THE EGYPTIAN FOUNDRYMEN SOCIETY (EFS) & THE CENTRAL

METALLURGICAL R&D INSTITUTE (CMRDI) - EgyCast 2015

1

Modelling of centrifugally cast bi-metal roll using Procast

software

M. N. Naguib, C. A. Saleh2 and R.M. Rashad

3

1 , 2 and 3 Mechanical Design & Production Department

Faculty of Engineering, Cairo University

ABSTRACT

The results of mathematical model of bimetallic roll produced by centrifugal casting with a horizontal

axis of rotation are presented. The mathematical background for the thermal, mechanical and fluid flow

analysis is demonstrated. The temperature dependence of the parameters of the layers is taken into account.

The residual stresses in the radial and tangential direction are obtained and compared with experimental

results. The effect of pouring time, shake out time, mold preheat temperature, cooling media and mold

rotational speed were studied and their effect on the residual stresses are presented.

Key Words: Bimetal, Centrifugal casting, Numerical simulation, Procast

1. INTRODUCTION

Bimetals are metallic parts that are composed of two layers of metals. The interest of using and

studying bimetals is increasing because of the improvement of features they add such as wear

resistance, corrosion resistance and cost reduction. They are used as blades, wires and rolls.

The scope of this study focuses on bimetallic rolls used for grain grinding. The bimetallic rolls used

for grain grinding have a direct influence on the flour extraction rate and on the quality of the end

product. The rolls are subjected to wear; hence it should have an outer hard shell and a tough core.

To achieve these properties, the rolls are fabricated from white cast iron as the outer shell material

and grey cast iron for the inner core.

The rolls are produced using the centrifugal casting method. During the solidification process,

residual stresses are developed due to temperature gradients, mechanical constraints imposed by the

mold during shrinkage of the cast metal and the phase transformation accompanied by volume

change. The formations of residual stresses affect the stress states of components under operational

conditions. In bimetallic grain grinding rolls, the level of residual stress has a major effect on the

life of the rolls. The prediction of residual stress formation in bimetallic roll would provide a tool to

assess and improve the rolls design. In order to define the proper casting conditions many

prototypes are casted and these trials have high and unnecessary cost. This was the main reason

behind using the computer simulation techniques for the casting process.

2

2. RESIDUAL STRESSES IN CENTRIFUGAL CASTING

It has been established that residual stresses in castings arise due to inhomogeneous plastic

deformation[1]. The non-uniform deformations occur due to variation in cooling rates in different

parts of the casting, hindrance to free contraction by the mold material and phase transformations

accompanied by volume changes.

Residual stresses are also set up due to temperature differences between the surface and the interior

of the casting during cooling. The magnitude of the residual stress is related to the overall rate of

cooling, and residual stresses arising from this source attain appreciable magnitudes only under

conditions of rapid cooling, for example quenching. Compositional and microstructural

heterogeneity can also cause residual stresses.

In fatigue applications, residual stresses can increase the limit of safe alternating stress or variable

stress, by reducing the mean stress in the component. This effect is clearly shown in the Goodman

diagram of Figure 1, where Goodman line is used to define a safe operating envelope for

applications involving a combination of alternating and mean stress. The permissible stress

amplitude increases as the compressive residual stress in the component increases, which applies

directly to our case. The alternating bending stresses result from the combination of gravity forces,

rotational forces and pressure force on the roll. The study of the stresses on the roll during operation

is beyond the scope of this thesis.

Figure 1: Goodman line showing the effect of the residual stress.

3

3. CASTING SIMULATION

The increasing power of computers and development of numerical methods in last 25 years

has helped researchers to better understand the governing principles of various material processing

operations [2]

Numerical simulation covers the following aspects:

Prediction of the pattern of solidification [3], indicating where shrinkage cavities [4]and associated

defects may rise.[5][6]

Effect of different parameters on the castings.[7]

Investigation of new casting techniques and processes (e.g. electromagnetic casting [8] and casting

of functionally graded materials [9])

Validation of numerical codes or software and comparing their results with each other or with

experimental results [10]

4. RESEARCH IN CENTRIFUGAL CASTING OF BIMETALLIC ROLLS

Many published researches were published that study the centrifugal casting of cylinders and rolls,

but not many of them were dedicated to the study of bimetallic rolls. Murthy et al [11] have

developed a mathematical model for computing the metallurgical phase transformations and

residual stresses in centrifugal casting of bimetal grinding rolls used for coal pulverizing. Lu et al

[12] have used Procast software in order to reduce segregation and shrinkage holes present in wet

type cylinder line which is an important component in internal combustion engine. Georgiev[13]

has studied the mathematical modeling of specific processes of a double castings formation by the

method of centrifugal casting with a vertical axis of rotation and Nannan et al have simulated the

core filling process of cast high speed steel roll.The mold filling of centrifugal casting of HSS

compound rolls [14] and in another study they developed a program to study the solidification

process of horizontal centrifugal casting coupled with eutectic carbides segregation [15].

