can often be used in place of "H" grades so
long as their composition (carbon and chromium) meet the "H"
limits. The grain size requirement may be satisfied by extra
5. "H" grades can be used as standard grades so long as their
carbon contents are 0.07% (304) or
6. It has become quite common for steel mills to supply "L"
heats when standard grades have been
7. If an application requires an "H" grade - generally for
high temperature applications - this must be specified at
time of order. Subject to availability Atlas Specialty
Metals will supply the required high carbon content steel,
but full compliance with "H" grade specification may require
8. All product is unambiguously traced through the Atlas Specialty Metals stock management system and marked with full identification. Certification can therefore be provided, which may enable alternative grade usage.
It is common practice for certain products including plate,
pipe and some bar to be stocked as “dual
AS 1210-1997 "Pressure Vessels"
AS 4041-1998 "Pressure Piping"
This article is taken from Atlas Tech Note 8