Mark Mor Luis is correct that choice of using E 347 -15 or E 309 Cb-15 would be better when choice of disssimilar (Carbon steel to SS 321 cladding) and also SS 321 with SS 304 plate was to be done. Better still if you could choose E 309-cb-16 or E 310-Cb-16 for the first run from carbon steel to S.S 321. E 310 will take care of additional dilution . Since we do not know the conditions of repair you are proposing these are thoughts. Yyou could keep for repair welding job of S.S 31 with S.S 304 plate for the HE # 3. My views on your crack problem : HE #1 crack : C-0.5 Mo channel box to nozzzle weld crack is PWHT not done properly (as you have diagnozed) HE # 2: Crack in partition plate to Channel box weld crack is also due to improper PWHT (as you have correctly diagnozed) HE # 3 : Crack in S.S. 321 to SS 304 plate, it could be micro cracks left over in cladded C.S to S.S 321 (originally and during repair welding done in 2002). Reason given by Debasis for KL attack and shrinkage stress etc will have to be diagnozed thoroughly. Knife Line attack in welds / HAZ - cause has to be diagnozed from other possibilities. As a general rule. Knife Line attack possibility is mainly due to excessive stresses in weld or HAZ. Has this been generated due to welding electrode/ welding current (i.e, dilution in the firt run) , mis-match stresses, improper choice of welding sequence, contribution of H2S (need not be when the equipment is in operation) but could be contributed during shut down periods. For your process temperature and pressure , C-0.5 Mo will not be affected by HIC. Nelson curve theory proposed by some others may not apply at all for this temperature and pressure conditions of max. 75 KSC and max, tube temperature of 310 o C. HE # 3: I guess you have to analyze from the number of shut downs the exchanger had seen during 2002-2005 period and possible entry of air contributing to polythionic acid corrosion cannot be ruled out at this stage. I would also guess that with already stresses present - whether due to mismatch or welding conditions,+ contributions of inadequate supervision of repair welding done in 2002 + nuber shut down conditions which accidental entry of air in the exchanger could lead to such cracks in S.S 321 cladded plates as well as in SS. 304 plates. This crack may look like KL also. X-ray stress analysis would throw more light - if you have time to anlayze further. For repair welding, pre-qualify welders and procedure for HE #3. For HE # 1 # 2 problem has been identified as improper PWHT. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Sept 26 E-mail:nishi@vsnl.com >For your third heat exchanger I am not so sure if welding >with E308L was the good option. Other option is to use >E347-15 or E309Cb-15 whose are stabilized electrodes like >your 321 clad. > >Regards > >Luis Marques > >Portugal

So, according to my understanding the crack that developed at the 304 ss is due to thermal expansion. But it was in service for 20 years is this impossible. SOHIC is not predicting because we did not have h2s liquid it is just a vapor. So, is there any way to reduce this thermal expansion?

I fully agree with views expressed by Mr. Srinivasan. All our discussion is based on the experiences & expertise of the individual concerned. In my case, I admit, experience is very limited and expertise is missing. I have joined the forum to learn from all of your experiences. A question, however, remains in my mind. Mark has informed that all the three Hex has similar operating condition at tube side. Then how come only the third HEx has SS 321 lining in Channel head ? Also the partition plate is of SS 304 in this case. Am I missing something ? Or may be the operating condition differs for the third Hex or the channel head was given cladding later on. Dear Mark Mor - Please clarify these points for me at your leisure. Regards