Thermal stress in a multilayer object

Hi.
Im trying to simulate the residual stress in a multimaterial sandwich structure and I have done it with the thermal stress module and with a time-dependant study.
I put all my doamins at an initial temperature of 100 °c, then I leave cool down by convection at 0°C. Every ten second I have the result for the Von Mises stress within my structure, in order to understand whether this stress reach the strenght of materials themselves.
Instead of finding that the stress increase as the temperature lowers (due to different thermal contraction), it results that the stress decreases in all layers.

1)Why?

2) What for, in case of a stationary study with steady temperature (T_initial=T_final), the stress is not zero?

Thanks to them can help me.