We are facing cracking at the three heat exchangers during the unit shutdown: First heat exchanger: material is carbon steel (1/2 Mo) cracking at the channel box to nozzles internal weld. Second heat exchanger: material is carbon steel (1/2 Mo) cracking at the channel box to partition plate internal weld. Third heat exchanger: material is carbon steel claded with stainless steel 321 and the partition plate is stainless steel 304 and the crack happened when weld these together at the HAZ. The service is kerosene with sour gas hydrogen sulfide (h2s).Since 1985 this is first time. Really I will not suspect cracking at these location since it were stress relieved . Any input will add value to me . Thanks Mark

Dear Mark, You have not mentioned the operating conditions of the HExs ( T, Flow , Pr) . HEx #1 : MOC is C-0.5 Mo. To mention though C-Mo steel is used in Old refineries, Fertilisers w.r.t. conditions of Old Nelson Curves but w.r.t number of Hydrogen induced failures worldwide ( API 941-1990 issue lists all these failures for user considerations) latest edition of API 941 does not include this C-0.5Mo material and discontinued. To mention suseptibility of HA is typically related to actual composition of Metal carbide w.r.t. MO and not the general bulk composition of C-0.5Mo steel and thus NT / QT HT gives satisfactory level of HA resistance and not the SR or Annealed one. Though as per different governing codes PWHT is NONMandatory below certain thickness but without proper PWHT most of C-0.5Mo material has very unpredictable HA resistance and are prone to failure. Also service side Hardness matters a lot. HEx #2 :- consider similar to HEx 1 HEx # 3 :- In this case the following reasonsa may attribute - (a) As 321 is a stabilised SS and 304 is unstabilised the morphology of their HAZ will be something different w.r.t. weld dilution effects and thus are prone to attacks. As your KERO is rich with Sour Gas H2S , if by any chance there are inclusions of carry over water and Oxygen and as you know solubility of H2S sharply decreases with increasing temperature leading to formation of gaseous H2S and chances of formation of sulphides which inturn can regenerate Polythionic Acid and at 1st the weld HAZ will be subjected to IGC Cracking. Also high Hardness matters. Others may please comment and contribute. regards DM

Dear All, What is the operating temp. of 3rd exchanger at tube side ? When you are installing a SS 304 partition plate, it is expected that the partition plate will be welded to CS base plate (of C/Box) and then welded to the SS 321 lining. What was the electrode/filler for SS 304 partition plate to CS base plate? E 312/E309 Mo / Inconel or some other filler/electrode ? In case the operating temp. is on higher side, aust. SS partition plate in CS/alooy steel (BCC) faces some stress due to uneven expansion problem. 'Hope this is not the case in your case. With better details (op. temp & weld details etc), we can all learn something from these shared experiences. 'Hope it helps. Regards