What is stainless steel ?
Any member of the family of alloy steels known as stainless steel, typically having 10 to 30 percent chromium. In combination with low carbon content, chromium offers exceptional heat and corrosion resistance. It is possible to add other elements, such as nickel, molybdenum, titanium, aluminum, niobium, copper, nitrogen, sulfur, or selenium, to improve corrosion resistance in a certain environment, boost oxidation resistance, or give something special.
The majority of stainless steels are initially melted in electric-arc or basic oxygen furnaces and then refined, mostly to reduce the carbon content, in another steelmaking vessel. In the argon-oxygen decarburization procedure, liquid steel is injected with a mixture of oxygen and argon gas. It is conceivable to reduce carbon to predetermined levels by oxidizing it to carbon monoxide by changing the oxygen to argon ratio without simultaneously oxidizing and losing costly chromium. In the initial melting process, less expensive raw materials, including high-carbon ferrochromium, may be employed.
Stainless steel comes in more than 100 different grades. In the family of stainless steels, the majority are divided into austenitic, ferritic, martensitic, duplex, and precipitation-hardening. The steels with the best corrosion resistance are typically austenitic steels, which range in chromium content from 16 to 26 percent to up to 35 percent nickel. They are nonmagnetic and cannot be heat-treated to become harder. The most popular kind is 304 grade, which has an 18% chromium and an 8% nickel content. Typical uses include the aviation, dairy, and food processing sectors. Standard ferritic steels are nickel-free and have chromium contents ranging from 10.5 to 27%; nevertheless, due to their low carbon content (less than 0.2%), they cannot be heat treated to harden them, making them suitable for less demanding anticorrosion uses like architectural and automotive trim. Martensitic steels normally have a chromium content of 11.5 to 18 percent, up to 1.2 percent carbon, and nickel may occasionally be added. They are used in cutlery, surgical tools, wrenches, and turbines and can be heat treated to become harder. They also have a moderate level of corrosion resistance. Duplex stainless steels combine austenitic and ferritic stainless steels in equal ratios; they have a chromium content of 21 to 27 percent, a nickel content of 1.35 to 8 percent, a copper content of 0.05 to 3 percent, and a molybdenum content of 0.05 to 5 percent. Due to their superior strength and corrosion resistance compared to austenitic and ferritic stainless steels, duplex stainless steels are frequently used in storage tank construction, chemical processing, and chemical transport containers. The strength of precipitation-hardening stainless steel is due to the addition of aluminum, copper, and niobium to the alloy in concentrations that are less than 0.5 percent of the alloy's total mass. Regarding corrosion resistance, it is comparable to austenitic stainless steel and comprises 15–17.5 percent chromium, 3–5 percent nickel, and 3–5 percent copper. Long shafts are built out of stainless steel that has undergone precipitation hardening.
How is Stainless steel made ?
In the later steps, a grade of stainless steel will undergo a different precise process. The way a grade of steel is formed, processed, and finished has a big impact on how it appears and functions.The molten alloy must first be made before you can produce a steel product that can be delivered.Because of this, the majority of steel grades start out in the same way.
Step 1: Melting
In an electric arc furnace (EAF), scrap metals and additives are first melted to create stainless steel. The EAF warms the metals for many hours to produce a flowing, molten mixture using high-power electrodes.Many stainless steel orders contain up to 60% recycled steel since stainless steel is completely recyclable. This assists in reducing the influence on the environment as well as costs.Depending on the type of steel produced, different temperatures will be required.
Step 2: Removing Carbon Content
Iron becomes harder and stronger with the help of carbon. However, too much carbon can lead to issues, such as precipitation of carbide while welding.
It is crucial to calibrate and reduce the carbon content of molten stainless steel to the appropriate level before casting.Foundries can regulate the carbon concentration in two different ways.Argon Oxygen Decarburization (AOD) is the first method. With only a small loss of other crucial elements, the carbon content of molten steel can be reduced by injecting an argon gas combination into it.
