We are having issues with a shaft for a new heavy service industrial application.

The shaft has an arm attached on the end. there is overhang to accommodate a grease seal. The arm has a device on the end with some significant weight, and through normal use, in an environment of intense vibration, it imparts some torsional force, but most of the force is parallel to the axis of the shaft. Basically it is like putting a vibrator motor out on the end of the arm, and then smacking it every so often with a carnival mallet. Another issue is we have a very limited area for the shaft bearings, so the shaft has been sized as large as we can go. This is also a harsh environment with lots of minerals and salt to contend with. So we are chrome plating.

Here is the problem. We selected 4140 hardened and tempered to 48 Rockwell, but the shafts are breaking, so we dialed back the hardness to 32-33 Rockwell and we are getting bending. I am not a metallurgist so I am wondering about some different steels and hardness that might work better for this project.