316L vs 316H Stainless Steel: Carbon Content, Creep Strength, and High-Temperature Selection

Ask any materials engineer to name the two most common austenitic stainless steels, and they will say 316L and 316H - often without realizing that these two "siblings" were designed for completely opposite jobs. 316L is the weldable, corrosion-resistant workhorse of chemical plants, marine equipment, and food processing. 316H is the high-temperature specialist, built to resist creep - the slow, permanent deformation that metals undergo when held under stress at elevated temperature for years or decades.

The entire difference between the two grades comes down to two variables: carbon content and grain size. 316L caps carbon at 0.03% to prevent weld sensitization. 316H requires carbon between 0.04% and 0.10% plus a coarse grain size of ASTM No. 7 or coarser, both of which maximize creep strength. This is not a "better or worse" comparison - it is a "right tool for the right temperature" comparison. Using 316L above 425°C (800°F) in pressure service risks creep failure; using 316H for a welded chemical tank invites intergranular corrosion. Selecting the wrong grade in either direction has caused plant shutdowns, vessel ruptures, and catastrophic failures.

The decision rule in one sentence: below 425°C (800°F) and welded or corrosion-critical, choose 316L; above 425°C where creep strength and ASME Code high-temperature ratings matter, choose 316H. The two grades are complementary, not interchangeable - and confusing them is one of the most common material-selection errors in the process and power industries.

Chemical Composition - Carbon Content and the Grain Size Requirement

316L and 316H share the same core chemistry - 16-18% chromium, 10-14% nickel, and 2-3% molybdenum - and differ in carbon content and grain size. 316L caps carbon at 0.03% (maximum weldability), while 316H specifies carbon of 0.04-0.10% plus a mandatory coarse grain size of ASTM No. 7 or coarser. The molybdenum in both grades is what separates the entire 316 family from 304: it provides resistance to pitting and crevice corrosion in chloride environments.

Both 316L (UNS S31603) and 316H (UNS S31609) are molybdenum-bearing austenitic stainless steels in the 300 series. The "L" in 316L stands for "Low Carbon" (≤ 0.03%). The "H" in 316H stands for "High Carbon" (0.04-0.10%), and it carries an additional requirement that no other 300-series grade has: a coarse grain size of ASTM No. 7 or coarser. This grain size requirement is what makes 316H fundamentally different from plain 316 with merely elevated carbon - the coarse grain is essential for creep resistance, and it is verified by microstructural examination on the mill test report.

Why Does Carbon Content Point the Two Grades in Opposite Directions?

Carbon plays two opposing roles in 316-series stainless steel. At high temperature, carbon strengthens the metal by forming fine chromium carbide precipitates that pin grain boundaries against creep deformation - this is why 316H needs 0.04% minimum carbon. But during welding, that same carbon migrates to grain boundaries and depletes chromium, causing intergranular corrosion (sensitization) - this is why 316L caps carbon at 0.03%. In short: carbon is 316H's source of high-temperature strength, and 316L's absence of carbon is its source of weld integrity. You cannot have both benefits in a single chemistry - which is why the two grades exist.

To understand this trade-off, picture carbon atoms as doing two different jobs at two different temperatures. In the 425-870°C (800-1,600°F) sensitization range - reached in the heat-affected zone of a weld - carbon reacts with chromium to form chromium carbides at grain boundaries, stealing chromium from the surrounding metal and creating microscopic corrosion paths. 316L's ultra-low carbon means there is not enough carbon to cause this damage, so welded 316L stays corrosion-resistant without post-weld heat treatment.

But at sustained high temperature (roughly 500-700°C), the story reverses. Creep - the slow stretching of metal under load - happens primarily by grain boundaries sliding past each other and by dislocation movement. A fine dispersion of chromium carbide particles at the grain boundaries acts like pins that lock the boundaries in place, dramatically slowing creep. This is exactly the strengthening mechanism that 316H's higher carbon provides. 316L, with almost no carbon to form these pinning carbides, creeps faster at high temperature. Adding to the advantage, 316H's coarse grain size means fewer grain boundaries per unit volume - and since grain boundaries are the weak link in creep, fewer boundaries equal better creep resistance.

The Grain Size Requirement - What Makes 316H Truly "H"

The grain size requirement of ASTM No. 7 or coarser is the defining feature that separates 316H from ordinary high-carbon 316. Coarse grains reduce the total grain-boundary area, and since creep deformation is concentrated at grain boundaries, coarse-grained material resists creep far better than fine-grained material of the same chemistry. This is why a material test report for 316H must show the grain size - a 316 material with the right carbon but a fine grain is NOT 316H and will not perform the same in creep service.

