Part II texts for additional reading

1. POWDER METALLURGY LOOKS TOWARDS THE FUTURE

Developments and advances in powder metallurgy, a technology created some 50 years ago, can save manufacturing industry great amounts of valuable materials. Powder metallurgy is a cheap alternative to many conventional manufacturing processes.

When components, simple or complex, require precision and high quality at a comparatively low lost — powder metallurgy can provide the solution of the problem. An important feature of powder metallurgy is that it can provide the industry with such material compositions which are not achievable by any other means.

Components produced by the powder metallurgy process can go straight into the manufacturing cycle or, if required, undergo further processing, including heat treatment. Powder metallurgy is finding new applications in various industries — in electronics, aviation, machine-building, etc.

The unique physical properties of powder metallurgy parts enable oil to be retained in minute porous cavities within the part. This self- lubricating characteristic is long lasting and can eliminate other lubri­cation systems.

The research and production association for powder metallurgy has developed a number of processes for powder metallurgy components production. The source material there is metal powder which is sub­jected to high pressure to acquire a required shape and is then put to thermo-electric furnaces. The resultant parts are more durable and re­quire no additional machining.

Future processes to be introduced by the association will produce self-lubricating bearings, metal and nonmetal alloys and other materi­als with preset properties based on combinations of various powders. Metal powder with its unusual characteristic features and properties is listed in the category of new materials.

Notes:

minute porous cavities —крошечныепористыепустоты

self-lubrication characteristic — свойство, обеспечивающее самосма- зывание деталей

The uses of metals are based upon their physical or chem-ical prop­erties. Metals vary in density, hardness, heat and electrical conductivity and weight. The lightest is lithium and the heaviest is osmium. The hard­ness of metals depends upon the presence of other substances in them and the nature of metal itself. Metals are the best conductors of heat and electricity.

The most widely used metals are: iron, copper, zinc, tin, lead, mer­cury, silver and gold. The most important metal is iron.

Nowadays, aluminium, magnesium and sodium have be-come com­mon due to the development of electrochemical processes for their pro­duction.

Metals occur most commonly as oxides or sulphides in ores. They must be separated from gangue materials such as clay, silica, granite, etc.

3. Silent metals

A hundred years ago noise on the main streets of the world’s biggest cities did not exceed 61 decibels. Today it is 100 and more. Industrial noise is at very high level at many factories, and in some it reaches 90 to 110 decibels.

Noise is an ever growing inconvenience of modern life, and of it is generated by vibrations of metals. These vibrations not only cause noise but can also lead to “fatigue” and consequent failure of a structure.

Research into methods which can minimize vibrations in structures is therefore of considerable importance. There are two methods to re­duce vibration in an engineering design; either we make the structure so stiff and heavy that it cannot vibrate significantly, or we introduce “damp­ing” into the structure, that is, we have to introduce some mechanism for the absorption of energy within the system.

To apply damping coating is standard practice today. The damping coatings are usually made of plastics and are applied to sheet-metal shells such as car bodies. This method is often cheap and the advantage is that the coating can be applied precisely where damping is required. But these damping coatings may be efficient for certain sound frequencies and temperatures.

So metallurgists were interested in the possibility of metals that are strong and tough enough to be used in structures. But they must also possess a high inherent damping capacity that is independent of fre­quency and less temperature-dependent that of plastics.

Scientists want to combine some of the properties, which charac­terize steel, with high damping capacity of lead and to produce a mate­rial that could be used to minimize noise and vibration. This can, in fact, be done with several materials, the most outstanding of which are alloys of manganese and copper. These alloys can be stronger than ordi­nary steel, with similar toughness and hardness, yet than that of steel.

However, noise and vibration are problems to be faced by engineers. It is seldom sufficient merely to replace a troublesome component with one of a high- damping alloy. The particular characteristics of these “high-damping” structural alloys should be properly employed. As a result, perhaps, the future will be a little quieter — in some respects at least!

Notes:

decibel — децибел^Ь) = 0.1 b; b (бел) — акустическая единица измерения damping — демпфирование

to sheet-metal shells — к наружным частям, сделанным из листового железа

a high inherent damping capacity — способность сильного внутреннего демпфирования yet — зд. но

it is seldom sufficient merely to replace — редко бывает достаточно лишь заменить at least — по крайней мере

Silicon is one of the most abundant elements found in the earth’s crust. It is second to oxygen in abundance.

Silicon never occurs free, but in combination with oxygen or with oxygen and metals. It forms a great variety of organic and inorganic compounds.

Elementary silicon is used as an alloying constituent to strengthen aluminium, copper, magnesium and other metals. It has a deoxidizing effect on steel.

Silica (quartz) is a crystalline form of silicon dioxide.

Silica bricks, made of nearly pure silica, are extensively used in metallurgical industry.

Silica is also used as one of the raw materials for the manufacture of common glass.

