Stainless Steel 309Cb
UNS S30940
AISI 309Cb (S30940) is a niobium-stabilized 23Cr-13Ni austenitic heat-resisting stainless in which Nb ties up carbon and prevents intergranular attack after high-temperature welding or service.
Reference datasheet — 309Cb is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the Annealed condition
309Cb tempers and conditions
309Cb is supplied in 5 conditions, soft to hard. Annealed is the default FabDigit quotes; the values on this page are for it.
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
AnnealedDefault solution annealed (1040–1120 °C, rapid cool) | Standard mill condition for plate, sheet, bar, pipe and fittings; maximum ductility and corrosion resistance. | 310 | 620 | 45 | 165 HB | — |
As-welded deposit | Condition of dissimilar-metal joints and stainless overlays on carbon/low-alloy steel; Nb stabilization is what lets the joint go into service unannealed. | 400 | 590 | 32 | 180 HB | 2.2 % IACS |
Cold drawn | Use for machined shafts, bolting and fixture pins where tighter tolerance and higher yield are wanted. | 480 | 720 | 30 | 15 HRC | 2.2 % IACS |
Quarter hard | Special-order strip/sheet temper when higher room-temperature strength is needed and forming demands are mild. | 515 | 860 | 20 | 25 HRC | 2.2 % IACS |
Half hard | Rarely stocked; only for flat springs/clips where strength outweighs formability and no hot service is involved. | 760 | 1,035 | 10 | 32 HRC | 2.2 % IACS |
Supply forms, finishes and certifications
Ways 309Cb is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
2B | Cold rolled, annealed, pickled and lightly skin-passed sheet | Surface finish |
HRAP | Hot rolled, annealed and pickled plate | Surface finish |
What is 309Cb?
AISI 309Cb (S30940) is a niobium-stabilized 23Cr-13Ni austenitic heat-resisting stainless in which Nb ties up carbon and prevents intergranular attack after high-temperature welding or service. Engineers use it for welded furnace and heat-exchanger assemblies operating to 1000 °C, with sheet, plate, bar, tube and matching filler available. Annealed properties are about 310 MPa yield, 620 MPa tensile, 45% elongation and 165 HB (85 HRB); formability (score 60) is below 304 and long holds at 700–850 °C cause sigma-phase embrittlement. Unstabilized 309S or 310S is easier to source when stabilization is not required. 309Cb has a density of 7.98 g/cm³ (0.288 lb/in³), heavier than 85% of stainless steel grades. It melts at 1,400°C (2,552 °F). 309Cb has a yield strength of 310 MPa (45 ksi) and a tensile strength of 620 MPa (89.9 ksi) — weaker than 63% of stainless steel grades. Elongation at break is 45% and the elastic modulus is 200 GPa (29 Msi).
What is 309Cb used for?
- high-temperature welded assemblies
- furnace components
- heat exchanger tubing
Advantages
- High service temperature — 1,000°C (1,832 °F), better than 88% of stainless steel grades
- Readily welded — 80/100, better than 80% of stainless steel grades
- Ductile — tolerates forming and impact — 45%, better than 76% of stainless steel grades
309Cb properties
Typical room-temperature values for 309Cb-Annealed; bars are the position among stainless steel grades.
Physical3
309Cb has a density of 7.98 g/cm³ (0.288 lb/in³), heavier than 85% of stainless steel grades. It melts at 1,400°C (2,552 °F).
Mechanical15
309Cb has a yield strength of 310 MPa (45 ksi) and a tensile strength of 620 MPa (89.9 ksi) — weaker than 63% of stainless steel grades. Elongation at break is 45% and the elastic modulus is 200 GPa (29 Msi).
Thermal6
309Cb is rated for continuous service to 1,000°C (1,832 °F). It conducts heat at 15.6 W/m·K (9.01 BTU/hr·ft·°F), better than 55% of stainless steel grades. Thermal expansion is 14.9 µm/m·K (8.28 µin/in·°F).
Electrical3
309Cb conducts electricity at 2.2 % IACS, worse than 61% of stainless steel grades.
Chemical & Environmental3
Corrosion resistance is good (66/100), better than 59% of stainless steel grades.
Sustainability4
Producing a kilogram of 309Cb takes about 60 MJ of energy and emits 4.8 kg of CO₂ — more than 70% of stainless steel grades. Typical recycled content is 60%.
Manufacturability7
309Cb's machinability is fair (38/100, better than 63% of stainless steel grades); weldability very good (80/100); formability good (60/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the stainless steel family
Yield strength against density for every stainless steel grade FabDigit runs. 309Cb is highlighted; hover a dot for its name, or open the interactive family chart.
Working with 309Cb
Is 309Cb easy to machine?