Grain size is measured by ASTM E112 and reported as a number; higher numbers mean finer grains. A typical annealed 316L has a grain size of ASTM No. 5-8, often on the finer side after solution annealing. 316H is deliberately produced with a grain size of ASTM No. 7 or coarser (typically No. 3-7 in practice), achieved through controlled solution-annealing temperature and time. This is one of the few cases in stainless steel where a "coarser" microstructure is "better" - a counterintuitive point that confuses engineers who learned that fine grain size improves room-temperature strength and toughness. The truth is that fine grain helps at room temperature, but coarse grain wins at creep temperatures.

[Source] ASTM A240/A240M - Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip. ASTM E112 - Standard Test Methods for Determining Average Grain Size.

Mechanical Properties

At room temperature in the annealed condition, 316H has slightly higher minimum tensile and yield strength than 316L (515 MPa vs 485 MPa tensile; 205 MPa vs 170 MPa yield), because the higher carbon content provides modest precipitation strengthening. However, this 6-15% difference is small, and both grades are ductile and formable. The room-temperature strength difference is NOT the reason to choose one grade over the other - the real differentiator appears only at elevated temperature.

Elevated-Temperature Strength and the ASME Allowable Stress Divide

The decisive mechanical difference appears above about 425°C (800°F). ASME Boiler and Pressure Vessel Code Section II, Part D lists allowable stress values for 316L only up to 800°F (427°C) - beyond that, 316L has no Code-listed allowable stress because its creep strength is not reliably guaranteed. 316H, by contrast, is listed up to 1,500°F (816°C). This single fact is the practical rule that governs the entire 316L vs 316H selection decision in pressure-vessel and piping design.

The reason for the 800°F (427°C) ceiling on 316L is creep. At temperatures above roughly 0.4 times the melting point (expressed in Kelvin), metals begin to deform permanently under loads well below their room-temperature yield strength. For austenitic stainless steel, this creep regime begins around 500°C (930°F), and by 600°C (1,112°F) it dominates design. Because 316L has neither the carbon nor the coarse grain to resist creep, ASME simply does not publish allowable stresses for it above 800°F - a designer literally cannot use 316L for a Code-stamped pressure component at 600°C. 316H, engineered specifically for creep, retains Code-listed allowable stress all the way to 1,500°F (816°C).

[Source] ASME Boiler and Pressure Vessel Code, Section II, Part D - Properties (Metric). Values shown are representative; consult the current Code edition for design.

Creep Strength

Creep is the slow, permanent deformation of a metal held under stress at high temperature - even at a stress far below its room-temperature yield strength. A pipe that easily holds 100 bar at 20°C can, at 600°C, slowly swell, thin, and rupture over months or years under the same pressure. Creep is the dominant failure mechanism in high-temperature equipment such as boiler tubes, superheater headers, refinery furnace tubes, and steam piping - and it is precisely the failure mode that 316H is engineered to resist and 316L is not.

To understand creep intuitively, think of stretching a piece of warm caramel or a soft cheese. Pull gently and it stretches slowly and permanently - it does not snap back. Metals do the same thing at high temperature: the atoms inside the metal gain enough thermal energy to move and rearrange under load. Over time, this atomic movement accumulates as permanent deformation. At room temperature, this movement is so slow it is irrelevant over a human lifetime. At 600°C, it happens fast enough to rupture a pressure vessel in a few years - which is why creep, not yield strength, governs high-temperature design.

Creep is typically characterized by the creep rupture strength: the stress that causes fracture after a specified time at a specified temperature, commonly reported at 100,000 hours (about 11.4 years). For example, a material might have a 100,000-hour creep rupture strength of 100 MPa at 600°C, meaning that a component loaded to 100 MPa will, on average, survive 100,000 hours before rupture. Design codes use these creep rupture values, divided by a safety factor, to set the allowable stress at high temperature - which is why 316L has no allowable stress above 800°F.

Why 316H Resists Creep Better Than 316L

316H resists creep through two reinforcing mechanisms that 316L lacks: (1) higher carbon (0.04-0.10%) forms fine chromium carbide precipitates at grain boundaries, which pin the boundaries and prevent them from sliding; (2) coarse grain size (ASTM No. 7 or coarser) means fewer grain boundaries per unit volume, and since grain boundaries are the weak link in creep, fewer boundaries equal less creep. 316L has neither mechanism, so it creeps significantly faster at high temperature.

