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What is Tube Bending? Working, Types, Applications of Tube Bending

Written by Anup Kumar Dey in Construction

Tube bending is a mechanical fabrication process to bend tubes permanently from their straight form. In many applications, bent tubes are required to fulfill certain specific tasks. So, tube bending is a necessary forming method for industries. Various instrument items use bent tubes as one of their component parts. Bent tubes are also found in stair railings, automotive parts, furniture frames, handles, air conditioning equipment, and many other places. In piping, plumbing, and pipeline industries pipe and tube fittings in the form of bent pipes and tubes are widely used whenever a flow direction change is required. In this article, we will learn about tube bending, their working principle, terminologies used for tube bending, and types.

What is Tube Bending?

Tube bending is a manufacturing process used to shape metal tubes or pipes into specific angles or curves without altering their structural integrity. This process involves the application of force to the tube, typically through specialized equipment like hydraulic or mechanical benders, to achieve the desired bend radius and angle. Tube bending is commonly employed in industries such as pipelines, automotive, aerospace, and construction, where precise and durable tubing shapes are required for components like exhaust systems, roll cages, pipeline network, and structural frameworks. The technique ensures that the tube maintains its strength and functionality while conforming to the required design specifications.

A wide range of metal tubing, including aluminum, stainless steel, mild steel, brass, and titanium, can be bent into various shapes and configurations for specific purposes. Common bending shapes include L bends, U bends, S bends, and coil bends. These shapes are formed by applying force to straight metal tubing, which stresses and reshapes it to achieve the desired configuration.

How does Tube Bending work?

To start the bending process, the tube is fixed at two points. The bending action is then enforced using a rotating press, roller, or die. The tube material experiences a combination of tensile and compressive force when the tool advances. The tube bending process is dependent on various parameters like:

  • the material of the tube
  • used tooling
  • bending geometry required
  • applied pressure for bending
  • lubrication, etc

Depending on the bending technique employed, the tube undergoes several physical changes per area. The outer side of the bend experiences tensile forces and creates elongation and wall thinning whereas the inner side of the bend experiences compressive forces and creates wrinkling and thickening of the wall.

The tube‘s cross-section undergoes a phenomenon known as ovality which is defined as the distortion of the tube‘s cross-section from the original round shape after bending.

After the tube bending process, the product may undergo various other fabrication processes like cutting and deburring, slotting, notching, and welding.

Terminologies used in Tube Bending

Let’s learn some of the terminologies frequently used in tube bending:

  • Center-Line Radius: It is the distance from the center of the curvature to the tube centerline.
  • Outside Diameter: The outside diameter is the distance between the two outermost points on the pipe or tube‘s cross-section having the same centreline.
  • Inside Diameter: Inside diameter indicates the size of the tube’s hole and is calculated as Outside diameter-2 X wall thickness.
  • Wall Thickness: It is the difference between the outside and inside radius of a tube. When choosing a die for a tube bending method, tube thickness, and outside diameter are the most important considerations.
  • Degree of Bend: It is the angle formed by bending the tube and is measured in degrees. The degree of bend indicates the “sharpness” of the bend.
  • Wall Factor: It denotes the relative wall thickness and is defined as the ratio of the outside diameter of the tube and its wall thickness. Wall factor determines if a tube is “thick-walled” or “thin-walled”. Tubes having lower wall factors are easier to bend and vice versa.
  • D of the Bend: The “D of the bend” is the term used by tube fabricators that denotes the ratio of the CLR of the bend to the tube‘s outside diameter. The higher the bend’s D, the easier it is to form bends with tighter radii.
  • Springback: Springback is the tube’s tendency to return to its original shape while bending. Springback is influenced by several factors like the material‘s stiffness, tensile strength, wall thickness, type of tooling, and bending technique used. Harder materials and smaller CLRs produce a greater springback.

Types of Tube Bending

The tube bending process can be classified depending on various parameters. In general, the following types of tube bending are popular:

Based on the forming methodology, there are two types of tube bending processes:

  • Form-bound tube bending where the die geometry influences the forming process and
  • Freeform tube bending where the forming is dependent on the tube’s movement through the tooling.

Based on the working temperature condition tube bending process is grouped as

  • Cold bending and
  • Hot bending.

Hot tube bending uses heat energy and is performed at higher temperatures whereas cold tube bending is done at room temperatures.

Depending on the tools used during the tube bending, there are various types of tube bending processes as mentioned below:

Press Bending

In the press bending process, the tube is fixed at two points and a ram/die is forced against the tube to take the shape of the bend. The bend characteristics are governed by the external dimensions of the cylindrical ram.

Press bending is widely used for symmetrical parts. The process is quick and does not require lubrication and cleaning. But, making a smaller degree of bend is difficult in this method. As this tube bending method is very difficult to control, it is only employed where a uniform cross-section is not required.

Rotary Draw Bending

Rotary draw bending is a highly precise tube bending method that gives minimal ovalization. This process is used in pipe fittings, instrument tubing, handrails, and automotive and aerospace parts. This method uses a set of interlocking dies. The tube is internally supported using a mandrel during the process.

A bend die that imparts the bend to the tube by rotating it around its outline determines the bend radius. Until the desired bend angle is obtained, the rotation is continued.

Types of Tube Bending Processes
Fig. 1: Types of Tube Bending Processes

Compression Bending

Compression bending has a simpler setup but is limited to circular hollow sections only. A mandrel is not used for supporting which may cause the outside surface to flatten slightly.

Roll Bending

The roll bending method is suitable for creating bends with large CLR. Consisting of two stationary rotating rollers and a moving roller in a triangular pattern, the roll bending is used for large tube components and structural applications. The stationary rollers rotate in the opposite direction of the moving roller. As the tube moves back and forth on the rotating rollers, the bend radius is gradually formed.

