HH, HT and HU castings: composition, properties and uses

HH, HT and HU are cast heat-resistant iron-chromium-nickel grades in ASTM A297. HH (25Cr-12Ni) suits steady oxidising service. HT (17Cr-35Ni) and HU (19Cr-39Ni) carry more nickel for carburising atmospheres and repeated heating and cooling. Together they make most furnace trays, fixtures, rolls, hangers and radiant tubes.

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

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What are HH, HT and HU?

All three are cast iron-chromium-nickel heat-resistant alloys in ASTM A297, the general-application specification for heat-resistant steel castings. HH is the 25% chromium, 12% nickel grade. HT and HU are the high-nickel grades, at about 35% and 39% nickel, made for parts that are heated and cooled over and over in carburising or neutral atmospheres. Between them they cover most furnace hardware: trays, baskets, fixtures, rolls, hangers and radiant tubes.

The chemistry moves in one direction across the three. Chromium goes down a little, nickel goes up a lot. More nickel buys a steadier austenitic structure, better resistance to carbon pickup and better tolerance of thermal cycling. It also costs more.

What is the composition of HH, HT and HU?

The ranges below are the ASTM A297 limits, in weight percent. Iron is the balance in each.

Element	HH	HT	HU
Carbon	0.20 to 0.50	0.35 to 0.75	0.35 to 0.75
Chromium	24.0 to 28.0	15.0 to 19.0	17.0 to 21.0
Nickel	11.0 to 14.0	33.0 to 37.0	37.0 to 41.0
Manganese	2.00 max	2.00 max	2.00 max
Silicon	2.00 max	2.50 max	2.50 max
Molybdenum	0.50 max	0.50 max	0.50 max
Phosphorus	0.04 max	0.04 max	0.04 max
Sulphur	0.04 max	0.04 max	0.04 max

Source: ASTM A297 grade table, as reproduced by Total Materia and in published ACI casting-grade chemistry tables. Check the edition named on your purchase order, since limits can change between revisions.

ASTM A297 sets room-temperature tensile minimums too. Total Materia lists 515 MPa tensile, 240 MPa yield and 10% elongation for HH, and 450 MPa tensile with 4% elongation for HT. Those numbers say little about life in a furnace. They are acceptance tests for the heat, not design data at temperature.

What are the equivalents of HH, HT and HU?

HH: UNS J93503. EN 1.4837 (GX40CrNiSi25-12) is the usual European match.
HT: UNS J94605, often cross-referenced to N08605.
HU: UNS J95405, often cross-referenced to N08004.
ACI letters: the "H" series names come from the old Alloy Casting Institute. The second letter climbs roughly with nickel content.

EN 10295 has close relatives for the high-nickel grades too, but the ranges do not line up exactly. A cross-reference helps you match a drawing. It does not make two specifications interchangeable, so name the standard you need.

Where are HH, HT and HU used?

In heat-treatment furnaces, reheating furnaces and fired heaters, for parts that carry load while hot and are not pressure parts. Typical work:

Trays, baskets and grids that carry work through carburising, hardening and annealing furnaces.
Fixtures and jigs that hold parts during heat treatment and go into the quench with them.
Furnace rolls for roller-hearth and continuous furnaces. Our guide to furnace and galvanizing rolls covers how rolls are specified.
Radiant tubes, muffles and retorts in gas-fired heat-treatment furnaces.
Hangers, supports and tube sheets in fired heaters, where HH is the common choice.

When should you pick HH, HT or HU?

Pick by atmosphere and by how often the part is cycled.

HH for steady, oxidising service. Its high chromium gives good scaling resistance and it is the cheapest of the three. Supports, hangers, burner parts and static furnace structure are where it is used most.
HT when the part sees carburising gas or repeated heating and cooling. Trays, baskets, fixtures and rolls in carburising and hardening lines are the classic use.
HU when HT is not lasting. The extra nickel and chromium give more margin against carburisation and thermal shock, which matters for fixtures that are quenched with the load.

Past HU, the higher-nickel grades HW and HX take over for the severest cycling. For tube supports in fired heaters, look at HK40, and for supports in oil-fired heaters with fuel-ash attack, 50Cr-50Ni-Nb.

How do HH, HT and HU fail in service?

Furnace hardware rarely breaks from a single overload. It wears out by a few slow mechanisms.

Carburisation. Carbon from the furnace gas diffuses into the metal and forms chromium carbides. The skin turns hard and brittle, grows in volume and cracks. A failure study of carburising-furnace components presented at the ASM Heat Treat conference in 2015 found excessive carburisation, leading to metal dusting and then cracking, was the main cause of failure. Higher nickel slows carbon pickup, which is why HT and HU are used here rather than HH.
Thermal fatigue. Each heat and quench strains the casting, because thin and thick sections heat at different rates. Cracks start at corners and section changes and grow a little with every cycle. Even sections and generous radii matter as much as the grade.
Creep and distortion. Trays sag and rolls bow when the load is too high for the temperature. A distorted tray jams in a pusher furnace long before it cracks.
Oxidation. At the top of the range the oxide spalls with each cycle and the section thins. HH's higher chromium helps most here, in steady oxidising service.

What should an RFQ for HH, HT or HU castings include?

The grade and standard: ASTM A297 HH, HT or HU, or the EN number on your drawing.
The drawing, quantity, and whether it replaces an existing part.
Furnace type, operating temperature, atmosphere and whether the part is quenched with the load.
The life you get today and how the old part failed. A photo of a failed tray tells us more than a paragraph.
Testing: chemistry and tensile certificates, dye penetrant, radiography if needed, and any third-party inspection.

Our guide to getting a quote lists the rest.

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HH, HT and HU questions

Is HH the same as 309 stainless?

No. The chromium and nickel are similar, but HH carries 0.20 to 0.50% carbon, far more than wrought 309. That carbon adds strength at temperature and makes HH a casting-only grade.

Can HT replace HH in a carburising furnace?

Usually yes, and it is often the right upgrade. HT's 35% nickel resists carbon pickup and thermal cycling better than HH. Check that the lower chromium is acceptable if the atmosphere is also strongly oxidising.

Is HU worth the extra cost over HT?

Only when HT parts are failing early from carburisation or quench cracking. For trays that run in steady conditions, HT is usually enough.

Can these grades be welded?

Yes, new castings weld with matching high-alloy consumables. Parts that are heavily carburised after service are brittle and may crack when welded, so assess them first.