decay though I would not eliminate an erosion-corrosion process.
________________________
Weld decay is a form of intergranular corrosion, usually of stainless
steels or certain nickel-base alloys, that occurs as the result of
sensitization in the heat-affected zone during the welding operation.
The corrosive attack is restricted to the heat affected zone (HAZ).
Positive identification of this type of corrosion usually requires
microstructure examination under a microscopy although sometimes it is
possible to visually recognize weld decay if parallel lines are already
formed in the heat affected zone along the weld.
Cause? As in the case of intergranular corrosion, grain boundary
precipitation, notably chromium carbides in stainless steels, is a well
recognized and accepted mechanism of weld decay. In this case, the
precipitation of chromium carbides is induced by the welding operation when
the heat affected zone (HAZ) experiences a particular temperature range
(550oC~850oC). The precipitation of chromium carbides consumed the alloying
element - chromium from a narrow band along the grain boundary and this
makes the zone anodic to the unaffected grains. The chromium depleted zone
becomes the preferential path for corrosion attack or crack propagation if
under tensile stress.
Prevention - Weld decay can be prevented through:
* Use low carbon (e.g. 304L, 316L) grade of stainless steels
* Use stabilized grades alloyed with titanium (for example type 321) or
niobium (for example type 347). Titanium and niobium are strong carbide-
formers. They react with the carbon to form the corresponding carbides
thereby preventing chromium depletion.
* Use post-weld heat treatment.
______________________
Here is an article that might help
Best Regards,
Richard M. Beldyk PhD, PE, CWEng
Supervisor Quality Engineering
Alstom Power, Inc.
Chattanooga, TN USA
Original Message:
-----------------
From: saqib jalal sakib321@gmail.com
Date: Tue, 22 Dec 2009 08:01:50 +0500
To: materials-welding@googlegroups.com
Subject: [MW:3870] root cause anaylsis
dear all
i want to discuss one case study with you i am working in a chemical plant
and one of our plant is Nitric acid production in it on of our equipment is
stationary ( BLEACHING TOWER) whose function is to absorb oxides of nitrogen
from 50-60 nitric acid at average 70-90 centigrade by spraying water from
top and moving up the nitric acid from bottom. the main problem is that the
material of this tower is ss 321 and in shut down we observe severe
depletion in the thickness of shell especially near the HAZ of welds so
would you please tell me most probable cause of this failure and its most
possible remedy
--
with Regards
SAQIB JALAL
Inspection Engineer
PakArab Fertilizer Limited
Khanewal Road , Multan
office number +92-61-9061 EXT 3110
mobile number +92-321-4043556
--
with Regards
SAQIB JALAL
Inspection Engineer
PakArab Fertilizer Limited
Khanewal Road , Multan
office number +92-61-9061 EXT 3110
mobile number +92-321-4043556
--
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