Stainless Steel Tubing

Chapter 1: What is the principle behind stainless steel tubing?

This section will cover the nature of stainless steel tubing, including its production process and operational functions.

What is Stainless Steel Tubing?

Stainless steel tubing is a robust and long-lasting material known for its resistance to corrosion and low maintenance requirements. The dimensions and types of stainless tubing vary according to the specific needs of the application and its intended use.

Many people confuse piping and tubing, as both have a circular cross-section and are often thought to serve similar purposes. However, unlike pipes, which are typically round, tubes can also come in shapes like square and rectangular.

A key distinction between pipes and tubes lies in their applications. Piping is primarily used for transporting gases and liquids and is measured by its internal diameter (ID). In contrast, tubing serves structural roles and is measured by its external diameter and wall thickness.

Stainless steel is an excellent material for builders and manufacturers due to its strength, resistance to corrosion, and minimal maintenance requirements. For structural applications, stainless steel tubing must be precisely sized according to its outer diameter and wall thickness. The choice of tubing type depends on the specific application and can include fractional, hypodermic, square, or metric tubing.

Manufacturing of Stainless Steel Tubing

Various methods are used to produce stainless steel tubing, including:

Tube Mill Process

This procedure involves steps such as:

Rolling and Welding

During the rolling and welding process, stainless steel coils are continuously formed into tubes on a tube mill. Techniques like TIG, high-frequency, and laser welding are used to join the strip edges. The edges may need preparation to ensure a high-quality weld, depending on the welding method. For finishing, weld beads on both the inner and outer diameters of the tube are removed through methods such as scarfing, sanding, or bead rolling, depending on the final application and specifications.

Cleaning and Heating

The rolled stainless steel tubes are cut to the required lengths based on industry needs. They are then cleaned to eliminate any dirt. Subsequently, the tubes undergo heat treatment to relieve any stress induced during the welding and forming processes. This treatment is performed in a continuously rolling hearth furnace, which is equipped with temperature controllers and recorders. After heat treatment, the stainless steel tubes and pipes are straightened and pickled to remove surface scales.

Cold Drawing

In certain situations, the desired size may not be readily available from the mill. To achieve the correct size, the cold drawing process can be used. During this process, the tubes or pipes are coated with a soap and oxalic solution, which acts as a lubricant to reduce friction. The tube or pipe is then pulled over a bench using die plugs during the cold drawing operation.

Finishing Process

Next, the drawn-out tube or stainless steel pipe is cleaned, heat treated, pickled, and straightened. The final pipes or tubes are marked using computerized ink jet marking equipment. Before being sent on to stainless steel pipe exporters, the pipe or tube is labeled with the grade of material, heat number, size, and the stamp of a third party inspection. The resulting stainless tubes or pipes will be subjected to rigorous testing and quality control. This is followed by appropriate packing and shipping. There are several stainless steel pipe applications for which these pipes and tubes are employed.

Seamless Extrusion

Extrusion, gun drilling, and piercing are common methods for producing seamless stainless steel tubing. Among these, extrusion provides the most consistent exterior diameter and, consequently, the most concentric internal diameter. Additionally, extrusion is particularly suited for producing long-length seamless stainless steel coil tubing. The process begins with a hot-extruded seamless pipe as the base material.

Afterward, the material undergoes cold processing. At this stage, the raw material is formed into a long, straight shape using tube reducers known as pilger mills. Cold pilgering is a longitudinal cold-rolling process that reduces both the wall thickness and diameter of steel tubes in a single operation, maintaining consistent outside diameters and concentric inside diameters. This method can achieve cross-section reductions of more than 90% in a single cycle for certain material types.

During the reduction process, lubricants are applied to both the outside and inside diameters of the tube, which are later degreased in preparation for annealing. This method typically produces lengths ranging from 40 to 160 feet. Cold reduction is particularly efficient for producing long lengths of seamless tubing intended for coil form processing.

