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With so many different products like seamless pipes, welded pipes and flanges it might be overwhelming trying to pick which seamless steel pipe to use for your business applications, or even what they can be used for in the first place. There are many different properties of each pipe that give it a unique use in its respective industries, and if you’re going to master the different types of piping materials and what they’re used for, it’s important that you learn the different types, what they can be used for and how they can be applied to various industries.
To give you a hand, we’ve put together a simple article that details most of the regular uses for ERW steel pipe, why it’s used and how it could be applied to a generic industrial company.
Types of Stainless Steel Pipe
First, let’s dive into the many different types of stainless steel pipe so that we can judge which types to use for each different scenario.
Seamless Pipe
A seamless pipe is a pipe that does not contain any seams or weld joints. It’s capable of standing up to intense levels of pressure and also temperatures thanks to the metal itself. It’s used in a wide variety of different oil and gas applications, but they are also used in mechanical and engineering industries too. This makes seamless pipes rather versatile and they are always inspected with high levels of precision to ensure the quality of the material.
Welded Pipe
Welded pipes can be used in virtually every industry thanks to its flexibility. However, they fare better in corrosion resistance as opposed to withstanding pressure. Because of the materials used and the lightweight nature of welded pipes, they’re far more cost-effective than most other piping methods and offer the best value for your money.
Flanges
Pipe flanges are another important part of your overall piping setup. These come in many forms such as slip-ons, blinds, lap joints, threaded and semis. These are made from durable and sturdy materials to ensure their reliability and high-quality products are always used when creating pipe flanges.
Spectacle Blinds
For permanent or long-term solutions that allow for isolation of piping sections, spectacle blinds, spades and spacers are used for convenience. If a piece of machinery or piping section needs to be inspected, then spectacle blinds are used so that you can isolate a certain part of the piping in order to maintain a larger system. Since this is an incredibly important role that is crucial to your safety, it’s recommended to rigorously test the reliability of your spectacle blinds if you want to remain safe. Given that each use case is different, it’s recommended that you speak with a professional first before investing a lot of money into spectacle blinds.
Now we’ve taken a look at the different types of steel pipe on offer, let’s look at the industries they are used in.
Oil and Gas
Technology plays a massively important role in the oil and gas industries. The need for corrosion-resistant piping has increased over the past couple of years as the depths we explore for oil deepens and more pressure is being placed on the duplex and super duplex stainless steel pipes we use. These harsh environments can be incredibly corrosive and if the damage is left unchecked, the pipes would easily burst and become unusable for deeper depths. This is why it’s important to have sturdy pipes in the oil and gas industry and how it plays such a huge role in the success of this industry.
Nuclear Power
Obtaining usable energy from atomic nuclei is no easy task. All of the nuclear reactors we use today heat water to produce steam which is then used to spin turbines that provide us with energy and most of them are placed near the coast due to the cooling requirements. As such, much of the water being pumped in to cool down these reactors is from the sea, and it needs to be resistant to the corrosive nature of seawater. SSAW steel pipe is more than qualified to defend against corrosion from seawater, hence its importance in the nuclear power industry.
LNG
LNG (liquefied natural gas) is gas that has been temporarily converted into a liquid form for the sake of transportation. Its volume is greatly reduced to make it easier and cheaper to transport. In some cases, transporting natural gas via pipelines isn’t possible or cheap enough to be worth the cost, which is why it’s instead transported via a liquid form in cryogenic sea vessels and road tankers. As such, it’s important to have the right piping that provides safe and economical LNG transport to deliver natural gas to parts of the country or world that are difficult to reach.
Desalination
Desalination is the removal of salt and other minerals from a source of water. This is done to convert water into fresh water that is fit for humans to use or possibly irrigation. In most cases, salt is created as a by-product and desalination are used on many submarines and ships. Today, desalination is typically spoken about when attempting to bring fresh potable water to regions of the world where it is difficult. Saltwater and metals generally don’t mix very well, hence the importance of getting piping in the desalination industry that is able to withstand the natural corrosive properties of salt water.
