Stainless Steel HP Aged
Heat-treated state
UNS N08705
Aged = 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.
Values for the Aged 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 |
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 |
AgedThis page 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 Aged?
HP Aged is HP heat treated to Aged — reference condition for fitness-for-service and repair-weld assessment of tubes already run in a furnace — strength holds, ductility collapses. HP Aged has a yield strength of 330 MPa (47.9 ksi) and a tensile strength of 480 MPa (69.6 ksi) — weaker than 57% of stainless steel grades. Elongation at break is 3% and the elastic modulus is 185 GPa (26.8 Msi). HP Aged 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; Aged is available on request or by drawing note.
Advantages
- High service temperature — 1,050°C (1,922 °F), better than 91% of stainless steel grades
Limitations
- Limited ductility — 3%, worse than 100% of stainless steel grades
- Low fracture toughness — 22 MPa·√m (20 ksi·√in), worse than 100% of stainless steel grades
- Difficult to machine — 15/100, worse than 100% of stainless steel grades
- Limited formability — 3/100, worse than 100% of stainless steel grades
- Hard to polish — 40/100, worse than 99% of stainless steel grades
HP Aged properties
Typical room-temperature values for HP Aged; 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 Aged 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 Aged has a yield strength of 330 MPa (47.9 ksi) and a tensile strength of 480 MPa (69.6 ksi) — weaker than 57% of stainless steel grades. Elongation at break is 3% and the elastic modulus is 185 GPa (26.8 Msi).
Thermal4
HP Aged is rated for continuous service to 1,050°C (1,922 °F). It conducts heat at 13.5 W/m·K (7.8 BTU/hr·ft·°F), worse than 83% of stainless steel grades. Thermal expansion is 15.5 µm/m·K (8.61 µin/in·°F).
Electrical3
HP Aged conducts electricity at 1.8 % IACS, worse than 87% of stainless steel grades.
Chemical & Environmental3
Corrosion resistance is good (55/100), worse than 71% of stainless steel grades.
Sustainability4
Producing a kilogram of HP Aged 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 Aged's machinability is poor (15/100, worse than 100% of stainless steel grades); weldability poor (20/100); formability not recommended (3/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with HP Aged
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 Aged — frequently asked
What is HP Aged used for?
HP Aged 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 Aged?
HP Aged has a typical yield strength of 330 MPa (47.9 ksi) and a tensile strength of 480 MPa (69.6 ksi) — weaker than 57% of stainless steel grades. Strength varies by condition: see the 5 listed tempers.
Is HP Aged easy to machine?
Not especially — machinability is rated poor (15/100, worse than 100% 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 Aged be welded?
Not readily — weldability is rated poor (20/100). 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 Aged?
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 Aged 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 Aged?
HP Aged is rated for continuous use to about 1,050°C (1,922 °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.