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An introduction to Stainless Steel:

Stainless steel:

In metallurgy stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content. 

Stainless steel does not stain, corrode, or rust as easily as ordinary steel, but it is not stain-proof. 

It is also called corrosion-resistant steel or “CRES” when the alloy type and grade are not detailed, particularly in the aviation industry. 

There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. 

Stainless steel is used where both the properties of steel and resistance to corrosion are required.

Stainless steel differs from carbon steel by the amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.

A few corrosion-resistant iron artefacts survive from antiquity. A famous (and very large) example is the Iron Pillar of Delhi, erected by order of Kumara Gupta I around the year AD 400. Unlike stainless steel, however, these artefacts owe their durability not to chromium, but to their high phosphorus content, which, together with favourable local weather conditions, promotes the formation of a solid protective passivation layer of iron oxides and phosphates, rather than the non-protective, cracked rust layer that develops on most ironwork.

Stainless steel’s resistance to corrosion and staining, low maintenance, relatively low cost, and familiar lustre make it an ideal base material for a host of commercial applications. 


                                                                  

There are over 150 grades of stainless steel, of which fifteen are most commonly used. The alloy is milled into coils, sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment e.g. in sugar refineries, and as an automotive and aerospace structural alloy and construction material in large buildings. Storage tanks and tankers used to transport orange juice and other food are often made of stainless steel, due to its corrosion resistance and antibacterial properties. This also influences its use in commercial kitchens and food processing plants, as it can be steam-cleaned, sterilised, and does not need painting or application of other surface finishes.

There are four major types of stainless steel. Of these, austenitic is the most widely used type. It has a nickel content of at least 7%, which makes it very flexible. It is used in a range of household products, industrial piping and vessels, constructional structures and architectural facades.

Austenitic is the most widely used type. It has a nickel content of at least 7%, which makes it very flexible. It is used in a range of household products, industrial piping and vessels, constructional structures and architectural facades. 

Ferritic stainless steel has similar properties to mild steel, but better corrosion resistance. This type of steel is commonly used in washing machines, boilers and indoor architecture. 

Martensitic stainless steel is a very hard, strong steel. It contains around 13% chromium and is used to make knives and turbine blades.

There is also a duplex stainless steel that is a composite of austenitic and ferritic steels. This steel is both strong and flexible. Duplex steels are most commonly used in the paper, pulp and shipbuilding industries. They are also widely used in the petrochemical industry.

Stainless steel is a very versatile material. It can literally be used for years and remain stainless. Stainless steel products have a significantly longer lifespan than products made of other materials. There are less maintenance costs, and stainless steel also has a very high scrap value on decommissioning.



What is Stainless Steel Made of?

The major components of stainless steel are: iron, chromium, carbon, nickel, molybdenum and small quantities of other metals. On the basis of its crystalline structure, stainless steel can be broadly categorized into four different types. It is to be noted that iron is always the main constituent and the rest of the chemical substances are present in varying proportions in each type of stainless steel. However, in any of them, the chromium content of should not go below 11 percent. This is because; the rich content of chromium in stainless steel makes it highly corrosion resistant. Here, their chemical composition has been described in detail.

Austenitic Stainless Steel
More than seventy percent of the total stainless steel produced belongs to this category. It has at least 16 percent of chromium, 0.15 percent of carbon and a good amount of nickel and manganese along with iron. Such a composition enables it to maintain its distinct structure at any given temperature, from the cryogenic to its melting point. It is used in various cryogenic applications due to presence of nickel that prevents the brittleness of steel. Sometimes, elements like molybdenum or titanium are added in small amounts to bring about changes in certain properties to make it suitable for any specific application that requires corrosion resistance at extremely high temperatures. Usually, this steel does not show any magnetic properties.

