Steel SKH59 QT66-68
Hardness band
JIS SKH59 · AISI/SAE M42
QT66-68 = hardened & tempered, 66–68 HRC
The normal working hardness for M42/SKH59 drills, taps and end mills cutting hardened or abrasive workpieces.
Values for the QT66-68 condition
Other SKH59 tempers and conditions
SKH59 is listed in 6 conditions; see the grade overview at SKH59.
Compare all 6 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Hardened & tempered, 64–66 HRC | Austenitize ~1180–1190 °C and temper high (2–3 × 550–560 °C) when interrupted cuts or larger tools need extra toughness. | — | — | 0 | 65 HRC | — |
QT66-68This page Hardened & tempered, 66–68 HRC | The normal working hardness for M42/SKH59 drills, taps and end mills cutting hardened or abrasive workpieces. | — | — | 0 | 67 HRC | — |
ANNDefault Annealed | Standard stock condition for bar/blank supply — machine tool geometry, then harden and temper. | 600 | 900 | 12 | 25 HRC | 2.9 % IACS |
Annealed, cold drawn / centreless ground bar | Precision-ground annealed round stock for drill/tap/end-mill blanks where diameter tolerance matters. | 650 | 950 | 9 | 25 HRC | 3 % IACS |
Hardened & tempered, 68–70 HRC | High austenitizing (~1190–1200 °C) plus 540 °C tempers for maximum wear resistance on light, rigid, continuous cuts. | 2,000 | 2,100 | 0 | 69 HRC | 2.6 % IACS |
Hardened, tempered + surface treated | Core at 66–68 HRC with a nitrided or TiN/TiAlN surface for lower friction and longer tool life on abrasive work. | 2,000 | 2,200 | 1 | 67 HRC | 2.7 % IACS |
What is SKH59 QT66-68?
SKH59 QT66-68 is SKH59 heat treated to QT66-68 — the normal working hardness for M42/SKH59 drills, taps and end mills cutting hardened or abrasive workpieces. SKH59 QT66-68 has a yield strength of 600 MPa (87 ksi) and a tensile strength of 900 MPa (131 ksi) — stronger than 77% of steel grades. Elongation at break is 0% and the elastic modulus is 230 GPa (33.4 Msi). SKH59 QT66-68 has a density of 7.98 g/cm³ (0.288 lb/in³), heavier than 94% of steel grades. It melts at 1,290°C (2,354 °F). ANN is the default condition FabDigit quotes; QT66-68 is available on request or by drawing note.
Advantages
- High service temperature — 600°C (1,112 °F), better than 98% of steel grades
- Polishes to a fine finish — 80/100, better than 97% of steel grades
- High strength — 600 MPa (87 ksi), better than 77% of steel grades
Limitations
- Limited ductility — 0%, worse than 100% of steel grades
- Difficult to machine — 4/100, worse than 100% of steel grades
- Low fracture toughness — 16 MPa·√m (14.6 ksi·√in), worse than 99% of steel grades
- Poor heat conductor — 19 W/m·K (11 BTU/hr·ft·°F), worse than 97% of steel grades
- Limited formability — 10/100, worse than 95% of steel grades
SKH59 QT66-68 properties
Typical room-temperature values for SKH59 QT66-68; 11 are specific to this condition. Other listed values are SKH59 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
SKH59 QT66-68 has a density of 7.98 g/cm³ (0.288 lb/in³), heavier than 94% of steel grades. It melts at 1,290°C (2,354 °F).
Mechanical17
SKH59 QT66-68 has a yield strength of 600 MPa (87 ksi) and a tensile strength of 900 MPa (131 ksi) — stronger than 77% of steel grades. Elongation at break is 0% and the elastic modulus is 230 GPa (33.4 Msi).
Thermal6
SKH59 QT66-68 is rated for continuous service to 600°C (1,112 °F). It conducts heat at 19 W/m·K (11 BTU/hr·ft·°F), worse than 97% of steel grades. Thermal expansion is 10.6 µm/m·K (5.89 µin/in·°F).
Electrical3
SKH59 QT66-68 conducts electricity at 2.9 % IACS, worse than 93% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (20/100), better than 87% of steel grades.
Sustainability4
Producing a kilogram of SKH59 QT66-68 takes about 55 MJ of energy and emits 4.2 kg of CO₂ — more than 94% of steel grades. Typical recycled content is 25%.
