Diat Htt Lect-28
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Transcript of Diat Htt Lect-28
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HeatTreatmentTechnology
Dr.
Santosh
S.
Hosmani
DEPT. METALLURGY & MATERIALS SCIENCE,
COLLEGE OF ENGINEERING, PUNE
Heat
Treatment
of
Issue of Sensitisation in Stainless Steels
,
20wt% Ni and between 0.03 and 0.1wt% C.
The solubility limit of carbon is about 0.05wt% at 800 C, rising to 0.5wt% at
1100 C. Therefore, solution treatment between 1050 C and 1150 C will
take all of the carbon into solution and rapid cooling from this temperature
range will give a supersaturated austenite solid solution at roomtemperature.
However, slow cooling or reheating within the range 550800 C will lead to
the re ection of carbon from solution usuall as the chromiumrich carbide
Cr23C6, even when the carbon content of the steel is very low (
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Issue of Sensitisation in Stainless Steels
FIGURE: Complete intergranular cracking of the sensitized type 304
stainless steel in 400 hours of exposure to high purity water at
2800C. The figure shows (a) intergranular facets of the fracture
sur ace an secon ary n ergranu ar ranc es emana ngfrom the main intergranular crack.
Ref.: BARC HIGHLIGHTS. Engineering - Material Research
Issue of Sensitisation in Stainless Steels
More grain boundary
martensite is present.
The 1112% Cr-type ferritic stainless steels are susceptibile to sensitization
The ferrite-ferrite grain boundaries are sensitized, whereas the
ferrite-martensite phase boun daries are largely unattacked
.partially to austenite in the high-temperature heat-affected zone (HTHAZ) during
cooling, with the austenite transforming to martensite at lower temperatures. The
ferrite-martensite boundaries were generally observed to be unsensitized. The
results suggest that if enough austenite forms in the HTHAZ during cooling, it actsas acarbon sink to dissolve excess carbon. This prevents supersaturation of the
ferrite phase and subsequent carbide precipitation that could lead to sensitization
.sensitization during low heat input welding.
Ref.:M.L.Greeff andM.duToit:WeldingJournal(Nov.2006) 243.
Issue of Sensitisation in Stainless Steels
FIGURE: Sensitized 304 stainless steel
i e after service in dilute nitric acid. Note
stainless steel plate. Chromiumcarbides are indicated by chains of
dark particles along austenite grain
that the weld metal and the HAZ closest
to the weld metal are unaffected, but the
region of the HAZ that reached peak
oun ar es. empera ures n e range o o870C is severely attacked.
Ref.:WeldingHandbook,8thEdition,Vol.4,p.273
Issue of Sensitisation in Stainless Steels
Possible Resolution Treatment: after welding, the steel
can be reheated to 9501100 C to allow Cr C to
redissolve, and further precipitation is then prevented by
rapid cooling to avoid the Cshaped curve.
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Issue of Sensitisation in Stainless Steels
As shown in the timetemperaturesensitisation curves below, the precipitation
of carbides occurs over time at temperatures in the range of about 450 850 C.
The time for precipitation to occur is highly dependant upon the amount of
carbon present in the steel, so low carbon content increases resistance to this
pro em.
content below 0.03wt% is
possible by modern steel-
making methods involving
oxygen anc ng. or
complete immunity fromintergranular corrosion in
18/8 steels, a carbon level
of 0.02wt% should not beexceeded.
Recall yourself from the earlier lectures that:
Addition of metallic allo in elements exce t Co shifts TTT curves
towards right. But, we made little comment about effect of carbon.However, I believe that, for a constant concentrations of partitioning (or
carbide forming) metallic alloying elements, if we increase the carbon
content, TTT curves forcarbide formation will shift towards left (in the
similar way as mentioned on earlier slide). See also the next slide (this is
same slide which we discussed in the past) in support of this argument.
