Molybdenum Electrode Breakage: Cause Analysis and Prevention

A sudden drop to zero or a large fluctuation in furnace current during operation first makes the operator suspect the power supply or the control system. In an electric melting furnace, however, a good share of such events originates in electrode breakage. The impact of a broken electrode goes beyond the loss of power: the remnant may contaminate the melt, block the flow passage, and a short-circuit arc may burn through the refractory and cause leakage. Whether the response is appropriate depends on judging, within a short time, the nature of the fracture, the position of the remnant and whether a short circuit is present.

The fracture surface is the evidence with the most information. The fracture location, morphology, surface color and attached material narrow the cause down to one or two categories. Classify the fractures and prevention has a target; most incidents are not without traces, but the precursors never made it into the log sheet.

On-site diagnosis of a broken electrode: locate first, then act

After a current anomaly is found, it is not advisable to reclose immediately with power on. Three diagnostic steps should be completed first: confirm whether a single electrode has failed or the whole group has lost power, ruling out transformer, short-network and control system faults; observe the phenomena that may correspond to the fracture position, such as abnormal boiling at the melt surface, hot spots on the outer wall of the refractory in that area, or a sudden change in cooling-water return temperature; determine whether the remnant causes a pole-to-pole short circuit or a short circuit to the furnace shell, since short-circuit current will erode the refractory within a short time.

Only after confirming there is no short circuit and the melt is stable should it be considered whether the remaining electrodes can continue to run. If the remnant lies near the flow hole or the upwelling channel, it should be dealt with within the planned window even if the current is unaffected, because the remnant changes the local flow field and continues to dissolve.

Six causes and their fracture features

For standards, electrodes are controlled to YS/T 1312-2019, Molybdenum Electrodes; molybdenum rods follow ASTM B387 / GB/T 17792; molybdenum plate and disc follow ASTM B386 / GB/T 3876; acceptance is based on the quality certificate and measured records.

Composite fractures are common in practice: an oxidation groove thins the section first, and thermal stress triggers failure during heating up after an outage; or arcing at the clamp forms an erosion pit first, which then propagates under mechanical load. Diagnosis should identify the origin, not merely where the break is.

Thermal stress type: the cost of heating and cooling

The linear expansion coefficient of molybdenum is about 5.2×10⁻⁶/K, which differs from that of the refractory and the clamp. When temperature changes rapidly, radial and axial temperature gradients form inside the electrode, and the surface expands differently from the core, creating tensile stress. The through-wall section is constrained by the refractory at one end and exposed to the furnace at the other, so the constraint is asymmetric and this is where thermal stress concentrates. Near the melt line, the temperature above and below the surface also differs, so a gradient readily forms there too.

The typical trigger of this type of fracture is heating up after a power outage. During the outage the furnace temperature falls and the electrode and refractory contract; if the power is restored straight to the pre-outage level, the electrode surface heats quickly while the core is still cold, and thermal stress superimposed on assembly residual stress initiates a crack within hours. The practicable prevention is: restart after an outage from a low or medium level and ramp up according to the stages of the heating curve; avoid full-power operation before the melt level has recovered; and during planned shutdowns, cool slowly with the furnace rather than using forced ventilation for rapid cooling.

Another trigger is the first heating of a newly installed electrode. The cold clearance is calculated from the expansion difference; if the actual clearance is insufficient, the electrode is radially squeezed during heating, and the fracture surface shows indentations where it contacted the hole wall. Such fractures are easily misdiagnosed as material brittleness when the real cause is an assembly dimension problem.

Oxidation notching: chronic damage at the gas-melt interface

The electrode section above the melt oxidizes appreciably in air above 600 °C, and the molybdenum trioxide formed is volatile, so the surface thins continuously. When the melt level is stable, the thinning is relatively uniform along the height; when the level fluctuates, the interface band repeatedly experiences exposure and oxidation followed by immersion and dissolution, forming an annular groove. The diameter at the bottom of the groove decreases, the section modulus falls and the stress concentration factor rises, so fracture follows under thermal stress or mechanical load.

This type of fracture is fairly easy to recognize: an obvious annular depression is visible near the fracture, the surface carries grey-green, grey-white or layered spalling oxidation products, and the groove position usually corresponds to a common melt level height. During review, the level records should be retrieved to check whether the groove height matches the range of level fluctuation.

Prevention focuses not on the electrode itself but on the level and cooling: stabilize level control and reduce large fluctuations; ensure the flow and venting of the water-cooling jacket are normal and hold the gas-phase section temperature down to a range where oxidation is not significant; for furnaces that start and stop frequently, evaluate inert gas protection or an oxidation-resistant coating; and during maintenance, measure the diameter near the melt line and add any groove found to the replacement plan.

Mechanical load and electrical damage

Mechanical-load fractures are more common on side-entry electrodes with a long cantilever. The elastic modulus of molybdenum is about 315–330 GPa, so it is stiff with limited ductility, and the bending stress produced in the cantilever by its own weight, batch impact or manual handling cannot easily be released through deformation. Sagging against the wall, impact from batch material during charging, and loss of support after the clamp loosens all create stress concentration at the opening or the support point. Bending marks or indentations are often visible near the fracture, which is an important basis for distinguishing this type from thermal stress.

