Tungsten Crucible Applications and High-Temperature Properties

Tungsten Crucible Basics: Material Choice and Operating Envelope

A tungsten crucible is a high-temperature vessel formed, machined or welded from unalloyed tungsten and used to contain molten metal, oxide melt or a vapour source at temperatures where alumina, zirconia and graphite crucibles have already reached their practical ceiling. The commercial reason for paying a premium for tungsten is simple: it melts at 3422 degrees Celsius, the highest melting point of any metal in routine industrial service, and its vapour pressure stays low enough for vacuum duty well above 2000 degrees Celsius. In rare earth reduction cells, sapphire furnaces, electron-beam evaporation sources and quartz melting boats, that combination of melting point and low volatility is what keeps the melt clean.

For a purchasing engineer the melting point is only the headline figure. Whether a crucible survives a full production campaign depends on purity, sintered density, grain structure, wall-thickness uniformity, the way the vessel is supported in the hot zone, and the heating and cooling rates it is subjected to. Two crucibles with identical drawings and both quoted as 99.95% tungsten can behave very differently if one is 96% dense with a coarse recrystallised structure and the other is above 99% dense with a fine, uniformly worked grain. The first will show grain-boundary cracking after a handful of thermal cycles; the second may run for months. This article explains which characteristics actually matter, how they are measured, and how they map onto the main application families.

Grades and Purity: What the Purchase Specification Should Fix

Unalloyed tungsten for crucible service is normally specified against a refractory-metal mill product standard rather than a crucible-specific one, because the crucible is machined or formed from plate, sheet, bar or sintered billet. ASTM B760 covers tungsten plate, sheet and foil, and it is the reference most often cited on a crucible drawing because the chemical limits and the density requirement in that document are directly transferable to a finished vessel. ASTM B387 is the corresponding specification for tungsten and molybdenum alloy bar, rod and wire, and it applies to the rod stock used for strengthening rings, lip reinforcements and welded sub-assemblies. Where a crucible is assembled rather than machined from one piece, the purchase order usually invokes both documents so that every input stock form carries the same chemical guarantees.

The chemical requirement that matters most is not the tungsten figure itself but the interstitial and substitutional impurity limits behind it. A 99.95% minimum tungsten content leaves up to 500 ppm for everything else, and the distribution of those 500 ppm decides behaviour. Carbon above roughly 30 ppm promotes brittle carbide films on grain boundaries. Oxygen above 50 ppm forms volatile tungsten oxides during vacuum outgassing and produces porosity in the sintered body. Iron, nickel and cobalt are the classic contamination trio: they are picked up from tooling, sieving equipment and handling, they lower the ductile-to-brittle transition temperature, and they are the first thing a customer's glow-discharge mass spectrometry report will flag. Molybdenum is a legitimate residual rather than a contaminant, but above a few hundred ppm it indicates feedstock blending rather than deliberate alloying, and it depresses density slightly.

A defensible purchase specification therefore states the tungsten minimum, individual limits for the interstitial elements, a combined limit for iron, nickel and cobalt, a minimum density expressed as a percentage of the 19.3 g/cm3 theoretical value, and the analytical method to be used for each. It also states the acceptance basis: certified chemical analysis from the melt or powder lot, verified on the finished part by an independent laboratory on a sampling plan agreed at order stage. Writing these numbers into the order is the single most effective way to avoid a shipment of crucibles that look correct, measure correct, and crack in the first heat.

Physical and Mechanical Properties at Operating Temperature

At room temperature unalloyed tungsten behaves like a ceramic: hard, stiff, and brittle. The ductile-to-brittle transition temperature of commercial purity tungsten is commonly reported in the 200 to 400 degrees Celsius band, and it moves upward as the material recrystallises and as grain size coarsens. This is the reason crucible handling rules are so strict: a cold crucible dropped or struck on its rim, or clamped unevenly during installation, can initiate a crack that only propagates once the vessel reaches temperature. Wrought and stress-relieved product has better toughness than as-sintered product, which is why rolled plate is preferred for large-diameter vessels that must tolerate handling.

Above the transition range the material becomes genuinely ductile and its strength falls gradually with temperature. Tensile strength of wrought tungsten is in the region of 500 to 700 MPa at room temperature and drops to roughly 150 to 250 MPa at 1500 degrees Celsius, so a crucible is never designed on room-temperature strength. Design is driven by creep and by thermal stress. A tungsten crucible in continuous service at 2000 degrees Celsius will creep under its own weight and under the weight of the charge unless the wall is supported by an outer liner, a graphite susceptor or a segmented support cage. This is why drawings for large vessels specify a support band position and a permitted ovality, not just a wall thickness.

