TIG Welding Stainless Steel Basics
In comparison with the welding of mild steel, for example, the austenitic stainless steels have several characteristics that require some revision of welding procedures that are considered standard for mild steel. The melting point of the austenitic grades stainless steels is lower, so less heat is required to produce fusion. 300 Series stainless steels electrical resistance is higher than that of mild steel so less electrical current (lower heat settings) is required for welding. These stainless steels also have a lower coefficient of thermal conductivity, which causes a tendency for heat to concentrate in a small zone adjacent to the weld. The austenitic stainless steels also have coefficients of thermal expansion approximately 50% greater than mild steel, which calls for more attention to the control of warpage and distortion in the heat affected zone.
During the TIG welding of stainless steels, the temperatures of the base metal adjacent to the weld reach levels at which micro-structural transformations occur. The degree to which these changes occur, and their effect on the finished weldment in terms of resistance to corrosion and mechanical properties, depends upon alloy content, thickness, filler metal, joint design, weld method, and welder skill. Regardless of the changes that take place, the primary objective in TIG welding stainless steels is to provide a sound joint with qualities equal to or better than those of the base metal.
To ensure success when TIG welding stainless steel, it is important to have good heat control, gas coverage and travel speeds.</p> Typically, TIG welding stainless steel requires a DC power source and pointed tungsten (any type except pure). Like aluminum or any metal to be welded for that matter, it should be free of oil, paint and/or dirt prior to welding to achieve optimal results. Stainless steel should be wire brushed between welding passes with a dedicated stainless steel wire brush that has not had contact with mild steel to help remove potential inter-pass oxides.
Filler rod is recommended on applications with a base material thicker than gauge 18 and will be contingent upon joint design. For example, outside corners may not require filler rod, but an inside joint will. Most TIG applications require overmatching of the filler rod. That is, a filler rod with higher strength properties should be used. For example, on 304 series austenitic stainless steel, an ER308 rod should be used. Typically, austenitic stainless steel filler rods are available in diameters ranging from .035 to 5/32 (.9-to 4.0-mm) and chosen according to the joint design, welding parameters and applications.
TIG Welding Stainless Steel : Filler Rod Selection
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