The Bessemer converter

 

In the Bessemer process of making steel air is blown through the molten pig iron, and the oxygen of the air com�bines with the carbon, manganese, and silicon of the pig iron. This action generates heat and frees the iron from the major part of its impurities thus converting the iron into steel.

The Bessemer converter, in which the process takes place, is a pear-shaped tilting vessel made of steel plates and lined with heat-resisting bricks and clay. The top of the converter is cut off to form a mouth through which molten metal is charged and discharged. In capacity converters range from five to twenty-five tons of molten metal. In the bottom of the vessel are a number of holes through which air is blown at a pressure of fifteen to twenty pounds per square inch.

Manganese, silicon, and carbon in the molten pig iron have varying attraction or "affinity" for oxygen, so they do not burn at the same time. Silicon and manganese have a greater affinity for oxygen than does carbon, and conse�quently they burn out first.

When the air blast is turned on, a shower of sparks bursts from the mouth of the converter. Immediately thereafter appear short ruddy flames and a dense cloud of reddish-brown fumes caused by the burning of the silicon and man�ganese in the iron. In about five minutes this part of the refining action is accomplished, and the next stage, the removal of carbon, begins.

The ruddy flames become more luminous, changing to a yellowish white, and increase in length until the tongue of flame extends for about  thirty   feet.  So brilliant  are the flames caused by the burning of the carbon that they some�times light up the sky with a glare visible for miles.

For about ten minutes the glare continues, and during that time the converter emits a deep roar caused by the violent generation of gas within it.

Suddenly the flame drops, a signal of great importance to the operator because it tells him that the blast of air must be diminished and the metal removed from the converter. Iron itself has an affinity for oxygen and it is only because silicon, manganese, and carbon have greater affinities that the iron has not previously burned out. If the operator al�lowed the blast to continue, the converter would eventually contain nothing but iron oxide and slag.

What remains in the converter after the blow is com�pleted, is a molten iron, very low in carbon, manganese, and silicon but containing gases and some iron oxide as the result of the great volume of air blown through it. But this is an unsatisfactory condition of the metal. To make the metal useful and free it from the remaining gases and iron oxide, ferro-manganese is added to the molten steel which then becomes quite ready for the use.

Bessemer steel is used because of the low cost of the pro�cess. No extraneous fuel is used, and a heat of 15 to 25 tons can be made in 10 to 15 minutes. Bessemer steel is fre�quently refined additionally in the open hearth or electric furnace when it is called duplex steel.

To-day we have a new, more perfect technology of con�verting pig iron into steel in which the blast of air is re�placed by a jet of nearly pure oxygen.