The Ultimate Guide About Weld Backing What is Weld Backing? In a nutshell, weld backing simply means the system of placing a backing material at the root or base of a weld joint to facilitate a complete joint penetration. The backing strip placed at the base of a weld joint will prevent the molten metal from dripping or burning through the open root. This way, it ensures that the metal retains its 100% thickness. Weld backing can be either permanent or temporary. In a permanent backing, the backing material should be similar to the material of the welded joint. And as the name suggests, it becomes a permanent part of the weld joint. Why? Because here, the backing strip is fused with the root of the weld joint. Hence, it becomes intact with the weld and is not easily removable. In contrast, temporary backing materials are usually made from a ceramic substance or copper material. In this method, the backing strip does not blend with the joint’s root. Hence, these backing materials are easily removable. Temporary weld backing is also known as removable backing. Now, you may ask why is there a need to remove backing strips from a weld. Well! The reasons could be many. However, one major reason is that a temporary backing strip can collect moisture, leading to corrosion. In another case, a backing strip can keep a weld joint from bending due to the additional applied support. If this happens, the weldment (unit formed by welding two pieces together) may not bend uniformly, thereby causing it to break. Why do you Need Weld Backing? From the above definition of weld backing, you may have already understood why exactly you need weld backing. The answer is simple. Weld backing will facilitate full joint penetration. It also ensures that the metals retain their complete thickness by preventing the dripping of molten substances. Weld backing may help prevent incomplete penetration, especially during consumable electrode processes like SAW, MIG, MMA, FCAW, and MAG welding. The backing material supports and protects the root weld to promote full penetration of the weld metals. In some cases, weld backing may also eliminate the need to grind or gouge an open root joint. A weld backing system also becomes necessary during a one-sided welding process. One-sided welds are usually preferable by welders as these processes save tons of time. But the only setback is that one-sided welds are difficult to accomplish. Hence, a weld backing material provides enough support for the molten root, allowing the welder to weld from only one side. Not just that, but weld backing also helps in trapping inert gas, which goes on to create an optimal welding pool to ensure proper side-wall fusion. This goes a long way in decreasing the presence of any weld defects. One of the most common welding problems is porosity. And a weld backing system may also have some positive effects in preventing it. Porosity occurs when gases like oxygen, nitrogen, and hydrogen get trapped in the weld metal. This is caused by insufficient gas shielding or the presence of rust, moisture, paint, or grease. The solid backing material can provide enough support and shield to the molten weld metal, thereby preventing porosity. What kind of weld backing you need completely depends on the type of welding process and materials you’re working with (more information on this is below). Types of Weld Backing Weld backing is all about supporting molten weld metal, especially during submerged arc welding processes. However, weld backing isn’t the only method through which weld metal support can be obtained. There are five different methods to do so – Non-fusible backing, weld backing, root backing, submerged arc flux backing, and fusible metal backing. Hence, let’s take a closer look at all the different types of backing, after which we will talk about the most common weld backing types/materials. Non-Fusible Backing Non-fusible backing is a temporary backing system where the backing material is removed after the weld has solidified. In this type of backing, a copper backing material is usually used in welding steel. Out of all, electrolytic copper is the most satisfactory material for backing up a weld. It is particularly useful in welding gage-thickness materials. In this type of backing, a backing copper material is placed on the base of metals when full weld penetration must be accomplished in one pass. Furthermore, non-fusible backing is also applicable to metals that do not have an adequate mass to support the molten metal. Let’s now look at how copper backing can help in supporting molten weld metal. If you don’t already know, copper is a good heat conductor. Thus, it can cool down the molten weld metal faster than other counterparts, thereby providing support without fusing with the weld metal. However, while using copper backing, you need to ensure that the backing material is placed tightly against the base of the weld joint to prevent the flow of weld metal from the inside. You can also groove the copper backing material to support complete weld metal penetration. Weld Backing and Root Backing Weld backing and Root backing are the two most common types used for providing weld metal support. Both weld backing and root backing are permanent weld backing