Control of Carbide Precipitation
The amount of carbide precipitation is reduced by promoting rapid cooling. Fortunately, the copper chill bars, skip welding and other techniques needed to control distortion in sheet metal help reduce carbide precipitation. Annealing the weldment at 1900°F (1038°C) or higher, followed by water quench, eliminates carbide precipitation, but this is an expensive and often impractical procedure. Therefore, when weldments operate in severe corrosive applications or within the sensitizing temperature range, either ELC or stablilized grades are needed.
Another remedy is to use stabilized stainless steel base metal and filler materials which contain elements that will react with carbon, leaving all the chromium in solution to provide corrosion resistance. Type 321 contains titanium and Type 347 contains niobium (columbium) and tantalum, all of which are stronger carbide formers than chromium.
The degree of carbide precipitation increases with:
1. Higher carbon content (for example, because 301 and 302 grades have a maximum carbon content of 0.15% they are more susceptible to carbon precipitation than grade 304 which has a maximum carbon content of only 0.08%).
2. Time at the critical mid-range temperatures – a few seconds at 1200°F (649°C) can do more damage than several minutes at 850°F (454°C) or 1450°F (788°C). Welding naturally produces a temperature gradient in the steel. It ranges from melting temperature at the weld to room temperature some distance from the weld. A narrow zone on each side of the weld
remains in the sensitizing temperature range for sufficient time for precipitation to occur. If used in severely corrosive conditions, lines of damaging corrosion appear alongside each weld.
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