Magic of Stainless Steel

Types of Stainless Steel

are essentially alloys of iron with more than 10.5% chromium. These grades may contain additional elements of nickel, manganese, carbon, nitrogen and silicon. They can further be modified for special purposes by addition of molybdenum, titanium, niobium, silicon, sulphur etc. A wide range of these grades have been developed based on specific requirements. These are classified into following categories based on their micro structure:

• Austentic Stainless Steel

  Austenitic Stainless Steel grades are characterized by superior corrosion and oxidation resistance, weldability, ductility and toughness compared to ferritic and martensitic stainless steel grades for similar levels of chromium. These grades exhibit excellent resistance to atmospheric corrosion, withstand attack of organic acids (e.g. acetic, lactic, citric etc.), exhibit good resistance to oxidizing acids (e.g. nitric acid), and fair resistance to mineral acids (e.g. sulfuric acid). They are well suited for severe forming. Austenitic Stainless Steel grades are nonmagnetic in annealed condition but depending on composition, they may become mildly magnetic when cold worked. They also exhibit excellent low temperature ductility and impact strength. Austenitic Stainless Steel grades can be readily fabricated by bending, drawing, spinning, punching, drilling, machining and welding and can be readily polished to a high finish. These attributes make them very versatile and popular for diverse applications in a variety of industries.



• Martensitic Stainless Steel

  Martensitic Stainless Steel grades are plain chromium grades containing 11.5 % to 18% of chromium with relatively high carbon content (0.1% - 1.2%). Initially developed for cutlery, these are well suited for applications requiring high hardness and resistance to abrasion and erosion. These grades are magnetic and display fair cold forming characteristics. Although these can be hardened by air cooling, oil quenching is sometimes used to assure uniform hardening. These grades can be welded but require stress relieving after welding. They exhibit their best corrosion resistance in the hardened condition and perform well in mildly corrosive, environments. Martensitic Stainless Steel grades are commonly used for knife blades, turbine blades, surgical instruments, fasteners, shafts etc.



• Ferritic Stainless Steel

  Ferritic Stainless Steel grades are non-hardenable plain chromium grades with chromium content varying from 10.5% to 28% and with low carbon content. These are magnetic and exhibit a better resistance to corrosion than martensitic grades. These grades are employed in applications where the desired formability, weldability and corrosion resistance is between those of martensitic, and austenitic types. The ferritics can be polished or buffed to achieve high luster.



• Duplex Stainless Steel

  Duplex Stainless Steel grades contain relatively high chromium (between 18% and 28%) and moderate amounts of nickel (1% to 8%). This combination of ferritic and austenitic structures is called duplex. Many of these grades contain molybdenum (1% to 5%) and nitrogen (0.05% to 0.3%). Some duplex Stainless Steel grades also contain manganese (up to 5%), copper (up to 2%) and tungsten (up to 2%. These grades exhibit high resistance to stress, corrosion cracking and chloride ion attack and have higher yield strength than that of austenitic or ferritic steel grades. These properties combined with suitable design lead to material saving. High quality fabrication and welding are possible if the operator is trained well. These grades are used in marine applications, offshore platforms, paper and pulp industry, chemical, petrochemical and desalination plants.

Stainless steel is often confused with mild steel or carbon steel. While carbon steel is a less flexible alloy of steel, stainless steel is an alloy of iron and has minimum 10.5% Chromium as a chief component.

The increased resistance to corrosion in stainless steel is due to the naturally occurring chromium-rich oxide film formed on its surface. Although extremely thin, this inert film is adherent to the metal and is extremely protective in a wide range of corrosive media. The film is rapidly self repairing, in the presence of oxygen, if damaged by any external force.

Metals are extracted from their natural and most stable states as ores. This consumes a great amount of energy and effort. All these efforts adversely affect our environment in one way or another. Therefore, the more we recycle the scrap/end-of-life metals, the better off we will be.

• Recycling of Stainless Steel Scrap

  Did you know that 90% of stainless steel is recycled?
  
  Stainless steel’s long service life, 100% recyclability, and its raw materials make it an excellent environmental performer.
  
  Stainless steel objects rarely become waste at the end of their useful life. Recycled stainless steel objects are systematically separated and recovered to go back into the production process through recycling. Stainless steel is actively recycled on a large scale around the world by recyclers who collect and process scrap (recycled stainless steel) for re-melting all around the world.
  
  Stainless steel contains valuable raw materials like chromium and nickel which makes recycling economically viable.