Hydroforming

Hydroforming is a metal shaping process that uses two dies and pressurized liquid to form straight metal tubes into various shapes. Typically, round tubes are used due to their flexibility in creating different designs. The process involves placing the metal tube between the die halves and then applying pressurized fluid inside the tube. As the pressure increases, the liquid forces the tube to take the shape of the die. After achieving the desired form, the die is separated, and the new part is removed. Hydroforming produces high-quality, aesthetically pleasing parts with excellent shape retention and durability.

Bending Springs

Bending springs are used to bend softer workpieces having small diameters. A strong and flexible spring is inserted inside the tube walls from its end to the center of the bend radius. For positioning and easy removal, a wire is sometimes attached to the ends of the spring.

All the above-mentioned bending processes are performed at room temperature and hence fall into the cold tube bending category.

The heat energy used during hot tube-bending techniques enhances the tube‘s plastic deformation. Heat induction bending, Sand packing hot-slab bending, etc are examples of Hot tube bending which are normally used for bending polymeric tubes like PVC, CPVC, and ABS.

Heat Induction Bending

Heta induction bending is a hot tube bending process where the tube is supported on the front end and the bend clamp is placed in between the rotating arm and the rear end. After clamping the front end of the tube in the pivot arm, it is pushed gradually from the rear end. The tube passes through an induction coil where heat is supplied at a point tangent to the bend radius. The pivot arm is made approximately equal to the workpiece bend radius. Depending on the material of the tube, the working temperature can range from 430°C – 1,200°C. Once the bending is done, the tube piece is quenched in water spray or air.

Heat induction bending creates minimal wall thinning and ovality and can be applied for a wide range of pipe sizes and thicknesses. However, the operation is costly.

Sand Packing Hot-Slab Bending

This is one of the oldest hot tube bending methods that is still employed in many fabrication sites. In this technique, fine sand is filled inside the tube and then both tube ends are sealed. The sand-filled tube is then heated and at around 870° C, the pipe is placed in a slab with pins set on it. A mechanical force is applied to the tube to bend it around the pins. The sand inside the tube maintains its original cross-section.

Lubrication

To reduce friction, Lubricant must be used before the insertion of the dies. Proper lubrication prevents premature wear and increases the life of the tooling. Lubricants are available as a paste or gel. They have unique formulations for different tubing materials. Usually, lubrication is applied on the inside and outside of the tube, bending mandrels, contact points of wiper dies, and bending springs. For heavy-duty tube bending operation, a more concentrated lubricant application may be required for heavier-duty bending. At the end of the bending process, the remaining lubricant is cleaned from the tube and die surfaces.

Common Materials Used in Tube Bending

The choice of material for tube bending is crucial, as it affects the ease of bending, the quality of the finished product, and the application of the bent tube. Here are some common materials used:

Aluminum

Aluminum is a lightweight, corrosion-resistant metal with excellent workability. It is commonly used in aerospace, automotive, and construction applications due to its favorable properties. Aluminum tubes can be easily bent and formed, making them suitable for various designs.

Stainless Steel

Stainless steel is known for its strength, durability, and resistance to corrosion. It is widely used in industries such as food processing, pharmaceuticals, and marine applications. While stainless steel requires more force to bend compared to aluminum, its strength and resistance to environmental factors make it a preferred choice for many applications.

Mild Steel

Mild steel, or low-carbon steel, is a versatile material known for its ease of fabrication and affordability. It is commonly used in construction, automotive, and furniture industries. Mild steel tubes are relatively easy to bend, making them suitable for a wide range of applications.

Brass

Brass is an alloy of copper and zinc, known for its machinability and aesthetic appeal. It is often used in decorative applications, plumbing, and electrical fittings. Brass tubes can be bent with precision, and their appearance adds a touch of elegance to the finished product.

Titanium

Titanium is a high-strength, lightweight metal with excellent corrosion resistance. It is used in aerospace, medical, and high-performance engineering applications. While titanium is more challenging to bend due to its strength, its superior properties make it valuable for specific applications.

Applications of Tube Bending

Tube bending is employed in a wide range of industries and applications. Here are some notable examples:

Pipeline Industry

In the pipelines industry, the pipes are bent into specific angles using very large diameter bends for pigging purposes. Cold bending is performed and care is considered such that wrinkling does not occur.

Automotive Industry

In the automotive industry, tube bending is used to create components such as exhaust systems, roll cages, and suspension parts. Bending techniques allow for precise shaping of tubes to fit within the confines of vehicle designs while maintaining structural integrity and performance.

Aerospace Industry

The aerospace industry relies on tube bending for manufacturing parts such as hydraulic lines, fuel systems, and structural components. The high strength-to-weight ratio of materials like aluminum and titanium, combined with precise bending techniques, ensures that components meet rigorous safety and performance standards.

Construction Industry

In construction, tube bending is used for creating structural elements, handrails, and supports. Bending techniques allow for customization of tubes to fit architectural designs and structural requirements, enhancing both functionality and aesthetics.

Furniture Design

Tube bending is also utilized in furniture design to create stylish and functional pieces. Bended metal tubes are used in the frames of chairs, tables, and other furniture items, offering a modern and sleek look while providing strength and durability.

Medical Devices

In the medical field, tube bending is used to manufacture components for medical devices, such as catheters, stents, and surgical instruments. Precision bending ensures that these components meet strict performance and safety standards.

Overall, tube bending is a critical process in manufacturing and fabrication, allowing metal tubes to be shaped into various forms for use in a multitude of applications. This process is essential in industries ranging from automotive and aerospace to construction and furniture design. As a versatile technique, tube bending offers numerous possibilities for designing components with precise dimensions and complex geometries.

304 vs 316 Stainless Steel: Difference between 304 and 316 Stainless Steel

Written by Anup Kumar Dey in Engineering Materials

Both 304 and 316 stainless steels are austenitic grade stainless steels having very good corrosion resistance. Also popular as A2 by the International Organization for Standardization ISO 3506; 304-grade stainless steel is the most widely used SS grade. It accounts for more than 50% of all stainless steel produced in this world. According to use, the 316-grade stainless steel comes into the second position. As per ISO 3506, 316 grade is popular as A4. At a quick glance, both 304 and 316 stainless steel grades may seem to be similar as there are many similarities in properties of both 304 and 316 grades. But there are some distinct differences. In the following section, we will find out the differences between 304 and 316 grades of stainless steel.

