Stainless Steel HK Aged
Heat-treated state
UNS J94224
Aged = thermally aged / ex-service
Reference condition for remaining-life and repair-weld assessment: carbide coarsening and σ-phase leave the alloy strong but nearly brittle at room temperature.
Values for the Aged condition
Other HK tempers and conditions
HK is listed in 5 conditions; see the grade overview at HK.
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Solution annealed ~1150 °C, rapid cooled | Occasionally specified for corrosion-service or weld-repaired A743 HK castings to dissolve grain-boundary carbides and restore ductility. | 300 | 490 | 20 | 170 HB | 1.9 % IACS |
A351/A743 HK30, static cast, 0.25–0.35 %C | Lower-carbon HK for pressure-containing castings where ductility and weld repair matter more than creep strength. | 310 | 500 | 15 | 180 HB | 1.9 % IACS |
FDefault As fabricated | Default supply condition for furnace hardware, tube supports and trays — used as-cast with no post-cast heat treatment. | 345 | 520 | 12 | 190 HB | 1.9 % IACS |
A351/A297 HK40, 0.35–0.45 %C, as-cast | The workhorse high-carbon variant — pick it for maximum creep-rupture strength in reformer/cracking furnace service. | 360 | 540 | 10 | 200 HB | 1.9 % IACS |
AgedThis page Thermally aged / ex-service | Reference condition for remaining-life and repair-weld assessment: carbide coarsening and σ-phase leave the alloy strong but nearly brittle at room temperature. | 380 | 520 | 3 | 17 HRC | 2 % IACS |
What is HK Aged?
HK Aged is HK heat treated to Aged — reference condition for remaining-life and repair-weld assessment: carbide coarsening and σ-phase leave the alloy strong but nearly brittle at room temperature. HK Aged has a yield strength of 380 MPa (55.1 ksi) and a tensile strength of 520 MPa (75.4 ksi) — stronger than 52% of stainless steel grades. Elongation at break is 3% and the elastic modulus is 183 GPa (26.5 Msi). HK Aged has a density of 7.75 g/cm³ (0.28 lb/in³), lighter than 65% of stainless steel grades. It melts at 1,340°C (2,444 °F). F is the default condition FabDigit quotes; Aged is available on request or by drawing note.
Advantages
- High service temperature — 1,150°C (2,102 °F), better than 99% of stainless steel grades
Limitations
- Limited ductility — 3%, worse than 100% of stainless steel grades
- Limited formability — 2/100, worse than 100% of stainless steel grades
- Difficult to machine — 20/100, worse than 98% of stainless steel grades
- Hard to polish — 45/100, worse than 96% of stainless steel grades
- Difficult to weld — 25/100, worse than 92% of stainless steel grades
HK Aged properties
Typical room-temperature values for HK Aged; 23 are specific to this condition. Other listed values are HK reference data and may not represent this condition. Bars are the position among stainless steel grades.
Physical3
HK Aged has a density of 7.75 g/cm³ (0.28 lb/in³), lighter than 65% of stainless steel grades. It melts at 1,340°C (2,444 °F).
Mechanical15
HK Aged has a yield strength of 380 MPa (55.1 ksi) and a tensile strength of 520 MPa (75.4 ksi) — stronger than 52% of stainless steel grades. Elongation at break is 3% and the elastic modulus is 183 GPa (26.5 Msi).
Thermal6
HK Aged is rated for continuous service to 1,150°C (2,102 °F). It conducts heat at 14 W/m·K (8.09 BTU/hr·ft·°F), worse than 80% of stainless steel grades. Thermal expansion is 15 µm/m·K (8.33 µin/in·°F).
Electrical3
HK Aged conducts electricity at 2 % IACS, worse than 77% of stainless steel grades.
Chemical & Environmental3
Corrosion resistance is fair (52/100), worse than 76% of stainless steel grades.
Sustainability4
Producing a kilogram of HK Aged takes about 53 MJ of energy and emits 4.5 kg of CO₂ — less than 67% of stainless steel grades. Typical recycled content is 65%.
Manufacturability8
HK Aged's machinability is poor (20/100, worse than 98% of stainless steel grades); weldability poor (25/100); formability not recommended (2/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with HK Aged
Is HK easy to machine?
Machine with rigid setups, positive-rake carbide or ceramic inserts, low speed and heavy positive feed to cut under the work-hardened skin; interdendritic chromium carbides are abrasive and aged material machines far worse than as-cast.
