The Background of Stainless Steel
If you are looking for information on stainless steel fabrication, it pays to have a little background information on the subject. Stainless steel is alloy that contains iron and it is used in many ways. Note that an alloy is a substance that contains two or more chemical elements. Stainless steel is valuable because it contains a high level of chromium. This makes it highly resistant to stain or rust. As a rule, 12-20 percent of the alloy must be chromium. This is the acid test and it is an indication that your stainless steel has the right quality. It is not easy for manufacturers of this product to produce inferior quality stainless steel and pass them of as authentic products. This is because there are standards manufacturers are expected to maintain. More than 57 alloys are recognized as standard alloys. Stainless steel is used in many industries and they are also used in different applications. Some of these include rail cars, snow skis, building exteriors and chemical processing plants. Stainless steel is also used in the automobile industry. The engine, the chassis, tubing for fuel lines, fasteners and exhaust trim are all made of stainless steel.
Every year, approximately 200,000 tons of stainless steel is used in the food processing industry in the North American continent. This is nickel-containing stainless steel and it is used for food storing, cooking and handling equipment. This particular type of stainless steel is also used in different stages of food collection, processing and storage. In the context of the food and beverage industry, some of the advantages of stainless steel are durability, corrosion resistance and economy. In addition, this substance is easy to clean and also helps preserve food flavor. According to official figures from the US Department of Commerce, total shipments in 1992 totaled over 1.5 tons. In fact, the exact figure was 1,514,222 tons.
There are different types of stainless steel and they are categorized according to their microstructure. Duplex stainless steels contain equal amounts of austenite and ferrite. Stainless steels of this variety have superior resistance to cracking due to chloride stress corrosion. They also provide much better resistance to crevice corrosion and pitting. For this reason, duplex stainless steels are used extensively in refineries, sea water piping installations, gas processing plants and pulp and paper plants.
One of the most common forms of stainless steel is the austenitic stainless steel. This type contains 6 percent nickel and austenite. Austenite is a carbon-containing iron with a cubic structure. It has relatively good corrosion resistance and it is highly ductile (ductility is the ability of the material to bend without breaking). Other forms of stainless steel are Ferritic stainless steel and Martensitic stainless steel. The former is difficult to weld but it has a better resistance to stress corrosion. The latter contains iron with a needle-like structure.
Raw materials for stainless steel are readily available. Some of these raw materials are carbon, nickel, iron ore, silicon, chromium, manganese and nitrogen. The properties or qualities of the final alloy are tailored by simply varying the amounts of these natural elements. For instance, nitrogen improves corrosion resistance and this makes it ideal for use in duplex stainless steels. Nitrogen also improves ductility and this makes it a vital component of stainless steel.
The Manufacturing Process
The manufacturing of stainless steel or stainless steel fabrication involves a number of processes. The first step is to melt the stainless steel.
To manufacture stainless steel, the raw materials like chromium, silicon, iron ore and nickel are melted in an electric furnace. This process takes between 8 and 12 hours. The next step is to cast the mixture into one of several shapes. These may include billets, blooms or slabs. The steel is heat treated then it is cleaned and polished to give it the perfect finish. After this step, the steel is packaged and sent to manufacturers. The manufacturers join and weld the steel to get the desired shapes and the product is ready for the market.
1. Melting and Casting
The raw materials are melted in an electric furnace at very high temperature. This process takes 8-12 hours and when it is finished, the product is cast into semi-finished forms. These are basically billets round or square shapes of 1.5 inches or 3.8 centimeters thick) or blooms (rectangular shapes). These semi-finished forms also include rods, slabs and tube rounds.
The next step is the forming operation. This begins with hot rolling. During this process, the steel is heated and passed through huge rolls. Slabs are formed into strip, sheet and plate. Blooms and billets are formed into bar and wire. Bars are available in different grades and come in squares, rounds, hexagons or octagons. Wire, plate and strip are usually measured in centimeters and inches. The measurement for wire is usually up to 0.5 inch (1.27 centimeters) in diameter or size. Plate is usually over 10 inches wide (25.4 centimeters) and 0.1875 inch (.47 centimeters) thick. Sheet is less than 0.1875 inch (0.47 centimeters) thick and 24 inches (61 centimeters) wide. Strip is less than 0.185 inch (0.47 centimeters) thick and less than 24 inches (61 centimeters wide).
