Hi Brian, I think someone screwed the pooch in specifying high phosphorous steel for a welded part.
i used to see things like this a lot in work for machine shops where they would free machining steels for welded parts.
great for ease of machining but not so good for welding.
I would imagine high P steel is way cheaper than other gear steels like 8620 but even if you able to baby it and get it welded with no visible cracks, you have to wonder about underbead cracks and such.
I think another alloy would be the way to go.