The material you reference is classified P4 in ASME Section IX. I would assume that the appropriate construction standard is ASME B31.1.

Table 132 (in B31.1) requires PWHT unless one of the notes provides an exception. The exception that applies to your situation is:
(1) Welds in pipe or attachment welds to pipe complying with the following: nominal material thickness of 1/2" or less, specified carbon content of 0.15% or less and preheat of at least 250F.

Nominal material thickness is defined in 132.4.3: the thicker of the materials being joined at the weld.

132.2.b states the following: When parts of two pifferent P numbers are joined by welding, PWHT shall be that specified for the material requiring the higher PWHT temperature. WHen a nonpressure part is welded to a pressure part (your situation) and PWHT is required for either part, the maximum PWHT temperature shall not exceed the maximum acceptable for the pressure retaining part.

Charles Hall

Your supports are already welded to the pipe and the situation is now irreversible.
But, as a matter of fact, welding supports to the piping is NOT a good engineering practice. Good engineering practice recommends that no structural members be welded to the pipes. So, the supports should be bolted or clamped instead of welded.
You can look at the piping supports typical arrangements prepared by renowned engineering firms or to the catalogs published by reputable piping supports manufacturers and you'll see that none of them is welded; they're all bolted, clamped or otherwise fixed with no weldments.
But as now you've already run into that situation, I agree with chall: Yes, you must stress relieve those welds according to the applicable Code.
Giovanni S. Crisi
São Paulo - Brazil

Hello A.H.Faraji!

Is there any particular reason why the support was to be welded on to the pipe? The reason I ask this question is because, as Giavonni stated, It's a bad engineering practice to do so!!!

The only advice I have for you is to PWHT that support, and if possible, DO NOT repeat the same engineering practice on any of the other supports!!! G. S. Crisi has spoken the word on pipe supports so, It's best for you to adhere to his advice!!!
I apologize for being so blunt but, I strongly feel that it's necessary in this case!!!

Respectfully,

SSBN727 Run Silent... Run Deep!!!