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Cooling rate in 800 to 500uC range fromdimensional analysis
A. Arora*1, G. G. Roy2 and T. DebRoy1
Structure and properties of steel welds are affected by their cooling rates in the 800 to 500 uC
range. The available cooling rate correlations are based mostly on heat conduction equation that
ignores convective heat transfer and are inaccurate or empirical in nature and valid only for a
limited range of welding conditions. Numerical heat and fluid flow models can accurately
calculate cooling rates, but they are not widely available. In the present paper, the authors
propose and test a generalised correlation for cooling rate developed through dimensional
analysis, valid for a wide range of welding processes, operating parameters and materialproperties.
Keywords: Cooling rate, Dimensionless correlation, Arc welding, Laser welding, Steels
Introduction
The properties of steel welds13 are affected by theircooling rates in the 800 to 500uC range where the phase
transformations important for the evolution of final
weld microstructure occur.4 The cooling rates of welded
samples are often calculated using analytical solutions of
heat conduction equations,5
because they are widelyavailable and easy to use. For three-dimensional heat
flow, the cooling rate R is commonly obtained from the
heat conduction equation as follows5
R~2pk T{Ti
2
Qg(1)
where kis the thermal conductivity of the weld material,T is the temperature at which the cooling rate is
calculated, Ti is the interpass temperature, Q is the heat
input per unit length of weld deposit and g is the arc
efficiency. However, Svensson et al.4 have observed that
such calculations based on heat conduction model areinadequate in representing experimental cooling curves
for a wide range of welding conditions. It is also worth
noting that other heat flow models such as that of
Rosenthal6 have also been found inappropriate for
application to the fusion zone.4
Calculations that ignore convective heat transfer withinthe weld pool overestimate the spatial variation of
temperature because they do not consider the mixing of
relatively hot and cold liquid metal. As a result, these
calculations also overestimate the cooling rate, which is
the product of the spatial gradient of temperature and the
solidification velocity. It will be shown subsequently in
this paper that the errors resulting from the use of heatconduction models can be unacceptably high becauseconvection is often the dominant mechanism of heattransfer within the weld pool. The significant discrepan-cies between the cooling rates computed from thecorrelation based on heat conduction model5 and theexperimentally measured cooling rate values available in
reference 4 are shown in Fig. 1. The cooling rates fromthe heat conduction equation required values of boththermal conductivity and arc efficiency. The values of arcefficiency were taken from the values provided inSvensson et al.4 However, Ref. 4 did not include anyvalue of thermal conductivity. In the calculations withheat conduction equation the authors used a value of27 W m21 K21 for simplicity. It should be noted that theuncertainty in the thermal conductivity values is insignif-icant when compared with the errors in the computedvalues of the cooling rates in Fig. 1. In fact, thermalconductivity value used is near the lower end of thetemperature dependent thermal conductivity of steels. A
higher value of thermal conductivity would result in evenhigher discrepancy between the experimentally measuredcooling rate4 and the values computed5 from the heatconduction equation. It can be observed that heatconduction model5 overestimates the experimental cool-ing rate4 by a factor of 25 in many cases.
Because of the difficulties in obtaining reliable coolingrates from heat conduction models, Svensson et al.4
suggested the following form of empirical correlationsfor computing cooling rate R
R~C1 DT
C2
Qg(2)
where DT is the temperature difference, and C1 and C2are the empirical constants whose values vary dependingon the welding processes and parameters. They pro-posed 27 sets of values for C1 and C2 for various weldingconditions. Each set of values of C1 and C2 is valid for aspecific set of welding conditions considered in their
1Department of Materials Science and Engineering, The PennsylvaniaState University, University Park, PA 16802, USA
*Corresponding author: email [email protected]
2Department of Metallurgical and Materials Engineering, Indian Institute ofTechnology, Kharagpur, India
2010 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 30 April 2010; accepted 8 June 2010DOI 10.1179/136217110X12720264008394 Science and Technology of Welding and Joining 2010 VOL 15 NO 5 42 3
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empirical correlations, and cannot be used for weldingconditions other than the ones used to derive these
correlations. Furthermore, the 27 correlations covered
cooling rates only up to 48 K s21. In other words,equation (2) is not applicable to many of the high energy
density processes that use a high welding speed andinvolve a high cooling rate.
The values of Peclet number for heat transfer withinthe liquid pool, which represents the ratio of heat
transfer by convection to conduction, have been found
to be much greater than one in all welding processes,especially for low thermal conductivity materials such as
steels.712 Although several numerical models of con-vective heat transfer and fluid flow have been reported in
the literature that can accurately calculate cooling rates,these models are complex and not always widely
available. Therefore, there is a need for a generalisedcorrelation that can calculate reliable cooling rates for a
wide range of welding processes, workpiece materials
and welding parameters.
The nature of the fluid flow and the resulting
convective heat transfer affects the temperature profilesin the weld pool and its geometry. The weld geometry can
be characterised by its aspect ratio which is the ratio of
the width to depth of the weld pool. The aspect ratioaffects the cooling rates for various welding processes.
