research on ball forging by ring rolling process · pdf fileto verify whether the proposed...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 12 (2016) pp 7823-7828

© Research India Publications. http://www.ripublication.com

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Research on ball forging by ring rolling process

Jong Hun Kang and Hyoung Woo Lee*

Assistant Professor, Department of Mechatronics Engineering, Jungwon University, Munmu-ro 85, Geosan Gun, Chungbuk,, South Korea.

Abstract

This study focuses on the forging of balls, a key component of

ball valves used in petrochemical plants. Balls for ball valves

come in a wide range of diameters, from a few millimeters to

1500 mm. The existing method of ball forging uses simple

dies in a free forging press, but can be time-consuming, as it

requires extensive post-processing. This study proposes a

method of forging with minimal post-processing by

combining free forging and ring rolling. In ball forging based

on ring rolling, the preform design plays an important role.

Finite element analysis was performed for the ball forging

process, and a method for the preform design was derived

based on the results. The preform design and the proposed

forging method were successful in high-precison ball forging

with less post-processing.

Keywords: Ball Valve, Forged Ball, Finite Element Analysis,

Ring Rolling, Preform design

INTRODUCTION Ball valves, used under high-pressure conditions in

petrochemical plants, are generally used to block the flow of

fluids. The structure of a typical ball valve is shown in Figure

1. Balls for ball valves are usually manufactured by casting or

forging, and come in various sizes from a few millimetres to

1500 mm in diameter. Balls smaller than 12" are produced by

die forging, while those larger than 14" rely on casting. The

latter is cheaper to produce, but requires surface welding due

to pores remaining on the surface after processing. While

some attemps have been made to replace casting with forging,

the disadvantage of forged products is that more machining is

required because of their lower dimensional accuracy.

This study proposed a new manufacturing method that

combines free forging and ring rolling for balls larger than

14", so as to reduce the amount of raw input materials by

minimizing the amount of machining after forging. As shown

in Figure 2, the forged ball has a hole through the middle for

fluids to flow, and is shaped in the form of a pivoting sphere.

From Figure 2, we can see that the forged ball weighs more

than 2,000 kg. The forging method used to produce such

heavy products is open die forging. However, because open

die forging suppresses the use of dies, it not only results in a

poor dimensional accuracy of forged products, but also low

productivity. Several attempts have been made to enhance

dimensional accuracy by introducing dies to free forging.

Choi et al. employed finite element analysis to optimize the

shape of round-shaped products [1], while Kim et al. used

finite element analysis to optimize the preform design of

crank throw for large ship engines [2]. Tamura et al.

eliminated forging defects caused by die shapes in the free

forging process and examined the dimensional control of

forged round billets [3].

Figure 1: The structure of ball valve

(a) Shape of forged ball

https://www.google.co.kr/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwiU0PzlxaTMAhWi5KYKHcwvBXwQjRwIBw&url=https%3A%2F%2Fwww.yppc.co.kr%2Fimg_view.asp%3Fimg%3Dvalve%2Fproduct_img%2Fvalve_04.jpg&bvm=bv.119745492,d.dGY&psig=AFQjCNFXN8_VLrRnoZgDqbHSqOsFC27BuQ&ust=1461493464491716



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(b) Dimension of forged ball

Figure 2: Dimension and weight of the forged ball

This study presents a preform design based on free forging for

the high-precision forging of spherical products, and proposes

a forging method based on ring rolling. Shivpuri et al. and

Johnson et al. performed ring rolling experiments on

rectangular rings to improve dimensional accuracy [4,5],

while Kim, Kim, Davey and Kim simulated ring rolling for

profiled rings using finite element analysis [6-10].

To obtain the final product by ring rolling, a sphere-shaped

preform design is needed. Kwon et al. presented a preform

design based on ring rolling and proposed a method of high-

precision forging using free forging [11]. This study examined

whether ring rolling can be used to fill the rectangle, ball, and

hexagon cross-sections obtained by free forging. Finite

element analysis was employed, and a method of preform

design with complete filling of forged balls was proposed.

After applying ring rolling to the preform designs, the

dimensional accuracy of the final products was assessed. The

proposed method was found to be more efficient, as it reduces

the amount of processing time.

DESIGN OF PROFILE RING ROLLING PROCESS

Small-sized products are usually produced by die forging,

while larger products rely on casting or open die forging. The

die forging method consists of blocking, finishing, piercing,

and flash trimming. The free forging method forges a donut

shaped blank and inserts a mandrel for the preform design,

and uses simple ball-shaped dies to obtain the final product.

Compared to die forging, the free forging method is more

time-consuming as workpieces are rotated several times to

achieve the desired shape. The conventional manufacturing

methods are shown in Figure 3.

The die forging method is limited by the press forging load as

the forging weight rapidly increases with dimension. Since the

free forging method falls under gradual forging, it is not as

restricted by the forging load. However, the repeated forging

of a single product is time-consuming, and the quality of the

forged product is highly dependent on the skills and

experience of the workman.

