Type 316
UNS S31600
Molybdenum-alloyed austenitic stainless steel · ASTM A240/A240M-26 — plate, sheet and strip · ASTM A276/A276M-25 — general-purpose bars and shapes · ASTM A479/A479M-25 — bars and shapes for boilers and pressure vessels
Type 316, UNS S31600, is a chromium-nickel-molybdenum austenitic stainless steel. Its molybdenum addition gives better resistance than Type 304 to chloride-induced pitting and crevice corrosion, but it is not immune to chloride attack or stress-corrosion cracking. Product specification, form, condition, dimensions and welding exposure are essential parts of the designation: ASTM A240 governs flat products, A276 general-purpose bars and shapes, and A479 pressure-purpose bars and shapes. For welded corrosion-resistant construction, the lower-carbon Type 316L is usually preferred unless the completed fabrication will be suitably solution annealed or the service has been otherwise qualified.
- 2.00–3.00% MoDefining alloy addition · Improves localized-corrosion resistance compared with Type 304.
- 515 MPa tensile; 205 MPa 0.2% yieldAnnealed minimum strength · Common minimum values for the cited ASTM product specifications; dimensional and product-specific provisions still apply.
- 0.08% maximumCarbon limit · This is Type 316, not the 0.030% maximum-carbon Type 316L.
- Austenitic; not hardenable by conventional quench-and-temper heat treatmentMetallurgical condition · Strength can be increased substantially by cold work.
- Pitting, crevice corrosion and SCC remain possible in chloride servicePrincipal limitation · There is no universal safe chloride concentration or service temperature.
- 316L base material and low-carbon filler are commonly preferredWelded construction · Standard-carbon 316 can become sensitized during unsuitable thermal exposure.
Overview
- Designation system
- AISI/SAE type designation with UNS identifier
- Product forms
- Plate, Sheet, Strip, Hot-finished bar, Cold-finished bar, Rounds, squares and hexagons, Hot-rolled or extruded shapes
- Condition
- Annealed or solution-annealed, Cold-finished where permitted by the product specification, ASTM A479 strain-hardened Level 1 or Level 2 when explicitly specified
- Density
- 8 g/cm³ (Representative room-temperature value; not an ASTM acceptance requirement.)
How the designation and specifications work
“316” identifies the alloy type, while S31600 is its UNS composition identifier. Neither designation by itself defines dimensions, surface finish, delivery condition, testing, tolerances or certification. Those requirements come from the applicable product specification and purchase order.
| Specification | Scope relevant to Type 316 | Practical significance |
|---|---|---|
| ASTM A240/A240M | Chromium and chromium-nickel stainless plate, sheet and strip for pressure vessels and general applications | Use for flat-rolled products. ASTM A480/A480M supplies the associated general requirements, including dimensional tolerances, finishes and other ordering provisions. |
| ASTM A276/A276M | Hot- or cold-finished bars and hot-rolled or extruded shapes, excluding bars for reforging | General-purpose bar and shape specification. The requested finish and condition must be stated; the list of condition symbols in A276 does not mean every condition applies to every stainless grade. |
| ASTM A479/A479M | Hot- and cold-finished bars and shapes for boiler and pressure-vessel construction | Use where the pressure-purpose scope is required. It includes solution-annealed austenitic material and provisions for specially ordered strain-hardened Type 316. |
| ASTM A480/A480M and A484/A484M | General requirements for flat products and for bars/shapes respectively | These companion standards contain important tolerances, workmanship, testing, retest, marking and ordering provisions; they are part of a complete procurement specification. |
ASME SA specifications are Code-adopted counterparts, not automatically interchangeable labels. Confirm the ASME edition and Code construction requirements when pressure-equipment certification is required.
Chemical definition
| Element | ASTM A240 Type 316 | ASTM A276/A479 Type 316 | Comment |
|---|---|---|---|
| Carbon | ≤0.08 | ≤0.08 | The principal distinction from S31603/316L, which limits carbon to 0.030%. |
| Manganese | ≤2.00 | ≤2.00 | Maximum. |
| Phosphorus | ≤0.045 | ≤0.045 | Maximum. |
| Sulfur | ≤0.030 | ≤0.030 | Maximum; ordinary 316 is not a free-machining grade. |
| Silicon | ≤0.75 | ≤1.00 | A product-standard difference that should not be hidden by quoting one universal composition table. |
| Chromium | 16.00–18.00 | 16.00–18.00 | Forms and maintains the passive film. |
| Nickel | 10.00–14.00 | 10.00–14.00 | Stabilizes the austenitic structure and supports fabrication and corrosion performance. |
| Molybdenum | 2.00–3.00 | 2.00–3.00 | The defining addition relative to Type 304. |
| Nitrogen | ≤0.10 | ≤0.10 | Maximum. |
| Iron | Balance | Balance | Subject to residual elements and normal steelmaking practice. |
Composition must be evaluated against the exact product specification and edition stated on the order. Meeting an informal “316 chemistry” range alone does not establish compliance with A240, A276 or A479.
