Stainless Steel HP
UNS N08705
HP is a 25Cr-35Ni heat-resistant cast stainless steel (UNS N08705) to ASTM A297, A351 and A743, produced statically or centrifugally and available in niobium- and microalloy-modified versions (HP-Nb, HP-MA, HP50WZ).
Reference datasheet — HP 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 AC-CENT condition
HP tempers and conditions
HP is supplied in 5 conditions, soft to hard. AC-CENT 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 |
|---|---|---|---|---|---|---|
AC-CENTDefault As-Cast, centrifugally cast tube | Standard supply condition for cracking/reformer furnace tubes — fine columnar structure, bored ID, best creep-rupture consistency. | 280 | 520 | 10 | 190 HB | 1.7 % IACS |
Niobium-modified (≈1 % Nb), as-cast | Default upgrade for ethylene furnace radiant coils: NbC strengthening raises 100 000 h rupture strength markedly over plain HP. | 300 | 550 | 8 | 200 HB | 1.7 % IACS |
Tungsten + zirconium modified (ASTM A608), as-cast | A608 variant with ~5 % W for solid-solution creep strength — used for heavily loaded reformer/pyrolysis tubes and supports. | 310 | 585 | 8 | 210 HB | 1.6 % IACS |
Microalloyed Nb + Ti (± Zr/rare earth), as-cast | Top-of-range cracking-tube alloy: micro-additions refine and stabilise the carbide network for the longest rupture life and best carburisation resistance. | 320 | 570 | 7 | 205 HB | 1.7 % IACS |
Thermally aged / ex-service condition | Reference condition for fitness-for-service and repair-weld assessment of tubes already run in a furnace — strength holds, ductility collapses. | 330 | 480 | 3 | 225 HB | 1.8 % IACS |
Supply forms, finishes and certifications
Ways HP is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
AC-STAT | As-Cast, static (sand/investment) casting | Surface finish |
What is HP?
HP is a 25Cr-35Ni heat-resistant cast stainless steel (UNS N08705) to ASTM A297, A351 and A743, produced statically or centrifugally and available in niobium- and microalloy-modified versions (HP-Nb, HP-MA, HP50WZ). Its high creep-rupture strength at extreme temperature makes it the standard for ethylene cracking furnace tubes and reformer fittings, with 280 MPa yield, 520 MPa tensile, 10% elongation, 190 HB and service to about 1100 °C. Elongation is the lowest in the family, and carburized or aged tube material must be machined back or buttered before repair welding. HK is the lower-cost alternative when creep strength is not critical. HP has a density of 7.9 g/cm³ (0.285 lb/in³), heavier than 68% of stainless steel grades. It melts at 1,330°C (2,426 °F). HP has a yield strength of 280 MPa (40.6 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 75% of stainless steel grades. Elongation at break is 10% and the elastic modulus is 190 GPa (27.6 Msi).
What is HP used for?
- ethylene pyrolysis furnace tubes
- reformer outlet fittings
- petrochemical furnace hardware
Advantages
- High service temperature — 1,100°C (2,012 °F), better than 96% of stainless steel grades
Limitations
- Hard to polish — 40/100, worse than 99% of stainless steel grades
- Limited ductility — 10%, worse than 98% of stainless steel grades
- Limited formability — 8/100, worse than 97% of stainless steel grades
- Difficult to machine — 22/100, worse than 95% of stainless steel grades
- Low electrical conductivity — 1.7 % IACS, worse than 93% of stainless steel grades
HP properties
Typical room-temperature values for HP-AC-CENT; bars are the position among stainless steel grades.
Physical3
HP has a density of 7.9 g/cm³ (0.285 lb/in³), heavier than 68% of stainless steel grades. It melts at 1,330°C (2,426 °F).
Mechanical16
HP has a yield strength of 280 MPa (40.6 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 75% of stainless steel grades. Elongation at break is 10% and the elastic modulus is 190 GPa (27.6 Msi).
Thermal4
HP is rated for continuous service to 1,100°C (2,012 °F). It conducts heat at 12.5 W/m·K (7.22 BTU/hr·ft·°F), worse than 90% of stainless steel grades. Thermal expansion is 15.5 µm/m·K (8.61 µin/in·°F).
Electrical3
HP conducts electricity at 1.7 % IACS, worse than 93% of stainless steel grades.
Chemical & Environmental3
Corrosion resistance is good (62/100), worse than 52% of stainless steel grades.
Sustainability4
Producing a kilogram of HP takes about 75 MJ of energy and emits 5.5 kg of CO₂ — more than 84% of stainless steel grades. Typical recycled content is 65%.
Manufacturability8
HP's machinability is poor (22/100, worse than 95% of stainless steel grades); weldability fair (40/100); formability not recommended (8/100).
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 stainless steel family
Yield strength against density for every stainless steel grade FabDigit runs. HP is highlighted; hover a dot for its name, or open the interactive family chart.