Figure 3: Conventional manufacturing method of forged ball

As shown in Figure 4, in the case of forged balls produced by

free forging, black surface defects can be observed on areas

where the raw material comes into contact with the die. This

can be resolved by making more allowance for machining in

the final dimension.

(a) Dies and operation of forged ball



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(b) section view of forging method

(b) Open die forged ball

Figure 4: Forged ball by open die forging method

To overcome the weaknesses of ball forging under the

existing free forging method, this study completed the

preform design and applied profile ring rolling for the final

product. Here, preform design is highly important as it affects

the shape and dimension of the final product.

Figure 5: Suggested manufacturing method of forged ball

The four methods of preform design reviewed in this study

are: (a) ring rolled preform, (b) donut-shaped forging based on

free forging, (c), and (d) free forging of the shape obtained in

(b) to get as close as possible to the desired spherical shape.

The first method of ring rolling uses typical rectangular blanks

to forge donut-shaped blanks, and performs ring rolling to

increase the internal and external diameters. This method is

known to have very high productivity. The second free

forging method produces a curved exterior due to barrelling

when forged by backward extrusion, and this facilitates filling

during ring rolling. The third method uses dies to elongate

donut-shaped blanks in the lengthwise direction, and forges

the workpiece to approach the final shape. The fourth method

is similar to the third, but involves more volumetric changes

in ring rolling and produces hexagonal shapes. Preforming

and the associated shapes under the four suggested methods

are presented in Figure 5.

Finite Element Analysis of Ring Rolling Process

To verify whether the proposed forging method can be used to

obtain the desired balls, finite element analysis was performed

under rigid viscoplastic conditions. The purpose of finite

element analysis was to estimate the filling rate of forged

balls, and calculations were carried out in Deform 3D. The

material of the ball used in this study was SA350 carbon steel,

which is usually used in petrochemical plants. The chemical

composition of SA350 is summarized in Table 1. The flow

stress for finite element analysis was calculated by Jmatpro

6.0, and Figure 6 shows the flow stress at 800℃, 900℃,

1000℃, and 1100℃.

Table 1: Chemical composition of SA350LF

C Si Mn P S

Spec. <0.3% 0.15~0.3% 0.6~1.35% <0.035% <0.04%

Actual 0.22% 0.25% 1.20% 0.03% 0.03%



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Figure 6: Flow stress of SA350LF calculated by Jmatpro 6.0

Table 2: Simulation condition of ring rolling process

Description Value

Material ASTM SA350LF2

Initial Temperature 1250℃

Tool Temperature 200℃

Main Roll Rotary Velocity 40RPM

Mandrel Feeding Velocity 0 ~ 1.5 mm/sec

Axial Roll Feeding Velocity 0 ~ 0.3mm/sec

Contact Heat Transfer Coefficient 11 kW/m2 K

The boundary conditions for the finite element analysis of the

ring rolling process are presented in Table 2. The material for

ring rolling is heated up to 1250℃, and the continuous forging

of the die results in a tool temperature of 200℃. For the initial

forging of the raw material, the temperature is gradually

increased, as the material may not rotate properly if the main

roll velocity is too high. The mandrel feeding velocity is also

gradually increased after the normal rotation of the raw

material and reduced upon completion of the forging process

to improve the dimensional accuracy of the final product. The

heat transfer coefficient between the raw material and the die

was the high-temperature forging heat transfer coefficient

provided by Deform.

The results of finite element analysis for the four proposed

preforms are shown in Figure 7. From analyzing the filling of

forged balls using finite element analysis, the preform

obtained by ring rolling the rectangular cross-section (a) was

in contact with the main roll only for the top, bottom, and both

ends of the ball. Unfilling was observed near the equator of

the ball. The donut-shaped preform obtained by free forging

(b) had a height (T) smaller than the forged ball, and was

completely filled near the equator. With only a slight increase

in length in the height direction, the desired forged ball was

not obtained. In the case of the ball-shaped preform (c), the

top, bottom, and both ends of the ball were in contact with the

main roll. Again, there was a high likelihood of unfilling near

the equator with limited volumetric displacement towards the

equator. Lastly, for the hexagonal preform (d), the forged ball

was attained with the external diameter of the ball coming into

contact with the main roll after filling the ball near the equator

during the initial forging.

(a) Rectangular section preform

(b) Donut shaped preform

(c) Ball shaped preform

(d) Hexagon cross section preform

Figure 7: Finite Element Analysis Results

The results of finite element analysis revealed that the shape

of the preform near the equator must be close to or the same

as the final shape, and the height must also be the same as the

forged ball. When forging preforms uses the main roll,

unfilling may occur near the equator if the two ends of the ball

are forged first. In other words, volumetric displacement

arising from the shifting of the mandrel must be greater than

that of the external area. Volumetric displacement near the

equator must be completed before the two ends of the ball

come into contact with the main roll. The dimensions of the

preform and ring rolled ball are given in Figure 8.



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Figure 8: Dimension of preform and ring rolled ball

For preforms to be completely filled, like the final forged ball,

the volume of the inner part shifts to the area in contact with

the main roll due to volume constancy. Since the changes in

the height of the ball and preform are negligible, the partial

volume of T1 and T2 must shift to the final forged ball. The

internal diameter of the preform and forged ball is given by

Eq. 1.

