I'm building a model to derive the temperature variation of the interanl surface of a bullet-like metal tip. The metal tip is immersed in water bath of constant temperature, say 40 degree Celsius; & the initial temperature of the internal surface is, say 20 degree Celsius.

I suppose to run with Heat transfer transient analysis. For the internal surface, should I set up the BC for internal surface in 2 step with the temperature of 1st step set at 20 degree, & then deactivate the BC in 2nd step. However, what is the time duration for the 1st step?