Which alloys are used for furnace rolls, radiant tubes and trays?

Most of this hardware is cast from the heat-resistant Fe-Cr-Ni grades in ASTM A297: HH (25Cr-12Ni), HK (25Cr-20Ni), HT (17Cr-35Ni) and HU (19Cr-39Ni), with HP-type (25Cr-35Ni) for the hottest duty. More nickel gives better resistance to carburization and thermal cycling. More chromium gives better oxidation resistance. Cobalt-base alloys go where a hot part also has to resist wear, such as skid riders and galvanizing bushes. The grades page lists what we pour, and the HK40 and HP-Nb pages cover two common furnace grades in more detail.

What do hearth rolls and furnace rolls do?

Hearth rolls carry strip, plate or slabs through a roller-hearth or continuous annealing furnace. A roll is a cast barrel, usually centrifugally cast, with end pieces and journals that sit outside the hot zone. It turns under load at furnace temperature for months at a time.

Why do hearth rolls fail?

The failure we are asked about most is a bent roll. A roll that stops turning, even briefly, creeps under its own weight and the load, and then marks the product. Next come build-up of oxide or metal on the barrel (pickup) that scars the strip, cracking where barrel meets end piece from thermal cycling, and wall loss by oxidation. Carburizing atmospheres add embrittlement over time, the same mechanism that cracks heat-treatment castings (Piekarski, 2010).

Grades usually specified: HK and HP-type for most roller-hearth service, and HT or HU where the roll sees heavy thermal cycling or a carburizing atmosphere.

What to put in an RFQ: drawing with barrel diameter, length and wall, end-piece and journal design, grade, furnace atmosphere and working temperature, the load on the roll, and whether the barrel is to be machined and balanced.

We cast roll barrels centrifugally and end pieces statically. Enquire about hearth and furnace rolls →

What do skid riders and buttons do in a walking-beam furnace?

In a walking-beam or pusher reheating furnace the slabs rest on water-cooled skid pipes. Riders and buttons are the cast pieces fixed on top of those pipes. They hold the hot slab off the cold pipe, which reduces the cold marks (skid marks) on the slab and protects the pipe from wear.

Why do skid riders fail?

A rider runs hot on top and cold underneath, under the full weight of the slab, with scale sliding across it. It fails by hot wear and flattening of the top face, by oxidation, and by cracking from the temperature gradient. Once a rider wears down or comes off, the skid pipe beneath takes the damage. Sahoo et al. (2020) describe skid pipe failure in a walking-beam furnace at an integrated steel plant.

Grades usually specified: heat-resistant Fe-Cr-Ni grades such as HK and HP-type in the cooler zones, and cobalt-base alloys in the soaking zone, where they hold their hardness and wear resistance at temperature (Birol, 2010). The choice is a trade between rider life and price.

What to put in an RFQ: rider or button drawing, skid pipe size and how the rider is fixed (welded, clamped or keyed), furnace zone, grade, and quantity per zone.

We cast riders, buttons and skid rails in Fe-Cr-Ni and cobalt grades. Enquire about skid riders and buttons →

What do U- and W-type radiant tubes do?

A radiant tube is a closed alloy tube with a burner firing inside it. The tube heats up and radiates to the load, while the combustion gas stays inside and leaves through the exhaust leg. The furnace atmosphere stays clean, which matters in carburizing, hardening and continuous annealing. U, W and single-ended shapes suit different furnace widths and burner layouts. Straight legs are usually centrifugally cast and joined by statically cast bends.

Why do radiant tubes fail?

The tube has combustion gas on the inside, often a carburizing atmosphere on the outside, and heats and cools every cycle. Typical failures are sagging and distortion by creep, cracks at welds and bends from thermal fatigue, carburization and oxidation that thin the wall, and local overheating where the flame touches the tube. Tian et al. (2018) analysed a perforated Cr28Ni48W5 radiant tube. Once a tube perforates, combustion gas leaks into the furnace.

Grades usually specified: HK and HP-type for most duty, and high-nickel HT or HU-type for carburizing furnaces with frequent cycling. Higher-alloy grades such as Cr28Ni48W5 are also used.

What to put in an RFQ: tube shape (U, W or single-ended), leg diameter, wall and length, bend geometry, grade, burner type and firing rate, furnace atmosphere, and how the tube is supported.

We cast radiant tube legs centrifugally and bends statically. Enquire about radiant tubes →

What do heat-treatment trays, baskets and fixtures do?

Trays, grids, baskets and fixtures hold parts as they go through carburizing, hardening, tempering or annealing. In a sealed-quench or pusher furnace the tray goes from the hot zone into an oil or polymer quench and back again, cycle after cycle. Fixtures hold gears, shafts or rings in a set position so they distort less.

Why do heat-treatment trays crack and distort?

