Stainless steels (SS) were invented to overcome the problem of corrosion which is a major concern of food and many other industries. The alloy of steel containing iron-chromium-nickel is known as stainless steels. Stainless steels typically contain between 9 and 30 percent chromium and varying amounts of nickel, molybdenum, copper, sulfur, titanium, niobium, etc., may be added to obtain the desired mechanical properties and service life. Stainless steel is considered noble metal for use in dairy industry. Stainless steels are classified based on the chemical composition and it provides information to overcome many types of corrosion. Some of the limitations of SS employed in food and dairy industry are attack by lactic and malic acids at elevated temperature and poor thermal conductivity. However, these limitations may be overcome by carefully selection and fabrication, optimized operating condition, care and maintenance of the equipment.
History
The works on material science at several places around the word lead to the discovery and development of the stainless steels. Stainless steels were invented by Stahl in the year 1912. This invention has helped a lot to improve corrosion resistivity and mechanical behavior of the material as compared to pure steels. These developments in metallurgy were not significantly to fulfill requirements of food industry. English metallurgist early in the year 1913 working on a project to improve rifle barrels accidentally discovered that adding chromium to low carbon steel gives it stain resistance. The first application for these stainless steels was in cutlery manufacturing in which the previously used carbon steel was replaced by the new stainless steels.
Among the iron-base alloys investigated were iron-chromium-nickel alloys with high chromium contents. It was found that specimens of alloys with more than 20% Cr did not rust in the laboratory for quite some time. It is subsequently concluded that at least 20% chromium was necessary to achieve resistance to oxidation or scaling. This was the starting point of the development of heat-resistant steels for applicable in chemical, food, dairy, beverage, bio-processing, pharmaceuticals industries.
Classification of Stainless Steel
Stainless steels are basically classified as austenitic, ferritic, martensitic, duplex and super-austenitic grades. Each of these main groups contains a number of alloys that are defined according to the chemical composition and specified in European and American International Standards. Apart from chromium, the alloy constituents molybdenum, nickel and nitrogen are of great importance to the corrosion resistance. Carbon will always be present to a certain degree, and it is important for the welding properties [6]. In addition, copper, manganese, sulphur, titanium and niobium are used as alloy constituents to impart certain properties. Stainless steel is typical wrought alloy AISI (American Iron and Steel Institution) series designations, includes: 200 (high manganese austenitic), 300 (austenitic), and 400 and 500 (ferritic and martensitic). Martensitic and ferritic steels are magnetic and martensitic steels are typically hardened by heat treatment and are not easily formable. Austenitic steels harden when cold worked. Duplex grades (austenitic/ferritic) are more resistant to stress corrosion cracking than austenitic and are tougher than ferritic grades.
Desirable Properties of Stainless Steel
The properties of stainless steel play an important role in the design of various equipment. The use of high quality stainless steel in fabrication of processing equipment helps not only to prevent corrosion but also ensures purity of food product handled in that equipment. In addition to this, stainless steels are easy to clean and maintain and a number of different products can be manufactured in the same equipment. If properly utilized, equipment made of stainless steel can be expected to last for many years. On a life-cycle basis, the alloys are often the most cost-effective.
Important characteristic to be considered in selecting the proper type of stainless steel for a specific application are listed below:
Corrosion resistance
Resistance to oxidation and sulfidation
Strength and ductility at ambient and service temperatures
Suitability for intended fabrication techniques
Suitability for intended cleaning procedures
Stability of properties in service
Toughness
Resistance to abrasion, erosion, galling, and seizing
Surface finish and/or reflectivity
Physical property characteristics such as magnetic properties, thermal conductivity and electrical resistivity
Total cost, including initial cost, installed cost, and the effective life expectancy of the finished product
Conclusion
Stainless steels of various grades are widely used as engineering material for the fabrication of equipment. Stainless steels are corrosive resistant but under certain conditions the materials can stain and can corrode. Dairy and food industries are concerned with reliability of equipment and product purity. To achieve these, stainless steels are often the economical and practical materials of choice for process equipment. It is necessary to select most suitable grade of stainless steel depending on the requirements for the specific application. The food and dairy processing industries, corrosion becomes a critical factor directly influencing equipment, and indirectly, overall equipment longevity. Key to minimizing corrosion impact needs better understanding of factors affecting corrosion as early as the equipment design phase and throughout subsequent processing operation.
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