Metal Story: Molybdenum (Mo) – Iron’s Ally
In order to make a tasty dish the cook adds different spices to it. In order to give steel valuable properties, the steelmaker uses various alloying elements.
Every spice serves a specific purpose. One improves the taste of the dish, another makes it fragrant and appetizing, a third makes it piquant, etc. It is hard to describe everything that spices do. But it is even harder to enumerate all the wonderful qualities that steel acquires when chromium, titanium, nickel, tungsten, molybdenum, vanadium, zirconium and other elements are introduced in it.
This chapter deals with molybdenum, one of the loyal allies of iron.
Molybdenum was discovered by the Swedish chemist, Karl Wilhelm Scheele, in 1778. That it was given a Greek name (from “molybdos”) was small wonder; many chemists had taken a look into the Greek dictionary before naming a new element. What was amazing was that “molybdos” meant “lead”. Why did molybdenum go by an assumed name? Why did it borrow it from lead?
The matter had a simple explanation, though. Ancient Greeks knew a mineral of lead, galenite, which they called “molybdena”. But there also existed another mineral, molybdenite, which was a “splitting image” of galenite. This resemblance confused the Greeks and they believed they were dealing with one and the same mineral, molybdena. Chemists in other countries shared this opinion. Therefore, when Scheele discovered an unknown element in that mineral he did not hesitate to call it molybdenum.
In 1783 the Swedish chemist P. H. Hjelm succeeded in extracting the element in the form of metallic powder, but it was contaminated with carbides. It was a whole century later that it had become possible to obtain pure molybdenum.
Like many of its “fellows” in the Periodic Table, molybdenum cannot stand any impurities and “protests” against them by drastically changing its properties. Thousandths or even tenthousandths of one per cent of oxygen or nitrogen make it very brittle. This is why many chemical handbooks printed at the beginning of the 20th century asserted that it was practically impossible to subject molybdenum to mechanical treatment. But in actual fact pure molybdenum is fairly ductile despite its great hardness, and is comparatively easily rolled and forged.
The beginning of molybdenum’s “service record” dates back several centuries when the mineral molybdenite was used to make slate-pencils (curiously, even today a pencil in Greek is called “molybdos”). Like graphite, molybdenum consists of myriads of tiny flakes and their size is so infinitesimal that if 1 600 of them are piled one on top of the other, they will make a 1 600-storey skyscraper … one micron high. It is precisely owing to these flakes that molybdenite “can” write and draw, leaving a greenish-greyish trace on paper.
In our day one will hardly come across molybdenite slates— the pencil industry has fully become the domain of graphite. But molybdenite has found another application. Before going into that, though, we would like to tell you about an incident that took place several years ago.
A team of Zaporozhets automobiles were being tested on the Semfiropol highway. Everything was going fine until suddenly one of the automobiles turned over at full speed, on an absolutely level stretch. Fortunately its occupants escaped unharmed. Nothing could be learned about the cause of the accident until the automobile was taken to pieces. And then it was found that one of the transmission gears which should have been freely rotating on a steel sleeve had become welded fast to it. Naturally that new “brake” acted immediately.
To prevent such accidents happening in future it was necessary to find an appropriate lubricant. This is where the ability of molybdenite to flake off came in handy. The flakes were to make a reliable lubricant on the rubbing components of the gear-box.
It is enough to dip a steel component in a solution containing only 2 per cent of molybdenite for its surface to become covered with a thin layer of an excellent solid lubricant. But this lubricant has a bad enemy — high temperature. When heated molybdenum disulphide (molybdenite) turns into molybdenic anhydride which, while being harmless to the components, does not have any lubricating ability, either. What next?
It was found that before covering the part with the disulphide it was necessary to treat it in a hot phosphate bath. In this case particles of disulphide penetrate the pores of the phosphate coating and a very thin lubricating film is formed on the surface of the component. This film is capable of withstanding colossal loads several tons per sq cm. Bushings coated with this lubricant were tested under most strenuous operational conditions and there was not a single case of welding. Since then the Zaporozhets cars have been crossing the country in all directions and back but not a single time has the notorious gear-box let the drivers down.