2ldd mb

1ldd mp

)( 12 lldd pb (1)

The external diameter of the area in contact with the main roll

is calculated on the volume constant condition by given by

Eq. 2.

22 bp VV

)446

(

22222

22pb

bpddTrr (2)

When the two ends of the ball come into contact with the

main roll, unfilling occurs near the equator of the ball. As

such, the condition shown in Eq. 3 must be satisfied.

11 bp VV (3)

The inequality of Eq. 4 is derived when the volumes in Eq. 3

are derived. 22

122

21

22221

21 32663444 bbbppppp dTrrdrrrr

(4)

If T1 in Eq. 4 is set as a dimension supported in free forging,

the maximum value of rp1 can be calculated, thus allowing

preforms to be designed as shown in Figure 8.

Here if T2 is too big like Figure 7(a), unfilling near the

equator of ring rolled ball appears. Therefore T2 is decided by

mandrel movement 2/)( 12 ll .

The dimension of preform which is calculated based on Eq. 2

and Eq.3 for 20" valve ball. The spherical diameter of 20" ball

is 780mm, inner diameter is 460mm and the height is 590mm.

The inner diameter of preform is decided by the capacity of

rolling machine. Considering rolling machine size and ball

dimension dp is set to be 250mm. The straight length of

preform T2 is assumed to be less than 1/3 of total height T and

decided to be 190mm. The calculated dimension of preform

based on decided value and Equations is given in Table.4.

Table 3: Calculated dimension of perform

Description Symbol Value

Ball spherical diameter D 781 mm

Ball inner diameter bd 460 mm

Ball height T 590 mm

Mandrel diameter md 250 mm

Ball inner clearance 2l 210 mm

Preform inner clearance 1l 30 mm

Preform taper height 1T 200 mm

Preform straight height 2T 190 mm

Outer radius of ball Vb2 2br 378.8 mm

Inner radius of ball Vb1 1br 255.9 mm

Straight radius of Preform Vp2 2pr > 340.9 mm

Taper radius of Preform Vp1 1pr < 212.1 mm

Verification of Suggested Manufacturing Method

With the preforms obtained from finite element analysis,

prototypes were developed to forge the final products by ring

rolling. Eq. 4 was applied to produce the preform shapes

shown in Figure 8, and they were tested for die filling. The

forging of preforms is similar to Figure 3(b), but the increase

in the dimension of the internal diameter through pipe forging

is eliminated since the external diameter is forged with an

increasing internal diameter during ring rolling. Donut-shaped

blanks were forged through free forging, and dies were used

to obtain preforms similar to the final shapes before carrying

out ring rolling. The preforming and final ring rolling process

are shown in Figure 9.

Figure 9: Preform manufacturing and ring rolling process

Through prototype development, the proposed blanks and ring

rolling method were found to be successful in producing the

desired forged balls. For the three forged balls produced by

ring rolling, dimension inspection was performed to examine

the variation in machining allowance. The measurements for

external diameter, internal diameter and thickness, as shown

in Figure 2(a), were compared with machining allowance. The



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results are given in Table 4. The external diameter, internal

diameter, and thickness had a clearance per surface of

25.5mm, 23mm, and 16.8mm on average. The distribution of

forging dimension was 8mm for the external diameter, 8mm

for the internal diameter, and 5mm for the height.

Table 4: Dimension inspection results of ring rolled ball

Location Machining

Dimension

Inspection Clearance per surface

Max. Min.

OD Ø555 Ø610 Ø602 23.5~27.5

ID Ø385 Ø343 Ø335 21~25

T 432 472 463 15.5~20

CONCLUSION

This study presented preform designs for ring rolling through

finite element analysis, and developed prototypes to assess the

validity of the proposed ring rolling method:

1) When forging large-sized balls by ring rolling, the

filling of forged balls is affected by the preform

design.

2) The height of preforms in the ring rolling process

must be the same or larger than that of the final

product. To prevent unfilling near the equator, the

gap between the external diameter near the equator

and the die must be larger than the gap between the

chamfer and the die.

3) The preform design limit was expressed in the form

of an equation, and actual ring rolling was performed

using the proposed preforms. Through the prototypes

and dimensional measurements, this study found that

the proposed method prevented unfilling and

achieved high dimensional accuracy.

ACKNOWLEDGEMENT

This study was performed as part of the "Development of

Manufacturing Technologies of the Main Shaft of 4MW Class

Offshore Wind Turbine for Asia Market Expansion" under the

Energy Technology Development Project (20153030023920).

REFERENCES

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[2] D.Y. Kim,Y.D. Kim,D.K. Kim, J.D.Kim, “A Study

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[7] Kim K.H., Suk H.G., Huh M.Y., 2007,“Development

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[8] Davey, K. and Ward, M.J., 2002, “The Practicalities

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[9] H.J. Kim, J.H. Kang and S.S. Kang, 2014, Preform

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research on ball forging by ring rolling process · pdf fileto verify whether the proposed...

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