Coiling Tube

On specially designed machinery, a straight length of tube is cold drawn into a coil shape. Cold drawing reduces the outside diameter, inside diameter, and wall thickness of the tube, causing it to lengthen as the product is drawn.

Main Drawing

Floating plug drawing involves pulling tubing through a conical die while inserting a floating plug into the inside diameter. The final inside diameter and wall thickness are determined by the size of the die and plug. This method produces a more uniform, smoother finish but is generally suitable only for light wall items.

Sink drawing entails pulling tubing through a conical die to determine the resulting inside diameter and wall thickness based on the drawing and tubing specifications. This method is used for high-pressure applications requiring a large relative wall thickness. Compared to floating plug drawing, the inside diameter is often somewhat rougher.

Lubricants are applied to the outside diameter and inside diameter of tube coils to aid in the drawing process. The lubricant is degreased from the tube for most end-use applications (medicinal, liquid transfer, etc.). In a large solvent degreasing tank, this is achieved in a single process.

Vacuum Annealing

Following the cold reduction phase, stainless steel tubing and coils are subjected to annealing to alleviate internal stresses and restore their crystalline structure, making them more amenable to additional cold processing. This treatment involves rapid cooling, which reverts the material from a hard, brittle state to a softer, more pliable form.

This annealing procedure is performed repeatedly until the tubing meets the specified dimensions for outside diameter, inside diameter, and wall thickness.

Hydrotest Tubing

Tubing of this kind must adhere to relevant ASTM standards and undergo rigorous inspections for any defects before delivery to the customer.

Once the tubing is fully shaped to meet the specified criteria, it undergoes hydrostatic testing to ensure its structural integrity. This testing method is crucial for verifying that the tubing meets all required specifications. Hydrostatic testing can provide various critical pieces of information, including:

• Mechanical properties
• Material flaws
• Specific hard spots which may fail when in the presence of hydrogen

Finishing Stainless Steel Tubing

Stainless steel coil tubing can be neatly wound onto spools for transport or straightened and cut to lengths specified by the customer. To ensure traceability and identify the manufacturer, products may be polished and marked with pertinent details.

Drawn Over Mandrel (Metal Spinning)

A mandrel is a small metal tool inserted into a tube to maintain its shape. It supports the tube during the drawing process, helping to avoid unwanted wrinkling. The tube is passed through a die with a smaller diameter than the tube’s original size. As the tube is drawn through the die, it contracts to match the die’s diameter. This method allows for precise tolerances and specifications.

DOM Tubing begins as CREW (Cold-Rolled ERW tube) or a Hollow. To begin, the bigger ERW tube is "tipped" so that a trolley jaw can grip it. The trolley is a platform that pulls the tube through the die like a conveyor belt. A carbide mandrel is put within the bore of the raw tube after it has been "tipped." The tube is then placed within a carbide die. The surface of the tube is then lubricated with a solvent. The trolley jaws then hold the tube at the pointed end, drawing the pipe through the die through the trolley hook.

The final dimensions of the tube, including both its outside and inside diameters, are determined by the die and mandrel. Essentially, a high-angle die reduces the thickness of a thick-walled tube, while a low-angle die increases the thickness of a thin-walled tube. The appropriate diameter-to-thickness (D/t) ratio is achieved by carefully selecting the die's position and angle.

Multiple Passes

The number of passes required during the DOM process can vary based on factors such as polishing needs, tube dimensions, and specific mechanical and metallurgical criteria. For instance, if significant reduction is needed for the final tube, multiple incremental passes might be necessary. Additionally, subsequent surface treatments might be required to meet the desired specifications.

Mandrel and Rod Removal

Once the initial drawing of the tube is complete, the mandrel must be extracted. One method involves applying pressure to the rotating tube as it moves through a series of rollers. This action compresses the tube, expanding it sufficiently to release the mandrel. Depending on the final specifications, additional drawing steps may be needed to achieve the desired finish.