Ferritic Stainless Steel
This stainless steel is mostly made of chromium, molybdenum and small amounts of nickel, carbon aluminium, titanium. Some of the types also include lead as a component. The chromium content lies between 10.5 to 27 percent. The main characteristics of these steels is that they have the power to resist corrosion even at high temperatures. This is possible on account of the large quantity of chromium present in it. However, its durability is comparatively lesser than austenitic steel. This type of steel possesses magnetic properties.

Duplex Stainless Steel
This can be described as a mixture of equal quantities of austenite and ferrite stainless steels. When used for commercial purposes, these elements can at times be in the ratio of 40/60. Hence, it has higher strength and resistance against pitting, stress corrosion and cracking. Both chromium and molybdenum are present in higher quantity in this type of steel as compared to austenitic stainless steel. The chromium content varies between 19 to 28 percent and molybdenum can go up to 5 percent. It has lesser amounts of nickel than those found in austenite stainless steel.

Martensitic Stainless Steel
This stainless steel is made of 12 to 14 percent of chromium, about 1 percent each of carbon and molybdenum and about 2 percent of nickel. Due to lower chromium content, it is more brittle than other stainless steels. It is a very tough and strong kind of steel due to the high amounts of carbon. It can be used in a wide range of applications because of its strength, but is not very effective in resisting corrosion.



The Manufacturing Process          

The manufacture of stainless steel involves a series of processes. First, the steel is melted, and then it is cast into solid form. After various forming steps, the steel is heat treated and then cleaned and polished to give it the desired finish. Next, it is packaged and sent to manufacturers, who weld and join the steel to produce the desired shapes.

Melting and casting

The raw materials are first melted together in an electric furnace. This step usually requires 8 to 12 hours of intense heat. When the melting is finished, the molten steel is cast into semi-finished forms. These include blooms (rectangular shapes), billets (round or square shapes 1.5 inches or 3.8 centimetres in thickness), slabs, rods, and tube rounds.

Forming

Next, the semi-finished steel goes through forming operations, beginning with hot rolling, in which the steel is heated and passed through huge rolls. Blooms and billets are formed into bar and wire, while slabs are formed into plate, strip, and sheet. Bars are available in all grades and come in rounds, squares, octagons, or hexagons 0.25 inch (.63 centimetre) in size. Wire is usually available up to 0.5 inch (1.27 centimetres) in diameter or size. Plate is more than 0.1875 inch (.47 centimetre) thick and over 10 inches (25.4 centimetres) wide. Strip is less than 0.185 inch (.47 centimetres) thick and less than 24 inches (61 centimetres) wide. Sheet is less than 0.1875 (.47 centimetres) thick and more than 24 (61 centimetres) wide.

Heat treatment

After the stainless steel is formed, most types must go through an annealing step. Annealing is a heat treatment in which the steel is heated and cooled under controlled conditions to relieve internal stresses and soften the metal. Some steels are heat treated for higher strength. However, such a heat treatment—also known as age hardening—requires careful control, for even small changes from the recommended temperature, time, or cooling rate can seriously affect the properties. Lower aging temperatures produce high strength with low fracture toughness, while higher-temperature aging produces a lower strength, tougher material.

Though the heating rate to reach the aging temperature (900 to 1000 degrees Fahrenheit or 482 to 537 degrees Celsius) does not effect the properties, the cooling rate does. A post-aging quenching (rapid cooling) treatment can increase the toughness without a significant loss in strength. One such process involves water quenching the material in a 35-degree Fahrenheit (1.6-degree Celsius) ice-water bath for a minimum of two hours.

The type of heat treatment depends on the type of steel; in other words, whether it is austenitic, ferritic, or martensitic. Austenitic steels are heated to above 1900 degrees Fahrenheit (1037 degrees Celsius) for a time depending on the thickness. Water quenching is used for thick sections, whereas air cooling or air blasting is used for thin sections. If cooled too slowly, carbide precipitation can occur. This build-up can be eliminated by thermal stabilization. In this method, the steel is held for several hours at 1500 to 1600 degrees Fahrenheit (815 to 871 degrees Celsius). Cleaning part surfaces of contaminants before heat treatment is sometimes also necessary to achieve proper heat treatment.

