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Cast steel


Cast steel
In the history of metallurgy of iron was three revolutionary coup that had a profound influence on the entire course of human history: the first took place in ancient times, when there were syrodutnye horns; the second took place in the Middle Ages, after the opening peredelochnogo process; the third came in the second half of the XIX century and was connected with the beginning of the production of cast steel. Steel has always remained the most necessary and desirable product of iron metallurgy, because it alone possesses the hardness and strength, which were required for the manufacture of tools, weapons and machine parts. But before you turn into the steel product, metal has been subjected to a number of labor-intensive operations. First, from the ore smelted iron. Then iron reduced in soft iron. Finally, by forging long-term iron Kritsa received from it the necessary steel part (or just blank it, which is then subjected to a final finish on machine tools). Production of soft iron and especially forging a long time remained the bottleneck in the process of iron processing. They took the most time and effort, but the results are not always proved satisfactory. Particularly acute, this problem began to be felt in the XIX century, when sharply increased demand for cheap steel. Naturally, many scientists and inventors have thoughts, which then expressed Bessemer: how to get a metal with properties of iron and steel, but in liquid form so that it can be used for casting? Resolution of the problem require decades of hard work of many steel companies. In this way, a number of important discoveries and inventions were made, each of which was an epoch in the history of iron processing.

By the end of the XVIII century redistribution of iron into a soft malleable iron occurs only in finery. This method, however, was uncomfortable in many ways. Gets in the metal it has been uneven - sometimes approaching in quality to the ductile iron, sometimes - to steel. In addition, the work required much time and physical strength. Since the fuel (coal) is in direct contact with the iron to it very high requirements, because any impurities affect the quality of the final product. Coal consumption was very high (an average of 1 kg of iron recovery could take up to 4 kg of coal). In the largest furnaces it can be 24 hours to receive no more than 400 kg of iron. Meanwhile, the market demanded more iron and steel. To meet these requests had to find a better way to iron alteration.

A significant step forward in this direction was proposed in 1784 by Englishman Cort puddling process in specially designed for this oven.

The fundamental unit puddling furnace was as follows. In the furnace of burning fuel. The products of combustion through the stone threshold fall in the furnace, where the hearth was loaded with glandular iron slag. Slag under the influence of flame passed into a dough and partially melted. With increasing temperature, iron begins to melt and impurities it burnt out due to oxygen, a prisoner in slags. Thus iron decarburization, ie turned into Critz sponge iron. An important difference from the puddling furnace bloomery forge lay in the fact that it may be used as fuel any fuel, including crude and cheap coal, and its volume was much larger. Thanks puddling furnaces iron became cheaper. However, in contrast to the finery furnace Cort did not require the forced injection. air and good traction are achieved through a high chimney. It was one of the reasons why the puddling furnaces are widely used all over the world. However, a significant drawback of these furnaces was that the air blew only the upper part of iron. In order to iron recovery was uniformly throughout the volume, accounted for periodically opening the oven and stir the iron. It was heavy manual labor. Furthermore, since the strength and possibilities have been limited to working, the furnace could not be too large. (To prevent agitation, Cort provided two pipes, one of which was located under the insert, and the second -. Its furnace end opened at the time when required to reduce the temperature.)

By the middle of the XIX century puddling furnaces ceased to satisfy the new needs of the industry. To keep up with demand, it was necessary to build on each large blast furnace several furnaces (an average of one blast furnace serviced ten puddling furnaces). This increases the cost and complexity of manufacture. Many inventors have thought about how to replace the puddling more perfect way to recover iron. Earlier this other problem was resolved English engineer Bessemer. Busy metallurgy Bessemer came after many years of work on the improvement of artillery and shells. He set a goal to find a way to produce high-quality cast steel, from which it would be possible to cast a cannon. Watching a lot of time in the melting of iron, he noticed that the solid reduced iron is produced first of all from the blower pipe. This gave him the idea of ​​getting steel reinforced by blowing air through the molten iron. His first experiments Bessemer held in closed crucible, which it is heated in a furnace with coke. The result exceeded all expectations. Less than an hour blow he received from top-notch cast steel. In addition, further experiments have shown that there is no need to enter into the metallurgical process of the heat outside. The fact that the cast iron contains its own fuel material as impurities: silicon, manganese, carbon - about 45 kg combustibles per ton of pig iron. His burning, they can significantly increase the melting temperature of the steel and to obtain a liquid.

