Adding additional metals and gases to SS alloy is frequently done for the following reasons:
- Good corrosion resistance
- Improve strength
- Improve weldability
- Improved formability
- Controlling magnetism
The steps involved in stainless steel production are given below:
- It all begins with melting waste metals and additives in an electric arc furnace (EAF). Also, temperature variations are regulated by the steel grade needed for manufacturing.
- The carbon content must be calibrated and reduced to the appropriate level before casting molten stainless steel.
- After carbon content calibration, it’s essential to ensure that the temperature and material’s chemistry are reasonably stable.
- Last but not least, the final product will determine the specific shape and characteristics. Here is a list of several popular patterns:
Depending on the production of different grades or forms of SS required, the procedure may involve many phases to get the desired geometry or characteristics. Below is an illustration of these stages:
Hot rolling
The hot rolling temperature is kept higher than the steel’s recrystallization temperature during this operation. To make geometries with the necessary material qualities while keeping the metal volume constant, the entire process regulates by varying the temperature.
Heat-treated metal is hot rolled between two rollers to flatten, expand, and keep a uniform thickness. The parts manufactured with this method are frequently employed in the metalworking sector as a final product or as inputs for subsequent processes.
Cold rolling
Where great accuracy and surface quality are essential, this method is chosen. In this process, the cold rolling temperature is kept below the material’s temperature of recrystallization. High dimensional precision is another benefit, as it aids in shielding the material from corrosion and enhances its wear resistance. High tolerances are achievable while the material’s strength and hardness increase by 20% during the cold rolling process.
Annealing
Most steel grades go through an annealing process after rolling. Annealing of stainless steel is performed at a temperature greater than 1040°C. However, certain types of stainless steel come under an annealing process at a controlled temperature that is below 1010°C by ensuring the fine grain size.
It also comprises controlled heating and cooling cycles. These cycles lower the internal stress that the entire procedure generates. The type and grade that has to be processed will affect the cost and time of the final product.
Descaling
In the course of heat treatment procedures, scales are created on a metal surface. The metal surface is stained by an oxide scale, which also roughens the part’s surface finish.
Therefore, the descaling process is used to remove undesired surface deposits from metal, and it is also a possible path in the pre-finish process, which also includes:
- Cleaning
- Stripping
- Pickling
Some of the physical methods used for descaling are given as follows:
- Wire brushes
- Extra blows
- Scraping devices
- Polishing
- Blasting
And two main chemical descaling methods are:
- Acid descaling typically with nitric acid
- Alkali descaling
Properties of stainless steel
Due to its desired qualities, stainless steel plays a significant role in the production of parts and components for several industrial sectors. Steel’s corrosion resistance is increased by almost 200 times because of its 10.5% chromium concentration. This ultimately results in higher impact resistance.
Its great strength and durability, resilience to maximum operating temperature, increased formability, longevity, environmental friendliness, and recyclable nature are additional benefits for customers.
Stainless steel is divided into several grades based on composition, physical characteristics, and intended uses. The series numbers 200, 300, 400, 600, and 2000 are the most often used. Austenitic chromium nickel alloys of types 304 and 316 are the most popular grades. Ferritic steel and martensitic chromium alloys of the 400 families are the sources of cutlery-grade stainless steels. Types 420 and 440 are used in surgical instruments and razor blade steels, respectively.
Some of the core properties offered by stainless steel are given below:
Weldability
The several varieties of stainless steel are recognized, and referrals on welding procedures and techniques that may be used to fabricate stainless steel parts without compromising the metal’s resistance to corrosion, oxidation, and other mechanical properties are provided.
Stainless steels are defined as iron-base alloys with more than 10% chromium, which gives the metal the corrosion-resistant qualities that make them so highly valued. The temperature of the base metal close to the weld rises to levels that cause micro-transformation during the welding of stainless steel. Depending on the following factors, the extent to which these changes occur and how they affect the completed weldment’s mechanical characteristics and corrosion resistance are determined:
- Alloy content
- Thickness
- Filler metal
- Joint design
- Weld method
While welding stainless steel, the main goal is to provide superior quality than other base metals to give a strong connection, regardless of the changes that occur. There are two basic considerations for welding, and they are as follows:
- Solidified weld metal
- In the heat affected zone (HAZ) the base metal is heated to high temperatures but less than melting temperature
Strength and Fatigue resistance:
Steel with a 10.5% chromium concentration is known as stainless steel. Stainless steel is highly resistant to corrosion and rust because of the reaction between chromium and oxygen in the environment that forms a protective coating. The quantity of chromium applied to the metal determines its corrosion resistance.
The following are some of the variables that affect corrosion on stainless steel:
- Another element that raises the risk of stainless steel rusting is the environment in which the metal is employed.
- Chlorine is quite corrosive when present.
