Steel HP9-4-20
UNS K91868
HP9-4-20 (UNS K91868) is the lower-carbon grade of the 9Ni-4Co ultra-high-strength steel family, heat treated to 1210 MPa yield and 1340 MPa tensile at 42 HRC with 17 % elongation.
Reference datasheet — HP9-4-20 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 QT-190 condition
HP9-4-20 tempers and conditions
HP9-4-20 is supplied in 5 conditions, soft to hard. QT-190 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 | 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 | — |
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?
HP9-4-20 (UNS K91868) is the lower-carbon grade of the 9Ni-4Co ultra-high-strength steel family, heat treated to 1210 MPa yield and 1340 MPa tensile at 42 HRC with 17 % elongation. It is selected for thick-section rocket motor cases, pressure vessels and structural forgings where fracture toughness and weldability (score 70, the best in this family) matter more than peak strength; plate, bar and forgings are the common forms. It is not corrosion resistant and is limited to about 400 °C service. HP9-4-30 is the choice when the extra 300 MPa of strength governs the design. HP9-4-20 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). HP9-4-20 has a yield strength of 1,210 MPa (175 ksi) and a tensile strength of 1,340 MPa (194 ksi) — stronger than 94% of steel grades. Elongation at break is 17% and the elastic modulus is 200 GPa (29 Msi).
What is HP9-4-20 used for?
- rocket motor cases
- thick-section structural forgings
- pressure vessels
Advantages
- Tough — resists crack growth — 150 MPa·√m (136.5 ksi·√in), better than 96% of steel grades
- High strength — 1,210 MPa (175 ksi), better than 94% of steel grades
- Polishes to a fine finish — 70/100, better than 87% of steel grades
Limitations
- 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
HP9-4-20 properties
Typical room-temperature values for HP9-4-20-QT-190; bars are the position among steel grades.
Physical3
HP9-4-20 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 has a yield strength of 1,210 MPa (175 ksi) and a tensile strength of 1,340 MPa (194 ksi) — stronger than 94% of steel grades. Elongation at break is 17% and the elastic modulus is 200 GPa (29 Msi).
Thermal6
HP9-4-20 is rated for continuous service to 400°C (752 °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 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 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's machinability is poor (28/100, worse than 91% of steel grades); weldability very good (70/100); formability poor (25/100).
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. HP9-4-20 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with HP9-4-20
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.
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 |
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.
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 HP9-4-20 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- K91868
How much does HP9-4-20 cost?
HP9-4-20 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.
HP9-4-20 — frequently asked
What is HP9-4-20 used for?
HP9-4-20 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?
HP9-4-20 has a typical yield strength of 1,210 MPa (175 ksi) and a tensile strength of 1,340 MPa (194 ksi) — stronger than 94% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is HP9-4-20 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 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?
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 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?
HP9-4-20 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 HP9-4-20 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.
How much does HP9-4-20 cost?
HP9-4-20 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 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; 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.