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51314-3794-Cracking of a Bi-Metallic Weld in HF Acid Service

Picture for Cracking of a Bi-Metallic Weld in HF Acid Service
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Product Number: 51314-3794-SG
ISBN: 3794 2014 CP
Author: Joyce Mancini
Publication Date: 2014
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A solid Monel nozzle welded into the carbon steel shell of a refinery HF acid settler failed due to H2 embrittlement cracking of the carbon steel along the bi-metallic weld fusion zone. This resulted in a thru-wall fracture opening up and propagating around the nozzle-to-shell weld on the vessel OD from the 12 o’clock to 6 o’clock position in a matter of hours. The metallurgical examination determined the root cause to be hydrogen induced stress cracking of carbon steel due to a combination of hard spots in the bi-metallic weld fusion zone galvanically-enhanced HF corrosion and corrosion product formation with operational stresses likely providing the final load element to propagate the thru-wall failure.Several design modifications were made to reduce the future risk of H2 embrittlement cracking coupled with inspection/monitoring enhancements to assure its future integrity. In addition inspection techniques were evaluated to ensure any future issues could reliably be detected.This paper reviews the causes of the failure the design modifications required to reduce the potential for future issues and the inspection methods identified that will allow for continued assurance of reliable service.  
A solid Monel nozzle welded into the carbon steel shell of a refinery HF acid settler failed due to H2 embrittlement cracking of the carbon steel along the bi-metallic weld fusion zone. This resulted in a thru-wall fracture opening up and propagating around the nozzle-to-shell weld on the vessel OD from the 12 o’clock to 6 o’clock position in a matter of hours. The metallurgical examination determined the root cause to be hydrogen induced stress cracking of carbon steel due to a combination of hard spots in the bi-metallic weld fusion zone galvanically-enhanced HF corrosion and corrosion product formation with operational stresses likely providing the final load element to propagate the thru-wall failure.Several design modifications were made to reduce the future risk of H2 embrittlement cracking coupled with inspection/monitoring enhancements to assure its future integrity. In addition inspection techniques were evaluated to ensure any future issues could reliably be detected.This paper reviews the causes of the failure the design modifications required to reduce the potential for future issues and the inspection methods identified that will allow for continued assurance of reliable service.  
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