Mechanism 1 - carbide pinning. In 316H, the carbon forms a dispersion of chromium-rich M₂₃C₆ carbides at the grain boundaries. These carbides act like pins driven through the grain boundaries, locking adjacent grains together and resisting the boundary sliding that drives creep deformation. 316L, with almost no carbon, forms far fewer of these pinning carbides, so its grain boundaries slide more freely under load at high temperature.

Mechanism 2 - coarse grain. Creep deformation concentrates at grain boundaries because boundaries are where the atomic structure is disordered. A fine-grained material has many boundaries (more weak links); a coarse-grained material has few. By requiring ASTM grain size No. 7 or coarser, 316H minimizes the number of weak links, directly reducing the creep rate. This is a rare case in metallurgy where coarser is better - the exact opposite of the fine-grain preference for room-temperature strength and toughness.

The combined effect is substantial. At 600°C (1,112°F), the 100,000-hour creep rupture strength of 316H is roughly 30-60% higher than that of 316L of equivalent base chemistry. In practical terms, a 316H boiler tube can carry the same steam load at 600°C for two to three times longer than a 316L tube - or carry a higher load for the same design life. For equipment with a 20-30 year design life, this difference is the difference between success and premature failure.

Creep Applications - Where 316H Earns Its Keep

316H is the standard material for elevated-temperature service in the 500-700°C (930-1,290°F) range, especially in power generation and refining: boiler superheater and reheater tubes, steam headers and steam piping, refinery furnace and reformer tubes, and high-temperature reactor vessels and heat-exchanger tubing. It is chosen specifically where long-term creep strength - not just short-term hot strength - is the governing design criterion.

High-Temperature Selection

The single most important number in the 316L vs 316H decision is 425°C (800°F). Below this temperature, 316L is fully acceptable and is usually the better choice because of its superior weldability and resistance to intergranular corrosion. Above this temperature, in any Code-governed pressure application, 316L is disallowed and 316H (or another creep-rated alloy) becomes mandatory. Memorize this threshold: it resolves the vast majority of 316L vs 316H questions.

The 800°F limit for L-grades is not arbitrary - it is written into ASME BPVC Section II, Part D, which simply does not list allowable stresses for 316L above 800°F. Because ASME Section VIII (pressure vessels) and ASME B31.3 (process piping) require designers to use these listed allowable stresses, a Code-stamped component cannot legally use 316L above 800°F. This is true even if a supplier tells you 316L "should hold up" at 600°C - the material may physically survive for a while, but it has no Code-approved stress basis, and its creep life is not guaranteed.

Temperature Selection Ladder

Oxidation - The Other High-Temperature Limit

Even 316H has an upper service temperature. Above about 870°C (1,600°F) in air, the protective chromium oxide scale on 316H begins to break down, and continuous service causes progressive oxidation (scaling) and loss of section. For continuous service above roughly 870°C, switch to a heat-resistant grade (such as 310S or 253MA) or a nickel alloy (such as Inconel 625 or 601). Between 816°C and 870°C, 316H may be used intermittently or under light load, but oxidation life must be evaluated case by case.

High-temperature corrosion is fundamentally different from the aqueous corrosion that dominates at low temperature. At low temperature, corrosion is electrochemical - it needs a liquid electrolyte. At high temperature, corrosion is oxidation - the metal reacts directly with oxygen, sulfur, or carbon in the gas. The chromium in 316H forms a protective Cr₂O₃ scale that resists oxidation up to roughly 870°C. Above that, the scale becomes less protective, and the underlying metal is progressively consumed.

This is why "high temperature" material selection has two independent questions: "Is the creep strength sufficient?" and "Will the metal oxidize away?" - and both must be answered before a grade is selected.

Welding and Sensitization

Here is the core irony of the 316 family: the very carbon that gives 316H its high-temperature creep strength is the same carbon that makes 316H vulnerable to weld sensitization. When 316H is welded and the heat-affected zone passes through 425-870°C, its carbon precipitates as chromium carbides at grain boundaries, depleting chromium and creating susceptibility to intergranular corrosion if the component is later exposed to a corrosive aqueous environment. 316L, with almost no carbon, welds cleanly without this risk - which is why 316L is the standard choice for welded, low-temperature, corrosive service.

This trade-off is why the two grades complement rather than compete. 316H components (boiler tubes, steam headers) operate above 500°C in steam or combustion gas - environments where aqueous intergranular corrosion does not occur, so sensitization is not a practical concern. 316L components (chemical tanks, marine piping, food equipment) operate below 425°C in aqueous or mildly acidic environments where intergranular corrosion is a real threat, so the low-carbon advantage is essential. Choosing the wrong grade inverts this logic and invites failure.

