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Procedia Engineering 81 (2014) 480 – 485

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/ ).

Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University doi:10.1016/j.proeng.2014.10.026

ScienceDirect

Available online at www.sciencedirect.com

11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014, Nagoya Congress Center, Nagoya, Japan

Influence of anvil shape of surface crack generation

in large hot forging process

Takefumi Arikawaa,*, Daisuke Yamabe

b, Hideki Kakimoto

c

a Research & Department Section, Technical Development Department, Steel Casting & Forging Division, KOBE STEEL, LTD., 2-3-1,

Shinhama, Arai-cho, Takasago-city, Hyogo, 676-8670, JapanbTurbo Machinery Engineering Section Rotating Machinery Engineering Department Compressor Division, KOBE STEEL, LTD., 2-3-1,

Shinhama, Arai-cho, Takasago-city, Hyogo, 676-8670, Japan c Mechanical Working Research Section Material Research Laboratory, KOBE STEEL, LTD., 1-5-5, Takatsukadai, Nishi-ku,Kobe, Hyogo, 651-

2271, Japan

Abstract

In hot free forging process of large products, surface cracks occur occasionally. When these cracks occur, they must be

removed by hot scarfing process after stopping forging process temporarily. Final products have no cracks on those surfaces

after applying the hot scarfing process. Hot scarfing process, however, cuts into productivity due to additional time consuming.

Since surface cracks are regarded as significant defects, suppression of the surface crack generation is important issue. A lot of

research has been studied about mechanism and criteria of forging crack generation [1]~[8]. In hot forging process, however,

there are a few researches [9]~[14]. Then the mechanism and criteria of crack generation during the hot forging process are not

clear at this moment. There are various factors for crack generation, such as reduction ratio, forging temperature, anvil shape,

ingot surface integrity and so on. The goal of this research is to clarify the mechanism of crack generation in actual forging

processes. The derived mechanism is applied to design anvil shapes by using numerical forging simulation.

© 2014 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of Nagoya University and Toyohashi University of Technology.

Keywords: Hot forging; Foging defect; Surface crack; Surface defect; Anvil shape

* Corresponding author. Tel.: +81-79-445-7131; fax: +81-79-445-7244.

E-mail address: [email protected]

© 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/ ).

Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University

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481Takefumi Arikawa et al. / Procedia Engineering 81 (2014) 480 – 485

1. Introduction

In hot free forging process of large products, surface cracks occur occasionally. When these cracks occur, they

must be removed by hot scarfing process after stopping forging process temporarily. Final products have no cracks

on those surfaces after applying the hot scarfing process. Hot scarfing process, however, cuts into productivity due

to additional time consuming. Since surface cracks are regarded as significant defects, suppression of the surface

crack generation is important issue. The mechanism and criteria of crack generation during the hot forging processare not clear at this moment. There are various factors for crack generation, such as reduction ratio, forging

temperature, anvil shape, ingot surface integrity and so on. The goal of this research is to clarify the mechanism

of crack generation in actual forging processes. The derived mechanism is applied to design anvil shapes by using

numerical forging simulation.

2. Investigation of actual forging process

The target actual forging process is the cogging process which forms a round bar from a steel ingot. One of the

mechanism hypotheses of surface cracks generation is illustrated in Fig. 1. In the cogging process, minor defect

was generated on the surface that has been forged. It was observed to occur at anvil lap part during cogging

process. When tensile stress is applied to minor defect during forging operation, it is expected that a crack will be

generated from the minor defect which is regarded as a stress concentration point. It is important to prevent minordefect generation on the surface.

Fig. 1. Mechanism hypotheses of surface crack generation.

Fig. 2. Illustration of minor defect generation in cogging process.

Anvil lap part

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482 Takefumi Arikawa et al. / Procedia Engineering 81 (2014) 480 – 485

3. Validation of mechanism hypotheses of surface crack generation

3.1. Confirmation of minor defect generation in cogging process

In order to confirm minor defect generation in cogging process, small size cogging test was conducted. The

examination process was shown in Fig. 3. Cross section shape of the billet was square. The billet was heated up to

1250 °C and was kept 1hr. After that it was cooled down to about 800 °C. 800 °C was the temperature that wasobserved a lot of surface defects in actual cogging process. Then, the billet was forged 23% reduction ratio. The

billet was cut at the center of the longitudinal cross-section after the cogging test.