The aspect ratio depends on the type of material, thenature of the heat sources for various welding processes,
the power density of the heat source and the nature andamount of the surface active elements. The presence of a
surface active element in steel reverses the fluid motionwithin the weld pool and makes it narrow and deep, with
a low aspect ratio. In many cases, a high energy density
welding process such as a keyhole mode laser or electronbeam welding process produces a deep and narrow weld
pool. A weld pool with low aspect ratio tends to dissipateheat primarily in the width and length directions. Thesedirections are often larger than the thickness of the
workpiece, and a low aspect ratio may result in a low
cooling rate for a given welding speed. On the contrary, asmall hemispherical pool may dissipate heat in all three
dimensions yielding a high cooling rate.13
Here the authors propose and test a non-dimensional
correlation of cooling rate. The proposed correlation
considers convective heat transfer in the weld pool
indirectly by considering the aspect ratio of the weld
pool as an independent variable. It is shown that the
proposed correlation can overcome the overestimations
of cooling rates by the heat conduction equations.
Furthermore, the proposed correlation is shown to be
fairly accurate for a wide range of cooling rates resultingfrom various welding processes, welding variables and
workpiece material properties.
Dimensional analysis
Buckingham-p theorem was used to determine the
appropriate dimensionless numbers that affect the cool-
ing rate. This theorem states that an equation with n
number of variables each expressed by k fundamental
units, such as mass M, length L, time Tand temperature
h can also be expressed by n2k dimensionless num-
bers.
14
In this study, the following eight variables thataffect the cooling rate are defined with symbols and
dimensions in terms of fundamental units within
parenthesis. These are density of the material r
(ML23), thermal conductivity of the solid k
(MLT23h21), specific heat of solid CP (L2T22h21),
temperature at which cooling rate is calculated minusthe preheat temperature T2T0 (h) and absorbed heat
input per unit length q5gQ (MLT22). The other
variables are depth D (L), width W (L) of the weld
pool and the cooling rate R (hT21). Considering four
fundamental units M, T, L and h, the authors can derive
the four dimensionless numbers, as defined below
p1~W
D(3)
p2~k
rWC1:5
P T{To 0:5
(4)
p3~q
W2rCP T{To (5)
p4~RW
C0:5
P T{To 1:5
(6)
Two meaningful dimensionless numbers p1 and p2 canbe derived from these four dimensionless numbers,
where p1~W=D, which is the aspect ratio of the pool,
and p2~Rq= k T{To 2
h i. Combining the relevant
dimensionless numbers, namely, p1,p2, the authors get
an expression of reference cooling rate at any tempera-
ture T as follows
Rref~k T{To
2
q
W
D
(7)
Average time for cooling from T1 to T2 may be
calculated by integrating equation (7) with respect totime and may be expressed as follows
DtT1=T2~2 1= T2{To {1= T1{To
k=q W=D (8)
The average cooling rate between T1 to T2 may be
expressed as
1 Comparison of average experimental cooling rates4 in
temperature range of 800500uC with estimated cooling
rates using heat conduction model correlation5
Aro ra et al. Cooling rate in 800 to 500uC range from dimensional analysis
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Raverage~ak T1{T2
2q 1= T2{T0 {1= T1{T0
W
D
b(9)
The empirical constants a and b are introduced forfitting equation (9) with the experimental data.
Results and discussion
The values of the constants a and b were determined byplotting the experimental average cooling rate4,1524
between 800 and 500uC (R8/5) against the expression inthe parenthesis on right hand side in equation (9). The
data used for the calculation are provided in Table 1. It
is assumed that the cooling rates in the fusion zone donot vary spatially. This means that cooling rate at one
location in the weld pool can be assumed to be same as
that at a location away in the same weld pool. A powerlaw fit, represented by the solid line in Fig. 2, yieldeda514?491 and b50?477. The final expression for theaverage cooling rate R8/5 is as follows
R8=5~14:491
300 k
2q 1= 1073{T0 {1= 773{T
W
D
0:477(10)
where T0 is the preheat temperature in Kelvin.
Table 1 Data used for calculation of cooling rate for experimental cases considered
No.Heat input per unitlength q, J m21
Preheat temperatureT0, K
Aspect ratioW/D
Thermal conductivityk, W m21 K21 Reference
1 2?796106 298 0?6 27 42 2?796106 373 0?6 27 43 2?796106 473 0?6 27 4
4 2?
776
10
6
523 0?
