Which alloys are used for reformer and furnace castings?

Most furnace castings come from the cast heat-resistant Fe-Cr-Ni family: HH, HK40 (25Cr-20Ni), HT and HP-type grades (25Cr-35Ni). For tubes the usual choice today is HP modified with niobium and microalloying elements. Where fuel ash attacks supports and hangers, 50Cr-50Ni-Nb (IN657 type) is used.

ASTM A297 covers iron-chromium and iron-chromium-nickel castings for general heat-resistant service, with grades including HH, HK and HT. ASTM A608 covers centrifugally cast Fe-Cr-Ni tubes for pressure service at high temperature. ASTM A351 covers austenitic castings for pressure-containing parts. The 50Cr-50Ni-Nb grade is listed in ASTM A560. Which one applies depends on whether the part holds pressure and how it is cast.

What do radiant tubes do in a fired heater?

Radiant tubes, called catalyst tubes in a steam reformer, carry the process gas through the firebox and take the heat from the burners. In a reformer they are filled with catalyst. They are the hottest pressure parts in the furnace, so their alloy and wall thickness set how long the furnace runs.

Why do radiant tubes fail?

Radiant tubes fail mainly by creep: slow deformation under pressure at high temperature, which ends in cavities along grain boundaries, cracks and rupture. Short overheating events speed this up sharply. Published failure analyses of HP-Nb reformer tubes find creep cavities next to the chromium carbides, and cracks near circumferential welds where the wall runs hottest.

Ethylene cracking coils add carburization. Coke on the inside wall feeds carbon into the metal, and carburization combined with creep leads to bulging, bowing and cracking. High-nickel alloys can hide carburization from magnetic inspection, and carburized tubes become hard to weld.

Which alloy is used for reformer tubes?

Reformer tubes are centrifugally cast, usually in HP modified (25Cr-35Ni-Nb) with microalloying additions such as titanium. HK40 was the original reformer tube alloy. Niobium-modified grades and then HP-mod replaced it because they give higher creep strength, and microalloyed HP allows thinner walls. Small additions of titanium, niobium and zirconium form fine, stable carbides that resist creep.

What to put in an RFQ: grade and specification (for example ASTM A608 grade), bore and minimum sound wall, length, machining of the bore, design temperature and pressure, and the weld-end preparation.

We cast radiant and heater tubes centrifugally for fired heaters and industrial furnaces. Enquire about furnace tubes →

What do U-bends and Y-pieces do in a furnace coil?

U-bends (return bends) turn the flow between two straight tubes. Y-pieces (wyes) split or join flow between passes. They are cast fittings welded into the coil, so each one adds two or three welds in the hot zone.

Why do return bends and wyes crack?

Bends and wyes see the same creep and carburization as the tubes, plus bending stress from thermal expansion at every start-up and shutdown. Cracks often start at the welds that join them to the tubes. In aged material, secondary carbides and sigma phase make the metal brittle, and a study of service-aged HK40 concluded it was unsuitable for weld repair.

Bends and wyes are normally cast in the same grade family as the tubes they join, so the welds match.

What to put in an RFQ: bend radius or centre-to-centre distance, bore and wall to match the tube, grade matched to the coil, and weld-end bevel.

We cast U-bends and Y-pieces, statically or centrifugally as the shape suits. Enquire about bends →

What does an outlet manifold do in a steam reformer?

The outlet manifold, or collector, gathers hot reformed gas from every catalyst tube and sends it on to the transfer line. Small pigtails or short pieces connect each tube to it. Cast tees, reducers and x-mas tree assemblies make up the collector system.

Why do reformer outlet manifolds fail?

Manifolds fail at welds. One published failure analysis found leaks in the weld fusion zone of a primary reformer manifold after about 22,000 hours, caused by pipe and filler metal with high silicon in the presence of niobium, which formed a brittle Laves phase. Pigtails on the roof also suffer thermal fatigue: temperature swings drive cracks that start on the bore and grow out to a leak.

Manifold castings are usually in HP-type or other Fe-Cr-Ni heat-resistant grades chosen with the pipe and filler metal as one system.

What to put in an RFQ: the drawing, grade and chemistry limits (silicon included), the pipe and filler metal it will be welded to, design temperature and pressure, and required NDT.

We cast manifolds and x-mas trees. Enquire about manifolds →

What do tube supports and support ladders do?

Tube supports and ladder-type supports hold radiant tubes in position along the wall or floor of the firebox. They carry the weight and keep the tube spacing, and the tubes slide on them as they expand. They sit in the hot flue gas without any process cooling inside them, so they often run hotter than the tubes.

Why do furnace tube supports fail?

Supports fail by creep and distortion, by oxidation, and by fuel-ash hot corrosion when the heater burns oil with vanadium and sodium. Ash containing vanadium pentoxide and sodium sulphate attacks ordinary 25Cr-20Ni supports. When a support sags or breaks, tubes bow and touch, and they overheat.

API 560 covers tube-support design for refinery fired heaters, including stress curves for support materials. The cast 25Cr-20Ni grade (HK type) is a common support material. In heavy fuel-ash service, 50Cr-50Ni-Nb is the usual step up. It resists vanadium attack but is costly and less ductile.

