Steel 45B-QT550
Heat-treated state
GB 45B
QT550 = 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.
Values for the QT550 condition
Other 45B tempers and conditions
45B is listed in 10 conditions; see the grade overview at 45B.
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 |
QT550This page 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-QT550?
45B-QT550 is 45B heat treated to QT550 — 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. 45B-QT550 has a yield strength of 740 MPa (107 ksi) and a tensile strength of 900 MPa (131 ksi) — stronger than 83% of steel grades. Elongation at break is 14% and the elastic modulus is 210 GPa (30.5 Msi). 45B-QT550 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). HR is the default condition FabDigit quotes; QT550 is available on request or by drawing note.
Advantages
- Low embodied carbon — 1.9 kg CO₂/kg, better than 92% of steel grades
- High strength — 740 MPa (107 ksi), better than 83% of steel grades
Limitations
- Limited ductility — 14%, worse than 75% of steel grades
45B-QT550 properties
Typical room-temperature values for 45B-QT550; 14 are specific to this condition. Other listed values are 45B reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
45B-QT550 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-QT550 has a yield strength of 740 MPa (107 ksi) and a tensile strength of 900 MPa (131 ksi) — stronger than 83% of steel grades. Elongation at break is 14% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
45B-QT550 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-QT550 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-QT550 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-QT550's machinability is fair (45/100, worse than 65% of steel grades); weldability fair (40/100); formability fair (40/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with 45B-QT550
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.
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.
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 | — |
45B-QT550 — frequently asked
What is 45B-QT550 used for?
45B-QT550 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-QT550?
45B-QT550 has a typical yield strength of 740 MPa (107 ksi) and a tensile strength of 900 MPa (131 ksi) — stronger than 83% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is 45B-QT550 easy to machine?
Reasonably — machinability is rated fair (45/100, worse than 65% 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-QT550 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-QT550?
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-QT550 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-QT550?
45B-QT550 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.
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. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.