Stainless Steel HP-NB
Grade
UNS N08705
HP-NB = 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.
Values for the HP-NB condition
Other HP tempers and conditions
HP is listed in 5 conditions; see the grade overview at HP.
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 |
HP-NBThis page 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 |
What is HP-NB?
HP-NB is HP in the HP-NB grade — default upgrade for ethylene furnace radiant coils: NbC strengthening raises 100 000 h rupture strength markedly over plain HP. HP-NB has a yield strength of 300 MPa (43.5 ksi) and a tensile strength of 550 MPa (79.8 ksi) — weaker than 68% of stainless steel grades. Elongation at break is 8% and the elastic modulus is 190 GPa (27.6 Msi). HP-NB 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). AC-CENT is the default condition FabDigit quotes; HP-NB is available on request or by drawing note.
Advantages
- High service temperature — 1,120°C (2,048 °F), better than 99% of stainless steel grades
Limitations
- Limited ductility — 8%, worse than 99% of stainless steel grades
- Hard to polish — 40/100, worse than 99% of stainless steel grades
- Difficult to machine — 20/100, worse than 98% of stainless steel grades
- Limited formability — 7/100, worse than 97% of stainless steel grades
- Low fracture toughness — 45 MPa·√m (41 ksi·√in), worse than 95% of stainless steel grades
HP-NB properties
Typical room-temperature values for HP-NB; 22 are specific to this condition. Other listed values are HP reference data and may not represent this condition. Bars are the position among stainless steel grades.
Physical3
HP-NB 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-NB has a yield strength of 300 MPa (43.5 ksi) and a tensile strength of 550 MPa (79.8 ksi) — weaker than 68% of stainless steel grades. Elongation at break is 8% and the elastic modulus is 190 GPa (27.6 Msi).
Thermal4
HP-NB is rated for continuous service to 1,120°C (2,048 °F). It conducts heat at 12.2 W/m·K (7.05 BTU/hr·ft·°F), worse than 91% of stainless steel grades. Thermal expansion is 15.5 µm/m·K (8.61 µin/in·°F).
Electrical3
HP-NB conducts electricity at 1.7 % IACS, worse than 93% of stainless steel grades.
Chemical & Environmental3
Corrosion resistance is good (63/100), better than 53% of stainless steel grades.
Sustainability4
Producing a kilogram of HP-NB 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-NB's machinability is poor (20/100, worse than 98% of stainless steel grades); weldability fair (38/100); formability not recommended (7/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with HP-NB
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.
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.
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 | — |
HP-NB — frequently asked
What is HP-NB used for?
HP-NB 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-NB?
HP-NB has a typical yield strength of 300 MPa (43.5 ksi) and a tensile strength of 550 MPa (79.8 ksi) — weaker than 68% of stainless steel grades. Strength varies by condition: see the 5 listed tempers.
Is HP-NB easy to machine?
Not especially — machinability is rated poor (20/100, worse than 98% 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-NB be welded?
With care — weldability is rated fair (38/100). 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-NB?
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-NB 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-NB?
HP-NB is rated for continuous use to about 1,120°C (2,048 °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.
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. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.