Tungsten Toque Bars are required to open certain doors hidden throughout the game. You can craft Tungsten Torque Bars at any work bench for 20 Tungsten, but because you consume the bar every time you use one, there's an inherent gamble for every door your decide to open.
On this page I will list the locations of all the Torque Bar Doors, and what rewards you get for opening each door.
During the CMS Greely optional mission, you will run into your first Torque Bar door. It's worth noting that you also get the 20 required Tungsten to craft your first Torque Bar during this mission.
During the Conning Tower optional mission
In the Aft Station