Austenite
Austenite
Austenite
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CHAPTER 6
because their greater thermal expansion coefficient tends to cause the protective oxide
coating to spall.
2. They can experience stress corrosion cracking (SCC) if used in an environment to which
they have insufficient corrosion resistance.
3. The fatigue endurance limit is only about
30% of the tensile strength (vs. ~50 to 60%
for ferritic stainless steels). This, combined
with their high thermal expansion coefficients, makes them especially susceptible to
thermal fatigue.
However, the risks of these limitations can be
avoidable by taking proper precautions.
Introduction
Austenitic stainless steels have many advantages from a metallurgical point of view. They
can be made soft enough (i.e., with a yield
strength about 200 MPa) to be easily formed by
the same tools that work with carbon steel, but
they can also be made incredibly strong by cold
work, up to yield strengths of over 2000 MPa
(290 ksi). Their austenitic (fcc, face-centered
cubic) structure is very tough and ductile down
to absolute zero. They also do not lose their
strength at elevated temperatures as rapidly as
ferritic (bcc, body-centered cubic) iron base alloys. The least corrosion-resistant versions can
withstand the normal corrosive attack of the
everyday environment that people experience,
while the most corrosion-resistant grades can
even withstand boiling seawater.
If these alloys were to have any relative
weaknesses, they would be:
Fig. 1
Fig. 2
Lean Alloys
Lean austenitic alloys constitute the largest
portion of all stainless steel produced. These are
principally 201, 301, and 304. Alloys with less
than 20% chromium and 14% nickel fall into
this unofficial category. Since they are stainless,
it is generally taken for granted that these alloys
will not corrode, and these alloys have sufficient corrosion resistance to be used in any indoor or outdoor environment, excluding coastal.
These grades are easily weldable and formable
and can be given many attractive and useful surface nishes, so they are very much generalpurpose alloys. Table 1 lists some typical compositions of the most commonly used lean
austenitic alloys. These typical compositions
vary with end use, raw material cost factors, and
the preference of a given manufacturer. The
compositions of standard alloys are often netuned to the intended end use. In this table, the
word drawing indicates higher nickel for lower
work hardening, while tubing indicates alloys
with higher sulfur to facilitate gas tungsten arc
welding (GTAW) penetration. Tensile indicates
lower alloy levels to increase the work-hardening rate for material that is intended to be used
in the cold-worked, high-strength condition.
316L is included in its most common tubing end
use chemistry even though it is a corrosion-resisting alloy because it is so pervasively used as
a service center sheet item.
The main difference among the lean
austenitic alloys lies in their work-hardening
rate: the leaner the alloy, the lower the austenite
stability. As unstable alloys are deformed, they
transform from austenite to the much harder
martensite. This increases the work-hardening
rate and enhances ductility since it delays the
onset of necking since greater localized
Wrought alloys generally have cast counterparts that differ primarily in silicon content.
Versions that require enhanced machinability
have a high content of controlled inclusions,
suldes, or oxysuldes, which improve machinability at the expense of corrosion resistance.
Carbon is kept below 0.03% and designated an
L grade when prolonged heating due to multipass welding of heavy section (greater than
about 2 mm) or when welds requiring a postweld stress relief are anticipated.
Table 1 Typical compositions of the most commonly used lean austenitic alloys
Alloy
Designation
Cr
Ni
Mo
Mn
Si
Other
Other
Other
201
201 drawing
201LN
301 tensile
301 drawing
303
304
304 drawing
304 extra drawing
304L tubing
305
321
316L
S20100
S220100
S20153
S30100
S30100
S30300
S30400
S30400
S30400
S30403
S30500
S32100
S31603
0.08
0.08
0.02
0.08
0.08
...
0.05
0.05
0.06
0.02
0.05
0.05
0.02
0.07
0.07
0.13
0.4
0.04
...
0.05
0.04
0.04
0.09
0.02
0.01
0.0
16.3
16.9
16.3
16.6
17.4
...
