Molybdenum Crucibles in Superalloy Melting: Observations from Service

Vacuum melting of nickel-base and cobalt-base superalloys demands more from a crucible than heat resistance. The melt contains reactive elements such as aluminium, titanium and chromium that react with the crucible material to varying degrees, while large charge lumps and long melting cycles add thermal shock on top of mechanical impact. How a molybdenum crucible performs under such conditions depends more on how it is used than on the material alone.

Actual Conditions in a Vacuum Induction Furnace

In an induction furnace the crucible is heated on the outside by the induction coil and holds the charge on the inside, so it sits in a temperature gradient with the outer wall hotter than the inner wall; in the early stage of melting this difference is the source of thermal stress. Under vacuum molybdenum evaporates slowly but is sensitive to oxygen and carbon, and oxides and residual oil brought in by the charge react with the crucible surface during heat-up.

The length of the melting cycle matters as well. A long cycle increases the time at high temperature, the grains grow and strength falls; a short cycle raises the frequency of thermal shock and the probability of crack initiation. A balance has to be found between the two according to furnace type and alloy grade.

Crucible shape also affects melt flow. Straight-walled and tapered crucibles produce different flow fields under induction stirring: in a straight-walled crucible the velocity near the wall is lower and heat is carried poorly to the centre, while in a tapered crucible the bottom corner must be prevented from becoming a stagnant zone. Differences in crucible configuration between furnace types are described under applications.

Interfacial Reaction Between Melt and Crucible

Melts containing titanium and aluminium readily form a reaction layer on the inner wall. Where the layer is uneven, demoulding stress is uneven too; where it is too thick, spalled material becomes a source of inclusions. Control measures include lowering the superheat, shortening the refining time and improving charge surface cleanliness.

The extent of reaction can be judged from changes in colour and roughness of the inner wall. A uniform dark grey is normal; local bright spots, pitting or an adherent layer that is hard to remove indicate heavier reaction, and the process parameters and charge condition should be reviewed rather than power simply increased.

When the proportion of return scrap in the charge rises, surface scale and inclusions increase and the reaction layer grows faster. Limiting the return ratio, or shot-blasting the returns, are common countermeasures.

Charging and Thermal Shock

The effect of charging practice on crucible life is often underestimated. Large lumps dropped straight in concentrate the impact at one point on the bottom and cause local dishing and microcracks. A sound practice distributes the charge along the wall, adds light material before heavy, and lowers heavy pieces slowly in a basket where necessary.

The crucible should be preheated before use so that a cold crucible does not meet hot charge directly. The preheat temperature need not be high; the aim is to equalise the body temperature and reduce the temperature difference at the moment of charging.

A few workable habits:

• Preheat the charge before charging to reduce moisture and residual oil;
• Avoid sharp lumps striking the inner wall and the bottom corner directly;
• Evacuate to the set value and hold before melting to reduce reactions with residual gas;
• Raise power in stages to give the crucible wall time to equalise.

Basis for Replacement

Whether a crucible can still be used is judged on three points: whether wall thinning exceeds the agreed proportion, whether cracks on the inner wall are interconnected, and whether the rim is visibly oval or sagging. Any one of them triggers replacement. Continuing on appearance alone lets risk accumulate until it is released in a single melt.

A replaced crucible need not be scrapped immediately; it can be sectioned to see whether the cracks started from the inner or the outer wall, giving a reference for the charging and power-ramp practice of the next crucible. Specifications and material options for a new crucible can be checked on the product specifications page.

On the matching of the crucible with other components in superalloy melting, the inspection and judgement methods are documented on the quality system page.