UNS S38815 — High-Silicon Austenitic Stainless Steel
Introduction
UNS S38815 is a high-silicon, chromium-nickel austenitic stainless steel developed for demanding high-temperature and highly corrosive environments — particularly hot, concentrated sulfuric acid, sulfidizing atmospheres, and furnace/retort applications. Its unusually high silicon content (≈5–6.5%) produces a protective silica/chromia scale that improves resistance to carburization, sulfidation and scaling at elevated temperatures while maintaining good corrosion resistance in strong acids.
Key features / quick summary
- High Si (≈5.5–6.5%) for enhanced sulfidation, scaling and carburization resistance.
- Good resistance to hot concentrated sulfuric acid — often chosen for sulfuric-acid plant equipment.
- Typical wrought product forms: sheet, plate, pipe/tube, bar — per ASTM/ASME specifications.
- Solid general high-temperature performance (service up to several hundred °C; selected uses up to ~800–1000 °C depending on environment and duty).
Chemical Composition
| Element | C | Si | Mn | P | S | Cr | Ni | Mo | Cu | Al | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| % (Weight) | 0.030 max | 5.500 – 6.500 | 2.000 max | 0.040 max | 0.020 max | 13.000 – 15.000 | 13.000 – 17.000 | 0.750 – 1.500 | 0.750 – 1.500 | 0.300 max | Balance |
Mechanical Properties (annealed / room temp — published typical)
| Property | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Reduction of Area (%) | Hardness (BHN) | Impact (Joule) |
|---|---|---|---|---|---|---|
| Value | 540 min | 255 min | 30 min | – | – | – |
Physical / thermal properties (typical)
- Density: ≈ 7.7–7.9 g/cm³.
- Thermal conductivity and thermal expansion: typical of high-Ni austenitics; consult supplier data for design values.
- Maximum practical service temperatures vary with environment — mechanical limits often cited up to ~860 °C (mechanical), oxidation/corrosion behavior must be assessed case-by-case (hot acidic/sulfidizing services may limit long-term service temperatures to lower values, e.g., 250–400 °C continuous in some acid duties).
Applicable specifications & common forms
- ASTM / ASME: A240 / SA240 (plate/sheet), A213 / A213M (tubes), A403 / A403M (wrought fittings: WP-S38815), A959 references — check the specific product standard. Suppliers commonly supply plate, sheet, bar, pipe and fittings.
Grade-specification comparison (where S38815 sits vs. other common standards)
- UNS S38815 is a high-Si austenitic stainless grade distinct from 304/316 families. It is functionally similar (for some high-temp acid duties) to silicon-enriched grades such as EN 1.4828 / “310Si” type materials and specialized high-Si alloys (often listed in vendor cross-reference tables). Exact equivalents depend on national naming — check supplier cross-reference tables for the exact product form.
UNS S38815 Possible alternative/substitute grades (short list & guidance)
When selecting alternatives consider corrosion mechanism, temperature, mechanical load and weldability:
- UNS S32615 / S30601 (high-Si variants) — also used for hot concentrated acid service; sometimes considered in similar applications.
- EN 1.4828 / X15CrNiSi20-2 (310Si) — a silicon-stabilized 310 variant used for high-temperature oxidation/carburation resistance (may be a workable substitute for some furnace / scaling duties).
- High-nickel alloys (Ni-based) — if service involves stronger reducing acids, chloride stress/crevice concerns or higher temperatures where nickel-alloys (e.g., Alloy 625/825/etc.) are better suited, consider Ni-alloys after cost/corrosion tradeoff evaluation. (Refer to corrosion data for the actual acid/temperature.)
Rule of thumb: don’t substitute without checking the specific acid strength/temperature and mechanical duty — S38815 is chosen specifically for its high-Si behavior in sulfidizing and hot concentrated acid environments.
Corrosion resistance
- Excellent resistance to hot, concentrated sulfuric acid and good resistance to sulfidation and carburization because of the high Si that promotes a protective silica-enriched scale. Recommended in towers, coolers, piping and furnace components exposed to SO₂/SO₃ environments.
