chapter 2 -tta . ttt - diagrams
TRANSCRIPT
2. TTA / TTT – Diagrams 9
An essential feature of low
alloyed ferrous materials is
the crystallographic trans-
formation of the body-
centred cubic lattice which
is stable at room tempera-
ture (α-iron, ferritic struc-
ture) to the face-centred
cubic lattice (γ-iron, aus-
tenitic structure), Figure
2.1. The temperature,
where this transformation
occurs, is not constant but
depends on factors like
alloy content, crystalline structure, tensional status, heating and cooling rate, dwell times,
etc..
In order to be able to
understand the basic
processes it is necessary to
have a look at the basic
processes occuring in an
idealized binary system.
Figure 2.2 shows the state
of a binary system with
complete solubility in the
liquid and solid state.
If the melting of the L1 alloy
is cooling down, the first
crystals of the composition
c1 are formed with reaching
the temperature T1. These crystals are depicted as mixed crystal α, since they consist of a
compound of the components A (80%) and of B (20%). Further, a melting with the composi-
tion c0 is present at the temperature T1. With dropping temperature, the remaining melt is en-
© ISF 2002br-eI-02-01.cdr
Body- and Face-CenteredLattice Structures
Lattice constant0.286 nm
at room temperature
Lattice constant0.364 nmat 900 °C
a -Ironbody-centered
g -Ironface-centered
Figure 2.1
© ISF 2002br-eI-02-02.cdr
Binary System With Complete Solubilityin Liquid and Solid Phase
1
2
3
4
5
S
Li
So
A(Ni)
B(Cu)
L1L1
TsA
T1
T2
TsB
c1 c2 c3 c4c0 Time t
Tem
pera
ture
T
Tem
pera
ture
T
Concentration c
S+ a
a -
ba
ss
Figure 2.2
2. TTA / TTT – Diagrams 10
riched with component B, following the course of line Li (liquidus line, up to point 4). In paral-
lel, always new and B richer α-mixed crystals are forming along the connection line So
(solidus line, points 1, 2, 5). The distribution of the components A and B in the solidified struc-
ture is homogeneous since concentration differences of the precipitated mixed crystals are
balanced by diffusion processes.
The other basic case of complete solubility of two components in the liquid state and of com-
plete insolubility in the solid state shows Figure 2.3 If two components are completely insolu-
ble in the solid state, no mixed crystal will be formed of A and B. The two liquidus lines Li cut
in point e which is also designated as the eutectic point. The isotherm Te is the eutectic line.
If an alloy of free composition solidifies according to Figure 2.3, the eutectic line must be cut.
This is the temperature (Te) of the eutectic transformation:
S → A+B (T = Te = const.).
This means that the melt at a constant temperature Te dissociates in A and B. If an alloy of
the composition L2 solidifies, a purely eutectic structure results. On account of the eutectic
reaction, the temperature of the alloy remains constant up to the completed transformation
(critical point) (Figure 2.2).
Eutectic structures are normally fine-grained and show a characteristic orientation between
the constituents. The alloy L1 will consist of a compound of alloy A and eutectic alloy E in the
solid state.
You can find further in-
formation on transforma-
tion behaviour in relevant
specialist literature.
The definite use of the
principles occurs in the
iron-iron carbide diagram.