Computer Aided Design (CAD)

Today, Computer Aided Design (CAD) has replaced the traditional trial-and-error approach in die design. CAD technology has greatly enhanced the precision of die creation, leading to significant improvements in drawing processes. This advancement has resulted in increased production efficiency, reduced lead times, and lower costs. For instance, CAD allows for accurate calculations of die angles for both thick-wall and thin-wall tubes, eliminating unnecessary time and expense.

Finite Element Method (FEM)

The final product is analyzed using Finite Element Method (FEM) techniques, which take into account factors such as pressure, temperature, material properties, and other production parameters. Advances in modeling and simulation have largely superseded traditional trial-and-error methods.

MSC SuperForm is a notable software that utilizes FEM for this purpose. It allows designers and engineers to model material flow, forming pressures, stresses, and trimming processes. This data helps engineers and tool designers accurately determine die and mandrel shapes and angles, optimize lubrication, and implement other cost-saving measures.

Specifications of Stainless Steel Tubing

The ASTM International organization has set forth a range of standards for the stainless steel tubing industry. These standards are globally recognized to ensure consistency and quality. They cover aspects such as chemical composition (grades), tube wall thickness (inches), fractional radii and wall thickness (millimeters), working pressure (typically in psi), and weight (commonly in pounds per foot).

Chapter 2: What are the different types of stainless steel tubing?

Stainless steel tubing comes in various types and configurations, each differing in several ways. For instance, tubes can vary in quality, be either seamless or welded, have different radii, lengths, or wall thicknesses, and may be either polished or unpolished.

Pipe vs Tubing

While the terms "tube" and "pipe" are often used interchangeably, there is a key difference between them, particularly in terms of material organization and tolerancing. Tubing is primarily used for structural purposes, where the outer diameter is crucial for determining its strength and stability. In applications like medical devices, where precise outer diameters are necessary, tubing plays a critical role. Conversely, pipes are designed for transporting gases or liquids, and the internal capacity is more important to ensure efficient flow. The round shape of pipes helps them handle the pressure of the fluids passing through them effectively.

Classification of Pipe vs Tubing

Pipes are typically categorized by their nominal size and schedule, which dictates the wall thickness. They are ordered based on the Nominal Pipe Size (NPS) standard, with a specified schedule number indicating the wall thickness. Different pipe sizes can have the same schedule number, but the actual wall thickness will vary accordingly.

Tubes, on the other hand, are generally ordered by their wall thickness and outside diameter, though they can also be specified by outside diameter and inside diameter or by inside diameter and wall thickness. Tube thickness is often indicated by a gauge number, and the inside diameter (ID) is a theoretical measure.

While pipes are exclusively round, tubes can come in various shapes such as square, rectangular, or cylindrical. Pipes are designed to handle larger-scale applications and are available in sizes from 12 inches to several feet, whereas tubing is usually employed in smaller-scale applications.

Ordering of Pipe vs Tubing

While tubing is characterized by three dimensions—outside diameter, inside diameter, and wall thickness—only two of these dimensions can be specified with tolerances, leaving the third as a theoretical value. Tubing is usually ordered and maintained to stricter tolerances and specifications compared to pipe. Pipe is generally ordered based on the Nominal Pipe Size (NPS) standard, which includes a nominal diameter and schedule number. Both tubes and pipes can be cut, bent, flared, and fabricated to meet various requirements.

