Steel HP9-4-30
UNS K91972
HP9-4-30 (UNS K91972) is the higher-carbon member of the 9Ni-4Co low-alloy ultra-high-strength steel family, quenched and tempered to 1400 MPa yield and 1655 MPa tensile at 48 HRC with 13 % elongation.
Reference datasheet — HP9-4-30 is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the QT540 condition
HP9-4-30 tempers and conditions
HP9-4-30 is supplied in 5 conditions, soft to hard. QT540 is the default FabDigit quotes; the values on this page are for it.
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Annealed | Pick for rough machining of large forgings before final austenitize-quench-temper. | 790 | 1,000 | 18 | 31 HRC | — |
Normalized and tempered | Mill delivery condition for bar/plate that will be re-heat-treated by the part maker. | 1,000 | 1,200 | 16 | 38 HRC | 3.9 % IACS |
Q&T, double temper 565–595 °C | Choose when fracture toughness and damage tolerance outweigh peak strength. | 1,310 | 1,520 | 15 | 45 HRC | — |
QT540Default Q&T, austenitize 830–860 °C, oil quench, −73 °C refrigerate, double… | Standard aerospace condition — best strength/toughness balance for landing gear and structural forgings. | 1,400 | 1,655 | 13 | 48 HRC | — |
Q&T, double temper 480–510 °C | Use only where maximum strength is required and section/stress state tolerates reduced toughness. | 1,520 | 1,790 | 11 | 51 HRC | — |
What is HP9-4-30?
HP9-4-30 (UNS K91972) is the higher-carbon member of the 9Ni-4Co low-alloy ultra-high-strength steel family, quenched and tempered to 1400 MPa yield and 1655 MPa tensile at 48 HRC with 13 % elongation. Engineers use it for heavy aerospace forgings and rocket motor cases because it holds that strength with good fracture toughness and hardens through thick sections. It is not stainless and needs plating or paint, service is limited to about 400 °C, and machinability is modest (score 25). HP9-4-20 is the alternative when toughness and weldability outrank strength. HP9-4-30 has a density of 7.84 g/cm³ (0.283 lb/in³), lighter than 78% of steel grades. It melts at 1,420°C (2,588 °F). HP9-4-30 has a yield strength of 1,400 MPa (203 ksi) and a tensile strength of 1,655 MPa (240 ksi) — stronger than 96% of steel grades. Elongation at break is 13% and the elastic modulus is 200 GPa (29 Msi).
What is HP9-4-30 used for?
- landing gear forgings
- arresting gear components
- rocket motor cases
Advantages
- High strength — 1,400 MPa (203 ksi), better than 96% of steel grades
- Polishes to a fine finish — 70/100, better than 87% of steel grades
Limitations
- Difficult to machine — 25/100, worse than 93% of steel grades
- High embodied carbon — 3.4 kg CO₂/kg, worse than 93% of steel grades
- Poor heat conductor — 24 W/m·K (13.9 BTU/hr·ft·°F), worse than 90% of steel grades
- Limited formability — 15/100, worse than 87% of steel grades
- Low electrical conductivity — 3.8 % IACS, worse than 84% of steel grades
HP9-4-30 properties
Typical room-temperature values for HP9-4-30-QT540; bars are the position among steel grades.
Physical3
HP9-4-30 has a density of 7.84 g/cm³ (0.283 lb/in³), lighter than 78% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
HP9-4-30 has a yield strength of 1,400 MPa (203 ksi) and a tensile strength of 1,655 MPa (240 ksi) — stronger than 96% of steel grades. Elongation at break is 13% and the elastic modulus is 200 GPa (29 Msi).
Thermal6
HP9-4-30 is rated for continuous service to 400°C (752 °F). It conducts heat at 24 W/m·K (13.9 BTU/hr·ft·°F), worse than 90% of steel grades. Thermal expansion is 11.5 µm/m·K (6.39 µin/in·°F).
Electrical3
HP9-4-30 conducts electricity at 3.8 % IACS, worse than 84% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (20/100), better than 87% of steel grades.
Sustainability4
Producing a kilogram of HP9-4-30 takes about 45 MJ of energy and emits 3.4 kg of CO₂ — more than 93% of steel grades. Typical recycled content is 15%.
Manufacturability8
HP9-4-30's machinability is poor (25/100, worse than 93% of steel grades); weldability fair (45/100); formability poor (15/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. HP9-4-30 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with HP9-4-30
Is HP9-4-30 easy to machine?
