Embrittlement Flashcards
One step embrittlement
> decrease in toughness, increased DBTT, does NOT affect ductility
plain carbon and low alloy steels susceptible
impurities like Sb, Sn, P and S segregate on austenite GB during austenitization. during tempering, plate-like carbide precipitates form on top of the GB as well, leading to intergranular fracture and embrittlement
thin film of retained austenite is present between the martensite laths in as-quenched steels. Decomp of this retained austenite yields thin plates of carbides during tempering that form continuous brittle phase path, giving transgranular fracture and embrittlement of steel
How do you reduce one step embrittlement?
> avoid tempering in critical temp range (260C)
reduce levels of Sb, Sn, P and S in steel
add Ni, Si, or Al to inhibit growth of carbides
reduce amount of Mn and Cr
Two step embrittlement
> tempered in range of 375-575C, occurs during slow cooling
Fe-C alloys and plain carbon steels not susceptible
alloy steels, especially containing Cr and Ni are susceptible
increase in DBTT
Sb, Sn, P, As, Si, Mn increase susceptibility (Sb>Sn>P>As)
DBTT increases with higher hardness and grain size
intergranular fracture
How do you reduce two step embrittlement?
> add 0.5wt% Mo
have a finer austenite grain size to lower DBTT
embrittlement is reversible by removing from above 575C then rapid cooling to room temp
Aluminum Nitride Embrittlement
> precipitation of AlN on austenite GB, significant reduction of toughness
“rock candy fracture”
observed in AL stabilized low alloy steels with lots of Ni
Embrittlement due to environment
SEE PICTURES
> DBTT at low temperatures
BCC metals have sharp ductile to brittle transition
if a componenet is likely to experience low temp in service, the DBTT must be significantly BELOW the service temp
DBTT can’t be detected by tensile test because the rate of loading is low
no DBTT in FCC metals
Liquid Metal Embrittlement
> due to exposure to liquid metals like Pb, Cd, Sn, Zn, Li and Cu giving intergranular brittle fracture
intiation of fracture is not time dependent
can be avoided by eliminating the tensile loading or the liquid metal responsible
Hydrogen Embrittlement
> change in fracture mode from ductile to brittle
