Application of Numerical Simulation Technique to Casting Process of Valve Block

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-:;", ScienceDirect JOURNAL OF IRON AND SfEEL RESEARCH. INTERNATIONAL. 2009. 16(4): 12-17

Application of Numerical Simulation Technique to Casting Process of Valve Block MI Guo-fa' ,

LIU Xiang-yu" ,

WANG Kuang-fei' ,

FU Heng-zhi'

(1. School of Materials Science and Engineering. Henan Polytechnic University. Iiaozuo 454000. Henan. China; 2. Department of Mechanical Engineering. Chengde Petroleum College. Chengde 067000. Hebei , China)

Abstract: The numerical simulation technique was applied to the casting process of a valve-type part. The mold-filling and solidification stages of the casting were numerically analyzed. The filling behavior. solidification sequence. and thermal stress distribution were reproduced and the possible defects. such as cold shut and shrinkage. were predicted. Based on the simulation result. the double-gating system was replaced by a single-gating system. Meanwhile. the chills were used to regulate the solidification sequence of casting. To eliminate the cracks in the casting. the sand core was converted into a canulate one. By modifying the original process. the defects were eliminated and the casting with good quality was obtained. Key words: cast steel; valve-type part; numerical simulation; process modification

Valve-type parts are the nuclear components of power engineering, the properties of which influence the operation of assembling unit[l-3]. Comparing with other types of casting, the valve-type casting either possesses complex configuration such as flanges, bosses, and ribs, or possesses complex internal structure and conspicuous wall unevenness. These characteristics easily cause the defects such as crack. shrinkage, and dispersed shrinkage. Meanwhile. the valve-type parts usually service in the caustic media under high pressure and temperature, and clearly, stringent demands are placed on them to prevent leakage accidents[4.5]. The computer-based simulation technique shows great advantages over the conventional trialand-error methodologies for design and optimization, and more and more enterprises adopt this powerful tool. Using this method, the shrinkage defects can be forecast efficiently and its accuracy can reach a quantitative level. The occurrence of defects during the mold-filling stage such as entrapped air, entrapped slag, and cold shut can also be predicted. In the past. several scholars carried out researches in this field[6-IO]. In the present study, the numerical

simulation system ViewCast was used to simulate the temperature. velocity and stress fields during the filling and solidification stages of steel valve-body casting. The aim of the present study is to predict the location and volume of the defects, and then the original process is modified to improve the quality of casting. An ammonia valve-body with a mass of 180 kg was cast from 30 steel. The materials of molds and cores were resign-bonded sand and sodium silicatebonded sand, respectively. The pouring temperature was 1 600 "C. After the castings solidified entirely, some cracks were found at the root segment of the flanges and several castings were even damaged by the through crack, as shown in Fig. 1.

Fig. 1

(a) Outside; (b) Inside Crack passed through valve body casting

Foundation Item: Item Sponsored by the Innovation Fund for Outstanding Scholar of Henan Province of China (0621000700) E-mail: peter@hpu. edu, en; Revised Date: April 18. 2008

Biegraphy sMl Guo-fat 1966-). Male. Doctor. Professor;

Application of Numerical Simulation Technique to Casting Process of Valve Block

Issue 4

In the original scheme, the designers adopted two downsprues , which were placed at the rims of two large flanges, as shown in Fig. 2. Four flat top risers were placed at the top of the flanges.

• 13 •

is the pressure; C; is the specific heat of molten metal; A is the thermal conductivity; T is the temperature; L is the latent heat; and I, is the solid phase ratio at the solidification stage. 1. 2

Geometric model The three-dimensional geometric model of steel valve-body was input into the ViewCast system and then translated into the FD (finite difference) model, which consisted of 10 000 000 FD meshes, as shown in Fig. 3 (b). The step length of mesh generation was self-adjusting and the thinnest position of the casting was divided into at least three meshes to ensure the accuracy of numerical simulation. 1. 3 Fig. 2

1

Initial parameters of calculation The material properties of metal and mold are listed in Table 1 and Table 2, respectively.

