High-Temperature Deformation and Sagging of Molybdenum Electrodes: How to Handle It

Molybdenum electrodes bend after long service, a common sight in high-temperature furnaces. Slight sagging changes the electrode spacing and the current distribution; severe sagging can bring an electrode against the wall or onto a neighboring electrode and cause a short circuit. Deformation is not a sudden accident but the cumulative result of creep over time. Understanding its rhythm is the only way to judge clearly when an electrode can still be used and when it must be replaced.

For suspended electrodes, the sag curve and the life curve are often mirror images of each other. A high current density, insufficient cooling or an excessive span all make bending appear earlier; and once bending has formed, the local current density and temperature rise again, accelerating deformation further. Managing sag is essentially managing the product of temperature, load and time.

1. Why sagging is a creep problem

Molybdenum melts at about 2620 °C. When the service temperature reaches roughly 0.4 to 0.5 times the absolute melting temperature, the material enters the creep-sensitive range, which corresponds to about 1050 to 1300 °C. Glass melt temperatures usually fall within this range, and the hot end of the electrode sits in it for long periods. Grain boundaries then slide slowly and dislocations continue to climb, so deformation grows with time even at constant load.

The loads on an electrode are not of one kind. The suspended section carries its own weight in tension, the horizontal section carries a bending load, and an obliquely inserted electrode also takes lateral force from melt flow; once current is applied, Joule heating raises the bar temperature further and lowers the material strength. Creep deformation is fast at first, then enters a relatively stable stage, and finally accelerates again as the section thins and stress rises. The sudden bend seen on site is usually only the final accelerating stage; a long, slow accumulation has gone before it.

2. Four variables that drive the sagging rate

The sagging rate depends mainly on the actual hot-end temperature, the load carried per unit section, the bar diameter and the suspended span, and the metallurgical condition of the material. Of these four, temperature is the most sensitive: for every few tens of degrees of rise, the creep rate may change by a factor. Load and span are amplified through the bending moment: double the span, and the elastic deflection grows with the fourth power, with creep deflection magnified accordingly. A larger diameter raises the moment of inertia of the section but changes current distribution and heat dissipation. The metallurgical condition determines the creep resistance of the material at the same temperature.

A note on the applicable standards: electrode products are controlled to YS/T 1312-2019, Molybdenum Electrodes, for composition and tolerances; molybdenum rods follow ASTM B387 / GB/T 17792; molybdenum plate and disc follow ASTM B386 / GB/T 3876; special requirements are implemented under the technical agreement between both parties.

Material structure is often overlooked. Pure molybdenum recrystallizes during long high-temperature service, and creep resistance falls once the grains coarsen; at the same diameter and current density, a new electrode and one that has served two years may differ noticeably in sagging rate. TZM alloy raises the recrystallization temperature and high-temperature strength through solid-solution and dispersion strengthening, while lanthanum-doped molybdenum improves creep resistance through oxide dispersion, at the cost of a higher purchase price and greater processing difficulty. When selecting material, build the campaign length and the difficulty of replacement into the calculation rather than comparing unit price alone.

3. Design-stage calculation and allowances

Sag calculation can be split into an elastic deflection part and a creep addition. The elastic deflection can be estimated with the usual simply supported beam formula for a uniformly distributed load: it is proportional to the load and to the fourth power of the span, and inversely proportional to the elastic modulus and the moment of inertia of the section. The creep addition depends on temperature, time and the creep parameters of the material and is usually corrected with empirical coefficients or data from comparable duties. Design need not pursue an exact analytical solution, but both parts must be included rather than judging on room-temperature stiffness alone.

The control index commonly used in engineering practice is the length-to-diameter ratio of the suspended section. For conventional glass furnace electrodes this ratio should be kept within a certain range, and beyond it a mid-span support or guide should be added; the specific limit varies with electrode diameter, temperature, load and grade and is a reference range, to be confirmed by thermal calculation and the manufacturer's recommendation. The other index is the permissible deflection: back-calculate it from the lower limit of the clearance between the electrode and the furnace wall and the neighboring electrodes, reserve installation tolerance, thermal expansion and flow-induced swing, and what remains is the allowable amount of bending.

Design allowances also cover initial straightness and installation orientation. If the incoming electrode is slightly bent, install it with the bend facing the side where clearance is sufficient rather than forcibly straightening it; the support point should allow thermal expansion displacement, so that thermal stress is not converted into bending stress. For long-span electrodes, a guide ring or a high-temperature support may be placed at mid-span, but the support itself must not create a new current path or a local hot spot.

4. Coupling of current, electromagnetic force and flow field

Sagging is not only a mechanical problem. As current passes through an electrode, electromagnetic force arises between adjacent electrodes, and the larger the current and the smaller the spacing, the more pronounced the force; under AC supply the direction of the force changes with each current cycle, and long-term action causes fatigue and oscillation. Natural convection in the glass melt, bubbling and batch-charging impact also apply lateral load to the electrode. Individually none of these forces is large, but superimposed on high-temperature creep they change the direction of bending, turning a straight sag into an irregular bend.

