Research on ball forging by ring rolling process · PDF fileTo verify whether the proposed...
Transcript of 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
7823
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
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 12 (2016) pp 7823-7828
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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
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 12 (2016) pp 7823-7828
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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).
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