vgirolamo,
I believe your problem can be solved by applying a well known analytical solution for transient heat conduction in a semi-infinite solid.
If the metal is initially at a uniform temperature and the surface temperature is suddenly changed, then the analytical solution will provide the temperature at a given depth as a function of time.
Alternately, if your piece of metal is small, or thin, and is heated uniformly on all surfaces (e.g., a ball bearing suddenly immersed in a heating or cooling fluid), then an analytical solution is available by using a well known transient lumped parameter approach. This method assumes that the entire metal mass is heating up uniformly.
I can help you with the calculation and provide the equations if you can provie a few more details:
1. metal density, specific heat, and thermal conductivity;
2. method of heating
3. metal geometry (e.g., flat plate with finite thickness such as steel beams; spherical such as ball bearings, cylindrical such as rebar).