Current density affects sag indirectly through temperature. A high current density raises the tip temperature and lowers creep resistance; local overheating also changes the viscosity and flow field of the glass, further intensifying lateral force. Conversely, sag reduces the electrode spacing, the local current density rises and the temperature climbs again, forming a positive feedback loop. This feedback is the common mechanism behind many sudden short-circuit events, and it is the reason bending trends must be identified as early as possible in operation.

5. From slight bending to short circuit: how the damage propagates

The harm from sag propagates in stages. Early on only the electrode spacing changes and the power distribution becomes slightly uneven; in the middle stage visible bending appears, local current density rises and abnormal foam or hot spots may show near the melt surface; in the late stage electrodes approach each other and may discharge, bridge or touch the wall, causing short circuits, refractory erosion and even furnace leakage. The table below matches typical symptoms at each stage to handling principles for use during inspection.

The point of the staging is to buy time. Most short circuits are not without warning: inspection records usually show current drift and changes in verticality long before, but these were not treated as a trend. Moving from a single measurement to a curve over time makes the judgment far more reliable.

6. Monitoring and early warning in service

Sag can be monitored from three angles: geometry, electrical data and thermal imaging. For geometry, measure the verticality of the exposed section or its distance from a reference point at intervals, using a plumb line, a laser or a fixed scale; what matters is that the measuring points are fixed and the records continuous. For electrical data, observe the current in constant-power mode: if the current keeps rising while power is unchanged, the section is usually thinning or the spacing has changed. For thermal imaging, use an infrared camera on the exposed and wall-penetrating sections; abnormal hot spots usually appear on the inner side of the bend or at a poor contact.

1. Give every electrode a position number and record the verticality datum at installation.
2. Measure the offset of the exposed section once a month, and compare trends quarterly rather than looking at single values.
3. Record current, voltage and cooling water temperature at the same power so that supply fluctuations do not distort the data.
4. When offset growth accelerates, check current density, melt level, batch recipe and support condition.
5. For electrodes approaching the clearance limit, define the trigger conditions for load reduction or replacement.

Warning thresholds should be tied to the clearance design. If the lower clearance limit between the electrode and the furnace wall is a given number of millimeters, then after deducting installation tolerance and thermal expansion the margin available for bending is fixed; setting half of that margin as the warning line and arranging an inspection once it is exceeded is a reasonably safe practice. Thresholds should not be copied from a neighboring plant, because furnace type, electrode spacing and electrode diameter differ, and so does the permissible deflection.

7. Handling principle: why cold straightening is not allowed

A molybdenum electrode that has already sagged must never be forced straight at room temperature. After high-temperature recrystallization, molybdenum loses much of its toughness and ductility at room temperature, and cold straightening introduces tensile stress on the outer side of the bend and initiates cracks at grain boundaries; even if the appearance is temporarily straight, the interior is already damaged and the electrode will easily fracture once it is heated again in the furnace. Straightening on site with a jack, a pry bar or a hammer may appear to save a spare part, but in reality it turns a controllable replacement into an uncontrollable broken electrode.

The safer approach is downgraded service or planned replacement. If the bend is still within the permissible range, its development can be slowed by lowering the current density, improving cooling and adjusting the support; if the bend already affects electrode spacing or approaches the clearance limit, the electrode should be scheduled for replacement at the next planned maintenance window. Electrodes that must keep running should be operated at reduced load with closer monitoring, and spares should be kept ready so that a shutdown is not forced under unprepared conditions. Hot straightening is theoretically feasible but requires strict control of temperature, atmosphere and applied force, which site conditions can rarely guarantee, so it should not be treated as a routine measure.

8. Prevention and replacement criteria

• At the design stage, control the length-to-diameter ratio and add mid-span support or a guide where necessary, with allowance for thermal expansion displacement.
• In operation, keep current density in the lower-middle part of the range and avoid supply above the upper limit for long periods.
• Stabilize melt level and batch recipe to reduce sudden temperature changes and flow-field disturbances.
• For long spans and high-temperature sections, specify creep-resistant grades such as TZM or lanthanum-doped molybdenum.
• At installation, let any initial bend face the side with the larger clearance; do not force it straight.
• Keep monthly records of verticality and current drift, and judge by trend rather than by a single point.
• When bending approaches the clearance limit, current rises abnormally or discharge marks appear, add the electrode to the replacement plan.

The replacement decision should not rest on how much the electrode has bent alone; the location of the bend, the section loss and the remaining campaign also matter. If the bend is near the melt line and accompanied by obvious thinning, replace the electrode first even if the deflection is small; if the bend is in a low-temperature section and the section is intact, continued observation with support adjustment is acceptable. Evaluating geometry, electrical data and the production plan together is the way to avoid the dilemma of replacing too early and wasting an asset or replacing too late and stopping the furnace.

Sagging is an inevitable consequence of high-temperature creep, but its rate can be managed. A reasonable current density, sufficient stiffness, effective cooling and continuous monitoring keep bending within an acceptable range; cold straightening, full-load operation with a known defect and ignoring trends turn predictable wear into a sudden failure.