Delta Ferrite Promotion
Effect of specific alloying elements such as chromium, molybdenum, or silicon to promote solidification as high-temperature ferrite in order to prevent hot cracking in the weld metal.
Delta ferrite forms during solidification of weld metal and is basically a high-temperature phase that normally transforms during cooling. If certain alloying elements like chromium or molybdenum are in the steel, some of it can remain. This is often desired when welding stainless steels, because a certain amount of delta ferrite in the structure prevents hot cracking during solidification. You have to imagine it like this: without this ferrite content, the weld metal can crack during cooling. The exact amount depends on the composition of the base material and filler material. In practice, this means for you: make sure the filler matches the base material, then the ferrite content is usually right. If you have strongly ferrite-promoting elements in the base material, you usually don't have to worry about the weld metal cracking.