What is 304 stainless steel?

304 grade of stainless steel is an austenitic grade of stainless steel alloy consisting of 18 to 20% chromium and 8 to 10.5% nickel. Manganese, silicon, and carbon are the other major allowing elements with the base metal as iron. The presence of high amounts of chromium and nickel provides excellent corrosion resistance and because of this 304 stainless steel is widely used in

  • Auto moldings and trim
  • Storage tanks
  • Wheel covers
  • Heat exchangers
  • Storage tanks
  • Electrical enclosures
  • Kitchen equipment and appliances like diswasher, refrigerators, etc
  • Decorative trims
  • Water and sewage treatment
  • Fasteners
  • Piping and piping components, etc

What is 316 stainless steel?

316-grade stainless steel also contains a high amount of nickel and chromium. they also contain 2 to 3% molybdenum which increases the corrosion resistance further. So, 316 grades of stainless steel provide added corrosion resistance as compared to 304 grades. Common applications of 316 grade stainless are:

  • Stainless steel floats.
  • Pulp and paper mills
  • Heavy industrial, Chemical processing and storage equipment.
  • Refinery equipment.
  • Food and Beverage processing facilities with extreme environments.
  • Medical and pharmaceutical devices.
  • Offshore oil & gas
  • Marine environments, especially those with chlorides present.

What are the differences between 304 and 316 stainless steel?

From the above discussion, we know about the 304 and 316-grade stainless steels. Let’s find out the major differences between these two grades; 304 vs 316 stainless steel.

1. 304 vs 316 stainless steel: Composition

As stated above, the major difference in both stainless steel grades is in their composition. 304 stainless steel is composed of 18% chromium, 8% nickel. On the other hand, 316 stainless steel consists of 16% chromium, 10% nickel, and 2% molybdenum.

2. 304 vs 316 stainless steel: Corrosion resistance

316 grade of stainless steel possesses added corrosion resistance capability. The main weakness of 304 stainless steel is that they are susceptible to pitting corrosion in a high chloride environment or saline environment. Whereas, 316 grades, due to the presence of molybdenum and increased nickel content can easily resist chlorides and acids. Because of these 316-grade stainless steel is widely used for marine and chemical processing applications.

3. 304 vs 316 stainless steel: Cost

Due to the addition of additional nickel and molybdenum, 316-grade stainless steels are costly as compared to 304-grade stainless steels.

4. 304 vs 316 stainless steel: Melting point

The melting point of 304-grade stainless steel (1,399°C – 1,454°C) is slightly higher than 316-grade stainless steel (1,371 °C – 1,399 °C).

5. 304 vs 316 stainless steel: Mechanical properties

316-grade stainless steel has slightly greater strength, hardness, and elevated temperature properties. 304 stainless steel has better weldability and formability as the addition of molybdenum reduces the weldability and formability of 316 grade.

So, overall 316-grade stainless steel is considered superior to 304-grade stainless steel but that comes with an added price. So, the selection between 304 vs 316 stainless steel must be done considering the application. 316 grades to be selected only if the application involves highly corrosive environments (caustic solution, chlorine solution, highly acidic environment, seawater, etc).

What is 304L/316L stainless steel?

304L and 316L stainless steels are extra low carbon variations of those grades. By reducing the carbon amount in those grades, the carbide precipitation during welding is avoided. These low carbon grades provide similar corrosion resistance but their mechanical properties slightly degrade.

Difference between 304/316 and 304L/316L stainless steel grades: 304/316 vs 304L/316L

As defined above the letter “L” in 304L and 316L denotes lower carbon variation of those stainless steel grades. As per AISI the carbon content in 304 and 316 basic grades are maximum 0.08% maximum whereas the same in 304L/316L is 0.03% maximum.

The main purpose to maintain carbon levels below 0.03% is to avoid inter-crystalline corrosion at elevated temperatures (450 to 850°C) applications. Also, it is believed that lower carbon variants have better weldability as compared to their base variants.

What is the meaning of dual certified 304/304L or 316/316L stainless steel?

In recent times, manufacturers produce dual certified stainless steel grades. The term “dual certification” is most widely used to refer to the practice of certifying a material as meeting the requirements of a specification(s) as two different alloys. For example, “dual” certification is that of 304 and 304L (or 316 and 316L) meets the lower carbon range as per 304 L and minimum properties of regular grade 304.

Does Stainless Steel Rust or Corrode?

Written by Anup Kumar Dey in Engineering Materials,Maintenance

Stainless steel is renowned for its exceptional resistance to corrosion, making it a popular choice in various industries, including construction, manufacturing, and food processing. However, there is often confusion surrounding the potential for stainless steel to rust or corrode.

“Does Stainless Steel Rust?” This question sometimes arises in every material engineer’s mind whenever they learn about stainless steel. It is well known that stainless steel is specifically prepared to provide it with in-build corrosion resistance by adding a sufficient amount of chromium. So, it is believed that stainless steel is rust-resistant. In this article, we will find out the answer to the most popular question “Does stainless steel rust or corrode?”

Rust Resistance Mechanism of Stainless Steel

The mechanism of stainless steel’s corrosion resistance is as mentioned below:

Stainless steel contains more than 10.5% chromium. Surrounding oxygen of air reacts with this chromium and forms a thin stable chromium oxide layer that musks the steel surface. This chromium oxide layer works as a passive barrier film that does not allow the air and water to access the underlying metal and thus protects the surface from rusting. This is the reason that stainless steel is rust-resistant and this alloy is preferred for countless manufacturing applications.

