Cobalt alloy castings: Co-Cr-W wear and Co-Cr-Fe heat alloys

Cobalt-chromium alloy castings keep their hardness and their oxide skin at red heat, and they hold up in molten zinc. Co-Cr-W and Co-Cr-Mo wear alloys are used for wear faces and for galvanizing pot bushes and sleeves. A Co-Cr-Fe heat alloy, about half cobalt, is used for furnace parts such as skid riders that face thermal shock and slag.

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

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Why use a cobalt alloy casting?

Because cobalt-chromium alloys keep their hardness and their oxide skin at temperatures where hardened steels soften and nickel-chromium grades start to wear. They are the answer for parts that slide, rub or carry load while red hot, or that sit in molten zinc. Two families cover most of that work: cobalt-chromium-tungsten and cobalt-chromium-molybdenum wear alloys, and a cobalt-chromium-iron heat alloy for furnace parts.

We cast them to compositions similar to the well-known commercial cobalt grades and do not use the proprietary names. We pour to the published nominal chemistry or to the chemistry you specify.

What is the composition of the Co-Cr-W and Co-Cr-Mo wear alloys?

Cobalt is the base in each. The ranges below are the published nominal compositions, in weight percent. Other elements, mainly nickel, iron, silicon and manganese, make up small amounts.

Element	Type 6	Type 12	Type 21
Chromium	27 to 32	27 to 32	26 to 29
Tungsten	3 to 6	7.5 to 9.5	none
Molybdenum	minor	none listed	4.5 to 6.0
Carbon	0.9 to 1.4	1.4 to 2.0	under 0.35
Nickel	minor	minor	under 3.0
Hardness, HRC	36 to 45	45 to 51	27 to 40
UNS	R30006	R30012	R30021

Source: manufacturer data sheets for the equivalent commercial grades. Older datasheets give tungsten in type 6 as 4 to 6%. Type 21 work-hardens in service, up to about 48 HRC by the same source. Name the composition you need on the order.

More carbon and tungsten means more hard carbides: type 12 is harder and more wear-resistant than type 6, and less tough. Type 21 goes the other way, with low carbon and molybdenum in place of tungsten, so it takes impact and thermal shock better and wears faster under pure abrasion.

What is the Co-Cr-Fe heat alloy?

It is a cobalt-chromium-iron heat-resistant alloy of about 50Co-28Cr-21Fe. ASM's Alloy Digest describes it as resistant to thermal shock, oxidation and corrosion by hot gases, slags and molten copper, and lists it for metallurgical and glass-industry use. A niobium-bearing variant was developed for higher stress-rupture strength.

It has known limits. A US patent on furnace skid rails notes that the alloy can crack after holding at or below about 1,100 °C because of sigma phase, and that it welds poorly. Check that before choosing it for a part that runs cooler than it was designed for.

Where are cobalt alloy castings used?

Skid riders and skid buttons on the water-cooled skid pipes of walking-beam and pusher reheating furnaces. The slab sits on them at rolling temperature, so they must hold their shape and resist wear and oxidation while the pipe under them is cold.
Galvanizing pot hardware: bushes, sleeves and bearing parts for sink rolls and stabiliser rolls that run submerged in molten zinc.
Valve seats, trim and wear pads that must stay hard at red heat.
Hot-working tools and guides, and furnace and sinter-plant fixtures hit by thermal shock and slag.

Which cobalt alloy suits a skid rider or skid button?

Riders and buttons have been made in cobalt-base and in nickel-chromium cast alloys for decades. A US patent on skid beams describes older skid buttons as heat-resistant cobalt cast steel or nickel-chromium cast steel. The cobalt grades are chosen for the hottest zones, where the rider must resist creep, scaling and the abrasion of the slab.

The usual trade-off is zone by zone: cobalt where the slab is hottest, nickel-chromium grades where cobalt is not worth the price. Tell us the furnace zone, the slab temperature and the failure you see now, and we will tell you what we would cast.

Which cobalt alloy suits galvanizing pot bushes and sleeves?

Molten zinc dissolves iron and most steels fast, and the sink-roll bearing runs in it under strip tension with no real lubrication. Co-Cr-W wear alloys have been the conventional bearing material for this job. Patents on sink-roll bearings describe sleeves and half-bushings in these alloys as the baseline that newer designs are measured against. Type 6 is the usual starting point. A harder type is used where wear dominates, a tougher one where the bush cracks.

The bush and the sleeve wear as a pair, so it pays to buy them as a matched set and to tell us what the mating part is made of. Our guide to furnace and galvanizing rolls covers the roll side of the pot.

How do cobalt alloy castings fail?

Wear, abrasive and adhesive, which is what they are bought to resist. Wear rate depends as much on the mating surface and the temperature as on the grade.
Cracking from thermal shock or impact, more likely in the high-carbon types. Type 21 or the Co-Cr-Fe heat alloy is the usual answer.
Embrittlement from sigma phase in Co-Cr-Fe alloys held in the wrong temperature band.
Dissolution in molten zinc, slower than for steels but not zero.

What should an RFQ for cobalt alloy castings include?

The alloy by chemistry or type number, and the hardness you need.
The drawing, machining allowance and quantity. Cobalt alloys are hard to machine, so say which faces need finishing.
The service: furnace zone and temperature, or bath type and temperature for pot hardware.
The life of the current part and how it fails.
Testing: chemistry certificate, hardness, dye penetrant and dimensional report.

Our guide to getting a quote lists the rest.

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Cobalt alloy questions

Can you match a named commercial cobalt grade?

We cast cobalt alloys similar to the widely used commercial wear and heat grades, to their published nominal chemistry or to your own specification, and certify the chemistry we pour.

Is the type 6 or type 12 Co-Cr-W alloy better for wear?

Type 12 is harder, with more carbon and tungsten, and resists abrasion better. Type 6 is tougher and more forgiving of impact and thermal shock. Most general wear parts start with type 6.

Why not hardface a steel part instead of casting it solid?

Hardfacing works for many parts. A solid casting makes sense when the whole part wears, when it runs hot enough to undermine the bond with a steel base, or when it sits in molten zinc that would attack the steel through any defect.