There is not a magic number for the requirements of surface temperatures. You basically have to provide protection under the general duty clause because you simply can't state an absolute temperature for every individual situation. There is not simply a way of stating that 120 F as the limit to where the human body can accept the transfer of heat, people in AZ would be in real trouble. However, general wisdom would tell someone that a strip of steel at 1000 F is a bit too hot to touch without proper protection.
You will need to look at your particular situation and determine what the risks are to your employees. In my professional opinion, anything over 150 F should not be touched by unprotected human hands. Anything over that, and I would provide first, engineering controls to restrict the access of a person from contact with the surface. Second, if access cannot be restricted, limit the access to as little as possible. Lastly, provide PPE(hot mill gloves, arm guards, face shields, etc.) to the employees who have no other option but contact with the material.