Steel 70 QT50-55
Hardness band
AISI/SAE 1070
QT50-55 = q&T 50–55 HRC
High-hardness condition for saw blades, scrapers, wear strips and thin clips where wear beats toughness.
Values for the QT50-55 condition
Other 70 tempers and conditions
70 is listed in 10 conditions; see the grade overview at 70.
Compare all 10 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Solution annealed | Pick for blanking, deep drawing, machining or cold coiling before hardening. | 360 | 640 | 20 | 190 HB | — |
Hot rolled | Cheapest mill condition for bars and coil that will be re-heat-treated by the fabricator. | 440 | 780 | 12 | 230 HB | — |
Normalized | Uniform pearlitic structure for bar stock before machining and final hardening. | 480 | 830 | 11 | 23 HRC | 9.8 % IACS |
Cold drawn | Bright, dimensionally accurate bar for lightly stressed clips and pins; strength from drawing only. | 800 | 950 | 8 | 29 HRC | 9.6 % IACS |
Cold rolled hard strip | Thin strip for stamped flat springs, shims and clips that are only low-temperature stress relieved after forming. | 1,150 | 1,250 | 2 | 41 HRC | 10 % IACS |
Stress relieved | Low-temperature bake applied to cold-formed hard strip or wire springs to raise the elastic limit and stabilise set. | 1,250 | 1,350 | 3 | 43 HRC | 9.6 % IACS |
Q&T 40–45 HRC | Tougher spring temper for thicker leaf springs and parts with impact or bending after hardening. | 1,300 | 1,450 | 8 | 43 HRC | — |
Oil quenched and tempered spring wire | Uniform through-hardened wire for coil springs needing consistent properties in larger diameters and better relaxation resistance than cold-drawn wire. | 1,450 | 1,650 | 6 | 46 HRC | 9.6 % IACS |
QT45-50Default Q&T 45–50 HRC | Standard spring service condition: best balance of elastic limit, fatigue life and residual toughness. | 1,550 | 1,750 | 6 | 47 HRC | — |
QT50-55This page Q&T 50–55 HRC | High-hardness condition for saw blades, scrapers, wear strips and thin clips where wear beats toughness. | 1,700 | 1,950 | 4 | 52 HRC | — |
What is 70 QT50-55?
70 QT50-55 is 70 heat treated to QT50-55 — high-hardness condition for saw blades, scrapers, wear strips and thin clips where wear beats toughness. 70 QT50-55 has a yield strength of 1,700 MPa (247 ksi) and a tensile strength of 1,950 MPa (283 ksi) — stronger than 99% of steel grades. Elongation at break is 4% and the elastic modulus is 206 GPa (29.9 Msi). 70 QT50-55 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,410°C (2,570 °F). QT45-50 is the default condition FabDigit quotes; QT50-55 is available on request or by drawing note.
Advantages
- High strength — 1,700 MPa (247 ksi), better than 99% of steel grades
- Low embodied carbon — 1.9 kg CO₂/kg, better than 92% of steel grades
- Polishes to a fine finish — 70/100, better than 87% of steel grades
- High electrical conductivity — 9.6 % IACS, better than 78% of steel grades
Limitations
- Limited ductility — 4%, worse than 98% of steel grades
- Difficult to machine — 15/100, worse than 98% of steel grades
- Low fracture toughness — 28 MPa·√m (25.5 ksi·√in), worse than 97% of steel grades
- Limited formability — 10/100, worse than 95% of steel grades
- Limited service temperature — 200°C (392 °F), worse than 93% of steel grades
70 QT50-55 properties
Typical room-temperature values for 70 QT50-55; 13 are specific to this condition. Other listed values are 70 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
70 QT50-55 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,410°C (2,570 °F).
Mechanical16
70 QT50-55 has a yield strength of 1,700 MPa (247 ksi) and a tensile strength of 1,950 MPa (283 ksi) — stronger than 99% of steel grades. Elongation at break is 4% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal6
70 QT50-55 is rated for continuous service to 200°C (392 °F). It conducts heat at 46 W/m·K (26.6 BTU/hr·ft·°F), better than 71% of steel grades. Thermal expansion is 11.3 µm/m·K (6.28 µin/in·°F).