Quenching is the main cause. The thin sections cool faster than the thick ones, and the stress from that difference cracks the casting over repeated cycles. In a carburizing furnace the alloy also picks up carbon, which makes it harder and more brittle with each cycle, so cracks start more easily. Piekarski (2010) examined damaged heat-resistant castings from a carburizing furnace and links the cracking to carburization and thermal cycling. Trays also warp by creep, then jam in the furnace or no longer stack.

Grades usually specified: high-nickel grades such as HT and HU for carburizing and quench service, where nickel helps against both carburization and thermal fatigue. HK or HH for tempering and annealing, where there is no quench and the atmosphere is milder.

What to put in an RFQ: drawing of the tray, grid or basket, the furnace type, the quench medium, the atmosphere and working temperature, the load per tray, and whether the trays must stack or interlock with existing ones.

We cast trays, grids, baskets and fixtures as single pieces or in sections that bolt or weld together. Enquire about trays and fixtures →

What do sink rolls, stabilizer rolls, arms and bushes do in a galvanizing pot?

On a continuous hot-dip galvanizing line the strip dives into the zinc pot, wraps around the sink roll and turns back up. Stabilizer and correcting rolls just above it keep the strip flat and steady before it leaves the bath for the air knives. Cast arms hang the rolls in the pot, and the rolls turn in sleeve-and-bush bearings that run in the molten zinc itself.

Why do pot rolls and bushes fail?

Molten zinc dissolves iron and reacts with steel to form iron-zinc compounds. Xu et al. (2007) measured the attack on 316L, 410 stainless and carbon steel in a zinc bath. The roll surface roughens, and dross builds up and marks the strip. The bearings wear faster still, because they slide under load in a liquid metal with hard dross particles in it. Jiang and Wang (2022) studied the sliding bearings of a sink roll. When bushes wear the roll starts to wobble, the strip vibrates and coating weight drifts, and the line stops for a roll change.

Grades usually specified: austenitic stainless steels of the 316L type for roll bodies and arms, and cobalt-base or other wear-resistant alloys for sleeves and bushes.

What to put in an RFQ: roll or bush drawing, bath chemistry (zinc, zinc-aluminium or aluminium-zinc), bath temperature, line speed and strip tension if known, grade, and whether the bush mates with an existing sleeve.

We cast sink rolls, stabilizer rolls, arms and bushes. Enquire about galvanizing pot hardware →

What do charging and coke buckets do?

Charging buckets carry scrap or charge material to a furnace. Coke buckets carry hot coke on coke-oven and quench lines. Both are lifted, tipped and set down many times a shift, often while hot, and they take impact from the load.

Why do charging and coke buckets fail?

They crack at corners, lugs and lifting points from impact and repeated heating and cooling, and they lose wall to abrasion and oxidation. A bucket that warps no longer seats or tips cleanly.

Grades usually specified: heat-resistant Fe-Cr-Ni grades such as HH and HK where the bucket runs hot, chosen against the temperature, the load and the lifting design.

What to put in an RFQ: drawing, capacity, lifting and tipping arrangement, working temperature, grade, and whether the bucket is one casting or a fabricated assembly of cast panels.

Enquire about charging and coke buckets →

Frequently asked questions

What is the difference between HK and HT for furnace hardware?

HK is about 25% chromium and 20% nickel. HT is about 17% chromium and 35% nickel. HK resists oxidation well and costs less. HT, with more nickel, holds up better against carburization and repeated quenching, which is why it is common for trays and radiant tubes in carburizing furnaces.

Why does my radiant tube sag?

Creep. At furnace temperature the alloy slowly deforms under its own weight and any load from its supports. Sagging speeds up if the tube runs hotter than its design temperature, if a support is missing or has failed, or if the grade is too lean for the duty.

Can cracked heat-resistant castings be weld repaired?

New castings weld well. Castings that have been in service for a long time often do not, because carbides and sigma phase build up in the alloy and make it brittle. Odanović et al. (2010) judged a service-aged HK40 part unsuitable for weld repair for that reason. A metallographic check before repair tells you which case you are in.

Why use cobalt alloys for skid riders?

Cobalt-base alloys stay hard and resist wear at temperatures where Fe-Cr-Ni grades soften. In the soaking zone, where the rider is hottest and the slab heaviest, that usually means a longer life per rider.

What attacks sink rolls in a zinc pot?

Molten zinc dissolves iron from the roll and forms iron-zinc compounds on its surface. Aluminium in the bath changes how fast this happens. Dross particles in the bath add abrasive wear, mostly at the bearings.

Should trays be cast or fabricated?

Fabricated trays from wrought sheet are lighter and cheaper to start with. Cast trays carry more load and usually stand up to repeated quenching better, because the sections and joints can be shaped for it. The right answer depends on the load, the quench and how many cycles you need.

What do I need to send for a quote?

A drawing, the grade or the service conditions if the grade is open, quantity, and any inspection or test requirements. Our guide to getting a quote lists the rest.

Furnace rolls, radiant tubes and galvanizing pot rolls: what they do and why they fail