Characteristics of Pipe vs Tubing

Several key characteristics differentiate tubes from pipes:

• Shape - Tubes can be rectangular, square, or round. Pipes are always round.
• Measurement - A tube is usually ordered wall thickness and outside diameter. A pipe is usually ordered by use of nominal pipe size (NPS) standard and specifying schedule number and nominal diameter.
• Telescoping Abilities - Tubes can be telescoped and pipes cannot.
• Rigidity - Pipe is firm and cannot be made into a shape without special equipment. Aside from brass and copper, tubes can be made into a shape with some effort. Coiling and bending tubing can be done without extreme distortion, fracturing, or wrinkling.
• Applications - For instance tubes are used in medical devices which require a precise outside diameter. Outside diameter is vital since it will show how much it can hold as a firmness factor. Pipes are used for transporting liquids or gasses, making it vital to know the capacity. The circular shape of a pipe makes it efficient when sustaining pressure from the liquid.
• Metal Types - Pipe is only hot rolled. Tubes are hot and cold rolled.
• Size - Pipes serve larger applications. Where small diameters are required, tubing is commonly used.
• Strength - Tubes are stronger than pipes. They perform better in applications requiring strength and durability.

304 Stainless Steel Tubing

304 stainless steel tube is a cylindrical, hollow machine and piping component manufactured of a specific steel alloy and commonly used to transfer fluids and gasses. 304 stainless steel is commonly used in industrial, commercial, and even domestic contexts since it has the great tensile durability and strength that almost all steels are recognized for, and good forming and welding qualities.

Originally referred to as 18/8 stainless steel due to its composition of approximately 18% chromium and 8% nickel, 304 stainless steel's elemental makeup can vary. Typically, it includes carbon and iron, with chromium content ranging from 18% to 20% and nickel from 8% to 18%. For enhanced weldability, low-carbon variants like 304L are used, while high-carbon variants such as 304H offer improved performance at elevated temperatures. The specific alloy chosen for a tube application depends on its intended use.

Although the initial cost of 304 stainless steel tubing may be higher compared to some alternatives, its durable elemental composition significantly extends the tube's lifespan. This results in reduced long-term costs related to replacements and maintenance. Additionally, despite eventual repair or replacement needs, 304 stainless steel can be easily melted down and recycled, further reducing costs and environmental impact.

316 Stainless Steel Tubes

316 stainless steel tubes are cylindrical, hollow components made from a specific steel alloy grade, primarily used for transporting fluids and gases. Known for its high tensile strength and excellent forming and welding properties, 316 stainless steel is utilized in a variety of settings, including industrial, commercial, and residential applications. With a melting point ranging from 1,350 to 1,400 degrees Celsius, this material is suitable for high-temperature applications.

Grade 316 stainless steel is primarily composed of iron, with carbon being a crucial addition for steel production. To achieve stainless properties, chromium must be present at levels of 10% or more. What sets grade 316 apart is its molybdenum content, which enhances its corrosion resistance compared to many other alloys. Variants of grade 316, such as 316H, 316L, and 316Ti, are commonly available in finished products and tube stock forms.

Grade 316L, a low-carbon variant, offers improved weldability, whereas grade 316H, a high-carbon variant, is better suited for high-temperature applications. Additionally, 316Ti, which contains up to 0.5% titanium, also performs well in high-temperature conditions.

Corrugated Stainless Steel Tubing

A corrugated stainless steel tube, often abbreviated as CSST, is a hollow cylindrical object with a multi-layered construction. It serves as an alternative to traditional steel piping for transporting gases. This type of tubing is widely used in gas distribution systems, as well as in residential and commercial settings for gas heating, transport, and appliance connections.

Corrugated stainless steel tubes are gaining popularity due to their numerous advantages over traditional gas flow systems. Their excellent corrosion resistance allows them to handle both volatile gases and high pressure effectively.

The dual-layer construction of these tubes facilitates easy bending and shaping, which reduces the need for additional stainless steel fittings and connectors, thereby minimizing the risk of costly and dangerous gas leaks. The flexibility, lightweight nature, and compact size of CSST contribute to easier installation and lower labor costs, alongside material savings. While the tubing is generally hidden inside walls, the outer plastic coating, available in black or yellow, meets aesthetic and safety standards.

The tubing consists of two layers: an inner seamless stainless steel coil made from cold or hot rolled alloyed steel strips, and an outer plastic layer. The chromium in the stainless steel provides outstanding corrosion resistance, essential for many gas supply applications. The plastic coating enhances flexibility and provides an additional layer of protection against potential gas leaks if the stainless steel fails.