Manufacturability7
SKH59 QT66-68's machinability is not recommended (4/100, worse than 100% 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 SKH59 QT66-68
Is SKH59 easy to machine?
Machine only in the annealed condition with sharp, rigid setups and low speeds (≈25% of 1212 free-machining steel); after hardening, finish by grinding with SG/CBN wheels and generous coolant — grindability is poorer than M2 because of the Co and high Mo carbides.
Can SKH59 be welded?
Not a structural welding grade; repair or bimetal band-saw joining only by EB, laser or friction welding with preheat 500–550 °C and immediate re-temper to avoid quench cracks.
Can SKH59 be formed, bent or molded?
Hot work at 1050–1100 °C (stop above 900 °C) with slow furnace cooling and spheroidize anneal 850–870 °C; no meaningful cold formability.
What surface finishes work on SKH59?
Harden 1180–1200 °C in vacuum/salt, quench and triple temper 540–560 °C; finish with steam-oxide, black oxide, nitriding or TiN/TiAlN PVD (≤500 °C so as not to over-temper) and keep oiled — the 4% Cr does not make it stainless.
SKH59 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 |
|---|---|---|
| CJIS G4403 SKH59 / AISI M42 | 1 – 1.15 | 1.08 |
| Si | ≤ 0.5 | 0.3 |
| Mn | ≤ 0.4 | 0.25 |
| P | ≤ 0.03 | — |
| S | ≤ 0.03 | — |
| Cr | 3.5 – 4.5 | 3.9 |
| Mo | 9 – 10 | 9.5 |
| W | 1.2 – 1.9 | 1.5 |
| V | 0.9 – 1.4 | 1.15 |
| Cocobalt gives red hardness | 7.5 – 8.5 | 8 |
| Fe | balance | — |
SKH59 QT66-68 — frequently asked
What is SKH59 QT66-68 used for?
SKH59 QT66-68 is typically used for high-speed drills, cutting tools for hardened steel and gear cutters. In short: highest-hardness cobalt HSS; for hardened-material cutting; poorest grindability in family.
What is the yield strength of SKH59 QT66-68?
SKH59 QT66-68 has a typical yield strength of 600 MPa (87 ksi) and a tensile strength of 900 MPa (131 ksi) — stronger than 77% of steel grades. Strength varies by condition: see the 6 listed tempers.
Is SKH59 QT66-68 easy to machine?
Not especially — machinability is rated not recommended (4/100, worse than 100% of steel grades). Machine only in the annealed condition with sharp, rigid setups and low speeds (≈25% of 1212 free-machining steel); after hardening, finish by grinding with SG/CBN wheels and generous coolant — grindability is poorer than M2 because of the Co and high Mo carbides.
Can SKH59 QT66-68 be welded?
Not readily — weldability is rated poor (15/100). Not a structural welding grade; repair or bimetal band-saw joining only by EB, laser or friction welding with preheat 500–550 °C and immediate re-temper to avoid quench cracks.
What surface finishes work on SKH59 QT66-68?
Harden 1180–1200 °C in vacuum/salt, quench and triple temper 540–560 °C; finish with steam-oxide, black oxide, nitriding or TiN/TiAlN PVD (≤500 °C so as not to over-temper) and keep oiled — the 4% Cr does not make it stainless.
Can SKH59 QT66-68 be formed, bent or molded?
Hot work at 1050–1100 °C (stop above 900 °C) with slow furnace cooling and spheroidize anneal 850–870 °C; no meaningful cold formability.
What is the maximum service temperature of SKH59 QT66-68?
SKH59 QT66-68 is rated for continuous use to about 600°C (1,112 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between SKH59-ANN and SKH59 QT68-70?
ANN is the default condition — standard stock condition for bar/blank supply — machine tool geometry, then harden and temper. QT68-70: high austenitizing (~1190–1200 °C) plus 540 °C tempers for maximum wear resistance on light, rigid, continuous cuts. Yield strength is 600 MPa in ANN versus 2,000 MPa in QT68-70.
Sources
- JIS G 4403 High speed tool steel materials — JSA (2015)
- ASTM A600 Standard Specification for Tool Steel High Speed — ASTM (2016)
- ASM Handbook Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol. 16: Machining — ASM International (1989)
- M42 / HS2-9-1-8 cobalt high speed steel datasheet — Böhler / Erasteel (2022)
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.