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Chapter V texts for home reading

1. The russian metallurgist d. K. Chernov

The father of the branch of science concerned with the changes in structure of steels was the Russian metallurgist Dmitry Konstantinovich Chernov (1839-1921). Investigating the properties of steel after heating to various temperatures, Chernov first established that at definite temperatures steel undergoes certain changes altering its structure and properties. These '"critical temperatures" characterized by internal changes in the steel are now known all over the world as the "Chernov points". One of these points called by Chernov point a, is notable for the fact that steel heated below this point (about 700°C) cannot be hardened no matter how rapidly it is cooled. Another point, b, is characterized by the fact that as soon as the temperature of the steel reaches it (800 to 850°C), the steel rapidly passes from the coarse crystalline into the fine crystalline state, in which it possesses the best mechanical properties. If the temperature is raised still further, the metal crystals begin to increase again in size, and the higher the temperature, the more rapidly they grow.

The discovery of the critical points of steel was of very great importance for metallurgical theory and practice. Explaining the phenomena of tempering and hardening of steel and the structural changes taking place in steel when heated, it enabled accurate determination of the hardening temperatures and selection of favourable conditions of forging and other types of steel treatment, promoting improvement of its mechanical properties.

2. Oxygen in the bessemer converter

Fundamentally, the addition of oxygen to the Bessemer air blast should have two major effects: (1) to increase the rate of oxidation because of the higher partial pressure of oxygen in the system; (2) to increase the amount of heat available to melt cold materials. The use of oxygen for enriching the blast of the basic Bessemer converter or for changing the nature of the converter blast altogether, became a very important development in steel works. Reduction of nitrogen content in Bessemer steel by oxygen enrichment alone reaches a limit at about .006 per cent nitrogen. Further it is necessary to remove all nitrogen from the blast but, at the same time, maintain a blast composition which will oxidize metalloids in the steel without generating so much heat that the converter tuyeres are burned out. This can be done by a mixed blast of oxygen and superheated steam. By this method steel containing less than .003 per cent nitrogen is produced.

One potential source of difficulty in the oxygen-steam mixed blast process is the danger that condensate from the steam may attack the dolomite bottom of the converter. To avoid this, some designers recommend the use of thin copper tubes embedded in the bottom plate to act as protective liners for the tuyeres. Quite recently some experiments were made with the so-called "dual-blast" converter in which air is blown upward through ordinary tuyeres in the bottom while high-purity oxygen from

the top is directed downward at low impact pressure onto the surface of the bath. This oxygen is intended not so much to penetrate the metal as to flood the whole atmosphere above the converter bath With oxygen, thus promoting high concentration and reactivity of iron oxide in the basic slag.

By changing the number of open tuyeres in the bottom of the converter the ratio of oxygen going down to air going up can be varied within wide limits. The object of this departure is to develop a conversion process that will remove phosphorus from Thomas pig-iron before decarburization is complete. By passing oxygen downward onto the surface of the metal and the lime which are being agitated by the rising air blast, the necessary early liquefying of the lime is achieved and early dephosphorization takes place.

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