Type 321
UNS S32100
Titanium-stabilized austenitic chromium-nickel stainless steel · ASTM A240/A240M — plate, sheet and strip · ASTM A276/A276M — general-purpose stainless steel bars and shapes · ASTM A479/A479M — bars and shapes for boilers and other pressure vessels
Type 321 is an 18Cr-9Ni class austenitic stainless steel stabilized with titanium to retain resistance to intergranular corrosion after welding or prolonged exposure in the carbide-precipitation temperature range. Its principal value is in welded or thermally cycled equipment where ordinary Type 304 may sensitize, but it remains broadly similar to 304 in general aqueous corrosion and is not a chloride-resistant substitute for molybdenum-bearing grades. Product standard, form, condition and high-temperature design basis must accompany the S32100 designation.
- Titanium stabilizationDefining feature · Titanium is controlled relative to carbon plus nitrogen so that carbon preferentially forms titanium compounds rather than chromium-depleting grain-boundary炭化物.
- 17–19% Cr; 9–12% NiNominal alloy class · The detailed limits, including silicon and nitrogen, must be taken from the governing product specification.
- 515 MPa tensile; 205 MPa 0.2% yieldASTM A240 minimum strength · Applies to qualifying plate, sheet and strip; elongation and hardness requirements also apply.
- Welded and thermally cycled componentsPrimary service rationale · Especially useful when exposure in the sensitization range may be followed by aqueous corrosion.
- 321 is not automatically 321HHigh-temperature distinction · S32109 has controlled higher carbon and additional high-temperature requirements; dual certification must be demonstrated by the material certificate.
- Approximately Type 304-level wet-corrosion resistanceCorrosion limitation · Susceptible to chloride pitting, crevice corrosion and chloride stress-corrosion cracking.
Overview
- Designation system
- ASTM type designation with UNS identifier
- Product forms
- Plate, Sheet, Strip, Hot-finished bar, Cold-finished bar, Hot-rolled or extruded shapes
- Condition
- Solution-annealed, Condition A bar, Cold-finished after annealing where permitted, Product-specific cold-worked conditions only when explicitly specified
- Density
- 7.9 g/cm³ (Representative physical-property value for annealed Type 321; density is not a grade acceptance requirement.)
What the designation does—and does not—define
“Type 321” identifies the alloy family and UNS S32100 identifies its composition. Neither designation alone defines an acceptable product. ASTM A240/A240M, A276/A276M and A479/A479M cover different product forms and impose different mechanical, heat-treatment, dimensional and testing requirements.
| Specification | Scope relevant to S32100 | Practical consequence |
|---|---|---|
| ASTM A240/A240M | Plate, sheet and strip for pressure vessels and general applications | Use for flat-rolled product. ASTM A480/A480M supplies associated general requirements such as dimensions, finish, heat treatment, testing and permissible variations. |
| ASTM A276/A276M | Hot- or cold-finished bars and hot-rolled or extruded shapes | General bar and shape specification. Condition, finish and dimensions must be stated; it does not by itself establish compliance with a pressure-vessel material specification. |
| ASTM A479/A479M | Hot- and cold-finished bars and shapes for boilers and other pressure vessels | Pressure-purpose bar and shape specification with its own heat-treatment, mechanical-property and supplementary corrosion-test provisions. ASTM A484/A484M provides general requirements. |
Bar certified only to ASTM A276/A276M should not be treated as ASTM A479/A479M material without documentary confirmation that every applicable A479/A479M requirement has been met.
Composition and the stabilization mechanism
| Element | ASTM A240/A240M limit, mass % | Technical significance |
|---|---|---|
| Carbon | 0.08 max | Carbon is permitted above L-grade levels because titanium is used to stabilize it; actual carbon still influences the titanium requirement. |
| Chromium | 17.0–19.0 | Provides the passive film and oxidation resistance. |
| Nickel | 9.0–12.0 | Promotes and stabilizes the austenitic structure. |
| Manganese | 2.00 max | Steelmaking and austenite-balance constituent. |
| Silicon | 0.75 max | A240 flat-product limit; A276 and A479 bar specifications permit a different maximum, so the ordered specification must be checked. |
| Phosphorus | 0.045 max | Residual-element control. |
| Sulfur | 0.030 max | Residual-element control; standard Type 321 is not a free-machining grade. |
| Nitrogen | 0.10 max | A240 flat-product limit and part of the titanium stabilization calculation. |
| Titanium | 5 × (C + N) min; 0.70 max | The defining stabilization addition. The lower limit depends on the actual heat analysis rather than being one fixed titanium percentage. |
The table presents ASTM A240/A240M flat-product limits. Procurement of bar must use the chemistry table in the specified edition of ASTM A276/A276M or A479/A479M; minor limits are not necessarily identical across product specifications.