The development of the lubricating film is not the only good thing that molybdenite can do for a steel surface. For example, if a cutting tool is treated by molybdenite, it will become stronger and more durable. This marvellous property was immediately made use of by hairdressers. But back to molybdenum.
Owing to its high melting point and low expansion coefficient molybdenum is widely used in electrical engineering, radioelectronics and high-temperature engineering. The tungsten filament holders in a common electric lamp are made from molybdenum, just as many components of electron and X-ray tubes. Molybdenum coils heat powerful vacuum resistance furnaces where very high temperatures are developed.
Extremely valuable composite materials have been developed in the Institute of Problems of Materials Study under the Ukrainian Academy of Sciences. Ductile metals, such as aluminium, copper, nickel, cobalt or titanium are taken as the base material while the high-strength metals like tungsten and molybdenum are used as reinforcement fibres taking upon themselves most of the tensile load. The strength of tungsten- or molybdenum- reinforced nickel and cobalt increases almost threefold. Titanium reinforced by molybdenum is twice as strong as common titanium.
Several years ago American researchers developed an unusual type of glass: it changed its colour depending on the time of day, turning blue in sunlight and becoming transparent at night. This effect was produced by additions of molybdenum either in molten glass or by glueing a thin transparent film in between two layers of glass.
Molybdenum compounds have many uses: they increase the covering power of enamels; molybdenum dyes are used in ceramics and plastics industries, as well as in tanning and in the fur and textile industries; molybdenum trioxide is applied as a catalyst in the cracking of oil and in other chemical processes.
As you see, molybdenum has plenty to do. But we have not said a single word about its main job yet — all that has been discussed is its side-lines, so to say. At the beginning of this story we mentioned molybdenum as a loyal ally of iron. This is the aspect we would like to take up now, as more than 90 per cent of molybdenum produced in the world is consumed by the special steel industry. In Russia the first molybdenum-containing steel was smelted in 1886 at the Putilov Plant in St. Petersburg. But it must be said that the use of this element as a means of improving the quality of steel has a much longer history.
For a long time futile attempts had been made to unravel the secret of the extraordinary sharpness of Samurai swords. Generation after generation of metallurgists failed in all attempts to make steel which would be like that used in olden times to make side-arms in the Land of the Rising Sun. The first successful attempts were made by the great Russian metallurgist Pavel Anosov (1799-1851). It was established that the mysterious steel contained molybdenum which simultaneously improved the metal’s hardness and ductility, even though an increase in hardness, as was commonly believed, usually resulted in an increase in brittleness.
The combination of hardness and ductility is essential for armour steel. The armour on the first British and French tanks that appeared on the battlefields of the First World War in 1916 was manufactured from hard but brittle manganous steel. Alas, German shells went through those massive 75-mm-thick shields as easily as through butter. But it was enough to add only 1.5-2 per cent of molybdenum to make the tanks impregnable, despite the fact that the thickness of the armour sheet had been reduced to a third of what it had been before.
How was that magic transformation to be explained? The point is that molybdenum retards grain growth in the proress of crystallization of steel, hence, makes it fine-grained and homogeneous, all of which ensures the metal’s fine quality. Most alloy steels suffer from tempering brittleness. But molybdenum-containing steels do not fear this “complaint”, owing to which they can be subjected to heat treatment without any danger that internal stresses may develop. Molybdenum considerably increases steel hardenability. Such steel is also characterized by considerable strength at high temperatures and high creep resistance. Tungsten affects steel in much the same way, but molybdenum’s influence on the strength of steel is much greater: 0.3 per cent of molybdenum can replace 1 per cent of tungsten — a more expensive metal.
However, armour is not the only application of molybdenum steel. Cast from this steel are gun barrels, aircraft and automobile components, steam engines, turbines, cutting tools and razor blades. Molybdenum also improves the quality of cast iron by increasing its strength and wear resistance.
The high alloying capacity of molybdenum is explained by the fact that its crystalline lattice is identical with that of iron; the radii of their atoms are also roughly similar. And “kindred souls” can always find a common tongue. But molybdenum is close not only to iron. Its alloys with chromium, cobalt and nickel are characterized by a high resistance to acids and are used in the manufacture of chemical apparatus. Some alloys of the same elements have a high resistance to rubbing. Alloys of molybdenum with tungsten can replace platinum; alloys with copper and silver go into the manufacture of electric contacts.
