Steel 9255-QT370
Heat-treated state
AISI/SAE 9255 · UNS G92550
QT370 = oil quenched, tempered 370 °C
Balance point between peak strength and usable toughness for coil springs.
Values for the QT370 condition
Other 9255 tempers and conditions
9255 is listed in 14 conditions; see the grade overview at 9255.
Compare all 14 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Spheroidize annealed | Use when severe cold heading, deep drawing or heavy machining of the bar is required before hardening. | 430 | 700 | 26 | 14 HRC | 5.9 % IACS |
Solution annealed | Softest condition — pick for cold forming, machining or coiling before hardening. | 485 | 775 | 22 | 20 HRC | — |
Normalized | Grain refinement and uniform structure after forging, prior to final hardening. | 580 | 930 | 20 | 27 HRC | — |
Hot rolled | Mill-supplied bar or flat for spring makers who will hot-form and then quench and temper in house. | 620 | 1,000 | 15 | 30 HRC | 6.5 % IACS |
Cold drawn | Close-tolerance bar and wire; strength from drawing, still formable and heat treatable. | 760 | 900 | 10 | 26 HRC | 6.2 % IACS |
Oil quenched, tempered 650 °C | General quenched-and-tempered machine part condition when ductility matters more than spring action. | 1,030 | 1,130 | 15 | 37 HRC | — |
Oil quenched, tempered 540 °C | Tough structural temper for highly stressed bolts, tools and springs needing impact resistance. | 1,370 | 1,490 | 11 | 44 HRC | 6 % IACS |
Oil quenched, tempered 480 °C | Slightly softer, tougher spring temper for shock-loaded leaf springs. | 1,500 | 1,620 | 9 | 47 HRC | 5.9 % IACS |
QT425Default Oil quenched 885 °C, tempered 425 °C | Standard service temper for 9255/60Si2Mn springs — best combination of elastic limit, set resistance and fatigue life. | 1,620 | 1,760 | 8 | 50 HRC | — |
Quenched, tempered ~425 °C and shot peened | Specify for production springs: peening puts the surface in compression and typically raises fatigue life several-fold. | 1,620 | 1,760 | 8 | 50 HRC | 5.7 % IACS |
QT370This page Oil quenched, tempered 370 °C | Balance point between peak strength and usable toughness for coil springs. | 1,700 | 1,860 | 7 | 51 HRC | 5.8 % IACS |
Oil quenched, tempered 315 °C | Common high-hardness spring temper for torsion bars and heavy leaf springs. | 1,730 | 1,960 | 5 | 53 HRC | 5.7 % IACS |
Oil quenched, tempered 260 °C | Very high strength with marginally better ductility than a 205 °C temper. | 1,790 | 2,050 | 4 | 54 HRC | 5.6 % IACS |
Oil quenched 885 °C, tempered 205 °C | Maximum hardness and elastic limit; only for wear/torsion parts where near-zero ductility is acceptable. | 2,000 | 2,100 | 1 | 55 HRC | — |
What is 9255-QT370?
9255-QT370 is 9255 heat treated to QT370 — balance point between peak strength and usable toughness for coil springs. 9255-QT370 has a yield strength of 1,700 MPa (247 ksi) and a tensile strength of 1,860 MPa (270 ksi) — stronger than 99% of steel grades. Elongation at break is 7% and the elastic modulus is 205 GPa (29.7 Msi). 9255-QT370 has a density of 7.8 g/cm³ (0.282 lb/in³), lighter than 87% of steel grades. It melts at 1,400°C (2,552 °F). QT425 is the default condition FabDigit quotes; QT370 is available on request or by drawing note.
Advantages
- High strength — 1,700 MPa (247 ksi), better than 99% of steel grades
- Polishes to a fine finish — 70/100, better than 87% of steel grades
Limitations
- Difficult to weld — 8/100, worse than 99% of steel grades
- Limited formability — 6/100, worse than 98% of steel grades
- Difficult to machine — 20/100, worse than 97% of steel grades
- Limited ductility — 7%, worse than 96% of steel grades
- Low fracture toughness — 38 MPa·√m (34.6 ksi·√in), worse than 93% of steel grades
9255-QT370 properties
Typical room-temperature values for 9255-QT370; 22 are specific to this condition. Other listed values are 9255 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
9255-QT370 has a density of 7.8 g/cm³ (0.282 lb/in³), lighter than 87% of steel grades. It melts at 1,400°C (2,552 °F).
Mechanical16
9255-QT370 has a yield strength of 1,700 MPa (247 ksi) and a tensile strength of 1,860 MPa (270 ksi) — stronger than 99% of steel grades. Elongation at break is 7% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
9255-QT370 is rated for continuous service to 270°C (518 °F). It conducts heat at 42 W/m·K (24.3 BTU/hr·ft·°F), worse than 56% of steel grades. Thermal expansion is 11.8 µm/m·K (6.56 µin/in·°F).
