Steel, like cast iron, is, an alloy of iron and carbon, or iron, carbon and other metals. The dividing line between steel and cast iron is at a carbon content of 2.2 per cent, i. e., all iron with a carbon content greater than this amount is cast iron, and all under this amount is steel or wrought iron. The physical properties of steel are greatly influenced by the amount of carbon, alloying elements and impurities present. The process of manufacture has much to do with the value of the metal for various purposes.
The general influence of carbon on steel is to give the steel greater tenacity and also to render it harder and stiffer. Man ganese increases the tensile strength of steel while the ductility is probably somewhat decreased. Silicon, as an alloying element, tends to increase the tensile strength, but to decrease the elonga tion and reduction of area. Nickel has a strengthening effect without decreasing the ductility. Chromium tends to make steel intensely hard and to give it a high elastic limit in the hardened or suddenly cooled state, so that it is neither deformed per manently nor cracked by extremely violent shocks. Chromium accelerates the case hardening process. Vanadium seems to render the steel more homogeneous and to render the effects of the other elements greater than in steels without vanadium, but otherwise of a similar composition.
Steel is made from pig iron by four different methods. The Bessemer process is the cheapest and produces the largest quan tity. The Bessemer process is conducted in the converter shown in fig. 69. The crucible process and the cementation process produce only small quantities of steel supplying the demand for fine tools, watch springs, needles, etc. For constructional work the most reliable method is the open hearth. In the open hearth process a flame playing upon the open bath of the molten metal removes the impurities. In the open hearth process pig iron, scrap iron and iron ore are melted in regenerative, reverberatory furnaces. Without the regenerative principle a sufficient tem perature cannot be maintained to keep the charge properly fused after the impurities are oxidized. For this reason, air for com bustion is heated to over 1,000° F. before it enters the combustion chamber. Measured quantities of ore, iron scale or other oxides added to the bath of molten metals react with the impurities present and serve to keep the mass thoroughly agitated. Silicon, manganese and carbon of the pig having a greater affinity for oxygen, oxidize first, protecting the iron of the pig and scrap from oxidation. Any oxidized iron will form slag on coming into contact with silica.
The carbon is oxidized by reaction with the iron ore. Figure 70 shows an open hearth furnace equipped with an oil burner.
Oil as a fuel for open hearth furnaces has many advantages. The repair cost of the fuel oil burner is about 40 percent less than when gas is used. A more even temperature may be maintained because the heat of the furnace is easily regulated. When oil is used a different chemical reaction takes place in the furnace and a superior quality of steel is produced and a lower grade of scrap iron can be used. For these reasons many large steel plants in the East have equipped their furnaces with fuel oil burners. Fig.
71 shows an open hearth furnace at Erie, Pennsylvania, equipped with a water-cooled oil burner. Fig. 72 shows an open hearth fur nace in Pittsburgh, Pa., using swinging oil burners.
The equipment of open hearth furnaces with oil burners is inexpensive. One open hearth furnace having one burner for each end of the furnace must have a reversing stand for reversing the flow of the oil and when the furnace is acting as the atomizing agent. This reversing stand is located on the charging flood. It must also have a pumping system for pumping oil from the stor age tank and regulating the supply to the burner. In addition, it must have a reducing valve for regulating the atomizing and the necessary valves, tank and pipe. For firing open hearth furnaces a swinging burner is commonly used. A water-cooled burner is used when the end of the- furnace is so near to the the building that there is no room for a swinging burner or when the furnaces are close together. A circulation of water through a -inch pipe prevents the burner from being melted off by the heat of the furnace.
The pressure at which the oil is fed to the burner varies con siderably at different plants but oil at 45 pounds and air or dry steam for atomizing at 40 pounds will probably give the best re sults under the average conditions. The question of whether compressed air or dry steam is best for atomizing seems to be an open one. About one-half of the plants use steam and the other half air as an atomizing agent. It is very important, however, that the steam be dry and it is usually well to put a drip in the steam line near the furnace and in some cases provide for super heating the steam before it enters the burner. An air or steam pressure reducing valve should be put in the line to cut the com pressor or boiler pressure down to the proper point for atomizing.
The American Rolling Mill Company of Middletown, Ohio. in the .manufacture of its Armco Iron uses fuel oil in many of its operations. Fig. 73 shows the layout, of its plant with respect to fuel oil distribution. Fig. 74 shows the method of construction of its oil storage tank. Fig. 75 shows the method of charging open hearth furnaces at this plant.