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Grade comparison

HK40 vs HP-Nb: which grade for furnace tubes and fittings?

HK40 is a 25Cr-20Ni cast alloy. HP-Nb is 25Cr-35Ni with niobium added. HP-Nb has the higher creep strength and better carburization resistance, so it is the usual choice for reformer tubes and other hot, stressed pressure parts. HK40 costs less and still does the job for supports, hangers, tube sheets and lower-duty fittings.

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

We cast both grades, and the question buyers usually bring us is this: is the HP-Nb premium worth paying on this particular part? Below are the published differences, followed by our working answer.

How do HK40 and HP-Nb compare side by side?

The main difference is nickel: about 20% in HK40 and about 35% in HP. HP-Nb also carries niobium, which forms stable carbides. Together they give HP-Nb more creep strength and more resistance to carbon pickup. HK40 is cheaper and simpler to source.

Composition sources: ASTM A351 HK40 as reproduced by Total Materia, and ASTM A297 HP as reproduced by Austral Alloys. The niobium figure is the Schmidt + Clemens finding published by Niobium Tech, not a standard limit. The edition named on your purchase order governs.

Why does HP-Nb have higher creep strength than HK40?

HP-Nb has more nickel, which stabilises the austenite at temperature. Its niobium also forms fine, stable niobium carbides alongside the chromium carbides. Those carbides pin the grain and dendrite boundaries where creep damage starts. Schmidt + Clemens report that pushing niobium past about 1% costs creep ductility, and that is why microalloyed HP later added titanium and other elements on top.

In practice, the extra strength lets a designer pick between a thinner tube wall, which carries more heat into the process, and keeping the wall and gaining margin. Many reformers that started on HK40 moved to HP-Nb and then to microalloyed HP for that reason.

Which grade resists carburization better?

HP-Nb. Carbon diffusing in from the process gas or from coke on the bore makes the wall hard and brittle, and nickel slows that pickup. That is why ethylene cracking coils, which coke up between decokes, are made in HP-type and higher-nickel grades rather than HK40. Where carburization is the main threat, HK40 is usually the wrong starting point.

When should you choose HK40, and when HP-Nb?

Choose HP-Nb for parts that hold pressure at the top of the temperature range, or that see a carburizing atmosphere. Choose HK40 for parts that carry load but not internal pressure, at moderate metal temperatures, where cost matters more than the last margin of creep life. Where an existing furnace was designed around one grade, stay with it unless the duty has changed.

Choose HP-Nb when

• the part is a reformer catalyst tube, outlet header or cracking coil;
• the metal runs near the upper limit for HK40, or the plant wants to raise throughput;
• carburization or coking is expected;
• the designer wants a thinner wall for the same pressure;
• the original design and its wall calculations are based on HP-Nb.

Choose HK40 when

• the part is a tube support, hanger, guide or tube sheet with no internal pressure;
• you are replacing like for like in a furnace built around HK40;
• the fitting runs at moderate temperature and HP-Nb would be paying for margin you will not use.

Two further cases sit outside this choice. Where the heater burns fuel oil high in vanadium and sodium, hangers and supports often go to 50Cr-50Ni-Nb instead of either grade. And for new reformer tubes, microalloyed HP is now the common specification, as the HP-Nb grade page explains.

How do HK40 and HP-Nb age in service?

Both grades change in service. Their carbides coarsen and transform, and they lose ductility at room temperature long before they fail. The difference lies in the phases that form. HK40 tends to form sigma phase. HP-Nb forms G-phase from its niobium carbides.

HK40

Published examinations of service-exposed HK40 describe the fine secondary carbides coarsening, and sigma phase forming in the matrix. Sigma is hard and brittle. A study in Revista de Metalurgia of a cracked HK40 reformer part found sigma and needle-shaped carbides, and judged the material unsuitable for weld repair.

HP-Nb

A 2019 study in Metals of HP-Nb reformer tubes found primary M7C3 carbides converting to M23C6 at exposure temperatures of 910 °C and above. The niobium carbides transformed to G-phase, a nickel-niobium silicide, during creep exposure. Strength and elongation at room temperature fell as this went on. A 2023 failure analysis of two HP-Nb tubes from one reformer found that the titanium-free tube failed by creep cavitation after 46,000 hours. The titanium-bearing tube, with finer carbides, held up better.

For both grades, metal temperature governs life. A tube that runs a little hotter than design for long periods loses more life than the margin suggests.

What should an RFQ for HK40 or HP-Nb castings say?

Name the specification and the grade, not just the label. "HP-Nb" can mean several chemistries, so give the standard, the niobium range and any microalloy additions. Then give the part geometry and the service conditions, and list the tests you need.

• Grade and standard: ASTM A351 or A608 HK40; ASTM A297 or A608 HP with niobium range; or EN 1.4852.
• Drawing with tolerances, machining allowance and, for tubes, bore condition and minimum sound wall.
• Design temperature, pressure, atmosphere and target life.
• Casting route if fixed: centrifugal for tubes, static sand for fittings and supports.
• Tests: chemistry and tensile certificates, radiography, dye penetrant, PMI, and third-party inspection if required.
• Quantity, and any licensor, EPC or end-user approval needed.

Our quote checklist covers the full list, including what to send when there is no drawing.

HK40 vs HP-Nb questions

Can HP-Nb replace HK40 directly?

Usually yes on chemistry, since HP-Nb is the stronger grade. But the drawing may have been sized for HK40, and a stronger grade does not fix a design problem. Check the weld procedures and the connecting parts too, because welds between the two grades need suitable filler metal.

Can HK40 replace HP-Nb to save cost?

Not on pressure parts designed for HP-Nb. The wall was calculated with HP-Nb creep strength, and HK40 at the same wall would have less life. On lightly loaded supports, ask the designer first.

Is HK40 obsolete?

No. It is no longer the first choice for reformer tubes, but it is still widely specified for supports, hangers, tube sheets and replacement parts in furnaces built around it.

What is the difference between HP-Nb and microalloyed HP?

Microalloyed HP adds titanium and other carbide formers on top of niobium. That gives a finer carbide dispersion and higher creep strength, especially above 1,000 °C. It is the usual grade for new reformer tubes.

Can aged HK40 or HP-Nb be weld repaired?

Sometimes. Aged material is less ductile and may hold sigma or G-phase, which can crack during welding. Get a metallurgical check of the actual part, such as a replica or a sample, before planning a repair.

Which ASTM standard covers centrifugally cast HK40 and HP tubes?

ASTM A608 covers centrifugally cast iron-chromium-nickel tubing for pressure service at high temperature, and it includes both HK40 and HP.

Sources

1. Total Materia: ASTM A351 Grade HK40, chemical composition
2. Austral Alloys: ASTM A297 HP composition
3. ASTM A608/A608M, centrifugally cast Fe-Cr-Ni tubing for high-temperature pressure service
4. Kirchheiner and Woelpert, Niobium in centrifugally cast tubes for petrochemical applications (Niobium Tech)
5. Haidemenopoulos et al., Aging phenomena during in-service creep exposure of heat-resistant steels, Metals 2019
6. Failure analysis of two HP-Nb tubes after 46,000 h of reformer service, Metals 2023
7. Odanović et al., weld repair suitability of a failed HK40 reformer part, Revista de Metalurgia (CSIC)