5. ROLL UNDER INVESTIGATION

The total length of the casting is 1070 mm, in order to obtain a clear length of 1000 mm drums. The

slag of the casting process represents about 70 mm length. The roll has outer and inner diameters of

129 and 49 mm respectively. The white cast iron thickness is about 25 mm and the grey cast iron

thickness is 55 mm.

The white cast iron layer is poured at 1420oC, the melting temperature is around 1270

oC.After the

white cast iron is poured in the mold, and when the inner surface temperature reaches 1150oC, the

grey cast iron layer is poured at 1420oC. The white cast iron at this case is in the mushy zone and

the grey cast iron is liquid. This results in diffusion of the grey iron in the white iron, which is

necessary to avoid any slipping between the layers in operation. The compound roll is left to cool in

the mold until the outer surface reaches a temperature of 500 degrees, and then it is shaken out to

reach room temperature. Figure 2 shows images for the rolls after production and finishing.

4

Figure 2: (a) Rolls after production (b) Rolls after finishing

6. NUMERICAL SIMULATION OF THE ROLL UNDER INVESTIGATION

The software used throughout this study is ProCAST. It is a FEM software that allows the

modeling of thermal heat transfer (heat flow), including fluid flow, mold filling, and stresses fully

coupled with the thermal solution

6.1 Thermal Model

A full assessment of heat transport in casting processes where liquid is present for part of the time

requires solution of heat, momentum and mass transport equations. However, in many applications

satisfactory accuracy can be obtained by considering only heat transport through conduction and

assuming that the liquid and solid regimes can be modeled as one domain[16]. The general form of

the heat conduction equation is shown in (1):

p

TC (T) k(T) T Q

t

(1)

where T is the temperature, k is the thermal conductivity, is the density, Cp is the specific heat and

Q is a volumetric heat source term associated with latent heat evolution.

6.2 Stress Model

To analyze the stress development in a component undergoing a phase transformation, the

thermo-mechanical behavior must be characterized. The different components of the thermo-

mechanical behavior are due to:

1) Metallurgical factors: the development of microstructure and its impact on the thermal field

5

2) Mechanical factors: the elastic/plastic behavior of the multiple phases developed in a

component

3) The interactions between stress/strain and phase transformation. [17]

For equilibrium to exist throughout a solid for three-dimensional problems, the following

differential equations must be satisfied:

xyx xzxf 0

x y z

(2)

yx y yz

yf 0x y z

(3)

zyzx zzf 0

x y z

(4)

Where fx, fy and fz are the components of the body force in the Cartesian coordinates.

The total macroscopic incremental strain, ijd for a material undergoing phase transformation

can be written in the form:

i e pl th tp

ij ij ij ij ijd d d d d (5)

wheree

ijd is the elastic strain, pl

ijd is the inelastic or plastic strainth

ijd is the thermal strain, and

tp

ijd is the transformation plasticity strain. The different relations for these strains are defined in the

following sections

6.3 Fuid Flow Model

The Navier-Stokes equations describe the motion of viscous fluid substances. These balance

equations arise from applying Newton's second law to fluid motion, together with the assumption

that the stress in the fluid is the sum of a diffusing viscous term (proportional to the gradient of

velocity) and a pressure term. In order to derive the equations of fluid motion, we must first derive

the continuity equation (which dictates conditions under which things are conserved), apply the

equation to conservation of mass and momentum, and finally combine the conservation equations

with a physical understanding of what a fluid is.

The most general form of the Navier-Stokes equation:

Dvp S f

Dt

(6)

The left hand side of the equation represents the force on each fluid particle and is equal to the

density multiplied by the normal derivative of the velocity. The normal derivative is the rate of

6

change the velocity on a particle in a velocity field. It must incorporate two things; the rate of

velocity change, dv

dt and

the change in position of other particle in the velocity field v

6.3 Modelling of the roll

The dimensions of the roll used are shown in figure 3. The type of model used is 2D axisymmetric

model. The total number of nodes is 7196, the total number of elements is 6740 and the element

type is wedge element.

