However, some of these coating processes cause the bronze material to become too brittle to use, and therein lies the conundrum. This tactic has been successful with topical and light thermal-diffusion coatings. Hence, many suppliers and users are looking at tool coatings to help bridge the gap and decrease wear. A bronze material normally is used for the mandrel and wiper dies when bending regular stainless steel, but the increased hardness of higher-end stainless adds more difficulty and more tool wear (see Figure 2). Using the optimal tool material and setup is the largest influence on successful bending. The focus starts with the right tool set. However, those changes then lead to deviations downstream in secondary processes like welding and cleaning. This leads to changes upfront in tool and lubricant selection. When it comes to bending high-end stainless materials, most of the attention is on the increased difficulty in bending. Both variables, along with bend angles and surface finish, help to determine the best lubricant for stainless steel bending. And, as the wall factor increases, so does the forming severity and need for better lubrication. When the D of bend decreases to less than 1.25, better lubrication is increasingly necessary to carry the increased frictional load. As the D of bend decreases, the bend becomes more challenging to achieve successfully. Likewise, the D of bend (CLR/OD) is a key factor. If the tube’s outside diameter (OD) remains constant, the wall factor (OD/t) increases, making the bend more challenging as the material along the outside of the bend elongates and the material along the inside compresses (see Figure 1). This results in a minimum wall thickness (t). Meanwhile, many high-volume suppliers desire to take any excess material out of the equation to save on process costs. When a tight radius is necessary, the process gets exponentially more difficult, requiring more performance from the lubricant. In some cases, the product designer can engineer the part to have a longer centerline radius (CLR) to reduce the bending severity. ![]() Higher stainless grades can be more challenging with respect to material properties, but they can be bent successfully. Because of these attributes, better-performing lubricants with good barrier and extreme-pressure (EP) protection, and in some cases smaller molecule sizes, can help to overcome the challenges associated with bending stainless steel tubing. Less lubricant and more substrate-to-tool surface contact usually translates to more heat, friction, and perhaps rips or tears. The surface is much smoother than carbon steel, leaving fewer and smaller pockets for lubricant to reside in during the bending process. The topography of stainless steels plays a role as well. Much of this is due to the increased strength and hardness. Replacing a carbon steel with a stainless alloy comes with tradeoffs, one of which is bending difficulty. Meanwhile, some ferritic grades such as 439 also are becoming more widely used for their strength, as well as heat and corrosion resistance. Alloys in the 300 series-austenitic, high-nickel grades-are making their way into high-volume applications such as automotive exhaust systems for appearance, strength, and corrosion resistance. While scores of stainless grades are available, alloy 304 accounts for nearly 50 percent of all stainless steel produced, which is followed by 316 (marine grade) and 409, a ferritic alloy used heavily in the automotive market. Selecting the right lubricant for your tube bending process can help you navigate the variables and contribute to process efficiency, consistency, and success. High-end stainless grades have made their way into more mainstream and high-volume applications and, as bend radii decrease and the desire for downstream process compatibility increases, the challenges compound. The lure, luster, and longevity of stainless beg for its use in tube and pipe, and it’s not just used in the exotic or aircraft markets anymore. Stainless steel has been around for a long time, and it is nothing new for tube applications.
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