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Rhenium, Zirconium, Hafnium and Other Refractory Metals

This series gathers the refractory and reactive metals outside molybdenum, tantalum and niobium. Rhenium melts at 3,186 °C, weighs 21.0 g/cm³ and, alone among the refractory metals, has no ductile-to-brittle transition — it bends cold and stays ductile after welding, which is why a few percent of it toughens tungsten and molybdenum. Zirconium (6.5 g/cm³) and hafnium (13.3 g/cm³) are chemical twins: zirconium is transparent to thermal neutrons and resists most acids, hafnium absorbs them and tips plasma-cutting electrodes. The list also carries beryllium structural and optical grades, chromium and ferrochrome, and the 50Cr-50Ni furnace-hanger alloys.

Popular rhenium, zirconium and hafnium grades

Compare in the explorer
GradeYield, MPaTensile, MPaDensity, g/cm³MachinabilityWeldabilityBest for
Pure Re3001,10021.02Not recommendedPoorThrust chambers, filaments, X-ray anode layers
W-26Re1,2001,50019.7Not recommendedPoorThermocouple leg and ductile hot wire
Zr 7022904506.51FairVery goodAcid-process vessels and heat exchangers
Zr 7054506206.64FairGoodZr-2.5Nb; twice the strength for pressure parts
Zircaloy-43304806.56PoorVery goodPWR fuel cladding and channel boxes
Hafnium Hf-1 (R02001)24045013.31PoorGoodControl rods and plasma-cutting electrode tips
S-200F2623701.85FairPoorStructural beryllium for stiff, light parts

All 42 rhenium, zirconium and hafnium grades

Choose rhenium, zirconium and hafnium for

The hottest rocket thrust chambers and nozzle throats (iridium-coated Pure Re), thermocouples above the reach of platinum types (W-26Re, Type C Thermocouple (W5Re/W26Re)), ductile filaments and springs at 2,000 °C, acid-process vessels and heat exchangers in acetic, hydrochloric, sulfuric and nitric acid (Zr 702, Zr 705), reactor fuel cladding and pressure tubes (Zircaloy-4, ZIRLO, Zr-2.5Nb), neutron-absorbing control rods and plasma-torch electrodes (hafnium), and mirrors and gimbals that must be stiffer and lighter than anything else (beryllium).

Look elsewhere when

A cheaper refractory metal reaches the temperature — Pure Mo or Pure W for furnace parts, since rhenium costs as much as some precious metals; the acid is one that zirconium cannot take, such as hydrofluoric acid, wet chlorine or ferric-chloride streams (Pure Ta); the corrosion is mild enough for titanium Grade 2 or Hastelloy C276; the beryllium part can be an aluminum-beryllium composite or a stiff aluminum alloy, avoiding the toxic-dust machining controls; or a stainless steel or nickel alloy survives the furnace duty (Inconel 601 in place of 50Cr-50Ni).

Yield strength against density, Rhenium & other refractory metals among every refractory metal grade

No materials in view have both of these properties.

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Rhenium, zirconium and hafnium by application

Extreme-temperature aerospace
Pure Re thrust chambers and nozzle throats, iridium-coated for oxidation and run to 2,200 °C; W-25Re and W-26Re for filaments and springs that must survive cold handling after hot service.
Thermocouples above 1,700 °C
Type C Thermocouple (W5Re/W26Re) to 2,300 °C in vacuum, hydrogen or inert gas, where Type S and B platinum couples have stopped; W-3Re and W-25Re form the Type D pair.
Acid-process equipment
Zr 702 plate, tube and forgings for acetic-acid, urea, hydrochloric and sulfuric plants; Zr 705 when the pressure code needs more strength; Zr 706 for heavy forming. Zr-2.5Nb doubles as the CANDU pressure-tube alloy.
Nuclear cores
Zircaloy-2 (BWR) and Zircaloy-4 (PWR) cladding, ZIRLO, M5, E110 and E635 as the low-corrosion niobium-bearing successors, Excel for advanced CANDU; hafnium control-rod plates and rods; Reactor Grade Zr for components requiring low hafnium.
Stiff, light optics and structure
S-200F and S-65 beryllium for gyroscope and satellite structure; I-70-H, I-220H and O-30H for mirrors and optical benches; AlBeMet AM162 where a machinable aluminum-beryllium composite gives most of the benefit with less hazard.

