2. TTA / TTT - Diagrams
2. TTA / TTT – Diagrams
9 An essential feature of low alloyed ferrous materials is
-Iron
-Iron
body-centered
face-centered
the crystallographic transformation
of
the
body-
centred cubic lattice which is stable at room temperature (α-iron, ferritic structure) to the face-centred cubic lattice (γ-iron, aus-
Lattice constant 0.364 nm at 900 °C
Lattice constant 0.286 nm at room temperature
tenitic
© ISF 2002
br-eI-02-01.cdr
structure),
2.1.
Body- and Face-Centered Lattice Structures
The
Figure
temperature,
where this transformation
Figure 2.1
occurs, is not constant but depends on factors like
alloy content, crystalline structure, tensional status, heating and cooling rate, dwell times, etc..
In
order
understand
to
be
able
the
to
basic
processes it is necessary to
S
have a look at the basic
TsA T1
processes occuring in an idealized
binary
system.
Figure 2.2 shows the state
L1
L1
1
T2
S+ 3 2
4 5
T e r u t a r e p m e T
Li So
TsB
T e r u t a r e p m e T
- ss
of a binary system with a
b
complete solubility in the liquid and solid state. If the melting of the L1 alloy
A (Ni)
c2
c0
c3
c4
B (Cu)
Concentration c
Time t
© ISF 2002
br-eI-02-02.cdr
Binary System With Complete Solubility in Liquid and Solid Phase
is cooling down, the first crystals of the composition
c1
Figure 2.2
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 composition c0 is present at the temperature T 1. With dropping temperature, the remaining melt is en-
2. TTA / TTT – Diagrams
10
riched with component B, following the course of line Li (liquidus line, up to point 4). In parallel, 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 structure 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 complete insolubility in the solid state shows Figure 2.3 If two components are completely insoluble 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 T e 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 (T e) of the eutectic transformation: S → A+B (T = Te = const.). This means that the melt at a constant temperature T e dissociates in A and B. If an alloy of the composition L 2 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 L 1 will consist of a compound of alloy A and eutectic alloy E in the solid state. You can find further inL1
L2
L1
L2
TsA
tion behaviour in relevant
S 1
2’
2 T re ut ar
specialist literature. T
TsB er tu
Li
Li
S+A e
formation on transforma-
ar e
p
p m e T
So Te
m e
S+B
The definite use of the T
3 4
principles occurs in the A+E
E
B+E
iron-iron carbide diagram. A br-eI-02-03.cdr
c1
ce Concentration c
B
Transformation behaviour
Time t © ISF 2002
Binary System With Complete Solubility in Liquid Phase and Complete Unsolubility in Solid Phase
Figure 2.3
of carbon containing iron in the equilibrium condition is described by the
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-Fe 3C). Per definition, iron carbide is designated as a structure constituent with cementite although its stoichiometric composition is identical (Fe 3C). 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 cementite is encouraged with increasing cooling rate and decreasing carbon content. In a double diagram, the stable melt + - solid solution
system is shown by a
dashed, the metastable by
solid sol.
melt + graphite
melt
Fe3C (cementite)
solid sol. melt + austenite
a solid line, Figure 2.4.
melt + cementite
austenite
C ° re ut
The
metastable
phase
ra
austenite + graphite austenite + cementite
e p m e T et ir u b
diagram is limited by the formation of cementite with
The
stoichiometry
the
lir e p
stable equilibrium metastable equilibrium
Mass % of Carbon © ISF 2002
br-eI-02-04.cdr
Stable and Metastable Iron-Carbon-Diagram
formed carbide phase can be read off at the top X-
ferrite + graphite ferrite + cementite et
strict of
d el
ferrite
a carbon content of 6,67 mass%.
e
austenite + ferrite
Figure 2.4
coordinate of the molar carbon content. In accordance with the carbon content of Fe 3C, cementite is formed at a molar content of 25%. The solid solutions in the phase fields are designated by Greek characters. 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 determined 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,
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 stabilized 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 contents according to the stable system. In the following, the most important terms and transformations 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 binary 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 temperature of 1147°C and a carbon concentration of 4.3 mass%, the eutectic phase called Ledeburite 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 Ledeburite 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 structure. α- and δ-phases are cubic body-centered (CBC lattice) and γ -phase is cubic facecentered (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 cubic 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 difference 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 δferrite 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 alloying elements as manganese, chromium, nickel and silicon. Principally the equilibrium diagram Fe-C applies also to such multi-component systems. Figure 2.5 shows a
Ac3
schematic cut through the Ac1e
three phase system Fe-M-C.