Transformation behaviour
of carbon containing iron
in the equilibrium condi-
tion is described by the
© ISF 2002br-eI-02-03.cdr
Binary System With Complete Solubility in Liquid Phaseand Complete Unsolubility in Solid Phase
TsA
Te
2’
L1 L1L2 L2
1
2
3
4
S+A
So
S
S+B
Li Li
A+E E B+E
B
TsB
c1 ce
Tem
pe
ratu
reT
Te
mp
era
ture
T
Concentration c Time tA
Figure 2.3
2. TTA / TTT – Diagrams 11
stable phase diagram iron-graphite (Fe-C). In addition to the stable system Fe-C which is
specific for an equilibrium-close cooling, there is a metastable phase diagram iron cementite
(Fe-Fe3C). During a slow cooling, carbon precipitates as graphite in accord with the stable
system Fe-C, while during accelerated cooling, what corresponds to technical conditions,
carbon precipitates as cementite in agreement with the metastable system (Fe-Fe3C). Per
definition, iron carbide is designated as a structure constituent with cementite although its
stoichiometric composition is identical (Fe3C). By definition, cementite and graphite can be
present in steel together or the cementite can decompose to iron and graphite during heat
treatment of carbon rich alloys. However, it is fundamentally valid that the formation of ce-
mentite is encouraged with increasing cooling rate and decreasing carbon content. In a dou-
ble diagram, the stable
system is shown by a
dashed, the metastable by
a solid line, Figure 2.4.
The metastable phase
diagram is limited by the
formation of cementite with
a carbon content of 6,67
mass%. The strict
stoichiometry of the
formed carbide phase can
be read off at the top X-
coordinate of the molar
carbon content. In accordance with the carbon content of Fe3C, cementite is formed at a mo-
lar content of 25%. The solid solutions in the phase fields are designated by Greek charac-
ters. According to convention, the transition points of pure iron are marked with the character
A - arrêt (stop point) and distinguished by subjacent indexes. If the transition points are de-
termined by cooling curves, the character r = refroidissement is additionally used. Heat-up
curves get the supplement c - chauffage. Important transition points of the commercially more
important metastable phase diagram are:
- 1536 °C: solidification temperature (melting point) δ-iron,
- 1392 °C: A4- point γ- iron,
Stable and Metastable Iron-Carbon-Diagram
© ISF 2002br-eI-02-04.cdr
melt +
- solid solutiond
d -+g-solid sol.
d -solid sol.
melt
melt +graphite
Fe C
(cementite)3
melt +cementite
melt +austenite
austenite
austenite + graphiteaustenite + cementite
ledeburite
austenite +ferrite
ferrite
perlite
stable equilibriummetastable equilibrium
ferrite + graphiteferrite + cementite
Mass % of Carbon
Tem
pera
ture
°C
Figure 2.4
2. TTA / TTT – Diagrams 12
- 911 °C: A3- point non-magnetic α- iron,
with carbon containing iron:
- 723 °C: A1- point (perlite point).
The corners of the phase fields are designated by continuous roman capital letters.
As mentioned before, the system iron-iron carbide is a more important phase diagram for
technical use and also for welding techniques. The binary system iron-graphite can be stabi-
lized by an addition of silicon so that a precipitation of graphite also occurs with increased
solidification velocity. Especially iron cast materials solidify due to their increased silicon con-
tents according to the stable system. In the following, the most important terms and transfor-
mations should be explained more closely as a case of the metastable system.
The transformation mechanisms explained in the previous sections can be found in the bi-
nary system iron-iron carbide almost without exception. There is an eutectic transformation in
point C, a peritectic one in point I, and an eutectoidic transformation in point S. With a tem-
perature of 1147°C and a carbon concentration of 4.3 mass%, the eutectic phase called Le-
deburite precipitates from cementite with 6,67% C and saturated γ-solid solutions with 2,06%
C. Alloys with less than 4,3 mass% C coming from primary austenite and Ledeburite are
called hypoeutectic, with more than 4,3 mass% C coming from primary austenite and Lede-
burite are called hypereutectic.
If an alloy solidifies with less than 0,51 mass percent of carbon, a δ-solid solution is formed
below the solidus line A-B (δ-ferrite). In accordance with the peritectic transformation at
1493°C, melt (0,51% C) and δ-ferrite (0,10% C) decompose to a γ-solid solution (austenite).