Molybdenum improves resistance to localized attack, but corrosion performance is not controlled by composition alone. Surface condition, inclusions, weld heat tint, fabrication contamination, crevices, temperature, oxidizing potential, chloride concentration and cleaning practice can dominate actual service life.
Delivery condition and mechanical properties
Type 316 is normally supplied in an annealed or solution-annealed condition. Solution treatment dissolves chromium-rich carbides and restores corrosion resistance and ductility; rapid cooling is used to prevent their reprecipitation. A typical governing minimum solution temperature for Type 316 products is 1040°C (1900°F), but the exact heat-treatment requirements and permitted processing route must be taken from the applicable product specification.
| Property | A240 flat product | A276/A479 bar or shape | Requirement type |
|---|---|---|---|
| Tensile strength | ≥515 MPa (75 ksi) | ≥515 MPa (75 ksi) | Specification minimum |
| 0.2% offset yield strength | ≥205 MPa (30 ksi) | ≥205 MPa (30 ksi) | Specification minimum |
| Elongation | ≥40% | ≥40% | Specification minimum; specimen geometry, product dimensions and applicable exceptions must be retained. |
| Hardness | ≤217 HBW or ≤95 HRB | Check the applicable table and condition in the ordered edition | Acceptance limit, not a typical operating hardness |
Do not use these annealed minima for cold-worked stock, fasteners, wire, tubing, forgings or other products governed by different specifications. Strength and ductility can also vary with bar size, test orientation and permitted finishing operations.
Austenitic stainless steel has a rounded stress-strain curve and no sharp yield point, so yield strength is reported as an offset proof strength. Cold work raises yield and tensile strength, hardness and springback while reducing ductility. ASTM A479 provides specially controlled strain-hardened Type 316 levels for applications requiring elevated strength; the required level must be stated explicitly and should not be inferred from a generic description such as “cold drawn.”
Corrosion behavior and material selection
Type 316 is selected over Type 304 principally where molybdenum provides useful additional resistance to pitting and crevice corrosion. It performs well in many food-processing, chemical-processing, freshwater, atmospheric and mildly chloride-bearing environments. It is often called “marine grade,” but that expression is not a service guarantee: continuously wetted seawater, warm stagnant chlorides, deposits, gasket crevices and poorly cleaned welds can cause rapid localized attack.
| Mechanism | Type 316 behavior | Engineering consequence |
|---|---|---|
| General corrosion | Good in many neutral and mildly reducing environments; suitability varies strongly with chemical concentration, impurities and temperature | Use environment-specific corrosion data or testing rather than a generic stainless-steel rating. |
| Pitting and crevice corrosion | Better than Type 304, but susceptible in chloride-bearing environments | Avoid stagnant zones and tight unsealed crevices; consider higher-molybdenum, duplex or other alloys when exposure is severe. |
| Chloride stress-corrosion cracking | Austenitic 316 can crack when tensile stress, a susceptible temperature range and a chloride environment coincide | Residual welding and forming stresses matter. Alloy upgrade, stress reduction or environmental control may be necessary. |
| Intergranular corrosion | Possible after sensitizing thermal exposure because S31600 permits up to 0.08% carbon | Prefer 316L for welded corrosion service unless the assembly will be solution annealed or resistance is otherwise demonstrated. |
| Galvanic corrosion | Usually relatively noble when passive | The coupled less-noble metal may corrode rapidly, especially with an unfavorable cathode-to-anode area ratio. |
There is no defensible universal chloride limit for Type 316. Assessment must include temperature, concentration, pH, oxidants, deposits, aeration, flow, geometry, stress and surface condition.