Certain specific alloying additions increase the corrosion resistance properties of stainless steel. For example, molybdenum. Because of the presence of molybdenum grade 316 stainless steel is more corrosion resistant as compared to grade 304 stainless steel.

Does Stainless Steel Rust?

Yes, Stainless steel does rust or corrode in certain situations. There are various factors that can corrode or develop rust in stainless steel. Some of the stainless steel corrosion mechanism are mentioned below:

Does Stainless Steel Rust
Rust on Stainless Steel Surfaces

Pitting Corrosion in Stainless Steel:

Certain types of stainless steel can rust when exposed to strong chlorides. For example, the corrosive nature of chlorine in seawater or pool water can cause the stainless steel to rust or corrode. This is known as pitting corrosion in stainless steel. This type of rusting of stainless steel can be prevented by:

  • Using the superior grade of stainless steel (for example SS316 is resistant to chlorides).
  • Applying a specialized coating to prevent direct contact with the chloride environment.
  • Proper cleaning of the surface.

Galvanic Corrosion due to Dissimilar Stainless Steel Alloys:

Sometimes, two different grades of dissimilar alloys are welded together to prepare some custom products. This creates a galvanic current flow between them and the anode starts to corrode. This phenomenon is known as galvanic corrosion. The weld filler can also cause galvanic corrosion. This type of stainless steel corrosion or rusting can be prevented by:

  • avoiding joining dissimilar metals.
  • adding a coating to seal the metal off for electron flow.

General Corrosion of Stainless Steel:

When the stainless-steel surface comes into contact with an acid-based material, general corrosion may occur in stainless steel. It creates uniform metal loss from the entire surface. Stainless steel components having a pH value of lower than 1 have a greater tendency to be attacked by general or uniform corrosion.

Transplanting of Plain Iron or Steel onto Stainless Steel:

Sometimes residue from iron or steel parts is transferred to stainless steel components. For example, while cleaning stainless steel utensils or cookware using steel wool or wire brush. If these residue particles are exposed to humid air or moisture, rusting starts. This type of stainless steel rust can be prevented by thorough cleaning of the surfaces to avoid the deposition of iron or normal steel over the SS surface.

Applying Temperature Extremes to Stainless Steel:

When stainless steel is exposed to extreme temperatures (750 to 15500F) the rust-resistance capability reduces. For example, during welding (or heat treatment) of stainless steel, high heating and cooling generate a process called sensitization where carbon and chromium bond together to form carbides. Due to these, the amount of chromium reduces in stainless steel making it prone to rusting. To prevent stainless steel rusting due to temperature extremes the product must be operated in its operating temperature range.

Chemical Corrosion in Polluted Air

In prolonged exposure to polluted air (containing larger amounts of sulfides, oxides, and hydrogen oxide), in the case of condensed water, sulfuric acid, nitric acid, and acetic acid liquid spots are formed on the stainless steel surface causing chemical corrosion.

Removing Rust from Stainless Steel Surfaces

There are various methods by which the rust from the stainless steel surface can effectively be removed. For example, By dissolving the rust or iron oxide using weak acids like phosphoric acid or acetic acids can help substantially to get rid of the rust. These acids do not react with the parent metal and simply dissolved the rust to provide a cleaner surface.

Preventing Stainless Steel from Rusting

The best effective way to prevent stainless steel from rusting is to powder coat the stainless steel surface by depositing a dry-colored powder using electrostatic charges. Upon heating the surface, the powder coating cures to a hard finish, which results in a full coating of the bare stainless steel in an attractive and protective layer. Also, stainless steel passivation will reduce the rust or corrosion in stainless steel material.

What can cause stainless steel to rust?

In general, stainless steel with its chromium oxide layer works as corrosion-resistant material. However, in certain environments when that chromium oxide layer is damaged, stainless steel can rust. Certain chlorides, cleaners, high humidity, mechanical abrasions, or high salinity environments can damage the protective layer of stainless steel, making it prone to rust formation.

Does stainless steel rust in water?

No, in normal water, stainless steel does not rust. You must have experienced the same with your stainless steel kitchen utensils. They are washed every day with water but do not produce rusting.

Does stainless steel rust over time?

If stainless steel is maintained properly and kept away from harsh environments it will not rust.

What does saltwater do to stainless steel?

With constant exposure to saltwater, stainless steel can rust over time.

Factors Influencing Stainless Steel Corrosion

Although stainless steel is highly resistant to corrosion, it is understood that certain factors can impact its corrosion resistance:

  • Environment: The specific environment in which stainless steel is exposed plays a crucial role in its corrosion resistance. Factors such as temperature, humidity, exposure to chemicals, and the presence of chlorides or acids can influence the corrosion behavior of stainless steel. Severe environments with high chloride concentrations, such as coastal areas, can pose challenges to stainless steel’s corrosion resistance.
  • Surface Finish and Cleaning: The surface finish of stainless steel affects its corrosion resistance. Smooth, polished surfaces are more resistant to corrosion than rough or pitted surfaces. Regular cleaning and removal of contaminants, such as dirt, oils, and debris, help maintain the protective passive film on the stainless steel surface, ensuring optimal corrosion resistance.
  • Alloy Composition: The composition of stainless steel, including the percentage of chromium, nickel, and other alloying elements, influences its corrosion resistance. Higher chromium and nickel content generally improve corrosion resistance. The selection of the appropriate stainless steel grade for a specific environment is crucial to ensure optimal performance.

Comparative Advantage of Stainless Steel

When compared to other metals, stainless steel offers distinct advantages in terms of corrosion resistance:

  • Iron and Mild Steel: Unlike iron and mild steel, which are highly susceptible to rusting, stainless steel forms a protective chromium oxide layer that prevents rust and corrosion. This makes stainless steel a more durable and long-lasting material.
  • Aluminum: While aluminum is known for its corrosion resistance, stainless steel often surpasses it in harsh environments, particularly when exposed to chlorides or acidic conditions.
  • Galvanized Steel: Galvanized steel, which is coated with a layer of zinc, offers corrosion resistance but may be less durable in certain aggressive environments compared to stainless steel.