In the case of alloy steels containing carbide-forming elements such as Cr, Mo,
, an , e prec p a on agrams ave wo c ear y separa e ranges o
pearlite and intermediate transformations. Each of the ranges is characterized
by its own C-shaped curves.
en e car on con en o s ruc ura s ee s s up o . . , e s agetransformation is shifted to the right relative to the stage II transformation;if the
carbon content is higher, stage I is found to the left of stage II.
49
0.8%C steel 1.13%C steel
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0.8%C steel 0.38%C alloy steel
But, for En 8 (0.4%C) steel, time indicated by dashed line
is smaller than 0.8%C steel why??
0.8%C steel 0.4%C steel (En 8)
The possible reasons could be as follows (this is as per my thought /
opinion):
It is known that the position of TTT diagram can be affected by prior
austenitic grain-size (i.e. grain-boundary-area per unit volume). In
case of 0.4% C steel there is a formation of ferrite rains in the. ,
austenite grains prior to the carbide formation (here, carbide is
cementite, Fe3C). Due to the formation of ferrite, total grain-boundary-
area per unit volume increases. Therefore, available sites for the
formation of carbide (here, Fe3C) increase, i.e. kinetics of the carbide
formation accelerates.
, . ,
side (compared to eutectoid steel) is possibly due to the other metallic
alloying elements in the steel (and may be not due to the decrease in
.
I strongly think that: for the alloy steel with increase in carbon content(without disturbing other alloying elements concentrations), if there is
no other phase formation (like, above mentioned ferrite formation prior
to carbide formation), TTT curve should move towards left
Issue of Sensitisation in Stainless Steels
elements, Nb, Ti niobium and
titanium form carbides which are
much more stable than Cr C so
they preferentially combine with the
available carbon and thus lessen the
opportunity for Cr23C6to nucleate.
TiC and NbC are much less soluble in
austenite than is chromium carbide,
so the will form at much hi her
temperatures as relatively stable
particles. These should remain
relatively inert during commercial
heat treatments involving solution
temperatures no higher than 1050
C, and thus minimize the possible
nucleation of Cr23C6.
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HT of Austenitic Stainless Steels
HT of Austenitic Stainless Steels
Before going into the different heat treatments for austenitic stainless
steels, it is important to remember two particularly important
properties of these steels: (i) low thermal conductivity and (ii) high
thermal expansion coefficient.
Thethermal conductivityof the austenitic stainless steels is low; about
one fifthof the value for pure iron andone thirdof the conductivity of an
AISI 1025 carbon steel.
higherthan for pure iron or of an AISI 1025 carbon steel.
speeds, the high thermal expansion coefficient requires special careconcerning the spacing between pieces to be treated.
FIGURE: Main thermal
rea men s an
transformations that
occur in austenitic
between room
temperature and the
.
(Ref.: A.F. Padilha, R.L. Plaut,
and P.R. Rios: ISIJ
n erna ona apan , ,
143, 2003.)
SOLUTION ANNEALING
Solution annealin is the heat treatment mostfrequently specified for austenitic stainless steels,
before their actual usage. The main objective of this
treatment, as the name implies, is todissolve the
FIGURE: Grain boundary M23C6
p ases a ave prec p a e ur ng e
thermomechanical processing of the material,
especially the chromium-rich carbides of the
M23C6-type, where M = Cr, Fe, Mo. prec p a es n an aus en cstainless steel observed using
transmission electron microscopy. As the precipitation of M23C6 occurs in the 450 to
900 C temperature range, the lower temperature
limit for solution annealing should be over 900 C.
Carbides should be completely dissolved but they
dissolve slowly. Grain growth limits the maximumsolution-annealing temperature. In particular,
,
recrystallization, must be avoided.
FIGURE: Optical micrograph showing secondary
recrystallizationstart in a titanium-stabilized austenitic
stainless steel after solution annealing
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Martensitic Transformation during Cooling:
s
chemical composition is:
This E uation su ests that man austenitic stainless steels, when
cooled to cryogenic temperatures, will form alpha prime () martensite.