The transformation of the γ-solid solution takes place at lower temperatures. From γ-iron with
C-contents below 0.8% (hypoeutectoidic alloys), a low-carbon α-iron (pre-eutectoidic ferrite)
and a fine-lamellar solid solution (perlite) precipitate with falling temperature, which consists
of α-solid solution and cementite. With carbon contents above 0,8% (hypereutectoidic alloys)
secondary cementite and perlite are formed out of austenite. Below 723°C, tertiary cementite
precipitates out of the α-iron because of falling carbon solubility.
2. TTA / TTT – Diagrams 13
The most important distinguished feature of the three described phases is their lattice struc-
ture. α- and δ-phases are cubic body-centered (CBC lattice) and γ-phase is cubic face-
centered (CFC lattice), Figure 2.1.
Different carbon solubility of solid solutions also results from lattice structures. The three
above mentioned phases dissolve carbon interstitially, i.e. carbon is embedded between the
iron atoms. Therefore, this types of solid solutions are also named interstitial solid solution.
Although the cubic face-centred lattice of austenite has a higher packing density than the cu-
bic body-centred lattice, the void is bigger to disperse the carbon atom. Hence, an about 100
times higher carbon solubility of austenite (max. 2,06% C) in comparison with the ferritic
phase (max. 0,02% C for α-iron) is the result. However, diffusion speed in γ-iron is always at
least 100 times slower than in α-iron because of the tighter packing of the γ-lattice.
Although α- and δ-iron show the same lattice structure and properties, there is also a differ-
ence between these phases. While γ-iron develops of a direct decomposition of the melt (S
→ δ), α-iron forms in the solid phase through an eutectoidic transformation of austenite (γ →
α + Fe3C). For the transformation of non- and low-alloyed steels, is the transformation of δ-
ferrite of lower importance, although this δ-phase has a special importance for weldability of
high alloyed steels.
Unalloyed steels used in industry are multi-component systems of iron and carbon with alloy-
ing elements as manganese, chromium, nickel and silicon. Principally the equilibrium dia-
gram Fe-C applies also to
such multi-component sys-
tems. Figure 2.5 shows a
schematic cut through the
three phase system
Fe-M-C.
During precipitation, mixed
carbides of the general
composition M3C develop.
In contrast to the binary
system Fe-C, is the three Description of the Terms Ac , Ac , Ac1b 1e 3
Ac3
Ac1e
© ISF 2002br-eI-02-05.cdr
Figure 2.5
2. TTA / TTT – Diagrams 14
phase system Fe-M-C characterised by a temperature interval in the three-phase field α + γ +
M3C. The beginning of the transformation of α + M3C to γ is marked by Aclb, the end by Acle.
The indices b and e mean
the beginning and the end
of transformation.
The described equilibrium
diagrams apply only to low
heating and cooling rates.
However, higher heating
and cooling rates are pre-
sent during welding, con-
sequently other structure
types develop in the heat
affected zone (HAZ) and in
the weld metal. The struc-
ture transformations during
heating and cooling are described by transformation diagrams, where a temperature change
is not carried out close to the equilibrium, but
at different heating and/or cooling rates.
A representation of the transformation
processes during isothermal austenitizing
shows Figure 2.6. This figure must be read
exclusively along the time axis! It can be
recognised that several transformations
during isothermal austenitizing occur with e.g.
800°C. Inhomogeneous austenite means
both, low carbon containing austenite is
formed in areas, where ferrite was present
before transformation, and carbon-rich aus-
tenite is formed in areas during transforma-
tion, where carbon was present before
transformation. During sufficiently long an-
nealing times, the concentration differences
are balanced by diffusion, the border to a ho-
TTA Diagram forIsothermal Austenitization
© ISF 2002br-eI-02-06.cdr
s
°C
Figure 2.6
TTA-Diagram forContinuous Warming
ASTM4; L=80µm ASTM11; L=7µm
20µm 20µm
© ISF 2002br-er02-07.cdr
Te
mp
era
ture
Time
Figure 2.7
2. TTA / TTT – Diagrams 15
mogeneous austenite is passed. A growing of the austenite grain size (to ASTM and/or in
µm) can here simultaneously be observed with longer annealing times.
