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MIROSŁAW ŁOMOZIK*
NUMERICAL MODELLING OF THERMAL CYCLES IN STEEL
DURING SURFACING AND WELDING WITHOUT
PREHEATING AND THEIR COMPARISON
WITH EXPERIMENTAL DATA
By means of the ANSYS 5.6 programme thermal fields in the heat affected zone (HAZ)
regions of overlay-welded and welded steel without preheating have been modelled and multiple
thermal cycles numerically calculated. The designed geometric models are of the two-dimensional
type with 3-D characteristics. The number of overlay-welded and welded layers was varied from 1
to 6. After numerical calculations for the individual overlay-welded and welded layers, the course
of thermal cycles was analyzed for the 1st, 3rd and 6th layer. Such a procedure followed from
practical utilization of the computation results. ASME regulations recommend welding of
minimum 6 layers to get the desired tempering effect of hardened microstructures in the HAZ,
which are characterized by unfavourable toughness. Parameters of thermal cycles in the HAZ’s of
overlay-welded and welded steel, calculated numerically and measured experimentally, were
compared. Moreover the effect of temperature field asymmetry was raised, which is caused by the
heat flow from successive layers of the overlay weld.
Key words: thermal welding cycle, Heat Affected Zone, cooling time, numerical modelling
1. INTRODUCTION
The post-weld heat treatment (PWHT) is an important problem, as is the case at
welding of components in the power industry. The objective of PWHT is the
maximum reduction of welding residual stresses and the improvement of heat
affected zone (HAZ) and weld metal toughness. However, in the case of repair
welding of big objects PWHT may cause serious difficulties. In those situations
an effective means of ensuring proper high HAZ toughness is the application of
so-called “temper bead welding”. The method consists in such a means and
sequence of laying individual weld beads, to temper the martensitic
microstructure in the HAZ of previously welded beads, by the controlled
_______________________
*
Dr inż. - Instytut Spawalnictwa, Gliwice
welding thermal cycle. The temper bead welding technique enables lowering of
hardness of martensitic and upper bainitic microstructures, which occur in the
coarse grain region of the HAZ, and in consequence, toughness improvement of
these welded joint regions.
To replace effectively the traditional PWHT by the temper bead welding
technique it is essential to gain knowledge on the temperature distribution
at individual weld beads (layers) and on the influence of welding thermal cycles
on structural transformations in the coarse grain HAZ regions.
2. MATHEMATICAL MODELLING OF TEMPERATURE FIELDS AT
VARIOUS HAZ REGIONS BY MEANS OF THE ANSYS PROGRAM
Results of numerical computations, performed by the application of the
ANSYS program with a partition ability of the identified area into 32 thousand
nodes and elements, are presented in this paper. Modelling of thermal fields in
the HAZ region of steel has been performed under the influence of multiple
welding thermal cycles with different parameters (maximum temperatures Tmax
and t8/5 cooling times). The designed two-dimensional models with
three-dimensional attributes have mapped multilayer welding und surfacing
without preheating. The number of weld layers has been changed from 1 to 6.
The technological input data (welding method, welding current I [A], arc voltage
U [V], welding speed [cm/s], time passed between welding of successive layers,
initial and interpass temperature [oC] and the joint thickness [mm]) have been
taken from the results of research projects [8] and [9]. The physical properties of
modeled steels, such as: mass density DENS (γ) [kg/mm3], thermal
conductivity KXX (λ) [cal/mm·s·oC] and specific heat c [cal/kg·oC] were taken
from the literature (DENS, KXX – designations used in the ANSYS program)
[2, 4, 6].
Thermal cycles measured during TIG overlay-welding under argon shielding
are presented in the research report [1]. The
course of the experiment was as follows. On
the SM400A steel plate six layers have been
successively overlay-welded, one after another.
The first layer consisted of six runs, and every
next layer of one run less. The scheme of the
overlay process is shown in Fig. 1.
Fig. 1. Scheme of the overlay welding [8]
Rys. 1. Schemat sposobu układania napoiny [8]
For numerical calculations a geometrical model has been used, which is shown
in Fig. 2 and 3. The model has represented the experimental conditions.
Fig. 2. General view of the model – three
dimensional version [10]
Rys. 2. Widok ogólny modelu – wersja
przestrzenna [10]
Fig. 3. Model for numerical calculations.
