I think many experts and valve practitioners here are trying to understand your question, whilst I got the impression that you only want to hear the answer and not really understand it.
So, no offense, but there is a gap of communication there.
Obturator asking for valve application meaning is this for water, for sour service application, slurry, abrasive application or else? what is the temperature and pressure?
Reason for the inquiry is what kind of corrosion/erosion mechanism due to the medium which is potential threat for valve trim. And whether hard facing are indeed required.
There are thousands pages of material study which discuss over hard facing (ENP, tungsten carbide included), of which we cannot summarize it in this forum. You have to do this research by yourself.
Hardness of material is a trade off with regards to its application.
General material such as carbon and low alloy steel alone have relatively low hardness, hence more susceptible for erosion and/or corrosion. Applying only carbon/low alloy on trim (wedge and/or seat) > eventually will be corroded/eroded>later stage than you will have excessive leak rate.
However the harder the material, it not necessarily good. Above 22 HRC material might be prone sulfide corrosion cracking and hydrogen stress cracking. Therefore NACE MR0175 suggest to have maximum hardness of 22 HRC.
I am not metallurgist expert, suggest to read ENP, Tungsten carbide and Stellite datasheet. Below link is old thread in this forum.
It is a common practice to have Seat treated as sacrificial part due to it is relatively easier to be replaced/repaired than wedge/ball/plug. Hence for seat is either without hard facing or if indeed hardfaced then usually it has lower hardness in comparison with its wedge (for example if wedge with Stellite 21, then seat is with Stellite 6).
Both parts (seat and wedge) to be required for hardfacing is usually for confirmed abrasive (not just corrosive) medium.
In your case, from different posting, you mentioned this is for water. Thus not susceptible for abrasion or sour service corrosion.
Sour service hardness requirement is 22 HRC
Alumunium bronze (your valve) hardness is 27 HRC
ENP hardness 68-72 HRC, but what for since there is minimum to no risk associated with abrasion/corrosion
Stellite 6 hardness is 35-40 HRC. Again what for.
For water only (neutral or even brackish), Alumunium bronze alone is sufficient. No need for hardfacing.
PS: in 2010 I did repair valve for water treatment plant in Indonesia , valve was dated 1920 (Dutch made) with Alumunium Bronze wedge and Seat. No corrosion found, and only small wear due to 90 years in service. Solid material selection (and good workmanship when valve was made).
