Some compositions of stainless steel are prone to intergranular corrosion when heated to around 700 °C, chromium present in the steel, reacts with the carbon content (some compositions of chromium carbide are Cr23C6, and Cr7C3) forming chromium carbide which precipitates at the grain boundaries, depleting the grain edges of chromium, reducing their corrosion resistance. Steel in such condition is called sensitized. Steels with carbon content 0.06% undergo sensitization in about 2 minutes , while steels with carbon content under 0.02% are not sensitive.
Weld decay:
Causes of weld decay is precipitation of Chromium Carbides at the grain boundaries, induced by the welding operation in the region of the heat affected zone where the temperature is in the range (550oC~850oC). The precipitation of chromium carbides consumes chromium from a narrow band of the metal along the weld and this makes the zone anodic . The chromium depleted zone thus is the preferred zone for corrosion or crack propagation if the material undergoes tensile stress.
Knife line attack:
For stabilized stainless steels and alloys, carbon reacts with the stabilizers (Ti or Nb) during welding thereafter weld decay does not occur in the heat affected zone. In the event of a subsequent heat treatment or welding, however, precipitation of chromium carbide is possible in this narrow zone adjacent to the weld making it susceptible to intergranular corrosion. As the name "knife line attack" implies, this is limited to a small zone, often a few micrometres wide. This zone being very near the weld is less noticeable.
Prevetion of weld Decay:
Using low carbon (e.g. 304L, 316L) stainless steels
Using stabilized SS alloyed with titanium (for example type 321) or niobium (for example type 347)as they are are stronger carbide- formers than chromium. Thus they will form the carbides preferentially preventing chromium depletion.
Use post-weld heat treatment.
Using stabilized SS alloyed with titanium (for example type 321) or niobium (for example type 347)as they are are stronger carbide- formers than chromium. Thus they will form the carbides preferentially preventing chromium depletion.
Use post-weld heat treatment.
Prevention of Knifeline Attack:
Heating the weld to 1065 degreeC to re-stabilize the material by dissolving the precipitated chromium carbide.
IGSCC( inter granular Stress corossion craking in a BWR) a case study:
A boiling water reactor (BWR) has a metal structure inside the reactor vessel known as the core shroud whish serves the purpose of a barrier between the core and the annulus and provides vertical and lateral support for the core plate, top guide and shroud head and has functions associated with in-vessel emergency core cooling system (ECCS) distribution.
The first cracks in the core shroud were reported in 1990. Since then similar cracks were detected in more Nuclear Power plants. Ultrasonic tests were carried out for evaluating the depths of the cracks. Cracks were found within the base material or in the heat-affected zone of the welds connecting the support ring to the shroud.
The first cracks in the core shroud were reported in 1990. Since then similar cracks were detected in more Nuclear Power plants. Ultrasonic tests were carried out for evaluating the depths of the cracks. Cracks were found within the base material or in the heat-affected zone of the welds connecting the support ring to the shroud.
Safety implications:
The shroud is important for safety of the nuclaer during both normal operation and design basis accidents
Presence of these cracks reduce the structural capacity of the shroud during a seismic event or a large pipe break.
The perceived risk is relatively low due to slow propagation rate of the crack and continuous monitoring.
Causes:
The causes for the shroud cracking failures was high carbon content in the material, unfavourable heat treatment and unfavourable weld procedures. This caused intergranular stress corrosion cracking.
The shroud is important for safety of the nuclaer during both normal operation and design basis accidents
Presence of these cracks reduce the structural capacity of the shroud during a seismic event or a large pipe break.
The perceived risk is relatively low due to slow propagation rate of the crack and continuous monitoring.
Causes:
The causes for the shroud cracking failures was high carbon content in the material, unfavourable heat treatment and unfavourable weld procedures. This caused intergranular stress corrosion cracking.