Electrical3
70 QT50-55 conducts electricity at 9.6 % IACS, better than 78% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (8/100), worse than 99% of steel grades.
Sustainability4
Producing a kilogram of 70 QT50-55 takes about 25 MJ of energy and emits 1.9 kg of CO₂ — less than 82% of steel grades. Typical recycled content is 30%.
Manufacturability6
70 QT50-55's machinability is poor (15/100, worse than 98% of steel grades); weldability poor (15/100); formability not recommended (10/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with 70 QT50-55
Is 70 easy to machine?
Machine in the annealed/spheroidized state (≈45–55 % of 1212) with sharp carbide or HSS tooling; above 45 HRC use grinding, EDM or CBN turning.
Can 70 be welded?
Effectively unweldable for structural joints — 0.7 %C gives brittle martensite; if unavoidable use 200–300 °C preheat, low-hydrogen consumables and immediate temper, and expect loss of spring temper in the HAZ.
Can 70 be formed, bent or molded?
Blank, bend and coil in the annealed or cold-rolled-hard state with generous bend radii along the grain; harden by oil quench from 800–830 °C then temper 380–480 °C, or stress relieve 200–300 °C for formed hard strip.
What surface finishes work on 70?
No corrosion resistance: specify oiling, phosphate + oil, black oxide, zinc or Zn-Ni plating (with 190–220 °C bake to avoid hydrogen embrittlement), or e-coat; shot peening markedly improves spring fatigue life.
70 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 699 / GB/T 1222; SAE 1070 is 0.65–0.75 | 0.67 – 0.75 | 0.71 |
| Si | 0.17 – 0.37 | 0.27 |
| Mn | 0.5 – 0.8 | 0.65 |
| Crresidual | ≤ 0.25 | — |
| Niresidual | ≤ 0.3 | — |
| Curesidual | ≤ 0.25 | — |
| P | ≤ 0.04 | — |
| S | ≤ 0.04 | — |
| Fe | balance | — |
70 QT50-55 — frequently asked
What is 70 QT50-55 used for?
70 QT50-55 is typically used for flat springs, clips and light coil springs. In short: low-cost carbon spring steel; good strip formability; shallow hardening only.
What is the yield strength of 70 QT50-55?
70 QT50-55 has a typical yield strength of 1,700 MPa (247 ksi) and a tensile strength of 1,950 MPa (283 ksi) — stronger than 99% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is 70 QT50-55 easy to machine?
Not especially — machinability is rated poor (15/100, worse than 98% of steel grades). Machine in the annealed/spheroidized state (≈45–55 % of 1212) with sharp carbide or HSS tooling; above 45 HRC use grinding, EDM or CBN turning.
Can 70 QT50-55 be welded?
Not readily — weldability is rated poor (15/100). Effectively unweldable for structural joints — 0.7 %C gives brittle martensite; if unavoidable use 200–300 °C preheat, low-hydrogen consumables and immediate temper, and expect loss of spring temper in the HAZ.
What surface finishes work on 70 QT50-55?
No corrosion resistance: specify oiling, phosphate + oil, black oxide, zinc or Zn-Ni plating (with 190–220 °C bake to avoid hydrogen embrittlement), or e-coat; shot peening markedly improves spring fatigue life.
Can 70 QT50-55 be formed, bent or molded?
Blank, bend and coil in the annealed or cold-rolled-hard state with generous bend radii along the grain; harden by oil quench from 800–830 °C then temper 380–480 °C, or stress relieve 200–300 °C for formed hard strip.
What is the maximum service temperature of 70 QT50-55?
70 QT50-55 is rated for continuous use to about 200°C (392 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 70 QT45-50 and 70-A?
QT45-50 is the default condition — standard spring service condition: best balance of elastic limit, fatigue life and residual toughness. A: pick for blanking, deep drawing, machining or cold coiling before hardening. Yield strength is 1,550 MPa in QT45-50 versus 360 MPa in A.
Sources
- GB/T 1222 — SAC / (2016)
- SAE J403 Chemical Compositions of SAE Carbon Steels (1070) — SAE International (2014)
- ASTM A684/A684M High-Carbon Cold-Rolled Strip — ASTM (2021)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels — ASM International (1990)
- Machinery's Handbook (spring materials, machinability ratings) — Industrial Press (2020)
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.