henry, curios to butter both members. I guess I would lean to the buttering of CS member. It also hard to think this would need PWHT, but code is code

vignesh5585,

First, I am remiss,  WELCOME TO THE AWS WELDING FORUM!!

That information helps considerably. 

If you are working to ASME then many factors come into play that should be carefully considered.  Code application, Project application, Job Specifications/Contract Documents with General Notes, and much more.  Information that we don't have access to. 

While we could offer some general guidelines, as Henry already did, we are not the ones with the liability.  You must consider all the possibilities in house and come to a procedure that your company, and ultimately the customer, are comfortable with. 

I'm unsure of the tenure of the question, are you the customer, contractor, a welder with questions, etc?  But a forum like this is not where you want to get advice with liablilty consequences.  Many on here will tell you that the advice just given by Al and myself is a bunch of unnecessary legal mumbo jumbo and just do it such and such a way.  It has worked for them for 20 years and ....ooooppppssss... Who built the fertilizer plant in Texas????  Who built the pipelines that have exploded recently??  Who put up the Biggee crane that came down??  Who built .... Well, hopefully you get the idea.  You can not be too safe and we are here for the protection of all parties involved and especially the Public Safety. 

Good luck.

Have a Great Day,  Brent

Hi Vignesh

Looks like I read your post incorrectly in terms of the pipe schedules. - OK

The SS 14" Sch 120 is 27.79mm thick, so after tapering the thicker CS pipe down to that thickness, then according to ASME B31.3, you will need to perform PWHT on the CS. No thickness of 316 SS will require PWHT. Strictly speaking, the weld will need PWHT according to the code. See 331.2.3(b):

(b) Heat treatment of welded joints including both
ferritic and austenitic components and filler metals shall
be as required for the ferritic material or materials unless
otherwise specified in the engineering design.

You will notice that it gives you an "escape clause": "unless otherwise specified in the engineering design"

To clarify how this clause could be implemented, see 331.2.2:

331.2.2 Exceptions to Basic Requirements. As indi-
cated in para. 331, the basic practices therein may require
modification to suit service conditions in some cases.
In such cases, the designer may specify more stringent
requirements in the engineering design, including heat
treatment and hardness limitations for lesser thickness,
or may specify less stringent heat treatment and hard-
ness requirements, including none.
When provisions less stringent than those in para.
331 are specified, the designer must demonstrate to the
owner’s satisfaction the adequacy of those provisions
by comparable service experience, considering service
temperature and its effects, frequency and intensity of
thermal cycling, flexibility stress levels, probability of
brittle failure, and other pertinent factors. In addition,
appropriate tests shall be conducted, including WPS
qualification tests.
-------------------------------------
This means that you can dispense with the PWHT requirement, as long as the appropriate engineering has been done to confirm that the resultant weld will be able to perform the specified service required of it.

Other options are:

1) Simply perform the PWHT. Most modern "L" grade austenitic stainless steels will not sensitise during the PWHT process, so would be OK, as long as you do not apply too long a heating cycle. This would obviously have to be proven during the welding procedure qualification testing. Also note that due to the large difference in thermal expansions between the CS and SS, there will still be rather high residual stresses in the welded joint following the PWHT, but certainly any hardened zones in the CS should be tempered. Because of the resultant high residual stresses, there may in fact not be much benefit to the PWHT, so it is often better to take other measures, and show through "engineering" that PWHT is not needed, or desirable. (See discussion above.)
2) Use a buttering technique. Here you would typically use an alloy that does not need PWHT, but can withstand it, to butter the CS weld surface. (Typically Ni based alloys of the "Inconel" type.) After buttering the weld surface of the CS, perform the required PWHT on the buttered end of the CS pipe. Then using the same Inconel filler, weld the SS pipe to the buttered CS pipe. No PWHT need be applied to the final joint.

As noted by some of the other people in this post, these types of situations need to be evaluated on an individual basis, to make sure that the resultant weld will perform the service required. This will need a detailed knowledge of the service / design conditions of the pipe.

Hope this helps.

Regards
Niekie