Welding Practice for 316L vs 316H

Can You Weld 316L to 316H?

Yes - 316L and 316H are metallurgically compatible and can be welded together, but the filler metal and service temperature must be considered together. For a low-temperature joint, use ER316L filler so the weld metal resists sensitization. For a high-temperature creep joint, the weld metal must have matching carbon and the same creep strength as the 316H base metal - using a low-carbon filler in a creep joint creates a weak link that will creep-fail first. Consult the applicable code (ASME B31.3, Section VIII) for the specific filler metal requirements of high-temperature 316H joints.

This is a subtle but important point that engineers often miss. In a welded joint at room temperature, the weld metal is usually stronger than the base metal, so a slightly weaker weld is still acceptable. But in creep service, the weld metal and heat-affected zone can be the creep-weakest parts of the joint. A 316L filler deposited into a 316H creep joint has lower carbon and finer grain than the 316H base metal, so it will creep and rupture first - a phenomenon known as "creep life reduction at welds." For critical high-temperature welds, the filler chemistry, grain structure, and even the weld heat treatment must be carefully matched to preserve creep life.

Corrosion Resistance - Aqueous vs High-Temperature

For general corrosion, pitting, and crevice corrosion resistance in aqueous environments, 316L and 316H are essentially equivalent, because these properties are governed by chromium, nickel, and molybdenum - which are identical in the two grades. Both have a PREN (Pitting Resistance Equivalent Number) of approximately 24-25, making both suitable for mild chloride exposure but not for severe seawater service. The corrosion difference between the grades appears only after welding (sensitization) and at high temperature (oxidation).

The molybdenum content (2-3%) is what gives the entire 316 family its advantage over 304. Molybdenum dramatically improves resistance to pitting and crevice corrosion in chloride environments. This is why 316/316L/316H are the standard "marine-grade" and "chemical-grade" stainless steels, while 304 is the standard "general-purpose" grade. Since 316L and 316H share the same molybdenum content, their resistance to chloride pitting is identical - the "L" or "H" designation has no bearing on this property.

Intergranular Corrosion - Where the Grades Diverge After Welding

After welding, the corrosion behavior of 316L and 316H diverges sharply. 316L resists intergranular corrosion in the as-welded condition because its low carbon prevents chromium carbide precipitation in the heat-affected zone. 316H, with 0.04-0.10% carbon, sensitizes readily during welding, and a welded 316H component exposed to a corrosive aqueous environment below about 400°C will suffer intergranular attack at the weld heat-affected zone. This is why welded 316H is reserved for high-temperature, non-aqueous service.

Intergranular corrosion (IGC) is the failure mode that the "L" grades were created to prevent. It occurs when chromium carbides form at grain boundaries during welding, depleting the adjacent metal of chromium below the ~12% passive-film threshold. The depleted boundaries then corrode preferentially, causing the metal to lose strength and eventually fail along grain boundaries. 316H is deliberately high in carbon, so it is inherently sensitization-prone. For any welded component that will see water, process chemicals, or even humid air at low temperature, 316L - not 316H - is the correct choice. Conversely, for a boiler tube at 600°C in dry steam, sensitization is irrelevant, and 316H is correct.

High-Temperature Oxidation and Carburization

At high temperature, the relevant corrosion mechanisms shift from aqueous attack to oxidation, sulfidation, and carburization. Here, 316H performs as well as (or slightly better than) 316L, because oxidation resistance is governed by chromium - identical in both grades - and the higher carbon of 316H can even improve resistance to carburization by pre-saturating the surface with carbon. Both grades form a protective Cr₂O₃ scale up to roughly 870°C, above which a heat-resistant or nickel-based alloy is required.

In furnace and reformer service, carburization - the absorption of carbon from the process gas into the metal - is a common degradation mechanism that embrittles the alloy. Because 316H already contains more carbon, it has a higher carbon "capacity" before carburization becomes harmful, giving it a slight edge over 316L in carburizing atmospheres. This is another reason 316H is the preferred grade in refining and petrochemical furnace service. For sulfidizing (high-H₂S) atmospheres at high temperature, however, neither grade is ideal, and a higher-nickel alloy such as Incoloy 825 or Inconel 625 should be considered.

Cost and Availability

316L and 316H are priced essentially identically - the cost difference is negligible (typically within 2-5%, and often zero) because both grades are produced by the same AOD refining route and the difference is only a modest carbon adjustment plus a grain-size control step. This means cost should never drive the 316L vs 316H decision; the choice should be made entirely on the basis of operating temperature, welding, and corrosion requirements.

