Grade page

HK40 castings: composition, properties and uses

HK40 is a cast heat-resistant stainless alloy of about 25% chromium, 20% nickel and 0.4% carbon, covered by ASTM A351 for static castings and ASTM A608 for centrifugally cast tubes. It is used for furnace parts that carry load at high temperature: tube supports, hangers and tube sheets in fired heaters, and historically the tubes of steam reformers and cracking furnaces.

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

Enquire about HK40 Request the full datasheet →

What is the composition of HK40?

HK40 is an iron-base alloy. The ranges below are the limits for grade HK40 in ASTM A351, in weight percent. Iron is the balance.

Element	ASTM A351 HK40, wt %
Carbon	0.35 to 0.45
Chromium	23.0 to 27.0
Nickel	19.0 to 22.0
Manganese	1.50 max
Silicon	1.75 max
Molybdenum	0.50 max
Phosphorus	0.040 max
Sulphur	0.040 max

Source: ASTM A351, grade HK40, as reproduced in the Total Materia database. Centrifugally cast tubes are bought to ASTM A608, whose HK40 silicon and molybdenum limits differ slightly. The edition named on your purchase order governs.

The carbon is what sets HK40 apart from wrought 310 stainless, which has a similar chromium and nickel level but far less carbon. At 0.35 to 0.45% carbon the alloy solidifies with a network of chromium carbides between the dendrites, and that network is what gives the casting its creep strength. The same carbon makes the alloy unsuitable for forging or rolling, so HK40 parts are made as castings.

What are the equivalents of HK40?

UNS: J94204.
EN / DIN: 1.4848, GX40CrNiSi25-20 (EN 10295). A close match, not an identical specification.
ASTM: A351 HK40 for static castings, A608 HK40 for centrifugally cast tubes. ACI, the old Alloy Casting Institute, is where the "H" letter series comes from.
Not the same as A297 HK. ASTM A297 also lists a grade HK (UNS J94224) with a much wider carbon range. If a drawing says "HK" without "40", ask which one is meant.

Cross-references help match a drawing to a grade. They are not proof that two specifications are interchangeable, so name the standard you need.

Where is HK40 used?

HK40 goes where a part has to hold its shape under load at temperatures up to about 1,000 °C in a clean oxidising or mildly reducing gas. In fired heaters and industrial furnaces that means:

tube supports, tube sheets and hangers in the radiant and convection sections;
furnace tubes and the fittings that join them, where the designer has not moved to an HP grade;
furnace structural parts, burner parts and fixtures.

For many years HK40 was also the standard material for steam reformer catalyst tubes and ethylene cracking tubes. In most new reformer and cracker designs it has since been replaced by niobium-modified HP grades, for reasons covered below. Our guide to reformer furnace castings explains which parts of a reformer furnace are cast and in which grades. The full range of shapes we pour is on the castings page. Ni-Chrome casts furnace tubes for fired heaters and industrial furnaces, along with internals and fittings.

How does HK40 fail in service?

Published failure analyses of HK40 tubes point to four mechanisms, often acting together.

Creep damage. A Japanese survey of about 70 HK40 catalyst tubes in steam reformers found longitudinal creep fissures in the base metal and near girth welds, and circumferential fissures in the girth weld metal (Journal of the Japan Petroleum Institute).
Sigma phase and embrittlement. Long exposure at temperature forms brittle sigma phase. A failed HK40 tube examined after about 55,000 hours showed sigma phase with needle-like carbides, and cracks running along grain boundaries; the authors judged it unsuitable for weld repair (Revista de Metalurgia).
Secondary carbide precipitation. In cracking furnaces, HK40 tubes embrittled through secondary M₇C₃ chromium carbides forming along the grain and cell boundaries of the columnar cast structure (Kirchheiner and Woelpert, Schmidt + Clemens, published by Niobium Tech).
Overheating. A study of modified HK40 reformer tubes that cracked after four years traced the failure to overheating caused by choked catalyst restricting flow (CSIR-NML, Jamshedpur).

The common thread is temperature. Creep life falls steeply as metal temperature rises, so a local hot spot or a long run above design shortens life sharply. Static parts such as supports and hangers usually see lower stress than pressure tubes, which is one reason HK40 remains common for them.

When should you choose HK40 over HP-Nb?

HP-Nb carries about 35% nickel and a niobium addition. The industry moved to it because it is stronger in creep. Schmidt + Clemens report that IN 519, an early niobium-bearing successor to HK40, had about 25% higher creep strength at 1,000 °C, and creep ductility of 8 to 10% after 10,000 hours at 900 °C against about 3% for HK40.

HK40 is still the reasonable choice when:

the part is a support, hanger or tube sheet loaded mainly by its own weight and the tubes it carries;
metal temperature stays clear of the top of HK40's range;
the plant is replacing like for like and the original design was HK40;
cost matters more than the extra margin, since HK40 contains much less nickel than HP grades.

Move to HP-Nb when the design temperature or stress rises, when wall sections must be thinner, or when carburising gas is present, since the niobium-bearing HP grades also resist carburisation better.

Which casting route suits HK40?

As general practice, round tubes are cast centrifugally in a spinning metal mould and bought to ASTM A608. Supports, hangers, tube sheets, fittings and other shaped parts are cast statically in sand moulds and bought to ASTM A351. The route is set by the shape of the part. Tell us what the part is and we will propose the route.

What should an RFQ for HK40 castings include?

The standard and edition: A351 HK40, A608 HK40, or EN 1.4848.
The drawing with tolerances, machining allowance and finished weight if known.
Operating temperature, atmosphere (oxidising, carburising, sulphur-bearing) and whether the part sees thermal cycling.
Quantity, and whether this is a replacement or a new design.
Testing: chemistry and tensile certificates, radiography, dye penetrant, and any third-party inspection.
Marking, documentation format and any EIL, licensor or end-user approval needed.

Enquire about HK40 Request the full datasheet →

HK40 questions

Is HK40 the same as 310 stainless?

No. Chromium and nickel are similar, but HK40 has 0.35 to 0.45% carbon. That carbon gives it much better creep strength and means it exists only as a casting.

Can service-aged HK40 be weld-repaired?

Sometimes, but aged material with sigma phase is brittle and may crack. Published work found that solution treatment restored room-temperature ductility but not creep-rupture life. Have the aged metal assessed first.

Why did reformers and crackers move from HK40 to HP grades?

Higher creep strength and creep ductility. The niobium-bearing grades that followed HK40 lasted longer at the same temperature or allowed thinner walls, and niobium also improved carburisation resistance.