Steel 40MnB
GB 40MnB
40MnB is a medium-carbon manganese-boron structural steel used in the quenched-and-tempered condition, and is a common substitute for 40Cr in shafts and bolts.
Reference datasheet — 40MnB 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 QT condition
40MnB tempers and conditions
40MnB is supplied in 11 conditions, soft to hard. QT is the default FabDigit quotes; the values on this page are for it.
Compare all 11 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Solution annealed | Softest state, per GB/T 3077 hardness delivery requirement; choose for deep machining or cold heading of bolt blanks. | 350 | 610 | 21 | 185 HB | — |
Hot rolled | Cheapest mill condition for stock that will be re-heat-treated after machining. | 420 | 700 | 16 | 220 HB | — |
Normalized | Uniform fine pearlitic structure for machining stock or as pre-treatment before quenching. | 480 | 780 | 16 | 21 HRC | 8 % IACS |
Cold drawn | Bright, close-tolerance bar for shafts and pins used without further heat treatment. | 620 | 760 | 10 | 225 HB | — |
Q&T | Pick when impact toughness and machinability matter more than peak strength (large shafts, dynamically loaded parts). | 730 | 900 | 15 | 27 HRC | — |
QTDefault Quenched and tempered | Standard specification condition and the usual stocked/supplied state for shafts, arms and grade 8.8–10.9 fasteners. | 850 | 1,030 | 12 | 31 HRC | — |
Induction hardened | Choose for shaft journals and spline flanks needing wear resistance with a tough Q&T core. | 850 | 1,030 | 11 | 55 HRC | 7.5 % IACS |
Q&T to 33–38 HRC | Higher-strength Q&T band for gear blanks, splined shafts and wear-loaded parts. | 980 | 1,150 | 11 | 35 HRC | — |
Q&T to 40–45 HRC | Use where wear and high yield are needed and section is thin; toughness begins to fall off. | 1,230 | 1,400 | 9 | 43 HRC | — |
Q&T | For high-hardness wear parts and small high-strength fasteners; low ductility, notch-sensitive. | 1,380 | 1,620 | 8 | 48 HRC | — |
As-Quenched | Intermediate state only — full martensite hardness but brittle; must be tempered before service. | 1,550 | 2,000 | 2 | 56 HRC | 6.6 % IACS |
What is 40MnB?
40MnB is a medium-carbon manganese-boron structural steel used in the quenched-and-tempered condition, and is a common substitute for 40Cr in shafts and bolts. Default Q&T properties are 850 MPa yield, 1030 MPa tensile, 12 % elongation and 300 HB (31 HRC), with a maximum service temperature of 400 °C and machinability rated 52; conditions on file range from hot-rolled and annealed through QT-550 and induction-hardened. At 0.40 % C with boron the weldability is poor and 200–300 °C preheat plus a 550–600 °C temper is mandatory, and the steel offers no corrosion resistance. 45B is the plain-carbon boron alternative where lower hardenability is acceptable. 40MnB has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,420°C (2,588 °F). 40MnB has a yield strength of 850 MPa (123 ksi) and a tensile strength of 1,030 MPa (149 ksi) — stronger than 88% of steel grades. Elongation at break is 12% and the elastic modulus is 210 GPa (30.5 Msi).
What is 40MnB used for?
- shafts
- bolts
- connecting rods
Advantages
- High strength — 850 MPa (123 ksi), better than 88% of steel grades
Limitations
- Limited ductility — 12%, worse than 84% of steel grades
40MnB properties
Typical room-temperature values for 40MnB-QT; bars are the position among steel grades.
Physical3
40MnB has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
40MnB has a yield strength of 850 MPa (123 ksi) and a tensile strength of 1,030 MPa (149 ksi) — stronger than 88% of steel grades. Elongation at break is 12% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
40MnB is rated for continuous service to 400°C (752 °F). It conducts heat at 43 W/m·K (24.8 BTU/hr·ft·°F), better than 51% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
40MnB conducts electricity at 7.8 % IACS, worse than 57% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (11/100), worse than 73% of steel grades.
Sustainability4
Producing a kilogram of 40MnB takes about 26 MJ of energy and emits 2 kg of CO₂ — less than 64% of steel grades. Typical recycled content is 25%.
