Steel C55 +QT (Quenched and tempered)
Heat-treated state
EN C55 · AISI/SAE 1055 · JIS S55C · GB 55
General-purpose toughened condition (≈830 °C quench, 550–660 °C temper) for shafts, cams and gears.
Values for the +QT condition
Other C55 tempers and conditions
C55 is listed in 10 conditions; see the grade overview at C55.
Compare all 10 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Annealed | Pick for maximum machinability, cold forming or deep drilling before final hardening. | 350 | 640 | 21 | 195 HB | — |
+NDefault Normalized | Standard stock/reference condition (≈820–860 °C air cool): uniform fine pearlitic structure and the property basis of GB/T 699 and EN 10083-2. | 400 | 750 | 16 | 220 HB | — |
Hot rolled | Cheapest stock condition for parts that will be re-heat-treated after machining or roughing. | 420 | 780 | 14 | 225 HB | — |
+QT | Use for larger diameters where hardenability limits the achievable core strength. | 430 | 770 | 17 | 21 HRC | 8.4 % IACS |
+QTThis page Quenched and tempered | General-purpose toughened condition (≈830 °C quench, 550–660 °C temper) for shafts, cams and gears. | 550 | 870 | 15 | 26 HRC | — |
Induction hardened | Best choice for wear surfaces on shafts, rolls and guideways where a tough core is still required. | 550 | 870 | 12 | 57 HRC | 8.4 % IACS |
Cold drawn | Choose for close-tolerance bright bar and higher as-supplied yield strength without heat treatment. | 650 | 830 | 9 | 24 HRC | — |
high-temper toughened condition | Order when the drawing calls out a through-hardness band for machined shafts and gear blanks. | 700 | 960 | 14 | 30 HRC | — |
low-temper wear condition | For cams, wear plates, blades and flat springs needing hardness with some residual toughness. | 1,150 | 1,350 | 9 | 43 HRC | — |
Hardened, as-quenched | Intermediate state only — maximum hardness but brittle; always temper before service. | 1,600 | 2,000 | 1 | 62 HRC | 7.2 % IACS |
What is C55 +QT?
C55 +QT is C55 heat treated to +QT — general-purpose toughened condition (≈830 °C quench, 550–660 °C temper) for shafts, cams and gears. C55 +QT has a yield strength of 550 MPa (79.8 ksi) and a tensile strength of 870 MPa (126 ksi) — stronger than 71% of steel grades. Elongation at break is 15% and the elastic modulus is 210 GPa (30.5 Msi). C55 +QT has a density of 7.84 g/cm³ (0.283 lb/in³), lighter than 78% of steel grades. It melts at 1,420°C (2,588 °F). +N is the default condition FabDigit quotes; +QT is available on request or by drawing note.
Advantages
- Low embodied carbon — 1.9 kg CO₂/kg, better than 92% of steel grades
Limitations
- Low fracture toughness — 60 MPa·√m (54.6 ksi·√in), worse than 78% of steel grades
- Difficult to machine — 40/100, worse than 77% of steel grades
C55 +QT properties
Typical room-temperature values for C55 +QT; 13 are specific to this condition. Other listed values are C55 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
C55 +QT has a density of 7.84 g/cm³ (0.283 lb/in³), lighter than 78% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
C55 +QT has a yield strength of 550 MPa (79.8 ksi) and a tensile strength of 870 MPa (126 ksi) — stronger than 71% of steel grades. Elongation at break is 15% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
C55 +QT is rated for continuous service to 400°C (752 °F). It conducts heat at 47 W/m·K (27.2 BTU/hr·ft·°F), better than 75% of steel grades. Thermal expansion is 11.4 µm/m·K (6.33 µin/in·°F).
Electrical3
C55 +QT 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 C55 +QT 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
C55 +QT's machinability is fair (40/100, worse than 77% of steel grades); weldability poor (30/100); formability poor (30/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with C55 +QT
Is C55 easy to machine?
Machine in the annealed or normalized state with rigid setups, carbide tooling and 80–150 m/min; above ~35 HRC switch to CBN/ceramic or grind.
Can C55 be welded?
Poor weldability (CE ≈ 0.7) — preheat 250–350 °C, low-hydrogen consumables, interpass control and immediate stress-relief/temper at 600–650 °C to avoid HAZ cracking.
Can C55 be formed, bent or molded?
Cold forming only in the spheroidize-annealed condition with generous bend radii (≥2t); hot forge 1100–850 °C followed by controlled cooling or normalizing.
What surface finishes work on C55?
No corrosion resistance — protect with oil, phosphate, black oxide, zinc/nickel plating or paint; grinds and polishes well when hardened.
C55 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 |
|---|---|---|
| CEN 10083-2 C55 / GB/T 699 55 | 0.52 – 0.6 | 0.55 |
| Si | ≤ 0.4 | 0.25 |
| Mn | 0.6 – 0.9 | 0.75 |
| PC55E ≤0.030 | ≤ 0.05 | — |
| SC55E ≤0.035; C55R 0.020–0.040 | ≤ 0.05 | — |
| Crresidual | ≤ 0.4 | — |
| Niresidual | ≤ 0.4 | — |
| Moresidual | ≤ 0.1 | — |
| Cr+Mo+Nisum limit, EN 10083-2 | ≤ 0.63 | — |
| CuGB/T 699 residual limit | ≤ 0.25 | — |
| Fe | balance | — |
C55 +QT — frequently asked
What is C55 +QT used for?
C55 +QT is typically used for hand tools, flat springs, wear plates, agricultural blades and forged discs. In short: hardest of the C-series; spring and wear-part steel; poor weldability, high hardness.
What is the yield strength of C55 +QT?
C55 +QT has a typical yield strength of 550 MPa (79.8 ksi) and a tensile strength of 870 MPa (126 ksi) — stronger than 71% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is C55 +QT easy to machine?
Not especially — machinability is rated fair (40/100, worse than 77% of steel grades). Machine in the annealed or normalized state with rigid setups, carbide tooling and 80–150 m/min; above ~35 HRC switch to CBN/ceramic or grind.
Can C55 +QT be welded?
Not readily — weldability is rated poor (30/100). Poor weldability (CE ≈ 0.7) — preheat 250–350 °C, low-hydrogen consumables, interpass control and immediate stress-relief/temper at 600–650 °C to avoid HAZ cracking.
What surface finishes work on C55 +QT?
No corrosion resistance — protect with oil, phosphate, black oxide, zinc/nickel plating or paint; grinds and polishes well when hardened.
Can C55 +QT be formed, bent or molded?
Cold forming only in the spheroidize-annealed condition with generous bend radii (≥2t); hot forge 1100–850 °C followed by controlled cooling or normalizing.
What is the maximum service temperature of C55 +QT?
C55 +QT 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 C55 +N and C55-IH?
+N is the default condition — standard stock/reference condition (≈820–860 °C air cool): uniform fine pearlitic structure and the property basis of GB/T 699 and EN 10083-2. IH: best choice for wear surfaces on shafts, rolls and guideways where a tough core is still required. Yield strength is 400 MPa in +N versus 550 MPa in IH.
Sources
- EN 10083-2 Steels for quenching and tempering — Non-alloy steels — CEN (2006)
- EN 10132-4 Cold rolled narrow steel strip — spring and hardenable steels — CEN (2000)
- ASTM A29/A29M & SAE J403 (AISI 1055) — ASTM / SAE (2020)
- JIS G 4051 (S55C) — JSA (2016)
- ASM Handbook Vol.1 Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.4 Heat Treating / Vol.16 Machining — ASM International (1991)
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.