3. Heat treatment
After the stainless steel is formed, most of these products go through a process called annealing. This is a heat treatment process in which steel is first heated then cooled under the right conditions to soften the metal and relieve internal stress. The heat treatment process can lead to different results. For instance, the higher-temperature aging process produces a tougher material with a lower strength while a lower aging temperature produce high strength steel with low fracture toughness.
Note that the heating rate to reach the required aging temperature does not usually affect the properties of steel but the cooling rate does. The required heating temperature for the aging process is 900-1000degrees Fahrenheit (482 to 537 degrees Celsius). On the other hand, a particular cooling process requires water quenching the material in an ice-water bath for at least two hours. The required temperature for this process is usually 35-degree Fahrenheit (1.6 degree Celsius.
Heat treatment and water quenching are technical processes and this is why they should be done the right way to get perfect results. The type of heat treatment you use depends on the type of steel you are dealing with. Ferritic, martensitic and austentic steels require different heating processes. Austenitic steels are usually heated to temperatures of 1900 degrees Fahrenheit and above (1037 degrees Celsius). Note that water quenching is used for thick sections while air blasting or air cooling is used for thin sections. Cleaning part surfaces before heat treatment is a great idea because this helps get rid of contaminants. This also helps you achieve the proper heat treatment results.
Annealing leads to a scale or a build-up to form on the steel. When this happens, the build-up can be removed through a number of processes. Two of the most common processes to remove this build-up are pickling and electro-cleaning. In electro-cleaning, an electric current is used to get rid of the scale. For the picking process, a nitric-hydrofluoric acid bath is used to descale the steel.
Cutting the steel is a vital process and it is necessary for two reasons. First, it enables you get the desired blank shape or size. It also helps you trim the parts to the required size. Mechanical cutting of stainless steel is done by a number of methods. These include straight shearing and circle shearing. Straight shearing is accomplished by guillotine knives while circle shearing is done by circular knives. Horizontal and vertical cutting can also be accomplished by using high speed blades. Blanking and nibbling are also used to cut irregular shapes. Cutting can also be accomplished by flame cutting and plasma jet cutting.
Surface finishing is a vital part of the stainless steel fabrication process. Some surfaces require smooth finishing while other products require rough finishing.
Finishing can also be categorized in accordance with the specific result at the end of the process and the method used for finishing. Examples are the dull finish, the bright finish and the mirror finish. Some processes used in finishing are buffing and tumbling.
7. Manufacturing at the fabricator or end user
After the finished goods are packed and shipped to end user or the fabricator, a number of other steps are required. A little more shaping is accomplished by some methods such as press forming, roll forming, extrusion, press drawing, forging and extrusion. Further heat treating and cleaning processes are also required.
The process of joining different parts of stainless steel together is accomplished by welding. In this context, the most common forms of welding are fusion welding and resistance welding. After the different parts of the stainless steel are joined by welding, the welded areas are usually cleaned and polished.
Stainless steel in Australia must meet the specifications developed by the Australian Standards. This applies to mechanical properties like toughness and corrosion resistance. In some cases, metallography is used to ensure that the products meet the right standards.
The use of stainless steel and super stainless products is expanding in the global market. This has led to new and better facilities to ensure that the right quality is delivered. In addition, the new Clean Air Act means that stainless steel manufacturers will comply with the new laws.
In addition, environmental laws are forcing refineries and petrochemical industries to recycle secondary cooling water in closed systems. In response to this demand, manufacturers are developing corrosion resistant steel products. It is expected that the use of duplex stainless steel tubing will increase because it costs less than other materials.
It is estimated that stainless steel usage per vehicle will rise from the present 55to 66 pounds (25 to 30 kilograms) to above 100 pound (45 kilograms) by the beginning of the new century. It is also expected that stainless steel will be used for new applications like long-life exhaust systems, brake lines and composite bumpers.
In addition, new products are being developed with better quality and super strength. Some of these are the ferritic iron-base alloys that contain 8-12 percent Cr for magnetic applications. There is also austenitic stainless steel that has extremely low sulphur content. This is used in the manufacture of pharmaceuticals and semiconductors.
Extensive research is still going on but the effort of committed researchers is yielding positive results already. Japanese researchers have been able to develop a powerful corrosion-resistant steel. This product returns to its original shape upon heating so this makes it truly unique. It is expected that this application will be useful for fire alarms, springs, clips, pipe fittings and fasteners and clamps. There is also an improved martensitic stainless steel that is just right for instrument rolling-contact bearings. This product reduces vibration levels and has improved life expectancy. This product also has much better surface finish when compared to other materials.