The overlay welding area with
a finite element mesh [10]: 1 – base material,
2 – melted weld metal, 3 – solidified weld metal
Rys. 3. Model przyjęty do obliczeń
numerycznych. Obszar napawania po podziale na
elementy skończone z zaznaczeniem [10]:
1 – materiału podstawowego, 2 – materiału
stopionego, 3 – materiału stopiwa po
zakrzepnięciu
The following input data were used for numerical computations:
a) General data:
- base material: unalloyed steel,
- welding filler metal: alloyed,
- overlay-welding method: TIG without preheating, six layers,
- plate thickness: t = 20 mm.
b) Technological data: – Table 1.
Table 1
Experimental conditions for TIG surfacing and welding [8]
Eksperymentalne parametry napawania / spawania metodą TIG [8]
Welding heat
input
Q [kJ/cm]
5,5
9,4
14,8
1)
Welding current
Arc voltage
Welding speed
I [A]
100
150
200
U [V]
9,2
10,5
12,3
v [cm/min]
15 1)
15 1)
15 1)
average value of welding speed.
c) Physical properties: – Tables 2 and 3.
Initial
temperature
[oC]
20
20
20
Room temperature data (20 oC) [2, 5, 6]
Zależności odniesione do temperatury otoczenia (20 oC) [2, 5, 6]
Physical value
Table 2
Material 1
base material
(unalloyed steel)
Material 2
melted weld metal
(alloyed steel)
Material 3
solidified weld metal
(alloyed steel)
0,79 × 10-5
0,70 × 10-5
0,70 × 10-5
0,96 × 10-2
0,2 × 10-3
0,4 × 10-2
174
160
160
Mass density DENS *)
[kg/mm3]
Thermal conductivity
KXX *) (λ)
[cal/(mm·s·oC)]
Specific heat c
[cal/(kg·oC)]
Where: *) designations used in the ANSYS program.
Table 3
Thermal conductivity at various temperatures for the base material and weld metal [2, 5, 6]
Zależności współczynnika przewodzenia ciepła w funkcji temperatury dla materiału
podstawowego i materiału stopiwa [2, 5, 6]
Thermal conductivity KXX (λ)
Base material 1
Weld metal 3 (layer 1 – 5, Fig. 1)
20
0,0096
0,0040
Temperature [oC]
600
1200
0,0075
0,0054
0,0032
0,0024
1500
0,0005
0,0002
Numerical computations have been performed for the six overlay welded
layers 1 to 6, based on the model presented in Fig. 3. A general view of the
models and calculated thermal fields for the individual weld layers contains
Table 4.
3. NUMERICAL MODELLING RESULTS FOR OVERLAY WELDING
After numerical computations have been performed for the individual
overlay-welded layers, the course of thermal cycles has been analysed for the
first, third and sixth layer. The reason was the necessity of practical utilization of
the calculated results. The ASME regulations [1] recommend a minimum of six
welded (overlay-welded) layers to get the desired tempering effect of hardened
microstructures, which occur in the HAZ and are characterized by unfavourable
toughness.
Results of numerical calculations for the overlay-welded layer No. 1 are
presented in Fig. 4 ÷ 6, for the layer No. 3 – in Fig. 7 and 8, and for the layer
No. 6 – in Fig. 9 and 10.
Table 4
General view of the models and calculated thermal fields for the individual weld layers [10]
Widok ogólny modeli i obliczonych numerycznie pól temperatury dla kolejnych napawanych
warstw [10]
No of overlay weld
layer
1
2
3
4
5
6
General view of the model
Numerically calculated thermal field
Fig. 4. Welded layer No. 1. Temperature distribution
in nodes and fragment of temperature field
Rys. 4. Warstwa napawana nr 1. Rozkład temperatur
w węzłach i fragment pola temperatur
Fig. 5. Welded layer No. 1. Thermal
cycles in nodes from Fig. 4 with Tmax =
1310, 1190 and 1095 oC
Rys. 5. Warstwa napawana nr 1.
Widok ogólny cykli cieplnych
wybranych w węzłach o temperaturach
maksymalnych Tmax = 1310, 1190
i 1095 oC z rys. 4
Fig. 6. Welded layer No. 1. Example of determination
of cooling time between 800 and 500 oC (t8/5) for
thermal cycles from Fig. 5
Rys. 6. Warstwa napawana nr 1. Przykład
wyznaczenia czasów stygnięcia w zakresie
temperatur pomiędzy 800 a 500 oC (t8/5) dla cykli
cieplnych z rys. 5
Fig. 7. Welded layer No. 3. Thermal
cycles in nodes with Tmax =875, 804
and 740 oC
Rys. 7. Warstwa napawana nr 3.
