Molybdenum Machining

Intoduction – Molybdenum the Element.

Carl Wilhelm Scheele was the first to identify Molybdenum in 1778, though the metal was know in prehistory. In ancient times, however, Molybdenum ores were often confused with lead ores. Hence the name which derives from an ancient Greek word for lead.

Molybdenum is mostly used in the making of high strength steel alloys, since it readily forms stable alloys and has one of the highest melting points of any element. It is not found in nature as a pure element rather it is found in various oxides.

The free element, which is a silvery metal with a gray cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys, and for this reason most of world production of the element (about 90%) is in making many types of steel alloys, including high strength alloys and superalloys.

Wrought Molybdenum metal possesses unique properties which make it useful in a number of manufacturing industries. Its most useful characteristic is its high melting point,

Molybdenum is a key element in certain biolgical processes and is a required element in all higher organisms.

Molybdenum Machining

The grain structure of the metal has a significant effect on how it can be machined. As sintered or crystallized the grain structure tends to be larger and the molybdenum can be machined similarly to cast iron. After it has been worked molybdenum is similar to stainless steel. However, it will tend to break on the edges if tools become dull, which can be an issue since molybdenum is very abrasive.

Molybdenum machines with the crumbling chip which is characteristic of hardened SAE 1040 steel. While it is possible to machine molybdenum with high speed steel tools, tungsten carbide tools are generally recommended for better tool life.

Turning and Millingr
For rough finishing Molybdenum can be machined at up to 200 feet per minute, using angled tools and using sulphur based cutting oil, or chlorinated oils if the application is for the semiconductor industry. For finishing work the metal can be machined up to 400 feet per minute, the depth of cut needs to be at least 0.005” to avoid excessive tool wear.
Since Molybdenum has a tendency to chip, it should be held tightly and tools should be rigidly supported with a little vibration as possible. Face milling requires the use of carbon tipped cutters and when edge milling it recommended that the molybdenum the clasped between steel plates while being machined to avoid chipping.
Drilling, Tapping and Threadingr
Carbide drills should be used with high speed drills and cutting oils should be used when drilling Molybdenum. A maximum of 60% of the drilled hole can be threaded if chipping is to be avoided.
Certain tapping processes require that the Molybdenum be heated to above 325 degrees Fahrenheit, however it should not be heated above 500 degrees unless in a protective, non oxidizing atmosphere.
Sawingr
Molybdenum saws easily with power bend saws and hacksaws. Approximately 1/8” should be allowed for kerfs end 3/16” for the camber of heavier sections. Molybdenum may also be abrasive saw cut.
Electric discharge machining (EDM) and Electrochemical machining (ECM)
EDM works quite well with Molybdenum as does ECM. With EDM having a stock removal rate of 0.5 inches per minute, and ECM capable of up 1 inch per minute. ECM is very useful where ultra-fine finishes are required.
Bendingr
Molybdenum can be bent up to 180 degrees at room temperature and can be formed into complex shapes if heated properly.
Weldingr
Pure molybdenum can be welded in a similar way to tungsten, provided this is done in a suitable atmosphere and it is chemically cleaned.

Applications for Molybdenum

Almost 90% of Molybdenum is used in alloys and compounds. In particular, it used to produce steel alloys and super alloys for use in the manufacture of military armor, aircraft parts, electrical contacts, industrial motors and electrical filaments. Many of these applications rely on the high melting point of Molybdenum, as well as its ability to withstand extreme temperatures without significant expansion or softening.

In steel alloys Molybdenum is used in percentages ranging from 0.25% to 8%, where its high corrosion resistance and weldability are assets. And it is sometimes used to replace Tungsten, because of its lower density and price.

Some Molybdenum superalloys are used highly corrosive environments.
Other applications for Molybdenum, include as a fertilizer, as X-ray source filaments, as a lubricant, as a semiconductor and a hydrocracking catalyst. In is also used in heating elements, as an adhesive, in soda glass and thin layer chromatography.

Conclusionr
Molybdenum is used in many industries; over 40,000 tonnes are used in steel alloy manufacture alone. The properties of Molybdenum that make it attractive include its high melting point, corrosion resistance, and the ability to withstand high temperatures without deforming.

Machining molybdenum requires a little care to avoid chipping, but is other wise straight forward.