Tungsten Crucible Manufacturing: Powder to Leak-Tight Vessel

Process Route Overview: Why the Drawing Decides the Method

A tungsten crucible is never made by a single universal process. The manufacturing route is a consequence of four variables on the customer drawing: outside diameter, wall thickness, depth-to-diameter ratio and the specified density. A shallow 120 mm dish for an evaporation source and a 500 mm deep reduction vessel with a 12 mm wall both end up as tungsten, but they share almost no process steps. This is why a supplier quoting a crucible should quote a route at the same time, and why a buyer who specifies only "pure tungsten, 99.95%" is leaving the most consequential decision to someone else.

Four industrial routes dominate. Press-and-sinter produces near-net-shape bodies from powder and is the only economic way to make thick walls at large diameter. Press, sinter and roll produces plate that is later machined or spun, giving the highest density and the best toughness. Flow forming, also called spinning, converts a disc or a preform into a thin-walled shell with circumferential grain flow and is the standard route for open-top crystal growth vessels. Welded assembly joins a spun or machined shell to a formed base and is used where a monolithic billet would exceed available press capacity or become uneconomic. Each route is a chain of thermal and mechanical steps, and each step leaves a signature in the finished metallurgy.

The common thread is that every step must be controlled by recorded parameters, because tungsten gives no visible warning before it fails. A crucible with 3% residual porosity, a 40 ppm oxygen content and a partly recrystallised wall looks identical to a sound one under good lighting. The process record is therefore part of the product, and a serious supplier will retain furnace charts, pressure logs and rolling schedules against the vessel serial number for the warranty period and beyond.

Step One: Powder Selection, Blending and Contamination Control

The starting point is tungsten powder, typically reduced from tungsten oxide in hydrogen and supplied at 99.95% W or better. Particle size is normally expressed as a Fisher sub-sieve size in the range of 2 to 5 micrometres, and the choice is a trade-off: finer powder sinters faster and reaches higher density, but it also adsorbs more oxygen and is harder to handle without dust loss and segregation. Coarser powder packs better in the die but needs a higher sintering temperature to close porosity, which drives grain growth and reduces toughness in the finished part.

The powder lot is characterised before use. Chemical analysis covers the interstitial elements carbon, oxygen and nitrogen, the metallic impurities iron, nickel, cobalt, aluminium, silicon and molybdenum, and the tungsten balance. Oxygen is the number to watch because it is both a quality indicator and a process variable: it depends on how the powder was stored and on how long it was exposed to humid air. Apparent density and tap density determine how much powder fills a given rubber bag, and hall flow or a simple sieve check catches agglomeration. Powder that has been stored in a humid warehouse will sinter to a lower density no matter how the furnace is programmed.

Blending and handling are the main sources of contamination. Sieving through stainless steel screens, mixing in stainless vessels and manual handling with bare hands all transfer iron, nickel and chromium. Production lines that hold low interstitial and low iron figures use tungsten or tungsten-lined contact surfaces, dedicated sieves, glove boxes or at minimum glove-and-mask discipline, and a magnet trap ahead of the press. Because these elements cannot be removed later, contamination control at this stage is the cheapest quality assurance in the entire process.

Step Two: Consolidation by Cold Isostatic Pressing

For crucible preforms the standard consolidation method is cold isostatic pressing. Powder is filled into a shaped rubber or polyurethane bag mounted on a steel mandrel, the bag is sealed and evacuated, and the assembly is immersed in a pressure vessel where hydraulic pressure compacts the powder uniformly from all directions. Applied pressure is typically in the 150 to 250 MPa range, and the uniformity is the reason the method is preferred over die pressing: a uniaxially pressed thick-walled cylinder has a density gradient from the punch face to the centre, and that gradient survives sintering as a dimensional distortion and as a zone of residual porosity.

Bag design is a craft in itself. The bag must be oversized to account for compaction shrinkage, which for tungsten is substantial, and the mandrel must be tapered or collapsible so the preform can be withdrawn without damage. Fill density must be even; a bag filled in one pour will show a density step at the fill line that later appears as a ring-shaped hardness variation. After pressing, the green preform is strong enough to be handled but fragile at the edges, and it is usually given a light machining operation to true the rim and the bore before sintering.

