Steel is basically up of an alloy of carbon and iron, where the presence of carbon may vary from 0.25% to 1.5% and hence steel is primarily classified as low carbon steel (around 0.25% carbon), medium carbon steel (between 0.25% to 0.75% carbon) and high carbon steel (0.75% to 1.5% carbon).
Steel may be manufactured through the following principle methods:
- Cementation Process
- Crucible Process
- Bessemer Process
- Open-hearth Process
- Electric Process
Let’s look at these one by one.
Cementation Process: In this process wrought iron bars are introduced in a furnace in between powdered charcoal layers and are subjected to a very high temperature – about 7000 Celsius for about a week to fortnight depending upon the required quality of the steel. The conditions slowly diffuse carbon into iron and cause the carbon to become dissolved in the iron, raising the carbon percentage. Steel obtained from this process is called “blister steel" due to the blister-like marks formed on the surface due to the evolved gases during the manufacturing process. The carbon amount here is usually around 0.75% to 1.5%.
Crucible Process: The process involves heating of either blister steel fragments or short lengths of wrought iron bars mixed with charcoal inside fire clay crucibles. The resulting molten steel is allowed to run through iron moulds. Such steel is called “cast iron." Cast steel is extremely hard and perfectly homogenous. These are specifically used for making cutting tools and the finest cutlery items.
Bessemer Process: In this process pig iron is melted in a cupola and poured into Bessemer converter which is pear shaped and has a steel shell lined with refractory material. It’s pivoted on trunnions so as to facilitate tilting, pouring or charging.
Once the above converter is charged with molten pig iron, a strong thrust of air is blasted across the molten mass for about 20 minutes through nozzles provided at the bottom of the vessel. The process oxidizes all traces of the carbon and silicon present, leaving the converter with pure iron.
After this the blasting of air is stopped and the specified amount of ferro-manganese is added to it for the sake of including the recommended content of carbon and manganese to the steel.
The air blasted procedure is again initiated for some time, ensuring perfect mixing of the alloy.
The converter is then tilted so that the molten material can be discharged into the ladles. In the final step the molten alloy is shifted into rectangular moulds where it’s obtained in the form of solid ingots.
Open Hearth Process: The specialty of open-hearth furnaces is the extreme heat that can be obtained from them due to their regenerative process. The charge of pig iron, steel scrap, iron ore, and flux are together kept in a shallow container with a flame burning above it. The process is initiated inside reverbaratory gas-fired regenerative furnaces for greater efficiency.
Regenerators are placed below the furnace and positioned in two pairs. The pairs are heated alternately through the passage of hot gases given out from the furnace in their route to the chimney. This heat is retained by the regenerators and is reversed and given back to the furnace. This heat exchange procedure helps the furnace to maintain high temperatures even with less fuels.
Once the furnace is charged with pig iron, pure oxidizing ores like haematite are added to it from time to time, which helps oxidization and the removal of impurities like silicon, carbon, and manganese in the pig iron. Spiegel is also introduced when the carbon content becomes less than 0.1%, and ferro-manganese after the metal is tapped out into the ladle. Ferro-manganese becomes important for restoring malleability and also for carburizing the iron.
Electric Process: In this process electric arc or electric high frequency furnaces are used. In electric arc furnaces which are more common among the two processes, high voltage electric arc struck between carbon electrodes and the charge becomes the source of a very high temperature. The charge is collected directly from an open hearth furnace, the intense arc heat keeps the charge in its molten state, and the impurities are removed in the form of slag.
The high frequency furnace is based on the principle that when high frequency alternating current is applied to steel, eddy currents starts flowing in them. If this induction is made very strong, it can heat up the steel and melt it.
Electric furnaces are more advantageous compared to the other steel manufacturing processes due to the absence of evolving gases, fumes, etc., which normally become a major problem with fuel operated furnaces.
Image Credit: From the book "An introduction to the study of metallurgy", By Sir William Chandler Roberts-Austen