5. ALLOYS

The word alloy comes from a French word meaning ‘to combine’. When metals in a molten state unite and make what seems to be a single substance, they are said to form an alloy. For example, brass is an alloy of copper and zinc. Most alloys are known to be made by melting the metals together but some are made by electro-chemical methods and a few by compressing the powdered metals together.

Men first learned about the simple metals like copper and tin. Then, perhaps after, a fire, they found that a different substance was formed if copper and tin were melted together. This substance, called bronze, was found to be more useful than either of the metals by themselves, for when two or more metals form an alloy the result is a substance which has different properties from those of the original metals.

Metallurgists are known to have produced many kinds of alloys which can be used in several different ways.

In the homes of ancient people copper was used to make tools and weapons but it was too soft to be really suitable. It soon lost its sharp

edge or bent if it struck something hard. The discovery of bronze gave a harder and more useful metal. Later iron was discovered and used in­stead of bronze.

When iron was obtained from the iron ore varying amounts of car­bon were left in the metal produced. More recently in history it became possible to obtain iron with a definite amount of carbon in it. When this metal was made red-hot and cooled quickly by plunging it into cold water, it became very much harder than the original iron. This metal was called steel. Steel is not really an alloy, it is rather like one. We know the name of steel to be used now for any iron which contains from 0.1 to 1.7% of carbon. After more study and experiment men discovered many effects of alloying different metals with steel. The results are called the alloy steels.

6. ALUMINIUM

Aluminium, which is sometimes called aluminum, is the typical metal in the third group in the periodic classifica-tion of the elements. Aluminium is the most abundant of the metals and the most widely distributed. It is found in feld-spars, micas, kaolin, clay, bauxite, cryo­lite, alunite, corun-dum and certain gems. Compounds of aluminium have been known for many years and they were recognized as being derived from a metal that had not been isolated.

Aluminium has a very low density, 2.7; it is used in construction when a metal is required and weight is an important consideration. It is ductile, malleable, and can be rolled. Its tensile strength is low in com­parison with that of iron; it cannot be machined and polished readily and does not yield good castings. These defects can be overcome by alloying it with other metals. Alloys of copper and alumin-ium which contain from 5 to 10 per cent of the latter are called aluminium bronzes. They have a fine yellow colour resembling gold and are used in making imitation jewelry and statuary.

On account of its low electrical resistance, aluminium is used in certain cases in wires and cables as conductors.

Copper was used in prehistoric times for making weapons and tools and later was alloyed with tin to form bronze which was the most im­portant metal of the Greeks and Romans. It was replaced for these pur­poses by iron and steel. Various grades of copper are used for engineer­ing purposes. The great development of the electric industries has re­sulted in such extensive uses of the metal that it now ranks next to iron in importance.

The copper alloys are more widely employed. The alloy-ing of cop­per with other elements increases the strength of the metal in some cas­es and improves the anticorrosive and antifriction properties in others. Copper alloys comprise two main groups — brasses and bronzes. Alloys of copper and zinc are called brasses. Alloys of copper with a number of elements including tin, aluminum, manganese, iron and beryllium are called bronzes.

8. ANCIENT STEEL-MAKING SECRET

When two metallurgists at Standford University were trying to pro­duce a “superplastic” metal they became interested in the secret of Damascus steel, the legendary material used by numerous warriors (войны) of the past, including Crusaders (крестоносцы). Its formula had been lost for generations.

Analyses of new steel revealed properties almost identical to those they found in Damascus steel, although their own plastic steel had been produced by present-day methods.

The remarkable characteristics of Damascus steel became known to Europe when the Crusaders reached the Middle East in the 11^thcen­tury. They discovered that swords (меч) of the metal could split (рассечь) a feather (перо) in air and at the same time retain their edge sharp through many battles.

The secret of Damascus steel was known in many parts of the an­cient world, especially in Persia, where some of the finest specimens were produced. For eight centuries the Arab sword makers kept the se­cret about their technique and methods. And with the invention of fire­arms (огнестрельноеоружие), the secret was lost and it was never ful­ly rediscovered.

The two metallurgists carried out a lot of researches. When they real­ized that they might be close to the discovery of a new material, a sword fancier (знаток), at one of their demonstrations, pointed out that Damas­cus steel, like their own product, was very rich in carbon. This led them to conduct a comparative analysis of their steel and those of the ancient weap­ons. As a result, it was found that a basic requirement was a high carbon content. The two metallurgists believed it had to be from 1 to 2 per cent, compared to only a part of 1 per cent in ordinary steel. Their research showed how to make steel of even greater hardness than Damascus steel.

9. STRENGTH OF MATERIALS

One of the most important problems in strength of mate-rials is to determine the mechanical conditions which cause solids in engineer­ing structures to deform or to fracture. There is a variety of states of stress in which solid bodies either greatly change their shapes or fail by fracture. The perma-nent or plastic deformation in ductile metals may develop either suddenly or quite gradually when the stresses increase. This depends on whether a definite yield stress characterizes the mate­rial or no definite stress exists at which it starts to deform permanently. An observed yield point depends also on the previous stressing and plas­tic deformation of the ma-terial. It is necessary to point out as well that the tempera-ture is of determining influence on the magnitude of the forces which are required to deform solid bodies.

