Thermal Shock Resistance of Molybdenum Electrodes: Analysis and Improvement Measures

Furnace ignition and heat-up, an unexpected power failure, a sudden change in liquid level or reheating after maintenance all subject a molybdenum electrode to large temperature changes in a short time. Thermal shock resistance determines whether it can survive these moments instead of breaking suddenly on some night shift. Molybdenum conducts heat well and has a low expansion coefficient, so its inherent properties are not bad, but room-temperature brittleness, surface defects and high-temperature oxidation often cancel that advantage. Understanding where thermal shock comes from is the only way to turn improvement measures into design and operating practice.

Thermal shock damage is essentially constraint stress caused by a temperature gradient exceeding the load-bearing capacity of the material at that moment. Different parts of an electrode heat at different rates: the hot end expands more, the cold end less, and tensile and compressive stresses arise between them. When the stress at a crack tip reaches the critical value, the crack propagates and the electrode cracks or spalls. This process depends on material parameters as well as on geometry, heating rate and surface condition; it cannot simply be blamed on poor material.

1. The physical picture of thermal shock damage

The usual way to assess a material's thermal shock resistance is to look at the ratio of thermal stress to strength. Thermal stress is roughly proportional to elastic modulus, thermal expansion coefficient and temperature difference, and inversely proportional to thermal conductivity; the stronger and tougher the material, the larger the temperature difference it can withstand. Molybdenum's expansion coefficient is on the order of 5 × 10⁻⁶ per °C, and its thermal conductivity near room temperature is about 130-140 W/(m·K); both help to keep thermal stress low. But molybdenum's elastic modulus is high, so the stress generated by a given temperature difference is not small; low expansion and high conductivity merely move the starting line forward and do not mean it can be quenched or heated at will.

In practice, electrode failure is often not fracture after uniform heating of the whole body but cracking caused by local constraint. Near a water-cooling jacket, for example, the hot end is far hotter than the cold end and the axial temperature gradient concentrates in the through-wall section; likewise, when the liquid level drops, a previously immersed section is suddenly exposed to the furnace atmosphere, oxidizes rapidly and cools, and the outer layer contracts against the constraint of the inner layer, creating tensile stress. Such local gradients are more dangerous than an overall temperature difference because they act at stress concentrations such as section changes, thread roots or surface grooves.

2. Molybdenum's material baseline and two real weaknesses

Molybdenum is a body-centered cubic metal with a melting point of about 2,620°C; its high-temperature strength and creep resistance are well balanced among the refractory metals. It has a low thermal expansion coefficient and high thermal conductivity, so in theory it withstands rapid heating and cooling better than most ceramics. But two weaknesses must be faced: first, low-temperature brittleness, since pure molybdenum has limited toughness near room temperature, and a high impurity content or recrystallization raises the ductile-to-brittle transition temperature further; second, high-temperature oxidation, since molybdenum begins to oxidize noticeably in air above about 400-500°C and the oxides formed are volatile, leaving grooves and thinning on the surface.

These two weaknesses amplify each other. Surface oxidation grooves reduce the effective section and create stress concentrations; when thermal shock arrives, cracks initiate at the grooves, and low-temperature brittleness denies them plastic blunting, so they propagate faster. Recrystallization is another variable: an electrode serving for a long time at high temperature grows its grains, and room-temperature toughness and thermal shock resistance decline. The same heat-up curve may therefore be survivable for a new electrode but not for an electrode in mid-life, so reheating strategies must differ.

3. Typical scenarios of thermal shock cracking

Cracking on site rarely happens during steady operation; it occurs far more often in the windows when conditions change. The table below maps common scenarios, shock sources and high-risk locations so that each can be covered item by item in the operating procedures. The contents are empirical summaries; specific temperature limits should be determined from electrode size, atmosphere and structure.

Inspection and acceptance criteria for the above items: electrode products are controlled to YS/T 1312-2019, Molybdenum Electrodes; molybdenum rods follow ASTM B387 / GB/T 17792; molybdenum plate and discs follow ASTM B386 / GB/T 3876. A quality certificate accompanies each delivery, and batches are traceable.

The table shows that most scenarios are not unpredictable; the problem is that operating procedures do not treat rate of temperature change as an independent control variable. Many furnaces specify a total heat-up time but no staged rates; they have a hold-temperature contingency for power failures but no rule on when protective gas is introduced and when it stops. Closing these gaps will clearly reduce thermal shock risk.

4. Setting heat-up and cool-down curves

The core of a heat-up curve is to keep the electrode temperature gradient within what the material can withstand at all times. In the low-temperature range the material has low toughness, so heating should be slow; in the mid-temperature range oxidation risk rises and atmosphere control is needed; in the high-temperature range thermal stress is relatively controllable, but local overheating must still be avoided. The table below gives an adjustable reference framework of typical values; actual rates should be determined from electrode diameter, furnace wall structure, cooling conditions and the manufacturer's installation manual.

A curve is not a document that ends when it is written; it must be used together with measurement. Thermocouples should be placed on the exposed section, the through-wall section and the cooling jacket return line, with readings logged at fixed intervals during heat-up; if one section rises noticeably faster than the set value, pause or slow the ramp. Judging electrode temperature from the furnace thermocouple alone is not enough, because the electrode generates its own heat and local temperature may exceed the furnace atmosphere. New and old electrodes should use different curves, and the difference should be written into the maintenance procedure.