Conclusion

Stainless steel’s resistance to corrosion sets it apart from many other metals. Its ability to form a protective chromium oxide layer enables it to withstand a wide range of corrosive environments. However, the corrosion resistance of stainless steel can be influenced by factors such as the environment, surface finish, cleaning practices, and alloy composition. By understanding these factors and selecting the appropriate stainless steel grade for specific applications, engineers and manufacturers can harness the full potential of stainless steel and ensure its long-term durability and performance.

Is Stainless Steel Magnetic? Does Stainless Steel Show Magnetic Properties?

Written by Anup Kumar Dey in Engineering Materials

Stainless steel is a widely used material known for its exceptional corrosion resistance and durability. However, there is often confusion surrounding its magnetic properties. “Is Stainless Steel Magnetic?” when asked this question by material engineers, a mixed response is obtained. Some say Stainless steel is non-magnetic and some say it is magnetic. In this detailed article, we delve into the question of whether stainless steel is magnetic, exploring the factors that influence its magnetism and shedding light on the various types of stainless steel and their magnetic behavior.

What is Stainless Steel?

Stainless Steel is a group of ferrous alloy materials containing more than 11% chromium to prevent it from rusting. Other elements of stainless steel are carbon, molybdenum, nitrogen, aluminum, sulfur, silicon, nickel, copper, niobium, selenium, etc. The chromium present in stainless steel creates a passive film to protect the underlying material from corrosion. The addition of nickel enhances corrosion resistance and imparts other desirable properties.

What Makes a Material Magnetic?

The magnetism of stainless steel depends on its specific composition and crystal structure. While stainless steel is primarily composed of iron, which is a ferromagnetic material, the addition of specific alloying elements can influence its magnetic properties.

For a material to be magnetic

  1. it must contain a ferromagnetic material like iron, nickel, cobalt, terbium, gadolinium, dysprosium, etc.
  2. crystal structure should be such that it has partially-filled electron shells (the incomplete inner core of electrons) that create a magnetic dipole moment. The Body-centered cubic lattice of ferritic microstructure shows this property.

Is Stainless Steel Magnetic?

As mentioned above, the magnetic property of Stainless Steel will depend on the crystal structure of the alloy. Stainless Steel, being a ferrous material contains iron, a ferromagnetic material. Now to possess magnetism, it needs to satisfy the second criterion mentioned above.

Is Stainless Steel Magnetic?

Stainless steels available in the market are classified into the following four groups:

Ferritic Stainless Steel:

One type of stainless steel is ferritic stainless steel, which contains a high concentration of chromium and little to no nickel. Ferritic stainless steel is generally magnetic due to its crystal structure, which consists of ferrite. The magnetic properties of ferritic stainless steel make it suitable for applications where magnetic attraction is desired, such as in magnetic storage devices or magnetic separators.

Austenitic Stainless Steel:

Another common type of stainless steel is austenitic stainless steel, which contains higher amounts of nickel and chromium. Austenitic stainless steel is non-magnetic in its annealed (softened) state. The high nickel content and specific crystal structure, known as austenite, suppress the ferromagnetic properties of iron. However, some austenitic stainless steel grades can exhibit weak magnetic response due to cold working or specific manufacturing processes.

Martensitic and Duplex Stainless Steel:

Martensitic stainless steel and duplex stainless steel are two additional types of stainless steel that can display varying degrees of magnetism. Martensitic stainless steel, which contains higher carbon content, can be magnetic. Duplex stainless steel, with a mixed microstructure of austenite and ferrite, can exhibit magnetic properties, depending on its composition and processing.

Is Ferritic Stainless Steel Magnetic?

Ferritic Stainless steel has a ferritic structure in its microstructure. At room temperature they are arranged as a BCC structure, meaning eight atoms are arranged at the corners of the unit cell, connecting to one lattice point in the middle of the structure. Such crystal arrangement shows magnetic properties. So, Ferritic stainless steels are magnetic. However, the magnetic strength of ferritic stainless steel is not as strong as conventional irons. Hence, they can be used as soft magnets. This is why stainless steel grades 409, 430, and 439 show magnetic properties.

Note that, at temperatures above Currie point temperature the ferritic BCC structure is converted into FCC and the magnetic property is lost.

Is Austenitic Stainless Steel Magnetic?

Austenitic Stainless steel consists of a high amount of austenite. Their crystal structure is arranged as an FCC structure, meaning a cube with an atom at each of the cube’s eight corners and an atom at the center of each of the six faces. Such grades of stainless steel are paramagnetic and do not show any magnetic properties. This is the reason that most popular austenitic stainless steel grades like SS 304, SS 316, etc are non-magnetic. Learn about the differences between 304 and 316 stainless steel by clicking here.

Is Martensitic Stainless Steel Magnetic?

All martensitic (body-centered tetragonal crystal structures) and most precipitation-hardenable stainless steels exhibit ferromagnetic properties and hence, they are magnetic. Because of the induced stresses by the hardening transformation, these stainless steel grades show permanent magnetic properties when magnetized in hardened conditions. Because of this martensitic stainless steel grades like grades 410, 420, and 440 show magnetic properties. The table in the following figure (Fig. 1) provides the magnetic properties of some ferritic and martensitic stainless steels.

Magnetic Properties of Ferritic and Martensitic Stainless Steels
Fig. 1: Magnetic Properties of Ferritic and Martensitic Stainless Steels

Is Duplex Stainless Steel Magnetic?

Duplex stainless steels (DSS) consist of a mixture of ferrite and austenite and are usually magnetic. The ferritic structure contributes to this magnetism. But, due to the fact that most DSS contains more austenite than ferrite, their magnetic strength is normally less.