6 27 45 1?746106 298 0?8 27 46 1?756106 373 0?6 27 47 1?756106 523 0?6 27 48 8?516105 473 0?6 27 49 9?306105 523 0?6 27 410 2?526106 298 0?8 26?8 1511 3?946106 298 1?45 27?1 1612 3?156106 298 1?5 27?1 1613 2?406106 298 1?6 27?1 1614 4?646105 298 1?5 30 1715 1?446106 298 1?2 21 1816 2?256106 298 0?9 21 1917 3?736106 298 1?3 27 2018 3?516106 298 1?3 27 20
19 3?076106
298 1?3 27 2020 2?136106 298 1?3 27 2021 2?006106 298 1?3 27 2022 1?756106 298 1?3 27 2023 1?616106 298 1?3 27 2024 1?51610
6 298 1?3 27 2025 1?326106 298 1?3 27 2026 1?306106 298 1?3 27 2027 1?226106 298 1?3 27 2028 1?066106 298 1?3 27 2029 2?896106 298 1?60 36?4 2130 6?986105 298 1?5 40 2231 3?786105 298 1?2 40 2232 6?686105 298 1?20 40 2233 1?316106 298 1?15 40 22
34 1?686106 473 1?10 40 2235 7?046105 298 1?50 40 2236 1?356106 298 0?8 40 2237 1?556106 298 1?50 21 2338 1?666106 298 1?45 21 2339 1?806106 298 1?40 21 2340 1?926106 298 1?40 21 2341 2?056106 298 1?35 21 2342 2?17610
6 298 1?25 21 2343 2?306106 298 1?25 21 2344 1?55610
6 298 1?50 21 2345 1?666106 298 1?45 21 2346 1?806106 298 1?40 21 2347 1?926106 298 1?35 21 2348 2?056106 298 1?30 21 2349 2?756104 298 0?45 27 2450 3?676104 298 0?37 27 2451 4?596104 298 0?36 27 2452 5?526104 298 0?32 27 2453 6?446104 298 0?30 27 2454 7?366104 298 0?33 27 2455 8?206104 298 0?33 27 24
Aro ra et al. Cooling rate in 800 to 500uC range from dimensional analysis
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Figure 3 shows the computed cooling rates forvarious aspect ratios of the weld pool. Here it is
observed that the ratio of cooling rate estimation by
equation (10) to the experimental cooling rate is close to
1 for all values of the aspect ratio. This means that the
proposed correlation in equation (10) estimates thecooling rates that are in good agreement with
the experimentally measured cooling rates. In contrast,the ratio of cooling rate estimated by the heat
conduction model to the experimental cooling rate is
much greater than 1 for many cases. Especially, the
cooling rates estimated from the heat conduction model
are 1520 times greater than the experimental cooling
rates for aspect ratios ,0?5, i.e. for the case of keyhole
mode of welding. In situations where the keyhole forms,the conduction equation can be modified by consideringa line heat source and the cooling rates will be somewhatdifferent from those shown in Fig. 3. However, thecooling rates calculated by equation (1) for high aspectratios also do not produce satisfactory cooling rates asobserved from Fig. 3. In contrast, cooling rates com-puted from equation (10) agree well with the experi-
mental values of cooling rates both for the keyhole modewelding with aspect ratio ,0?5 and for other welds withmuch higher aspect ratios. The shape of the weld poolsometimes deviates considerably from a simple hemi-sphere. For such unusual shapes,12,25 where W/D maynot fully represent the weld pool shape, equation (10)may not be strictly valid.
The calculated average cooling rates R8/5 fromequation (10) are plotted against the reported experi-mental cooling rates in Fig. 4. The computed valuesrepresent heat input values in the range of 36104 to46106 J m21 and cooling rates from 5 to y120 K s21.It is observed that satisfactory agreement is achieved
between the cooling rates calculated from equation (10)and the experimental cooling rates. A linear fit with unitslope is shown in Fig. 4. The correlation coefficient R2
for the data shown in Fig. 3 is 0?965, where a R2 value of1 would mean an exact match between the estimated andthe experimental values.
Conclusions
A correlation based on dimensional analysis is proposedfor the calculation of cooling rates in the 800 to 500uCrange for steel welds. The correlation is tested for a widerange of welding processes, operating parameters and
material properties by comparing the computed coolingrates with the corresponding experimental results. Thecorrelation has been tested for heat input range between36104 and 46106 J m21, and cooling rate values fromy5 to 120 K s21. The fit between the experimentalcooling rates and the values computed from the proposedequation showed a correlation with R2 value of 0?965.
2 Power law correlation between experimental average
cooling rate4,1524 from 800 to 500uC and expression in
x axis: all symbols are defined in text
3 Ratio of average cooling rate between 800 and 500uC
estimations by heat conduction model5 to experimental
cooling rate4,1524 and ratio of estimation by equa-
tion (10) to experimental cooling rates,4,1524 with
respect to weld pool aspect ratio W/D
4 Comparison of estimations of average cooling rates
between 800 and 500uC by heat conduction model and
proposed correlation (equation (10)) with respect to
experimental cooling rates: experimental cooling rates
are for various welding processes from literature4,1524
Aro ra et al. Cooling rate in 800 to 500uC range from dimensional analysis
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Science and Technology of Welding and Joining 2010 VOL 15 NO 5 42 7
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