SS309 Stainless Steel
SS309 is a high-chromium, high-nickel austenitic stainless steel developed for elevated-temperature service, offering substantially better oxidation resistance than conventional 18-8 stainless steels.
SS309 Grade Information
SS309, commonly designated AISI Type 309, is an austenitic chromium-nickel stainless steel with higher chromium and nickel contents than SS304. The increased alloy content provides improved resistance to oxidation and scaling at elevated temperatures.
For Spiral Wound Gaskets, SS309 may be considered for metallic winding or compatible ring components where elevated-temperature oxidation resistance is required and the alloy is chemically compatible with the process medium.
| Common Designation | SS309 / AISI Type 309 |
|---|---|
| UNS Number | S30900 |
| Material Family | Heat-resistant austenitic chromium-nickel stainless steel |
| Common Product Specification | ASTM A240 / A240M |
| Distinction from SS309S | SS309 permits higher carbon content; SS309S is the lower-carbon variant |
| Typical Gasket Use | Metallic winding and compatible ring components for suitable elevated-temperature service |
SS309 Chemical Composition
Representative ASTM A240 composition limits for Type 309. Exact requirements should be confirmed against the applicable specification edition, product form and certified material documentation.
| Element | Symbol | Composition, % |
|---|---|---|
| Carbon | C | 0.20 max. |
| Manganese | Mn | 2.00 max. |
| Silicon | Si | 1.00 max. |
| Phosphorus | P | 0.045 max. |
| Sulfur | S | 0.030 max. |
| Chromium | Cr | 22.00 – 24.00 |
| Nickel | Ni | 12.00 – 15.00 |
| Iron | Fe | Balance |
SS309 Mechanical Properties
Representative room-temperature requirements for annealed Type 309 flat products. Values can vary with product form, thickness, processing condition and governing specification.
| Property | Requirement | Unit |
|---|---|---|
| Tensile Strength | 515 min. | MPa |
| 0.2% Offset Yield Strength | 205 min. | MPa |
| Elongation | 40 min. | % |
| Brinell Hardness | 217 max. | HBW |
| Rockwell B Hardness | 95 max. | HRB |
Mechanical values shown are reference requirements, not design allowables. Final engineering values must come from the governing material specification and certified material test report.
SS309 Physical & Service Information
Reference physical properties and important service considerations for SS309 material selection.
| Density | Approximately 7.9 g/cm³ |
|---|---|
| Elastic Modulus | Approximately 200 GPa at room temperature |
| Thermal Conductivity | Approximately 15–16 W/m·K near 100°C |
| Mean Thermal Expansion | Approximately 15–16 µm/m·K over the lower-temperature range |
| Melting Range | Approximately 1400–1450°C |
| Structure | Austenitic |
| Elevated-Temperature Behaviour | Higher chromium and nickel contents provide substantially better oxidation and scaling resistance than standard 18-8 stainless steels |
| Oxidation Service | Commonly selected for service approaching approximately 1000°C in oxidizing atmospheres; allowable temperature depends on duty cycle, atmosphere and applicable design code |
| Corrosion Behaviour | Provides good general corrosion resistance, but SS309 is primarily selected for its high-temperature properties rather than severe aqueous or chloride corrosion service |
| Magnetic Behaviour | Generally non-magnetic in the solution-annealed condition; cold work can introduce some magnetic response |
| Heat Treatment | Normally solution annealed followed by rapid cooling; the alloy is not hardened by heat treatment |
| Spiral Wound Gasket Selection | Select only where SS309 is compatible with the process medium, pressure, filler material and required elevated-temperature service |
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