Liquefied gases especially nitrogen, are widely used in refrigerating engineering. Terrible frost — nearly 200°C below zero — is needed in order to keep it in its liquid state. Under such a temperature ordinary steel becomes as fragile as glass. Containers for the storage of liquid nitrogen are made from a special cold-resistant steel. However, this steel too had one drawback which it had proved impossible to eliminate for a long time. This drawback was that its welds were not strong enough until molybdenum was added to the metal. Formerly the additions used during welding included chromium which, as it was found, caused the welds to crack. Research showed that molybdenum prevented cracking. As a result of numerous experiments it was finally established that the addition of 20 per cent of molybdenum was an optimal addition, enabling the welds to withstand minus 200°C frost as easily as the steel itself.
Not long ago metallurgists developed a new remarkable alloy, comochrome, from cobalt, molybdenum and chromium, which proved to.be an excellent material for human “spare parts”. It is absolutely harmless for the organism and is willingly applied by surgeons when damaged joints have to be replaced.
Agriculture is another field where molybdenum has found application. In 1965 a group of Soviet researchers was awarded the Lenin Prize for a study of the biological role of trace elements and their use in agriculture. Introduced in microscopic doses in soil or in animal feeds some elements literally work magic. Molybdenum is one of these miracle workers. Infinitesimal quantities of this element substantially increase the yield of many crops and improve their brands. Leguminous crops are particularly strongly influenced by molybdenum. The yield from peas processed by ammonium molybdate is 30 per cent greater than usual. Molybdenum concentrations that are formed in the plants’ tubers improve assimilation of nitrogen from the atmosphere— a process absolutely essential for plant development. Molybdenum is conducive to increase in the content of proteins, chlorophyl and vitamins in plant tissues. It is an interesting fact that this element is pernicious for some weeds.
Extraordinary research was carried out at Osaka University in Japan. Scientists there analyzed, by means of most sophisticated equipment, some remains of burnt human hair and arrived at the conclusion that its colour depended on the presence of traces of certain metals. For example, fair hair was rich in nickel and golden in titanium. If redheads feel like complaining about their hair, they must blame molybdenum — it is precisely this element, according to the Japanese researchers, that gives the hair its red colour. There is every reason to suppose, therefore, that had there really existed a “redheaded league”, so expertly dealt with by Sherlock Holmes, the symbol of molybdenum should have adorned its emblem.
Unfortunately, this element sometimes becomes involved in affairs that can never be regarded as beneficial to mankind. The “negative” aspects of molybdenum were revealed by Soviet scientists after they had returned from a lengthy marine expedition.
The expedition on board the Mikhail Lomonosov set out from Vladivostok at the end of 1966. Its task was to study the level of radioactive contamination in various sections of the World Ocean. Month after month the boat was plying water expanses and all the time the sensitive Geiger counters were “doing duty” on its board. They were like frontier guards ready to intercept radioactive “guests” any moment.
One day the boat was preparing to cross the equator at one of the most desolate regions of the Pacific Ocean. Day and night the blades of the ventilator on board rotated with great speed and directed thousands of cubic metres of sea air to the filters capable of catching dust particles as small as a few hundredths of one micron. At certain intervals the filters were burned together with the dust they had collected. The radioactivity of the ash was determined by means of highly sensitive instruments. Suddenly the Geiger counters registered unusual “excitement”: the ashes contained the radioactive isotopes molybdenum-99 and neodymium- 147. These isotopes have a very short life (the half-life of the molybdenum-99 is only 67 hours). The scientists’ measurements and calculations revealed that the “unwelcome guests” were produced on December 28, 1966. Indeed, as the New China News Agency reported, China carried out a nuclear test on that day, and within a few days the radioactive particles had been blown over an area of thousands of miles around.