Can HK be welded?
Weldable by SMAW/GTAW with matching HK or ER310-type filler; preheat 150–260 °C, limit heat input, avoid restraint — service-aged castings need carbide-dissolution anneal or buttering to avoid HAZ cracking.
Can HK be formed, bent or molded?
Cast only (sand, investment or centrifugal); no cold forming — design generous fillets and controlled section changes to limit hot tearing and shrinkage in the wide freezing range.
What surface finishes work on HK?
Normally supplied shot-blasted or as-cast; grind/machine sealing faces, no anodizing or plating needed since the Cr₂O₃ scale is self-protective — avoid halide-bearing cleaners.
HK 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 |
|---|---|---|
| CA297 HK; A351 HK30 = 0.25–0.35, HK40 = 0.35–0.45 | 0.2 – 0.6 | 0.4 |
| Mn | ≤ 2 | 1.2 |
| Si≤1.75 in A351/A608 variants | ≤ 2 | 1.3 |
| P | ≤ 0.04 | — |
| S | ≤ 0.04 | — |
| CrA351 HK30/HK40: 23.0–27.0 | 24 – 28 | 25.5 |
| Ni | 18 – 22 | 20 |
| Moresidual, not intentionally added | ≤ 0.5 | — |
| Fe | balance | — |
HK Aged — frequently asked
What is HK Aged used for?
HK Aged is typically used for reformer/pyrolysis furnace fixtures, radiant tube supports, kiln furniture and heat-treat trays. In short: most-used heat-resistant casting; strength and oxidation to 1150 °C; widest foundry availability.
What is the yield strength of HK Aged?
HK Aged has a typical yield strength of 380 MPa (55.1 ksi) and a tensile strength of 520 MPa (75.4 ksi) — stronger than 52% of stainless steel grades. Strength varies by condition: see the 5 listed tempers.
Is HK Aged easy to machine?
Not especially — machinability is rated poor (20/100, worse than 98% of stainless steel grades). Machine with rigid setups, positive-rake carbide or ceramic inserts, low speed and heavy positive feed to cut under the work-hardened skin; interdendritic chromium carbides are abrasive and aged material machines far worse than as-cast.
Can HK Aged be welded?
Not readily — weldability is rated poor (25/100). Weldable by SMAW/GTAW with matching HK or ER310-type filler; preheat 150–260 °C, limit heat input, avoid restraint — service-aged castings need carbide-dissolution anneal or buttering to avoid HAZ cracking.
What surface finishes work on HK Aged?
Normally supplied shot-blasted or as-cast; grind/machine sealing faces, no anodizing or plating needed since the Cr₂O₃ scale is self-protective — avoid halide-bearing cleaners.
Can HK Aged be formed, bent or molded?
Cast only (sand, investment or centrifugal); no cold forming — design generous fillets and controlled section changes to limit hot tearing and shrinkage in the wide freezing range.
What is the maximum service temperature of HK Aged?
HK Aged is rated for continuous use to about 1,150°C (2,102 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between HK-F and HK-SA?
F is the default condition — default supply condition for furnace hardware, tube supports and trays — used as-cast with no post-cast heat treatment. SA: occasionally specified for corrosion-service or weld-repaired A743 HK castings to dissolve grain-boundary carbides and restore ductility. Yield strength is 345 MPa in F versus 300 MPa in SA.
Sources
- ASTM A297/A297M — Steel Castings, Iron-Chromium and Iron-Chromium-Nickel, Heat Resistant, for General Application — ASTM International (2019)
- ASTM A351/A351M — Castings, Austenitic, for Pressure-Containing Parts (HK30, HK40) — ASTM International (2022)
- ASTM A608/A608M — Centrifugally Cast Iron-Chromium-Nickel High-Alloy Tubing for Pressure Application at High Temperatures — ASTM International (2018)
- ASTM A743/A743M — Castings, Iron-Chromium, Iron-Chromium-Nickel, Corrosion Resistant, for General Application — ASTM International (2019)
- ASM Specialty Handbook: Heat-Resistant Materials — Heat-Resistant Cast Steels — ASM International (1997)
- ASM Handbook Vol. 1: Properties and Selection — Irons, Steels, and High-Performance Alloys — ASM International (1990)
- Steel Castings Handbook, 6th ed. — Steel Founders' Society of America / ASM (1995)
- GB/T 8492 (ZG40Cr25Ni20) (2014)
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.