Diagrammatic chart of original scheme

2

Experimental

1. 1

Mathematical model The flow of liquid metal is assumed to be incompressible Newtonian fluid and the governing equations at the filling and solidification stages are given below. Navier-stokes equation au au au au, p:.,-- + u -J + v -) + w ::;- - I = - -) + pg + ( at (X 'y o z: (X 2 , a2u + J2 U + a u I P,ax (1) I 2 J y 2 J Z2

«»

, av + u J v + v av + w av 'l =_ap+ g p [ at ax ay Jz, Jy P y

C I aT +u aT +v JT +w aT] p P at Jx Jy az 2T] AI' aZTz +JzT+a z +L JI. ay2 az ax at

Original scheme Fig. 4 reveals the simulation result of mold filling. From Fig. 4 (b), it can be seen that two fluid streams merge at the bottom of the valve-body. which is the thinnest region in the casting. Owing to the heat transmission between the liquid metal and mold, the temperature of liquid front falls sharply.

(2)

ap p ~w+u dW+ v dU!+w aw:, =--+pg.+ at ax ay Jz! az

au + Jv + Jw =0 Jx Jy dz Heat transfer equation

2. 1

+

J2v+az~+azvl P I ax 2 ayz az2 , a2w + azw + aZwj P,I ax z Jyz azz " Continuity equation

Simulation Results and Discussion

(3)

(a) Three-dimensional model;

Fig. 3 Table 1

(4)

Material property of 30 steel

Material property

Value

Density/j kg

7870

e

Viscosity/r kg

rn ,) >

m-

I • ,-I)

Specific heat/ (j • kg

1 •

where, p is the density; u , u , and ware the velocity vectors; t is the time; P is the dynamic viscosity of the liquid metal; g,r' gy' and g. are the gravitational acceleration vectors in the directions x , y, and z; P

Latent heat/ (j • kg-

6. 2X 10 '

K-

Thermal conductivity/f W

(5)

( b) Meshed model

Models of valve body casting

I)

s

172

1)

m- I

•

K

I)

50. 7 0.255

Modulus of elasticity/GPa

205

Shear modulus/GPa

80

Liquidus temperature/K

1 803.0

Solidus temperature/K

1 763.0

Poisson's ratio

0.29

Journal of Iron and Steel Research. International

• 14 •

Table 2 Resin-bonded sand

Material properties of mold Densit y /Lkg

>

m

1 600

3)

Specific heat/(j • kg-- I

i. 07

• K-I)

Thermal conductivity/(W· m- I Sodium silicate-bonded sand

Densit y/Lkg

v

m

Specific heat/ (J

t

6551


ss

(c)

K

I)

o. 7 2 583

I •

K -I

Coefficient of heat transfer between mold and casting/ (W • m -

(a) - ] .

•

")

• kg-

\. 202

)

Thermal conductivity/ (W • m - I

Fig. 4

Vol. 16

2 •

-5.6151

•

K-

K-

I)

I )

o. 36 580


Temperature distribution in mold-filling stage of original scheme

Consequently. this may cause the defects such as cold shut and misrun in the bottom of casting, and the turbulence of the liquid metal may cause the slag entrapped in the mold-filling stage. When the mold cavity was filled entirely. the solidification simulation followed and the results are shown in Fig. 5. Fig. 5 indicates the volume and position of shrinkage defects of the original scheme. It is seen that the feeding system is not effective and therefore some isolated liquid regions are created in the casting. These regions solidify slower than other positions of casting, and at the final stage of solidification, they do not have enough liquid metal to feed. As a result, the shrinkage defects are formed at these regions. The stress analysis was applied to the original scheme and the results are given in Fig. 6, which are the thermal stress distribution of the casting at 250 s. Fig. 7 depicts the temperature distribution at the same time. The results indicate that at 250 s after mould filling during solidification stage, the tensile stress zone is formed at the root segments of flanges (Fig. 6), at about 1 500 'C (Fig. 7), At this temperature, the metal is in the semi-solid state with poor strength, and the concentration of stress leads to the crack of the casting.