Widok ogólny cykli cieplnych
wybranych w węzłach o Tmax = 875,
804 i 740 oC
Fig. 8. Welded layer No. 3. Determination of t8/5
cooling time for thermal cycles from Fig. 7
Rys. 8. Warstwa napawana nr 3. Wyznaczenie
czasów stygnięcia t8/5 dla cykli cieplnych z rys. 7
Fig. 9. Welded layer No. 6. Heat flux
propagation [11] after welding
termination
Rys. 9. Warstwa napawana nr 6.
Strumień rozpływania się ciepła [11]
po zakończeniu napawania
Fig. 10. Welded layer No. 6. Thermal field numerically
calculated after 120 seconds of the
cooling stage
Rys. 10. Warstwa napawana nr 6. Obliczone numerycznie
pole temperatur po upływie 120 sekund fazy stygnięcia
4. DEVELOPMENT OF THE GEOMETRIC MODEL CONCEPT AND
GENERATION OF THE COMPUTATION MESH FOR WELDING
PROCESS MODELLING
One of the most important stages of the modelling process was the adoption
of an optimal, from the practical point of view, geometrical model of the welded
joint. The final shape of the geometrical model was decided by the welding
practice and repair regulations of welded joints by means of the temper bead
technique. The following course of reasoning was approved. In a hypothetical
structural part under service (e.g. a power plant structure) a crack has been
detected, which causes a risk for the farther save operation. The defect has to be
repaired by welding (Fig. 11).
Fig. 11. Example of a crack in a hypothetical
structural part and its removal by machining
[10]
Rys. 11. Przykład pęknięcia w hipotetycznej
konstrukcji i jego usunięcie za pomocą
obróbki mechanicznej [10]
For numerical calculations the geometric model shown in Fig. 12 and 13 was
applied.
Fig. 12. General view of geometric model with
the finite element mesh divided into: 1 – base
material, 2 – melted weld metal, 3 – solidified weld
metal
Rys. 12. Widok ogólny modelu geometrycznego po
podziale na elementy skończone z zaznaczeniem:
1- materiału podstawowego, 2 – materiału
stopionego, 3 – materiału stopiwa po
przekrystalizowaniu
Fig. 13. 3-D model as in Fig. 12 with
the finite element mesh
Rys. 13. Model przestrzenny jak na rys. 12
z podziałem na elementy skończone
5. MODELLING OF THERMAL FIELDS IN THE HAZ
AT WELDING CONDITIONS
The following input data have been taken for numerical calculations.
a) General data:
- base material (1): unalloyed steel,
- filler metal (3): alloyed steel,
- welding method: TIG, without preheating, six layers in the weld,
- plate thickness: t = 40 mm.
b) Technological data: welding parameters as in Table 1.
c) Physical properties: room- and elevated temperature data as given in Tables 2
and 3.
6. NUMERICAL MODELLING RESULTS FOR WELDING
Results of numerical calculations for the welded layer No. 1 are presented in
Fig. 14 ÷ 16, for the layer No. 3 – in Fig. 17 and 18, and for the layer No. 6 – in
Fig. 19 ÷ 22.
Fig. 14. Welded layer No 1. Temperature
distribution in nodes and temperature field
Rys. 14. Warstwa spawana nr 1. Rozkład
temperatur w węzłach wraz z polem temperatur
Fig. 15. Welded layer No 1. Thermal cycle in a
node at a distance of 2,8 mm from the welding
groove bottom with a temperature of 1318 oC
Rys. 15. Warstwa spawana nr 1. Widok ogólny
cyklu cieplnego wybranego w węźle w
odległości 2,8 mm od dna rowka spawalniczego
o temperaturze maksymalnej 1318 oC
Fig. 16. Welded layer No 1. Determination of
t8/5 cooling time for the thermal cycle in Fig. 15
Rys. 16. Warstwa spawana nr 1. Wyznaczenie
czasu stygnięcia t8/5 dla cyklu cieplnego
z rys. 15
Fig. 17. Welded layer No 3. The weld
penetration depth into the base material
determined by the 1500 oC isotherm
Rys. 17. Warstwa spawana nr 3. Zasada
sprawdzania głębokości wtopienia spoiny
w materiał podstawowy w oparciu o izotermę
1500 oC
Fig. 18. Welded layer No 3. Heat flux
propagation 30 s after welding termination
Rys. 18. Warstwa spawana nr 3. Strumień
rozchodzenia się ciepła po zakończeniu
spawania (po czasie 30 s)
Fig. 19. Welded layer No 6. Temperature
distribution in nodes
Rys. 19. Warstwa spawana nr 6. Rozkład
temperatur w węzłach
Fig. 21. Welded layer No 6. The temperature
Fig. 20. Welded layer No 6. Thermal cycles in
field after 30 s from start of welding
nodes with a maximum temperature of 573, 500
Rys. 21. Warstwa spawana nr 6. Pole temperatur
and 449 oC
Rys. 20. Warstwa spawana nr 6. Widok ogólny po upływie 30 sekund od momentu rozpoczęcia
spawania
cykli cieplnych w węzłach o temperaturze
maksymalnej 573, 500 i 449 oC
Fig. 22. Welded layer No 6. Sector of the 3-D model.