Figure 3: Roll dimensions

The mold initial temperature is set to 70 ºC and it is air cooled by convection of 10 W/m2.oC. The

white cast iron layer is poured at 1420oC while its melting temperature is around 1270

oC. When the

inner surface temperature of the molten reaches 1150oC, (this is after 120 seconds from pouring the

outer layer) the grey cast iron layer is poured at 1420 oC also. So the white iron initial temperature

is set to 1420 ºC and air cooled. The same initial temperature was also used for the grey iron.. The

compound roll is left to cool in the mold until reaching a temperature of 500 degrees, and then it is

shaken out to reach room temperature. Air gap formation is taken into account

7

7. RESULTS AND DISCUSSION

The numerical results are obtained at the same conditions of casting process. The comparison

between the tangential and radial stresses obtained from the model and those measured

experimentally are shown in Figures 4 . The experimental measurements were obtained through

using the hole drilling method according to the procedure and steps listed in ASTM E-837

Figure 4: Comparison between the Numerical and Experimental Tangential and Radial Stresses at

different radii

7.1 Effect of Changing the Shakeout Temperature

The casting is originally shaken out of the mold after the outer surface reaches 500oC. The

simulation was run while changing the shakeout temperature to be 600oC and 400

oC. The residual

stresses are obtained and compared. Figures 5 show these comparisons

Figure 5: Comparison between the Numerical Results for the Tangential and Radial Stresses at

Different Shakeout Temperatures (MPa) Versus Radius (mm)

As shown above, the higher the shakeout temperature the higher are the residual stresses. This can

be explained this by the fact that the cooling rate of the casting is slower in the mold when

compared with cooling in the air

8

7.2 Effect of Changing the Mold Speed

The mold speed according to the production procedure is 1000 rpm. The simulation is run at speeds

700 rpm and 1300 rpm. The residual stresses are obtained and compared together. Figures 6 shows

these comparisons.

Figure 6: Comparison between the Numerical Results for the Tangential and Radial Stresses at

Different Mold Speeds (MPa) Versus Radius (mm)

The figure indicates that increasing the mold rotational speed increases the values of the stresses.

Increasing the rotating speed increases the solidification rate and consequently increases the stresses

7.3 Effect of Changing the Pouring Time of the inner metal

The inner metal is originally poured when the temperature of the inner surface of the outer

metal reaches 1150 o

C. The simulation is run at different pouring time i.e. at different temperature

of the inner surface of the outer metal. The time steps chosen are 80 sec and 120 sec (1200 o

C and

1100 o

C respectively). The residual stresses are obtained and compared together. Figure 7 shows

these comparisons

Figure 7: Comparison between the Numerical Results for the Tangential and Radial Stresses at

Different Inner Layer Pouring Times (MPa) Versus Radius (mm)

The arlier pouring time (i.e. pouring the inner material when the temperature of outer metal is

high) leads to decreasing the stresses due to the decrease of the thermal stresses and the overall

stresses.

9

7.4 Effect of Changing the the Cooling Medium

The mold is originally air cooled. The effect of changing the cooling medium from air to

water is studied and the results are shown in Figure 8

Figure 8: Comparison between the Numerical Results for the Tangential and Radial Stresses at

Different Cooling Media (MPa) Versus Radius (mm)

Cooling with water instead of air increases the stresses due to the fact that the cooling rate in case

of water is larger which will consequently lead to increase of the residual stresses.

7.5 Effect of Changing the Preheat Temperature

Originally, the mold temperature is 70oC. The mold is heated to 300

oC and the effect of the preheat

temperature is studied. Figures 5.26 and 5.27 show the effect of increasing the preheat temperature

on the tangential and radial stresses generated in the roll respectively.

Figure 9: Comparison between the Numerical Results for the Tangential and Radial Stresses at

Different Preheat Temperature (MPa) Versus Radius (mm)

Preheating the mold to 300oC instead of 70

oC decreases the level of residual stresses due to the fact

that preheating the mold will decrease the cooling rate

10

8. CONCLUSIONS

In the present investigation a bimetallic roll was studied experimentally and numerically. Hardness,

microstructure and residual stresses are obtained experimentally. Centrifugal casting of bimetallic

drum has been simulated. Residual stresses and cooling curves for different points within the drum

are obtained numerically. The mold speed and the pouring time of the inner metal are changed and

the effect of the change was demonstrated. The following conclusions are drawn from the

investigation:

1) The centrifugal casting process of bimetallic roll has been simulated using Procast software.

It is found that compressive tangential residual stresses are generated at the outer surface of

the roll. This reduces the corrosion effect and enhances the fatigue life.

2) The model shows compressive radial stress near the interface of the two metals. This

reduces the tendency for separation.

