James,
The maths is simple but probably not very useful for your application.
Tensile strength is measured in Pascals and a Pascal is a Newton per metre squared. If a rod has tensile strength T [N/m^2] and diameter d [m] then it will support a weight which is a product of its tensile strength and area, i.e. W = T * pi * d^2 / 4 [N]. The material will permanently deform long before it breaks.
Taking your example of T = 600MPa and d = 0.004m then W is around 7500 N which corresponds to a mass of 750 kg at the Earth’s surface.