But it must be noted for the sake of justice that molybdenum plays a very modest role in this dangerous game. We hope that nuclear tests are going to be banned completely, and then molybdenum will be seen no more in this unseemly role but will be engaged strictly in serving humane purposes. And you have already seen that mankind does need molybdenum for many purposes and in large amounts. What are the reserves of this element on the planet?
The share of molybdenum in the earth’s crust is 0.0003 per cent. In the extent of its natural occurrence it holds a modest place in the Mendeleyev Table — in the fourth dozen. But still molybdenum deposits are found in many parts of the world.
While at the beginning of this century molybdenum production was only a few tons, it went up almost 50 times already during the First World War (armour was in great need!). Afterwards the output of molybdenum ores dropped sharply but around 1925 a fresh rise in production was registered. By 1943 (i. e. during the Second World War) it reached Us maximum — 30000 tons. Hence, the name “war metal” as this metal is sometimes referred to.
In the Soviet Union a large deposit of molybdenite was discovered in 1934 by Vera Flerova, a geology student, in the Baksan river canyon in North Caucasus. That was a remarkable event in the history of the national rare metals industry. Two years later a mine was already under construction there. Unfortunately, Vera Flerova was not fated to see the town of Tyrnyauz which owed its birth to her, springing up high in the mountains, on the spot she had discovered. Driven by her constant thirst for exploration, this courageous young woman had a fatal incident in the mountains, in 1936. One of the squares in Tyrnyauz and a mountain peak bear her name today. And away from the busy thoroughfares, on a lonely mountain slope there stands a modest obelisk in commemoration of Vera Flerova. A little away from it and above, trolleys carrying the wonderful molybdenite ore are slowly moving along steel wire ropes, as though symbolizing her feat.
Molybdenum ores are for the most part processed into ferromolybdenum used in the metallurgy of high-grade steels and special alloys. The first industrial attempts to produce ferromolybdenum were made at the end of the 19th century. In 1890 a process whereby this alloy was made by reducing molybdenum oxides was developed. But that was all that was done in the way of ferromolybdenum production in pre-revolutionary Russia. In 1929 S.S. Shteinberg and P. S. Kusakin obtained an alloy containing 50-65 per cent of molybdenum by a silico-thermal process. The successful experiments carried out by V. P. Yelyutin in 1930-31 made it possible later to introduce this process in metallurgy.
However, thechnology needs pure molybdenum, as well. But it was a long time before articles from pure molybdenum could be made. Why? Wasn’t the method of producing comparatively pure molybdenum powder known since long in the past? The reason was molybdenum? high melting point which prevented metallurgists from transforming the powder into solid metal by smelting. Other ways had to be found. In 1907 the first molybdenum filament was produced in laboratory conditions. For that molybdenum powder was mixed with a sticky organic substance and drawn through a die. The sticky thread which the experimenters now had was placed in a hydrogen armosphere and current was passed through it. As could only be expected, the thread warmed up and the organic substance burned away, while the metal particles melted into a thread (hydrogen was needed to prevent molybdenum from oxidation).
Three years later a patent was issued on the production of high-melting metals in the process of powder metallurgy which is still in use today. Metallic powder is pressed, melted, then rolled or drawn and the strip or wire is ready for use.
The production of molybdenum wire was started in the Soviet Union in 1928. Three years later the Moscow Electrozavod Plant was turning out as much as 20 million metres of it.
During the last few years molybdenum has also begun to be produced by vacuum arc melting and zone and electron-beam melting.
We have already mentioned the fact that the reserves of molybdenum in the earth’s crust are not large. Perhaps they are going to be exhausted after some time and humanity will be faced with the problem of finding new sources of this valuable metal.
So far there is no cause for worry over the fate of the future generations. For it is a fact that the seas and oceans also contain enormous reserves of many elements. If the sea treasures were to be shared among all the inhabitants of the planet, then all people would become fabulously rich. Suffice it to say that Neptune’s “coffers” conceal something like three tons of gold per earthling, to say nothing of other treasures. Quite a goldmine, isn’t it? As for molybdenum, we would have about 100 tons each.
Mankind is still trying to find a key to the blue “coffers” of Neptune and the day is not far off when it succeeds.
Source: Tales About Metals, S. Venetsky