Unlike 304L vs 304 - where the L-grade carries a small premium over standard 304 - the 316L vs 316H comparison is roughly cost-neutral. Both grades command the same base alloy price (driven by the 2-3% molybdenum content, which is the expensive ingredient), and the incremental cost of achieving 316H's carbon range and coarse grain is minor. This is fortunate, because it removes price from the equation entirely: engineers can select purely on technical merit, which is exactly how a safety-critical decision should be made.

Availability

316L is the most widely stocked and available stainless steel in the world, available in every product form (sheet, plate, bar, pipe, tube, fittings, flanges) from essentially every stainless steel distributor and mill. 316H is less common but still readily available, particularly in the tubular product forms (ASTM A213/A312 TP316H) and plate (ASTM A240 316H) used in power and refining applications. For high-temperature pressure applications, 316H availability is well established through mills serving the boiler and petrochemical industries.

A practical note: because 316L is so dominant in the market, many distributors and fabricators default to it - even for applications that should use 316H. When ordering for high-temperature service, explicitly specify "316H" and verify on the material test report that (1) carbon is 0.04-0.10%, and (2) grain size is ASTM No. 7 or coarser. Material that meets only the 316 chemistry (with 0.08% max carbon) but lacks the grain-size requirement is NOT 316H and will not deliver the required creep performance.

Frequently Asked Questions

Q: What does the "H" in 316H stand for?

A: "H" stands for "High Carbon." 316H specifies a carbon content of 0.04-0.10% (higher than 316L's 0.03% max) plus a coarse grain size of ASTM No. 7 or coarser. Together, these two features maximize creep strength for high-temperature service. The "H" designation was introduced to distinguish creep-rated 316 from ordinary 316 and from low-carbon 316L.

Q: Is 316H stronger than 316L?

A: At room temperature, 316H has slightly higher minimum strength (515 MPa vs 485 MPa tensile; 205 MPa vs 170 MPa yield). But the meaningful difference is at high temperature: above 425°C (800°F), 316H retains far higher creep strength than 316L, and 316H is the only one of the two with Code-listed allowable stress values above 800°F. For room-temperature applications, the strength difference is too small to matter.

Q: Can I use 316L instead of 316H at high temperature?

A: Not for ASME Code pressure service above 425°C (800°F). 316L has no Code-listed allowable stress above this temperature because its low carbon and fine grain cannot guarantee creep resistance. For temperatures above 425°C in pressure applications, use 316H. For non-pressure or lightly loaded applications, 316L is sometimes used up to about 450°C, but this is outside Code practice and should be done with engineering caution.

Q: Is 316H weldable?

A: Yes, 316H is weldable, but its higher carbon makes the heat-affected zone susceptible to sensitization (chromium carbide precipitation) during welding. This is acceptable for high-temperature, non-aqueous service, where aqueous intergranular corrosion does not occur. Do not use welded 316H in low-temperature corrosive aqueous service - use 316L instead.

Q: What is the maximum service temperature of 316H?

A: 316H has ASME Code allowable stress listed up to 816°C (1,500°F). However, for continuous service in air, oxidation becomes limiting above about 870°C (1,600°F). In practice, 316H is most commonly used in the 500-700°C (930-1,290°F) creep range. Above roughly 870°C continuous, switch to a heat-resistant grade or nickel alloy.

Q: What is the maximum service temperature of 316L?

A: For ASME Code pressure service, 316L is limited to 425°C (800°F). This is not a corrosion limit but a creep-strength limit: above 800°F, 316L's creep resistance is not reliably guaranteed, so no allowable stress is published. For any pressure-retaining component above 800°F, use 316H.

Q: 316L vs 316H vs 316 - how do I choose?

A: Use 316L for welded, corrosion-critical, low-temperature (below 425°C) service - chemical tanks, marine piping, food equipment. Use 316H for high-temperature (425-816°C) creep service - boiler tubes, steam piping, furnace tubes. Use plain 316 (0.08% max carbon) mainly as a general-purpose grade when neither extreme applies, though in practice 316L has largely replaced 316 for welded applications.

Q: How do I verify I am receiving 316H and not 316 or 316L?

A: Check the material test report (MTR). For 316H, the MTR must show: (1) carbon 0.04-0.10%, and (2) grain size ASTM No. 7 or coarser. Handheld XRF (PMI) cannot distinguish 316/316L/316H because it cannot measure carbon or grain size. Carbon must be verified by OES or laboratory chemical analysis, and grain size by metallographic examination per ASTM E112. Request an EN 10204 Type 3.1 certificate to confirm the material is true 316H.