Stainless Steel HK40
Grade
UNS J94224
HK40 = 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.
Values for the HK40 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 |
HK40This page 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 |
What is HK40?
HK40 is HK in the HK40 grade — the workhorse high-carbon variant — pick it for maximum creep-rupture strength in reformer/cracking furnace service. HK40 has a yield strength of 360 MPa (52.2 ksi) and a tensile strength of 540 MPa (78.3 ksi) — weaker than 51% of stainless steel grades. Elongation at break is 10% and the elastic modulus is 185 GPa (26.8 Msi). HK40 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; HK40 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 — 10%, worse than 98% of stainless steel grades
- Limited formability — 8/100, worse than 97% of stainless steel grades
- Hard to polish — 45/100, worse than 96% of stainless steel grades
- Low electrical conductivity — 1.9 % IACS, worse than 84% of stainless steel grades
- Difficult to weld — 46/100, worse than 77% of stainless steel grades
HK40 properties
Typical room-temperature values for HK40; 22 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
HK40 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
HK40 has a yield strength of 360 MPa (52.2 ksi) and a tensile strength of 540 MPa (78.3 ksi) — weaker than 51% of stainless steel grades. Elongation at break is 10% and the elastic modulus is 185 GPa (26.8 Msi).
Thermal6
HK40 is rated for continuous service to 1,150°C (2,102 °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 µm/m·K (8.33 µin/in·°F).
Electrical3
HK40 conducts electricity at 1.9 % IACS, worse than 84% of stainless steel grades.
Chemical & Environmental3
Corrosion resistance is good (60/100), worse than 58% of stainless steel grades.
Sustainability4
Producing a kilogram of HK40 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
HK40's machinability is poor (27/100, worse than 83% of stainless steel grades); weldability fair (46/100); formability not recommended (8/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with HK40
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 | — |
HK40 — frequently asked
What is HK40 used for?
HK40 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 HK40?
HK40 has a typical yield strength of 360 MPa (52.2 ksi) and a tensile strength of 540 MPa (78.3 ksi) — weaker than 51% of stainless steel grades. Strength varies by condition: see the 5 listed tempers.
Is HK40 easy to machine?
Not especially — machinability is rated poor (27/100, worse than 83% 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 HK40 be welded?
With care — weldability is rated fair (46/100). 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 HK40?
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 HK40 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 HK40?
HK40 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.