Cool-down also needs management. If the protective gas is shut off and the furnace door opened immediately after shutdown, incoming cold air quenches the glowing electrode surface, the outer layer contracts against the inner layer, and cracks readily form. A safer sequence is to reduce power and then disconnect in stages, maintain protective gas until the electrode temperature is below the oxidation threshold, and only then ventilate slowly; avoid forced air cooling in the high-temperature range and, if necessary, wrap the exposed section in insulation.

5. Oxidation grooves and atmosphere limits

Oxidation of molybdenum in oxygen-containing atmospheres is an important precursor to thermal shock cracks. Above about 400-500°C, oxides formed on the surface volatilize, leaving pits and circumferential grooves; the higher the temperature and the faster the gas flow, the faster the grooves develop. The reason the liquid level line is a weak zone is precisely that it combines high temperature, furnace gas scouring and liquid level fluctuation. The way to improve it is not to repair grooves after they appear but to keep the temperature and atmosphere in this section below the oxidation threshold, or to isolate it structurally.

Protective atmosphere selection must fit the actual furnace. Nitrogen and argon are commonly used to exclude air; hydrogen is reducing but demands tighter control of dew point, purity and safety interlocks. When hydrogen is used, the dew point should be kept at a low level, the piping must be protected against flashback, and the gas shut-off sequence must match the cool-down sequence. Whatever the gas, the key is to keep the electrode surface under positive pressure or effectively covered, and to prevent air ingress caused by opening the furnace door, aspiration through sight holes and seal failure. In batch-operated furnaces, gas flow during shutdown is often more easily overlooked than during operation.

Vacuum or low-pressure environments also need attention. Oxide volatilization increases at low pressure, and if the atmosphere contains traces of oxygen or moisture the electrode surface will still thin slowly. For such duties, confirm the electrode has cooled to a safe temperature before evacuation, and keep it dry or under inert gas during shutdown. Treating atmosphere as a concern only during operation is a management gap behind repeated thermal shock and oxidation incidents.

6. Influence of structure and water-cooling design

Structural design can actively reduce thermal shock stress. Electrode sections should avoid abrupt changes, with rounded or tapered transitions at diameter changes; thread roots need a sufficient fillet radius to reduce stress concentration; the through-wall section can be fitted with a heat-insulating sleeve or transition sleeve to spread the temperature drop over a longer distance rather than concentrating it in a short segment. For suspended electrodes, the support point should allow some thermal expansion displacement, avoiding additional thermal stress from over-constraint.

The water-cooling jacket shapes the temperature gradient and is also a source of thermal shock risk. In normal operation it removes axial heat leakage and protects the clamp and seals; once it leaks, cooling water contacts the glowing electrode, local quenching directly induces cracks, and steam accelerates oxidation. The design should ensure reliable sealing and sufficient creepage distance between the water path and the electrode, with a drain at the low point of the jacket so leaks are easy to find; operation should monitor flow, return temperature and pressure, and on an abnormality reduce power first and then investigate rather than watching at full load.

7. Operating checklist for shutdown, reheating and maintenance

1. Before shutdown, record the current, temperature and appearance of each electrode, and mark any existing cracks or grooves.
2. Reduce power on the curve; avoid a sudden power cut that causes an abrupt temperature change in the melt and the electrode.
3. Maintain protective gas until the electrode temperature falls below the oxidation threshold; confirm gas flow and purity are normal.
4. Keep the furnace dry during shutdown, preventing rain, condensate or cooling water from entering the chamber.
5. Before reheating, inspect the electrode surface, threads, welds and cooling jacket, and confirm there are no new cracks, leaks or blockages.
6. During reheating, ramp at a reduced rate on the aged-electrode curve, preheat at low power, then load in stages.
7. During reheating, measure temperature more frequently; if a temperature difference or current anomaly appears, pause immediately and resume only after the cause is found.

The value of this checklist is that it turns experience into steps that can be handed over. Staff turnover, night-shift operation and outsourced work are all high-risk points for thermal shock incidents; lists, records and signatures are what ensure key actions are not omitted. New employees should also be told clearly why rapid cooling is not allowed and why the furnace door cannot be opened immediately after a power failure, rather than simply told to comply.

8. Inspection and acceptance of incoming and in-service defects

Thermal shock resistance depends not only on operation but also on the condition of the incoming electrode. Before installation, check the surface for grinding cracks, chipped edges, laps and oxide scale, the thread roots for machining marks, and the weld areas for porosity or undercut. For critical electrodes, penetrant, ultrasonic or radiographic inspection can be added; for slender bars, straightness should also be checked to avoid residual stress from forced correction during installation.

• Cracks on the surface, longitudinal or circumferential, deeper than the permitted limit: do not install.
• Continuous circumferential grooves near the liquid level line with obvious section thinning: assess for downgrading or replacement.
• Cracks at the thread root or loosening of the thread engagement: stop using it; do not force it tight with higher torque.
• Visible defects in welds or the heat-affected zone: handle to the weld acceptance standard; do not run with a defect.
• New cracks after shutdown not re-checked before reheating: do not heat up on the original curve.
• Electrode clearly recrystallized with lower room-temperature toughness: reduce the reheating curve rate and replace if necessary.

There is no single-point solution to thermal shock management. The material provides the baseline, the structure distributes stress, the atmosphere suppresses oxidation, operation controls the rate, and inspection screens out damaged parts; all five are required. Treating thermal shock resistance as a material property easily leads to neglect of the operating window; treating it as a system capability is what leads to the right decisions at the critical moments of ignition, power loss and reheating.