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The Influence of Peak Temperature and Deformation on Welding CCT Diagram of Eutectoid Carbon Steel
M.Maalekian1,a, M.L.Lendinez1,b, E.Kozeschnik1,c, H.P. Brantner2,d and
H.Cerjak1,e 1Institute for Materials Science, Welding and Forming, Graz University of Technology;
Kopernikusgasse 24, 8010 Graz, Austria 2Voestalpine Schienen GmbH, Technologie: Forschung & Entwicklung, Kerpelystrasse 199, 8700
Leoben, Austria [email protected],
[email protected], [email protected]
Keywords: Welding CCT diagram, austenite decomposition, hot deformation, thermo-mechanical processing
Abstract. The welding continuous cooling transformation (WCCT) behavior of eutectoid carbon
steel was investigated in different peak temperatures and in the undeformed and deformed
conditions. The corresponding WCCT and welding continuous cooling compression transformation
(WCCCT) diagrams were constructed by means of dilatometric and metallographic analyses in
addition to hardness measurements. It was found that the higher austenitizing temperature slightly
accelerates pearlitic transformation, i.e., it shifts the WCCT diagram to shorter times. Furthermore,
heavy hot deformation of austenite could strongly promote the formation of pearlite, that is, the
WCCCT diagram moved toward the top left corner compared to the WCCT diagram, while
martensite start temperature was lowered slightly, which is a characteristic of a displacive
transformation mechanism.
Introduction
The austenitizing temperature and soaking time each affect the grain size of the austenite, hence
modifying the subsequent transformation characteristics on cooling. The austenitizing temperature
also affects the composition of austenite if the steel contains strong carbide-forming elements and
consequently undissolved carbides may be present. Care should be taken, therefore, when adapting
the diagrams for austenitizing conditions different from those indicated [1]. For instance, phase
transformations occurring during welding are usually far away from equilibrium and differ
markedly from those experienced during heat treatment and thermomechanical processing.
Moreover, conventional continuous cooling transformation (CCT) diagrams exhibit transformation
characteristics of austenite that has been homogenized by a relatively long soak at high constant
temperature. For this reason, these diagrams cannot be readily adapted to the -welding process and
the difference between welding CCT and conventional CCT diagrams is more drastically, the faster
heating and cooling rates are applied [1, 2].
In addition to the influence of fast temperature cycles, in some solid state welding operations, such
as pressure gas welding, forge welding, flash butt welding, friction welding and upset welding, steel
parts are subject to high pressures leading to macroscopic deformation of the samples and a
subsequent phase transformation under stress and plastic deformation.
Attempts to obtain the influence of austenite deformation on the onset of transformation and CCT
diagrams have been carried out over a long period of time, see e.g. refs. [3-11]. However, according
to the authors knowledge, little or no efforts to investigate the influence of pre-deformation on
welding CCT (WCCT) diagrams have been undertaken. Hence, objective of this work is the
construction of WCCT diagrams for a pearlitic carbon steel with different peak temperatures and
austentizing times, and the assessment of the influence of plastic deformation of austenite on the
onset of phase transformation.
Advanced Materials Research Vols. 15-17 (2007) pp 1008-1013Online available since 2006/Feb/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.15-17.1008
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 143.107.104.200, University of Sao Paulo, SAO PAULO - SP, Brazil-20/05/13,17:08:10)
Experimental
The material employed in this work is a eutectoid carbon steel with pearlitic structure. The chemical
composition is summarized in Table 1. Simulation of the weld thermal cycle was performed on a
Gleeble-1500 testing machine. Specimens used for weld thermal cycle and deformation experiments
were machined in 12 mm diameter and 110 mm length. The specimens were heated up to peak
temperatures of either 1300°C or 1100°C for 1s or 2.5s and then free cooled. In order to control the
cooling rate, different heating zone from 15 mm to 90 mm were used. The test temperature was
monitored using a thermocouple welded on the center of the specimen surface. For the deformation
experiments, the specimens were heated up to 1100°C, held for 1 s, and subsequently cooled down
with different controlled cooling rates. Plastic deformation was carried out with 120MPa pressure
applied for 3 s above 850°C in austenite region. Figure 1 shows schematically the weld thermal
cycles with and without plastic deformation. A combination of optical microscopy, dilatometric
analysis, Vickers hardness and microhardness tests was used to construct the WCCT diagram and to
assess the influence of compressive deformation.