What to put in an RFQ: drawing, grade, fuel type and its vanadium and sodium content if fired on oil, design metal temperature, and any proof-load or dimensional checks.

We cast tube supports and support ladders, including in HP-Nb. Enquire about tube support ladders →

What do tube hangers do in a reformer furnace?

Tube hangers suspend radiant tubes from the roof or hold them to the wall. In top-supported heaters the hangers carry the full weight of the coil. If a hanger creeps or cracks, its load drops onto the tubes below.

Why do tube hangers fail?

Hangers fail by creep elongation, distortion and fuel-ash corrosion, the same mechanisms as supports. Sodium sulphate and vanadium pentoxide ash cause hot corrosion when low-grade fuels are burnt. Cast 25Cr-20Ni is a common hanger material, and 50Cr-50Ni-Nb is recommended where ash attack is severe.

What to put in an RFQ: drawing, grade, roof or wall position, fuel analysis, design load and metal temperature, and marking requirements for grade and heat.

We cast roof and wall tube hangers with the grade and heat number cast in. Enquire about tube hangers →

What does a tube sheet do in a fired heater?

A tube sheet is a cast plate with a pattern of holes that guides and supports a row of tubes, often in the convection section or at the ends of a reformer box. It keeps the tubes on pitch while letting them slide through as they expand.

Why do tube sheets fail?

Tube sheets fail by distortion, cracking between holes, oxidation and fuel-ash corrosion. A warped sheet binds the tubes so they cannot slide, which loads them in bending. Tube sheets for fuel-ash service appear on the 50Cr-50Ni-Nb application lists of alloy makers; otherwise they are cast in heat-resistant grades such as HK40 or HH.

What to put in an RFQ: drawing with hole pattern and tolerances, grade, which section of the heater, fuel type, and whether holes are cast or bored.

We cast reformer and heater tube sheets with the hole pattern bored. Enquire about tube sheets →

What are peep doors, observation doors and access doors for?

Peep doors and observation doors let operators look into the firebox at the flames and the tubes while the heater runs; the names are used loosely, and the drawing decides the design. Access doors let people in at shutdown. Both close an opening in the hot casing, so their inner face sees firebox heat.

Why do furnace doors fail?

Doors and their frames warp, scale and crack from repeated heating and cooling, especially when opened in service. A distorted door leaks air into the firebox or hot gas out.

The grade depends on how hot the inner face runs. We cast peep doors, observation doors and access doors, with their frames, to the grade on the purchaser's drawing. Enquire about peep doors →

What to put in an RFQ: drawing, grade, hot-face temperature, and whether frame and door are supplied together.

Enquire about furnace doors →

Frequently asked questions

What is the difference between HK40 and HP modified?

HK40 is a 25Cr-20Ni cast alloy. HP is 25Cr-35Ni, and HP modified adds niobium, often with microalloying elements such as titanium. The extra nickel and the fine carbides give HP-mod higher creep strength, which is why it replaced HK40 for most reformer tubes.

What does microalloyed mean for a reformer tube?

It means small additions of elements such as titanium, niobium or zirconium. They form a fine dispersion of carbides that stay stable at high temperature and slow creep. Makers use the extra strength to offer thinner walls.

When should supports and hangers be in 50Cr-50Ni-Nb?

When the heater burns fuel oil with significant vanadium, sodium and sulphur. The ash from such fuel corrodes 25Cr-20Ni hangers. The 50Cr-50Ni-Nb grade resists that attack, at higher cost and lower ductility.

What is carburization in a furnace tube?

Carburization is carbon diffusing into the tube wall from a carbon-rich process gas or from coke on the bore. It makes the metal brittle and hard to weld, and with creep it causes bulging and cracking. It is a main concern in ethylene cracking coils.

Can aged reformer castings be weld repaired?

Often not easily. Long service precipitates secondary carbides and, in some alloys, sigma phase, which make the metal brittle. A published study of a cracked HK40 reformer part concluded it was unsuitable for weld repair. Repair decisions need a metallurgical assessment of the actual part.

What is the difference between static and centrifugal casting for furnace parts?

Centrifugal casting spins the mould, which suits round parts such as tubes, liners and cylinders. Static sand casting suits any shape, such as supports, hangers, tube sheets and fittings. We use whichever suits the part.

Which ASTM specification covers centrifugally cast furnace tubes?

ASTM A608 covers centrifugally cast iron-chromium-nickel high-alloy tubing for pressure service at high temperature. ASTM A297 covers general heat-resistant castings, and ASTM A351 covers austenitic castings for pressure-containing parts.

Does API 560 cover tube supports?

Yes. API 560 covers fired heaters for general refinery service, including the design of tube supports, and gives stress curves for tube-support materials.

Why do reformer pigtails crack?

Pigtails flex as the tubes and manifold expand and contract, so they carry bending stress at every start-up and shutdown. Temperature swings cause thermal fatigue cracks that start on the bore and grow outward to a leak.

Sources

Further reading: Förster, reformer pigtail thermal fatigue · Revista de Metalurgia, HK40 weld repair and sigma phase

Need one of these parts cast? See the castings we make and our alloy grades, or send us a drawing.

Reformer and radiant furnace castings: what each part does and why it fails