• Scrap collection

  The use of stainless steel scrap is fundamental to the steel-making process. There are two types of scrap - reclaimed scrap (old scrap) and industrial scrap (new scrap). Reclaimed scrap includes industrial equipment, tanks, washing machines and refrigerators that have reached the end of their service life. Industrial scrap includes industrial returns or production off-cuts from manufacturing by industrial engineering and fabrication sources.
  
  Today, stainless steel is made up of approximately 60% recycled content including:
  • 25% reclaimed scrap
  • 35% industrial scrap
  • 40% new raw materials
  
  The useful service life of stainless steel products is long. So, the availability of scrap is dependent on levels of production from decades ago. With an average content of 25% of old scrap, stainless steel is close to the theoretical maximum content of material from end-of-life products.



• Recycling the scrap

  Specialised expertise and sophisticated technology is needed in recycling to separate and prepare each type of alloy for re-melting. A recycling processor feeds the scrap into a large shredder to break it into smaller pieces. It is then chemically analysed and stored by type. This process may include ‘blending’ the scrap into chrome steels, nickel alloys, and other types of stainless steels. After blending into piles for specific customer requirements, the scrap is then loaded into containers for export to overseas mills.



• The global market for scrap

  Virtually, all Australian stainless steel scrap goes overseas. There’s a small market for stainless steel scrap in Australia for use in the foundries business. Foundries often use profile off-cut or plate material scrap products.
  
  Majority of stainless steel scrap in Australia is exported in a year to stainless steel mills in countries including China, South Korea, Taiwan, India, and Japan.
  
  For mills, scrap is important because recycled stainless steel contains valuable raw elements including chromium, nickel, and molybdenum that are gathered, processed, and reused in the production process. The more scrap used in furnaces by mills, the less raw materials are required in the production process.



• Stainless steel mills

  Scrap along with other raw materials, ferro-chromium (chrome/iron), ferro-molybdenum (molybdenum/iron), and nickel are blended into an electric furnace.
  
  After melting, impurities are removed, the molten metal is refined and the chemistry is analysed to determine what final adjustments are necessary for the specific type of stainless steel being produced.
  
  The molten stainless steel is then cast into slabs or billets before production of plate, sheet, coil, wire, and other forms for use by industrial manufacturers.



• The stainless returns to you

  Industrial manufacturers produce stainless steel items that you use everyday including cutlery, pots and pans, kitchen sinks, and many architectural, industrial and other components. At each stage of the production process, stainless steel retains its basic properties and utility value. Unlike many industrial and engineering materials, stainless steel may be returned to its original quality in the supply chain without any degradation.
  
  You can be assured that even after its long service life, your environmentally-efficient stainless steel will always return to you bright, shining and new as ever!

Speaking strictly in economic terms, cost is always associated with Value of Money. The cheapest alternative may not always be the best option to choose when the entire life of the product is taken into account. Therefore, life cycle costing becomes an important key.

Life Cycle Cost (LCC) analysis is a means of quantifying the choice of materials for a product or construction, with the aim of selection of the most economic alternative. Stainless steel normally comes with an initially higher cost of material, but its longer life due to excellent wear and corrosion resistance, low requirement of maintenance, and recyclability compensate for the higher initial cost and make it an environment friendly material or ‘Green Material’.

• Why is LCC important?

  With the advent of new technology in every stream of engineering and the fast pace of the infrastructural and industrial progress, engineers are faced with daunting challenges in designing and constructing robust structures and machineries that are long lasting and low in maintenance. It is therefore necessary that engineers take the total life cycle costing for their designed structures into account.
  
  Corrosion is the major culprit for loss of value and reduction in useful life of any structure. It causes plant shut downs, waste of valuable resources, loss or contamination of products, reduction in efficiency and costly maintenance. Awareness with respect to corrosion is very less in our country. A NACE International study estimates the global cost of corrosion to be USD 2.5 trillion, equivalent to roughly 3.4% of the global Gross Domestic Product (GDP).
  
  Stainless steel, by virtue of its special chemical composition, is one such value added material for construction that offers long lasting and low maintenance structures. A good understanding of life cycle costing (LCC) is important to make informed choices and create values that will serve for years to come.



• How LCC is calculated?

  Corrosion is the major culprit for loss of value and reduction in useful life of any structure. Material costs are assessed with their implications. For example, initial outlay, maintenance and its frequency, downtime effects, production losses, repair, replacement, and other operationally related costs such as manpower and energy consumption.



LCC = Acquisition Cost + Fabrication and Installation Cost + Maintenance Costs (periodic) + Replacement Costs (periodic) + Cost of Lost Production (periodic) - Residual (Scrap) Value.