Will using window in Welch's method introduce artificial peak in estimated PSD?
It is not any artifact. When you calculate the DFT of your signal you will get following magnitude spectra:


So you can clearly see that around $25 \mathtt{Hz}$, and $50 \mathtt{Hz}$ there is some harmonic content present. 

Reason why you don't see your harmonics on PSD in first case is that you are not using any windowing. Probably you are aware of side-lobes attenuation in characteristic of the window functions. For rectangular window it is approximately $13.3 \mathtt{dB} $ and for Hanning you get $31.5 \mathtt{dB} $. It is therefore possible that sidelobes will cover some harmonics of your signal. 

Please take a look on your figures; for rectangular window lowest value of noise is approx. $-14$, whereas for Hanning you are getting around $-18$. 

Below you can see frequency domain representation of these two windows.



For more info about windows, please refer to:


  F. J. Harris - On the Use of Windows for Harmonic Analysis with the DFT