Mining and Minerals
Mining is the process of extracting materials from the earth. The materials covered are usually in the form of metals both common and precious and it’s something that has been done for thousands of years to acquire beneficial resources. The mining industry is one where stainless steel has made it easier and more profitable to take on larger operations. Whether it’s used in the actual mines itself where workers and machines dig at the earth, or in the bunk beds and mess halls where staff sleep, it’s used in a variety of different applications to provide safety and reliability.
There’s no voodoo when it comes to welding stainless steel tube and pipe – proper selection of filler metals, joint preparation, cleanliness and welding processes help ensure that the final product meets the designated quality standards and retains its intended corrosion resistant qualities. There are, however, evolutions to well-established processes and techniques that allow pipe fabricators to increase productivity without sacrificing the corrosion resistance of the stainless steel.
This article will cover the basics of welding stainless steel tube and pipe for applications ranging from high purity food and beverage, pharmaceutical and petrochemical pipe to oil and gas applications. Within those basics, we will present best practices and new wrinkles on established methods that may help drive productivity in your shop while improving or maintaining the desired corrosion resistance.
As a note: critical applications where processes are certified should not be altered without going through appropriate certification processes. Each process detailed in this article has been certified in critical applications and is meant to spur ideas as to how to move your own welding practices forward.
Filler Metal Selection Critical in Controlling Carbon Levels
Selecting filler metal for pipe fittings is about enhancing the properties of the weld and meeting the requirements of the application. Filler metals with an “L” designation, such as ER308L, provide a lower maximum carbon content, which can help retain corrosion resistance in low carbon stainless alloys. As an example: if you weld a 304L base metal with a standard 308 filler metal, you’ll actually raise the carbon content of that joint and increase the chance of corrosion. In high purity applications – food, beverage, pharmaceutical – that low carbon content is critical for maintaining corrosion resistance. Conversely, a filler metal with an “H” designation provides higher carbon content for applications requiring greater strength, particularly at high temperatures. Filler metals with higher silicon levels, such as ER309LSi, increase weld puddle fluidity, improve tie-ins and increase travel speeds for greater productivity. The 309 series filler metals are also particularly adept at joining dissimilar stainless steels and in overlay applications.
When welding stainless steels, it’s also important to select a filler metal with low trace (or “tramp”) elements. These are residual elements in the raw materials used to make filler metals. They include tin, antimony, arsenic, phosphorus and sulfur, and can have strong effects on corrosion resistance.
Controlling Sensitization with Filler Metals, Interpass Temperature Control
Sensitization is the primary cause of the loss of corrosion resistance and is affected by the chemistry of the base material and filler metal, as well as the temperatures at which the weld cools. Chromium oxide is the “stainless” layer of stainless steel. If you raise the carbon levels in the weld and neighboring heat affected zone, it forms chromium carbides, which tie up the chromium, preventing the formation of chromium oxide. This in turn allows the steel to corrode or it will not have the intended corrosion resistance.
There are three primary ways to combat sensitization: the first is to use a low carbon base and filler metal to reduce or eliminate carbon in the welding application. This method, however, is not always practical as carbon is a vital alloying ingredient in some applications.
The second is to minimize the time the weld and heat affected zone spend at temperatures conducive to sensitization. That range will vary depending on whom you ask, but a general consensus puts that range between 500- and 800-degrees Celsius. The shorter the time spent in that temperature zone, the less damage that accrues from the heat of welding. As such, it is important to adhere to maximum interpass temperatures identified in welding procedures. The goal in multi-pass applications should be to use as few passes as possible and weld at the lowest heat input possible to achieve faster cooling.