- Scheduled maintenance is a straightforward and trusted preventative method to keep stainless steel from corrosion. Additionally, an environment like saline water can accelerate the deterioration of stainless steel.
Stainless steel is susceptible to certain types of corrosion which are listed below:
- General corrosion is the most predictable and easiest to handle. It is characterized by a uniform loss of the entire surface.
- Galvanic corrosion refers to a situation where metals react with each other.
- Pitting corrosion leaves cavities and holes in metal due to the presence of chloride.
- Crevice corrosion when two surfaces connect, a fissure between them might experience corrosion. A metal or a non-metal, or even two metals, might experience it.
Heat resistance
Oxidation resistance and heat resistance are one of the most important characteristics of any stainless steel alloy. High temperatures frequently degrade steel alloys’ ability to resist oxidation, causing them to rust and lose some of their structural integrity.
If high-temperature oxidation resistance or other requirements such as high-temperature strength are needed, stainless steel is the material of choice. The wet corrosion resistance of stainless steel and its performance at elevated temperatures are both greatly enhanced by the high chromium content.
In general, a material’s creep strength is used to represent how strong it is at high temperatures. The capacity of a material to withstand deformation after being exposed to high temperatures for a long time. The austenitic stainless steel performs very well in this regard. Both the martensitic and precipitation-hardening families of stainless steel can achieve high strength through thermal expansion treatments. However, exposure to these grades at temperatures higher than those used for their heat treatment will permanently soften them, so they are also rarely used at high temperatures.
Good polishing performance
Finishing is the last phase in the manufacturing process. Its primary goal is to obtain highly accurate produced components that adhere to the necessary technical requirements for form, surface roughness, geometry, and precision of measurements. The following two primary finishing methods are used to polish stainless steel:
Mechanical polishing
Mechanical polishing is the process of smoothing a surface with mechanical tools and abrasives. Typically, there are three primary phases, and they are as follows:
- Grinding
- Polishing
- Buffing
Mechanical polishing removes imperfections and extraneous markings like grinding lines, scratches, pits, and surface flaws that might damage a part’s operation and appearance. It is a very specialized technique, and a professional operator is preferable.
The following are a few advantages of mechanically polishing stainless steel:
- Reduce product adhesion
- Improves surface cleanability
- Enhance the aesthetic appeal
- Increase material durability
- High thermal conductivity
- High maximum operating temperature
- Moderate corrosion resistance
- Efficient in mildly corrosive environments
- Increase brightness, color, and reflectivity
Electrochemical polishing
It is suitable for compact products with complicated and irregular geometries that need finishing. This method of polishing metal surfaces produces results that are not conceivable with other methods. It entails racking the material before submerging it in an electrolyte solution with a predetermined temperature. This is an amalgam of phosphoric acid and concentrated sulfuric acid. During this procedure, the material is positively charged, and the electrolyte serves as a conductor, removing surface flaws and leaving behind a polished and smooth surface. Before drying the component, any remaining electrolyte is eliminated.
Some important variables must be taken into account while electro-polishing stainless steel:
- Electrolyte chemical composition
- The temperature of the electrolyte (76.6°C – 82.2°C)
- Duration of electrical charge
- The density of the electrical current
- Composition of metal alloy
The following are some advantages of electropolishing stainless steel are:
- Offers 35 times more corrosion resistance than unprocessed part
- Hygienically protected surface
- Less surface drag and friction
- Little to no risk of surface distortion
- Increase the life of material/part
Physically, stainless steel has a smooth surface that is not easily scratched or dented. Making it ideal for applications where the finished look matters, the chemical compositions of stainless steel do not significantly change during heat treatment and welding processes. Overall, stainless steel is an incredibly versatile material that offers superior polishing performance along with durability and corrosion resistance. Its ease of cleaning and maintenance makes it an excellent choice for a variety of decorative and functional projects.
Here we will cover the fundamentals of stainless steel. Let’s now explore some critical characteristics relating to various stainless steel grades.
Stainless Steel Types:
Because of its versatility, stainless steel works well with many different construction materials. Engineers have been exploring the concept of alloys for many years by introducing and eradicating certain metals. They could make many stainless steel grades or kinds with various corrosion resistance properties and uses.
Thousands of different stainless steel alloys exist, however, they can be categorized concerning their crystalline structure, which describes how their atoms are arranged.
The five major types of stainless steel are briefly explained below:
Austenitic stainless steel
Austenitic stainless steel possesses an austenitic structure. It implies that the cells adopt the shape of a cube while preserving an atom at each corner and one in the center. It is also known as a face-centered crystal structure in crystallography. By alloying steel with nickel, nitrogen, or manganese to withstand the microstructure at all ranges of temperature, from very low to melting point. Any heat treatment cannot make them harder. However, cold working provides greater strength and durability.
