B31.3 will not determine anything specific to H2S service other than a general addressing of corrosive services that can be physically harmful. But these are not metallurgical or mechanical requirements due to the specifics of a particular medium such as H2S. This is essentially left up to NACE, or project specifications. B31.3 determines requirements based more upon temperatures, pressures, and cylicity.

For a 7 mm thickness, B31.3 does not prescribe any PWHT.
Now, as js55 has pointed out, B31.3 requirements are based on metallurgical and mechanical strength considerations, not corrosion.
It's a well known fact that a stressed metal is less resistant to corrosion, so it may be possible that, although not necessary from a mechanical or metallurgical point of view, the PWHT is necessary to improve the corrosion resistance of the pipe material.
In this case, the project specifications should state whether or not PWHT is necessary, and they (the specifications) must be followed.
If H2S is a problem, ASTM A-106 Gr. B is certainly NOT the suitable material for the pipe. A-106 is plain carbon steel, whose corrosion resistance is very low.
Back in my days of erector engineer, I took part in the construction of an oil refinery in Argentina, where H2S was also a problem. All crude oil piping was made of A-335 Gr P5 (5% chromium steel), to withstand H2S corrosion.
Now, it's also possible that the piping designer has adopted a suitable corrosion allowance (i.e., an extra pipe thickness) to take up the corrosion and leave untouched the base metal, i.e., the one that must withstand internal pressure. This you should check with the project engineer, although the corrosion allowance is usually stated on the project specifications.
Giovanni S. Crisi