Stainless Steel HK
UNS J94224
HK is a 25Cr-20Ni heat-resistant cast stainless steel (UNS J94224) to ASTM A297, A351 and A743, ordered as HK30, HK40 or centrifugally cast HK40-CC and in as-cast, solution-annealed or aged conditions.
Reference datasheet — HK 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 F condition
HK tempers and conditions
HK is supplied in 5 conditions, soft to hard. F 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 |
|---|---|---|---|---|---|---|
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 |
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 |
Supply forms, finishes and certifications
Ways HK is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
HK40-CC | Centrifugally cast tube | Surface finish |
What is HK?
HK is a 25Cr-20Ni heat-resistant cast stainless steel (UNS J94224) to ASTM A297, A351 and A743, ordered as HK30, HK40 or centrifugally cast HK40-CC and in as-cast, solution-annealed or aged conditions. It is the workhorse of the family, combining 345 MPa yield, 520 MPa tensile, 12% elongation and 190 HB with oxidation resistance and creep strength to about 1150 °C. Sections are cast, not formed, and aged material loses ductility and machines poorly, so weld repairs need carbide-dissolution annealing or buttering. HP is the alternative where higher creep-rupture strength is required. HK 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). HK has a yield strength of 345 MPa (50 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 54% of stainless steel grades. Elongation at break is 12% and the elastic modulus is 185 GPa (26.8 Msi).
What is HK used for?
- reformer/pyrolysis furnace fixtures
- radiant tube supports
- kiln furniture
- heat-treat trays
Advantages
- High service temperature — 1,150°C (2,102 °F), better than 99% of stainless steel grades
Limitations
- Hard to polish — 45/100, worse than 96% of stainless steel grades
- Limited ductility — 12%, worse than 95% of stainless steel grades
- Limited formability — 10/100, worse than 95% of stainless steel grades
- Poor heat conductor — 13 W/m·K (7.51 BTU/hr·ft·°F), worse than 85% of stainless steel grades
- Low electrical conductivity — 1.9 % IACS, worse than 84% of stainless steel grades
HK properties
Typical room-temperature values for HK-F; bars are the position among stainless steel grades.
Physical3
HK 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 has a yield strength of 345 MPa (50 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 54% of stainless steel grades. Elongation at break is 12% and the elastic modulus is 185 GPa (26.8 Msi).
Thermal6
HK is rated for continuous service to 1,150°C (2,102 °F). It conducts heat at 13 W/m·K (7.51 BTU/hr·ft·°F), worse than 85% of stainless steel grades. Thermal expansion is 15 µm/m·K (8.33 µin/in·°F).
Electrical3
HK conducts electricity at 1.9 % IACS, worse than 84% of stainless steel grades.
Chemical & Environmental3
Corrosion resistance is good (62/100), worse than 52% of stainless steel grades.
Sustainability4
Producing a kilogram of HK 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's machinability is poor (30/100, worse than 70% of stainless steel grades); weldability fair (50/100); formability not recommended (10/100).
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. HK is highlighted; hover a dot for its name, or open the interactive family chart.
Working with HK
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.
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 |
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.
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 HK in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- J94224
How much does HK cost?
HK is a premium-cost stainless steel ($$$), cheaper than about 51% 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.
HK — frequently asked
What is HK used for?
HK 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?
HK has a typical yield strength of 345 MPa (50 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 54% of stainless steel grades. Strength varies by condition: see the 5 listed tempers.
Is HK easy to machine?
Not especially — machinability is rated poor (30/100, worse than 70% 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 be welded?
With care — weldability is rated fair (50/100). 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?
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 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?
HK 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.
How much does HK cost?
HK is a premium-cost stainless steel ($$$), cheaper than about 51% 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 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; 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.