So from the above discussions, it is clear that stainless steel can be magnetic or non-magnetic and it fully depends on the crystal structure and composition. But, The magnetic behavior of stainless steel varies considerably from paramagnetic in fully austenitic stainless steel grades to permanent magnetic behavior in the hardened martensitic stainless steel grades.

Factors Influencing Magnetism in Stainless Steel

Several factors can affect the magnetic behavior of stainless steel. These include:

  • Alloy Composition: The presence and concentration of alloying elements, such as nickel, chromium, and manganese, significantly impact the magnetic properties of stainless steel. Higher nickel content tends to reduce magnetism, while higher chromium or manganese content can increase magnetism.
  • Crystal Structure: The crystal structure of stainless steel, such as austenite, ferrite, or martensite, plays a critical role in determining its magnetic behavior. Different crystal structures can affect the alignment and mobility of electrons, thereby influencing magnetism.
  • Cold Working: Cold working processes, such as rolling, bending, or forging, can introduce strain into the stainless steel structure. This strain can alter the crystal lattice and create magnetic regions in typically non-magnetic stainless steel, leading to a weak magnetic response.
  • Heat Treatment: Specific heat treatments, such as annealing or quenching, can modify the crystal structure of stainless steel and affect its magnetic properties. Annealing can restore non-magnetic behavior while quenching may induce martensitic transformation and increase magnetism.

Why does the magnetic property of stainless steel matter?

The magnetic capabilities of stainless steel material are always inferior to conventional magnetic material. Hence, Stainless steels are usually not used based on their magnetic behavior. However, the magnetic or nonmagnetic capabilities can sometimes influence the fabrication and use of these alloys. Also, in magnetic materials electric current behaves differently.

Which types of stainless steel are magnetic?

The below-mentioned stainless steel types are usually magnetic:

  • Grade 409, 430, and 439 ferritic Stainless Steel.
  • Grades 410, 420, and 440 martensitic Stainless Steel.
  • Grade 2205 Duplex Stainless Steel, etc.

Should a magnet stick to stainless steel?

Magnets stick to materials having magnetic properties. Some grade of stainless steel has weak magnetic properties whereas some do not. For example, austenitic stainless steel grades like 304 or 316 do not provide magnetic properties. On the other hand, ferritic stainless steel grades are ferromagnetic, and thus magnets stick to certain ferritic and martensitic stainless steel grades.

What stainless steels are not magnetic? Do magnets stick to 304 stainless steel?

Austenitic stainless steel grades are not magnetic. Stainless steel grade 304 and 316, being non-magnetic, does not show magnetism, and thus magnets do not attract them.

Magnetic Testing of Stainless Steel

To determine the magnetic properties of stainless steel, various testing methods can be employed. One common method is the use of a magnet to assess the attraction or repulsion between the stainless steel sample and the magnet. If the stainless steel exhibits a strong magnetic response, it is likely ferritic or martensitic stainless steel. Conversely, if the stainless steel shows no or weak magnetic properties, it is likely austenitic or duplex stainless steel.

It is worth mentioning that magnetic testing is not always conclusive, as some stainless steel grades may exhibit a weak magnetic response due to specific processing conditions or cold working. Therefore, chemical analysis and material specifications are often necessary to accurately identify the stainless steel grade and its magnetic behavior.

Is kitchen stainless steel magnetic?

There is a general concept that all materials containing iron must be magnetic. However, that is not always true. To show magnetic properties their crystal structure should have a partially filled electron shell. The FCC structure austenitic stainless steel does not show magnetic properties. Kitchen stainless steels are produced mainly from the austenitic grade of stainless steels and hence, kitchen stainless steels are usually non-magnetic.

Is type 430 stainless steel magnetic?

Yes, the ferritic 430 grade of stainless steel shows magnetism and attracts magnets.

Stainless Steel vs Aluminum: 12 Major Differences between Stainless Steel and Aluminum

Written by Anup Kumar Dey in Engineering Materials

Aluminum and Stainless steel are two widely used materials in various industries. Wherever lightweight is a requirement, aluminum seems to be a good metallic option. Stainless steel is widely used as a corrosion-resistant ferrous metallic alloy. In this article, we will explore 12 major differences between stainless steel and aluminum.

1. Stainless Steel vs Aluminum: Appearance

Stainless steel is shiny whereas aluminum usually has a dull texture. However, weather can change appearances. Still, stainless steel will be brighter as compared to aluminum.

2. Stainless steel vs Aluminum: Composition

Aluminum is non-ferrous metal whereas stainless steel is ferrous where iron is the main constituent element. Pure aluminum is metal but stainless steel is an alloy.

3. Stainless Steel vs Aluminum: Strength to Weight Ratio

Even though stainless steel is stronger than aluminum because of its almost one-third weight It is one of the preferred materials in the automobile and aircraft industries. Aluminum has approximately half the strength compared to stainless steel. So, the strength-to-weight ratio of aluminum is better than stainless steel and this aspect makes aluminum a better choice in various industries.

4. Aluminum vs Stainless Steel: Corrosion Potential

In a normal environment, aluminum has very good corrosion resistance and does not rust. However, in some extremely acidic and base environments, it corrodes rapidly and can even result in catastrophic failure.

Stainless steel is an alloy of iron, chromium, nickel, manganese, and copper. The chromium is purposely added to generate a protective film and provide corrosion resistance. Also, as stainless steel is non-porous, it increases its corrosion resistance further.

5. Stainless Steel vs Aluminum: Thermal and Electrical Conductivity

When comparing both metals in terms of thermal conductivity, aluminum is better as compared to stainless steel. Because of this reason, aluminum is used widely for air conditioners and car radiators. However, stainless steel has high thermal resistance as compared to aluminum and thus can be used at high temperatures.

Aluminum has better electrical conductivity as compared to stainless steel.

6. Aluminum vs Stainless Steel: Cost

When comparing in terms of weight, Aluminum usually is costlier than stainless steel. However, when compared in terms of volume, due to low density, Stainless steel is costlier than aluminum.

Also, the aluminum refining process is less complex than producing stainless steel. This also reduces the cost of aluminum.

