Steel 45B
GB 45B
45B is a medium-carbon plain steel with a boron addition that improves hardenability over standard 45# steel at minimal cost increase.
Reference datasheet — 45B 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 HR condition
45B tempers and conditions
45B is supplied in 10 conditions, soft to hard. HR is the default FabDigit quotes; the values on this page are for it.
Compare all 10 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Solution annealed | Choose for cold heading, deep machining or when maximum softness before hardening is required; GB/T 3077 delivery hardness limit ≤217 HBW. | 350 | 600 | 20 | 187 HB | — |
HRDefault Hot rolled | Standard mill-supplied condition for 45B round bar; used as feedstock for machining plus later quench-and-temper or induction hardening. | 400 | 700 | 16 | 197 HB | — |
Normalized | Used to refine grain and homogenize forgings/bar before machining or hardening; gives uniform ~200 HB structure. | 410 | 690 | 18 | 14 HRC | 8.6 % IACS |
Cold drawn | Pick for close-tolerance bright bar and better surface finish where the part runs as-drawn without heat treatment. | 620 | 780 | 10 | 22 HRC | 8.5 % IACS |
Q&T | Use the 840 °C water quench with 550 °C temper that underpins the GB/T 3077 minima (Rm ≥835, ReL ≥685 MPa) for shafts and links needing the best all-round strength/toughness balance. | 740 | 900 | 14 | 27 HRC | — |
Induction hardened | Choose for shaft journals, splines and sprockets needing a 1–4 mm wear case with a tough Q&T core; boron helps the case harden fully. | 740 | 900 | 12 | 56 HRC | 7.8 % IACS |
Q&T 28–32 HRC | Typical machinable-strength ordering band for medium-loaded shafts, pins and machined links. | 800 | 960 | 13 | 30 HRC | — |
Q&T 35–40 HRC (temper ≈400–450 °C) | Higher-strength band for tillage tools, wear links and highly loaded fasteners where some toughness is traded for strength. | 1,080 | 1,250 | 10 | 38 HRC | — |
Q&T 45–50 HRC (low temper ≈250–300 °C) | Wear-oriented condition for plough shares, cultivator tines and blades; low impact toughness, no welding. | 1,350 | 1,550 | 7 | 47 HRC | — |
As-quenched | Intermediate state only — full martensite with high hardness but brittle and highly stressed; must be tempered before service. | 1,550 | 2,000 | 2 | 56 HRC | 7.8 % IACS |
What is 45B?
45B is a medium-carbon plain steel with a boron addition that improves hardenability over standard 45# steel at minimal cost increase. It is supplied hot-rolled, annealed, normalized and cold-drawn, and is quench-tempered to bands from QT550 through 28–32, 35–40 and 45–50 HRC, or induction hardened; the default condition gives 400 MPa yield, 700 MPa tensile, 16 % elongation and 197 HB, with machinability 55 and formability 40. Weldability is only fair at CE ≈ 0.55–0.65 and the steel has no corrosion resistance. Where deeper hardenability is required, step up to 40MnB. 45B 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). 45B has a yield strength of 400 MPa (58 ksi) and a tensile strength of 700 MPa (102 ksi) — weaker than 60% of steel grades. Elongation at break is 16% and the elastic modulus is 210 GPa (30.5 Msi).
What is 45B used for?
- shafts
- gears
- structural bolts
Advantages
- Low embodied carbon — 1.9 kg CO₂/kg, better than 92% of steel grades
45B properties
Typical room-temperature values for 45B-HR; bars are the position among steel grades.
Physical3
45B 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).
Mechanical15
45B has a yield strength of 400 MPa (58 ksi) and a tensile strength of 700 MPa (102 ksi) — weaker than 60% of steel grades. Elongation at break is 16% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
45B is rated for continuous service to 400°C (752 °F). It conducts heat at 46 W/m·K (26.6 BTU/hr·ft·°F), better than 71% of steel grades. Thermal expansion is 11.6 µm/m·K (6.44 µin/in·°F).