The third is to use filler metals with special alloying ingredients to prevent the formation of chromium carbides. For instance, titanium and niobium can be alloyed into the filler metal and help prevent reactions between chromium and carbon. These elements also have strong effects on strength and toughness, limiting the applications in which they are useful. They also do not provide any benefit to the areas farthest away from the weld in the heat affected zone.
Shielding Gas Critical in Retaining Corrosion Resistance
Welding stainless steel tube and pipe traditionally requires a back purge of argon. In non-critical applications, where cost is a driving factor, nitrogen can also be used as a back purge but it may lead to the formation of some nitride compounds in the weld root, which sacrifices some corrosion resistance. This can be an acceptable trade-off in applications such as stainless steel piping for large compressed air systems and hydraulic fluid systems where water is not normally present inside the pipes and the risk of corrosion from the inside is low.
Straight argon is recommended for gas tungsten arc welding (TIG) of stainless steel tube and pipe. Shielding gas selection for wire processes is more complicated.
Traditionally, MIG welding has relied on mixtures of argon and carbon dioxide, argon and oxygen, and 3 gas mixtures based on helium, argon and carbon dioxide. These mixes usually contain mostly argon or helium, with carbon dioxide comprising less than 5 percent of the total gas mix. That is because the carbon dioxide can decompose in the arc and contribute carbon to the weld pool, creating a sensitized weld that is vulnerable to corrosion. Pure argon isn’t used with the MIG processes because it doesn’t easily support a stable welding arc. Other trace constituents like carbon dioxide and oxygen can serve this role. Argon and oxygen gas mixes can only be used to weld in the flat position because the oxygen makes for a very fluid molten weld puddle. Argon/carbon dioxide in combination with Pulsed MIG can be used to weld in all positions, as can the Tri-Mix shielding gas mixtures.
Flux-cored wires for welding stainless steel are designed to run on traditional 75/25 percent argon/carbon dioxide mixes. The flux ingredients prevent the carbon contributed by the shielding gas from contaminating the weld and the fluxing action of the slag covering scavenges the excess carbon and keeps it from entering the weld deposit. 304 stainless steel can be successfully welded using the Regulated Metal Deposition (RMD?) process without a back purge. This is not true for duplex stainless steels. These must be purged with an inert gas such as argon.
Weld Preparation and the Importance of Fit-Up
A discussion on welding stainless steel tube and pipe is not complete without a discussion on joint preparation. The normal trappings of welding stainless steel apply: use dedicated brushes, files and grinders that never touch carbon steel or aluminum. Cleanliness is critical. Even trace elements of foreign materials incorporated into the weld joint can cause flaws and lead to reduced corrosion resistance and strength. Because stainless steel is so sensitive to heat input to maintain its properties – both in shape and corrosion resistance – the way the pipe is cut and beveled can also have a detrimental effect on the weld. Any gap or lack of fit-up requires the welder to add more filler metal and can slow the welding process down, leading to buildup of heat in the affected area. You want as close to perfect fit-up as possible, especially on sanitary and high-purity tubing.
The problem of corrosion of steel pipes in hot water supply systems is very relevant in Russia. In a number of cases, the accelerated corrosion of pipelines and fittings installed in hot water supply networks is observed. The purpose of this study is to investigate the mechanism of corrosion of steel zinc-coated and non-galvanized pipes in hot water supply systems, to analyze the causes of corrosion and to develop methods for identifying active corrosion processes occurring in water supply systems. According to literature review the microstructure and composition of the zinc coating and the methods of its application are studied. A SEM/EDS study of the structure and elemental composition of inner zinc coating on new and used steel galvanized pipes is conducted. This study shows the nonuniform distribution of zinc layer which can lead to a rapid corrosion of both the zinc layer and the metal substrate and the development of pitting corrosion. A structure and composition of corrosion sediments formed on the inner surface of pipes was studied. The different causes and factors of steel corrosion in hot water are considered and discussed. A method of corrosion identification of galvanized pipes and heat exchange equipment based on the determination of corrosion products, in particular zinc and copper, was proposed.
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