Manufacturability6
40MnB's machinability is fair (52/100, better than 52% of steel grades); weldability poor (32/100); formability fair (35/100).
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 steel family
Yield strength against density for every steel grade FabDigit runs. 40MnB is highlighted; hover a dot for its name, or open the interactive family chart.
Working with 40MnB
Is 40MnB easy to machine?
Machine in annealed or normalized state (180–235 HB) with carbide tooling; at 300 HB Q&T expect ~55 % of 1212 rates, and finish-grind anything above 40 HRC.
Can 40MnB be welded?
Poor weldability at 0.40 %C with boron — preheat 200–300 °C, low-hydrogen consumables, slow cooling and temper at 550–600 °C after welding; avoid welding on Q&T parts near critical sections.
Can 40MnB be formed, bent or molded?
Hot forge 1150–850 °C then normalize; cold heading/upsetting of bolt blanks only after spheroidize annealing, and boron content makes it strongly quench-hardening in oil.
40MnB 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 |
|---|---|---|
| C | 0.37 – 0.44 | 0.4 |
| Si | 0.17 – 0.37 | 0.27 |
| Mn | 1.1 – 1.4 | 1.25 |
| BAcid-soluble boron; provides hardenability in place of Cr | 5×10⁻⁴ – 0 | 0 |
| Crresidual | ≤ 0.3 | |
| Niresidual | ≤ 0.3 | |
| Curesidual | ≤ 0.25 | |
| P | ≤ 0.03 | |
| S | ≤ 0.03 | |
| Fe | balance |
What surface finishes work on 40MnB?
No corrosion resistance — protect with black oxide, phosphate + oil, zinc electroplate (bake to avoid hydrogen embrittlement above 1000 MPa), Dacromet or paint; tempered martensite polishes and hard-chromes well.
Surface treatments FabDigit runs on 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).
Organic Coating
Black / Phosphate
Thermal Spray
How much does 40MnB cost?
40MnB is a budget-cost steel ($), cheaper than about 90% 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.
40MnB — frequently asked
What is 40MnB used for?
40MnB is typically used for shafts, bolts and connecting rods. In short: direct 40Cr replacement; 1030 MPa Q&T strength; poor weldability, oil-hardening.
What is the yield strength of 40MnB?
40MnB has a typical yield strength of 850 MPa (123 ksi) and a tensile strength of 1,030 MPa (149 ksi) — stronger than 88% of steel grades. Strength varies by condition: see the 11 listed tempers.
Is 40MnB easy to machine?
Reasonably — machinability is rated fair (52/100, better than 52% of steel grades). Machine in annealed or normalized state (180–235 HB) with carbide tooling; at 300 HB Q&T expect ~55 % of 1212 rates, and finish-grind anything above 40 HRC.
Can 40MnB be welded?
Not readily — weldability is rated poor (32/100). Poor weldability at 0.40 %C with boron — preheat 200–300 °C, low-hydrogen consumables, slow cooling and temper at 550–600 °C after welding; avoid welding on Q&T parts near critical sections.
What surface finishes work on 40MnB?
No corrosion resistance — protect with black oxide, phosphate + oil, zinc electroplate (bake to avoid hydrogen embrittlement above 1000 MPa), Dacromet or paint; tempered martensite polishes and hard-chromes well.
Can 40MnB be formed, bent or molded?
Hot forge 1150–850 °C then normalize; cold heading/upsetting of bolt blanks only after spheroidize annealing, and boron content makes it strongly quench-hardening in oil.
What is the maximum service temperature of 40MnB?
40MnB is rated for continuous use to about 400°C (752 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 40MnB-QT and 40MnB-Q?
QT is the default condition — standard specification condition and the usual stocked/supplied state for shafts, arms and grade 8.8–10.9 fasteners. Q: intermediate state only — full martensite hardness but brittle; must be tempered before service. Yield strength is 850 MPa in QT versus 1,550 MPa in Q.
How much does 40MnB cost?
40MnB is a budget-cost steel ($), cheaper than about 90% 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
- GB/T 3077 (Alloy structural steels) — SAC / (2015)
- GB/T 699 / GB/T 1172 — SAC (1999)
- (Mechanical Design Handbook) (2018)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels — ASM International (1990)
- ASM Handbook Vol.4: Heat Treating — ASM International (1991)
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.