Calculated thermal field for the 6-th layer after 125 s
of the cooling phase
Rys. 22. Warstwa spawana nr 6. Wycinek modelu
przestrzennego. Obliczone numerycznie pole
temperatur dla szóstej warstwy po upływie 125
sekund fazy stygnięcia
7. DISCUSSION OF RESULTS
In Table 5 thermal cycle parameters (maximum temperature Tmax and t8/5
cooling time) in the HAZ region, obtained by numerical calculations for overlay
welding and welding, are compared with those determined experimentally.
The data obtained by numerical modelling and presented in Table 5, enable
anticipation of thermal processes development in welded joints. From data in
Table 5 it is evident, that beginning from the 4th layer up to the 6th layer, in the
HAZ under the weld the maximum temperatures of the thermal cycles are lower
than 800 oC, and the analysis of the increasing cooling time is not justified from
the point of view of metal science.
Table 5
Comparison of overlay welding and welding thermal cycle parameters for the HAZ region
obtained by numerical calculations and determined experimentally [10]
Porównanie parametrów cyklu cieplnego napawania i spawania w obszarach SWC uzyskanych
numerycznie i eksperymentalnie przy napawaniu / spawaniu [10]
Layer No
in the
overlay
weld /
weld
Welding thermal cycle parameters determined:
numerically
overlay welding
Tmax
[oC]
t8/5/CT
[s]
d
[mm]
1310
30
0,5
1190
31
1,5
1095
32
2,5
1052
37
1,0
1009
37
1,5
968
38
2,0
875
39
1,0
804
46
2,0
740
58
3,0
4
685
61
5
522
6
440
1
2
3
experimentally
for overlay welding
welding
Tmax
[oC]
t8/5/CT
[s]
d
[mm]
Tmax
[oC]
t8/5/CT
[s]
d1
[mm]
1318
22
2,8
1336
20
1,2
1031
26
1,4
1008
30
1,9
808
48
2,1
783
60
2,8
1,0
674 /
728 1)
91
0,7
684
66
3,5
117
1,0
649 /
810 1)
72
0,7
628
69
4,2
-
2,0
449 /
880 1)
-
2,1
442
73
5,0
Where: d – distance from the weld groove bottom into the HAZ,
d1 – distance from the thermocouple tip to the fusion line,
CT – cooling time for thermal cycles with the maximum temperature
Tmax lower than 800 oC, determined for the Tmax – 500 oC
temperature range,
1)
– maximum temperature value of the thermal cycle after taking into
consideration the thermal inertia.
In real welding conditions the thermal field shows asymmetry, which is
caused by heat transfer during welding of consecutive beads in the layer. An
attempt was made in numerical calculations to find, how does the thermal field
asymmetry influence the thermal conditions, which accompany the heating and
cooling process of an overlay-welded specimen. An example of thermal field
asymmetry in an overlay-welded layer is shown in Fig. 23 and 24.
Fig. 23. Numerically calculated temperature
distribution, which illustrates the thermal
field asymmetry as a result of heating during
welding of consecutive beads in layer No 1
after 40 seconds
Rys. 23. Obliczony numerycznie rozkład
temperatur ilustrujący asymetrię pola
powstającą w wyniku nagrzewania podczas
układania kolejnych ściegów napoin
w warstwie 1 po upływie 40 sekund
Fig. 24. Numerically calculated
temperature distribution in layer No 1
after 120 seconds of the cooling phase
Rys. 24. Numeryczny rozkład temperatury
w warstwie 1 po upływie 120 sekund fazy
stygnięcia
Figure 23 shows, that just after finishing the overlay-welding of six beads
in the first layer a differentiation of temperatures occurs, the highest temperature
is recorded in the last bead (on the right), and the lowest – in the first one (on the
left in Fig. 23). After 120 seconds of the cooling process the temperatures level
out in the hot specimen, and the asymmetry in the temperature field distribution
decays (see Fig. 24). It results, that the analysis of thermal processes in the
central part of the overlay-welded specimen can be done without taking into
consideration the influence of the temperature field asymmetry (the reading of
temperature values and thermal cycles is done along the vertical symmetry axis
of the overlay-welded specimen).
