In order to effectively TIG weld stainless steel, it important to consider carbide precipitation.
TIG Weld Stainless Steel : Carbide Precipitation
What is Carbide Precipitation? Carbide precipitation occurs when the chrome and carbon in 300 series stainless steel is drawn out of the material and reacts to the atmosphere. It occurs around between 800 to 1650 degrees Fahrenheit (426 to 899 degrees Celsius), so care should be taken to keep the weld zone below that range or in an inert atmosphere (via argon shielding gas).
A characteristic of an annealed austenitic stainless steels such as 304, is its susceptibility to an important micro-structural change if it is exposed to temperatures within an approximate range of 800-1650F. Within this range, chromium and carbon form chromium carbides, and these precipitate out of the solid solution at the boundaries between the grains. The rapidity of carbide development depends on a number of factors. The actual metal temperature between the range of 800-1650F is one factor. Chromium carbides form most rapidly at about 1200F, and the formation falls off to nil at the upper and lower limits. Another factor is the amount of carbon originally present in the material, the higher the carbon content the more pronounced the action. Time at temperature is a third factor.
The effect of carbide precipitation on corrosion resistance is to reduce the chromium available to provide corrosion resistance. Because low-carbon content reduces the extent to which carbide precipitation occurs, the low-carbon austenitic grades may be preferred for weldments to be used in highly corrosive service. 304 with a maximum carbon content of 0.08% is widely used. Also available are low-carbon 304L, 316L, and 317L with 0.03% carbon.
Heat and Travel Speed
Generally speaking, there are three causes of carbide precipitation: heat, travel speed and gas. Specifically, too hot of a TIG weld, too slow of travel speed and/or inadequate shielding gas coverage can individually, or in combination, cause the problem.
Some methods of preventing carbide precipitation when TIG Welding Stainless Steel:
- Stainless steel requires 1/3rd less amps for every thousandths of an inch of material thickness compared to mild steel.
- Maintaining an appropriate travel speed helps prevent an excess amount of heat from entering the TIG weld.
- Choose the correct tungsten and filler rod diameter.
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