Table1. Chemical composition of the steel [wt. %]
C Mn Si Cr Ni S
0.75 1.02 0.28 0.11 0.05 0.015
Fig. 1. Schematic illustration of thermal and thermo-mechanical cycles: (a) weld thermal cycle and
(b) weld thermal cycle with plastic deformation of austenite prior to transformation
Results and discussion
Figure 2 displays representative optical micrographs of the investigated material in the undeformed
condition, i.e. austenitised at 1300°C for 2.5s. The figure reveals a pronounced variation of
microstructure as cooling time, t8/5 (the time interval required for the specimens to cool from 800°C
to 500°C), is increased from 4.5s to 76s. At the high cooling rate (t8/5=4.5s), the transformed
microstructure is completely martensitic (Fig.2-a). As the cooling rate is reduced, the first bainite
and pearlite islands are formed (Fig.2-b) and, consecutively, at lower cooling rates, the amount of
transformed pearlite increases (Fig.2-c and d).
Based on the microstructural characterization, hardness, microhardness and dilatometry data, the
WCCT diagram was constructed for the steel with parent austenite phase in unstrained condition
(see Fig. 3,). The bainite region is indicated with a dashed line, because the start and end points
could not be clearly identified by dilatometry due to the low quantity of bainite (<4%). However,
this line was revealed by microstructure and microhardness analyses.
Time
TPeak
100°C/s
thold
Tem
per
atu
re
Free cooling
Tpeak= 1300 or 1100 °C
thold= 1 or 2.5 sec
(a) 1100°C 1s
100°C/s
Tem
per
atu
re
Controlled
cooling
3s
Time
Pre
ssu
re
120Mp
a
TPeak
2s γ -region
Time
(b)
Advanced Materials Research Vols. 15-17 1009
Fig. 2. Light micrographs showing the microstructure evolution in undeformed condition
austenitized at 1300°C for 2.5s with different cooling rates: (a) t8/5=4.5s (b) t8/5=10.5s (c) t8/5=18s
(d) t8/5=76s
0
100
200
300
400
500
600
700
800
900
1000
1 10 100 1000
Time (s)
Tem
pera
ture
(°C
)
M
P
A
et
Ms
B<4%
832
325
331606778809
287
310
Tpeak: 1300°C
thold: 2.5 sec
Grain size:ASTM 2
A: Austenite
P: Pearlite
B: Bainite
M: Martensite
: HV 10
: t8/5
4.5 10.57 18 41.5
52
76
331
Fig. 3. WCCT diagram of the eutectoid carbon steel at 1300°C- 2.5 sec
Figure 4 demonstrates the influence of peak temperature on the WCCT diagram. The higher
austenitizing temperature shifts the diagram to longer times. In other words, hardenability is
increased by raising austenitizing temperature which can be attributed to grain coarsening and
homogenization phenomena. Incomplete homogenization resulting from lower peak temperatures
supports austenite that exhibits accelerated transformation during cooling. At sites progressively
closer to the weld, higher hardenability and longer transformation times are promoted by higher
peak temperatures, which speed up the process of alloy and carbide dissolution. Next to the weld, in
(a) (b)
(c) (d)
1010 THERMEC 2006 Supplement
the region subjected to the highest austenitizing temperatures, the onset of rapid grain coarsening
introduces an additional increase in hardenability [1].
0
100
200
300
400
500
600
700
800
900
1000
1 10 100 1000
Time (s)
Tem
pera
ture
(°C
)
P
B<4%
A
M
1300°C- 1sec
1100°C- 1sec
Fig. 4. Influence of peak temperature on WCCT diagram
The influence of hot deformation (compression) of austenite, carried out at a temperature range of
850-1000 °C, can be derived from Figure 5. The doted lines in Figure 5 represent the welding
continuous cooling compression transformation (WCCCT) diagram for the present eutectoid steel.