During exposure in the sensitization range, unstabilized austenitic stainless steel can precipitate chromium carbides at grain boundaries, leaving adjacent regions depleted in chromium and vulnerable to intergranular attack. Type 321 uses titanium, which has a stronger affinity for carbon, to reduce this mechanism. Stabilization improves resistance to sensitization; it does not increase resistance to chloride pitting or make the alloy immune to every form of intergranular attack.
Specified room-temperature properties depend on product standard
| Product specification and condition | Tensile strength min | 0.2% yield strength min | Elongation min | Other requirement |
|---|---|---|---|---|
| ASTM A240/A240M plate, sheet and strip | 515 MPa (75 ksi) | 205 MPa (30 ksi) | 40% in 50 mm (2 in.) | 217 HBW or 95 HRBW maximum |
| ASTM A276/A276M bar and shape, Condition A | 515 MPa (75 ksi) | 205 MPa (30 ksi) | 40% in 50 mm or 4D | 50% reduction of area minimum; applicable table notes and specimen provisions govern |
| ASTM A479/A479M bar and shape, annealed | 515 MPa (75 ksi) | 205 MPa (30 ksi) | 30% in 50 mm or 4D | 40% reduction of area minimum |
These values are specification minima, not typical design properties. The precise requirements and exceptions in the edition invoked by the purchase order remain controlling.
The differing elongation and reduction-of-area requirements illustrate why values from flat product and bar specifications must not be merged into one universal property set. Cold work can raise strength substantially while reducing ductility and may introduce measurable magnetic response. Such properties are condition-, size- and process-dependent and must not be assumed for material ordered simply as S32100.
Elevated-temperature use
Type 321 is principally selected where fabrication or service exposes the steel to approximately 425–900°C and resistance to subsequent intergranular corrosion is important. Typical applications include expansion joints, bellows, exhaust components, furnace hardware, burner ducts, heat shields and welded process equipment. These examples describe established usage rather than universal suitability.
| Issue | What Type 321 offers | What still controls selection |
|---|---|---|
| Sensitization resistance | Titanium stabilization limits chromium-carbide-related grain-boundary depletion during welding and intermediate-temperature exposure. | Stabilization condition, time-temperature history, welding cycle and severity of the subsequent corrosive environment. |
| Oxidation and scaling | Useful oxidation resistance in air at elevated temperature; published typical limits are commonly around 850–925°C depending on continuous or intermittent exposure. | Atmosphere, cycling, sulfur or halogen contamination, deposits, section thickness and acceptable scaling rate. |
| Load-bearing service | Austenitic structure retains useful hot strength. | Code allowable stress, creep and creep-rupture data, design life, weld strength reduction and the applicable construction code. |
| Creep-range service | S32100 may be usable where permitted by the design code. | For sustained high-temperature loading, S32109 Type 321H is normally the grade requiring evaluation because its carbon and grain-size controls support creep strength. |
An oxidation limit is not an allowable design temperature. Pressure equipment must use the correct ASTM or ASME product specification and the allowable-stress tables, temperature limits and fabrication rules of the governing construction code.
Corrosion performance and limitations
In the solution-annealed condition, general aqueous-corrosion behavior is broadly comparable with Type 304. Type 321's advantage appears after welding or intermediate-temperature exposure: properly stabilized material is less likely to become susceptible to intergranular corrosion. This advantage should not be mistaken for higher general corrosion resistance.