Some of the limiting conditions on which the design of machine parts must be based are: a) the load or loads under which the first per­manent distortion begins to develop if the material has a sharply defined yield stress; b) the stresses or loads under which the permanent portions of the strains will not exceed certain small limiting strain values; c) the maxi-mum permissible elastic displacement or deflection in a part of a construction or in a whole construction; d) the load causing fracture, including failure through fatigue fracture, etc.

Elevated and very low temperatures are to be considered in dealing with permissible loads, deflections and displace-ments. The design of machines operating at high pressures and speeds, at normal or low tem­peratures, raises quite a number of problems. Several of the criteria on which the limiting values of the loads must be based depend on the time or on the velocities with which the small permanent strains may form.

One of the first problems of the mechanics of the plastic states of crystalline solids is to determine from observations the shape of the lim­iting surface of yielding for various materials.

Most solid materials withstand very high hydrostatic pres-sures with­out fracture if the pressure acts uniformly from all sides as it does in a fluid surrounding the solid. Materials with a loose or porous structure, such as wood, undergo con-siderable permanent deformation under high hydrostatic pressure and remain in their volume after the pressure is released. Wood when sufficiently condensed in this manner will no long­er float in water. The crystalline solids, such as metals, under these con­ditions are compressed chiefly in an elastic way by very small amounts.

With respect to their compressibility the impervious poly-crystal- line or amorphous solids behave like liquids. They are elastically com­pressible bodies and withstand high hydrostat-ic pressure to almost any possible value without suffering a permanent distortion after the pres­sure is released. In less compact solid materials subjected to fluid pres­sure, however, marked evidences of failure have repeatedly been ob­served, e. g. in marble cylinders which were exposed to hydrostatic pres­sure and in wood, which is compressed into irregular shapes because of its cellular anisotropic structure. If cer-tain precautions are not taken when such materials are ex-posed to high fluid pressure, the liquid used to exert stress, may penetrate the material through the fine fissures or cracks which it contains. Glass balls which are exposed for a short time to very high fluid pressure do not break at the maximum pressure but either during the period of decrease of the pres-sure or later after it was rapidly released. The small amounts of fluid which penetrate through the invisible fine surface cracks into the outermost layers of the balls cannot escape fast enough from the cracks when the pressure is quickly re-leased. The crumbling through the external pressure of fluid can de­stroy the structure of weaker materials, such as marble and sandstone.

This can be prevented if the test specimen is covered by a very thin flex­ible foil of brass or copper which does not permit the penetration of fluid into the cracks of the material.

In contrast to this behaviour under high pressure it is certain that solids under a uniform tension acting in all directions are able to resist only certain definite pressures. The so-called brittle materials, e. g. glass, cast metals, most natural rocks, under uniaxial or multiaxial tension break suddenly and do not deform permanently appreciably under such stresses before they fracture.

10. HEAT TREATMENT OF IRON AND STEEL

The heat treatment of steel after its formation is rather important. The treatment of steel is now an elaborate science of its own, and it is, of course, closely connected with the contents of the steel. The heat­ing and the quenching are designed to bring about the arrangement of the crystalline structure in the desired way, to give the required proper­ties. For example, steels containing a high percentage of carbon may be quenched after being heated to a high temperature to make them hard, and then moderately tempered to restore toughness.

No steel is used today that has not been tested. The simplest exam­inations are those made with magnifying glass or a microscope.

Spectroscopic or chemical analysis, X-ray examination, and mag­netic or electrical tests may be applied. Specimens are also subjected to physical tests for toughness, strength, and hardness.

Notes:

heat treatment — термообработка

is now an elaborate science of its own — является сейчас самостоятельной тщательно разработанной наукой closely connected with... — непосредственно связанной с... to bring about — зд. произвести

in the desired way — в нужном порядке, нужным образом to make them hard — чтобы сделать их твердыми are subjected to — подвергаются

11. HOT WORKING

When the mechanical working of metal or alloy takes place above the recrystallization temperature, the process is known as hot working.

In a hot-worked metal or alloy the grains are being continually formed as hot working continues, and the size of grains depends on the temperature. The smallest grains are formed when the temperature is just above the recrystallization temperature, and this is usually the best finishing temperature for hot working operations.

Hot rolling and forging are very widely used in the fabrication of metal shapes, and, even when the finishing operation to be cold work­ing, the preliminary shaping is done by hot working.

The temperature of the metal for hot working depends on the metal or alloy, and there is a best temperature range and a best finishing tem­perature in each case.

Notes:

hot working — горячая обработка

takes place — происходит

as hot working continues — по мере того, как происходит горячая обработка