Stainless steel is metallurgically defined as an alloy with 11% chromium. The metal is popularly used in domestic and industrial appliances, since it does not corrode, stain or rust as easily as regular steel. Although the metal is stain-less, it is not stain-proof. The alloy is also referred to as CRES or corrosion-resistant steel, especially when the alloy is not graded. It is now available in various grades and surface finishes to suit industrial and domestic requirements and the environment to which the metal is expected to be exposed to. Different grades of stainless steel have different amounts of chromium to generate the desired film of chromium oxide. It is this chemical reaction between chromium and atmospheric oxygen that prevents surface corrosion, and that along the internal structure.
Making Stainless Steel
Stainless steel is made from iron ore, silicon, chromium, carbon, nickel, manganese and nitrogen. Its manufacturing comprises a series of processes. The raw materials are first melted in an electric furnace. They are subjected to at least 12 hours of intense heat. Thereafter the mixture is cast into either blooms, slabs or billets, before taking on a semi-solid form. This initial form of steel is then processed via 'forming' operations that include hot-rolling into bars, wires, sheets and slabs. From here, the steel is subjected to annealing. This is a heat treatment in which the metal is first heated and then cooled under extreme, controlled conditions. The metal is thus treated for internal stresses and is duly softened and strengthened. This segment of processing the metal is also referred to as 'age hardening'. It requires careful monitoring of temperature and heating and cooling times. The aging temperature seriously affects metal properties; while lower temperatures generate high-strength and low-fracture toughness, higher temperatures result in a lower strength, but tougher material. The heat treatment involved in the manufacturing of stainless steel depends on the type and grade of steel being produced. Annealing or the heat treatment leads to the development of a scale. The scale can be removed via several methods such as:
- Pickling or the use of nitric-hydrofluoric acid bath.
- Electro-cleaning or the application of an electric current, using phosphoric acid and a cathode.
De-scaling of the material is introduced into the manufacturing process at different times, depending on the type of steel being produced. While the bar and wire forms have to be additionally treated with hot rolling, forging and extruding, the sheet and strip forms go through annealing after hot rolling. Cutting operations, in the manufacturing process, are very essential to obtain the desired shape and size of the end product. Mechanical cutting involves the use of guillotine knives and high speed steel blades for blanking (punching out the shape by shearing) and nibbling (cutting out a series of overlapping holes). It is also cut via flame cutting, a process that involves the use of a flame generated by oxygen, propane and iron powder. The plasma jet cutting method uses an ionized gas column to melt and cut the metal. Surface finish, the final step in the manufacture of stainless steel, is critical to obtain the smooth and reflective surface that the metal is popular for. This final stage offers the product the desired corrosion resistance and gets the metal ready for further specific industrial manufacturing steps, as required. In the surface finish stage, the metal is subjected to treatment according to the physical appearance desired; either a dull finish, bright finish or mirror finish. Manufacturing of end products involve further shaping via heat-rolling, pressing, forging and extrusion. The material is then joined via welding (fusion and resistance) and given the desired shape. In-process quality control is monitored throughout the manufacture and fabrication of stainless steel. The material is constantly checked for optimum mechanical properties, to survive antiquity.