Rough machine in the annealed condition; at 46–50 HRC use rigid setups, negative-rake carbide or CBN, low speed/high feed, and avoid grinding burn (temper etch inspect).
Can HP9-4-30 be welded?
Best of the UHS steel family for fusion welding — GTAW/EB with matching 9Ni-4Co filler, clean low-hydrogen practice, then re-temper (or full re-heat-treat) the weldment.
Can HP9-4-30 be formed, bent or molded?
Hot forge 1050–1200 °C with controlled finish temperature; cold forming is impractical at strength level — bend/straighten only in the annealed state.
HP9-4-30 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 | 0.29 – 0.34 | 0.31 |
| Ni | 7 – 8 | 7.5 |
| Co | 4.25 – 4.75 | 4.5 |
| Cr | 0.9 – 1.1 | 1 |
| Mo | 0.9 – 1.1 | 1 |
| V | 0.06 – 0.12 | 0.09 |
| Mn | 0.1 – 0.35 | 0.22 |
| Si | ≤ 0.2 | 0.1 |
| P | ≤ 0.01 | |
| S | ≤ 0.01 | |
| Fe | balance |
What surface finishes work on HP9-4-30?
Cannot be anodized; use Zn-Ni or Cd plate, IVD aluminium, or phosphate + epoxy primer, with 190 °C/23 h hydrogen-relief bake after plating and shot peening of fillets.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Organic Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel HP9-4-30 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- K91972
How much does HP9-4-30 cost?
HP9-4-30 is a high-cost steel ($$$$), pricier than about 78% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
HP9-4-30 — frequently asked
What is HP9-4-30 used for?
HP9-4-30 is typically used for landing gear forgings, arresting gear components and rocket motor cases. In short: weldable 9Ni-4Co steel; thick-section toughness; rocket case forging grade.
What is the yield strength of HP9-4-30?
HP9-4-30 has a typical yield strength of 1,400 MPa (203 ksi) and a tensile strength of 1,655 MPa (240 ksi) — stronger than 96% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is HP9-4-30 easy to machine?
Not especially — machinability is rated poor (25/100, worse than 93% of steel grades). Rough machine in the annealed condition; at 46–50 HRC use rigid setups, negative-rake carbide or CBN, low speed/high feed, and avoid grinding burn (temper etch inspect).
Can HP9-4-30 be welded?
With care — weldability is rated fair (45/100). Best of the UHS steel family for fusion welding — GTAW/EB with matching 9Ni-4Co filler, clean low-hydrogen practice, then re-temper (or full re-heat-treat) the weldment.
What surface finishes work on HP9-4-30?
Cannot be anodized; use Zn-Ni or Cd plate, IVD aluminium, or phosphate + epoxy primer, with 190 °C/23 h hydrogen-relief bake after plating and shot peening of fillets.
Can HP9-4-30 be formed, bent or molded?
Hot forge 1050–1200 °C with controlled finish temperature; cold forming is impractical at strength level — bend/straighten only in the annealed state.
What is the maximum service temperature of HP9-4-30?
HP9-4-30 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 HP9-4-30-QT540 and HP9-4-30-ANN?
QT540 is the default condition — standard aerospace condition — best strength/toughness balance for landing gear and structural forgings. ANN: pick for rough machining of large forgings before final austenitize-quench-temper. Yield strength is 1,400 MPa in QT540 versus 790 MPa in ANN.
How much does HP9-4-30 cost?
HP9-4-30 is a high-cost steel ($$$$), pricier than about 78% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- AMS 6526 — Steel Bars, Forgings and Tubing 7.5Ni-4.5Co-1.0Cr-1.0Mo (0.29-0.34C), Consumable Electrode Melted — SAE International (2017)
- AMS 6523 — Steel Sheet, Strip and Plate, 9Ni-4Co-0.30C — SAE International (2016)
- ASM Handbook Vol. 1 — Properties and Selection: Irons, Steels, and High-Performance Alloys (Ultrahigh-Strength Steels) — ASM International (1990)
- MMPDS / MIL-HDBK-5 — Metallic Materials Properties Development and Standardization, 9Ni-4Co steels — Battelle / FAA (2023)
- ASM Handbook Vol. 19 — Fatigue and Fracture — ASM International (1996)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.