PetroStreet

whats factors we will consider in making a decision whether to use SS or CS......?

There are many factors but mainly the below ones are considered:
1. Corrosive service
2. Low temperature service (exemption from impact test) and associated risk of brittle fracture
3. High temperature service (before going to SS, designer may consider many other materials like low alloy steels)
4. Life span

Agreed with arcpro. just add a couple of dimensions for material selection:
5. Required Mechanical Properties (strength, hardness, weldability, etc)
6. Material Cost

I am also uploading a very useful paper on material selection criteria. Hope it would be of help and future reference.

Regards - Qaisar

In material A-106 and B-425 what does "A" and "B" stands for, also what "106" and "425" stands for?

Regards

A as per ASTM standards stands for ferrous metals and B for non-ferrous.
106 or 425 or any number is the assigned standard to a certain material / product by ASTM Committee.

I have seen a numbering system in UNS and ASTM E527 is a standard practice for the same.
I do not know whether this '106', '425' stand for anything meaningful and related to something like material classification etc. Probably not.