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Metal heat treatment
Metal heat treatment is a kind of craft to heat pieces of metals at the suitable temperature in some medium and to cool them at different speed after some time.
The metal heat treatment is one of the important crafts in the machine-building, comparing with other technologies, the heat treatment seldom changes the form of the work pieces and chemical composition of the whole .it improve the serviceability of the work piece through changing their micro- work pieces, chemical composition, or surface. Its characteristic is improving inherent quality of work pieces which can not be watched by our eyes.
In order to make the metal work piece have mechanics , physics and chemical property which are needed, besides the use of many materials and various kinds of crafts which are shaped , the heat treatment craft is essential. Steel is a wide-used material in the mechanical industry, its complicated micro-composition can be controlled through the heat treatment , so the heat treatment of the steel is a main content of the metal heat treatment . In addition aluminium, copper, magnesium, titanium and their alloys also can change their mechanics , physics and chemical property through the heat treatment to make different serviceability.
During the process of development from the Stone Age to the Bronze Age and to the Iron Age, the function of the heat treatment is gradually known by people. As early as 770 B.C.~222 B.C., the Chinese in production practices had already found the performance of the copper and iron changed by press and temperature . White mouthfuls of casting iron’s gentle-treatment is a important craft to make farm implements.
In the sixth century B.C., the steel weapon was gradually adopted. In order to improve the hardness of the steel, quench craft was then developed rapidly. Two sword and one halberd found in YANXIA, Hebei of China , had “MA structure” in its micro-composition which was quenched.
With the development of quenching technology, people gradually found the influence of cold pharmaceutical on quality of quenching. Pu yuan a people of the Three Kingdoms(now, Shanxi province Xiegu town)made3000 knives for Zhu Ge-liang.the knives were quenched in Chengdu according to legend. This proved that the chinese had noticed the cooling ability of waters with different quality in ancient times, and the cooling ability of the oil and urine at the same time were found. People found a sword in Zhongshan tomb which were up to the Western Han Dynasty (B.C. 206 -A.D. 24 ),in whose heart department carbon was about 0.15-0.4%, but on whose surface carbon was about more than 0.6%.this has shown the use of the carburization craft. But as the secret of individual's " craftsmanship " at that time, the development was very slow.
In 1863, Britain metallo graphy expert and geologist's discoverity that six kinds of different metallography organizations existed in the steel under the microscope, proved that the inside of steel would change while heating and cooling. the looks of steel at the high temperature would change into a harder looks when urgently colded. Frenchmen Osmon established Allotropic theory , and Englishmen Austin first made the iron- carbon looks picture .these tow theories set the theoretical foundation for the modern heat treatment craft . Meanwhile, people also studied the metal protection in the heating to avoid the metal's oxidizing and out of carbon in the course.
1850~1880s, there were a series of patent to use kinds of gases to heat (such as hydrogen , coal gas , carbon monoxide etc. ). Englishman's Rec obtained the patent of bright heat treatment of many kinds of metal in 1889-1890.
Since the 20th century, the development of metal physics and transplantation application of other new technologies,make the metal heat treatment craft develop on a large scale even more. A remarkable progress was carburizition of gas in a tube of stoves in industrial production during 1901~1925; 1930s the appeariance of the electric potential different count and then the use of carbon dioxide and oxygen made stove carbon of atmosphere under control . In 1960s, hot treatment technology used the function of the plasma field, developed the nitrogen, carburization craft.The application of laser , electron beam technology, made the metal obtain new method about surface heat treatment and chemical heat treatment.
The metal heat treatment craft
The heat treatment craft generally includes heating, keeping and cooling and sometimes only heating and cooling two progresses . The course links up each other.
Heating is one of the important processes of the heat treatment . There are a lot of heating methods of the metal heat treatment . the first heat source were the charcoal and coal , then liquid and gaseous fuel. The application of the electricity is easy to control the heating, and no environmental pollution. the heat source could be heated directly or indirectly by the use of salt or metal of melting or the floating particle.
While metal heated, the work piece in air , is often oxidized or take off carbon ( steel's surface carbon content reduces).this does harm to the metal's surface performanc which is heated. Therefore metal should heat in the the vacuum or the melted salt, in controlled atmosphere or protected atmosphere . Sometimes it is heated in the protect means of coating or pack .
Heating temperature is one of the important craft parameters of the heat treatment craft , choosing and controling heating temperature is a main matter of guaranting heat treatment quality. Heating temperature may change according to the different purposes of the heat treatment and different metal materials , but usually it is up to the temperature at which high temperature frame could be abtained.it must keep some time at the high temperature to make the inside and outside of the metal reach the some heating level,so that its micro-frame would turn out wholely.we call this period of time "keep-heat"time. There is no "keep-heat"time when adopting density heating and surface heat treatment of high energy because of the rapidity. But the chemical heat treatment often need much more time to sustain the heat .
