Ferritic stainless steels

Question 1

Charatcteristics of ferrite SS

Answer 1

> comp outside gamma loop
up to 30% Cr
no Ni
carbon is an impurity
if Cr content isn’t high, susceptible to sensitization
not heat treatable
cheapter than austenitic SS (no Ni)
good corrosion, oxidation and cracking resistance
poor ductility, toughness, and weldability
susceptible to DBTT
can’t be strengthened by cold working due to low rate of work hardening
low DBTT because BCC, not suitable for cryogenic applications
DBTT decreases with decrease in grain size and CC
Nb/Ta locks up carbon in their carbides, decreases DBTT, reduces sensitization

Question 2

Sigma formation in ferritic SS

Answer 2

> occurs at 500-850C if held for long time or cooled slowly
brittle intermetallic compound
precipitation of sigma depletes matrix of Cr and reduces corrosion resistance