Metallurgy Of Cast Iron

KKKKCTS OK BLAST TKM I'KR ATURES. 77
After this limit is reached, it would seem that too great a body of the ore was suddenly brought to such a swollen, gummy state, as to retard the proper ascent of the blast and gases. The first factor to give notice that a furnace is getting " hot " is an increase in the temperature of the gases and the refusal of the stock to descend as rapidly as when the furnace is working in a normal condition. To retard the increase of heat or lower the temperatures to the best point, it has been found that increasing the blast pressure would often bring a "hot furnace" back to its normal working. By this method a greater volume of blast is admitted, which having a lower temperature than the incandescent stock in the furnace, naturally cools it down. Then, again, a plan is now largely adopted in having arrangements made so that cold blast can be turned on at a moment's notice. This "brings a furnace 'round" more quickly and in a much better manner than by increasing the pressure of the regular blast which, it should be understood, will have its temperatures lowered as much as is practical before being admitted. It is chiefly with brick hot-blast stoves that arrangements are provided for admitting cold blast to cool off u furnace, as these carry higher temperatures of blast than iron hot-blast stoves, as can be seen by referring to Chapter XI. The causes leading to "hot" furnaces can be traced to excess of fuel, often brought about by using larger percentages than, ordinary, which may be called for by reason of having to use small, or what is thought to be inferior coke or fuel, and again in burdening a furnace with fuel in order to raise the silicon in the iron or guard against " scaffolding " or "slips" from the use of fine ores, etc. It may also beency :|,