Electrical3
45B conducts electricity at 8.6 % IACS, better than 66% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (10/100), worse than 86% of steel grades.
Sustainability4
Producing a kilogram of 45B takes about 25 MJ of energy and emits 1.9 kg of CO₂ — less than 82% of steel grades. Typical recycled content is 30%.
Manufacturability7
45B's machinability is good (55/100, better than 62% of steel grades); weldability fair (40/100); formability fair (40/100).
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. 45B is highlighted; hover a dot for its name, or open the interactive family chart.
Working with 45B
Is 45B easy to machine?
Machines like 1045 at 170–230 HB; use coated carbide, 120–180 m/min turning, and machine before hardening — above 35 HRC expect heavy tool wear.
Can 45B be welded?
Fair only: CE ≈ 0.55–0.65, so preheat 200–300 °C, use low-hydrogen consumables and temper after welding; avoid welding hardened parts.
Can 45B be formed, bent or molded?
Hot forge 1150–850 °C then normalize; cold heading/upsetting requires spheroidize-annealed stock — boron steel is widely used for cold-headed bolts.
45B 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 |
|---|---|---|
| CGB/T 3077 boron series | 0.42 – 0.49 | 0.45 |
| Si | 0.17 – 0.37 | 0.27 |
| Mn | 0.6 – 0.9 | 0.75 |
| Bsoluble boron; Ti/Al added to fix N so B stays effective | 8×10⁻⁴ – 0 | 0 |
| Crresidual limit | ≤ 0.3 | 0.1 |
| Niresidual limit | ≤ 0.3 | 0.1 |
| Curesidual limit | ≤ 0.25 | 0.1 |
| P | ≤ 0.03 | 0.02 |
| S | ≤ 0.03 | 0.02 |
| Fe | balance |
What surface finishes work on 45B?
No corrosion resistance — specify phosphate + oil, black oxide, zinc or Zn-Ni plating (bake to avoid hydrogen embrittlement above 1000 MPa), or paint.
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 45B cost?
45B 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.
45B — frequently asked
What is 45B used for?
45B is typically used for shafts, gears and structural bolts. In short: boron-upgraded 1045; low-cost cold-heading bolt steel; fair weldability only.
What is the yield strength of 45B?
45B has a typical yield strength of 400 MPa (58 ksi) and a tensile strength of 700 MPa (102 ksi) — weaker than 60% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is 45B easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% of steel grades). Machines like 1045 at 170–230 HB; use coated carbide, 120–180 m/min turning, and machine before hardening — above 35 HRC expect heavy tool wear.
Can 45B be welded?
With care — weldability is rated fair (40/100). Fair only: CE ≈ 0.55–0.65, so preheat 200–300 °C, use low-hydrogen consumables and temper after welding; avoid welding hardened parts.
What surface finishes work on 45B?
No corrosion resistance — specify phosphate + oil, black oxide, zinc or Zn-Ni plating (bake to avoid hydrogen embrittlement above 1000 MPa), or paint.
Can 45B be formed, bent or molded?
Hot forge 1150–850 °C then normalize; cold heading/upsetting requires spheroidize-annealed stock — boron steel is widely used for cold-headed bolts.
What is the maximum service temperature of 45B?
45B 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 45B-HR and 45B-Q?
HR is the default condition — standard mill-supplied condition for 45B round bar; used as feedstock for machining plus later quench-and-temper or induction hardening. Q: intermediate state only — full martensite with high hardness but brittle and highly stressed; must be tempered before service. Yield strength is 400 MPa in HR versus 1,550 MPa in Q.
How much does 45B cost?
45B 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 / China National Standard (2015)
- GB/T 699 (Quality carbon structural steels, for 45# comparison) — SAC (2015)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.4: Heat Treating (boron hardenability) — ASM International (1991)
- (Handbook of Mechanical Engineering Materials) (2018)
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.