The difficulty with weld overlay on a duplex is that you will tend to form lots of detrimental secondary phases. So when you do eventually get failure of the overlay the metal exposed will be much more susceptible to corrosion than the bulk alloy.
Some of the low temp surface treatments that caburize, nitride, or nitrocarburize below 600F can be used on these alloys. The treatment times are long (20-50 hrs) and the layers are thin. But they are very corrosion resistant and they are very hard.
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