Austenitic stainless steels

Question 1

Characteristics

Answer 1

> non-magnetic
austenite stable up to melting point, no solid state phase transf so not heat treatable
high rate of work hardening, can be strengthened by cold working
FCC so no DBTT
good resistance to corrosion, good creep resistance and formability

Question 2

Composition

Answer 2

> main alloys are Cr and Ni, but can contain Mn, Mo, Ti, Nb
Ni: austenite stabilizer, increases ductility and toughness, improves corrosion resistance
C: undesired impurity, letter “L” in AISI (XXXL)
Mo: increases strength through solid soln. hardening, increased corrosion resistance
Nb/Ta: eliminates problems with sensitization of steel
Mn: austenite stabilizer, eliminates hot shortness and increases solid soln. hardening

Question 3

AISI 2XX series austenitic SS

Answer 3

> chromium-magnanese-nickel austenitic SS
more Mn and less Ni than in 3XX SS
2XX cheaper than 3XX because less Ni
less corrosion resistance in 2XX
typical comp: 0.15 wt%C max; 5.5-10wt% Mn; 3.5 to
6 wt%Ni; 16-19 wt% Cr

Question 4

Chromium Carbide formation

Answer 4

> Cr23C6 precipitates on GB in austenitic SS, and can completely dissolve above 950C
precipitates form if in temp range of 500-950C

Question 5

Sensitization

Answer 5

> Cr23C6 nucleates preferentially on high angle GB
formation of Cr23C6 depletes Cr around it, so steel is no longer stainless in those regions–>intergranular corrosion
twin boundaries not preferred for nucleation sites

Question 6

How do we eliminate sensitization?

Answer 6

> add Ti or Nb/Ta, which are strong carbide former and bond with carbon
decrease CC (AISI 304L and 316L)