Understanding the manufacturing process of stainless steel
Due to its versatility, there is a wide range of manufacturing techniques for stainless steel products. To better understand the benefits of the different types of this metal, it will be useful to review some of these processes.
This is why in the following Tisco Steel article we will tell you about the manufacturing process of stainless steel, which will vary depending on the finish or benefits required.
What does stainless steel consist of?
As you probably already know, stainless steel is an alloy containing iron, a substance made up of two or more chemical elements. The percentage of chromium and other components will vary depending on the grade of steel required.
This metal is also known as CRES, or corrosion resistant steel, especially when the alloy is ungraded. It is available in a variety of grades and finishes to meet the industrial, domestic and environmental requirements faced by the metal. It is popular for use because it does not corrode, stain or rust as easily as ordinary steel.
Different grades of stainless steel have different amounts of chromium to produce the required chromium oxide film which prevents the spread of rust. Depending on its composition, this material can be used in an almost infinite number of applications and industries, including, to name just a few.
Bulk material handling equipment
Building facades and roofs
Automotive components (exhaust pipes, trim parts, engines, chassis, fasteners, fuel lines, etc.)
Chemical processing plants (scrubbers and heat exchangers)
Pulp and paper manufacturing
Petroleum refining
Water supply pipes
Consumer goods
Shipbuilding and maritime
Sporting goods (toboggans) and transport (horse-drawn carriages)
Raw materials
Stainless steel is made from some of the basic elements found in the earth: iron ore, chromium, silicon, nickel, carbon, nitrogen and manganese. The properties of the final alloy are tailored by changing the quantities of these elements. For example, nitrogen improves tensile properties, such as ductility, and also corrosion resistance, making it valuable in duplex stainless steels.
You may also be interested in. Types of corrosion faced by stainless steels.
Fabrication of stainless steel
The manufacture of stainless steel involves a number of processes.
Foundry
Understanding the manufacturing process of stainless steel
First, the raw material is melted in an electric furnace and subjected to intense heat for approximately 12 hours.
The mixture is shaped into rectangular plates, slabs or bar blanks, which then take on a semi-solid form.
This initial form of steel is processed through forming operations, including hot rolling into bars, wires, plates and sheets.
The manufacture of the final product requires further forming by hot rolling, stamping, forging and extrusion. The material is then welded together and shaped into the desired shape.
Annealing and heat treatment
The metal is then annealed by heat treatment, where it is first heated and then cooled under extremely controlled conditions to relieve internal stresses and soften the metal. This process is also known as precipitation hardening and is often used to provide greater strength.
The above processes need to be carefully monitored as even small changes in temperature, time or recommended cooling rates can seriously affect the performance of the final product.
For example, low temperatures can result in high strength and low fracture toughness (i.e. the metal will be more brittle rather than ductile), while high temperatures can result in a tougher but lower strength material (more ductile than brittle). Rapid cooling can provide a tough steel without any significant loss of strength.
The heat treatment used for stainless steel depends on the type and grade of steel being produced.
Rust removal
Annealing or heat treatment produces a precipitate known as scales. These deposits can be eliminated by different methods, such as pickling (bathing with nitric-hydrofluoric acid), electro-cleaning (applying current with phosphoric acid and a cathode), etc.
The material descaling process is carried out at different times, depending on the type of steel produced.
Although stainless steel wire and stainless steel bars are traditionally treated by hot rolling, there are many who consider hot rolling to be undesirable.
Descaling
Annealing or heat treatment produces a deposit, known as scale. These deposits can be removed by different methods, such as pickling (bathing with nitric-hydrofluoric acid), electro-cleaning (applying an electric current with phosphoric acid and a cathode), etc.
The material descaling process is carried out at different times, depending on the type of steel produced.
While stainless steel wire and bars are traditionally processed by hot rolling, forging and extrusion, plates and strips must be descaled after hot rolling and before annealing and descaling.
On the other hand, stainless steel sheet or plate must undergo an initial annealing and descaling step immediately after hot rolling. After cold rolling (passing through the rolls at a relatively low temperature), which produces a further thickness reduction, they are again annealed and descaled. A final cold rolling step then prepares the steel for final processing.
Cutting
The cutting operation during the manufacturing process is essential to obtain the desired shape and size of the final product.
Mechanical cutting is achieved by various methods, including straight cutting with guillotines, circular cutting with horizontally and vertically placed circular blades, sawing with HSS blades, and cutting presses and mills.
Punch cutting uses metal punches and dies to punch and shape by shearing, while planing machines punch a series of overlapping holes to form irregular shapes.
Cutting can also be done with a torch, which is a clean and fast process involving the use of a flame produced from oxygen, propane and iron powder.
The plasma flame cutting method uses a column of ionised gas and an electric arc to melt and cut the metal.
Finishing and refining
Understanding the manufacturing process of stainless steel
Surface finishing, the final step in the manufacture of stainless steel, is essential to achieve the smooth, reflective surface that makes this metal so popular.
This final step provides the required corrosion resistance to the product and prepares the metal for other specific industrial manufacturing steps as required.
In this surface treatment stage, the metal is treated according to the desired physical appearance: a dull finish, a glossy finish or a mirror finish.
Certain surface finishes also make stainless steel easier to clean, which is obviously important for hygienic applications.
The smooth surfaces obtained by polishing also provide better corrosion resistance. Rough surface finishes, on the other hand, often require lubrication for the application as well as facilitating other manufacturing steps.
A non-glossy surface can be obtained by hot rolling, annealing and descaling.
Bright finishes are obtained by hot rolling followed by cold rolling on polishing rolls.
A highly reflective finish is produced by cold rolling in combination with annealing and grinding and polishing in a controlled atmosphere furnace.
Finally, a mirror finish is produced by polishing with progressively finer abrasives, followed by extensive polishing.
During the manufacturing process, quality control must be monitored by constantly checking the optimum mechanical properties of the material. The constant attention to the smallest detail makes stainless steel a universally applicable material.
Although it has a relatively high strength and work-hardening rate, stainless steel is sufficiently ductile to be bent, machined, welded, deep drawn or spun.