I want to grow crystal at 40 degree C. If I want to use the evaporative crystallisation, and since the solvent I will be using is water, is it alright if I heat the solution up to 100 degree C to make the solution evaporate to form crystals?

I am not an accomplished crystal grower but my experience is this. If you don't care what size the crystals are then heating the solution up to 100 C will get things started faster. That will lead to a lot of small crystals. If you are trying to grow "single crystals" then slow is the word and I don't think heating the solution is the thing to do.