Cooling is an indispensable step in the craft course of heat treatment too . cooling methods are different because of crafts , mainly at controling the speed of cooling. generally anneals is slowest in speed, the cooling normalizing is a little fast in speed, the quenched cooling is much faster in speed. But there are different demands according to the kindof steel, for example empty hard steel can be cooled with normalize as quick as the speed by hard quench .
The metal heat treatment craft can be divided into whole heat treatment , surface heat treatment and chemical heat treatment.Every kind could be divided into different crafts according to heating medium , heating temperature and cooling method. The same kind of metal adopting different heat treatment crafts can get different organizations which have different performance . The steel is the widest-used metal on the industry, and its micro- organization is the most complicated, so the steel heat treatment craft is various in style.
The whole heat treatment is to change the whole mechanics performance of work piece through heating the work piece wholely and then cooling at the proper speed. The whole heat treatment of steel roughly has four basic crafts of annealing , normalizing , quenching and flashing back .
Annealing means heating the work piece to the proper temperature ,then adopting different temperature retention time according to the material and size of work piece and then cooling slowly, whose purpose is to make the metal organization to achieve or close to the balance state, obtain good craft performance and serviceability, or prepare for quench further. normalizing is to cool in the air after heating the work piece at suitable temperature , its result is similar to annealing except that the organization out of normalizing are more refined which is often used to inhance the cutting performance of the material and is occationally used for the final heat treatment of material which are not high-requested. .
Quenching is to cool work piece which has been heated and kept in warm fast in the cold medium as water , oil , other inorganic salts ,or organic aqueous solution and so on . The steel quenched becomes hard and fragile too. To reduce its fragility , we must first keep the quenched piece of steel in a certain temperature which is higher than room temperature but lower than 650℃for a long time,and then cool it again. this progress is called the flashing back . Annealing , normalizing, quenching , flashing back is " four fires " in the whole heat treatment . the quenching contact close to flashing back ,and they are often used together.
" Four fire "is divided into kinds of heat treatment crafts by different heating temperatures and diferent ways of cooling. What is " quality adjust " is a kind of craft combining "quench" with "high-temper a ture flash back" to make the work piece obtain certain intensity and toughness. Some alloy saturation out of quench can improve its hardness, intensity, electricity and magnetism after it is kept in the high proper temperature for a little long time . Such heat treatment craft is called “effective dealing”.
Deformation-heat-treatment is the combination of pressure-deformation and heat treatment on work piece ,this mothod could enhance its intensity; and vacuum-heat-treatment is that work piece is heated in atmosphere or vacuum.It can make the work piece not oxidize or take off carbons , keep its surface bright and neat and improve its performance. At the same time ,it can carry on the chemical heat treatment by the pharmaceutics.
Surface heat treatment on work piece is only to heat its cover to change the metal-layer's mechanics performance. In order to only heat the layer of work piece without making too much heat spreading into the inside, the heat source used must be of high density of energy , namely it can offer greater heat energy on the unit's area of the work piece and make its layer or parts reach high temperature in short-term or instantaneously. The main method of the surface heat treatment is "flame quenching" and "reaction heat" treatment and the heat source used commonly are flame as oxygen acetylene or propane, reaction electric current, laser and electron beam,ect.
The chemical heat treatment is to alter the chemical composition, organization and performance of the top layer of work piece.The difference between Chemical and surface heat treatment is that the latter just change the chemical composition of the top layer of work piece . The former is to set the work piece heating in the medium (the gas , liquid , solid ) including carbon , nitrogen or other alloying elements,and then to keep it warm for longer time, thus to make elements as the carbon,nitrogen,boron and chromium,etc permeate through the top layer of work piece.Sometimes after permeation, there is other heat treatment craft to carry on such as quenching and flashing back . The main method of the chemical heat treatment include carbon,nitrogen, and metal permeation.
The heat treatment is one of the important processes in machine components and tool and mould manufacture. Generally speaking, it guarantees and improves various kinds of performance of the work piece , for instance wear proof and anti-corrosion. It also improve the organization and state of the tough work piece to ensure various kinds of cooling and heating work.
For example tin are annealed for a long time to turn into malleable cast iron which is of plasticity. proper heat treatment craft can prolong the gear wheel's service life at double or dozens of times than these without heat treatment ; In addition, the cheap carbon steel with some alloying elements permeated will own the alloy steel performance whose prices hold high so that it can replace some heat-resisting steel , stainless steel ; all tool and mould need to be through the heat treatment before in use..
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