What are Stainless Steel Pipes and Their Types

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Stainless steel pipes are metal tubes that are made of steel alloys that contain chromium, which is a passive film on the surface that prevents rusting. This is what makes stainless steel one of the most preferred materials. SS pipes can withstand reducing acids and chloride-rich environments because of this quality. Industrial firms rely on stainless steel pipeevery day for transporting liquids, gases and slurry, and its composition can be adjusted to suit very different service conditions.

Manufacturing Processes of Stainless Steel Pipe : Welded Vs. Seamless

There are two main production methods for stainless steel pipe: welded and seamless. Pipe that has been welded is produced out of an unending coil or plate that is folded into a circular cross-section by the use of roller or bending equipment. The sides are welded using electric welding and submerged arc welding (also known as HFW and SAW), resistance welding or high frequency welding (also known as EFW and ERW). Using filler metal makes welded production economical, and welded pipe is generally less expensive than seamless pipe because it requires less intensive forming. 

Seamless stainless steel pipe, by contrast, is produced from a solid round billet. The billet can be pierced to an empty hole and then extruded or rolled to its size. Because there is no longitudinal seam, seamless SS pipe is favoured for high-pressure applications such as oil and gas transmission and refinery piping. These pipes are also not uniform with regard to strength along the circumference due to failure to provide a welded joint. The decision between welded and seamless pipe often hinges on cost, pressure requirements and available sizes.

Classification of Stainless Steel Pipe by Alloy Grade

The corrosion resistance and mechanical properties of a stainless steel pipe are heavily dependent on the alloy composition. International standards have different nomenclatures to name grades; the most used include the German DIN, the European EN and American ASTM. A summary of some of the similar grades is presented in the table below.

These grades show the variation of the same chemical composition in the names of the standards. Grade TP304 and TP304L (low carbon) are common in general corrosive service and TP316 and low-carbon variants have better chloride resistance. TP321 contains titanium to stabilise the alloy against sensitisation during welding.When specifying SS pipe, it is important to identify the correct grade because small changes in alloying elements can significantly alter corrosion resistance and mechanical properties.

Standards and Specifications of SS Pipes

Pipes used in industries are produced under strict requirements in order to ensure unvaried quality. One of the most popular standardisation organisations is the American Society of Testing and Materials (ASTM). More than one hundred ASTM standards govern the chemical composition and manufacturing processes of stainless steel pipes.Different ASTM specifications are designed for distinct service conditions; for example:

• A106- high temperature carbon steel pipe.
• A335 - high temperature seamless ferritic steel pipe.
• A333 - welded and seamless alloy steel pipe to use in low-temperature service.
• A312 - general corrosion-resistant and high-temperature austenitic stainless pipe.

The right specification will give the stainless pipe the right chemical and mechanical specifications that it will need when it will be in service. An example is the A312 which includes seamless and welded austenitic SS pipe applied in high temperature and corrosive conditions. The seamless duplex and super duplex pipes are welded with A790, but the electro-fusion welded duplex pipes are considered as A928.

ASTM A312 Pipes

There are many industries where ASTM A312 pipes are used which include oil and gas industries, chemical processing industries, food processing industries, pharmaceutical and construction industries. The specification permits both seamless and welded manufacturing methods; seamless pipes are extruded through a mandrel or pierced from a solid bar, whereas welded pipes are made by joining stainless plates or coils.

There are many grades of A312 pipes. The alloy that is used in general corrosive and low-temperature applications is Grade TP304/304L. TP316/ 316L incorporates molybdenum to enhance resistance to hostile chemicals and high temperatures. The other grades like TP321, TP347 and TP904L are specialised and require stabilisation against sensitisation or high resistance to corrosion. The alloying elements are usually chromium, nickel and molybdenum with controlled portions of carbon, manganese, silicon and nitrogen in order to gain desirable properties. Mechanical tests, such as tensile, hardness and non-destructive are done to ensure that the pipe is within the required strength and toughness requirements.

Duplex and Super duplex: A790 and A928

Although conventional austenitic stainless pipe can be used in most cases, duplex and super-duplex stainless steel can be used to provide a high strength and enhanced corrosion resistance. ASTM A790 covers seamless duplex and super-duplex pipes of quality services in terms of high corrosion, temperature and pressure. 

Duplex stainless steel is defined as having two microstructures, that is, austenite and ferrite. The resulting balanced structure provides the duplex pipes with a higher strength than the conventional austenitic stainless steel with good toughness and ductility. Duplex pipes have also excellent resistance to pitting, crevice corrosion, stress-corrosion cracking in the chloride-rich environment. Super-duplex grades also increase the resistance to corrosion and mechanical strength. Due to its qualities, duplex and super-duplex pipes are applied in marine works, oil and gas exploration, desalination plants and other corrosive environments.

When comparing duplex pipe with conventional stainless pipe, the key differences lie in microstructure, corrosion resistance and strength. Duplex pipes are two-phase in structure and they are stronger and more resistant to the chloride-induced corrosion, compared to traditional SS pipe which has a single-phase microstructure and is adequate in corrosion-resistance in many applications. Traditional SS pipe is still simpler to weld and comes in many forms 304 and 316, duplex pipe can also be weldable, but requires special attention to welding parameters to prevent damaging stages.

Application-Based Types of Stainless Steel Pipes

Stainless steel pipe can also be categorised by its end use. The following groups illustrate how variations in surface finish, dimensions and tolerances adapt stainless pipe to particular roles:

• Sanitary Pipe: Sanitary stainless pipe is used in hygienic applications such as food processing, pharmaceuticals and biotechnology. It provides a liquid internal surface, close dimensional permissibility and good corrosion resistance that make it favorable in the transporting of liquids where contamination is not to occur. Sanitary pipes are usually ASTM A270-compliant, material and finish.
• Mechanical Pipe: Mechanical stainless pipe is used as structural components such as bearings, hollow shafts or cylinders where shape and machinability are important. These pipes are available in round, square or rectangular sections and are produced to the standards of A554 and ASTM A511. They are preferred in automotive and agricultural equipment because of their easy machinability and tolerance holding capabilities.
• Polished Pipe: Polished stainless steel pipe is finished to a smooth, reflective surface. This pipe is applied in domestic and architectural applications to reduce friction and wear on moving components, minimize contamination on equipment surfaces and offer an attractive look. Decorative stair rails, furniture and other aesthetic uses The polished pipes do not need another coating and are commonly used in decorative stairs.

The Bottom Line

The stainless steel pipes are now inevitable in most industries because of their resistance to corrosion, strength and flexibility. The composition and the processing path that the different stainless steel pipes follow during their manufacture defines their performance regardless of whether it is of a welded or a seamless type. DIN, EN and ASTM systems of international grades allow a structure to be used to specify the correct alloy with the most common grades being 304, 304L, 316 and 316L. The manufacturing and testing of these pipes to high-temperature and corrosive service is defined by standards such as ASTM A312 and duplex and super-duplex pipes to more demanding services are specified by A790 and A928.

At Amardeep Steel Centre, we supply a comprehensive range of stainless steel pipe solutions that meet global quality and performance standards. Having decades of experience, strong emphasis on technical quality, and customer-oriented approach, we assist industries to select the appropriate materials to gain many-years operational success.

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