Type 416 Martensitic stainless steel is a free-machining grade that is weldable providing the process, or filler metal, does not supply hydrogen that can react with the sulfur or selenium in the base metal to produce porosity. Due to the amount of sulfur that enters the weld due to dilution, the filler metal selection should provide weld metal that can tolerate these elements without hot cracking. An E312 series stainless steel is one of the better choices. A nickel-alloy filler metal cannot be used due to the formation of low melting nickel eutectics, which cause hot cracking. Personally, I would recommend the 312 series over the 420 or the 309.

Chuck