Stainless steel’s corrosion resistance makes it a popular choice for important pipe applications. However, improper welding can decrease the pipes’ corrosion resistance. To ensure the metal retains its corrosion resistance, follow these five tips for welding stainless steel pipes.
Tip 1: Choose a low carbon filler metal
When welding stainless steel, it is important to select a filler metal with low trace elements, which are residual elements from the raw materials used to make filler metals, such as antimony, arsenic, phosphorus and sulfur. These elements can significantly impact the material’s corrosion resistance.
Tip 2: Pay attention to solder preparation and proper assembly
Proper joint preparation and assembly are crucial in controlling heat input and maintaining material properties when working with stainless steel. Uneven fit and gaps between parts can cause the torch to remain in one position for longer, requiring more filler metal to fill the gaps. This buildup of heat can cause overheating in the affected area, compromising the integrity of the part. Additionally, poor fit can make it challenging to achieve the necessary weld penetration and close gaps. Ensure that the fit of the stainless steel parts is as close to perfect as possible.
Additionally, cleanliness is crucial when working with this material. Even the slightest amount of contamination or dirt in a weld can cause defects that reduce the strength and corrosion resistance of the final product. To clean the substrate before welding, use a brush specifically designed for stainless steel and not used on carbon steel or aluminum.
Tip 3: Control Sensitization Through Temperature and Filler Metal
To prevent sensitization, it is crucial to carefully select the filler metal and control the heat input. When welding stainless steel, it is recommended to use a low carbon filler metal. However, in some cases, carbon may be necessary to provide strength for specific applications. It is essential to control the heat input, particularly when low-carbon filler metals are not available.
Tip 4: Understand how shielding gas affects corrosion resistance
Gas tungsten arc welding (GTAW) is the traditional method for welding stainless steel pipes, which typically involves a back purge with argon to prevent oxidation on the back side of the weld. However, wire welding processes are becoming increasingly popular for stainless steel pipes. It is important to understand how different shielding gases can affect the material’s corrosion resistance.
When welding stainless steel using the gas metal arc welding (GMAW) process, a mixture of argon and carbon dioxide, argon and oxygen, or a three-gas mixture (helium, argon and carbon dioxide) is traditionally used. These mixtures primarily contain argon or helium and less than 5% carbon dioxide. This is because carbon dioxide can contribute carbon to the weld pool and increase the risk of sensitization. It is not recommended to use pure argon for GMAW on stainless steel.
Flux-cored wire for stainless steel is designed to be used with a conventional mixture of 75% argon and 25% carbon dioxide. The flux includes ingredients that prevent carbon contamination from the shielding gas during welding.
Tip 5: Consider different processes and waveforms
As Gas Metal Arc Welding (GMAW) processes have developed, they have made welding stainless steel tubing and pipe simpler. Although the Gas Tungsten Arc Welding (GTAW) process may still be necessary for some applications, advanced wire processes can offer comparable quality and greater productivity in many stainless steel applications.
Welds on the inside diameter (ID) of stainless steel made with GMAW Regulated Metal Deposition (RMD) are of similar quality and appearance to the corresponding welds on the outside diameter (OD).
Miller’s Regulated Metal Deposition (RMD) is a modified short-circuit GMAW process that can eliminate the need for back purge in certain austenitic stainless steel applications. This can save time and money compared to using GTAW with backpurge, especially on larger pipes. The RMD root pass can be followed by pulsed GMAW or flux-cored arc welding filler and cap passes.
The RMD process uses precisely controlled short-circuit metal transfer to produce a calm, stable arc and weld pool. This technique reduces the likelihood of cold laps or lack of fusion, minimizes spatter, and enhances the quality of the pipe root pass. Precisely controlled metal transfer also ensures consistent droplet deposition and facilitates control of the weld pool, resulting in better management of heat input and welding speed.
Unconventional processes have the potential to increase welding productivity, with welding speeds of 6 to 12 in/min achievable using the RMD. The pulsed GMAW process helps maintain the performance and corrosion resistance of stainless steel by increasing productivity without applying additional heat to the part. Additionally, the reduced heat input of the process helps control deformation of the substrate.
This process offers shorter arc lengths, narrower arc cones, and less heat input than conventional jet pulse delivery. Furthermore, the closed-loop nature of the process virtually eliminates arc drift and tip-to-workpiece distance variations. This technique simplifies weld pool control for both in-situ and out-of-situ welding. Combining pulsed GMAW for filler and cap passes with RMD for the root pass enables the welding process to be completed using a single wire and gas, eliminating the need for process changeover time.
Post time: Jan-26-2024