hardness vs. yield strength
Unfortunately, if you want to determine tensile strength based on hardness the best you can do is make an estimate. Hardness and tensile strength are indicators of a metal's resistance to plastic deformation so they can be related by an equation. ISO 10808 will give you conversion equations for various common steels (I know ASTM has a standard relating the two but I can't think of its number). One must understand though that these conversions are approximations and the multiplication factors can and will vary considerably.
A very general rule of thumb for most steels, the HB and the tensile strength are related by (note: this is a very rough estimate):
tensile strength (MPA) = 3.45 * HB
This will give a rough estimate of the tensile strength of the material. If you need to know this information for a critical application, I would highly recommend that you perform the tensile testing.