During precipitation, mixed carbides of the general composition M3C develop. © ISF 2002
br-eI-02-05.cdr
In contrast to the binary Description of the Terms Ac1b , Ac1e , Ac3
Figure 2.5
system Fe-C, is the three
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 A cle. The indices b and e mean the beginning and the end of transformation. The described equilibrium
°C
diagrams apply only to low heating and cooling rates. However, higher heating and cooling rates are present during welding, consequently other structure
s
types develop in the heat
© ISF 2002
br-eI-02-06.cdr
affected zone (HAZ) and in
TTA Diagram for Isothermal Austenitization
the weld metal. The structure transformations during
Figure 2.6
heating and cooling are described by transformation diagrams, where a temperature change is not carried out close to the equilibrium, but ASTM4; L=80µm
at different heating and/or cooling rates.
ASTM11; L=7µm
A
representation
processes
during
of
the
transformation
isothermal
austenitizing
shows Figure 2.6. This figure must be read 20µm
20µm
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
e r u t a r e p m e T
formed in areas, where ferrite was present before transformation, and carbon-rich austenite is formed in areas during transformation, Time br-er02-07.cdr
© ISF 2002
TTA-Diagram for Continuous Warming
Figure 2.7
where
carbon
was
present
before
transformation. During sufficiently long annealing times, the concentration differences are balanced by diffusion, the border to a ho-
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 austenitizing, two microstructure photographs are shown, both with different grain size classes to ASTM. Figure 2.8 shows the relation between the TTA and the Fe-C diagram. It's obvi-
Ac3
ous that the Fe-C diagram is only valid for infinite long
Ac1e Ac1b
dwell times and that the TTA diagram applies only for one individual alloy.
Figure 2.9 shows the dif© ISF 2002
br-eI-02-08.cdr
ferent
Dependence Between TTA-Diagram and the Fe-M-C System
time-temperature
passes during austenitizing and
Figure 2.8
subsequent
cooling
down. The heating period is com Ac3
posed of a continuous and
continuous
an isothermal section. Ac1e
Ac1b
During cooling down, two
isothermal
different ways of heat control can be distinguished: 1. : During continuous temperature Heating and Cooling Behaviour With Several Heat Treatments
Figure 2.9
control
a
© ISF 2002
br-eI-02-09.cdr
cooling is carried out with a constant cooling rate out of
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), followed by an isothermal holding until all transformation processes are completed. After transformation will be cooled down to room temperature.
Figure
2.10
shows
the
time-temperature diagram of a isothermal transformation 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 metastable austenite, marked © ISF 2002
br-eI-02-10.cdr
Isothermal TTT-Diagram of Steel C45E (Ck 45)
with
an
A.
The
areas
marked with F, P, B, und M represent areas where fer-
Figure 2.10
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 martensitic 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
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
27 19 40
as the steel Ck 15. Here you can see that all transformation 370
are
strongly postponed in rela-
170
235 220
processes
tion to the mild steel. A
Time © ISF 2002
br-eI-02-11.cdr
Continuous TTT-Diagram of Steel C15E (Ck 15)
completely
martensitic
transformation
is
carried
out up to a cooling time of
Figure 2.11
about 1.5 seconds, comChemical composition %
C
Si
Mn
P
S
Al
0 ,13
0 ,3 1
0 ,5 1
0 ,0 23
0 ,0 09
0, 01 0
1000 °C 900
Cr
Mo
Ni
V
1 ,5
0, 06
1 ,55
< 0, 01
pared with 0.4 seconds of
austenitizing temperature 870°C (dwell time 10 min) heated in 3 min
Ck 15. In addition, the
Ac3
800
completely diffusion con-
Ac1
700 e r u t a r e p m e T
A+C
600
F
47
25
22
10
75 75 25 25
75
75 75 55 67
trolled transformation proc-
P
5
esses of the perlite area
25
500 MS
B
23
60
400
72
55
M
37
are postponed to clearly 30
300
22 9
longer times.
2
200 100 417
400
396
314 304 287 268 251 224 192
167
152
151
0 -1
10
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0
10
1
10
2
3
10
10 Time
Continuous TTT-Diagram of Steel 15 CrNi 6
Figure 2.12
4
10
5
10
s © ISF 2002
6
10
The hypereutectoid steel C 100
behaves
completely
different, Figure 2.13. With this carbon content, a pre-
2. TTA / TTT – Diagrams
18 eutectoid ferrite formation cannot still be car-
C Si 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
Chemical composition %
ried out (see also Figure 2.3).