The ability to form alpha prime () martensite becomes more significant
during cooling after sensitization. M23C6 precipitation at grain
boundaries causes depletion of chromium, carbon, and other alloying
elements in the vicinity of the grain boundaries. This leads to a higher
Mstemperature, making the material more susceptible to the formation
o a p a pr me mar ens e c ose o gra n oun ar es ur ng coo ng.
For epsilon () martensite no equations like above Equation are
.
The most frequent case of martensite formation at room temperature in
Stain-Induced Martensitic Transformation:
- .
Widely used empirical equations that relate the Mdtemperature with the
chemical composition is:
dtrue tensile strain of 30%. For the majority of austenitic stainless steels,
the Md temperature is above room temperature. For epsilon ()
martensite such em irical e uations are not available..
Susceptibility of the austenite to form martensite and the amount of
martensite formed increases with decreasing deformation temperature.
When stainless steels containing deformation-induced martensite areannealed, the martensite may revert to austenite. This reversion usually
occurs at temperatures about 100 C lower and for shorter times than
those required for the recrystallization of the deformed stainless steel.
The formability of the austenitic alloys is influenced greatly by
martensitic transformation during straining.
HT of Ferritic Stainless Steels
HT of Ferritic Stainless Steels
Ferritic stainless steel ingots have a coarse grain size, are relatively
brittle, and should not be submitted to thermal shocks.
Cast plates are ground, slowly heated, and hot rolled into strips.
- , , .
Cold rolling, used to obtain intermediate gages, is followed by
- .
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SOLUTION ANNEALING
Ferritic stainless steels are normally used only after solution annealing.
The solution-annealing temperature varies substantially according to the
steel type.
The first-generation steels are treated at lower temperatures, forexample, AISI 430 steel is treated in the 705 to 790 C temperature
ran e and the AISI 446 in the 760 to 830 C ran e..
The second-generation steels are treated at somewhat higher
temperatures, for example, AISI 409 is treated in the 870 to 925 C
range.
The third-generation steels, such as the AISI 444, are treated at even
, .
Steels with higher chromium contents, such as the super-ferritic, should
be water cooled in order to avoid the 475 C embrittlement due to the
formation of intermetallic alpha prime () phase.
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HT of Duplex Stainless Steels
Ferriticaustenitic stainless steels with a duplex microstructure can be
classified into two subgroups:
. oys w ow-car on con en . w . w , requen y
mechanically worked and heat treatable
2. Alloys with high-carbon content (0.3 wt% C 0.5 wt%), used in theas-cast condition or after solution annealing.
Duplex steels of higher carbon content show lower toughness and ductility
u ave an exce en wear res s ance.
Duplex steels of lower carbon content have better formability and
.
Duplex stainless steels are susceptible tothree types of embrittlement:
HT of Duplex Stainless Steels
1. Embrittlement caused by the presence of a carbide network,
particularly in the austenite, in alloys with higher carbon content
. m r emen cause y prec p a on o e n erme a c -p ase,
475 C embrittlement of ferrite
3. Embrittlement caused by precipitation of the -phase, particularlyin the ferrite
FIGURE: Schematic TTT diagram showing precipitation of sigma (), alpha prime (),
and other phases in duplex stainless steels.
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SOLUTION ANNEALING
. -
annealing temperature should be sufficiently fast, generally into water, in
order to avoid precipitation (see schematic TTT diagram on previous
slide es eciall the 475 C embrittlement due to intermetallic al ha,
prime () phase.After welding, the solution-annealing treatment is recommended, followed
.
FIGURE: Ferrite brittle cleavage
fracture (475 C embrittlement)
duplex stainless steel. Scanningelectron microscopy with secondary
electrons. A=austenite; F=ferrite.