| Mode | Assessment for Type 321 | Engineering implication |
|---|---|---|
| General aqueous corrosion | Generally similar to Type 304 in many mildly corrosive environments. | Base selection on actual chemical concentration, temperature, contaminants and operating state. |
| Intergranular corrosion | Improved resistance following welding or sensitizing thermal exposure because of titanium stabilization. | For severe service, consider specified corrosion testing and whether a stabilization treatment is required. |
| Pitting and crevice corrosion | Limited resistance in chloride-bearing media; no deliberate molybdenum addition. | Do not select 321 merely because the environment contains chlorides. Compare with 316L, duplex or higher-alloy material as conditions require. |
| Chloride stress-corrosion cracking | Susceptible when tensile stress, chlorides and sufficient temperature occur together. | Residual weld or forming stress and evaporation or concentration zones require particular attention. |
| High-temperature gas corrosion | Performance varies markedly with oxygen potential, sulfur, halogens, deposits and combustion contaminants. | Air oxidation data cannot safely be transferred to process gases, flue gas or molten-salt environments. |
Heat treatment, welding and fabrication
| Operation | Guidance | Important limitation |
|---|---|---|
| Solution annealing | A479 bar requirements use a minimum of 1040°C (1900°F) for Type 321 followed by water quenching or rapid cooling as prescribed. Commercial product guidance commonly uses approximately 1040–1100°C. | The governing product specification and section size control the actual cycle. Slow cooling can permit unwanted precipitation. |
| Stabilization treatment | A separate treatment near 870–900°C may be specified after solution treatment for particularly severe elevated-temperature/intergranular-corrosion service. | It is not an automatic requirement for every ASTM 321 product. The cycle and acceptance criteria should be agreed before ordering or fabrication. |
| Thermal hardening | Not hardenable by quenching and tempering. | Strength increases primarily through cold work; cold-worked properties are not represented by annealed specification minima. |
| Fusion welding | Generally weldable by conventional stainless-steel processes; preheat is normally unnecessary. | Control heat input, distortion, cleanliness and interpass practice in accordance with the qualified welding procedure. |
| Filler metal | Type 347 filler is commonly used because niobium stabilization is retained more reliably through the welding arc than titanium stabilization. | Consumable selection must follow the qualified welding procedure, service environment and construction code. |
| Cold forming | Good ductility and formability, with significant work hardening and springback. | Severe forming can raise strength, reduce ductility and create residual stresses; heat treatment may be needed depending on service. |
| Machining | Machinability is broadly similar to other standard austenitic grades. | Use rigid tooling, positive cuts, suitable coolant and feeds that avoid dwelling in the work-hardened surface. |
| Surface restoration | Remove heat tint, embedded carbon-steel contamination and fabrication scale using qualified cleaning, pickling or mechanical procedures. | Poor post-fabrication surface condition can dominate corrosion performance even when the bulk alloy is correctly specified. |
Annealed Type 321 is essentially nonmagnetic, but cold working and some weld microstructures can produce a magnetic response. Magnet testing is therefore not a reliable standalone method of grade verification. Positive material identification also has limitations because portable X-ray fluorescence does not reliably quantify carbon and may not fully verify the titanium-to-carbon-plus-nitrogen stabilization relationship.
Procurement and verification
| Item | Why it matters |
|---|---|
| Complete specification and edition | ASTM A240/A240M, A276/A276M and A479/A479M are not interchangeable; stating the year or revision fixes the contractual requirements. |
| Grade and UNS number | State Type 321 / UNS S32100. Do not accept a generic description such as “18-8 stabilized stainless.” |
| Product form, dimensions and tolerances | Determines which specification and general-requirements standard apply. |
| Delivery condition and finish | State annealed or other permitted condition, hot- or cold-finished bar, sheet finish, edge condition and any final-anneal requirement. |
| Pressure-code status | For coded equipment, state the required ASME specification, code edition, allowable-stress basis and certification requirements. |
| Mechanical testing and orientation | Specify supplementary tests, specimen orientation or elevated-temperature tests where the base specification does not provide what the design requires. |
| Intergranular-corrosion testing | Invoke the applicable supplementary requirement and ASTM A262 practice when service severity or project rules require it; identify whether the specimen must be sensitized. |
| Heat treatment after fabrication | State any stabilization or solution treatment requirement before manufacture because it can affect dimensions, surface condition and mechanical properties. |
| Material test report | Check heat analysis, titanium stabilization relationship, mechanical results, heat treatment, product specification, edition, heat number and traceability. |
| 321/321H dual certification | Accept only when chemistry, grain size, heat treatment, testing and documentation satisfy both designations and the governing product/code requirements. |
Substitution should be reviewed against the complete product specification and service requirements. Similar corrosion behavior or a cross-reference table is not sufficient evidence of interchangeability.
Sources
- ASTM A240/A240M-26 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and StripASTM International
- ASTM A276/A276M-25 — Standard Specification for Stainless Steel Bars and ShapesASTM International
- ASTM A479/A479M-25 — Standard Specification for Stainless Steel Bars and Shapes for Use in Boilers and Other Pressure VesselsASTM International
- ASTM A480/A480M — General Requirements for Flat-Rolled Stainless and Heat-Resisting SteelASTM International
- ASTM A484/A484M — General Requirements for Stainless Steel Bars, Billets, and ForgingsASTM International
- Stainless Steel Grade Sheets — Type 321worldstainless
- Design Guidelines for the Selection and Use of Stainless SteelsNickel Institute
- Alleima 321 Aerospace — Technical DatasheetAlleima
- Outokumpu Core Range DatasheetOutokumpu
Ask MetalMate about Type 321
Discussing: Type 321 / UNS S32100
AI assistant · Responses are generated and can contain errors; verify specifications before acting.