- Good resistance to pitting and crevice corrosion compared with many 300-series in some chloride environments — but actual performance is environment-specific. Nickel Institute guidance and specialty-alloy literature recommend S38815 for hot, concentrated acids where conventional 304/316 fail.
Design note: Always confirm corrosion rates with immersion/coupon data or vendor data for the exact acid concentration and temperature — published guidance highlights S38815’s niche suitability for hot concentrated H₂SO₄ services.
Heat resistance & service temperature
- S38815 shows good oxidation and scaling resistance in sulfidizing/oxidizing atmospheres at elevated temperatures; some vendor literature cites intermittent/continuous exposure up to several hundred °C and sometimes up to ~800–1100 °C for scaling/corrosion resistance depending on duty and environment. For long-term mechanical load at high temperature, designers should use conservative allowable stresses and consult supplier data.
Welding & fabrication
- Weldability: S38815 is generally weldable with appropriate filler metals and welding procedures, but its high silicon content influences HAZ chemistry and can cause heterogeneous microstructure in welds — careful procedure qualification and HAZ control are recommended. Recent microstructural studies show oxidation/segregation in HAZs of high-Si welds; thus welding procedure development and post-weld inspection are important for critical parts.
- Filler metal: select filler compatible with the base chemistry and service (consult filler metal vendor and welding procedure specifications). Nickel-based fillers may be used in severe corrosive services.
- Forming / machining: High Si can reduce hot workability and machining ease compared with conventional 300-series; many producers use specialized melting/refining (AOD/VOD + ESR/VAR) and processing to produce workable material. Shop trials and tool selection (and possibly slower feeds / heavier tooling) are typical considerations.
Fabrication tips/cautions (practical)
- Specify mill/test certificates and confirm heat treatment condition on purchase paperwork.
- For welded pressure parts, qualify weld procedures with the actual wall thickness and joint design.
- For hot corrosive service (e.g., sulfuric acid towers), consider cathodic/anodic protection or metallurgical alternatives if exposures exceed validated limits.
Applications
- Sulfuric acid plants (towers, piping, coolers) — hot concentrated acid service.
- Furnace and kiln components (sintering belts, radiant tubes, retorts) exposed to sulfidizing/oxidizing atmospheres and carburizing gases.
- Heat exchangers, boiler tubes and superheaters where high temperature corrosion/oxidation resistance is needed.
- Chemical process equipment, valves and fittings where acid resistance and high temperature scaling resistance are required.
Procurement & specification checklist (what to call out on PO / drawing)
Exact UNS number: UNS S38815.
ASTM/ASME product spec: e.g., ASTM A240 (plate), A213 (tubes), A403 (butt-weld fittings) as applicable.
Product form & condition: plate/sheet/bar/pipe — annealed/solution-annealed.
Chemical limits: include the element limits (Si range critical).
Mechanical property acceptance: list minimum Rm / Rp0.2 and elongation (or call out mill cert requirement).
Welding/filler metal & PWHT requirements if part of the scope.
Short technical references (selected)
- MakeItFrom — material page for UNS S38815 (mechanical/thermal properties summary).
- Vendor / supplier datasheets (typical chemical & mechanical ranges): lkalloy / lksteelpipe / gravitycast / manufacturer product pages.
- Nickel Institute – alloy selection guidance for sulfuric acid (discussion of high-Si alloys including UNS S38815).
- Recent microstructure/weldment study noting HAZ/precipitation behavior of S38815 (welding consequences).
Final recommendations (practical)
- Use UNS S38815 when the service is hot, concentrated sulfuric acid, sulfidizing atmospheres or where carburization/scale resistance is vital. Confirm supplier test certificates and request vendor guidance for welding and high-temperature duty. For chloride, reducing acid or higher temperature mechanical stresses, evaluate Ni-alloys and other high-performance stainlessites as alternative options. Always validate with corrosion/compatibility tests for the specific acid concentration, temperature and flow conditions.
