November 10, 2023
Welding is the process by which two pieces of metal can be joined together. The welding process involves more than just bonding two components, as in brazing and soldering. Instead, by using extreme heat and sometimes adding other metals or gases, it causes the metal structures of the two components to merge and become one. The welding process involves applying heat and pressure to the materials being joined, in addition to the filler material, to form a molten pool that cools and creates a strong bond.
Common Stainless Steel Welding Methods
Though the welding process itself has undergone significant changes with the advent of new technologies, it's important to understand the differences between all existing welding techniques to make informed decisions when selecting the right process. There are various methods and processing techniques for welding stainless steel, but in the routine production processes of stainless steel fabrication plants, the most common welding methods include:
Gas Tungsten Arc Welding (GTAW or TIG)
This is the most widely used method due to its versatility, high quality, and the aesthetic appearance of the finished weld. It's capable of welding at low currents, resulting in low heat input. Its ability to add filler wire when needed makes it an ideal choice for root runs on thin sheets, thick plates, and pipe single-side welding. The process is easily mechanized, and its capability for autogenous welding, whether or not filler wire is added, makes it suitable for pipeline and orbital welding processes. Pure argon gas is the most commonly used shielding gas, but argon-rich mixtures with added hydrogen, helium, or nitrogen can also be used for specific purposes. Inert backing gases protect the weld from oxidation and ensure corrosion resistance in single-side welding.
Plasma Arc Welding (PAW)
A derivative of the TIG process, PAW involves a nozzle system designed to produce a narrow, concentrated transfer plasma arc with deep penetration characteristics. It's primarily used in mechanized systems, requiring high-speed, high-production automated welding for square butt joints up to 8mm thick. For thicker square edge butt joints, the combination of PAW/TIG and filler wire becomes essential to ensure complete weld surface. For thicknesses greater than 10mm, a partial V-preparation of the PAW root run is used, followed by multi-pass joint filling. Argon gas backing is necessary to protect the underside of the weld for corrosion resistance.
Shielded Metal Arc Welding (SMAW or MMA)
MMA electrodes are manually operated and represent one of the oldest arc processes due to their adaptability to various materials. The electrode coating types are designed to offer performance characteristics, making them suitable for diverse welding applications. The most commonly used acid rutile-coated electrodes produce a spray arc metal transfer, self-releasing slag, and aesthetically pleasing weld profiles with fine ripples. Minimal post-weld dressing is necessary. They are primarily used for root runs and positional welding for fillet and butt joints. Electrodes with this coating type can be used in appropriate positions but are limited in application and size, typically up to a maximum of 3.2mm.
Basic-coated electrodes produce weld metal with higher integrity, fewer slag inclusions, and porosity, which is useful for fixed pipe weldments. However, their slag removal and weld appearance are not as aesthetically appealing as the acidic rutile types. Special-coated electrodes are produced for specific applications, such as vertical down and high recovery manual welding. Electrode sizes range from 2.5 to 5.0mm (types 308L, 347, and 316L are also available in 1.6 and 2mm diameters).
Gas Metal Arc Welding (GMAW or MIG/MAG)
This semi-automatic welding process can be used manually or automatically and involves continuous feeding of consumable solid wire electrodes and a shielding gas rich in argon. It's utilized for its high productivity features when welding thin materials using the "short-circuit" metal transfer mode or thicker materials with "spray arc" transfer. Power sources producing pulse current have been developed to enhance welding metal quality for positional welding and a clean weld appearance. Gas mixtures containing oxygen, helium, carbon dioxide, etc., have been developed to improve arc stability and weld pool "wetting" characteristics.
Flux-Cored Arc Welding (FCAW or FCW)
A form of MIG/MAG process where solid wire consumables are replaced by tubular wires filled with flux (FCW) or metal powders (MCW), compatible with the same type of equipment. It produces two types of wires, one for all-position capability and another for higher deposition down-hand welding applications. Compared to MMA or MIG/MAG processes, there is a higher possibility of weld deposition and weld metal coverage. This method significantly reduces post-weld cleaning and dressing.
Submerged Arc Welding (SAW)
A fully mechanized process using wire and flux powder, shielded arc, with high deposition rates, rapid travel speed, and welding quality. Applications include continuous down-hand fillet and butt welds in thicker section plates, pipes, and vessels, as well as stainless steel cladding of carbon steel components, particularly for long seam or long run cases. Strip electrode electro-slag processes can also be used for cladding, offering some advantages over SAW.
Electric Resistance Welding (ERW)
Resistance spot and seam welding are generally limited to mass production welding of thinner materials, where welded joint types and resulting gaps do not compromise the expected corrosion resistance during use.
The energy concentration achieved at the focal point of the laser beam is extremely strong and capable of producing deep penetration welds in thick stainless steel with minimal component distortion. This process involves high capital cost equipment and is reserved for large-scale manufacturing production.
Welding joints must be designed to withstand the forces they'll experience during their lifespan. This means the joint's design depends on the type and magnitude of the expected loads acting on the weld. Some types of welding joints are designed to withstand extreme shear loads, while others are intended for extreme torsional forces. Joint types used for welding include butt, lap, corner, T, and edge joints.
All welders need to respect the work and equipment they use. Here's a list of safety equipment and precautions:
- Wear welding gloves.
- Use protective goggles - darker lenses are needed for arc welding than for gas welding.
- Have appropriate ABC-rated fire extinguishers nearby when welding.
- Maintain a clean work area free of flammable materials and obstacles.
- Install smoke extraction devices to ensure they don't harm physical or mental health.