Heavy deformation of austenite to a true strain of about ε ≈ 0.8 accelerates the pearlite
transformation, which shifts its existence region in the WCCT diagram to higher cooling rates and
increased start-finish temperatures. Simultaneously, the martensite start temperature is lowered
slightly.
no deformation
with deformation
0
100
200
300
400
500
600
700
800
900
1000
1 10 100 1000
Time (s)
Tem
pera
ture
(°C
)
P
B<4%
A
M
Ms=235°C
Ms=225°C
Tpeak = 1100°C
thold = 1 sec
Fig. 5. Influence of deformation of austenite on WCCT diagram
The effect of hot plastic deformation on the non-isothermal decomposition of austenite to pearlite is
evident from a comparison of the WCCT and WCCCT diagrams, Fig. 5. It has been widely reported
[7,10-14] that the heavy deformation of austenite may give rise to an increase in the ferrite/pearlite
nucleation site, and, thus the nucleation rate, due to the high density formation of dislocations,
substructures, twinning and deformation bands or shear bands. This proposition emerges to be
sustained by the present study where the pearlite grain size obtained after transformation from the
deformed austenite is greatly finer than that acquired from undeformed austenite. Besides, austenite
deformation leads to an increase in the austenite grain-boundary area per unit volume [7,10] known
to serve as strong pearilte nucleation sites [7,10]. Thus, the number of convenient sites for the
formation of new nuclei (pearlite) is much higher than in the original material. In addition to
Advanced Materials Research Vols. 15-17 1011
increasing the heterogeneous nucleation sites density, the strain energy of produced defects raises
the austenite free energy [7], which, in turn, leads to a specific reduction in the critical free energy
for pearlite nucleation. As a result, new phase (pearlite) starts to nucleate earlier in comparison with
the decomposition of the same material without straining.
Moreover, Figure 5 shows that, at lower cooling rates, the WCCT and WCCCT curves are closer to
each other. The reason for this is not clear for the authors; however, it may be attributed to
recrystalization and grain growth phenomena. Severe deformation was carried out at a temperature
range of 850-1000 °C. Austenite non-recrystalization temperature (Tnr) is the temperature below
which the austenite recrystalization stops entirely for the given thermo-mechanical conditions [13].
Although, the deformation temperature range is lower than the Tnr, which can be calculated by a
formula proposed by Boratto et al [15] based entirely on chemical composition of the steel, it must
be mentioned that the Tnr is not only dependent on the material characteristics but is also dependent
on many processing parameters [13], such as strain rate, strain and cooling rate. Therefore, at such a
high temperature range when low cooling rates are used, recrystalization and grain growth may
occur. Consequently, the state of the austenite may become similar to the undeformed condition
[10]. Hence, the transformation curves for both cases-with and without deformation- become closer
(see Fig. 5).
Figure 5 also indicates that the martensite start temperature for deformed austenite is slightly lower
than that for undeformed austenite. This can be explained by mechanical stabilization phenomenon
[14,16,17]. Martensitic (displacive) transformation involves the coordinated movement of atoms
(glissile interfaces), which can be hindered or rendered sessile on encountering defects such as
dislocations or grain boundaries. Thus, martensite transformation is retarded as a result of small size
of austenite grains which were deformed heavily prior to the transformation. As a result, plastic
deformation of austenite hinders the growth of martensite, giving rise to lessening in the
transformed fraction although the heterogeneous nucleation rate is increased in correspondence with
the larger number of grains and defect density [10, 16]. This retardation of transformation by plastic
deformation is known as mechanical stabilization [14,16,17].
Conclusions
The welding continuous cooling transformation (WCCT) diagram of eutectoid carbon steel has been
determined using dilatometry, metallography and hardness measurements. The effects of
austenitizing temperature and prior γ-deformation on WCCT diagram have been studied
experimentally. Based on this investigation, the following main conclusions can be drawn:
1) The WCCT diagram shifted slightly to longer times when the peak temperature increased
from 1100 °C to 1300°C.
2) Plastic deformation of austenite brought about a significant decrease in hardenability due to
enhanced formation of pearlite microstructure. In addition, the final microstructure appeared
to be finer.
3) The transformation curves for both cases- with and without γ-deformation - at lower cooling
rates were found to be closer, supposedly due to the recrystalization and grain growth.
4) The martensite start temperature (Ms) was slightly lowered by severe deformation of
austenite as a result of mechanical stabilization of martensite.
Acknowledgements
The authors are gratefully acknowledge the financial support of this work as a part of K-net JOIN
granted by the Federal Ministry of Economy and Labour, Austria. The authors also are very grateful
to Professor A. Kulmburg for helpful discussions.
1012 THERMEC 2006 Supplement
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Advanced Materials Research Vols. 15-17 1013
THERMEC 2006 Supplement 10.4028/www.scientific.net/AMR.15-17 The Influence of Peak Temperature and Deformation on Welding CCT Diagram of Eutectoid Carbon
Steel 10.4028/www.scientific.net/AMR.15-17.1008
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