Grade L vs Grade H Stainless Steel Pipe: What the Suffix Changes in the Standard

A pipe specification readingTP316L and one reading TP316H look like the same material with a different letter. They are not. The suffix changes the carbon limit, the minimum tensile and yield strength, the annealing temperature and, for the H grades, it introduces a mandatory grain size requirement.

Every figure below comes from ASME BPVC 2023 Section II Part A-1, SA-312/SA-312M. Conditions on size or wall thickness are stated.

Carbon is the whole story

Carbon is what the L and H suffixes control. In austenitic stainless steel, carbon drives chromium carbide precipitation at grain boundaries when the material is heated into the 600-900 °C range, which is sensitisation. Chromium bound as carbide is chromium no longer available to form the passive oxide film, and the grain boundary becomes the weak point.

SA-312 Table 1 sets three distinct carbon limits across the austenitic grades:

Across the carbon column the pattern is unambiguous. The L grades carry a carbon maximum. The plain grades carry a higher maximum. The H grades carry a carbon range with a mandatory floor.

That floor is the important part. A grade specified as 0.08 max carbon can arrive at 0.02. A grade specified as 0.04-0.10 cannot. The H grade is a deliberate guarantee that the material has enough carbon to form austenite properly during welding, and enough of it to maintain creep strength at elevated temperature.

Note D: the L grade is not one number

SA-312 Table 1 note D modifies the 0.035 % carbon maximum on TP304L and TP316L: for small diameter or thin walls or both, where many drawing passes are required, a carbon maximum of 0.040 % is necessary. Small outside diameter tubes are those under 0.500 in. [12.7 mm] in outside diameter; light wall tubes are those under 0.049 in. [1.20 mm] average wall thickness, or 0.044 in. [1.10 mm] minimum wall thickness.

So the L grade is not one chemistry. On small-diameter or light-wall pipe, 0.040 % is the applicable ceiling. Specifying 0.035 max on small-diameter pipe creates a condition the specification itself anticipates as difficult to meet.

Strength: the L grade gives some up

SA-312 Table 4 sets minimum tensile and yield strength. The L grades are lower, not higher.

The drop from 75 to 70 ksi tensile and 30 to 25 ksi yield is the arithmetic consequence of the carbon ceiling. This is the trade the L grade makes: post-weld corrosion resistance bought with about 5 ksi of tensile and 5 ksi of yield.

TP321 and TP321H also carry a wall thickness condition. Above 0.375 in. [9.50 mm] seamless, the minimum drops to 70 [485] ksi and 25 [170] ksi for TP321, and 70 [480] MPa with 25 [170] ksi for TP321H. Quoting only the 75 [515] ksi figure is incomplete for heavy-wall seamless pipe.

The N suffix works differently again. TP316N raises tensile to 80 [550] ksi and yield to 35 [240] ksi, with nitrogen at 0.10-0.16 % rather than the 0.10 % max applying to TP316 and TP316L. TP316LN returns strength to 75 [515] ksi and 30 [205] ksi while keeping the higher nitrogen range.

Grain size: only the H grades carry a requirement

SA-312 Table 5 is a short table, and it is the clearest single answer to what the H suffix is for. Grain size determinations under ASTM E112 are required for these grades and no others in the common austenitic range:

SA-312 section 11.4 requires grain size determinations on each heat treatment lot for the grades in Table 5. No L grade appears in that table.

The requirement exists for creep. Austenitic stainless steel creeps under sustained elevated-temperature load, and creep resistance is strongly related to grain size: coarse grains mean fewer grain boundaries, and grain boundaries are where creep cavitation and cracking initiate. A coarse ASTM grain size number is a low number, so grain size 7 is coarser than grain size 4. The H grade guarantees a deliberately coarse-grained structure because coarse grains resist creep.

Annealing temperatures differ between the two

SA-312 Table 2 sets the annealing requirement, and the H grades do not all share the same treatment.

SA-312 section 6.2 requires all pipe to be furnished in the heat-treated condition per Table 2, then carves out an exception: other than for grades S33228, S30815, S31272 and the H grades, seamless pipe immediately following hot forming may be quenched or rapidly cooled, provided the temperature after hot forming is not less than the minimum specified. The H grades are excluded from that shortcut and get the full defined treatment.

Table 2 note D also requires rapid cooling at a rate sufficient to prevent re-precipitation of carbides, as demonstrated by the capability of pipes to pass ASTM A262 Practice E. The manufacturer need not run that test unless it is specified on the purchase order. The certification basis for carbide prevention is therefore a capability the mill asserts, not a test you receive by default.

Choosing between them

Use the L grade for welded fabrication in corrosive service. Post-weld heat-affected-zone corrosion is why 304L and 316L exist as distinct grades, and the low carbon maximum is the mechanism. For pipe welded into a system then exposed to acid or chloride condensate, the L grade is the correct default.

Use the H grade for elevated-temperature service. Creep strength and grain size are the reason. Continuous service well above ambient, particularly in the 550 °C range and above, is where the ASTM E112 grain size requirement and the higher strength minimums start to matter.

Use stabilised grades TP321 and TP347H where the service is both hot and welded. These tie titanium or niobium to carbon during annealing, and SA-312 supplementary requirement S6 requires a stabilisation heat treatment after the solution anneal, at a temperature agreed between purchaser and vendor. They are also the only 300-series grades in Table 5 besides the H plain grades, so they carry a grain size requirement.

Do not accept a bare suffix in a quotation. Write the grade and UNS number together, TP316L / S31603 or TP316H / S31609. The UNS number removes ambiguity about what the mill is supplying, and it is the number that must appear on the material test report.

For grade identification before ordering, or for stock in a specific grade and size, send us the specification and quantity and we will confirm what the mill will actually certify. Our nickel alloy seamless pipe is supplied by grade and UNS designation with full material certification, and the complete pipe and tube catalogue covers the austenitic, duplex and nickel alloy families. Request a quotation through the contact page or on WhatsApp.