Thermal expansion is low, at roughly 4.5 x 10-6 per Kelvin, which is about a third of that of austenitic stainless steel but still enough to matter across a 400 mm diameter: a 2000 degree rise produces close to 3.6 mm of diametral growth. Clearances in the hot zone must accommodate that growth, and any refractory or graphite component in contact with the crucible must be allowed to slide rather than constrain it. Thermal conductivity is high, 160 to 175 W/(m K) at room temperature, which helps equalise wall temperature and reduces the thermal gradient that generates stress during ramp-up, but it also means heat is drawn rapidly out through any cooled support.

Property	Typical value, unalloyed tungsten 99.95% W	Why it matters for a crucible
Density	19.25 to 19.30 g/cm3 theoretical; 19.0 g/cm3 minimum specified	Residual porosity controls outgassing and melt contact area
Melting point	3422 degrees Celsius	Sets the upper limit of melt and evaporation duty
Boiling point	About 5555 degrees Celsius	Low vapour pressure keeps vacuum melts free of crucible carry-over
Thermal conductivity	160 to 175 W/(m K) at 20 degrees Celsius	Flattens wall gradients during ramp-up and cool-down
Coefficient of thermal expansion	4.4 to 4.6 x 10-6 per Kelvin	Sets hot-zone clearances and support tolerances
Electrical resistivity	5.3 to 5.6 micro-ohm centimetre at 20 degrees Celsius	Relevant for induction and resistance heated arrangements
Young's modulus	400 to 410 GPa at 20 degrees Celsius	High stiffness concentrates thermal strain at geometric features
Vickers hardness	300 to 430 HV as sintered; 380 to 480 HV as rolled	Proxy for work history and machinability
Recrystallisation range	About 1200 to 1400 degrees Celsius	Above this range grain growth starts and toughness falls
Ductile-to-brittle transition	About 200 to 400 degrees Celsius, purity and structure dependent	Defines handling and pre-heat rules before service

Product Forms: Sintered, Stamped, Spun, Machined and Welded

Crucibles reach the same drawing by five different routes, and the route determines both the price and the failure mode. Sintered crucibles start as a cold isostatically pressed powder preform close to final shape, are sintered in hydrogen above 2000 degrees Celsius, and are then machined to the final bore, rim and base radius. This route gives the thickest wall and the best dimensional control at large diameter, at the cost of a coarser grain structure and lower toughness than wrought product. Stamped crucibles are deep drawn from tungsten sheet in the 0.5 to 3 mm range; they are light, cheap in small sizes, and well suited to evaporation sources and laboratory work, but the wall is thin and the drawn corner is a stress raiser.

Spun or flow-formed crucibles are produced by rotating a disc or preform against a mandrel at elevated temperature, which produces a dense, circumferentially oriented grain structure with good hoop strength. This is the preferred route for medium and large open-top vessels used in sapphire and oxide melting. Machined-from-plate crucibles are cut from rolled tungsten plate and bored on a lathe; the result has the best combination of density and toughness, and the process suits shallow, wide vessels and liners. Welded assemblies combine a machined or spun shell with a separately formed base and molybdenum rod reinforcement, and they are used where a single-piece billet would be uneconomic, typically above 400 mm diameter.

Build route	How the wall is formed	Typical wall thickness	Typical outside diameter	Structural character	Preferred duty
Sintered and machined	Cold isostatic press plus hydrogen sintering, then turning	3 to 15 mm	50 to 400 mm	Equiaxed, coarse grain, thick section	Rare earth reduction, heavy melt loads
Stamped and drawn	Deep drawing of sheet, annealed between passes	0.5 to 3 mm	30 to 200 mm	Thin wall, worked, corner stress raisers	Evaporation sources, laboratory melts
Spun or flow formed	Hot flow forming on a mandrel	0.8 to 5 mm	80 to 500 mm	Circumferential grain, high hoop strength	Sapphire and oxide crystal growth
Machined from rolled plate	Boring and turning of wrought plate	2 to 20 mm	60 to 350 mm	Dense, fine grain, best toughness	Shallow vessels, liners, high-cycle duty
Welded assembly	Shell plus base joined in inert atmosphere, rod reinforcement	1.5 to 10 mm	200 to 600 mm	Joint-controlled, needs weld qualification	Very large vessels, replacement liners

