Pure moly, MoLa or TZM?

A heating band, a thin radiation shield and a loaded hearth rail face different demands. Molybdenum is an established hot-zone material; its chemistry and processing can be adjusted to suit each component.

These are selection starting points, not exclusive uses or a universal ranking. Solar Manufacturing documents the MoLa-element, MoLa-hot-face and TZM-hearth combination in its all-metal hot zones. That is evidence of established OEM use, not a life or capacity guarantee for another design. Solar Manufacturing · Hot zones

What does “pure moly” mean?

It means commercially pure molybdenum with controlled residual elements. Elmet’s linked pure-Mo sheet specifies 99.95% minimum Mo and individual impurity limits. It also states that its analysis is based on powder lots; finished-part chemical and mechanical tests are not routinely performed to the cited ASTM requirements. Request the certification your purchase actually needs. Elmet · Pure molybdenum material data sheet

Plansee lists at least 99.97% Mo on a metallic-purity basis excluding tungsten. The accounting basis matters: a larger headline percentage alone does not prove a cleaner or better furnace component. Compare the actual elements, limits and analysis basis on the current certificates. Plansee · Molybdenum: material properties and alloys

Why it works in a vacuum hot zone

Molybdenum combines a high melting point, low vapor pressure, relatively low thermal expansion and useful thermal conductivity. Its electrical resistance changes with temperature. Elmet’s reference data illustrate these properties and their temperature dependence. Elmet · Physical and mechanical properties of molybdenum

In an element, section and length must match the electrical design; conductivity alone does not determine heating performance. In shielding, layer arrangement, spacing, surface condition and attachment matter alongside the metal. A melting point is never the furnace’s allowable operating temperature.

Lanthanated moly: shape retention and handling after heat

MoLa contains deliberately dispersed lanthanum oxide, La₂O₃, within the molybdenum. Plansee describes a fibrous, layered structure after recrystallization that supports creep resistance and room-temperature ductility after hot service. Elmet also reports a higher recrystallization temperature and generally better post-recrystallization ductility than pure moly. Plansee · Molybdenum: material properties and alloys · Elmet · Lanthanated molybdenum

• Heating elements: resistance to time-dependent deformation is useful where a thin band must maintain its position between supports. Better retained ductility can also matter during maintenance. Supports, joints and local overheating still govern the assembly.
• Hot-face shielding: the thin inner layer must survive repeated heating and remain serviceable. MoLa is worth evaluating for this duty; Solar explicitly specifies it for the hot face. That does not mean every cooler shield layer requires the same grade. Solar Manufacturing · Hot zones

“Doped” does not identify the grade

Elmet also supplies potassium-doped HCT moly, so “doped moly” and “lanthanated moly” are not interchangeable names. Ask what the addition is. Elmet · Lanthanated molybdenum

Plansee lists ML with 0.3 wt% La₂O₃, and MLR/MLS with 0.7 wt% and different supplied microstructures. These are producer designations. Elmet’s MoLa sheet gives 0.7 wt% La₂O₃ nominal. Specify the producer’s grade, addition and condition instead of assuming every “MoLa” product is equivalent. Plansee · Molybdenum material definitions · Elmet · MoLa sheet products data sheet

The intentional oxide addition is part of the engineered material. Its lower Mo percentage does not make it inferior “dirty pure moly.” Likewise, an oxygen limit that includes oxygen in La₂O₃ cannot be compared directly with a pure-Mo oxygen limit without reading the basis. Elmet · MoLa sheet products data sheet

TZM: carrying the load while hot

TZM is titanium-zirconium-molybdenum with a controlled carbon addition. Elmet describes a nominal composition of 0.50% titanium, 0.08% zirconium and 0.03% carbon, with molybdenum making up the balance. Its rod sheet specifies ranges rather than those single nominal values. Elmet · TZM alloy · Elmet · TZM rod products data sheet

Solid-solution and fine-particle strengthening improve hot strength and creep resistance compared with pure moly. This makes TZM a useful candidate for hearth rails and posts subjected to sustained loads. Plansee · Molybdenum material definitions · Plansee · Molybdenum: material properties and alloys

“Stronger” does not establish a new load rating. Specify workload plus fixtures, support span, cross-section, temperature, hold time and acceptable long-term deflection. Ask for creep data for the supplied product condition. Room-temperature tensile strength alone cannot answer how a rail will behave after many loaded cycles.

Choose the grade and the finished part together

1. Duty: element, shield, fastener or structural support; normal and maximum component temperature, atmosphere, cycle and time at temperature.
2. Chemistry: exact producer/grade, purity basis, deliberate additions, impurity limits and required lot traceability.
3. Condition: sheet, rod or plate; thickness; rolling direction; stress-relieved or recrystallized supply; forming, joining and surface finish.
4. Performance evidence: resistance/geometry for elements, distortion and attachment details for shields, and creep/deflection under load for hearths.
5. Compatibility: process vapors, workpiece/fixture contact and maintenance history. Elmet’s screening table identifies oxidation and carbon-related reactions; an upgraded grade does not remove the need to evaluate the operating environment. Elmet · Chemical reactions of molybdenum

These are established commercial material families. The practical decision is which grade and construction address the actual failure mechanism or service requirement. A specific life extension or load increase needs evidence for that application.