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The wear blade is heat treated and very hard. Your weld shrinks and pulls at it, it won't give so the weld pulls loose. 309 will do the same but will take a little longer.
Preheat the area where you intend to weld (no less than 400'). Heat a large enough area to prevent your weld from quenching or have some one standing by to add heat as soon as you finish. Hold the preheat for a few minutes (30 if you can), wrap to cool slowly. Your other option is to do a PWHT of the entire piece. The material will no longer be as strong or as hard but it should be plenty tuff.