There are two main reasons for the improved cracking resistance when
ferrite is formed during solidification of stainless steel welds.
*Ferrite has the bcc crystal structure as opposed to the fcc crystal
structure of austenite. The bcc structure allows more rapid diffusion
of impurity elements (and other elements that can form eutectic
constituents) in stainless steels. This reduces the segregation
tendency of impurity elements along weld metal grain boundaries and
reduces the volume fraction of liquid films coating the grain
boundaries at the end of solidification. A reduction in the volume
fraction of liquid films limits the propagation of solidification
cracks.
*Austenite/ferrite interfaces are interlocking ("tortuous") as opposed
to fairly straight austenite/austenite interfaces. This reduces the
wetting of liquid films along the austenite/ferrite weld metal grain
boundaries at the end of solidification, and provides a tortuous path
that is more resistant to fracture than the straight austenite/
austenite boundaries.
Ferrite number is typically controlled to balance solidification
cracking resistance, corrosion performance, and low temperature
toughness.
A low FN such as 0.5 is not a sufficient volume fraction to provide
the effects mentioned above to promote solidification cracking
resistance. Impurity control and proper selection of weld procedure
is generally required for welding such materials.
A FN of 15 will have excellent solidification cracking resistance but
at the cost of poor low temperature toughness in cryogenic
applications. Also the presence of this much ferrite will promote
sigma phase formation during elevated temperature service or prolonged
heat treatments. This effectively embrittles the weld metal.
Therefore a FN of "3 to 8" would provide an optimal balance of all of
these properties.
Hope this helps,
Jeffrey Sowards, Ph.D.
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