HP40 Centrifugal Casting: 25Cr-35Ni Heat-Resistant Alloy for Reformer Tubes & High-Temperature Service

HP40 is a centrifugally cast heat-resistant alloy with nominal composition 25% chromium, 35% nickel, balance iron — designated as "Grade HP" under ASTM A608 (Iron-Chromium-Nickel Heat-Resistant Castings for General Application). The high nickel content provides excellent creep strength and oxidation resistance at temperatures up to 1,050°C. The "40" suffix is industry shorthand reflecting the carbon level (approximately 0.40% C target).

HP-Nb (also called HP modified or HP-Nb modified) is HP40 with the addition of approximately 1.5% niobium. The niobium precipitates as Nb-carbides during high-temperature service, stabilizing the microstructure and improving creep-rupture life by 15-25% at temperatures above 950°C. Maximum service temperature increases from 1,050°C (HP40) to 1,150°C (HP-Nb). Niobium addition adds approximately 10% to the alloy cost.

ASTM A608 / A608M (Standard Specification for Centrifugally Cast Iron-Chromium-Nickel High Alloy Tubing for Pressure Application at High Temperatures) is the primary standard. HP40 falls under Grade HP within this standard. Related standards: ASTM A351 for static castings, ASTM A790 for duplex stainless tubes, ASTM A335 for ferritic tubes. EN equivalent: EN 10295 (1.4837 grade).

Design life for HP40 reformer tubes is typically 100,000 hours (approximately 11.5 years of continuous operation) at design temperature and stress. Actual service life depends on operating conditions — feedstock contamination, temperature excursions, thermal cycling. Standard practice in refinery operations is risk-based inspection (RBI) with periodic remaining-life assessment (RLA) using replica metallography to estimate remaining creep life.

Solution annealing at approximately 1,150°C followed by rapid cooling is standard for HP40 castings going into reformer service. 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, HP40 castings can be welded using matching alloy filler material (typically HP40 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 HP40 itself but may be specified for joints to relieve residual stress. We supply tubes pre-welded with pigtail connections or as separate pieces for site welding by buyer fabrication contractor.

Per ASTM A608 Grade HP: Cr 24-28%, Ni 33-37%, C 0.35-0.75%, 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 34-35%, C 0.38-0.45%. For HP-Nb grade: additional Nb 1.0-1.5% target. Actual chemistry from each heat reported on EN 10204 3.1 mill certificate.

As-cast OD surface from centrifugal casting is typically Ra 6.3-12.5 μm (250-500 μin). After OD machining, standard finish is Ra 1.6-3.2 μm (63-125 μin). ID finish after rough machining (to remove impurity layer) is typically Ra 3.2-6.3 μm. Mirror polish (Ra below 0.8 μm) available for specific applications but adds significant cost — rarely needed for reformer service.

Yes. Long-term creep-rupture test data available on our HP40 and HP-Nb test heats. Typical data points: 950°C wall temp / 16 MPa stress / 100,000 hour rupture life (HP40); 950°C / 20 MPa / 100,000 hour (HP-Nb). Data conforms to API 530 design methodology. We can supply specific creep data on request for customers running remaining-life assessment (RLA) modeling.

No fixed piece-count MOQ. For HP40 reformer tubes with existing standard tooling: orders from 1 tube ship from stock. New custom tooling (non-standard OD/ID): project economical at 12-20 tubes of same geometry. Spare-parts orders for refinery / hydrogen plant maintenance ship at any quantity from existing tooling library. Bulk catalyst-charge orders (200-400 tubes) get production-line priority.