For thin-walled and shallow crucibles the alternative is die pressing or, for very small dishes, powder metallurgy pressing followed by sintering without machining. Stamped crucibles bypass powder entirely: they are deep drawn from tungsten sheet, with intermediate vacuum anneals between draws to restore ductility. Each anneal partially recrystallises the wall, so the number of draw-and-anneal cycles is a quality variable that should be fixed in the process sheet rather than left to the press operator.

Step Three: Sintering in Hydrogen and Densification

Sintering is the step that turns a chalky preform into metal. Tungsten is sintered in a hydrogen atmosphere furnace, usually with molybdenum or tungsten heating elements and shielding, at temperatures from about 2000 to 2300 degrees Celsius with holds of several hours. Hydrogen does two jobs: it reduces the residual oxide on the powder surfaces so that particle contacts become metallic rather than oxide-mediated, and it carries away the water vapour produced by that reduction. Dew point is therefore a critical control parameter, and a furnace with a wet atmosphere will produce a low-density, high-oxygen body no matter what the thermocouple reads.

Densification proceeds by neck growth between particles, closure of open porosity and finally elimination of closed pores. Commercial crucibles are normally delivered at 97 to 99% of the 19.3 g/cm3 theoretical density, with the higher figure needed for vacuum duty where internal porosity becomes a virtual leak. Where the highest density is required, sintering is followed by hot isostatic pressing at 1400 to 1700 degrees Celsius under 150 to 200 MPa of argon, which closes residual pores without the grain growth that a longer sinter cycle would cause.

The sintering cycle also sets grain size, and grain size sets toughness. A short, hot cycle gives a fine grain and good toughness but may not close porosity at the centre of a thick wall. A long cycle closes porosity but produces a coarse equiaxed structure with a high ductile-to-brittle transition temperature. Suppliers resolve this tension by tailoring the profile to the wall thickness, and the profile should be recorded and reported per lot rather than described as a generic "high temperature sinter".

Step Four: Hot Working, Forming and Stress Relief

Wrought tungsten is produced by rolling or forging sintered billet at elevated temperature, typically starting in the 1400 to 1600 degrees Celsius range and finishing lower as the section thins. Rolling breaks down the as-sintered grain structure, closes residual porosity and aligns the grain flow, producing plate with a density at or above 99% of theoretical and markedly better toughness than sintered product. Plate for crucibles is normally cross-rolled so that properties are reasonably isotropic in the plane, because a strongly directional plate produces a vessel that cracks along one preferred orientation.

Flow forming achieves a similar densification in a rotational geometry. A disc or a cup preform is clamped against a rotating mandrel and worked by rollers that thin the wall and lengthen it, with the workpiece held at a temperature high enough to keep the material ductile. The result is a shell with circumferential grain flow and good hoop strength, well matched to the hoop stress that a heated open-top crucible experiences. Wall thickness control in flow forming depends on roller path and mandrel geometry, and the achievable tolerance is wider than for machining, typically a few tenths of a millimetre on a 3 mm wall.

Every hot working operation leaves residual stress, and every machining operation adds more. Stress relief annealing in vacuum at 900 to 1100 degrees Celsius is applied after forming and again after rough machining, so that the final finish cut removes a stress-balanced layer rather than a stressed one. Skipping this step is a common cause of a vessel that changes shape during the first thermal cycle, or that develops cracks at a machined corner where tensile residual stress and thermal stress add together.

Step Five: Machining, Welding and Surface Finishing

Tungsten is machined with carbide tooling for roughing and with ceramic or polycrystalline diamond tooling for finishing, at low surface speed and with generous coolant or, for vacuum-grade parts, with no coolant at all so that no fluid is absorbed into surface porosity. The material behaves like grey cast iron in that it produces a segmented chip and a somewhat abrasive wear pattern, and it work-hardens at the surface if the tool rubs. A dull insert leaves a smeared, torn layer that later becomes a thermal fatigue initiation site, so tool change intervals and a final light finishing pass are process controls, not cosmetic choices.

