stress corrosion cracking in 316 stainless steel 2

[rsauk]

We have a pressure vessel which exhibits micro cracking in the vessel external surface (underneath lagging).<br>Can we safely weld io this surface.

 

[butelja]

The cracking is most likely chloride induced stress cracking.&nbsp;&nbsp;When it is external, the most usual source for the chlorides is the insulation.&nbsp;&nbsp;Weld repair of major cracks can sometimes be done.&nbsp;&nbsp;However, usually the heat of welding will cause crack growth and/or new cracks to form.&nbsp;&nbsp;<br><br>If the cracking is extensive, it could lead to catastrophic failure of the vessel.&nbsp;&nbsp;In general, if a vessel has chloride stress cracking, it will need to be replaced.

 

[rsauk]

Thanks for your observations which concurr with my opinions.<br>However,the situation I found myself in with regard to this pressure vessel working at 0.7barg at temp. of 50 degrs.C was complicated by the fact that this cracking was discovered when replacing a vessel leg support, This support required the welding of reinforcement pads to vessel shell (15mm.thk) and bottom dishing (25mm.thk)<br>It is true that grinding operation (preparation) exacerbated micro cracks,(this confirmed by DPI. examination)<br>The choice to be made during shutdown period was either continue with fitting of reinforcement pads or replace section of vessel shell.<br>It was my opinion that without further examination it was impossible to decide extent of chloride attack and therefore size of patch.<br>Before carrying out welding of pads,weldability tests was carried out on shell using 316welding rod (mma)<br>DPI. examination of test run indicated weld metal free of cracking.<br>Final DPI. of welded pads (fillet welds) show welds free from cracking in welds albeit plate material around welds do show signs of scc.<br><br>What is your considered opinion of this action and could you also expand on the possibility of catastrophic failure.<br><br>Finally, this vessel wil be monitored for leakage!.<br>I understand that good practice is sometimes to give outside surface a good 'wash' before re-instating correct type of insulation or vapour barrier.<br>Do you have any information on this?<br><br>regards.