Machine like 310: rigid setups, positive-rake sharp carbide, heavy feeds at low-to-moderate speed (~25–40 % of 1212 rates), flood coolant, and avoid dwelling — the alloy work-hardens fast and Nb carbides are abrasive.
Can 309Cb be welded?
Readily welded by GTAW/GMAW/SMAW with matching 309Cb, ER309LNb or 309L filler; no post-weld anneal is required thanks to Nb stabilization, but keep interpass temperature low and avoid long holds at 700–850 °C to limit sigma phase.
Can 309Cb be formed, bent or molded?
Cold forms and roll-bends well but needs about 50 % more press force than carbon steel with large springback; hot work at 1150–1200 °C and finish above 950 °C, then solution anneal at 1040–1120 °C with rapid cooling.
309Cb chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| CNb is set at ≥10×C | ≤ 0.08 | 0.06 |
| Si | ≤ 1 | 0.6 |
| Mn | ≤ 2 | 1.5 |
| P | ≤ 0.05 | — |
| S | ≤ 0.03 | — |
| Cr | 22 – 24 | 23 |
| Ni | 12 – 15 | 13.5 |
| NbNb+Ta, minimum 10×C (stabilizer) | ≤ 1.1 | 0.8 |
| Nreported when specified | ≤ 0.1 | — |
| Fe | balance | — |
What surface finishes work on 309Cb?
Pickle and passivate (ASTM A967) or grit-blast plus acid clean after hot forming; can be ground, brushed or electropolished — anodizing does not apply, and high-temperature service will grow a tight dark oxide scale.
Surface treatments FabDigit runs on stainless steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Passivation
Organic Coating
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Stainless Steel 309Cb in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- S30940
How much does 309Cb cost?
309Cb is a premium-cost stainless steel ($$$), cheaper than about 51% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
309Cb — frequently asked
What is 309Cb used for?
309Cb is typically used for high-temperature welded assemblies, furnace components and heat exchanger tubing. In short: nb-stabilized 309; sensitization-free after welding; 1000 °C service ceiling.
What is the yield strength of 309Cb?
309Cb has a typical yield strength of 310 MPa (45 ksi) and a tensile strength of 620 MPa (89.9 ksi) — weaker than 63% of stainless steel grades. Strength varies by condition: see the 5 listed tempers.
Is 309Cb easy to machine?
Not especially — machinability is rated fair (38/100, better than 63% of stainless steel grades). Machine like 310: rigid setups, positive-rake sharp carbide, heavy feeds at low-to-moderate speed (~25–40 % of 1212 rates), flood coolant, and avoid dwelling — the alloy work-hardens fast and Nb carbides are abrasive.
Can 309Cb be welded?
Yes — weldability is rated very good (80/100). Readily welded by GTAW/GMAW/SMAW with matching 309Cb, ER309LNb or 309L filler; no post-weld anneal is required thanks to Nb stabilization, but keep interpass temperature low and avoid long holds at 700–850 °C to limit sigma phase.
What surface finishes work on 309Cb?
Pickle and passivate (ASTM A967) or grit-blast plus acid clean after hot forming; can be ground, brushed or electropolished — anodizing does not apply, and high-temperature service will grow a tight dark oxide scale.
Can 309Cb be formed, bent or molded?
Cold forms and roll-bends well but needs about 50 % more press force than carbon steel with large springback; hot work at 1150–1200 °C and finish above 950 °C, then solution anneal at 1040–1120 °C with rapid cooling.
What is the maximum service temperature of 309Cb?
309Cb is rated for continuous use to about 1,000°C (1,832 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 309Cb Annealed and 309Cb-CD?
Annealed is the default condition — standard mill condition for plate, sheet, bar, pipe and fittings; maximum ductility and corrosion resistance. CD: use for machined shafts, bolting and fixture pins where tighter tolerance and higher yield are wanted. Yield strength is 310 MPa in Annealed versus 480 MPa in CD.
How much does 309Cb cost?
309Cb is a premium-cost stainless steel ($$$), cheaper than about 51% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- ASTM A240/A240M — Chromium and Chromium-Nickel Stainless Steel Plate, Sheet and Strip (S30940) — ASTM International (2023)
- ASTM A479/A479M — Stainless Steel Bars and Shapes for Boilers and Pressure Vessels — ASTM International (2021)
- ASTM A312/A312M — Seamless, Welded and Heavily Cold Worked Austenitic Stainless Steel Pipes — ASTM International (2022)
- AWS A5.9 / A5.4 — Bare and Covered Corrosion-Resisting Chromium-Nickel Electrodes (309Cb/309LNb) — American Welding Society (2017)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.13B — Corrosion: Materials — ASM International (2005)
- 309/309S/309Cb heat-resisting stainless product data — Cleveland-Cliffs (AK Steel) / Outokumpu (2021)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.