Add the following to a .cmd file and then run it:
FOR /D %%d in (*) do (
cd %%d
FOR /D %%w in (welding) do (
cd %%w
forfiles /S /M *.* /C "cmd /c rename @file @fname.wld"
cd ..
)
cd ..
)
Note: Create a test directory first to make sure the script works for your configuration. Do not test it on your live files.