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Metal Casting Technologies : June 2010
Back to the his is a sequel to my previous article about how I became a foundryman and made steel castings. This is about how acid-electric steel should be made. Steel made in an arc-furnace is considered superior in quality to steel made in an induction furnace. The main difference is that an arc furnace is a refining furnace -- alloying elements can be subtracted or added as the case requires -- while an induction furnace is merely a melting furnace -- alloys can only be added to the melt and that's it. It is similar to a cooking pot where ingredients can only be added to the broth. It can be seen that the charge for the arc-furnace is not as sensitive as the charge needed for the induction furnace, because bad elements can be taken out. I sometimes jokingly say that with the arc-furnace we can have "garbage in and good steel out." The acid-electric complete oxidation melting pro- cess There are two ways of making acid-electric steel: the complete-oxidation process and the partial-oxidation process. The first one carries out oxidation of the elements to as far as they will go, while the partial-oxidation process oxidizes the elements up to a certain level by catching the carbon content on its way down and "kills" the melt. In both cases, the required alloy additions are then made to meet composition specifications. It will be noted that the partial-oxidation process is less expensive in power consumption; however, it is the more "tricky" to handle metallurgy-wise. Many steel foundries using the acid-electric practice have found it necessary to employ the complete-oxidation process in melting their steel. This is because the complete-oxidation process makes better and slightly superior steel. Most of the cast steel produced in these foundries must pass minimum mechanical specification requirements, and many are subjected to radiographic and magnetic inspection. The process consists of melting at high voltages which are gradually lowered as the melting progresses. The arcs that each electrode makes with the metal charge are erratic and inconsistent (on and off) until there appears a pool of molten metal where the electrodes can maintain their arcs. The voltage is lowered and kept at the most acceptable minimum level when the metal is completely melted. The heat from the arcs, besides heating the metal directly, also heat up the furnace wall and the furnace roof which all contribute to the heat needed for melting the metal. During the melting operation, and the following boiling period (called the carbon- boil), the oxidation reaction of C + FeO → Fe + CO occurs, in which the iron oxide is supplied by the rust on the charge and by iron ore or mill scale added to the melt, or even, by gaseous oxygen injected by an oxygen lance into the melt. Adding these oxygen-bearing charges to the almost fully molten charge results in a more vigorous boiling action (which is responsible for cleaning the steel of impurities and unwanted gases). The carbon boil is very effective, since it reduces the FeO content in the metal toward the equilibrium value, which is another way of saying that the possible content of nonmetallic inclusions is reduced. Moreover, the carbon boil is an additional security against the inclusion of hydrogen and nitrogen, since these gases are able to leave the bath only through the formation of CO bubbles. The melt-down carbon will fall anywhere from 0.18 to 0.40 percent. Usually, carbon at melt-down is around 0.20 percent. The manganese content at melt-down will be about 0.25 to 0.35 percent, and the silicon content will be 0.13 to 0.20 percent for the standard practice which calls for the addition of oxygen (in the form of iron ore, mill scale, or gas) after all the charge is melted. The amount of iron ore used ranges from 0.75 to 4 percent of the weight of the charge. The average is about 1.25 to 1.75 percent. The amount of iron ore added will depend on the carbon content of the charge. Chromium ore and manganese ore are sometimes used as oxidizing agents. The chromium ore is used, of course, only in the manufacture of chromium-bearing cast steels. The addition of iron ore to the melted charge will produce a vigorous boiling action and the carbon, silicon, and manganese contents of the bath will be materially reduced. It is usual practice to reduce carbon to approximately 0.10 percent. The actual carbon content is not as important as maintaining a vigorous boil for 15 to 25 minutes. The carbon drop will indicate whether or not this requirement has been carried out. It should be pointed out that a practice of complete oxidation can be employed even though the carbon content of the bath at the end of the boil is How to make good steel for castings - part 2 T By Prof. John H. D. Bautista, Consultant, Philippine Metalcasting Association, Inc. 44 www.metals.rala.com.au