Steel HP9-4-20 QT-LT
Heat-treated state
UNS K91868
QT-LT = q&T low temper
Pick when peak strength (~1450–1550 MPa) is needed and some toughness can be traded away; not the usual aerospace release condition.
Values for the QT-LT condition
Other HP9-4-20 tempers and conditions
HP9-4-20 is listed in 5 conditions; see the grade overview at HP9-4-20.
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Annealed | Supply/soft condition for heavy machining, cold straightening and deep forming before final hardening. | 760 | 965 | 20 | 29 HRC | — |
Normalized and tempered | Intermediate condition for large forgings/plate before final austenitize-quench-temper, or where moderate strength suffices. | 930 | 1,100 | 19 | 34 HRC | 3.2 % IACS |
Welded joint, post-weld heat treated (re-austenitized or tempered) | Use for fabricated pressure vessels and rocket cases — weld metal matches ~90–100 % of base plate strength after PWHT. | 1,140 | 1,275 | 13 | 41 HRC | 3.1 % IACS |
QT-190Default Q&T 190 ksi | Standard aerospace condition: best balance of ~1310 MPa strength with exceptional fracture toughness and weld repairability. | 1,210 | 1,340 | 17 | 42 HRC | — |
QT-LTThis page Q&T low temper | Pick when peak strength (~1450–1550 MPa) is needed and some toughness can be traded away; not the usual aerospace release condition. | 1,280 | 1,500 | 14 | 46 HRC | — |
What is HP9-4-20 QT-LT?
HP9-4-20 QT-LT is HP9-4-20 heat treated to QT-LT — pick when peak strength (~1450–1550 MPa) is needed and some toughness can be traded away; not the usual aerospace release condition. HP9-4-20 QT-LT has a yield strength of 1,280 MPa (186 ksi) and a tensile strength of 1,500 MPa (218 ksi) — stronger than 95% of steel grades. Elongation at break is 14% and the elastic modulus is 200 GPa (29 Msi). HP9-4-20 QT-LT has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F). QT-190 is the default condition FabDigit quotes; QT-LT is available on request or by drawing note.
Advantages
- High strength — 1,280 MPa (186 ksi), better than 95% of steel grades
- Polishes to a fine finish — 70/100, better than 87% of steel grades
Limitations
- Limited service temperature — 175°C (347 °F), worse than 99% of steel grades
- High embodied carbon — 3.4 kg CO₂/kg, worse than 93% of steel grades
- Difficult to machine — 28/100, worse than 91% of steel grades
- Poor heat conductor — 30 W/m·K (17.3 BTU/hr·ft·°F), worse than 83% of steel grades
- Limited ductility — 14%, worse than 75% of steel grades
HP9-4-20 QT-LT properties
Typical room-temperature values for HP9-4-20 QT-LT; 10 are specific to this condition. Other listed values are HP9-4-20 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
HP9-4-20 QT-LT has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
HP9-4-20 QT-LT has a yield strength of 1,280 MPa (186 ksi) and a tensile strength of 1,500 MPa (218 ksi) — stronger than 95% of steel grades. Elongation at break is 14% and the elastic modulus is 200 GPa (29 Msi).
Thermal6
HP9-4-20 QT-LT is rated for continuous service to 175°C (347 °F). It conducts heat at 30 W/m·K (17.3 BTU/hr·ft·°F), worse than 83% of steel grades. Thermal expansion is 11.8 µm/m·K (6.56 µin/in·°F).
Electrical2
HP9-4-20 QT-LT has an electrical resistivity of 5.5×10⁻⁷ Ω·m.
Chemical & Environmental3
Corrosion resistance is poor (15/100), better than 75% of steel grades.
Sustainability4
Producing a kilogram of HP9-4-20 QT-LT takes about 42 MJ of energy and emits 3.4 kg of CO₂ — more than 92% of steel grades. Typical recycled content is 20%.
Manufacturability7
HP9-4-20 QT-LT's machinability is poor (28/100, worse than 91% of steel grades); weldability very good (70/100); formability poor (25/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with HP9-4-20 QT-LT
Is HP9-4-20 easy to machine?