Fabrication, welding and machining
| Operation | What matters for Type 316 |
|---|---|
| Cold forming | Generally very formable and ductile, but it work-hardens rapidly. Forming loads and springback are higher than for carbon steel, and heavily formed regions can become somewhat magnetic. |
| Hot forming | Use procedures that avoid prolonged residence in sensitizing or intermetallic-forming temperature ranges. A full solution anneal may be required after severe hot work when optimum corrosion resistance is needed. |
| Welding | Readily weldable by common arc processes and normally requires neither preheat nor post-weld heat treatment solely to prevent hardening. Low-carbon 316L consumables such as ER316L or E316L are commonly used for corrosion-resistant fabrication. |
| Weld shielding | Protect the root from oxidation where the root contacts the process fluid. Severe oxidation or “sugaring” creates rough, chromium-depleted surfaces with impaired corrosion resistance. |
| Post-fabrication cleaning | Remove heat tint, scale, embedded iron, weld spatter, grinding contamination and residues by a qualified cleaning, pickling and/or passivation procedure appropriate to the application. |
| Machining | Tough, relatively gummy and strongly work-hardening. Use rigid setups, sharp tools, positive cutting action, adequate feed and effective coolant; avoid dwelling or rubbing. |
| Thermal cutting and grinding | Use stainless-dedicated tools and abrasives. Carbon-steel contamination can create rust staining and localized corrosion initiation sites. |
Filler-metal selection must also account for service corrosion, dilution, joint design, required strength and any dissimilar material. A nominally matching filler is not automatically the best choice for every environment.
Physical and magnetic characteristics
| Property | Representative value | Status |
|---|---|---|
| Elastic modulus | Approximately 193–200 GPa at room temperature | Typical engineering value, not an ASTM grade acceptance requirement |
| Poisson's ratio | Approximately 0.30 | Typical |
| Thermal conductivity | Approximately 15 W/m·K at room temperature | Typical; much lower than carbon steel |
| Mean thermal expansion | Approximately 16 × 10⁻⁶/K from 20–100°C | Typical; relatively high compared with carbon steel |
| Specific heat | Approximately 500 J/kg·K at room temperature | Typical |
| Density | Approximately 8000 kg/m³ | Typical |
Temperature-dependent design values should come from the applicable design code or qualified material dataset.
Fully annealed Type 316 is generally only weakly magnetic, but “non-magnetic” should not be treated as an acceptance requirement unless a permeability limit and test method are specified. Cold work can produce strain-induced martensite, and weld metal commonly contains some ferrite for hot-cracking resistance; either can increase magnetic response.
Specifying and purchasing Type 316
| Item | Why it matters |
|---|---|
| Product specification and edition | Select A240, A276 or A479 according to product form and service. Do not place pipe, tubing, forgings, fittings or fasteners under an unsuitable bar or plate specification. |
| Designation | State both Type 316 and UNS S31600 to reduce ambiguity. |
| Units system | ASTM and ASTM M requirements are separate systems and must not be mixed when assessing conformity. |
| Product form and dimensions | State thickness or diameter, width, length, shape and applicable tolerances. |
| Condition and finish | Examples include annealed plate, cold-finished bar, centerless-ground bar or an explicitly identified A479 strain-hardened level. |
| Surface finish | For flat products, specify the required ASTM A480 finish or a project-specific finish and roughness requirement. |
| Certification | State the required material test report, heat traceability, chemical analysis, mechanical tests and marking. |
| Supplementary testing | Intergranular-corrosion testing, product analysis, special inspection, ultrasonic examination or additional mechanical testing must be invoked where required. |
| Pressure-code status | If ASME construction applies, specify the correct SA material specification, Code edition and certification requirements. |
| Welded service | Do not assume S31600 is interchangeable with S31603. State 316L or dual certification explicitly when required by design or fabrication documents. |
A certificate showing low carbon does not by itself establish dual certification. The material must meet every chemical, mechanical, heat-treatment, product-form and documentation requirement of both designations under the specified standard.
Sources
- ASTM A240/A240M-26 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General ApplicationsASTM 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-25b — Standard Specification for General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and StripASTM International
- ASTM A484/A484M-24 — Standard Specification for General Requirements for Stainless Steel Bars, Billets, Shapes, and ForgingsASTM International
- The Nickel Advantage — Corrosion Resistance and JoiningNickel Institute
- Fabricating Stainless Steels for the Water Industry: Guidelines for Achieving Top PerformanceNickel Institute
- Comparison of 304 or 316 and 304L or 316L Type Compositions and Effect on Corrosion ResistanceBritish Stainless Steel Association
- An Introduction to the Corrosion Resistance of Stainless SteelsBritish Stainless Steel Association
- Structural Design of Stainless SteelBritish Stainless Steel Association
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