18.3
18.4
18.3
18.3
18.8
17.7
16.4
4.5
5.4
4.5
6.8
7.4
...
8.1
8.6
9.1
8.1
12.1
9.1
10.5
0.2
0.02
0.2
0.2
0.02
...
0.3
0.3
0.3
0.3
0.2
0.03
2.1
7.1
7.1
7.1
1.0
1.7
...
1.8
1.8
1.8
1.8
0.8
1.0
1.8
0.45
0.5
0.45
0.45
0.45
...
0.45
0.45
0.45
0.45
0.60
0.45
0.50
0.001 S
0.001 S
0.001 S
0.001 S
0.007 S
...
0.001 S
0.001 S
0.001 S
0.013 S
0.001 S
0.001 S
0.010 S
0.03 P
0.30 P
0.03 P
0.03 P
0.03 P
...
0.03 P
0.03 P
0.030 P
0.030 P
0.02 P
0.03 P
0.03 P
0.2 Cu
0.6 Cu
0.5 Cu
0.3 Cu
0.6 Cu
...
0.3 Cu
0.3 Cu
0.4 Cu
0.4 Ci
0.2 Cu
0.4 Ti
0.4 Cu
Fig. 3
(a) Iron-chromium phase diagram at 8% nickel; (b) iron-nickel phase diagram at 18% chromium
Fig. 4
Variation of martensite formation with temperature and true strain for 304. Source: Ref 7
(Eq 3)
(Eq 4)
(Eq 5)
(Eq 6)
Fig. 6
Fig. 5
tion in austenite, and diffusion rates are sufficient for carbon and chromium to segregate into
precipitates. The solubility of carbon in austenite is over 0.4% at solidication but decreases
greatly with decreasing temperature. The solubility is given by (Ref 12):
log (C ppm ) = 7771
6272
T (K )
(Eq 7)
Fig. 7
Fig. 8
Fig. 10
Fig. 9
Figure 9 shows that the local chromium depletion is such that the chromium level can become low enough that it has not even enough to
be stainless and certainly much lower corrosion
resistance than the surrounding area. This zone,
because it is lower in chromium, also has very
unstable austenite and is quite prone to martensite formation. Figure 10 shows how the locus
of precipitation changes with time and temperature. Carbon relatively far from grain boundaries in the interior of grains remains in supersaturation until much longer times and much
greater supersaturation since bulk diffusion is
required for the nucleation and growth of these
precipitates.
The key observation is that any solid-state
precipitation of a chromium-rich precipitate
will necessarily cause local chromium depletion
and a resulting loss of corrosion resistance.
Fig. 11
stable phase when the solubility limit is exceeded. The solubility is over 0.15% in austenite, so its precipitation seldom has the possibility of occurring, but it does become an issue in
ferritic stainless steels in this regard, for which
solubility is much lower. Manganese and
chromium increase the solubility of nitrogen in
austenite.
Stabilization. Before carbon was easily lowered to harmless levels, it was found that adding
more powerful carbide formers than chromium
could preclude the precipitation of chromium
carbides. Titanium and niobium are the most
useful elements in this regard. They form carbides with solubility that follows the following
equation type:
log [ M] [ X] = + A H
RT
(Eq 8)
6780
T
9350
T
(Eq 9)
(Eq 10)
Oxides and suldes are more energetically favorable than are carbides and nitrides of these
metals. Thus, any additions made to form carbides must be sufficient to account for the prior
formation of these compounds. Nitrogen also
competes with carbon for available titanium or
niobium. Thus, for successful gettering of all
carbon, there must be sufficient titanium or niobium to combine stoichiometrically with all
these species present.
This requires in rough terms that titanium exceed four times the carbon plus nitrogen, or that
niobium exceed eight times, after accounting
for the oxygen and sulfur. It would be a mistake
Fig. 12
Variation of hardness with depth and therefore carbon content in colossal supersaturation
(Eq 11)
This is consistent with the similar thermodynamic interaction coefficients that carbon and
nitrogen share with regard to chromium.