As a normal practice metallography spot measurements are being taken after polishing and etchning at the surface.If any grain boundary cracks noticed and classified as IGSCC/TGSCC ,is it possible to measure the whether it has penetrated to full thickness or only superficial?Normally in furnace tubes in ASS service polythionic SCC is common phenomenon due to exposure of tubes in fsulphur enviornment . Is it mandatory to take another spot measurement inside the tubes before making any judgment to discard

Gopal In-situ replica metallography ( done mostly from outside) will give you indication of the depth of cracking (IGSCC or TGSCC) to a great extent. Simple D.P. test will give corroborative indication if the crack is deep enough or only at surface deep. You should recognize that inside of the coil will not be accessible for a good micro-examination. Even if you have access for inside of the tube, achieving a satisfactory in-situ metallographic quality polishing and etching, viewing at site and taking a good replication is almost not possible.. The best option available to you to assess the degree of degradation of IGSCC or TGSCC is to cut a ring sample or cut a small spool pipe of the tube to get a cross section micro profile. For this, cutting a sample ring or sample spool piece addition at the cut portions of the tube is a must. Boat sampling and plugging , suggested by Debasis, is not resorted by most users under normal circustances - due to complications of new plug welding with old tube welding with known surface or sub-surface IGSCC or TGSCC . Besides this will not give an aesthetic look at the plug welded sample location. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt ltd India Aug 06,2005 E-mail:nishi@vsnl.com Boat sampling is done for heavy wall thickness - multi layer vessels - where practical difficulty is experienced for taking samples. >Giving final judgement from one spot is not recommended in >principle. Normally in-situ metallography is done from >outside. If the cause of IGSCC/ TGSCC is from external >corrosion source then to assess the depth of propagation of >microcracks you may have to collect boat/ plug samples by >specific metallurgical scooping device maintatining the >design thickness and then evaluate the full cross section of >the boat and also may check the bottom face of the boat to >get idea about crack propagation. If the cause of OD surface >crack from tube inside process ( like PTA etc.), then >definitely it has traversed full thickness. At that time it >is better to cut a small tube ring and assess the entire >cross section. > > >