I never paid attention to what they use but if you search carbidizer in knife forums you should still be able to find threads on guys making their own. I remember seeing quite a few of them.
They were popular amongst knife makers for carbizing the lock bar face on titanium lock bars as well as carbidizing the edge for titanium blades. Titanium alloys don't make for great knife blades due to very poor wear resistance & is why they carbidize them to help but some people like to make them. These days many knife makers have switched to steel insert lock bar faces so not sure if many knife makers still use carbidizers anymore.