Rough machine annealed and finish after heat treatment; at 40–44 HRC use rigid setups, sharp carbide or CBN, low speed and heavy positive feed to avoid work hardening.
Can HP9-4-20 be welded?
Excellent for a UHS steel — GTAW/EB welded with matching filler, no preheat needed for thin sections; keep interpass low, use clean low-hydrogen practice and re-temper (or full re-austenitize) after welding.
Can HP9-4-20 be formed, bent or molded?
Form hot (900–1150 °C) or in the annealed condition; cold forming after hardening is not practical — bend radii must be generous and springback is high.
What surface finishes work on HP9-4-20?
Grind/polish carefully to avoid grinding burns; protect with cadmium, IVD aluminium or electroless nickel plus epoxy primer, and hydrogen-relief bake at ~190 °C after any plating or acid cleaning.
HP9-4-20 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 | 0.17 – 0.23 | 0.2 |
| Ni | 8.5 – 9.5 | 9 |
| Co | 4.25 – 4.75 | 4.5 |
| Cr | 0.65 – 0.85 | 0.75 |
| Mo | 0.9 – 1.1 | 1 |
| V | 0.06 – 0.12 | 0.09 |
| Mn | 0.1 – 0.35 | 0.25 |
| Si | ≤ 0.2 | 0.1 |
| P | ≤ 0.01 | 0.01 |
| S | ≤ 0.01 | 0 |
| Febalance; vacuum induction melted + vacuum arc remelted | balance | — |
HP9-4-20 QT-LT — frequently asked
What is HP9-4-20 QT-LT used for?
HP9-4-20 QT-LT is typically used for rocket motor cases, thick-section structural forgings and pressure vessels. In short: most weldable UHS steel; toughest 9Ni-4Co grade; thick-section pressure grade.
What is the yield strength of HP9-4-20 QT-LT?
HP9-4-20 QT-LT has a typical yield strength of 1,280 MPa (186 ksi) and a tensile strength of 1,500 MPa (218 ksi) — stronger than 95% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is HP9-4-20 QT-LT easy to machine?
Not especially — machinability is rated poor (28/100, worse than 91% of steel grades). Rough machine annealed and finish after heat treatment; at 40–44 HRC use rigid setups, sharp carbide or CBN, low speed and heavy positive feed to avoid work hardening.
Can HP9-4-20 QT-LT be welded?
Yes — weldability is rated very good (70/100). Excellent for a UHS steel — GTAW/EB welded with matching filler, no preheat needed for thin sections; keep interpass low, use clean low-hydrogen practice and re-temper (or full re-austenitize) after welding.
What surface finishes work on HP9-4-20 QT-LT?
Grind/polish carefully to avoid grinding burns; protect with cadmium, IVD aluminium or electroless nickel plus epoxy primer, and hydrogen-relief bake at ~190 °C after any plating or acid cleaning.
Can HP9-4-20 QT-LT be formed, bent or molded?
Form hot (900–1150 °C) or in the annealed condition; cold forming after hardening is not practical — bend radii must be generous and springback is high.
What is the maximum service temperature of HP9-4-20 QT-LT?
HP9-4-20 QT-LT is rated for continuous use to about 175°C (347 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between HP9-4-20 QT-190 and HP9-4-20-ANN?
QT-190 is the default condition — standard aerospace condition: best balance of ~1310 MPa strength with exceptional fracture toughness and weld repairability. ANN: supply/soft condition for heavy machining, cold straightening and deep forming before final hardening. Yield strength is 1,210 MPa in QT-190 versus 760 MPa in ANN.
Sources
- AMS 6523 — Steel, Plate 9Ni-4Co-0.20C (HP 9-4-20) — SAE International (2018)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels and High-Performance Alloys (Ultrahigh-Strength Steels) — ASM International (1990)
- MMPDS-17 Metallic Materials Properties Development and Standardization (9Ni-4Co steels) — Battelle/FAA (2022)
- UNS K91868 — Metals & Alloys in the Unified Numbering System — SAE/ASTM (2017)
- ASM Handbook Vol.19: Fatigue and Fracture (fracture toughness of UHS steels) — ASM International (1996)
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.