Corrugated stainless steel tubing, like other stainless steel tubing products, comes in various shapes and grades to suit different applications.

Hypodermic Tubing

Hypodermic tubing is a typical medium for administering medicine or medicines under the skin. Hypodermic tubing, whether used for injection or removal, enables a quick supply of fluids. As a result, this stainless steel tubing is commonly used to make hypodermic needles for medical applications. It is a thin, cylindrical, hollow length of stainless steel, usually 316 or 304 grade, with a puncturing point at one end to form a needle.

The size of hypodermic tubing significantly impacts its applications. For instance, large-diameter hypodermic tubes are preferred for blood donation procedures because their wider bore allows for a faster blood draw compared to smaller or average-sized tubes. Conversely, small-diameter hypodermic tubes are unsuitable for blood donation, as their narrow size can damage red blood cells, rendering the blood unusable. Besides medical applications, hypodermic tubing is also utilized in various fields such as electrical, mechanical, and chemical industries for functions like spacers, cannulas, guide wires, and capillary tubes.

Stainless Steel Pipes

A stainless steel pipe is a cylindrical conduit made from stainless steel, designed to carry substances such as water, gas, oil, or other materials. Its excellent resistance to corrosion makes it ideal for use in environments with high temperatures, corrosive fluids, and high levels of contaminants.

Stainless steel's advantageous properties make it the preferred material for pipe fabrication. Its resistance to corrosion, low maintenance needs, ease of cleaning, and long-term cost-effectiveness contribute to its widespread use. Despite a higher initial investment compared to other metals, stainless steel offers lower maintenance and replacement costs over time.

Stainless Steel Fittings

Stainless steel pipe fittings are essential components used to connect or seal the ends of pipes and tubing, facilitating various configurations such as offsets, in-line connections, mounting, and multiport setups. Since bending pipes can be labor-intensive and costly, fittings are often employed to join two straight sections of pipe at angles necessary for different layouts.

As crucial elements in flow and plumbing systems, pipe fittings must efficiently handle all fluids, gases, and particulates that might be present. Stainless steel is a favored material for these fittings due to its exceptional strength, durability, and resistance to corrosion in challenging environments.

Stainless steel pipe fittings are widely utilized across various industries, including paper and pulp, petrochemical, automotive, water treatment, dentistry, medical, food processing, semiconductor, and aerospace sectors. In fields such as food processing and healthcare, where hygiene and ease of cleaning are critical, stainless steel is often preferred due to its sanitary properties. The long-term benefits of stainless steel, such as enhanced durability and reduced maintenance costs, often outweigh its higher initial expense. Even though fittings might eventually require repair or replacement, stainless steel's recyclability further contributes to cost savings and waste reduction.

Stainless Steel Square Tubing

Stainless steel square tubing consists of a hollow, pipe-like structure with four equal-length sides, made from various steel alloys. While these tubes can serve in gas and fluid transfer applications similar to their cylindrical counterparts, they are predominantly employed in structural and support frameworks due to their unique shape and strength properties.

Stainless steel square tubing is used in the construction, architectural, building, material handling, mounting, metal working, and framing sectors for a wide range of goods such as shelving, support beams, racks, grids, stepladders, partitions, and rails. While other materials may be utilized, stainless steel is particularly well suited to these applications.

Although various materials can be used, stainless steel is particularly advantageous for these applications. Despite its higher initial cost compared to other materials, stainless steel's inherent durability extends the lifespan of the tubing, reducing long-term expenses associated with replacements and maintenance. Even though square tubing will eventually require repair or replacement, its recyclability helps to minimize costs and environmental impact.

Stainless Steel Tubes

Stainless steel tubes are hollow, cylindrical structures used both as components in machinery and as conduits for fluid transport. The prevalent use of stainless steel in tubing is attributed to its beneficial properties, making it the preferred material for manufacturing these tubes.

