Been meaning to post this for quite some time, but never got around to it.

First noticed the phenomenon maybe 10 years ago when stainless steel valves started dropping in price.

As long as the system is under pressure, everything is fine.
during depressurizing and pulling vacuum, everything is fine.
When charged with ammonia, the valves start leaking between the housing and insert.

After the first few times, I made it a habit to check/tighten the bolts on all stainless valves while the system was under full vacuum, and it's not uncommon to be able to tighten the bolts 2-3 full turns.

Now stainless steel has a lower tensile strength than carbon steel, and I am guessing that it is a combination of the valves being alternated between warm and cold, stretching the bolts, and maybe the gasket getting squashed while pulling vacuum.

Now, knowing that stainless steel bolts doesn't have the same tensile strength as carbon steel, I'm wondering if we will be looking at valve inserts popping out after 10-20 years when the bolts have been tightened again and again

Anyone else observed this?