Sensitization

1. Sensitization
Sensitization is the loss of alloy integrity. It results from chromium
depletion in the vicinity of carbides precipitated at grain boundaries.
This causes the steel or alloy to become susceptible to intergranular
corrosion or intergranular stress corrosion cracking (SCC).
Sensitization requires specific combinations of:
• Temperature
• Time
• Composition
• Corrosion

2. • Sensitization happens when a stainless steel is held at a temperature of
425 - 815°C and chromium carbides precipitate at the grain
boundaries.
• This precipitation happens because the carbides are insoluble at these
temperatures.
• In order for the carbide to precipitate, it must obtain chromium from
the surrounding metal.
• This means that there is a chromium-depleted zone around the grain
boundaries.

3. • Sensitization is particularly important in welded metals.
• This is because the welding zone experiences temperatures in the
range that can cause sensitization, which causes localized intergranular
corrosion at the weld site.
• If a sensitized alloy is exposed to a corrosive environment, the areas
near the grain boundaries are preferentially attacked.
As the corrosion proceeds, the grains fall out and the metal loses
strength.
• Sensitization can be prevented by:
• Reducing the carbon content
• Adding stabilizers such as niobium or titanium
• Reducing the time of exposure to the critical temperature range

4. Hot cracking
• Hot cracking is mainly due to high amounts of elements with low melting
temperatures in the base material.
• Slightly elevated amounts of these elements is usually not enough to cause
cracking, but rather it couples with other issues such as improper width to
depth ratio of the weld bead.
• Some elements used in steel making like sulfur and phosphorus have low
melting points.
• Because of these low melting points these elements are pushed to the center
of the weld bead as it is solidifying.
• These elements can then get trapped in the middle of the weld when the
puddle does not stay in a molten state long enough. When this occurs we
get a centerline crack as the one pictured below.