Welded crucibles require a joint design that keeps the weld away from the highest-stress region, usually a machined lip and a matching base flange rather than a simple butt joint at the corner. Welding is done by electron beam in vacuum or by tungsten inert gas welding inside a chamber with an inert or reducing atmosphere, and the workpiece is held at elevated temperature to reduce the thermal gradient across the joint. Tungsten welds are prone to grain-boundary cracking when the joint is restrained, so pre-heat, a matched fixture that allows contraction, and a post-weld stress relief are standard. Reinforcement with molybdenum rod, either as an external cage or as a welded rim stiffener, is common on vessels above 300 mm diameter.

Finishing depends on duty. A bore that will contact a reactive melt is usually given a fine turned or ground finish so that it can be cleaned between campaigns. A vacuum-facing surface is degreased, pickled where the specification permits, and handled only with gloves. Edges are broken with a small radius rather than left sharp, because a sharp tungsten edge chips and a chip is a crack origin.

Process step	Typical equipment	Key parameters	What is controlled and recorded
Powder characterisation and blending	Sieving line, mixer, glove box	2 to 5 micrometre Fisher size, oxygen and iron limits	Lot chemistry, apparent density, contamination log
Cold isostatic pressing	Wet bag isostatic press	150 to 250 MPa, evacuated bag	Pressure curve, bag and mandrel identification
Green machining	Lathe with dust extraction	Rim and bore trued to sinter allowance	Green dimensions and weight
Sintering	Hydrogen atmosphere furnace	2000 to 2300 degrees Celsius, hold of several hours, low dew point	Full temperature and atmosphere chart, dew point, density result
Hot isostatic pressing	Argon HIP vessel	1400 to 1700 degrees Celsius at 150 to 200 MPa	Pressure and temperature record, density before and after
Rolling or flow forming	Reversing mill, flow forming lathe	Start around 1400 to 1600 degrees Celsius, cross-rolled plate	Reduction schedule, pass temperatures, grain flow direction
Stress relief	Vacuum annealing furnace	900 to 1100 degrees Celsius	Cycle chart per part, hardness check
Finish machining	Lathe with carbide and diamond tooling	Low surface speed, light final pass	Bore, wall, base, ovality, flatness report
Welding and finishing	Electron beam or inert chamber welding	Pre-heat, matched fixture, post-weld relief	Weld qualification, penetrant and leak test results

Step Six: Inspection, Defect Analysis and Release

Inspection is staged rather than final, because most defects are already irreversible by the time the part is finished. Powder chemistry is checked at receipt, green density and weight at pressing, density and hardness after sintering, dimensions after rough machining, and everything again at final. The classic release package for a vacuum-duty crucible contains chemical analysis traceable to the powder lot, a density figure from the immersion method, ultrasonic examination for internal voids and delamination, a dimensional report covering bore at three heights, wall thickness at eight positions, base thickness, ovality and flatness, a penetrant examination of welds, and a helium leak test.

Defect analysis is worth building into the process because the same visual symptom has different causes at different stages. A longitudinal crack that appears after sintering points to a pressing density gradient or an uneven fill. A crack that appears after machining points to residual stress or a rubbing tool. A crack that appears only after the first service cycle points to thermal shock or to a design that over-constrains expansion. Separating these by stage lets the supplier correct the process rather than simply replace the part.

