Steel 8640 · hardness band
Steel 8640 Q&T 38-42 HRC
Properties of the Q&T 38-42 HRC condition, compared with the other 8640 tempers.
High-strength condition (temper ≈425 °C) for heavily loaded pins, sprockets and fasteners where wear resistance outweighs toughness.
What is 8640 Q&T 38-42 HRC?
8640 Q&T 38-42 HRC is 8640 heat treated to Q&T 38-42 HRC — high-strength condition (temper ≈425 °C) for heavily loaded pins, sprockets and fasteners where wear resistance outweighs toughness. 8640 Q&T 38-42 HRC has a yield strength of 1,275 MPa (185 ksi) and a tensile strength of 1,380 MPa (200 ksi) — stronger than 95% of steel grades. Elongation at break is 14% and the elastic modulus is 205 GPa (29.7 Msi). 8640 Q&T 38-42 HRC has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,425°C (2,597 °F). HR is the default condition FabDigit quotes; Q&T 38-42 HRC is available on request or by drawing note.
Advantages
- High strength — 1,275 MPa (185 ksi), better than 95% of steel grades
- Low embodied carbon — 1.9 kg CO₂/kg, better than 92% of steel grades
Limitations
- Difficult to machine — 32/100, worse than 87% of steel grades
- Difficult to weld — 20/100, worse than 84% of steel grades
- Low fracture toughness — 65 MPa·√m (59.2 ksi·√in), worse than 76% of steel grades
- Limited ductility — 14%, worse than 75% of steel grades
8640 Q&T 38-42 HRC properties
Typical room-temperature values for 8640 Q&T 38-42 HRC — 12 properties are specific to this condition; the rest are grade-level values shared by every 8640 temper. Each bar shows where the value sits among the steel grades in FabDigit's library — further right is higher.
Physical3
8640 Q&T 38-42 HRC has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,425°C (2,597 °F).
Mechanical16
8640 Q&T 38-42 HRC has a yield strength of 1,275 MPa (185 ksi) and a tensile strength of 1,380 MPa (200 ksi) — stronger than 95% of steel grades. Elongation at break is 14% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
8640 Q&T 38-42 HRC is rated for continuous service to 400°C (752 °F). It conducts heat at 45 W/m·K (26 BTU/hr·ft·°F), better than 66% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
8640 Q&T 38-42 HRC conducts electricity at 7.8 % IACS, worse than 58% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (14/100), better than 70% of steel grades.
Sustainability4
Producing a kilogram of 8640 Q&T 38-42 HRC 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
8640 Q&T 38-42 HRC's machinability is poor (32/100, worse than 87% of steel grades); weldability poor (20/100); formability fair (35/100).
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Other 8640 tempers and conditions
8640 is also supplied in 9 other conditions. The full side-by-side table is on the 8640 overview.
- HRDefaultStandard as-rolled bar for parts that will be heat treated after machining; cheapest stock form.570 MPa yield · 20 HRC
- Q&T 48-52 HRCLow-temper (≈205–260 °C) condition for maximum hardness/wear in small sections; notch-sensitive and difficult to machine — grind to size.1,620 MPa yield · 50 HRC
- Q&T 38-42 HRCHigh-strength condition (temper ≈425 °C) for heavily loaded pins, sprockets and fasteners where wear resistance outweighs toughness.1,275 MPa yield · 40 HRC
- Q&TGeneral commercial condition: oil quench from ~830 °C and temper ~540 °C for a balanced strength/toughness shaft or gear blank.1,000 MPa yield · 35 HRC
- Q&T 32-36 HRCCommon shaft/gear-blank hardness giving high fatigue strength while still machinable with carbide tooling.1,000 MPa yield · 34 HRC
- Q&T 28-32 HRCTough mid-hardness condition (temper ≈600 °C) for drill-pipe tool joints, sour-service and welded-then-tempered assemblies.860 MPa yield · 30 HRC
- CDCold-drawn bar with better size tolerance, straightness and surface than HR; higher yield strength but residual stress can cause distortion on machining.760 MPa yield · 25 HRC
- NormalizedNormalized ~900 °C and air cooled: uniform fine grain and moderate strength; typical pre-treatment before machining and final Q&T of forgings.690 MPa yield · 27 HRC
- AnnealedSoftest condition (full/spheroidize anneal ~840 °C, furnace cool) — best for deep machining, cold forming or thread rolling before hardening.380 MPa yield · 184 HB
Working with 8640 Q&T 38-42 HRC
Is 8640 easy to machine?