Electrical3
9255-QT370 conducts electricity at 5.8 % IACS, worse than 79% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (11/100), worse than 73% of steel grades.
Sustainability4
Producing a kilogram of 9255-QT370 takes about 30 MJ of energy and emits 2.3 kg of CO₂ — more than 56% of steel grades. Typical recycled content is 30%.
Manufacturability8
9255-QT370's machinability is poor (20/100, worse than 97% of steel grades); weldability not recommended (8/100); formability not recommended (6/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with 9255-QT370
Is 9255 easy to machine?
Machine in the annealed/spheroidized state with rigid setups and carbide tooling; above ~45 HRC only grinding, hard turning with CBN or EDM is practical.
Can 9255 be welded?
Poor weldability — high carbon equivalent gives martensitic HAZ cracking; if unavoidable preheat 300–400 °C, use low-hydrogen consumables and post-weld temper, accepting loss of spring properties in the HAZ.
Can 9255 be formed, bent or molded?
Cold coil or bend only in the annealed condition with generous radii, or hot form at 900–1050 °C; heavy sections are normally hot formed, then oil quenched from 860–890 °C and tempered 400–500 °C, with decarburization control (protective atmosphere or grind-off allowance).
What surface finishes work on 9255?
Grind or shot-peen to remove decarburized skin, then phosphate + oil, black oxide, zinc or Zn-Ni plating (bake at 200 °C within 4 h against hydrogen embrittlement) or epoxy/powder paint.
9255 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 |
|---|---|---|
| CSAE J404 9255 | 0.51 – 0.59 | 0.55 |
| Simain strengthening addition | 1.8 – 2.2 | 2 |
| Mn | 0.7 – 0.95 | 0.82 |
| P | ≤ 0.04 | — |
| S | ≤ 0.04 | — |
| Fe | balance | — |
9255-QT370 — frequently asked
What is 9255-QT370 used for?
9255-QT370 is typically used for leaf springs, torsion bars, coil springs and agricultural equipment springs. In short: high-resilience spring steel; deep-hardening silicon grade; very limited cold formability.
What is the yield strength of 9255-QT370?
9255-QT370 has a typical yield strength of 1,700 MPa (247 ksi) and a tensile strength of 1,860 MPa (270 ksi) — stronger than 99% of steel grades. Strength varies by condition: see the 14 listed tempers.
Is 9255-QT370 easy to machine?
Not especially — machinability is rated poor (20/100, worse than 97% of steel grades). Machine in the annealed/spheroidized state with rigid setups and carbide tooling; above ~45 HRC only grinding, hard turning with CBN or EDM is practical.
Can 9255-QT370 be welded?
Not readily — weldability is rated not recommended (8/100). Poor weldability — high carbon equivalent gives martensitic HAZ cracking; if unavoidable preheat 300–400 °C, use low-hydrogen consumables and post-weld temper, accepting loss of spring properties in the HAZ.
What surface finishes work on 9255-QT370?
Grind or shot-peen to remove decarburized skin, then phosphate + oil, black oxide, zinc or Zn-Ni plating (bake at 200 °C within 4 h against hydrogen embrittlement) or epoxy/powder paint.
Can 9255-QT370 be formed, bent or molded?
Cold coil or bend only in the annealed condition with generous radii, or hot form at 900–1050 °C; heavy sections are normally hot formed, then oil quenched from 860–890 °C and tempered 400–500 °C, with decarburization control (protective atmosphere or grind-off allowance).
What is the maximum service temperature of 9255-QT370?
9255-QT370 is rated for continuous use to about 270°C (518 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 9255-QT425 and 9255-SA?
QT425 is the default condition — standard service temper for 9255/60Si2Mn springs — best combination of elastic limit, set resistance and fatigue life. SA: use when severe cold heading, deep drawing or heavy machining of the bar is required before hardening. Yield strength is 1,620 MPa in QT425 versus 430 MPa in SA.
Sources
- SAE J404 — Chemical Compositions of SAE Alloy Steels — SAE International (2021)
- ASTM A29/A29M — Steel Bars, Carbon and Alloy, Hot-Wrought — ASTM (2020)
- ASTM A689 — Carbon and Alloy Steel Bars for Springs — ASTM (2021)
- GB/T 1222 — (60Si2Mn) — SAC / (2016)
- Heat Treater's Guide: Practices and Procedures for Irons and Steels — ASM International (1995)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- SAE Spring Design Manual (HS-795) — SAE International (1996)
- Mysteel / 60Si2Mn — Mysteel, SMM (2025)
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.