Pipe welding basics
Welders often have a lot of questions about welding different kinds of pipe, from high-pressure and high-purity pipe for food and beverage industries to pipe for oil and gas applications. There are many common issues in pipe welding and fabrication that can lead to problems.
Focusing on some basic variables in the pipe welding process can help address these challenges — especially as pipe shops and field operations push to train new welders, work with new materials and increase quality and productivity. Learn more about nine common issues in pipe welding and tips for how to resolve them.
1. Cutting corners with cutting
When working with materials like stainless steel that are sensitive to heat input and more prone to distortion, a poor cut can lead to poor fit-up and create unnecessary gaps. Welders may then compensate by putting more filler metal into the joint. This added heat can cause distortion and reduce the corrosion resistance of the base metal. Poor material preparation also leads to longer weld-cycle times, higher consumable costs and potential repairs. Pay attention to proper part fit-up — this includes having a consistent root face (land), squareness of pipe, and bevel to wide or narrow. Consistency in these factors helps keep filler metal and heat input into the joint at a minimum.
2. Forgetting to grind the joint during weld prep
Operators may use plasma or oxy-fuel cutting to prepare material for welding. Both of these processes add a layer of oxide to the cut edge that must be removed before welding.
The oxides can also remain in the weld and cause porosity, inclusions, lack of fusion and other weld defects. It’s important to grind the joint down to the parent material prior to welding, as well as to grind the inside and outside diameters of the pipe to remove oxides and other potential contaminants.
3. Improper tacking
Tacking is critical to pipe fit-up, and best practices recommend cutting out or feathering the tack to ensure consistency of the final weld. Tacks left in the joint become consumed by the weld. If there is a defect in the tack or if the fitter used the wrong filler metal to tack the joint, there is a risk for weld defects. Cutting out the tacks helps eliminate this potential problem. Watch this video to learn more about preparing the pipe joint.
4. Treating joint prep the same for MIG and stick
Training welders is a priority for many shops and operations, and many welders bring past experiences with them to the job. One common mistake is using the same joint preparation for wire processes that are used for stick and TIG welding.
Welders experienced and trained in traditional stick often prep a joint with a heavy landing area to keep the gap as narrow as possible, whereas TIG welding typically has a knife edge land. TIG also typically has a slightly wider gap — about 1/8-inch root opening compared to a 3/32-inch opening for stick.
Compare this to common wire processes used in pipe welding. The Regulated Metal Deposition (RMD®) process from Miller will have a minimum of 1/8 inch to 5/32 inch for the gap, with a slight land (3/32 inch to knife edge). Welders will want to focus heat on the bevel to ensure proper tie-in to the sidewall.
It’s important to train welders on the specifics of each application so they understand differences in weld prep and technique for each process.
5. Using the wrong shielding gas flow rate or mix
There is a misconception that more shielding gas is always better for protecting the weld. However, too much shielding gas not only wastes gas and money, it can also cause problems like increased agitation of the weld puddle and a convection effect that sucks oxygen into the weld and possibly causes porosity. Each weld station should be outfitted with a regulator flow meter, and operators should understand how to set and adhere to recommended flow rates. Learn more about shielding gas flow and best practices.
In addition, buying mixed shielding gas is typically better than relying on mixing the gas with a flow regulator. There are a variety of shielding gases to choose from, so it’s recommended to check the Welding Procedure Specification to ensure the right gas is being used for the application.
6. Misunderstanding porosity
Welding power sources typically don’t cause porosity. To find the cause of porosity, welders should check all connections and parts, from the front of the welding gun back to the power source. Porosity is often caused by an interruption or problem with the gas flow, so that’s a good place to start. Other common culprits include loose connections, incorrect shielding gas, or improper material prep that results in contaminants in the weld.
7. Using the wrong drive roll or nozzle
It’s important to choose the proper type of drive roll for the wire being used. Flux-cored wires should be used with knurled drive rolls, while solid wires should be used with standard V drive rolls. When using copper coated flux-cored wire, U-groove drive rolls should be used.
Be sure to change out the drive rolls when the wire is changed. If a standard V drive roll is used with flux-cored wire, it will typically result in wire slippage. If drive roll tension is then tightened to accommodate that, it crushes the cored wire. Using a knurled drive roll with solid wire will chip the outer coating of the wire and plug the liner. When an operator feels the need to crank up wire tension, it’s often a symptom of something else wrong, such as the wrong drive roll type or a clogged liner. Watch this video to learn more about properly setting drive roll tension.
Choosing the proper nozzle size for the application is another important factor. For instance, a tapered nozzle and tapered tip are recommended with the RMD process, but a tapered nozzle can’t accommodate the gas flow requirements of pulsed MIG and could result in improper gas coverage. Be sure to understand which nozzles match with each process.
8. Choosing the wrong MIG gun for the application
If pulsed MIG welding will be used, the MIG gun must be able to handle the higher peak amperages of that process. Operations will often select a MIG welding gun based on the average amperage needed. But, purchasing a 250-amp gun when the application averages 250 amps means the gun will be subjected to considerably higher amperages during the peak of the pulsing cycle. If the gun isn’t designed for that peak amperage, it can burn out faster.
Similarly, most MIG guns are rated for use with 100% carbon dioxide shielding gas. That is fine for applications that weld with that gas, but the available amperage on the gun decreases as soon as a mixed gas is used, which frequently happens in pipe welding applications.
While lower amperage MIG guns are lighter and less expensive, they may not meet an operation’s pipe welding needs in the long run. Always choose a gun rated toward the higher end of welding needs.
9. Buying a machine that can’t handle the work
Pipe welding is its own animal. A 250-amp welding power source may provide the power and performance needed for some pipe welding applications — but perhaps not enough power for all the necessary jobs.
Smaller, less expensive machines have lower duty cycles and often fewer capabilities. If a shop or field operation is serious about pipe fabrication and wants to maintain high productivity, operating at higher duty cycles will ensure consistent use. It’s the difference between 250 amps at 20% duty cycle (two minutes welding in a 10-minute cycle) versus 250 amps at 100% duty cycle (10 minutes of continuous welding in a 10-minute cycle).
The Miller® PipeWorx 400 Welding System — with arc performance that’s optimized for fab shop pipe welding — is rated at 400 amps at 100% duty cycle, ensuring a strong, consistent arc all day, without having to stop in most process pipe scenarios. This is particularly helpful in applications where larger diameter wires and higher wire feed speeds are used and the power source continually works at higher amperage levels. The XMT® 350 FieldPro™ with Polarity Reversing welding system, rated at 350 amps at 60% duty cycle, is designed for the field, with exceptional arc performance to help maximize welding quality and improve productivity in field pipe welding.
More robust industrial welding systems also offer strong multiprocess welding capabilities, which are often critical in pipe welding applications that might require a stick or TIG root pass before switching to a wire process for the hot, fill or cap passes. Having these capabilities in one system helps reduce changeover time and costs and the hassles of using multiple pieces of equipment.
Pipe welding tips
Preparing the material properly and choosing the correct shielding gas and equipment are important factors in achieving the best results in pipe welding. Following these best practices when welding pipe in the shop or in the field can help operations avoid some common pitfalls — and optimize productivity and quality.