The influence of heating rate on austenitizing is shown in Figure 2.7. This diagram must only
be read along the sloping lines of the same heating rate. For better readability, a time pattern
was added to the pattern of the heating curves. To elucidate the grain coarsening during aus-
tenitizing, two microstructure photographs are shown, both with different grain size classes to
ASTM.
Figure 2.8 shows the rela-
tion between the TTA and
the Fe-C diagram. It's obvi-
ous that the Fe-C diagram
is only valid for infinite long
dwell times and that the
TTA diagram applies only
for one individual alloy.
Figure 2.9 shows the dif-
ferent time-temperature
passes during austenitizing
and subsequent cooling
down.
The heating period is com-
posed of a continuous and
an isothermal section.
During cooling down, two
different ways of heat con-
trol can be distinguished:
1. : During continuous
temperature control a
cooling is carried out with a
constant cooling rate out of
Dependence Between TTA-Diagramand the Fe-M-C System
Ac3
Ac1e
Ac1b
© ISF 2002br-eI-02-08.cdr
Figure 2.8
Heating and Cooling Behaviour WithSeveral Heat Treatments
Ac3
Ac1e Ac1b
continuous
isothermal
© ISF 2002br-eI-02-09.cdr
Figure 2.9
2. TTA / TTT – Diagrams 16
the area of the homogeneous and stable austenite down to room temperature.
2. : During isothermal temperature control a quenching out of the area of the austenite is
carried out into the area of the metastable austenite (and/or into the area of martensite), fol-
lowed by an isothermal holding until all transformation processes are completed. After trans-
formation will be cooled down to room temperature.
Figure 2.10 shows the
time-temperature diagram
of a isothermal transforma-
tion of the mild steel Ck 45.
Read such diagrams only
along the time-axis! Below
the Ac1b line in this figure,
there is the area of the me-
tastable austenite, marked
with an A. The areas
marked with F, P, B, und M
represent areas where fer-
rite, perlite, Bainite and
martensite are formed. The
lines which limit the area to the left mark the beginning of the formation of the respective
structure. The lines which limit the area to the right mark the completion of the formation of
the respective structure. Because the ferrite formation is followed by the perlite formation, the
completion of the ferrite formation is not determined, but the start of the perlite formation.
Transformations to ferrite and perlite, which are diffusion controlled, take place with elevated
temperatures, as diffusion is easier. Such structures have a lower hardness and strength, but
an increased toughness.
Diffusion is impeded under lower temperature, resulting in formation of bainitic and marten-
sitic structures with hardness and strength values which are much higher than those of ferrite
and perlite. The proportion of the formed martensite does not depend on time. During
quenching to holding temperature, the corresponding share of martensite is spontanically
formed. The present rest austenite transforms to Bainite with sufficient holding time. The right
Isothermal TTT-Diagramof Steel C45E (Ck 45)
© ISF 2002br-eI-02-10.cdr
Figure 2.10
2. TTA / TTT – Diagrams 17
detail of the figure shows the present structure components after completed transformation
and the resulting hardness at room temperature.
Figure 2.11 depicts the graphic representation of the TTT diagram, which is more important
for welding techniques. This is the TTT diagram for continuous cooling of the steel Ck 15.
The diagram must be read along the drawn cooling passes. The lines, which are limiting the
individual areas, also depict the beginning and the end of the respective transformation.
Close to the cooling curves, the amount of the formed structure is indicated in per cent, at the
end of each curve, there is the hardness value of the structure at room temperature.
