Why spend more when Induction can give the same quality?
Alloy steel is alloyed with a variety of elements in different amounts in order to improve its mechanical properties. There are low and high alloy steels but since the exact definitions of each of these have not been totally agreed upon, most experts refer to low alloy when talking about any alloy steels. As a general rule, this type of steel is manufactured either through the electric arc furnace process or the electric induction furnace process. Both have similarities and differences; below are a few of the comparisons between these two processes.
When it comes to nitrogen content, it is higher when making alloy steel through the electric arc furnace process than with the induction furnace process. This is because the arc furnace ionizes the nitrogen molecule to atoms that will be absorbed by the molten steel. With induction furnaces, the nitrogen content is lower although the oxygen content is higher. Finally, the alloy quick life value is lower in the induction furnace process than the arc furnace process. The oxygen can then be controlled via Vacuum Degassing.
Here are some other differences between these two processes:
- Alloy element smelting: The induction furnace has a higher metal recovery rate than the arc furnace.
- Alloy element smelting burn-out ratio: This is lower in the induction furnace process than the arc furnace process.
- Environmental concerns: Induction furnaces are better for the environment, in part because of arc furnace steelmaking results in a lot of waste residue, exhaust gas, noise, and energy.
- Since no graphite electrodes are used in an induction furnace there is no carbon pick up.
- In an induction furnace due to Eddy current churning effect, the molten steel is circulated automatically by electromagnetic action so the homogeneous product is ensured in the minimum time. It also helps to stir the charge materials and creates optimum heat transfer condition for melting of the charge materials.
The electric arc furnace is stronger when it comes to dephosphorization than the induction furnace is. With the induction furnace, the slag is cold and the temperature of the slag is maintained by molten steel. With the arc furnace, the slag is hot and the furnace itself keeps it hot. The arc furnace can complete the dephosphorization by the furnace slag, which also allows for sufficient expansion for the deoxidation process. This is why the ability of the arc furnace to de-phosphorize is better than the induction furnace. Arc is preferred when phosphorus requirements are less than 0.015.
As a general rule, arc furnaces are different from induction furnaces when it comes to the charged material, which is directly exposed to an electric arc. In addition, the current that flows in the furnace terminals passes through the charged materials. It is possible to use a wide variety of charged material in arc furnaces whereas in induction furnaces the charged material depends on the final grade of the material to be manufactured.
Does this mean that one of these processes is better than the other?
Not really. Arc furnaces are usually in large batches starting from 25000kgs and above capacity, whereas induction furnaces can be found even as low as 500kg capacity so it is more cost-effective in smaller batch sizes from 500kg to 20000kg batches.
Melting in arc furnace is necessarily followed by ladle refining and then vacuum degassing to make the final alloy steel whereas in induction it is possible to not do the refining and degassing and achieve the chemical composition in the induction furnace compromising the final quality of alloy steel. This leads to a general perception of the quality of alloy steel in induction vs arc. However, given the induction furnace process also does the ladle refining and vacuum degassing, steel made induction is much cheaper than made in arc helping users save massive costs and also order smaller batches.
Further, since no oxygen is lanced in the induction furnace the oxides formation is less thus inclusions are less; also the bath is continuously covered by slag thus less oxide formation.
Even though there are certain inherent inclusion generations during the various reactions taking place, which is true for both IF-> LRF-> VD and EAF-> LRF-> VD routes. In both, the process routes proper slag in LRF, effective de-oxidation/desulphurisation/ VD/ Soft Argon bubbling will produce good quality clean steel.
Also in EAF along with Steel Scrap, Pig Iron, Sponge Iron(DRI) are used which are having high impurities & oxides and further oxygen is blown to melt the heat. Therefore the burden of impurities, inclusions are high in EAF melting and subsequently more efforts are required in the refining process to produce clean steel. At the same time in the induction, route heats are melted with selected scrap without any blow of oxygen resulting in a burden of impurities/inclusions are less hence better refining. So as long as the steel is ladle refined and vacuum degassing after induction melting, the quality of steel is at par with arc furnace steel.