7. Stainless Steel vs Aluminum: Workability

Aluminum being soft is very easy to cut and form. On the other hand, Stainless steel is very difficult to form. For producing any customized shape, aluminum will take less energy and time as compared to stainless steel.

8. Aluminum vs Stainless Steel: Welding property

Aluminum is difficult to weld as compared to stainless steel. So, for applications involving an extensive amount of welding, stainless steel will be a better choice as compared to aluminum.

9. Aluminum vs Stainless Steel: Sparkability

While grinding stainless steel produces sparks whereas aluminum does not produce spark.

10. Aluminum vs Stainless Steel: Reactivity with food

Aluminum can affect the taste and color of certain foods. It is, therefore, stainless steel is preferred over aluminum in the food industry as stainless steel is non-reactive to foods.

11. Stainless Steel vs Aluminum: Occurrence

Aluminum is naturally available from bauxite ores. On the other hand, stainless steel is a fully man-made engineered alloy. Stainless steel is not naturally available.

12. Aluminum vs Stainless Steel: Magnetic Properties

Aluminum does not show magnetic properties but certain grades of stainless steel have magnetic properties.

The major differences between stainless steel and aluminum can be summarized below:

AspectStainless SteelAluminum
CompositionPrimarily iron, with chromium (≥10.5%) and nickelPrimarily aluminum, often with copper, manganese, silicon, or magnesium
DensityHigh (around 8,000 kg/m³ or 8 g/cm³)Low (about 2,700 kg/m³ or 2.7 g/cm³)
Strength-to-Weight RatioLower than aluminum but high in absolute termsHigher due to lower density, ideal for weight-sensitive applications
Corrosion ResistanceExcellent, especially with high chromium contentGood, but susceptible to galvanic corrosion when in contact with other metals
ConductivityLower thermal and electrical conductivityHigh thermal and electrical conductivity
Thermal ExpansionLower thermal expansion coefficientHigher thermal expansion coefficient
Magnetic PropertiesCan be magnetic (ferritic and martensitic types)Non-magnetic
MachinabilityMore difficult to machine due to the hardnessEasier to machine
FormabilityModerate to good, depending on the gradeExcellent, easily formed into shapes
WeightHeavyLight
DurabilityExtremely durable and wear-resistantDurable, but softer and can dent or scratch easily
Surface FinishCan have a high-quality, shiny finishTypically has a matte or anodized finish
RecyclabilityHighly recyclableHighly recyclable
CostGenerally more expensiveGenerally less expensive
Common UsesKitchenware, medical devices, automotive parts, construction materialsAerospace components, beverage cans, automotive parts, electronics
Environmental ResistanceBetter suited for harsh environments and high temperaturesBetter for applications requiring lightweight materials
WeldabilityGenerally good, especially with appropriate filler materialsExcellent, often used in structures that require extensive welding
MaintenanceLow maintenance, does not require coatingMay require protective coatings or anodization for enhanced durability
Tensile StrengthGenerally higher (around 400-500 MPa for austenitic grades)Lower (typically around 90-130 MPa for commercial grades)
Table 1: Stainless Steel vs Aluminum

Conclusion

Even though both aluminum and stainless have varying properties, both of them are highly useful materials. The selection between aluminum and stainless steel mainly depends on the application. So, the end-use dictates which material to be used for a specific application.

What are Industrial Scrubbers? Their Working, Types, and Advantages

Written by Anup Kumar Dey in Process

What are Scrubbers? | Scrubber Definition

Industrial Scrubbers are crucial pollution control devices that remove hazardous particles from industrial exhaust gas streams before releasing them into the atmosphere. Scrubbers are engineered equipment that uses liquid to remove particulate matter, solids, mist, and harmful gases. The liquid in its atomized form entrains particles and pollutant gases and effectively cleans them from the gas flow. Industrial Scrubbers can also be used to recover valuable products preventing loss of products. As scrubbers purify gases they are also popularly known as gas scrubbers.

Along with the capability of handling explosive and flammable gases safely, Industrial Scrubber can also provide cooling. Scrubbers for air pollution control devices, suffer from high levels of corrosion and produce slurry waste streams.

An industrial scrubber system usually incorporates a system fan, recycle pump, instrumentation and controls, mist eliminators, and exhaust stack. They can be designed as either vertical or horizontal. A scrubber material should be corrosion-resistant. The mechanical scrubber design depends on various parameters like:

  • operating process conditions
  • flow rate
  • temperature
  • concentration
  • removal efficiency
  • the chemicals being removed, etc

Depending on the chemicals or pollutants that need to be removed such as ammonia, sulfur, and chlorine, the scrubbing material is selected. Fig. 1 below shows a vertical gas scrubber in an oil and gas facility.

Gas Scrubber in Oil and Gas Facility
Fig. 1: Gas Scrubber in an Oil and Gas Facility

Applications of Industrial Scrubbers

Industrial mechanical scrubbers can be used for a variety of industrial processes involving gases. Any industry with a high level of air toxins or odors, or burning fossil fuels must install a properly designed scrubber system. Some typical examples of the application of industrial scrubbers are:

  • Acid Plants
  • Chemical Industry
  • Steel mills
  • Large Power Plants
  • Fertilizer plants
  • Surface treatment
  • Asphalt plants
  • Pharmaceutical industry
  • Waste incineration installations
  • Storage and transfer of chemicals

Functions of Industrial Scrubbers

The different types of industrial scrubbers help to protect the environment by eliminating hazardous chemicals and acids from polluted industrial exhaust gases. The main functions that industrial scrubbers serve are:

  • It ensures that the exhaust gas released into the environment will not cause an undue amount of pollution.
  • They reduce the chances of acid rain by removing harmful acid gases.
  • Local cleansing.
  • Fume abatement.
  • Heat recovery from hot gases.

