HP-Nb castings: composition, properties and uses

HP-Nb, also called HP modified or HP40 Mod.Nb, is a cast heat-resistant alloy of about 25% chromium and 35% nickel with a niobium addition, built on ASTM A297 grade HP. It has long been the standard material for centrifugally cast tubes in steam reformer and cracking furnaces, and microalloyed versions add titanium and other elements for more creep strength.

Published 17 Oct 2026 · Updated 17 Oct 2026 · Ni-Chrome Alloy Castings

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What is the composition of HP-Nb?

There is no single ASTM grade called HP-Nb. The base is ASTM A297 grade HP, whose published limits are below. Niobium, and in microalloyed versions titanium and other elements, are added to a range agreed between buyer and foundry or set by the purchaser's specification.

Element	ASTM A297 HP, wt %
Carbon	0.35 to 0.75
Chromium	24 to 28
Nickel	33 to 37
Manganese	2.00 max
Silicon	2.50 max
Molybdenum	0.50 max
Phosphorus	0.04 max
Sulphur	0.04 max
Niobium	Not specified in A297 HP; added by agreement

Source: ASTM A297 grade HP, as reproduced by Austral Alloys and Total Materia. The edition named on your purchase order governs.

On niobium level, Schmidt + Clemens report that the best compromise of creep strength and creep elongation in HP40 Mod.Nb was found at 0.8 to 1.0% niobium, with creep elongation above 10%. That is a published research finding, not a standard limit.

What are the equivalents of HP-Nb?

EN / DIN: 1.4852, G-X 40 NiCrNb 35 25, also written GX40NiCrSiNb35-25.
ASTM base grade: A297 HP, UNS N08705 (some databases list J95705). Centrifugally cast tubes in HP are bought to ASTM A608.
Trade and shop names: HP-Mod, HP40 Nb, HP-Micro or HP-MA for microalloyed versions, and producers' own alloy names.

Because the niobium and microalloy additions vary between specifications, two "HP-Nb" orders can differ. Name the specification, not only the label.

Where is HP-Nb used?

HP-Nb is used for parts that carry stress at the top of the cast-alloy temperature range, often in carburising or hydrocarbon atmospheres.

Steam reformer catalyst tubes and outlet systems, and ethylene cracking coils, are the classic applications. In reformers the tube runs hot under internal pressure for a design life commonly set at 100,000 hours.
Furnace tubes in fired heaters and industrial furnaces.
Fittings such as return bends and manifolds, and static furnace internals where HK40 or HH lacks margin.

Ni-Chrome casts furnace tubes for fired heaters and industrial furnaces, plus furnace internals and fittings, in HP-Nb. Our reformer furnace castings guide explains how the tubes, supports and fittings in a reformer fit together, and the castings page shows the shapes we pour.

How does HP-Nb fail in service?

The published work on service-exposed HP-Nb tubes describes damage that builds up through microstructural aging and creep.

Carbide aging. A 2019 study in Metals of HP-Nb reformer tubes found primary M₇C₃ carbides converting to the more stable M₂₃C₆ at exposure temperatures of 910 °C and above, and primary niobium carbides transforming to a nickel-niobium silicide, G-phase, during creep exposure. Room-temperature strength and elongation fell as this progressed. The authors treat these changes as precursors to creep damage.
Creep cavitation. A 2023 failure analysis of two HP-Nb tubes from the same reformer found one had failed after 46,000 hours against a 100,000-hour design life. Its fracture formed by creep pores joining into grain-boundary cracks. The second tube, which contained titanium, had finer and more discrete carbides and performed better.
Overheating. Running hotter than design dissolves the fine secondary carbides and coarsens the primary carbides at interdendritic boundaries, removing the strengthening the alloy depends on (IntechOpen review of reformer alloys).

As with HK40, life is governed by metal temperature. Small, sustained overheats cost more life than the design margin suggests.

When should you choose HP-Nb over microalloyed HP?

Microalloyed HP, introduced in the mid-1980s according to Schmidt + Clemens, adds titanium and other carbide formers to HP-Nb. The reason is that niobium alone can only be raised to about 0.8 to 1.0% in HP before creep ductility suffers, which caps how much carbide strengthening it gives. The microalloy additions produce a finer, more stable carbide dispersion and higher creep strength, particularly above 1,000 °C.

Plain HP-Nb remains reasonable when:

the original design and its wall thicknesses were based on HP-Nb;
the part is a static casting, fitting or support rather than a highly stressed tube;
the purchaser's specification calls for it.

Microalloyed HP makes sense when the designer wants thinner walls, higher throughput at the same tube count, or more margin at high temperature. Going the other way, HK40 is the lower-cost option for supports and hangers at moderate duty.

Which casting route suits HP-Nb?

As general practice, tubes are cast centrifugally in a rotating metal mould and bought to ASTM A608. Published work notes that the outer wall of a centrifugal tube solidifies with a columnar structure and the inner part equiaxed, and some specifications call for a minimum of 30% equiaxed structure from the inside. Fittings, manifolds and internals are cast statically in sand moulds.

What should an RFQ for HP-Nb castings include?

The full chemistry required: base standard (A297 HP, A608 HP, EN 1.4852), niobium range, and any microalloy additions such as titanium.
The drawing, with tolerances, machining allowance and bore condition for tubes.
Design temperature, pressure, atmosphere and expected design life.
Any macrostructure requirement for centrifugal tubes, such as minimum equiaxed fraction.
Testing: chemistry and tensile certificates, radiography, dye penetrant, creep-rupture testing if required, and third-party inspection.
Quantity, delivery needs and any EIL, licensor or end-user approval required.

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HP-Nb questions

Is HP-Nb the same as HP?

No. HP is the ASTM A297 base grade. HP-Nb adds niobium, which forms stable carbides and raises creep strength, creep ductility and carburisation resistance.

What does "microalloyed" mean for HP?

Small additions of titanium and other carbide-forming elements on top of niobium, giving a finer carbide dispersion and higher creep strength.

Why does aged HP-Nb lose ductility?

Carbides coarsen and transform, and niobium carbide turns into G-phase over long exposure. Published tests show room-temperature elongation falling as this happens.