Application Focus: Rare Earth Smelting and Reactive Metal Reduction

The largest single application for tungsten crucibles is the reduction and refining of rare earth metals. In calciothermic and lanthanothermic reduction, a rare earth fluoride or chloride charge is reduced inside a lined vessel and the resulting metal is consolidated as an ingot or as a sponge that is subsequently vacuum remelted. The melt is aggressive to oxides and to graphite, and the operating temperature in the reaction zone commonly sits between 1450 and 1700 degrees Celsius. Alumina crucibles fail by slag attack, graphite crucibles contribute carbon to the metal, and molybdenum crucibles soften and sag. Tungsten survives because it is thermodynamically stable against the rare earth metals, it does not wet them readily, and it does not introduce carbon.

The performance metric that matters in this duty is not melting point but campaign life per vessel, and that is controlled by wall thickness, support arrangement and the thermal cycle. A thick sintered wall tolerates more reduction cycles because there is material to lose to progressive surface reaction and to machining during refurbishment. Buyers in this segment normally keep a refurbishment schedule: the bore is cleaned, skimmed and re-measured after a fixed number of cycles, and the vessel is retired when the wall reaches a minimum calculated thickness. Specifying that minimum thickness on the drawing, together with the measurement positions, prevents arguments about when a crucible is worn out.

A second and growing use is in the melting of reactive and refractory metals and master alloys, including titanium and zirconium bearing charges where a ceramic crucible would contaminate the melt with oxygen. Here the vessel is usually a liner inside a water-cooled copper or graphite support, and the tungsten is chosen for its combination of non-reactivity and thermal shock tolerance in thin sections. Crucibles for this duty are frequently supplied with a controlled inside surface finish, because a rough bore traps oxide and makes cleaning between heats difficult.

Application Focus: Vacuum Coating, Crystal Growth and Quartz Melting

In physical vapour deposition, tungsten crucibles and boats act as resistive or electron-beam heated sources for aluminium, silver, gold, chromium and oxide evaporants. The requirement profile is different from a melting vessel: the wall is thin, 0.5 to 2 mm, the geometry is often a shallow dish or a lined boat, and the failure mode is local hot-spot erosion rather than bulk creep. Uniform wall thickness and freedom from porosity are critical because a porous wall wets the evaporant and eventually burns through. Evaporant capacity, contact area between the charge and the tungsten, and the power density delivered to the source determine source life far more than the certified purity does.

Sapphire and other oxide crystal growth uses tungsten crucibles as the melt container in resistance heated furnaces operating near 2050 degrees Celsius. At that temperature the crucible is in slow creep, the melt is corrosive to most refractories, and the crystal quality is sensitive to any metallic contamination carried from the crucible into the melt. Spun and machined vessels are both used; the choice depends on diameter and on whether the furnace design supports the wall from outside. Crucible ovality and base flatness are specified tightly here, because an out-of-round vessel changes the thermal field and produces off-axis growth.

Quartz and high-silica glass melting uses tungsten as an electrode and, in some furnace designs, as a liner in contact with the melt. Tungsten is chosen because it resists attack by molten silica at 1700 to 2000 degrees Celsius and does not discolour the glass. In all three of these application families the practical recommendation is the same: define the thermal cycle, define the support condition, then let the supplier select the build route, rather than fixing a build route and hoping it survives the cycle.

Design, Handling and the Failure Modes Seen in Service

Most premature tungsten crucible failures fall into four categories, and only one of them is a material defect. The first is thermal shock during heat-up: a cold vessel pushed into a hot zone, or heated at a rate the wall cannot follow, develops a hoop stress gradient that exceeds the low-temperature strength of the material. Pre-heating and a controlled ramp solve this, and the ramp rate should be written into the furnace recipe rather than left to the operator. The second is mechanical damage during handling and installation, typically a rim chip or a base bruise that becomes a crack origin; padded lifting gear, a dedicated installation cradle and a no-metal-on-metal rule prevent it.

