Steel S6 HT-AQ
Heat-treated state
AISI/SAE S6 · UNS T41906
HT-AQ = hardened, as-quenched
As-quenched martensite — an intermediate state only; must be tempered immediately to avoid cracking.
Values for the HT-AQ condition
Other S6 tempers and conditions
S6 is listed in 6 conditions; see the grade overview at S6.
Compare all 6 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
ANNDefault Annealed | Standard mill supply condition for bar and flats; machine and drill in this state before hardening. | 415 | 740 | 22 | 210 HB | — |
Hot rolled | Mill hot-worked stock before spheroidize annealing; only for re-forging or stock that will be fully heat treated. | 680 | 1,000 | 12 | 32 HRC | 3 % IACS |
Q&T 50–54 HRC | Pick when shock and chipping resistance outrank wear — heavy pneumatic tools, hot punches, shear knives on thick stock. | 1,400 | 1,700 | 10 | 52 HRC | — |
Q&T 54–56 HRC | Best all-round balance of toughness and wear for chisels, rivet sets and impact punches. | 1,550 | 1,900 | 7 | 55 HRC | — |
Q&T 56–58 HRC | Choose for punches and shear blades where abrasion dominates and impact energy is moderate. | 1,700 | 2,050 | 5 | 57 HRC | — |
HT-AQThis page Hardened, as-quenched | As-quenched martensite — an intermediate state only; must be tempered immediately to avoid cracking. | 1,800 | 2,200 | 1 | 61 HRC | 2.7 % IACS |
What is S6 HT-AQ?
S6 HT-AQ is S6 heat treated to HT-AQ — as-quenched martensite — an intermediate state only; must be tempered immediately to avoid cracking. S6 HT-AQ has a yield strength of 1,800 MPa (261 ksi) and a tensile strength of 2,200 MPa (319 ksi) — stronger than 99% of steel grades. Elongation at break is 1% and the elastic modulus is 205 GPa (29.7 Msi). S6 HT-AQ has a density of 7.75 g/cm³ (0.28 lb/in³), lighter than 93% of steel grades. It melts at 1,420°C (2,588 °F). ANN is the default condition FabDigit quotes; HT-AQ is available on request or by drawing note.
Advantages
- High strength — 1,800 MPa (261 ksi), better than 99% of steel grades
Limitations
- Difficult to machine — 5/100, worse than 100% of steel grades
- Difficult to weld — 3/100, worse than 100% of steel grades
- Limited formability — 2/100, worse than 100% of steel grades
- Limited ductility — 1%, worse than 99% of steel grades
- Limited service temperature — 150°C (302 °F), worse than 99% of steel grades
S6 HT-AQ properties
Typical room-temperature values for S6 HT-AQ; 22 are specific to this condition. Other listed values are S6 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
S6 HT-AQ has a density of 7.75 g/cm³ (0.28 lb/in³), lighter than 93% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical15
S6 HT-AQ has a yield strength of 1,800 MPa (261 ksi) and a tensile strength of 2,200 MPa (319 ksi) — stronger than 99% of steel grades. Elongation at break is 1% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
S6 HT-AQ is rated for continuous service to 150°C (302 °F). It conducts heat at 22 W/m·K (12.7 BTU/hr·ft·°F), worse than 93% of steel grades. Thermal expansion is 12.4 µm/m·K (6.89 µin/in·°F).
Electrical3
S6 HT-AQ conducts electricity at 2.7 % IACS, worse than 96% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (10/100), worse than 86% of steel grades.
Sustainability4
Producing a kilogram of S6 HT-AQ takes about 33 MJ of energy and emits 2.4 kg of CO₂ — more than 76% of steel grades. Typical recycled content is 40%.
Manufacturability7
S6 HT-AQ's machinability is not recommended (5/100, worse than 100% of steel grades); weldability not recommended (3/100); formability not recommended (2/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with S6 HT-AQ
Is S6 easy to machine?
Machine only in the annealed state (≈200 HB); the 2 % Si makes chips gummy — use sharp carbide or cobalt HSS, rigid setups, and finish-grind after hardening.
Can S6 be welded?
Not a structural weld grade; repair-weld only with matching or S-series filler at 300–400 °C preheat followed by an immediate full temper.
Can S6 be formed, bent or molded?
Forge 1040–1150 °C, stop above 900 °C, cool slowly and spheroidize anneal at 780–800 °C; cold forming is limited to light bending of annealed strip.
What surface finishes work on S6?
Harden 900–925 °C with a protective/neutral atmosphere or pack to control the strong decarburization tendency, oil quench, temper twice; finish with grinding plus black oxide, phosphate or nitride for rust protection.
S6 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 |
| Mn | 1.2 – 1.5 | 1.35 |
| Sihigh Si raises tempering resistance and hardenability; promotes decarburization | 2 – 2.5 | 2.25 |
| Cr | 1.2 – 1.5 | 1.35 |
| Mo | 0.3 – 0.5 | 0.4 |
| Vgrain refinement | 0.2 – 0.3 | 0.25 |
| P | ≤ 0.03 | — |
| S | ≤ 0.03 | — |
| Fe | balance | — |
S6 HT-AQ — frequently asked
What is S6 HT-AQ used for?
S6 HT-AQ is typically used for chisels, heavy-duty punches, shear blades and forming tools. In short: toughest wear-resisting S grade; oil-hardening impact steel; strong decarburization tendency.
What is the yield strength of S6 HT-AQ?
S6 HT-AQ has a typical yield strength of 1,800 MPa (261 ksi) and a tensile strength of 2,200 MPa (319 ksi) — stronger than 99% of steel grades. Strength varies by condition: see the 6 listed tempers.
Is S6 HT-AQ easy to machine?
Not especially — machinability is rated not recommended (5/100, worse than 100% of steel grades). Machine only in the annealed state (≈200 HB); the 2 % Si makes chips gummy — use sharp carbide or cobalt HSS, rigid setups, and finish-grind after hardening.
Can S6 HT-AQ be welded?
Not readily — weldability is rated not recommended (3/100). Not a structural weld grade; repair-weld only with matching or S-series filler at 300–400 °C preheat followed by an immediate full temper.
What surface finishes work on S6 HT-AQ?
Harden 900–925 °C with a protective/neutral atmosphere or pack to control the strong decarburization tendency, oil quench, temper twice; finish with grinding plus black oxide, phosphate or nitride for rust protection.
Can S6 HT-AQ be formed, bent or molded?
Forge 1040–1150 °C, stop above 900 °C, cool slowly and spheroidize anneal at 780–800 °C; cold forming is limited to light bending of annealed strip.
What is the maximum service temperature of S6 HT-AQ?
S6 HT-AQ is rated for continuous use to about 150°C (302 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between S6-ANN and S6 HT-AQ?
ANN is the default condition — standard mill supply condition for bar and flats; machine and drill in this state before hardening. HT-AQ: as-quenched martensite — an intermediate state only; must be tempered immediately to avoid cracking. Yield strength is 415 MPa in ANN versus 1,800 MPa in HT-AQ.
Sources
- ASTM A681 — Standard Specification for Tool Steels Alloy — ASTM International (2022)
- ASM Handbook, Vol. 1: Properties and Selection — Irons, Steels, and High-Performance Alloys (Tool Steels) — ASM International (1990)
- Tool Steels, 5th ed. (Roberts, Krauss, Kennedy) — ASM International (1998)
- SAE J438 / UNS T41906 designation data — SAE International (2017)
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.