Defect	Stage where it appears	Typical cause	Detection method	Process correction
Low sintered density	After sintering	Wet atmosphere, coarse powder, short hold	Immersion density, ultrasonic attenuation	Dew point control, powder specification, revised profile
Longitudinal crack	After sintering or machining	Fill density gradient, residual stress, dull tool	Penetrant, visual	Even bag fill, added stress relief, tool change interval
Ring-shaped hardness band	After sintering	Step in fill density at the fill line	Hardness traverse	Layered filling with vibration, bag redesign
Ovality out of tolerance	After finish machining	Asymmetric support in the furnace, stress imbalance	Three-point bore measurement	Furnace setter redesign, symmetric stock removal
Weld toe cracking	After welding or first cycle	Restrained joint, no pre-heat, no post-weld relief	Penetrant, leak test	Joint redesign, pre-heat, post-weld anneal
Virtual leak in vacuum	At leak test	Surface-connected porosity	Helium leak detector, rate-of-rise test	Higher density route, HIP, improved surface finish

Process Capability, Cost Drivers and What to Fix in the Purchase Order

The cost of a tungsten crucible is dominated by material recovery and by the number of high-temperature steps, not by machining time. A route that presses close to net shape wastes less expensive powder and needs fewer hours in the sintering furnace; a route that starts from rolled plate buys density and toughness but pays for the rolling and for the material machined away as swarf, which is recyclable but not free. Buyers comparing offers should therefore compare the route, the density, the number of thermal cycles and the inspection scope, not just the unit price per kilogram.

Lead time follows the same logic. Powder preparation and pressing are fast; sintering, HIP and any hot working are the bottlenecks, because furnace capacity is shared and cycles are long. A supplier who offers a short lead time on a thick-walled large-diameter vessel is either holding sintered billet in stock or proposing a route that skips a step. Both are legitimate, but the buyer should know which, since it determines what happens when a dimension changes after the order is placed.

The purchase order should fix the input standard for each stock form, the density and its measurement method, the grain-structure or hardness acceptance criterion if the duty is thermally cyclic, the dimensional tolerances including ovality and flatness, the non-destructive examination scope, the leak rate for vacuum service, and the documentation package. It should also fix the marking: a serial number stamped or engraved on the base, cross-referenced to the process record, so that a failure in service can be traced back to the actual furnace cycle that produced the part. That single requirement converts a commodity purchase into an auditable one.

FAQ

Q: Why is hydrogen used rather than vacuum for sintering tungsten crucibles?
Hydrogen reduces the oxide layer on the powder particles so that metallic contact forms between them, and it flushes away the water vapour that the reduction produces. Vacuum sintering of tungsten is possible but is far less effective at removing oxygen, and residual oxygen in the sintered body later causes porosity and outgassing in vacuum service.

Q: What density should a tungsten crucible reach before it is accepted?
The theoretical density of unalloyed tungsten is about 19.3 g/cm3, and commercial crucibles are normally delivered at 97 to 99% of that figure, which is roughly 18.7 to 19.1 g/cm3. Vacuum duty and evaporative duty justify the upper end of the range, and hot isostatic pressing is the usual way to get there without excessive grain growth.

Q: Can a tungsten crucible be repaired if it develops a crack?
Small, accessible cracks in a non-critical region can sometimes be ground out and blended, provided the remaining wall still meets the drawing limit. Cracked welds can be cut back and re-welded with the same qualified procedure. Cracks in the bore at the melt line are generally not repairable, because the repair itself introduces a new thermal stress concentration.

Q: How does flow forming differ from deep drawing for crucible shells?
Deep drawing stretches a sheet into a cup and thins the wall in a single or a few strokes, needing intermediate anneals. Flow forming progressively thins and extends the wall against a mandrel with rollers, producing circumferential grain flow and better hoop strength, and it can produce a thicker base than wall in one piece, which suits open-top melting vessels.

Q: What causes a tungsten crucible to change shape during its first production cycle?
The usual cause is residual stress left in the part by forming or rough machining and not relieved before the finish cut. When the vessel is heated, the stressed layer relaxes and the geometry moves. A vacuum stress relief at 900 to 1100 degrees Celsius between rough and finish machining, plus symmetric stock removal, prevents it.

Q: Which documents should accompany a finished tungsten crucible?
A certificate of analysis traceable to the powder lot, a density and hardness report, the sintering and any HIP cycle records, a dimensional report covering bore, wall, base, ovality and flatness, non-destructive examination reports for welds, a helium leak test result for vacuum duty, and a serial number that links the vessel to its full process history.