Steel SACM645 · heat-treated state · default condition
Steel SACM645 Q&T
Properties of the Q&T condition, compared with the other SACM645 tempers.
The standard stocked and pre-nitride condition: tough 850–1000 MPa core that stays dimensionally stable through the 500–540 °C nitriding cycle.
What is SACM645 Q&T?
SACM645 Q&T is SACM645 heat treated to Q&T — the standard stocked and pre-nitride condition: tough 850–1000 MPa core that stays dimensionally stable through the 500–540 °C nitriding cycle. SACM645 Q&T has a yield strength of 780 MPa (113 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 86% of steel grades. Elongation at break is 17% and the elastic modulus is 207 GPa (30 Msi). SACM645 Q&T has a density of 7.8 g/cm³ (0.282 lb/in³), lighter than 87% of steel grades. It melts at 1,425°C (2,597 °F).
Advantages
- High strength — 780 MPa (113 ksi), better than 86% of steel grades
- High service temperature — 450°C (842 °F), better than 86% of steel grades
Limitations
- Difficult to weld — 18/100, worse than 87% of steel grades
- High embodied carbon — 2.5 kg CO₂/kg, worse than 82% of steel grades
SACM645 Q&T properties
Typical room-temperature values for SACM645 Q&T — 14 properties are specific to this condition; the rest are grade-level values shared by every SACM645 temper. Each bar shows where the value sits among the steel grades in FabDigit's library — further right is higher.
Physical3
SACM645 Q&T has a density of 7.8 g/cm³ (0.282 lb/in³), lighter than 87% of steel grades. It melts at 1,425°C (2,597 °F).
Mechanical16
SACM645 Q&T has a yield strength of 780 MPa (113 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 86% of steel grades. Elongation at break is 17% and the elastic modulus is 207 GPa (30 Msi).
Thermal6
SACM645 Q&T is rated for continuous service to 450°C (842 °F). It conducts heat at 36 W/m·K (20.8 BTU/hr·ft·°F), worse than 72% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
SACM645 Q&T conducts electricity at 7.5 % IACS, worse than 68% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (15/100), better than 75% of steel grades.
Sustainability4
Producing a kilogram of SACM645 Q&T takes about 34 MJ of energy and emits 2.5 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 28%.
Manufacturability6
SACM645 Q&T's machinability is fair (42/100, worse than 72% of steel grades); weldability poor (18/100); formability poor (25/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Other SACM645 tempers and conditions
SACM645 is also supplied in 3 other conditions. The full side-by-side table is on the SACM645 overview.
- Q&TDefaultThe standard stocked and pre-nitride condition: tough 850–1000 MPa core that stays dimensionally stable through the 500–540 °C nitriding cycle.780 MPa yield · 28 HRC
- Q&T+NITFinal service condition for screws, barrels, spindles and gauges needing extreme surface hardness and wear/scuff resistance with no quench distortion.790 MPa yield · 67 HRC
- NUse as an intermediate grain-refining / stress-relieving condition on forged blanks; not the final delivery state.690 MPa yield · 26 HRC
- APick for deep machining, cold sawing or re-forging before the final Q&T + nitride cycle.430 MPa yield · 205 HB
Working with SACM645 Q&T
Is SACM645 easy to machine?
Machine fully in the Q&T condition (≈270 HB, ~45 % of 1212) with rigid setups and coated carbide; leave 0.05–0.10 mm grinding stock because nitriding must be the last operation.
Can SACM645 be welded?
Practically unweldable for structural joints — high Al and Cr give air-hardening and crack-prone HAZ; if unavoidable, preheat 300–350 °C, use low-hydrogen/matching filler and re-temper immediately, and never weld a nitrided surface.
Can SACM645 be formed, bent or molded?
Hot forge 1150–850 °C followed by slow cooling and annealing; cold forming is limited to minor straightening, and a stress relief at ~600 °C before nitriding prevents distortion.
What surface finishes work on SACM645?
Gas or plasma nitride 500–540 °C for 20–90 h to a 0.2–0.6 mm case of 900–1100 HV; remove the brittle white layer by light lapping/polishing; the nitrided surface takes only light oxidation or oiling — plating and anodizing are not applicable.
SACM645 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 |
|---|---|---|
| C | 0.4 – 0.5 | 0.45 |
| Si | 0.15 – 0.35 | 0.25 |
| MnJIS G 4202 | 0.6 – 0.9 | 0.75 |
| P | ≤ 0.03 | — |
| S | ≤ 0.03 | — |
| Cr | 1.3 – 1.7 | 1.5 |
| Mo | 0.15 – 0.3 | 0.22 |
| Alnitride-forming element, key to the grade | 0.7 – 1.2 | 0.95 |
| Niresidual, max | ≤ 0.25 | — |
| Curesidual, max | ≤ 0.3 | — |
| Fe | balance | — |
SACM645 Q&T — frequently asked
What is SACM645 Q&T used for?
SACM645 Q&T is typically used for nitrided drive and transmission shafts, extruder screws and barrels, gear and pinion working surfaces, precision machine spindles, cam shafts and cam followers and valve stems and spindles. In short: nitridable aerospace bar.
What is the yield strength of SACM645 Q&T?
SACM645 Q&T has a typical yield strength of 780 MPa (113 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 86% of steel grades. Strength varies by condition: see the 4 listed tempers.
Is SACM645 Q&T easy to machine?
Not especially — machinability is rated fair (42/100, worse than 72% of steel grades). Machine fully in the Q&T condition (≈270 HB, ~45 % of 1212) with rigid setups and coated carbide; leave 0.05–0.10 mm grinding stock because nitriding must be the last operation.
Can SACM645 Q&T be welded?
Not readily — weldability is rated poor (18/100). Practically unweldable for structural joints — high Al and Cr give air-hardening and crack-prone HAZ; if unavoidable, preheat 300–350 °C, use low-hydrogen/matching filler and re-temper immediately, and never weld a nitrided surface.
What surface finishes work on SACM645 Q&T?
Gas or plasma nitride 500–540 °C for 20–90 h to a 0.2–0.6 mm case of 900–1100 HV; remove the brittle white layer by light lapping/polishing; the nitrided surface takes only light oxidation or oiling — plating and anodizing are not applicable.
Can SACM645 Q&T be formed, bent or molded?
Hot forge 1150–850 °C followed by slow cooling and annealing; cold forming is limited to minor straightening, and a stress relief at ~600 °C before nitriding prevents distortion.
What is the maximum service temperature of SACM645 Q&T?
SACM645 Q&T is rated for continuous use to about 450°C (842 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between SACM645 Q&T and SACM645-A?
Q&T is the default condition — the standard stocked and pre-nitride condition: tough 850–1000 MPa core that stays dimensionally stable through the 500–540 °C nitriding cycle. A: pick for deep machining, cold sawing or re-forging before the final Q&T + nitride cycle. Yield strength is 780 MPa in Q&T versus 430 MPa in A.
Can FabDigit make parts in SACM645 Q&T?
Yes — SACM645 Q&T is available for CNC machining and sheet metal with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- JIS G 4202 Aluminium chromium molybdenum steels — Japanese Standards Association (2015)
- GB/T 3077 Alloy structural steels (38CrMoAl) — SAC (2015)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels — ASM International (1990)
- ASM Handbook Vol.4A — Steel Heat Treating (Nitriding) — ASM International (2013)
- Machinery's Handbook (machinability and nitriding steels) — Industrial Press (2020)
- 38CrMoAl — Mysteel / 1688 (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.