Beginning with the layer No 4, the effect of thermal inertia was observed.
For example, at the thermal cycle selected in a node with a maximum
temperature of 649 oC (Fig. 25), the temperature still rises and reaches the
maximum value of 815 oC. The effect of thermal inertia can be explained by
changes of the thermal conductivity KXX (λ) with temperature, as shown in
Fig. 26.
Fig. 25. Welded layer No 5. Thermal cycle
in a node with Tmax=649 oC and the effect
of thermal inertia
Rys. 25. Warstwa spawana nr 5. Widok
ogólny cyklu cieplnego wybranego
w węźle o Tmax = 649 oC. Widoczne
zjawisko bezwładności cieplnej
Fig. 26. Occurrence of the effect of thermal inertia in
dependence on the thermal conductivity value λ
of the material [7]
Rys. 26. Występowanie zjawiska bezwładności
cieplnej w zależności od wartości współczynnika
przewodzenia ciepła λ materiału [7]
It follows from Fig. 26a, that for materials characterized by great thermal
conductivity values (λ >> 1) the effect of thermal inertia does not occur. But the
effect can be observed for materials with thermal conductivity values λ much
lower than 1 at 20 oC (Fig. 26b) and its further decrease with increasing
temperature [7].
From comparison of numerically calculated overlay welding data with those
determined experimentally for the individual layers (Table 5) it is apparent, that
the thermal cycle parameters (maximum temperature Tmax and t8/5 / CT cooling
times) show nearly similar values.
From the results of numerical calculations performed for the welding process
it can be seen, that the cooling time increases for the consecutive welded layers.
8. CONCLUSIONS
1. The upper part of the HAZ cooling curve is characterized by the highest heat
abstraction, that is, the greatest temperature gradient, which slows down with
time (Fig. 15) and has an effect on the temporary conditions of allotropic
structural transformations and precipitations which occur at high, medium and
lower temperatures.
2. When welding a low carbon steel, the available austenite (γ) → ferrite (α)
transformation time is shorter than that available for the bainite transformation,
which occurs at lower temperatures. That is why the fraction of hardening
microstructures, especially bainite, can increase, which may deteriorate the HAZ
toughness.
3. By numerical modelling, values of thermal cycle parameters (maximum
temperatures and cooling times) comparable with the experimental ones have
been obtained. It can be concluded therefore, that on the basis of numerical
modelling results, the development and course of thermal processes in real
welded joints can be anticipated.
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Recenzent: prof. dr hab. inż. Edmund Tasak
MODELOWANIE NUMERYCZNE CYKLI CIEPLNYCH W STALI PODCZAS
NAPAWANIA I SPAWANIA BEZ PODGRZEWANIA WSTĘPNEGO
W ODNIESIENIU DO DANYCH EKSPERYMENTALNYCH
Streszczenie
Przedstawiono proces modelowania i wyniki obliczeń numerycznych przy użyciu programu
ANSYS 5.6. Przeprowadzono modelowanie pól temperatur w obszarach strefy wpływu ciepła
(SWC) stali w warunkach oddziaływania wielokrotnych cykli cieplnych cykli cieplnych spawania
o różnych parametrach. Badane modele matematyczne odwzorowywały proces napawania
i spawania wielowarstwowego w wariancie bez podgrzewania wstępnego. Utworzone modele
geometryczne są modelami płaskimi o cechach przestrzenności. Liczba napawanych i spawanych
warstw zmieniała się od 1 do 6. Po przeprowadzeniu obliczeń numerycznych dla poszczególnych
napawanych i spawanych warstw analizowano przebiegi cykli cieplnych dla pierwszej, trzeciej
i szóstej warstwy napoiny lub spoiny. Taki sposób postępowania wynikał głównie z praktycznego
wykorzystania wyników obliczeń. Przepisy ASME zalecają wykonanie minimum sześciu warstw
spawanych lub napawanych w celu uzyskania pożądanego efektu odpuszczania struktur
hartowniczych, które występują w obszarze SWC i charakteryzują się niekorzystnymi
własnościami plastycznymi. W rozdziale dotyczącym analizy wyników badań m.in. dokonano
porównania parametrów cyklu cieplnego napawania i spawania w obszarach SWC stali, które
uzyskano numerycznie z analogicznymi parametrami uzyskanymi eksperymentalnie. Ponadto
poruszono zagadnienie zjawiska niesymetryczności temperatury, które występuje w wyniku
nagrzewania podczas układania kolejnych ściegów napoiny.
Słowa kluczowe: cykl cieplny spawania, strefa wpływu ciepła, czas chłodzenia, modelowanie
numeryczne