What is difference between post heat and PWHT ?

What is difference between post heat and PWHT ?

Explain with details

1. Post heat we use before welding to reduce the cooling rate and Hydrogen level. Nd pwht we use after welding to reduce the residual Stress in weldment
2. But before welding we use pre heat my brother did pre and post heat same
3. Yes pre and post are same thing
4. How come pre and post same meaning . This is wrong my friend pre means before and post means after
5. Post heat is ....maintaining homogeneous structure and pwht reduced residual stresses...
6. Post heating use for maintain cooling rate of welding and PWHT use for stress refiling
7. Post heating will do for avoid h2 (HIC) crack after welding and temperature maintain below 425 C.
8. Post heating we do for slow cooling rate to avoid cracking in alloy steel and phwt for stress releavig
9.  Post heat and post weld heat treatment we use after welding....difference is that in both heat , 
10. post heat we use after welding finished in between 10 to 20 mint to reduce the hydrogen level to prevent from crack.... And PWHT we use to reduce residual stresses......
11.  Also we use different method for both heat to do....
12. And temperature is also different....in post heat temperature approximately 120 c and in PWHT Approximately 600 c
13. Post heat apply after finish welding before NDT, meanwhile PWHT applied after profile grinding and NDT. No more process applied after PWHT.
14. We have to do post heat immediately after welding to slow down cooling rate to prevent cracking and in alloy steel