Shielding Gas Selection For Stainless Steel Attachment |
Posted: August 6, 2018 |
The implementation of stainless steel, be it in the form of a stainless threaded steel bar or an anchor bolt made from it, has been growing especially in North America, over the past two decades. The number of consumables required to attach various steel objects have also been increasing gradually. As stainless steel applications are a bit different from the ones related to carbon steel, their welding requirements are also different. There are two major alloy types of stainless steel which are -
Both of them have unique properties and welding requirements.
Shielding GasThe shielding gas used Flux-cored arc welding (FCAW), Gas Metal Arc Welding ( GMAW) or Gas Tungsten Arc Welding (GTAW) can have a significant impact on the welding properties, quality and productivity of the welding operation. Some of the main functions of a shielding gas are -
For making the right choice for selecting the shield gas, it needs to be understood whether the results in welding will be achieved to the desired level or not. Chemical Composition of Shielding GasIn addition to a specific welding chemistry and soundness at accepted levels of productivity, weld color and shape, which are also known as bead appearance, are important metrics in defining attachment of a threaded rebar or austenitic alloys in general. The gas blends which are used to join steel alloys contain Argon due to its inert nature, its ability to start easy arc starting and its spray type metal transfer method. In some welding situations, Helium may be added to conduct more heat to the base metal to increase weld penetration and improve the weld puddle fluidity. This can result in less distortion, higher travel speeds and overall savings in weld costing. In FCAW and GMAW, Carbon Dioxide or Oxygen is added in the shielding gas to improve arc stability and weld puddle fluidity. On special case scenarios Hydrogen or Nitrogen are also added in measured amounts for improving bead appearance in austenitic stainless steel. To select a shielding gas, the materials to be joined, the welding process and the filler metal composition needed are deciding factors. FCAW of Austenitic and Ferritic Stainless SteelsFlux-cored wires are developed for implementation with shielding gas compositions. For Austenitic stainless steels, composition is generally 100 % CO2 or 75 % Argon and 25 % CO2. The slag covering of the weld limits carbon absorption. So in such scenarios, these shielding gases with high carbon dioxide content can be used. The chosen gas blend generally relies on the welding position and operation conditions. An Argon/CO2 blend generally provides the widest range of operation and the best operator appeal. With FCAW, Argon- 25 %, CO2 offers good authority for out-of-position welding and a reduced distortion rate compared with 100 percent CO2 shielding.
GMAW of Stainless SteelTwo Part BlendsThis two part blend for commonly used stainless steels conventionally use mixtures of Argon and Carbon Dioxide or Oxygen. They are ideal for conventional or pulsed spray transfer. If extra-low weld metal carbon content is required for maximum corrosion resistance, argon/oxygen (1-2)% can produce a spray like metal transfer.These welds have a tough oxide coating layer that might require post-weld cleaning. Three Part BlendsMaximum three-part blends work well in short-circuiting, spray and pulsed spray transfer. They provide excellent welding characteristics and high productivity. An example would be a rock bolt produced by three-part blend welding. Argon with (25-35)% Helium addition and (1-2)% Carbon Dioxide can provide an increase of 20 % or more in travel speed over two-part blends, enhanced control of distortion in thin materials and superior bead shape and color. This blend is the ideal choice for joining the carbon steel with stainless steel using a pulsed spray transfer. For 300 series material, optimal bead shape and color can be obtained with argon/CO2-hydrogen blends. The reducing atmosphere produced by (1-2) % Hydrogen addition minimizes the oxide on bead surface and helps enhance fluidity and penetration. According to experts, “Mixtures containing Nitrogen should be used with utmost care when joining stainless steel to carbon steel to ensure an appropriate microstructure is obtained.” ConclusionSo coming to the concluding stage that the Shielding Gas selection is very much crucial for joining of stainless steel, for example a threaded bar, made from it. In most Austenitic and Ferritic grades of stainless steel, aron/carbon dioxide blend improves performance and productivity. This holds true especially if traditional and pulsed spray transfer are used. When the appearance of bead is necessary, then three-part blends can help a lot while reducing overall costs. So properly supervised matching of shielding gas to the welding operation is a vital step towards world-class weld quality and productivity.
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