Process modification The double-gating system used in the original scheme caused hidden trouble of cold shut and mis-

run and the feeding system was not effective to feed the shrinkage of metal during the solidification stage. Consequently, the process was improved in three aspects, namely feeding system, gating systern, and mold.

Fig. 5

Viewer of shrinkage defect distribution

2. 2

Fig. 6

Stress distribution of the original scheme

Issue 4

Fig. 7

Application of Numerical Simulation Technique to Casting Process of Valve Block

Temperature distribution of original scheme

Fig. 8

The risers were placed on the crown face of flanges with pads, as shown in Fig. 2, which were difficult to cut. In the modified scheme, the risers were put on the flat surfaces of flange. To enhance the feeding capacity, the cylindrical risers were used instead of flat risers, because the former could supply more liquid metal during the solidification stage, as shown in Fig. 8 (a). For the purpose of enhancing the cooling capacities of hot spots, some chills were placed at the relevant positions in the mold. Fig. 8 (b) explains the positions of chills. The aim of the pouring system is to lead the liquid

Diagrammatic chart of riser

metal into the mold cavity rapidly and smoothly. From the simulation result, it can be seen that the faulty design of gating system led to underlying defects. Moreover, in the manual operation, the foundry workers found it difficult to ensure that two downsprues were filled at the same time using the double-gating system. Thus, In the modified scheme, the single-gating system has been adopted, which consists of pouring cup, downsprue , runners, and ingates , as shown in Fig. 9. Fig. 10 reveals the filling behavior of the liquid metal by use of the modified gating system, Using the single-gating system, the filling behavior of liquid metal becomes more reliable. The two runners are of the same size and have symmetrical distribution; therefore. the two liquid fronts reach the bottom of casting at the same time at a high temperature, higher than 1 550 ·C. This kind of filling mode can avoid the defects such as misrun and cold shut. Fig. 11 indicates the solidification sequence of casting using the risers and chills. The right color bars in the figure indicate the time and the unit is second. The solidified regions have been hidden for

• IS •

(a)

and chill

(b)

vivid observation. From Fig. 11, it can be found that the modified risers and chills can regulate the solidification sequence of casting effectively. Owing to the chilling effect, the bottom of the casting solidified first. By placing the chills at the root of the flanges. these areas also solidified faster than other regions. The solidification of the whole casting is from spherical surface to risers. There is ample hot metal in the risers, which can feed the shrinkage of liquid metal in the final stage of solidification. As a result, the volume of shrinkage defects can

Fig. 9

Diagrammatic chart of modified pouring system

Journal of Iron and Steel Research. International

• 16 •

(a) -2.69 s;

Fig.l0

(a) -100 s,

(b) -2.93 s;

(c) -3.65 s;

Vol. 16

(d) -13.3 s

Temperature distribution in mold-filling stage of modified scheme

(b) -200 s,

Fig. 11

CC) -300 s,

(d) -500 s,

(e) -800 s,

CD -1 100 s

Solidification time of critical regions in casting

be reduced to a certain amount. as shown in Fig. 12. Fig. 12 indicates that there are only two concentrated shrinkages in the casting and the rest are placed in the runners and risers. It can be concluded that the riser and 'chill are useful to eliminate the shrinkage defects in the casting. The structure of casting is complex. In more detail. there are hot spots at the junctions of flanges and spherical surface. Meanwhile. the placement of

Fig. 12 Position and volume of defects in casting with chill and modified riser

top risers on the flanges aggravates the hindering of the solid shrinkage of casting. According to practice. the inferior deformability of mold sand at an elevated temperature is the immediate cause of the casting's crack. In such a case. the infarctate sand core was converted to a canulate core (with a skeleton of metal in the sand), as shown in Fig. 13 (a). Meanwhile, to prevent the collapse of core sand, the cavity of the sand core was filled with dry sand. as shown in Fig. 13 (b).