methods in which the backing material becomes a part of the joint weld. In a weld backing system, the first weld pass (usually made by gas metal arc, submerged arc, or shielded metal arc) makes the backing for subsequent passes made from the same side or the opposite. Manual welds are usually suitable for backing in submerged arc welds when other backing methods are not feasible due to poor joint preparation, inaccessibility, or immobility of the weldment. When using a manual weld during backing, make sure that you pick a weld with suitable quality and the ability to bear all the heat applied. If not, the weld may break, leading to unwanted welding problems. Though weld backing is a permanent method, you can still remove the backing material by chipping, gouging, or machining once the submerged arc weld has been established. On removing the manual backing weld, a permanently submerged arc weld occupies its space. On the other hand, a root backing method is usually used for fillet welds, butt welds, and slot or plug welds. In a root-backed joint, the root face has enough thickness to support the incompletely penetrated first pass of the weld. Here, additional chilling or backing may also be often used. Submerged Arc Flux Backing Submerged arc flux backing is another temporary backing method. Among all the submerged arc flux grades, the UM 350, in particular, is more suitable for use as a weld backing flux. In this type of backing, the backing flux is placed evenly against the base of the weld joint by a water or air-inflated hose. The backing flux should be held between the bottom of the joint and the hose. Here, the granular material will conform to the back of the plate. Hence, compared to other kinds of backing, using a backing flux can tolerate greater fit-up irregularities. When the applied pressure exceeds that necessary to hold the granular material against the weld joint’s base, it tends to form a concave weld bottom. Fusible Metallic Backing The fusible metallic backing is a permanent backing method. Here, the weld penetrates the backing material and fuses with it, thereby becoming either a permanent or temporary part of the weldment. Before performing this type of backing, ensure that the contact surfaces are completely clean and held close together to avoid welding problems like leakage and porosity. The process of fusible metallic backing is rather simple. For this, you can use a backing strip (same as or compatible with the metal being welded), or you can locate a suitable joint so that a part of the structure itself forms the backing material. Common weld backing materials (weld backing types/methods) Permanent Backing Bar You are most likely to need a weld backing bar in electro-gas or electro-slag welding. Electroslag welding is a single-pass welding process, usually performed on thick materials in a vertical position. This type of welding is mostly used to join two low-carbon steel plates and sometimes for large cross-section aluminum busbars and structural steel materials. Since the process calls for high direct current (DC) voltage, a backing material usually goes at the back of the weld joint to shield and support the molten weld metal. The backing material can be either metal or non-metal, depending on the metal that’s being welded. As the name suggests, a permanent backing bar will become a fixed part of the weld joint. It is usually inexpensive and requires little to no special skills. Advantages of Backing • • • Easy to work with inexpensive requires few special skills Disadvantage of Backing Permanent backing bars may cause possible entrapment and flow disruption in pipeworks that involve the flow of gases or other products. From an aesthetic point of view, a permanent backing bar may appear undesirable as it becomes a permanent part of the weldment. Recommended Weld Backing Products When it comes to weld backing products, the choices could be endless. There are several brands out there that manufacture some of the best weld backing products. But this can be a double-edged sword situation, as too many choices can lead to confusion as to which backing product you should go for! Hence, to make things easier for you, we have picked up the two best weld backing products that could save you time and money. Have a look at the detailed review of each product and go for the one that works for you best! Weld Backing Symbols Each type of backing has its own symbol. The most common types of backing are removable backing and permanent backing. Here is an example of a permanent backing symbol. Notice the use of the letter M in permanent backing. Below you can find an example of a removable backing symbol. Notice the use of the letter R in removable backing. What is a Temporary Backing Bar A temporary backing bar is another type of backing bar that supports the molten weld metal once placed at the base of the weld joint. However, a temporary backing bar does not fuse with the weldment and can is easily removable after the welding process. Temporary backing bars are usually made from copper and are water-cooled to ensure that the bar does not fuse with the weld joint. While setting it up, a temporary backing bar is usually held in place by an additional fixture or tool. This makes