Vacuum Oxygen Decarburization (VOD) is the other technique that is employed. With this technique, molten steel is moved to a different chamber and heated while oxygen is pumped into the steel. After that, expelled gases are drawn out of the chamber by a vacuum, further lowering the carbon concentration. Both processes provide fine-grained control over the carbon concentration to guarantee a suitable combination and accurate properties in the finished stainless steel product.
Step 3: tuning
Following the reduction of carbon, temperature and chemistry are finally balanced and homogenized. This guarantees that the steel's composition remains constant throughout the batch and that the metal satisfies the specifications for the grade it is designed for.Samples are examined and tested. The mixture is then adjusted until it reaches the necessary standard.
Step 4: Forming or Casting
The foundry now has to produce the crude shape that will be used to cool and work the steel once it has been created as molten steel. The finished product will determine the precise form and size.
Typical forms include:
Blooms
Billets
Slabs
Rods
Tudes
The batch is then tracked through the numerous processes that will come after by marking the forms with an identifier.Depending on the targeted grade and final item or function, the following procedures will vary. Plates, strips, and sheets are made from slabs. Blooms and billets transform into wires and bars.
A steel may go through several of these processes more than once to provide the required appearance or qualities, depending on the grade or format specified.
The following steps are typically involved in the manufacturing process:
1. Hot rolling: This process occurs at temperatures above the steel's recrystallization temperature, helping to establish its basic physical specifications. The steel is kept pliable throughout the process by precisely controlling the temperature. Repeated passes gradually modify the dimensions of the steel, often requiring rolling through multiple mills to achieve the desired thickness.
2. Cold rolling: Precision is often required, and cold rolling is used in such cases. It takes place below the steel's recrystallization temperature and involves using supporting rollers to shape the steel. Although this method yields a more appealing and consistent finish, it can also deform the steel's structure. Heat treatment is sometimes necessary to restore the steel to its original microstructure.
3. Annealing: After rolling, most steel undergoes an annealing process. This involves cycles of controlled heating and cooling to remove internal stress and soften the steel. The specific temperatures and timeframes required for annealing depend on the grade of steel. Heating and cooling rates also impact the final product.
4. Descaling or Pickling: Scaling on the surface of the steel can occur during the various processes. This not only affects the appearance but also the weldability, durability, and stain resistance of the steel. To remove this scale and create the oxide barrier that provides stainless steel with its corrosion and stain resistance, descaling or pickling is performed. This can involve controlled heating and cooling in an oxygen-free environment or using acid baths. Depending on the final product, the metal may undergo additional rolling or extruding, followed by repeated annealing processes until the desired characteristics are achieved.
5. Cutting: Once the steel has been processed and prepared, it is cut according to the order specifications. Mechanical techniques such as cutting with guillotine knives, circular knives, high-speed blades, or pounding with dies are commonly used. Flame cutting or plasma jet cutting may also be employed for intricate shapes. The choice of cutting method depends on the requested steel grade and the desired shape of the final product.
6. Surface Finishing: Finishing is one of the final steps in the manufacturing process. There are various methods for achieving different finishes on stainless steel, ranging from matte to mirror. Acid or sand etching, sandblasting, belt grinding, belt buffing, and belt polishing are commonly used techniques. After the surface finishing process, the steel is assembled into its final shape and prepared for delivery to the customer. Often, large quantities of stainless steel are shipped and stored in coils and rolls for use in different production processes. The ultimate form of the steel will depend on the specific type of steel required and other order-specific factors.
In conclusion, it is essential to understand the appropriate stainless steel grades and types for different applications and situations. Whether you need a strong and corrosion-resistant steel for marine environments or a beautiful and easy-to-clean option for restaurants, there is a stainless steel alloy available to meet your needs. This knowledge will ensure long-lasting results and help reduce costs.