(a) Canulate sand core,

Fig. 13

(b) Cavity of core filled with dry sand

Photograph of modified sand core

Application of Numerical Simulation Technique to Casting Process of Valve Block

Issue 4

2. 3

Practical pouring after processes modification Fig. 14 shows the valve-body produced by the modified process. It can be seen that the surface of the casting has an integrated, smooth, and bright surface. After observational check, no cracks were found. The process modification can be considered to be successful.

• 17 •

(4) The infarctate sand core was converted into a canulate core to eliminate the thermal stress during the solidification stage of casting, and a sound casting was obtained using the modified process. (5) The ViewCast system can be used well to design and calculate the running and feeding system of cast steel parts. The example of valve block in this study indicated that the ViewCast system can be applied to the cast steel industrial output and can supply a kind of operative technology for the development of view-cast technique. References: [IJ

(a) Outside;

Fig.14

3

(b) Inside

[2J

[n.

Valve body cast by modified process

Conclusions

The overall process from filling stage to solidification stage for the valve block was numerically analyzed. In addition, various solutions to eliminate or reduce the casting defects were implemented. The following conclusions can be drawn from the present study. (l) The temperature field, flow pattern, and stress distribution in the two stages were obtained, and three kinds of potential defects including cold shut shrinkage and crack in the casting were predicted. (2) The modification was applied to the structure of the pouring system, and temperature distribution during the filling stage was reasonably adjusted. and the defects, namely cold shut and misrun , were eliminated. (3) The combined use of chill and riser was good for establishing a reasonable temperature gradient, which is beneficial to feeding the shrinkage of casting.

GB/T 12224-2005. General Requirements for Industrial Steel Valves [SJ (in Chinese>' ZHAO Zhan-liang, LIU Ming-liang , LI An-shun. et al. The Casting Technology for Large Scale Ductile Valve and Valve Body Foundry Technology, 2003, 24(3): 246 (in Chinese), LU Meng-yu , CHEN Shi-xi, Improvement of Foundry Process of Size 36 Inch Big Butterfly Type Valve [n. Foundry Technology. 2004, 25(5): 383 (in Chinese). ZHU Iiu-cheng , TANG Bai-zhi. The Technology Measures of Settling Cracks of Large-Scale Rectangular Valve Body [n. Hot Working Technology, 2001, 30(4): 56 (in Chinese), LIU Bing, MENG Xian-jia , WANG Yong-qin. Research on the Relationship Between the Cracking and Inclusion in the Cast Steel and Decreasing Measure [J]. Foundry Technology, 2005. 26(0): 854 (in Chinese>. HUANG Yin-ping, MA Min-t uan , HAO Wei, et al. Carrying Out CAD&'CAE Engineering Optimized on Large-Scaled Complex Castings [n. Foundry, 2006, 55(2): 1255 (in Chinese).

[3J

[4J

[5J

[6J

[7J

[8J

[9J

[10J

WANG Jun-qing , SUN Xun , GUAN Yang, et al. Numerical Simulation and Process Optimization of Large Castings [n, Foundry, 2006, 55(9): 916 (in Chinese). XU Z A, Mampaey F. Experimental and Simulation Study on Mold Filling Coupled With Heat Transfer [J]. AFS Transactions, 1994, 102(43): 181. XU Z A, Mampaey F. Experimental and Simulation Study on Mold Filling With Various Gating Systems [J]. AFS Transactions, 1996, 104(79): 159. ZHAO Hai-dong, LIU Bai-cheng , LIU Wei-yu, et al. Numerical Simulation of Microstructure of Spheroidal Graphite Iron Castings [n. Chinese Journal of Mechanical Engineering, 2000, 36(2) :76 (in Chinese).