Figure 2.12 shows the TTT
diagram of an alloyed steel
containing approximately
the same content of carbon
as the steel Ck 15. Here
you can see that all trans-
formation processes are
strongly postponed in rela-
tion to the mild steel. A
completely martensitic
transformation is carried
out up to a cooling time of
about 1.5 seconds, com-
pared with 0.4 seconds of
Ck 15. In addition, the
completely diffusion con-
trolled transformation proc-
esses of the perlite area
are postponed to clearly
longer times.
The hypereutectoid steel C
100 behaves completely
different, Figure 2.13. With
this carbon content, a pre-
Continuous TTT-Diagramof Steel C15E (Ck 15)
Time
27
40
19
370 235 220 170
© ISF 2002br-eI-02-11.cdr
Figure 2.11
MS
M
A+C F
B
5
5522
P
25
2
23
Ac3
Ac1
Chemicalcomposition %
SiC Mn P S Al Cr Mo Ni V
0,13 0,31 0,51 0,023 0,009 0,010 1,5 0,06 1,55 < 0,01
10-1
100
101
102
103
104 10
510
6
0
100
200
300
400
500
600
700
800
900
1000
s
°C
Tem
pera
ture
Time
austenitizing temperature 870°C(dwell time 10 min) heated in 3 min
25
4767
7510
75 7525
257575
60
7255
3730
22
9
417 400 396 314 304 287 268 251 224 192 167 152 151
Continuous TTT-Diagramof Steel 15 CrNi 6
© ISF 2002br-eI-02-12.cdr
Figure 2.12
2. TTA / TTT – Diagrams 18
eutectoid ferrite formation cannot still be car-
ried out (see also Figure 2.3).
The term of the figures 2.9 to 2.11 "austenitiz-
ing temperature“ means the temperature,
where the workpiece transforms to an austen-
itic microstructure in the course of a heat
treatment. Don’t mix up this temperature with
the AC3 temperature, where above it there is
only pure austenite. In addition you can see
that only martensite is formed from the aus-
tenite, provided that the cooling rate is suffi-
ciently high, a formation of any other
microstructure is completely depressed. With
this type of transformation, the steel gains the
highest hardness and strength, but loses its
toughness, it embrittles. The slowest cooling
rate where such a transformation happens, is
called critical cooling rate.
0
100
200
300
400
500
600
700
800
900
1000
°C
Tem
pera
ture
10-1
100
101
102
103
104
105
0
100
200
300
400
500
600
700
800
900
1000
s
°C
Te
mp
era
ture
Time
P
2 15
100
100 100 100 100 100 100
AC1e
AC1b100
MS
M RA 30»
914 901 817 366 351 283 236 215 214 177
180
A+C
AC1e
AC1b
MS
M
A P
C
100100
100
1005
100 100 100 100 100
RA 0»4
876 887 867 496 457 442 347 289 246 227 200
194
Continuous TTT-Diagramof Steel C100U (C 100 W1)
Chemicalcomposition %
Mn P S Cr Cu Mo Ni V
1,03 0,17 0,22 0,014 0,012 0,07 0,14 0,01 0,10 traces
C Si
austenitizing temperature 790°Cdwell time 10 min, heated in 3 min
austenitizing temperature 860°Cdwell time 10 min, heated in 3 min
© ISF 2002br-er02-13.cdr
Figure 2.13
Influence of Alloy Elementson Transformation Behaviour of Steels
Tem
pera
ture
Transition time
Low number of nuclei due tomelting, high temperature, longdwell time, coarse austenitegrain, C-increase up to 0,9%,Mn, Ni, Mo, Cr
High number of nuclei, lowhardening temperature,C-increase above 0,9%
Ar1
Ar3
Perlite 100%
Cr, V, Mo
Cr, V, Mo
Bainite
C, Cr, Mn, Ni, Mo, hightemperature, ferriteprecipitation in perlite
Low hardening temperature(special carbides),austenite above bainite
C, Mn, Cr, Ni, Mo, V, highhardening temperature, pre-precipitation in bainite
Martensite
Co, Al, deformationof austenite, lowhardening temperature
Ms
© ISF 2002br-er02-14.cdr
Figure 2.14
Temperature Influence onTransformation Behaviour of Steels
Te
mp
era
ture
1000
800
A
600
400
200
°C
MS
M
B
F
P
900°C1300°C
Str
uctu
re d
istr
ibu
tio
n
100
75
50
25
0
%M M
B B
10-1 1 10 10
210
3s
Cooling time (A to 500°C)3
© ISF 2002br-er02-15.cdr
Figure 2.15
2. TTA / TTT – Diagrams 19
Figure 2.14 shows schematically how the TTT diagram is modified by the chemical compo-
sition of the steel.
