Polished Molybdenum Rod: Grades, Surface Finishes and Tolerances
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What Polishing Changes on a Molybdenum Rod
Polished molybdenum rod is produced by removing material from the surface in a controlled sequence so that surface roughness and dimensional error both fall. Depending on the route, material is removed mechanically, chemically or electrochemically; the objective is the same, a bright, smooth, oxide-free surface with a uniform diameter. The benefit is practical rather than cosmetic. A smooth surface reduces friction and galling during feeding into a machining centre, removes the brittle oxide scale that can spall into a vacuum furnace, and gives a defined surface condition for subsequent coating or joining operations.
Product Families and Chemistry
Molybdenum rod is supplied in three material families, each aimed at a different service temperature. Composition ranges quoted below are the ranges normally agreed in purchase orders for these families.
| Family | Alloying additions | Typical service |
|---|---|---|
| Pure molybdenum | Mo 99.95% minimum | Furnace parts, electrodes, sputtering targets |
| Lanthanum-doped molybdenum (Mo-La) | La2O3 0.4-1.2% | High-temperature racks, heating elements |
| TZM | Nominally 0.5% Ti, 0.08% Zr, 0.02% C | Forging dies, hot-work tooling, high-temperature structural parts |
ASTM B387/B387M covers molybdenum and molybdenum alloy bar, rod and wire, while ASTM B386/B386M covers plate, sheet and strip. The two documents should be quoted together when a project uses both rod and sheet, because the mechanical property requirements and the sampling frequency differ between product forms.
Surface Condition Classes
Rod is classified by the finishing operation, and the class determines whether the customer must machine the surface before use.
| Class | Surface appearance | Practical consequence |
|---|---|---|
| Sintered | Silver-grey metallic lustre | Porosity may be present; normally forged or rolled before critical use |
| Forged | Dark oxide layer | Scale must be removed before vacuum service |
| Turned or ground | Metallic lustre, visible tool marks | Dimensional error reduced, roughness still measurable |
| Polished | Bright, uniform, low roughness | Ready for direct machining and clean assembly |
Dimensions, Density and Tolerances
Molybdenum has a density of approximately 10.2 g/cm3 and a melting point of 2,623 C, which makes it a candidate where tungsten would be unnecessarily heavy and where steel would soften. Commercial rod diameters run from about 0.4 mm to 100 mm. For ground and polished rod, a diameter tolerance of plus or minus 0.05 mm is achievable on small and medium sizes, and straightness is controlled by a maximum bow per unit length. Because molybdenum is brittle at room temperature after recrystallisation, rod is normally supplied in the stress-relieved condition and handled above a minimum temperature during forming.
Applications and Selection Logic
Polished rod is used for electrodes and electrode holders in glass melting and resistance heating, for sputtering targets and target backing, for high-temperature fasteners and supports in vacuum furnaces, and for machined components in the electronics and semiconductor sectors. The choice between pure molybdenum, Mo-La and TZM is made on the basis of recrystallisation temperature and required creep resistance: lanthanum doping raises the recrystallisation temperature and keeps a fine grain structure, and TZM offers the best strength at temperature where the component sees structural load.
Quality Control of a Polished Surface
Inspection covers chemistry, density, hardness, diameter and length, straightness, surface roughness and visual freedom from laps, cracks and inclusions. Roughness is measured on the finished surface, and results are recorded as an Ra value or as a comparator sample agreed with the customer. Where rod is intended for a vacuum or semiconductor environment, the surface is additionally checked for residual contamination after polishing.
Frequently Asked Questions
Q: Why specify polished rod instead of turned rod?
A: Polished rod gives a lower roughness value and a tighter diameter tolerance, which matters when the part is fed automatically, when it slides in a close-fitting holder, or when loose oxide could contaminate a vacuum process.
Q: What density should be expected?
A: Around 10.2 g/cm3 for fully dense wrought molybdenum. Sintered stock may measure lower because of residual porosity, so density is a useful indicator of whether the rod has been properly worked.
Q: Is Mo-La rod used for the same parts as TZM?
A: They overlap, but the logic differs. Mo-La is chosen to keep a stable fine grain structure at high temperature, while TZM is chosen where higher strength and creep resistance are needed under structural load.
Q: How should polished rod be handled after delivery?
A: Keep it dry, avoid contact with chloride-bearing materials, and support it over its full length to prevent bowing. Handling at room temperature is acceptable for stress-relieved rod, but recrystallised material should not be bent cold.
Q: Can polished rod be supplied to a customer drawing?
A: Yes. Machined diameters, lengths, chamfers and end conditions can be produced to drawing, with the diameter tolerance confirmed against the drawing rather than against a general catalogue figure.
