Maximum / Minimum Hardness For Ring Gaskets

I do not think 316 ss material may come to have 135 Brinell hardness. the lowest I've seen have been 153 Brinell.
And yes, if you found 316 ss metallic gasket with a hardness of 135 Brinell you can use it according to ASME 16.20 . However, I think there is a mistake somewhere. Error in the certificate, or the calibration of your hardness machine, or to the measuring method. (Are you using EQUOTIP?).

It is recommended that ring-joint gaskets be of a lesser

hardness than that of the mating flanges.

2.3.2 Hardness. Ring-joint gaskets of materials

listed in Table I shall have a hardness equal to or

less than that shown in Table I .

WP

It depends upon the relationship between the flange hardness and the gasket hardness:

• If the gasket were harder than the flange, the flange would distort into the gasket when the bolts are flogged-up and not the other way round.
• If that is unacceptable then use a softer gasket instead.

Only the <...we...> can determine the correct approach from the standards and guidelines to hand.