Working with HP
Is HP easy to machine?
Rigid setups, sharp positive carbide or ceramic inserts, low speed with heavy feed and flood coolant — the carbide network is abrasive and the austenite work-hardens rapidly; ID boring of centrifugal tubes is the normal finishing step.
Can HP be welded?
GTAW/SMAW with matching 25Cr-35Ni(-Nb) filler, low heat input, no preheat needed but keep interpass <150 °C; aged or carburised material must be buttered or the carburised layer machined off before repair welding.
Can HP be formed, bent or molded?
Supplied as-cast only — static sand casting for fittings and centrifugal casting for tubes; no cold forming, and hot straightening should be done above 1000 °C.
HP 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 |
|---|---|---|
| CASTM A297 HP; cracking-tube practice usually 0.40–0.50 | 0.35 – 0.75 | 0.45 |
| Mn | ≤ 2 | 1.2 |
| Sihigher Si improves carburisation resistance | ≤ 2.5 | 1.5 |
| P | ≤ 0.04 | |
| S | ≤ 0.04 | |
| Cr | 24 – 28 | 26 |
| Ni | 33 – 37 | 35 |
| Moresidual limit | ≤ 0.5 | |
| NbHP-Nb / HP-MA variants only; not required by base A297 HP | 0.5 – 1.8 | 1.2 |
| TiHP-MA microalloyed variant only | 0.03 – 0.25 | 0.1 |
| WHP50WZ (ASTM A608) variant only | 4 – 6 | 5 |
| ZrHP50WZ variant only | 0.05 – 0.6 | 0.2 |
| Fe | balance |
What surface finishes work on HP?
Shot blast plus pickling/passivation of casting skin; no anodising or plating — service protection comes from the self-forming Cr₂O₃/spinel oxide scale.
Surface treatments FabDigit runs on stainless 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).
Passivation
Organic Coating
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Stainless Steel HP in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- N08705
How much does HP cost?
HP is a high-cost stainless 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.
HP — frequently asked
What is HP used for?
HP is typically used for ethylene pyrolysis furnace tubes, reformer outlet fittings and petrochemical furnace hardware. In short: highest creep-rupture strength; standard for cracking tubes; nb/microalloy modifiable.
What is the yield strength of HP?
HP has a typical yield strength of 280 MPa (40.6 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 75% of stainless steel grades. Strength varies by condition: see the 5 listed tempers.
Is HP easy to machine?
Not especially — machinability is rated poor (22/100, worse than 95% of stainless steel grades). Rigid setups, sharp positive carbide or ceramic inserts, low speed with heavy feed and flood coolant — the carbide network is abrasive and the austenite work-hardens rapidly; ID boring of centrifugal tubes is the normal finishing step.
Can HP be welded?
With care — weldability is rated fair (40/100). GTAW/SMAW with matching 25Cr-35Ni(-Nb) filler, low heat input, no preheat needed but keep interpass <150 °C; aged or carburised material must be buttered or the carburised layer machined off before repair welding.
What surface finishes work on HP?
Shot blast plus pickling/passivation of casting skin; no anodising or plating — service protection comes from the self-forming Cr₂O₃/spinel oxide scale.
Can HP be formed, bent or molded?
Supplied as-cast only — static sand casting for fittings and centrifugal casting for tubes; no cold forming, and hot straightening should be done above 1000 °C.
What is the maximum service temperature of HP?
HP is rated for continuous use to about 1,100°C (2,012 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between HP AC-CENT and HP Aged?
AC-CENT is the default condition — standard supply condition for cracking/reformer furnace tubes — fine columnar structure, bored ID, best creep-rupture consistency. Aged: reference condition for fitness-for-service and repair-weld assessment of tubes already run in a furnace — strength holds, ductility collapses. Yield strength is 280 MPa in AC-CENT versus 330 MPa in Aged.
How much does HP cost?
HP is a high-cost stainless 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
- ASTM A297/A297M — Steel Castings, Iron-Chromium and Iron-Chromium-Nickel, Heat Resistant, for General Application — ASTM International (2019)
- ASTM A608/A608M — Centrifugally Cast Iron-Chromium-Nickel High-Alloy Tubing for Pressure Application at High Temperatures — ASTM International (2018)
- ASTM A351/A351M — Castings, Austenitic, for Pressure-Containing Parts — ASTM International (2018)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys (Heat-Resistant Castings) — ASM International (1990)
- Steel Castings Handbook — High Alloy Data Sheets, Heat Series — Steel Founders' Society of America (2004)
- Centrifugally cast HP/HP-Nb/HP-MA furnace tube alloy data — Kubota / Schmidt + Clemens / Manoir Industries (2022)
- API 530 — Calculation of Heater-Tube Thickness in Petroleum Refineries (creep data for 25Cr-35Ni-Nb) — American Petroleum Institute (2015)
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.