The influence of an increased austenitizing temperature on transformation behaviour shows
Figure 2.15. Due to the higher hardening temperature, the grain size of the austenite is
higher (see Figure 2.6 and 2.7).
This grain growth leads to
an extension of the diffu-
sion lengths which must be
passed during the trans-
formation. As a result, the
"noses" in the TTT diagram
are shifted to longer times.
The lower part of the figure
shows the proportion of
formed martensite and
Bainite depending on cool-
ing time. You can see that
with higher austenitizing
temperature the start of
Bainite formation together
with the drop of the mart-
ensite proportion is clearly
shifted to longer times.
As Bainite formation is not
so much impeded by the
coarse austenite grain as
with the completely diffu-
sion controlled processes
of ferrite and perlite forma-
tion, the maximum Bainite
proportion is increased
from about 45 to 75%.
Welding TTT-Diagramof Steel S355J2G3 (St 52-3)
0
100
200
300
400
500
600
700
800
900
1 2 4 6 8 10 20 40 60 80 100 200 400sTime
Te
mp
era
ture
449 420 400 363 334 324 270 253 251 249
222 215
243
°C
Chemicalcomposition %
SiC Mn P S Al N Cr Cu Ni
0,16 0,47 1,24 0,029 0,029 0,024 0,0085 0,10 0,17 0,06
Max. temperature 1350 °C Welding heat cycle
48
75
S355J2G3 (St 52-3)
55
© ISF 2002br-eI-02-16.cdr
B
Figure 2.16
Welding TTT - Diagramof Steel 15Mo3 (15 Mo 3)
0
100
200
300
400
500
600
700
800
900
°C
1 2 4 6 8 10 20 40 60 80 100 200 400sTime
Tem
pera
ture
440 431 338 285 255 234 224 210
208 200 178
A =861°Cc3
A =727°Cc1
MS
B
F
HV30
P
M
14 7487
95
99 83 77 60 38 15
1 7 19 4 8
32 45
17
53
32
Chemicalcomposition %
SiC Mn P S Mo
0,16 0,30 0,68 0,012 0,038 0,29
15 Mo 3 Max. temperature 1350 °C Welding heat cycle
© ISF 2002br-eI-02-17.cdr
Figure 2.17
2. TTA / TTT – Diagrams 20
Due to the strong influence of the austenitizing temperature to the transformation behaviour
of steel, the welding technique uses special diagrams, the so called Welding-TTT-diagrams.
They are recorded following the welding temperature cycle with both, higher austenitizing
temperatures (basically between 950° and 1350°C) and shorter austenitizing times.
You find two examples in Figures 2.16 and 2.17.
Figure 2.18 proves that the
iron-carbon diagram was
developed as an equilib-
rium diagram for infinite
long cooling time and that
a TTT diagram applies al-
ways oy for one alloy.
Relation Between TTT-Diagramand Iron-Carbon-Diagram
F
P
0
200
400
600
800
°C
1000
10-1
100
101
102
103
104
s ¥
B
M
0
200
400
600
800
°C
10000
0,451
%C
2
MSTe
mp
era
ture
Te
mp
era
ture
Time
0,5
© ISF 2002br-eI-02-18.cdr
Figure 2.18