The most frequently used stainless steel grades for tubing are 304 and 316. Stainless steel offers several benefits, including excellent corrosion resistance, easy cleaning, minimal maintenance, and overall cost-effectiveness. While the initial cost of stainless steel may be higher compared to other metals, its long-term maintenance and replacement costs are generally lower.

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Chapter 3: What are the applications and benefits of stainless steel tubing?

This section will explore the various applications and benefits of stainless steel tubing, as well as the factors to consider when choosing the right tubing for specific needs.

Applications of Stainless Steel Tubing

Stainless steel tubing is utilized across a wide range of applications, from industrial machinery to automotive exhaust systems. It serves primarily to facilitate the movement or control of gases and liquids. Stainless steel tubing is particularly effective in handling high temperatures and corrosive substances, making it adaptable for various uses. Its versatility allows it to be employed in numerous contexts, limited mainly by budget and creativity.

This type of tubing is suitable for a variety of environments and can withstand prolonged exposure to the elements without rusting or deteriorating. It finds applications in industries such as construction, outdoor equipment, automotive manufacturing, and kitchenware, thanks to its resistance to corrosion and durability.

Benefits of Stainless Steel Tubing

Stainless steel tubing offers several advantages over other materials. Its key benefits include exceptional durability, the ability to transport both cold and hot liquids and gases, resistance to chemical corrosion, and flexibility in terms of grades and dimensions. Additionally, it can be manufactured cost-effectively for low-pressure systems.

Drawbacks of Stainless Steel Tubing

Despite its advantages, stainless steel tubing comes with a higher manufacturing cost compared to other metals. The processes involved in producing stainless steel tubes are also expensive. The assembly of stainless steel components can be challenging, and care must be taken to avoid wear and tear during installation. If the chromium oxide protective layer is damaged, it can lead to increased friction and potential issues during assembly.

When assembling stainless steel parts, friction and heat can cause the components to bond at the molecular level, creating a rough surface that may require special anti-jamming compounds to minimize wear. Some stainless steel tubes may eventually need repair or replacement.

Considerations When Selecting Stainless Steel Tubing

Several factors should be considered when choosing stainless steel tubing:

Right Metal for the Operating Environment

The choice of steel grade depends on the specific conditions the tubing will be exposed to. Extreme pH levels, high stresses, temperatures, and crevice corrosion can affect performance. Austenitic steels, such as 304 and 316 alloys, maintain their strength and corrosion resistance across a broad temperature range.

Ductility, Strength, and Toughness

Strength: The maximum force that a metal can withstand before breaking or deforming.

Ductility: The ability of a material to be shaped, such as being drawn into a wire, without losing its strength or breaking.

Toughness: The capacity of a metal to absorb energy and deform before fracturing.

Form and Process

Austenitic stainless steel is available in various forms, including bars, wires, tubes, pipes, sheets, and plates. However, many products require additional machining or forming to meet specific application needs.

Customer Preferences

Designers often choose stainless steel for its aesthetic appeal, whether it features a shiny, matte, electro-polished, pickled, or black oxide finish. Austenitic stainless steel tubing can accommodate these treatments, including the commonly applied passivation process.

Material Cost and Availability

While high-performance austenitic stainless steel tubing may have a higher initial cost, it is a worthwhile investment due to its durability and suitability for its intended application. Using corrosion-resistant materials can reduce downtime, maintenance, and replacement costs. Life-cycle costing can provide a comprehensive comparison of various materials, assessing both current and future expenses.

Conclusion

Stainless steel tubing is a multifaceted product that is commonly utilized in structural applications. Stainless steel tubing diameters and variations vary greatly based on the application requirements and are widely employed in a wide range of industrial sectors. It is manufactured by three processes namely seamless, metal spinning, and tube milling. Although the names are sometimes used interchangeably, there is one significant distinction between tube and pipe, notably in how the material is arranged or ordered and the classification.

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