The third is chemical attack at the melt line, where slag, flux or oxide sits against the wall and reacts. This shows as a waisted or locally thinned section, and it is managed by keeping the melt line at a consistent height, by using a wash coat where the process permits, and by measuring wall thickness at the melt line at every refurbishment. The fourth is creep sagging of large unsupported vessels, which shows as a bulged wall or a dished base and eventually as a loss of concentricity that prevents the crystal or ingot from being withdrawn. The design answer is a support band, a segmented support cage, or a smaller diameter and a taller stack.

Grain structure deserves a separate note because it is invisible on a drawing. Tungsten that has been heated above its recrystallisation range for long periods develops large equiaxed grains, and those grains provide long, straight boundary paths for crack propagation. A crucible that was fine-grained and tough when delivered may be coarse and brittle after a hundred hours at 2100 degrees Celsius, and its remaining life is then governed by handling rather than by wall thickness. Suppliers who understand this will quote a maximum service temperature for continuous duty, distinct from the short-term peak the material can survive, and buyers should insist on both figures.

Inspection, Acceptance and Documentation

A tungsten crucible order should close with an inspection and test plan that is proportionate to the duty. For chemical composition, a certificate of analysis traceable to the powder or billet lot is the baseline, with independent verification by glow-discharge mass spectrometry or inductively coupled plasma mass spectrometry on a sample per lot. For density, the immersion method gives a bulk figure, while ultrasonic testing reveals internal voids and delamination that a bulk density number can hide. For dimensions, the plan should list bore diameter at three heights, wall thickness at a minimum of eight equally spaced positions, base thickness at centre and at the radius, ovality calculated as the maximum minus minimum diameter, and base flatness measured on a surface plate.

For welded vessels the plan adds a weld qualification record, a dye penetrant or fluorescent penetrant examination of the joint, and a helium leak test when the vessel will operate under vacuum; a leak rate tighter than 1 x 10-9 Pa m3/s is a common contractual figure for high-vacuum service. Surface condition is also an acceptance item: embedded tool marks, grinding burns and smeared metal from a dull insert all become sites for thermal fatigue, so a visual and penetrant check of the bore after final machining is worth specifying. Finally, the documentation package should include the mill certificates for every input form, the heat treatment record with furnace, atmosphere, temperature and hold time, the dimensional report and the non-destructive testing reports, all traceable to the serial number stamped on the vessel.

FAQ

Q: What purity should a tungsten crucible be ordered at for rare earth melting?
99.95% W minimum is the normal commercial baseline for unalloyed tungsten mill product, with individual limits on carbon, oxygen and nitrogen, and a combined limit on iron, nickel and cobalt. Higher purity grades are available but the practical benefit is limited unless the analytical method and sampling plan are agreed at order stage.

Q: What is the maximum continuous service temperature of a pure tungsten crucible?
The melting point is 3422 degrees Celsius, but continuous duty is normally limited to about 2000 to 2300 degrees Celsius in vacuum or inert gas because of creep and grain growth. Short excursions to 2600 degrees Celsius are possible in clean vacuum with adequate support, and the supplier should be asked to state both the continuous and the peak figure.

Q: Why does a tungsten crucible crack during the first heat even though it passed inspection?
The usual cause is thermal shock on a cold or unevenly heated wall. Commercial purity tungsten has a ductile-to-brittle transition between roughly 200 and 400 degrees Celsius, so a rapid ramp leaves part of the wall brittle while the rest expands. A controlled pre-heat and a slower ramp through the transition range usually eliminate the problem.

Q: Is a sintered crucible or a machined-from-plate crucible better?
Sintered vessels allow thicker walls and larger diameters at lower cost, and they suit high-melt-load reduction duty. Machined-from-plate vessels have higher density and better toughness in thin sections, and they suit high-cycle and handling-intensive duty. Spun vessels give the best hoop strength for large open-top crystal growth crucibles.

Q: How is wall thickness measured on a used crucible during refurbishment?
Ultrasonic thickness gauging from the outside, referenced to the original certified dimensions, is the standard shop-floor method. Measurements are taken at fixed marked positions including the melt line, and the vessel is retired when the minimum reading falls below the drawing limit. A written measurement record per vessel makes the retirement decision auditable.

Q: How should tungsten crucibles be packed and stored before installation?
They should be packed in a rigid crate with the bore supported, kept in a dry environment with a desiccant, and never stacked rim to base. Gloves should be worn during handling to avoid transferring salts and skin oils that later volatilise in vacuum, and the vessel should be visually checked for rim damage immediately before it enters the furnace.