HT of Martensitic Stainless Steels
Martensitic stainless steels are essentially FeCrC alloys, containing
chromium in the range of 11.5 to 18 wt% and carbon in range of the 0.1
o . w . s c rom um eve s ncrease , car on eve as o ncrease
also in order to stabilize austenite.
or e mar ens c s ee s, s, o
course, essential to form austenite
from which martensite is obtained. ,
complete austenitization, steels
containing 13 wt% chromium need
0.15 wt% carbon and to be heated
to at least 950C. Steels containing
more chromium, say 27wt%, need
to have carbon content higher than
0.3 wt% carbon and to be heated to
at least 1100C. Apart from carbon.
both nitrogen and nickel expand
the -loop,
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Other alloying elements frequently observed in martensitic stainless steels
are nickel and molybdenum. Nickel entitles the usage of lower carbon
, ,
resistance may be obtained. For soft but tougher martensitic steels
containing lower carbon levels, nickel content may reach 5 wt%.
o y enum a so mproves corros on res s ance, n a on o
improvement in toughness. It must be remembered that corrosionresistance in martensitic stainless steels is significantly lower compared to
.
Martensitic stainless steels may be subdivided into three subgroups: (a)
low-carbon steels for turbines;(b)medium-carbon steels for cutlery;
and(c)high-carbon wear-resistant steels.
The microstructure of each group is also characteristic: (a)martensitic
-
and (c) ultrafine martensitic microstructure containing primarycarbides, respectively.
(a)
(b)
(c)
Higher carbon steels, such as the AISI 440C, or nickel containing,
such as the AISI 431, may present large amounts (more than 30% in
volume ofretained austenite after uenchin .De endin on tem erin
temperature and chemical composition, especially the Cr/C ratio, several
carbides may precipitate, such as the M2X, M3C, M7C3, M23C6, and MC
types.
FIGURE: AISI 410 martensitic
stainless steel, quenched andtempered to 20 HRC. Microstructure of
FIGURE: AISI 420 martensitic
stainless steel. Microstructure oftempered martensite with intergranular
tempered martensite with fine-carbideprecipitates. Optical microscopy.
Etched with Villela.
and intragranular precipitates. SEMimage using secondary electrons.
Etched with Villela.
martensite laths
retained austenite
Ref.:
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As-quenchedAISI 440C
As-tempered
The Equilibrium Phase Diagram of440C Steel
Ref.: International Journal of Mechanical and MaterialsEngineering (IJMME), Vol. 4 (2009), No. 2, 123-126
Prior to final hardening and
tempering heat treatments,
martensitic stainless steels
areannealed in order to be
machined and cold worked.
For example, an AISI 410 is
annealed in the 750 to 900 Ctemperature range for 2 to 4h
.
high temperatures, their
stable structure is austenitic
is a stable mixture offerrite
and carbide.
FIGURE: Continuous cooling transformation
diagram forAISI 410 steel.
Prior to their final usage, martensitic stainless steels are submitted to the
same heat treatment sequence as that for carbon steels, namely they are
, ,
ductility and toughness.
The formation of a more stable (ferrite + carbides) microstructure is very
sluggish and the tendency toward martensite formation (high
hardenability) is very high.
,
air cooling, even for sections that are up to about 300mm in thickness.
Hardening media may be air or oil.
While oil cooling is preferred in order to avoid carbide precipitation, air
cooling may be needed to avoid distortions in more complex sections.
ar emper ng s a so poss e n s c ass o s ee s. ar ens e
hardness depends essentially on carbon content varying from about
35 HRC for a 0.1 wt% carbon to 60 HRC for 0.5 wt% carbon, thereon. .
For low-carbon martensitic steels, such as the AISI 410, the Msand Mftemperatures are relatively high, 350 and 250 C, respectively, and
decrease with increasin carbon content.
High-carbon steels may present retained austenite (more than about 30
vol.%) and a subzero treatment around -75 C, immediately after
hardenin is recommended. Double tem erin is also ver common., . .
Tempering temperature is determined by the required mechanical
properties.
FIGURE: Tempering
temperature effect on
mechanical properties
.
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