Dont expect to get a lot of replys other than questions.

Most folks will tell you this not a good practice and don't want to give advice that could destroy or injure.

More information is needed like size, load, speed and I for one do not recognize the material of the gear or the shaft.

And most of all why must you weld them? Welding will most likely weaken the shaft not to mention what problems the distortion from the heat will cause.

8620 is a pretty common gear material. Various amounts of CrNIMo which allows for it to be carborized. Never heard of the other material. Welding the 8620 might be difficult due to the high amount of surface carbon it contains. When you weld it you will more than likely destroy the carborized case which is put there during the carborization process.

What are you trying to do? What is the application? I have welded several gears to shafts for various projects around the farm but none of them have been safty critical. Really depends on what you are trying to do.

I think you might be referring to "EDT" It's a "stress proof" shaft material.
If so the only success I had welding it was with 312 stainless. This also works for joining CrMo cast materials to 304L. We preheat to 250F, 500F interpass temp, no post heat. Works for us, some pieces have been in service more than twenty years.

maybe i'm wrong but i think this guy has an idea of some sort & to build his experiment he has to weld a gear to a shaft (maybe a second time as the first time the weld cracked or something). if i had the information i'd post it, i don't but i'm sure someone reading this does.