3) The residual stresses obtained from the numerical solution are compared with residual

stresses at six measured locations

4) Leaving the casting to cool inside the mold is found to reduce the residual stresses

5) The pouring timing of the inner layer has been studied. It is found that the change of pouring

timing, when the inner surface of the outer layer is still in the mushy zone-has effect on the

residual stress generated within the drum. The earlier the inner layer is poured the less are

the residual stresses

6) Changing the mold rotating speed and its effect on the residual stresses has been studied. It

is found that increasing the mold speed tends to increase the residual stresses.

7) Changing the cooling medium of the mold is studied. Instead of cooling with air the mold is

cooled with water and it is found the cooling with air will result in smaller residual stresses

11

9. REFERENCES

[1] C. Balasingh, M. R. Seshdari, M. N. Srinivasan, and S. Ramasehan, “Effect of carbon

equivalent and inoculation on residual stresses in grey iron castings,” J. Mech. Work.

Technol., vol. 9, pp. 53–66, 1984.

[2] S. Koric and B. G. Thomas, “Thermo-Mechanical Model of Solidification Processes with

Abaqus,” 2006.

[3] K. T. Hock, “Numerical simualation of sand casting process - Thesis,” no. November, 1987.

[4] A. S. Sabau, “Numerical Simulation of the Investment Casting Process,” vol. 160, no. 04, pp.

1–11, 2005.

[5] K. O. Yu, J. a. Oti, M. Robinson, and R. G. Carlson, “Solidification Modeling of Complex-

Shaped Single Crystal Turbine Airfoils,” Superalloys 1992 (Seventh Int. Symp., pp. 135–144,

1992.

[6] F. Bonollo, G. Chiarmetta, N. Gramegna, and P. Parona, “NUMERICAL SIMULATION OF

SEMI-SOLID CASTING OF AN AUTOMOTIVE COMPONENT.”

[7] E. Kaschnitz, “Numerical simulation of centrifugal casting of pipes,” IOP Conf. Ser. Mater.

Sci. Eng., vol. 33, p. 012031, Jul. 2012.

[8] P. Barral, a. Bermúdez, M. C. Muñiz, M. V. Otero, P. Quintela, and P. Salgado, “Numerical

simulation of some problems related to aluminium casting,” J. Mater. Process. Technol., vol.

142, no. 2, pp. 383–399, Nov. 2003.

[9] J. W. Gao and C. Y. Wang, “Modeling the solidification of functionally graded materials by

centrifugal casting,” Mater. Sci. Eng. A, vol. 292, no. 2, pp. 207–215, Nov. 2000.

[10] S. M. Afazov, a. a. Becker, and T. H. Hyde, “FE prediction of residual stresses of investment

casting in a Bottom Core Vane under equiaxed cooling,” J. Manuf. Process., vol. 13, no. 1,

pp. 30–40, Jan. 2011.

[11] Y. V. L. N. Murthy, G. V. Rao, and P. K. Iyer, “Analysis of Residual Stresses and

Metallurgical Phase Changes in Centrifugally Cast Bi-Metal Grinding Rolls,” Trans. ASME,

vol. 119, no. August, pp. 290–297, 1997.

[12] S. Lu, F. Xiao, S. Zhang, Y. Mao, and B. Liao, “Simulation study on the centrifugal casting

wet-type cylinder liner based on ProCAST,” Appl. Therm. Eng., vol. 73, no. 1, pp. 510–519,

2015.

[13] G. E. Georgiev and I. Georgiev, “Institute of Metal Science, Equipment and Technologies

with Hydroaerodynamics Centre ‘Akad. A. Balevski’, Bulgarian Academy of Sciences, 67

Shipchenski Prohod Blvd, Sofia 1574, Bulgaria,” vol. 1, no. 133, pp. 187–191, 2012.

[14] S. Nannan, L. Yikun, B. Yunlong, K. Xiuhong, X. Lijun, and L. Dianzhong, “Computer

simulation of core fi lling process of cast high speed steel roll,” vol. 6, no. 4, pp. 314–318,

2009.

12

[15] N. Song, Y. Luan, Y. Bai, Z. a. Xu, X. Kang, and D. Li, “Numerical Simulation of

Solidification of Work Roll in Centrifugal Casting Process,” J. Mater. Sci. Technol., vol. 28,

no. 2, pp. 147–154, Feb. 2012.

[16] M. Rappaz, “Modelling of microstructure formation in solidification processes no,” Int.

Mater. Rev., vol. 34, no. 3, 1989.

[17] S. Denis, “Considering Stress-Phase Transformation Interactions in the Calculation of Heat

Treatment Residual Stresses To cite this version :,” J. Phys. IV, vol. 6, pp. 159–174, 1996.