Text 12. Heat Treatment of Steel
We can alter the characteristics of steel in various ways. In the first place, steel which contains very little carbon will be milder than steel which contains a higher percentage of carbon, up to the limit of about l,5°/o-Secondly, we can heat the steel above a certain critical temperature, and then allow it to cool at different rates. At this critical temperature, changes begin to take place in the molecular structure of the metal. In the process known as annealing, we heat the steel above the critical temperature and permit it to cool very slowly. This causes the metal to become softer than before, and much easier to machine. Annealing has a second advantage. It helps to relieve any internal stresses which exist in the metal. These stresses are liable to occur through hammering or working the metal, or through rapid cooling. Metal which we cause to cool rapidly contracts more rapidly on the outside than on the inside. This produces unequal contractions, which may give rise to distortion or cracking. Metal which cools slowly is less liable to have these internal stresses than metals which cool quickly.
_ - On the other hand, we can make steel harder by rapid cooling. We heat it up beyond the critical temperature,
' and then quench it in water or some other liquid. The rapid temperature drop fixes the structural change in the steel which occurred at the critical temperature, and makes it very hard. But a bar of this hardened steel is more liable to fracture than normal steel. We therefore heat it again to a temperature below the critical temperature, and cool it slowly. This treatment is called tempering. It helps to relieve the internal stresses, and makes the steel less brittle than before. The properties of tempered steel enable us tonse it in the manufacture of tools which need a fairly
hard steel. High carbon steel is harder than tempered steel, but it is much more difficult to work.
These heat treatments take place during the various shaping operations.
to relieve stresses снимать напря- to contract сжиматься, девать
же ниє усадку
to be liable to smth иметь склон- distortion искривление
ность к чему-л. Drop зд. Падение
AFTER-TEXT SECTION
Practice . Определите:
а) что общего между тремя видами термообработки;
б) в чем состоит различие между ними;
в) какие свойства приобретают стали в результате различных видов термообработки.
Text 13. Superhard Material from Powder
The development of the engineering, electronic, radio- technical and electrotechnical industries is unthinkable without materials with specific properties. They include, in particular, composite materials, i.e. materials that combine incompatible properties, plastics, etc
Powder metallurgy figures prominently in obtaining these and other .materials. Composite materials have been j produced by the hot compaction methods,, which are bet- \ ter in their properties, especially in durability, than j similar materials obtained abroad, The antivibration !
alloys of the "iron-copper" type made it possible to double (he durability of drilling tools. This increased labour productivity by 20 percent. Welding electrodes made of powders used for resistance welding are 9-12 times more durable than those made by conventional methods.
The operational principle of powder metallurgy is very well known — an item of necessary size is modeled, in a mould, out of very small metal grains. After that the item is put into an electrothermic furnace where the grains get sintered together.
The coefficient of the use of meta! grows by five times. True, powders more often than not cost more than metal-rolling. But labour input is much smaller and labour productivity—higher. As a result of that, an item made of powder is over three times cheaper. This saving is achieved on account of a complete or partial liquidation of machining the item, the reduction of metal consumption by half or more, and automation of pressing and sintering.
The time of operation of powder articles increases considerably. The sintered articles have already proved their advantages working in outer space, in deep sea conditions and inside various machines.
It has been estimated that the output of 1,000 tons of powder articles saves the labour of 190 workers, frees 80 metal-cutting machine tools and saves 2,000 tons of ferrous and non-ferrous rolled stock and some 1.5 million roubles.
Soviet scientists have attained considerable success in powder metallurgy. They were the first in the world to develop, using high-energy moulding methods, a new superhard material out of boron nitride. The priority of the USSR in developing the first article out of the so-called viscous ceramics has been recognized in many countries.
composite materials композиты conventional method обычный
hot compaction method метод го- метод
рячей прессовки item, article изделие
dnrability долговечность grain зерно
alloy сплав to sinter спекать
to double удваивать consumption потребление
welding сварка rolled-stock прокат
AFTER-TEXT SECTION
Practice 1, Вам необходимо сделать сообщение на тему «Порошковая металлугия. Какую информацию из текста вы включите в свое сообщение? В каких абзацах она содержится?
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