Top HK40 Centrifugal Casting: 25Cr-20Ni Heat-Resistant Alloy for Ethylene Cracking & Furnace Service

HK40 is a centrifugally cast heat-resistant alloy with nominal composition 25% chromium, 20% nickel, balance iron — designated as "Grade HK" under ASTM A608 (Iron-Chromium-Nickel Heat-Resistant Castings for General Application). The 25Cr-20Ni composition provides good oxidation and creep resistance at temperatures up to 1,050-1,100°C. The "40" suffix reflects the carbon level (approximately 0.40% C target). Primary application is ethylene cracking furnace tubes.

HK45 has the same nominal Cr-Ni composition as HK40 (25Cr-20Ni) but with higher carbon content (0.45% vs 0.40%). The increased carbon improves creep-rupture strength by 15-20% at temperatures above 950°C due to more primary chromium carbides forming during solidification. Cost premium is minimal (under 5%). HK45 is preferred for higher-severity cracking service where coil-wall temperatures approach 1,080°C.

ASTM A608 / A608M (Standard Specification for Centrifugally Cast Iron-Chromium-Nickel High Alloy Tubing for Pressure Application at High Temperatures) is the primary standard. HK40 falls under Grade HK within this standard. EN equivalent: EN 10295 (1.4848 grade) for centrifugal heat-resistant castings sold into EU markets. JIS equivalent: SCH22 under JIS G5122.

Design life for HK40 ethylene cracking tubes is typically 60,000-100,000 hours (7-11 years continuous operation). Actual life depends on coil-wall temperature, feedstock contamination, and decoking-cycle severity. Most ethylene plants run risk-based inspection (RBI) with periodic remaining-life assessment (RLA) using replica metallography to estimate remaining creep life before scheduled replacement.

Solution annealing at approximately 1,150°C followed by rapid cooling is standard for HK40 castings. Hold time at temperature is scaled to wall thickness (typically 1 hour per 25 mm of wall). The heat treatment dissolves coarse primary carbides and promotes a finer secondary carbide network during service, which provides creep strength. Documented thermal profile (time-temperature curve) issued with every batch.

Yes, HK40 castings can be welded using matching alloy filler material (HK40 filler rod or wire) under controlled conditions. Best practice is GTAW (TIG welding) with preheat 200-300°C and slow cooling. Post-weld heat treatment generally not required for HK40 itself but may be specified for stress relief. We supply tubes pre-welded with bends or as separate pieces for site welding by buyer fabrication contractor.

Per ASTM A608 Grade HK: Cr 24-28%, Ni 18-22%, C 0.35-0.45%, Si max 2.0%, Mn max 2.0%, P max 0.04%, S max 0.04%. We typically target tighter ranges than ASTM minimums: Cr 25-26%, Ni 19-21%, C 0.38-0.42%. Actual chemistry from each heat reported on EN 10204 3.1 mill certificate.

HK40 (25Cr-20Ni) is the standard alloy for ethylene cracking — service ceiling 1,050-1,100°C coil-wall temperature. HP40 (25Cr-35Ni) offers significantly better creep strength (16 vs 8 MPa at 1,000°C/100,000h) and higher service ceiling 1,150°C, but costs 25-35% more due to higher nickel content. For severe-service cracking, modern hydrogen reformers, and high-temperature operations, HP40 is preferred. HK40 remains standard for routine ethylene cracking replacement.

Yes. Long-term creep-rupture test data available on our HK40 test heats. Typical data points: 1,000°C wall temp / 8 MPa stress / 100,000 hour rupture life (HK40); 10 MPa for HK45. Data conforms to API 530 design methodology. We can supply specific creep data on request for customers running tube-life prediction or remaining-life assessment (RLA) modeling.

No fixed piece-count MOQ. For HK40 cracking tubes with existing standard tooling: orders from 1 tube ship from stock. New custom tooling (non-standard OD/ID): project economical at 15-25 tubes of same geometry. Spare-parts orders for cracking-furnace maintenance ship at any quantity from existing tooling library. Complete coil-set replacement orders (40-80 tubes per furnace) get production-line priority.