Types of Industrial Scrubbers

Depending on the working principles of industrial scrubbers, they are grouped into the following three classes:

  • Wet Scrubbers
  • Dry Scrubbers, and
  • Eletrostatic Precipitators

Wet Industrial Scrubbers

The wet industrial scrubber is one of the most basic forms of the industrial scrubber. They use a wet substance (Mostly water) to remove harmful substances. The wet substance is encapsulated in a metallic or composite container. When the contaminated gas is passed through the container, the water/wet substances absorb or dissolve the contaminants.

Wet scrubbers are manufactured in numerous configurations. With an increase in gas-to-liquid contact time inside the scrubber, the removal efficiency increases. Also, increasing surface area and mixing can improve the performance of wet industrial scrubbers.

Depending on the pollutants present in the gas exhaust, various chemicals with positively or negatively charged or non-charged chemicals are used in wet industrial scrubbers. In general, to remove particulate matter and dust, water is used. However, to eliminate airborne contaminants like acidic gases, wet chemicals are used.

Note that, high amounts of water vapor or moisture are added to the gas that is being expelled from the wet scrubbers. When this gas is released from vents, it is visible as a white smoke cloud.

Wet scrubbers are grouped into various types as follows:

  • Spray Tower wet scrubber
  • Cyclone Spray Chamber wet scrubber
  • Venturi wet scrubber
  • Orifice wet scrubber
  • Impingement wet scrubber

Dry Industrial Scrubbers

As the name suggests, dry scrubbers use dry substances to purify the gas. A dry industrial scrubber consists of a reaction center with dry sorbent or reaction material to absorb harmful contaminants from the polluted gas. There is a material component to remove the reaction product.

Dry scrubbers are widely used to eliminate acids found within gasses. To remove the acid from the gas, an alkaline dry sorbent is used. The sorbent is basic and neutralizes by reacting with the acid present in the gas. There are two different types of dry industrial scrubbers.

  • Dry sorbent injection process where the gas is mixed directly with the alkaline sorbent.
  • Spray dryer absorbent method where the contaminated gas is processed through a mist of the sorbent. The sprayer dryer produces a good mixture and efficient removal of hazardous pollutants.

The working of dry industrial scrubbers is almost similar to wet scrubbers except that dry reagents are used. Even though Dry scrubbers are not as effective as wet scrubbers, they are preferred because dry scrubbers create very little waste as compared to wet scrubbers. At the same time, the hassles of disposing of wet waste material are removed.

Electrostatic Precipitators or ESPs

Electrostatic precipitators use charged energy to remove dust and other contaminants from a gas stream. They match the polarity and type of charge to bind to and remove the pollutants from the gas. ESPs have a very high collection efficiency. Depending on the cleaning of the collected particles in collector plates, Electrostatic precipitators are of two types:

  • Dry ESPs where the collected particles are removed from collector plates as dry material by mechanical impulse or vibration.
  • Wet ESPs where the colletor plates are washed with water.

A plate precipitator is a typical example of a dry electrostatic precipitator. A sheet of metal is charged with a specific type of charge that runs parallel with the piping. When the gas passes through the plates, the dust and other contaminants are removed. Wet electrostatic precipitators aid in removing high moisture gases like sulfuric acid.

An electrostatic precipitator is usually comprised of gas distribution plates, discharge electrodes, collection surfaces, and rappers.

Products from Different Types of Industrial Scrubbers

Though the products of scrubbers are hazardous waste products, they can be treated to make them useful products. In many cases, the scrubber and downstream processing creates a solid slurry that is separated from the purified gas. The pollutant product is processed further to make it a useful end product.

Gypsum extracted from scrubbers used in coal power plants is an example of a useful product. This synthetic gypsum is used as a drywall component. Companies need to understand the byproducts of their scrubbers. They must be properly disposed of hazardous waste. Recycling waste is a great option. This will save us from hazardous waste disposal. Also, the byproduct will help in making money.

Advantages of Industrial Scrubbers

The main advantages that an industrial scrubber system provides are:

  • Effective prevention of pollutants.
  • Handling of flammable and explosive dust with little risk.
  • Providing gas absorption and dust collection in a single unit.
  • Cooling of hot gases.
  • Compact design helps in retrofitting any existing collection systems without requiring much space.
  • Corrosive gases and dust are neutralized.
  • Very low maintenance cost.

Disadvantages of Scrubbers

The main disadvantages of industrial scrubbers are

  • High corrosion potential.
  • The collected particulate material may be contaminated and unrecyclable.
  • Freezing protection may be required.
  • Waste sludge disposal may be expensive.

Frequently Asked Questions with Answers

  • What is industrial scrubbing?
    • The process of removing unwanted pollutants from industrial exhaust gas streams using industrial scrubbers or gas scrubbers is known as industrial scrubbing.
  • How does a scrubber work?
    • The industrial gas stream is passed through the scrubber where the hazardous contaminants are absorbed by the liquid/solid particles present. The pure clean gas is then vented to the environment.
  • What liquid is used in most commercial scrubbers?
    • Water is the most common liquid that is used in most industrial wet scrubbers.
  • How many types of scrubbers are there?
    • Broadly, there are two main types of scrubbers; wet scrubbers and dry scrubbers. Each of these two types of scrubbers has its subcategories.
  • What do scrubbers remove?
    • Industrial Scrubbers remove harmful and hazardous substances from industrial exhaust gases or flue gases.
  • How effective are scrubbers?
    • The efficiency of scrubbers depends on various factors. With proper design, scrubbers can show 95% efficiency in particle removal.
  • Where are scrubbers used?
    • A properly designed scrubber should be used in all industries with a high level of air toxins or odors, or burning fossil fuels.
  • Why do industrial factories use scrubbers?
    • To safely remove hazardous solids, mist, and harmful gases from exhaust gases.
  • What are scrubbers for acid rain?
    • The scrubbers that eliminate the acid-forming components (Sulphur, HLC, NOx, components) from the exhaust gases to prevent acid rain are known as scrubbers for acid rain.