Austenitic stainless steel offers the following advantages:
- Easy to clean
- Highly hygienic
- Good weldability
- Works well at elevated temperatures
- Excellent corrosion resistance
It is further divided into two groups, which are the 200 series and the 300 series. In the 200 series, manganese and nitrogen content is prominent. In the 300 austenitic stainless steel series, nickel is present in excessive amounts. The inclusion of carbon content in austenitic stainless steel is another distinctive feature. The maximum allowable carbon content is .08%. However, they do not have any minimum requirements.
Some of the common grades of the 300 series are:
- Type 304
- Type 316
- Type 310
- Type 321
Ferritic stainless steel
It is the most popular kind of stainless steel and is renowned for its strong magnetic properties. Due to its low nickel concentration, it is cost-effective and sparingly employed in industries. To improve the toughness and creep resistance chromium, molybdenum, niobium, and titanium alloys are incorporated. These alloys are ideally suited for applications where performance and cost are more important than aesthetics because they offer better corrosion resistance and weldability.
Tensile stress combined with a corrosive environment leads to a typical kind of steel deterioration known as stress corrosion cracking (SCC). Ferritic stainless steels have several microstructures, which significantly increase their resistance to chloride stress corrosion cracking. It is also a great choice in environments where chloride is prevalent. Its industrial usage is constrained by the loss of toughness and poor corrosion resistance in a welded region. Alloys are important in preventing the ferritic stainless steels welding issue. The best outcomes are attained by using mixed ferrite-martensite or ferrite-austenite microstructures. While used in welding applications where stress corrosion is a possibility, these combinations provide positive outcomes.
Some of the types of ferritic stainless steels are listed below:
- Type 409/410L
- Type 430
- Type 430Ti, 439, and 441
- Type 434, 436, 444
- Type 445, 446, 447
Martensitic
What is the difference between 2024 and 6061 aluminum alloy?
Due to its chemical composition, it is the most well-known type of stainless steel. About 11.5% to 18% of martensitic stainless steel comprises chromium. Up to 1.2% carbon and nickel are also frequent components. Its considerable carbon content gives strength to structural components, but its low nickel content makes it less corrosion-resistant than other variants.
There is also a little addition of secondary alloying elements, including manganese, molybdenum, and nickel. Through aging and heat treatment, It dramatically enhances the toughness and reinforcement of martensitic stainless steel.
These processes impart desirable qualities in martensitic stainless steel, making it an excellent choice for the manufacturing of mechanical valves, turbine components, medical tools, mechanical equipment, and a variety of other things.
Some of the types of martensitic stainless steels are listed below:
- Low carbon martensitic stainless steels
- High carbon martensitic stainless steels
- Type 410
- Type 420
- Type 440
Duplex stainless steels
It is a stainless steel grade made up of ferritic and austenitic grades. The two-phase microstructure of ferritic and austenitic stainless steel flakes is called duplex steel. In manufacturing duplex stainless steel, the proportion of ferritic and austenitic steel is about equal. The most popular duplex stainless steel is 2205 because of its superior corrosion resistance and good strength. The average composition, 22% chromium, and 05% nickel, is where the name derives from.
The most popular grade from super duplex stainless steels, which has remarkable corrosion resistance capabilities, is UR52N+. It differs from grade 2205 because it can withstand hot chlorides and intense reducing acids simply by adding carbon.
Some of the applications of duplex stainless steel are given as follows:
- Oil and gas industry
- Marine environment
- Pollution control equipment
- Chemical plants
- Transport and storage
Precipitation hardening
The material’s chemistry is the primary factor used to classify the grades and varieties of stainless steel. Precipitation hardening can use a combination of titanium, nickel, or aluminum. It is a method of stiffening and hardening malleable materials like steel.
The precipitation hardening generally involves three main steps and these are listed below:
- The metal must go under solution treatment. Metal should be heated at high temperatures to dissolve precipitates and alloying agents into a supersaturated solution.
- The metal will next be quenched to bring it down to room temperature. A slow cooling down produces a coarser grain size than a quick cooling down. A finer grain size eventually improves the final alloy’s performance.
- Precipitation hardening is the last phase. A tiny precipitate cluster emerges by dissolving the supersaturated solid solution and significantly strengthening the metal; when working with stainless steel, the metal is kept at a steady high temperature for a certain period, and air cooling prescribes until the room temperature is achieved.
A family of corrosion-resistant alloys includes several types of precipitation-hardening stainless steel. Some alloys have exceptional tensile strengths between 850 and 1700 MPa and yield strengths between 520 and 1500 MPa. The following is a list of the most popular alloys used in the PH process:
- 2000, 6000, and 7000 series aluminum alloys
- Zinc
- Magnesium
- Copper
- 02% of beryllium
This is all about the tech aspects of stainless steel. Let’s now discuss some of the applications of stainless steel.