In 1856 Bessemer publicly demonstrated his invention of the stationary converter. Converter had the appearance of a low vertical stove top closed vault with an opening for the outlet of gases. Side of the furnace was a second opening for pouring pig iron. The finished steel produced through the hole in the bottom of the oven (during his Converter scored clay). The blower tube (lance) are right next to the hearth. Since the converter has been fixed, a purge began before the poured iron. Otherwise, the poured metal to the lance. For the same reason it was necessary to conduct a purge until all the metal it has not been released. The whole process lasted less than 20 minutes. The slightest delay in the issue of giving the marriage. This inconvenience, as well as a number of other shortcomings stationary converter Bessemer forced to go to the rotary kiln. In 1860 he took out a patent for a new design of the converter, preserved in general terms to the present day.

Bessemer method was a real revolution in the field of metallurgy. For 8-10 minutes it turned Converter 10-15 tons of pig iron or steel, malleable iron, on that before it would take several days of puddling furnace or a few months of former finery. However, after the Bessemer method has been applied in an industrial environment, the results were worse than in the lab, and began to leave a very poor quality. Bessemer two years tried to solve this problem and finally found that the iron in his experiments contained little phosphorus, whereas the widely used in England iron smelted iron ores with high phosphorus content. Meanwhile, phosphorus and sulfur are not burnt out, together with other impurities; they fell from iron and steel in significantly reduced quality. That, and also the high cost of the converter, has led to the fact that the Bessemer process is very slowly being introduced into production. And 15 years later in England, most of the cast iron melted down in puddling furnaces. Much more widespread use of converters have received in Germany and the United States.

Along with the Bessemer process of steel production soon acquired a huge role open-hearth method. Its essence is that the cast iron alloyed with iron bar in a special regenerative furnace. This oven was invented and built in 1861 by German engineer Friedrich and William Siemens for the needs of the glass industry, but the most widespread in the industry. The composition of the furnace consisted of gas producers (generators or gas), oven itself with the Restorer of heat (or regenerators) to preheat the gas and air and casting department (yard). Generators and regenerators were interconnected channel system for special gas, air and combustion products. The last set aside in a chimney height of 40 m, provide the necessary thrust. The generators were located under the hearth or furnace sides. The regenerators are a special chamber for heating gas and air. Special variable valve directs the gas and air in one chamber or the other, and the combustion products are diverted into a pipe. Combustion takes place as follows. Gas and air are heated each in his cell, and then fed into the melting space where combustion occurs. The products of combustion, after passing over the hearth furnace, poured into the regenerators and gave most of his heat regenerators masonry, and then went into the pipe. To process takes place continuously, by means of valves directs air and gas in the one pair of regenerators or the other. As a result of deliberate heat furnace temperature reaches 1600 degrees, that is higher than the melting temperature of pure carbon-free iron. Creating a high-temperature furnace has opened up new horizons for metallurgy. By the middle of the XIX century in all industrial countries had huge stocks of scrap iron. Due to its high refractoriness could not be used in production. French engineer Emile and Pierre Martin (father and son) offered to fuse the scrap iron with iron in regenerative furnaces and thus obtain steel. In 1864 the factory Sireyl they led by Siemens carried out the first successful smelting. Then, this method has been applied everywhere.

Open-hearth furnaces converters are cheaper and therefore were more widespread. However, neither Bessemer or hearth method did not allow to produce high quality steel from ore containing sulfur and phosphorus. This problem remains unresolved and a half decades, until in 1878, English metallurgist Sidney Thomas did not come up to the converter to add 10-15% of lime. Thus formed slag, phosphorus capable of holding solid chemical compounds. As a result, phosphorus burned off together with other unnecessary impurities and the iron was converted to the high-grade steel. The value of the invention of Thomas was huge. It is possible to massively produce steel of phosphorus ores that were mined in large quantities in Europe.

In general, the introduction of the Bessemer and open-hearth processes made it possible to produce steel in unlimited quantities. Cast steel has quickly won a place in the industry, and since the 70s of the XIX century wrought iron is almost completely out of use. Already in the first five years after the introduction of open-hearth and Bessemer manufactures the world steel production increased by 60%.

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