Editor’s Note: This article is the second in a two-part series on the market and manufacturing of small diameter liquid transfer lines for high pressure applications. The first section discusses the domestic availability of conventional products for these applications, which are rare. The second part discusses two non-traditional products in this market.
The two types of welded hydraulic pipes designated by the Society of Automotive Engineers – SAE-J525 and SAE-J356A – share a common source, as do their written specifications. Flat steel strips are cut to width and formed into tubes by profiling. After the edges of the strip are polished with a finned tool, the pipe is heated by high frequency resistance welding and forged between pressure rolls to form a weld. After welding, the OD burr is removed with a holder, which is usually made of tungsten carbide. The identification flash is removed or adjusted to the maximum design height using the locking tool.
The description of this welding process is general, and there are many small process differences in actual production (see Figure 1). However, they share many mechanical properties.
Pipe failures and common failure modes can be divided into tensile and compressive loads. In most materials, the tensile stress is lower than the compressive stress. However, most materials are much stronger in compression than in tension. Concrete is an example. It is highly compressible, but unless molded with an internal network of reinforcing bars (rebars), it is easy to break. For this reason, steel is tensile tested to determine its ultimate tensile strength (UTS). All three hydraulic hose sizes have the same requirements: 310 MPa (45,000 psi) UTS.
Due to the ability of pressure pipes to withstand hydraulic pressure, a separate calculation and failure test, known as a burst test, may be required. Calculations can be used to determine the theoretical ultimate burst pressure, taking into account wall thickness, UTS and outside diameter of the material. Because J525 tubing and J356A tubing can be the same size, the only variable is UTS. Provides typical tensile strength of 50,000 psi with a predictive burst pressure of 0.500 x 0.049 in. The tubing is the same for both products: 10,908 psi.
Although the calculated predictions are the same, one difference in practical application is due to the actual wall thickness. On the J356A, the internal burr is adjustable to a maximum size depending on the pipe diameter as described in the specification. For deburred J525 products, the deburring process typically intentionally reduces the inside diameter by about 0.002 inches, resulting in localized wall thinning in the weld zone. Although the wall thickness is filled with subsequent cold working, the residual stress and grain orientation may differ from the base metal, and the wall thickness may be slightly thinner than the comparable pipe specified in J356A.
Depending on the end use of the pipe, internal burr must be removed or flattened (or flattened) to eliminate potential leak paths, mainly single wall flared end forms. While J525 is commonly believed to have a smooth ID and therefore not leak, this is a misconception. J525 tubing can develop ID streaks due to improper cold working, resulting in leaks at the connection.
Begin deburring by cutting (or scraping) the weld bead off the inside diameter wall. The cleaning tool is attached to a mandrel supported by rollers inside the pipe, just behind the welding station. While the cleaning tool was removing the weld bead, the rollers inadvertently rolled over some of the welding spatter, causing it to hit the surface of the pipe ID (see Figure 2). This is a problem for lightly machined pipes such as turned or honed pipes.
Removing the flash from the tube is not easy. The cutting process turns the glitter into a long, tangled string of sharp steel. While removal is a requirement, removal is often a manual and imperfect process. Sections of scarf tubes sometimes leave the tube manufacturer’s territory and are sent to customers.
Rice. 1. SAE-J525 material is mass-produced, which requires significant investment and labor. Similar tubular products made using SAE-J356A are completely machined in in-line annealing tube mills, so it is more efficient.
For smaller pipes, such as liquid lines less than 20 mm in diameter, ID deburring is usually not as important as these diameters do not require an additional ID finishing step. The only caveat is that the end user only needs to consider whether a consistent flash control height will create a problem.
ID flame control excellence begins with precise strip conditioning, cutting and welding. In fact, the raw material properties of J356A must be more stringent than J525 because J356A has more restrictions on grain size, oxide inclusions and other steelmaking parameters due to the cold sizing process involved.
Finally, I.D. welding often requires coolant. Most systems use the same coolant as the windrow tool, but this can create problems. Despite being filtered and degreased, mill coolants often contain significant amounts of metal particles, various oils and oils, and other contaminants. Therefore, the J525 tubing requires a hot caustic wash cycle or other equivalent cleaning step.
Condensers, automotive systems, and other similar systems require piping cleaning, and the appropriate cleaning can be done at the mill. The J356A leaves the factory with a clean bore, controlled moisture content and minimal residue. Finally, it is common practice to fill each tube with an inert gas to prevent corrosion and seal the ends prior to shipment.
J525 pipes are normalized after welding and then cold worked (drawn). After cold working, the pipe is normalized again to meet all mechanical requirements.
