These two grades -UNS S31000 and UNS S31008 - sit at the workhorse end of the heat-resistant austenitic stainless steel family. They are not exotic. But the gap between a well-specified S310S pipe that lasts a decade and a poorly procured one that fails in fourteen months comes down to decisions most spec sheets never address.
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Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
| Factor | S310 (UNS S31000) | S310S (UNS S31008) |
|---|---|---|
| Carbon content | ≤ 0.25% | ≤ 0.08% |
| EN designation | 1.4841 (X15CrNiSi25-21) | 1.4845 (X8CrNi25-21) |
| Room-temp tensile strength | Slightly higher (carbon solid-solution strengthening) | Baseline per ASTM A312 |
| Intergranular corrosion risk | High in the 425–860°C sensitization window | Significantly reduced |
| Weldability | Marginal - requires care with filler and procedure | Good - standard TIG/MIG/MAG applicable |
| Typical use | Non-welded high-temp components, static furnace parts | Welded pipe systems, fabricated assemblies, pressure piping |
Material Specification
Grade: AISI 310S / UNS S31008 / EN 1.4845 / W.Nr. 1.4845
Manufacturing standard: ASTM A312 (pipe) or ASTM A213 (tube) - specify TP310S designation
Type: Seamless / ERW / EFW - specify explicitly
Size: OD × wall thickness or NPS × schedule
Length: Single random (4.8–6.7m), double random (9–12m), or fixed length (specify)
End: Plain End (PE), Beveled End (BE) - specify bevel angle if non-standard
Surface finish: Pickled and passivated (standard), polished (specify grit), bright annealed
Quality Requirements
MTC: EN 10204 3.1 (3.2 if third-party witnessed)
Heat treatment: Solution annealed at 1040–1150°C, rapid cool
NDT: Hydrotest per ASTM A312; UT full-body for ≥4" NB seamless
PMI: 100% verification on all lengths
Grain size: ASTM E112, target 4–6 (if specified)
Intergranular corrosion: ASTM A262 Practice E (if service in sensitization range)
Manufacturing Routes: Seamless, Welded
S310S pipe arrives in three primary manufacturing forms, each with distinct cost, pressure rating, and quality profile:
Seamless Pipe (SMLS)
Produced by piercing a solid billet and then hot-rolling or cold-drawing to final dimensions. Seamless pipe has no longitudinal weld seam, which eliminates the weakest metallurgical link in the pressure boundary. For high-temperature pressure applications - boiler tubes, superheater tubes, reformer furnace tubes - seamless is the default. ASTM A213 TP310S (seamless boiler/superheater/heat exchanger tubes) and ASTM A312 TP310S (seamless pipe for general service) are the governing specifications.
Seamless S310S pipe costs 15–25% more than equivalent welded product, depending on size. The premium narrows for small diameters (under 2") where cold-drawing efficiency is high, and widens for large diameters (over 12") where billet availability and piercing capacity become constraints.
Welded Pipe (ERW / EFW)
Manufactured by rolling plate or strip into a cylindrical form and welding the longitudinal seam. Electric Resistance Welded (ERW) pipe uses resistance heating to fuse the edges without filler metal; Electric Fusion Welded (EFW) uses a consumable filler with TIG or plasma welding. ASTM A249 TP310S covers welded boiler/heat exchanger tubes; ASTM A358 TP310S covers EFW pipe for high-temperature general service.
Welded S310S pipe is viable for many non-code or low-pressure applications and is widely used in furnace structural components, ductwork, and exhaust systems. The weld seam, when properly made and annealed, achieves base-metal-equivalent mechanical properties. However, in cyclic thermal service, the weld seam HAZ (heat-affected zone) can become a preferential crack initiation site due to residual grain size differences.
Welding S310S: Filler Metal, Sigma Phase, and Post-Weld Integrity
Welding is where S310S pipe systems either succeed or fail in service. The grade itself is readily weldable by TIG (GTAW), MIG (GMAW), and SMAW (stick) processes. No preheat is required. But three specific issues demand attention.
Filler Metal Selection
The standard matching filler for S310S is ER310 (bare wire for GTAW/GMAW) or E310-15/E310-16 (covered electrode for SMAW). ER310 has the same 25Cr-20Ni chemistry as the base metal, producing a weld deposit that matches the oxidation resistance and thermal expansion behavior of the pipe.
However, matching fillers can be problematic in certain service conditions. If the welded joint will operate in the 650–900°C sigma-phase range for extended periods, the weld deposit - which solidifies with a dendritic structure and micro-segregation - can form sigma faster than the base metal. In these cases, a dissimilar filler like ERNiCr-3 (Alloy 82) or ERNiCrMo-3 (Alloy 625) may be specified. These nickel-base fillers are immune to sigma formation and provide superior creep ductility at the weld, though at higher filler cost.
| Filler Metal | Process | Chemistry (Cr-Ni-Fe) | Best For | Relative Cost |
|---|---|---|---|---|
| ER310 | GTAW / GMAW | 25Cr-20Ni-Fe balance | Matching chemistry, standard service | Baseline |
| E310-15 | SMAW | 25Cr-20Ni-Fe balance | Field welding, positional work | +10–15% |
| ERNiCr-3 (Alloy 82) | GTAW / GMAW | 20Cr-72Ni-Fe | Sigma-prone service, dissimilar joints | +200–250% |
| ERNiCrMo-3 (Alloy 625) | GTAW / GMAW | 22Cr-60Ni-9Mo | Severe carburizing/corrosive service | +300–350% |
Post-Weld Heat Treatment (PWHT)
S310S generally does not require PWHT after welding for most applications. The low carbon content (≤0.08%) minimizes sensitization risk, and the austenitic structure does not undergo the martensitic transformation that makes PWHT mandatory for some ferritic and martensitic grades.
The exception is when S310S has been heavily cold-worked before welding - for example, in cold-bent elbows or formed heads. Cold work introduces residual strain energy that can accelerate carbide precipitation during welding. In such cases, a solution anneal at 1050–1150°C followed by rapid quench restores the equilibrium microstructure. This is not a PWHT in the traditional sense; it is a full solution treatment that redissolves any precipitated carbides and eliminates residual stresses.
Weld Defect Indicators Specific to S310S
Three weld defect types recur in S310S pipe fabrication:
Hot cracking (solidification cracking): The 25Cr-20Ni composition has a wide solidification temperature range, making it susceptible to hot cracking if the weld pool is constrained and the sulfur/phosphorus content is at the high end of the spec window. Using filler with controlled low S/P (ER310 typically runs S ≤0.015%, P ≤0.020%) mitigates this.
Detectable in RT/UT as: Linear indications along the weld centerline, often branching.
Sensitization in HAZ: If the welding heat input is too high (travel speed too slow), the HAZ dwells in the 425–860°C sensitization range long enough for chromium carbide to precipitate. Control heat input to 1.0–1.5 kJ/mm for typical wall thicknesses up to 6mm.
