Sintering of 17-4PH

Hey there!
Maybe one of you has an explanation for the following observation:
when sintering a MIM 17-4PH sample - particle size and green density
remaining the same - in vacuum compared to hydrogen, I observed quite an increase in the starting temperature of sintering (about 80C difference). Why does the vacuum retard the start of sintering? 2nd observation was that a much lower difference in sintering temperature (only about 5-10C) occurred when repeating the experiment with 316L. Any ideas?
Thanks in advance Flipp
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