Working with rhenium, zirconium and hafnium

Machining
Rhenium work-hardens fiercely and is cut by EDM or grinding, with frequent anneals if it must be formed. Zirconium machines like titanium — sharp tools, low speed, heavy feed, flood coolant — and its fine chips and dust are pyrophoric, so swarf is kept wet and cleared often. Hafnium behaves the same. Beryllium is machined only in facilities with certified dust control; the metal itself is safe once finished.
Welding
Zirconium and hafnium weld cleanly by TIG or electron beam under full argon coverage, like titanium; the weld must stay silver — any straw or blue color means contamination. Rhenium and W-Re weld by electron beam and, unusually for this family, stay ductile. Beryllium is joined by brazing or bolting, not fusion welding.
Heat treatment and coatings
Zirconium is used annealed (Zr 705 stress-relieved after welding to avoid delayed hydride cracking); autoclaving in 400 °C steam grows the black protective oxide used on reactor parts. Rhenium runs in vacuum or under an iridium coating in oxidizing gas. Beryllium is stress-relieved after machining for dimensional stability.
Finishing
Pickling in nitric-hydrofluoric acid for zirconium and hafnium with attention to hydrogen pickup; electropolishing for vacuum and nuclear service; rhenium is used bare or coated. Beryllium mirrors are diamond-turned and nickel-plated before final polishing.

Other refractory metal series

Rhenium, Zirconium, Hafnium and Other Refractory Metals — frequently asked

What is rhenium, zirconium and hafnium?

This series gathers the refractory and reactive metals outside molybdenum, tantalum and niobium. Rhenium melts at 3,186 °C, weighs 21.0 g/cm³ and, alone among the refractory metals, has no ductile-to-brittle transition — it bends cold and stays ductile after welding, which is why a few percent of it toughens tungsten and molybdenum. Zirconium (6.5 g/cm³) and hafnium (13.3 g/cm³) are chemical twins: zirconium is transparent to thermal neutrons and resists most acids, hafnium absorbs them and tips plasma-cutting electrodes. The list also carries beryllium structural and optical grades, chromium and ferrochrome, and the 50Cr-50Ni furnace-hanger alloys.

Which grade of rhenium, zirconium and hafnium should I use?

Pure Re for iridium-coated thrust chambers, mass-spectrometer filaments, X-ray anode layers and the highest-temperature heater wire; W-26Re and W-5Re make the Type C Thermocouple (W5Re/W26Re) that reads to 2,300 °C in vacuum or hydrogen, and W-3Re and W-25Re are the ductile filament and Type D thermocouple wires. Zr 702 for chemical-process equipment — acetic acid, hydrochloric acid at any concentration, sulfuric acid to 70 %, boiling nitric acid; Zr 705 (Zr-2.5Nb) when the same equipment needs twice the yield strength. Zircaloy-2, Zircaloy-4, ZIRLO, M5 and E110 are the reactor fuel-cladding alloys and are bought only to nuclear specifications. Hafnium Hf-1 (R02001) and Nuclear Grade Hafnium (R1) for control rods and cutting-torch electrode inserts. S-200F and S-65 are structural beryllium for stiff, light aerospace and instrument parts; I-70-H and O-30H the optical grades for mirrors.

Which grade of rhenium, zirconium and hafnium is the strongest?

Type C Thermocouple (W5Re/W26Re) has the highest yield strength in the series at 1,750 MPa; W-3Re follows at 1,750 MPa. Values are for each grade's default condition.

Which grade of rhenium, zirconium and hafnium is easiest to machine?

AlBeMet AM162 (machinability fair, 45/100), then S-65 (45/100).

Zr 702 or Zr 705?

Zr 702 is unalloyed zirconium (yield about 210 MPa) and the standard for vessels, heat exchangers and piping in acids: best corrosion resistance, easiest to form and weld. Zr 705 is Zr-2.5Nb, with yield about 380 MPa, chosen when a pressure-code design would otherwise be too heavy. Zr 705 must be stress-relieved within 14 days of welding to avoid delayed hydride cracking at the joint, and it gives up a little corrosion resistance in oxidizing chlorides. For most chemical plant work Zr 702 is the answer.

When is rhenium worth its price?

When nothing else survives. Rhenium is among the scarcest metals in the crust and costs on the order of a precious metal, so it is used in grams: a thin iridium-coated rhenium thrust chamber that runs at 2,200 °C for thousands of seconds, the tip of a thermocouple, a filament, a sub-millimetre layer on an X-ray anode, or 3–6 % alloyed into a single-crystal turbine blade. For furnace hardware, shields and general high-temperature parts Pure Mo or Pure W does the job at a tiny fraction of the cost.