The normalizing, wire drawing and second normalizing steps require transporting the pipe to the furnace, to the drawing station and back to the furnace. Depending on the specifics of the operation, these steps require other separate sub-steps such as pointing (before painting), etching and straightening. These steps are costly and require significant time, labor and money resources. Cold-drawn pipes are associated with a 20% waste rate in production.
J356A pipe is normalized at the rolling mill after welding. The pipe does not touch the ground and travels from the initial forming steps to the finished pipe in a continuous sequence of steps in the rolling mill. Welded pipes such as J356A have a 10% wastage in production. All other things being equal, this means that J356A lamps are cheaper to manufacture than J525 lamps.
Although the properties of these two products are similar, they are not the same from a metallurgical point of view.
Cold drawn J525 pipes require two preliminary normalizing treatments: after welding and after drawing. Normalization temperatures (1650°F or 900°C) result in the formation of surface oxides, which are usually removed with mineral acid (usually sulfuric or hydrochloric) after annealing. Pickling has a large environmental impact in terms of air emissions and metal-rich waste streams.
In addition, the normalization of temperature in the reducing atmosphere of the roller hearth furnace leads to the consumption of carbon on the surface of the steel. This process, decarburization, leaves a surface layer that is much weaker than the original material (see Figure 3). This is especially important for thin wall pipes. At 0.030″ wall thickness, even a small 0.003″ decarburization layer will reduce the effective wall by 10%. Such weakened pipes can fail due to stress or vibration.
Figure 2. An ID cleaning tool (not shown) is supported by rollers that move along the ID of the pipe. Good roller design reduces the amount of welding spatter that rolls into the pipe wall. Nielsen tools
J356 pipes are processed in batches and require annealing in a roller hearth furnace, but this is not limited to. The variant, J356A, is completely machined in a rolling mill using built-in induction, a heating process that is much faster than a roller hearth furnace. This shortens the annealing time, thereby narrowing the window of opportunity for decarburization from minutes (or even hours) to seconds. This provides J356A with uniform annealing without oxide or decarburization.
Tubing used for hydraulic lines must be flexible enough to be bent, expanded and formed. Bends are necessary to get the hydraulic fluid from point A to point B, passing through various bends and turns along the way, and flaring is the key to providing an end connection method.
In a chicken-or-egg situation, chimneys were designed for single-wall burner connections (thus having a smooth inside diameter), or the reverse may have occurred. In this case, the inner surface of the tube fits snugly against the socket of the pin connector. To ensure a tight metal-to-metal connection, the surface of the pipe must be as smooth as possible. This accessory appeared in the 1920s for the nascent US Air Force Air Division. This accessory later became the standard 37-degree flare that is widely used today.
Since the beginning of the COVID-19 period, the supply of drawn pipes with smooth inner diameters has significantly decreased. Available materials tend to have longer delivery times than in the past. This change in supply chains can be addressed by redesigning end connections. For example, an RFQ that requires a single wall burner and specifies J525 is a candidate for replacing a double wall burner. Any type of hydraulic pipe can be used with this end connection. This opens up opportunities for using the J356A.
In addition to flare connections, o-ring mechanical seals are also common (see figure 5), especially for high pressure systems. Not only is this type of connection less leak-tight than a single-wall flare because it uses elastomeric seals, but it is also more versatile—it can be formed at the end of any common type of hydraulic pipe. This provides pipe manufacturers with greater supply chain opportunities and better long-term economic performance.
Industrial history is full of examples of traditional products taking root at a time when it is difficult for the market to change direction. A competing product – even one that is significantly cheaper and meets all the requirements of the original product – can be difficult to gain a foothold in the market if suspicions arise. This usually happens when a purchasing agent or assigned engineer is considering a non-traditional replacement for an existing product. Few are willing to risk being discovered.
In some cases, changes may not just be necessary, but necessary. The COVID-19 pandemic has resulted in unexpected changes in the availability of certain pipe types and sizes for steel fluid piping. The affected product areas are those used in the automotive, electrical, heavy equipment and any other pipe manufacturing industries that use high pressure lines, especially hydraulic lines.
This gap can be filled at a lower overall cost by considering an established but niche type of steel pipe. Selecting the right product for an application requires some research to determine fluid compatibility, operating pressure, mechanical load, and connection type.
A closer look at the specifications shows that the J356A can be equivalent to the real J525. Despite the pandemic, it is still available at a lower price through a proven supply chain. If solving final shape issues is less labor intensive than finding J525, it could help OEMs solve logistical challenges in the COVID-19 era and beyond.
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