Machine in the annealed or ≤32 HRC Q&T condition with carbide tooling and rigid setups; rough before hardening and leave grind stock for finish sizes.
Can 8640 be welded?
Weldable with low-hydrogen consumables but requires ~200–300 °C preheat, interpass control and post-weld temper below the original tempering temperature to avoid martensitic cracking.
Can 8640 be formed, bent or molded?
Cold forming only in the annealed state and with generous bend radii; hot forge at 1050–1200 °C then normalize before final heat treatment.
What surface finishes work on 8640?
No natural corrosion protection — apply black oxide, phosphate + oil, zinc or hard-chrome plating; bake plated parts above ~38 HRC to relieve hydrogen embrittlement.
8640 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 | 0.38 – 0.43 | 0.4 |
| Mn | 0.75 – 1 | 0.88 |
| Si | 0.15 – 0.35 | 0.25 |
| Ni | 0.4 – 0.7 | 0.55 |
| Cr | 0.4 – 0.6 | 0.5 |
| Mo | 0.15 – 0.25 | 0.2 |
| Pmax, SAE J404 wrought bar | ≤ 0.04 | — |
| Smax; 8640H/resulfurized variants differ | ≤ 0.04 | — |
| Fe | balance | — |
8640 Q&T 38-42 HRC — frequently asked
What is 8640 Q&T 38-42 HRC used for?
8640 Q&T 38-42 HRC is typically used for gears, shafts and bolts. In short: workhorse Ni-Cr-Mo bar.
What is the yield strength of 8640 Q&T 38-42 HRC?
8640 Q&T 38-42 HRC has a typical yield strength of 1,275 MPa (185 ksi) and a tensile strength of 1,380 MPa (200 ksi) — stronger than 95% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is 8640 Q&T 38-42 HRC easy to machine?
Not especially — machinability is rated poor (32/100, worse than 87% of steel grades). Machine in the annealed or ≤32 HRC Q&T condition with carbide tooling and rigid setups; rough before hardening and leave grind stock for finish sizes.
Can 8640 Q&T 38-42 HRC be welded?
Not readily — weldability is rated poor (20/100). Weldable with low-hydrogen consumables but requires ~200–300 °C preheat, interpass control and post-weld temper below the original tempering temperature to avoid martensitic cracking.
What surface finishes work on 8640 Q&T 38-42 HRC?
No natural corrosion protection — apply black oxide, phosphate + oil, zinc or hard-chrome plating; bake plated parts above ~38 HRC to relieve hydrogen embrittlement.
Can 8640 Q&T 38-42 HRC be formed, bent or molded?
Cold forming only in the annealed state and with generous bend radii; hot forge at 1050–1200 °C then normalize before final heat treatment.
What is the maximum service temperature of 8640 Q&T 38-42 HRC?
8640 Q&T 38-42 HRC 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 8640-HR and 8640-CD?
HR is the default condition — standard as-rolled bar for parts that will be heat treated after machining; cheapest stock form. CD: cold-drawn bar with better size tolerance, straightness and surface than HR; higher yield strength but residual stress can cause distortion on machining. Yield strength is 570 MPa in HR versus 760 MPa in CD.
Can FabDigit make parts in 8640 Q&T 38-42 HRC?
Yes — 8640 Q&T 38-42 HRC 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
- SAE J404 — Chemical Compositions of SAE Alloy Steels — SAE International (2021)
- ASTM A29/A29M — General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought — ASTM (2020)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.4 — Heat Treating (tempering curves for 86xx steels) — ASM International (1991)
- SAE J1268 hardenability / typical mechanical property tables for 8640 — SAE International (2019)
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.