1000
austenitizing temperature 790°C dwell time 10 min, heated in 3 min
°C 900
The term of the figures 2.9 to 2.11 "austenitiz-
800 AC1e
700
A+C
e r 600 u t a r e 500 p m e T
100
1 00
100
100
100
100
100 AC1b
100 P
where the workpiece transforms to an austen-
2 15
400
180
itic microstructure in the course of a heat
300 200
treatment. Don’t mix up this temperature with
MS M
100
RA30 914 901 817 366
351
283
236
215
214
the AC3 temperature, where above it there is
177
0 1000 °C 900
austenitizing temperature 860°C dwell time 10 min, heated in 3 min
800
only pure austenite. In addition you can see that only martensite is formed from the aus-
AC1e
700
C A
e r 600 u t a r e 500 p m e T 400
ing temperature“ means the temperature,
100
P
100
100
100
100
100
100
AC1b
tenite, provided that the cooling rate is suffi-
100
100 5
ciently
194
high,
a
formation
of
any
other
300 200
microstructure is completely depressed. With
MS RA0
100
this type of transformation, the steel gains the
M 876 887 867 496 457 442
0 -1
0
10
1
10
10
347 2
10
289 3
10
246
227 4
10
Time
br-er02-13.cdr
200
s
5
10
© ISF 2002
Continuous TTT-Diagram of Steel C100U (C 100 W1)
highest hardness and strength, but loses its toughness, it embrittles. The slowest cooling rate where such a transformation happens, is
Figure 2.13
called critical cooling rate.
Ar 3 Ar 1
Perlite
100%
Low number of nuclei due to melting, high temperature, long dwell time, coarse austenite grain, C-increase up to 0,9%, Mn, Ni, Mo, Cr
1000
800 er
A
High number of nuclei, low hardening temperature, C-increase above 0,9%
Cr, V, Mo
900°C 1300°C
°C
ut ar e
F
600 p
P
m e
Cr, V, Mo
T
C, Cr, Mn, Ni, Mo, high temperature, ferrite precipitation in perlite
B
MS 400
Bainite
M Low hardening temperature (special carbides), austenite above bainite e r u t a r e p m e T
200 100 % n iot
75
Ms u
M
M
B
B
bi tr
Martensite C, Mn, Cr, Ni, Mo, V, high hardening temperature, preprecipitation in bainite
si d
50
Co, Al, deformation of austenite, low hardening temperature er tu c rut S
25 0 -1
10 Transition time br-er02-14.cdr
10
2
10
s
br-er02-15.cdr
© ISF 2002
Temperature Influence on Transformation Behaviour of Steels
Figure 2.15
3
10
Cooling time (A 3 to 500°C) © ISF 2002
Influence of Alloy Elements on Transformation Behaviour of Steels
Figure 2.14
1
2. TTA / TTT – Diagrams
19
Figure 2.14 shows schematically how the TTT diagram is modified by the chemical composition 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 Max. temperature 1350 °C
S355J2G3 (St 52-3) C 0,16
Chemical composition %
Si 0,47
Welding heat cycle
Mn P S Al N Cr 1,24 0,029 0,029 0,024 0,0085 0,10
Cu 0,17
Ni 0,06
900 °C 800
formation. As a result, the
48
600 ut ar
500 e p m
"noses" in the TTT diagram
B
75
55
400 e
sion lengths which must be passed during the trans-
700 re
an extension of the diffu-
T
222
are shifted to longer times.
215
300
The lower part of the figure
200 449
420
400
363 334 324
270
253
251 249 243
shows the proportion of
100 0
1
2
4
6
8 10
20 Time
br-eI-02-16.cdr
40
60 80 100
200
s
400
© ISF 2002
formed
martensite
and
Bainite depending on cool-
Welding TTT-Diagram of Steel S355J2G3 (St 52-3)
ing time. You can see that
Figure 2.16
with
higher
austenitizing
temperature the start of 15 Mo 3
Max. temperature 1350 °C C 0,16
Chemical composition %
Si 0,30
Bainite formation together
Welding heat cycle
Mn P S Mo 0,68 0,012 0,038 0,29
with the drop of the mart-
900 °C 800
ensite proportion is clearly
Ac3=861°C Ac1=727°C
700
F
7
1
32 8 4
19
53
45
32
17
shifted to longer times.
P
600 re ut
ar 500 e p m
99
B
MS 14
74
77
60
38
As Bainite formation is not
15
87
so much impeded by the
95
400 e
83
T
208
M
200
178
300
coarse austenite grain as
200 440
HV30
431
3 38
285
255
234
224
210
with the completely diffu-
100
0
1
2
4
6
8 10
20 Time
40
60 80 100
s
400
© ISF 2002
br-eI-02-17.cdr
Welding TTT - Diagram of Steel 15Mo3 (15 Mo 3)
Figure 2.17
200
sion controlled processes of ferrite and perlite formation, the maximum Bainite proportion
is
increased
from about 45 to 75%.
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 W elding-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
2 %C 1
iron-carbon diagram was
0,45 0,5
developed as an equilib-
1000 0
°C 800
1000
rium diagram for infinite
°C
long cooling time and that
800 er
F
600 ut ra
P e
a TTT diagram applies alre
600 p m e
T 400
ut
B
ar
MS
e p
M
400
m e T
200 200 0 -1 10
10
0
10
1
10
2
10
3
s
10
4
Time
0 © ISF 2002
br-eI-02-18.cdr
Relation Between TTT-Diagram and Iron-Carbon-Diagram
Figure 2.18
ways oy for one alloy.