Descaling

Annealing causes a scale or build-up to form on the steel. The scale can be removed using several processes. One of the most common methods, pickling, uses a nitric-hydrofluoric acid bath to descale the steel. In another method, electrocleaning, an electric current is applied to the surface using a cathode and phosphoric acid, and the scale is removed. The annealing and descaling steps occur at different stages depending on the type of steel being worked. Bar and wire, for instance, go through further forming steps (more hot rolling, forging, or extruding) after the initial hot rolling before being annealed and descaled. Sheet and strip, on the other hand, go through an initial annealing and descaling step immediately after hot rolling. After cold rolling (passing through rolls at a relatively low temperature), which produces a further reduction in thickness, sheet and strip are annealed and descaled again. A final cold rolling step then prepares the steel for final processing.

Cutting

Cutting operations are usually necessary to obtain the desired blank shape or size to trim the part to final size. Mechanical cutting is accomplished by a variety of methods, including straight shearing using guillotine knives, circle shearing using circular knives horizontally and vertically positioned, sawing using high speed steel blades, blanking, and nibbling. Blanking uses metal punches and dies to punch out the shape by shearing. Nibbling is a process of cutting by blanking out a series of overlapping holes and is ideally suited for irregular shapes.

Stainless steel can also be cut using flame cutting, which involves a flame-fired torch using oxygen and propane in conjunction with iron powder. This method is clean and fast. Another cutting method is known as plasma jet cutting, in which an ionized gas column in conjunction with an electric arc through a small orifice makes the cut. The gas produces extremely high temperatures to melt the metal.

Finishing

Surface finish is an important specification for stainless steel products and is critical in applications where appearance is also important. Certain surface finishes also make stainless steel easier to clean, which is obviously important for sanitary applications. A smooth surface as obtained by polishing also provides better corrosion resistance. On the other hand, rough finishes are often required for lubrication applications, as well as to facilitate further manufacturing steps.

Surface finishes are the result of processes used in fabricating the various forms or are the result of further processing. There are a variety of methods used for finishing. A dull finish is produced by hot rolling, annealing, and descaling. A bright finish is obtained by first hot rolling and then cold rolling on polished rolls. A highly reflective finish is produced by cold rolling in combination with annealing in a controlled atmosphere furnace, by grinding with abrasives, or by buffing a finely ground surface. A mirror finish is produced by polishing with progressively finer abrasives, followed by extensive buffing. For grinding or polishing, grinding wheels or abrasive belts are normally used. Buffing uses cloth wheels in combination with cutting compounds containing very fine abrasive particles in bar or stick forms. Other finishing methods include tumbling, which forces movement of a tumbling material against surfaces of parts, dry etching (sandblasting), wet etching using acid solutions, and surface dulling. The latter uses sandblasting, wire brushing, or pickling techniques.

Manufacturing at the fabricator or end user

After the stainless steel in its various forms are packed and shipped to the fabricator or end user, a variety of other processes are needed. Further shaping is accomplished using a variety of methods, such as roll forming, press forming, forging, press drawing, and extrusion. Additional heat treating (annealing), machining, and cleaning processes are also often required.

There are a variety of methods for joining stainless steel, with welding being the most common. Fusion and resistance welding are the two basic methods generally used with many variations for both. In fusion welding, heat is provided by an electric arc struck between an electrode and the metal to be welded. In resistance welding, bonding is the result of heat and pressure. Heat is produced by the resistance to the flow of electric current through the parts to be welded, and pressure is applied by the electrodes. After parts are welded together, they must be cleaned around the joined area.

Quality Control

In addition to in-process control during manufacture and fabrication, stainless steels must meet specifications developed by the American Society for Testing and Materials (ASTM) with regard to mechanical properties such as toughness and corrosion resistance. Metallography can sometimes be correlated to corrosion tests to help monitor quality.

 

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