Electrical damage starts at the contact surface. When the clamp and electrode make poor contact, contact resistance rises, the local temperature climbs rapidly and arcing and melt pits appear; the pit acts as a notch and propagates into a crack during subsequent thermal cycles. The precursors of this fault are elevated clamp temperature, abnormal voltage jumps and discoloration of the contact face. Periodic re-tightening of the clamp, torque control and a clean contact face are low-cost preventive measures. Stray current from insulation failure also forms erosion pits on the electrode surface; these pits are mostly irregularly distributed and must be judged together with insulation test records.

Structural embrittlement and original defects

Pure molybdenum begins to recrystallize at 900–1100 °C, and glass furnace electrodes work at higher temperatures for long periods, so grains keep growing and room-temperature toughness and ductility fall. The fracture is intergranular with a pronounced granular appearance and almost no plastic deformation. Such fractures occur mostly during maintenance dismantling, furnace cleaning or abrupt temperature changes, and can also be triggered by thermal stress in normal operation. Dispersion-strengthened molybdenum delays grain growth but cannot prevent it completely, and dismantling must still treat the material as brittle.

Fractures of the original-defect type have their origin already present at delivery or installation: grinding cracks, machining damage at the thread root, internal porosity or lamination, and edge chipping from transport impact. The origin zone shows linear or point defects and the propagation zone is relatively smooth. Prevention relies on incoming inspection and pre-installation re-inspection: penetrant testing of ground surfaces, checking the thread profile for completeness, re-measuring the straightness of long electrodes, and ultrasonic spot checks on important batches.

Precursors of fracture and an inspection table

Putting the signals in the table into the daily inspection log costs little, and its value lies in turning the sudden into the predictable. A single signal is not conclusive, and a trend is more meaningful than an instantaneous value: a current that falls slowly for two consecutive weeks deserves investigation more than a single jump.

Response procedure after breakage

1. Reduce load or cut power immediately; confirm whether a single electrode has failed or the whole group has lost power, and rule out power supply and short-network faults.
2. Determine the position of the remnant: using the electrode insertion depth, the fracture height and the current signature, estimate whether the remnant remains in the hole, near the melt surface or has sunk to the bottom of the bath.
3. Determine whether a short circuit exists: observe voltage and current waveforms and hot spots on the furnace shell, and confirm with insulation testing if necessary.
4. Assess the state of the melt: take samples and observe bubbles, tinting and stones to judge the degree of contamination and whether the redox state needs adjustment.
5. Decide the response: with no short circuit and a stable melt, operation may continue until the planned window; with a short circuit or hot spots in the refractory, shut down for treatment as soon as possible.
6. Prepare for retrieval: dedicated hooks, high-temperature-resistant clamps, record forms and imaging equipment; define the working position and safety measures.
7. Analyze the remnant: measure length and diameter, photograph the fracture, and record its position and service duration for comparison with data from the same batch.
8. Check the batch: tighten inspection of other electrodes of the same batch and position, and replace them in advance if necessary.

A remnant that sinks to the bottom of the bath usually does not immediately degrade glass quality, but it continues to dissolve and changes the local flow field, and long residence at the bottom may form striations or stones. The retrieval decision must weigh the cost of a shutdown against the contamination risk, not be based only on whether it can be retrieved. Short-circuit arcing that erodes the refractory is the more urgent risk; once a hot spot is found on the furnace shell, the leakage risk should be addressed first.

Four-layer prevention system and case reviews

Prevention should be divided among four levels. The design level controls current density, support structure and cooling scheme, avoiding excessive cantilever length and local overheating; the procurement level keeps original defects out with non-destructive testing, metallography and dimensional acceptance; the installation level ensures alignment, clearance, torque and insulation; the operation level executes the heating curve, stabilizes the level and records precursor signals. If any one of the four is missing, the efforts of the other three are weakened.

Case 1: heating too fast after a power outage

A furnace lost power for several hours because of an external line fault, and after restoration the pre-outage power level was applied directly to catch up on output. Two days later an electrode broke in the through-wall section; the fracture was straight with no plastic deformation, and the position corresponded to a section with a large temperature gradient. The review confirmed that the restart had not followed the staged curve. The operating procedure was subsequently revised: after power restoration, run at a lower level first and load up gradually once the furnace temperature and melt level have recovered.

Case 2: notching from level fluctuation

Another case occurred in a furnace with intermittent charging. The level fluctuated periodically over a wide range, and an electrode broke at the melt line, with an obvious annular groove above the fracture and a grey-green oxide layer on the surface. After the charging rhythm was adjusted and a level interlock added, groove development on the new electrodes slowed noticeably. This case shows that the fracture position itself is a clue, and there is no need to wait for dismantling to determine the cause.

Case 3: arcing caused by a loose clamp

In the third case the origin was at the clamp. Hot re-tightening was not performed, the clamp contact pressure fell, arcing in service formed a melt pit, and two months later the electrode broke at the pit. Clear arc erosion spots were visible at the fracture origin. After hot re-tightening and infrared measurement were added to the monthly maintenance routine, similar failures decreased. Clamp problems are often mistaken for electrode material problems, yet temperature records can reveal them in advance.

The lessons of electrode breakage are mostly not new; what is often missing is turning known measures into a fixed routine. The diagnostic methods described here must be combined with the specific furnace type and duty, and parameters are subject to the contract.