it easier for removal after the completion of the welding process. However, using a temporary backing bar can prove to be time-consuming, particularly for low-volume manufacturing, due to the extra work of setting up and removing it after welding. Not only that, but a temporary backing bar also requires you to have proper water-cooling lines. Advantages of Backing Bar When it’s impossible to use a permanent backing bar in welding, a temporary backing bar comes in to save the day. Since a temporary backing bar is removed after the welding process, it reduces the risk of welding problems like porosity, moisture, rust, etc. Disadvantages of Backing Bar Time-consuming Backing Rings Backing rings are necessary when welding two pipes together. A small piece of the backing ring (compatible with the pipe’s material) is applied between the two pipes by welding to ensure that the pipes are properly aligned and prevent leakage. Backing rings are either square- or round-shaped and mostly made from rubber or metals such as carbon steel, aluminum, or chrome. Advantage Suitable when joining two pipes together. Disadvantage Some backing rings do not provide the correct axial spacing of the pipe. Hence, it results in weak weldments. Glass-Reinforced Fiber Backing Tape This is one of the most commonly used backing materials. Glass-reinforced fiber backing tapes are typically suitable for backing weld materials like stainless steel, alloy, nickel, carbon, copper, and titanium alloys. It is also suitable for use in MMA, GMAW, and GTAW welding processes. In this method, a self-adhesive inert tape is placed on the bottom of the weld joint. The backing tape is of aluminum foil fused with a band of woven, continuous-filament glass fiber cloth. The tape is further cut into a suitable length before sticking it to the base of the joints to be welded. This tape supports the right penetration of the weld joints as the fiber material prevents over-penetration of the fusion zone. Simultaneously, it helps shape the weld under the bead to create a more continuous and positive reinforcement that blends with the subject weld on both sides. Once the weldment has been secured, you can easily peel off the tape from the weld zone. Advantage It prevents over-penetration. A fiber backing tape is one of the simplest and most convenient ways of weld backing. It also eliminates the need for back-gouging and grinding and minimizes the risk of welding problems. Disadvantage A backing tape may not be suitable for some heavy-duty welding processes. Ceramic Tile Backing Strip Ceramic tile backing strips are designed in such a way as to meet all the requirements of slag processes like MMA welding, submerged arc welding (SAW), and flux-cored GMAW. A ceramic tile strip can be attached to the underside of the weld with adhesive tape. On melting, the tile recess below the weld contains the slag, thereby protecting and shaping the external weld bead. Advantage It helps in protecting and shaping the external weld bead. Easy to use and remove. Disadvantage It may not be suitable for some welding processes. Inert gas Inert gas was originally used to butt weld tubes during the GTAW process. However, presently, the use of inert gas has extended to linear butt welds. In this backing method, a welder provides nominal gas sealing at the ends of the weld joint seam. Subsequently, this gas sealing creates a gas pressure under the weld bead that acts as a support for the molten metal and shields the weld from contamination. Out of all the options, argon gas is the most common inert gas used for weld backing. In some cases, welders may also use nitrogen wherever there are no chemical or metallurgical restrictions. Advantage Provides good root profiles. Disadvantage Additional cost of buying and installing inert gas. Consumable Inserts Consumable or shaped inserts come in multiple materials designed to suit a wide range of weld metals. Using consumable inserts is quite easy. While depositing the root bead, you just have to melt the insert into the weld pool. As a result, this produces a consistent and firm root profile. The most common inserts are known as A-shape, J_, K-type, and EB- inserts. The standard insert materials mostly use common grades of stainless steel. With other materials like nickel alloys, you may not always find a suitable form. That’s because the availability of different inserts completely depends on the availability of raw materials. Advantage Provides a consistent root profile. Disadvantage Available in limited materials. Tips for Weld Backing In weld backing, one of the most important things you should look out for is the compatibility of the backing material with the metal being welded. This is even more important in the case of a permanent backing system since the backing material becomes a permanent part of the weldment. Subsequently, check that the backing material can bear the heating temperature. Always make sure the weld contact surfaces are completely clean before fixing the backing material on the weld. This eliminates the risk of any leakage and porosity. While using any kind of backing material, ensure that you fix the backing material tightly to the weld. This will better support the molten metal and also prevent porosity.