Table1. Chemical composition of test material (SF60) (wt%).

C Si Mn Cr

0.45 0.25 0.80 0.15

Fig. 3. Examination process.

The cross-sectional shape and metal flow of billet obtained from the experiment are shown in Fig. 4. It shows

minor defect was occurred at anvil lap part. Since minor defect area had metal flow, it was suggested the

possibility of preventing minor defect generation by metal flow control.

Fig. 4. Forged billet, (b) cross-sectional shape and (c) metal flow of billet obtained from experiment.

In order to confirm deformation behavior of forged surface, simulation is applied to the cogging process using

FE analysis (FORGETR

). The cross-sectional shape of billet obtained from FE analysis under the same conditions

with the experiment of Fig. 4 are illustrated in Fig. 5. The calculated result is good agreement with experiment one.

The deforming behavior of forged surface obtained from FE analysis is shown in Fig. 6. It shows that the minor

defect was formed at anvil lap part by material flow with stroke during forging.

100mm

90mm

90mm

Test billetHeating

(Temparature1250)Forging

(Reduction ratio:23%)

Anvil

Width =30mm

Edge radius= R2.5

(a) (b) (c)

20001mm

Minor defect

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Fig. 5. (a) Forged billet and (b) cross-sectional shape of billet obtained from FE analysis.

Fig. 6. Deforming behavior of forged surface obtained from FE analysis: (a) stroke=0 mm; (b) stroke=15 mm;(c) stroke=30 mm.

3.2. Confirmation mechanism of surface crack generation

In order to confirm the mechanism of surface crack generation, simulation is applied to the cogging process

using FE analysis (FORGE). Fig. 7(a) shows analysis model. Cross section shape of billet model is octagon which

is the shape of actual cogging process. Initial temperature distribution of billet model is shown in Fig. 7 (b). The

anvil stroke was 150mm and the feed per pass was 600mm during cogging simulation, where pass means one times

anvil stroke. Deformation resistance was acquired by high temperature tensile tests. Friction model is based on

Coulomb model and friction coefficient is assumed to be 0.15. Possibility of surface crack generation was

evaluated by Cockcroft and Latham ductile fracture evaluation parameter (C-value) [16]. C-value is given by

f

d C eq

0

max . (1)

wheremax is the maximum principle stress,eq is the equivalent stress andf is the equivalent strain.

The C-value contours during cogging process obtained from FE analysis is shown in Fig. 8. Maximum C-value

is generated at minor defect parts. It shows crack generation is easy to occur at minor defect part. The result was

suggested suppression of minor defect leads to prevent surface crack generation.

Fig. 7. Analysis model: (a) illustration of billet model and anvil model; (b) initial temperature distribution.

Billet

Anvil

(a) (b)

(a) (b) (c)Anvil

Billet

Minor defect

Anvil lap part

Forged area

Minor defect

(a) (b)1000

500

750

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484 Takefumi Arikawa et al. / Procedia Engineering 81 (2014) 480 – 485

Fig. 8. C-value contours during cogging process obtained from FE analysis.

4. Influence of anvil shape on minor defect generation

It is considered that suppression of minor defect is effective for preventing surface crack generation. Since the

minor defect was formed by material flow during forging, material flow control at anvil lap part is required in

order to suppress it. The anvil shape, especially the edge radius, is important to control material flow at anvil lap

part. In order to investigate influence of the edge radius on minor defect, simulation was applied to the cogging

process using FE analysis (FORGE). The analysis condition except reduction and anvil shape was the same as the

calculation Fig. 9. Reduction and anvil shape condition is shown in Table3. Reduction means stroke of the anvilduring forging.

Relation between reduction and minor defect depth obtained from FE analysis is shown in Fig. 10. The minor

defect depth is increased with increase of reduction. When the reduction is 150 mm, on the other hand, the minor

defect depth is reduced with increase of edge radius. Since the edge part shape of forged surface come close to flat

with increase of edge radius, the minor defect depth was reduced. In order to organize, therefore, the relation

between the edge part shape on forged surface and minor defect depth, aspect ratio was defined. The definition of

the aspect ratio illustrated in Fig. 10(a). The aspect ratio is the parameter that the edge part shape was regarded as

quantitative value. Relation between the aspect ratio and minor defect depth obtained from FE analysis is shown in

Fig. 10(b). The minor defect depth is increased with increase of aspect ratio. In order to minimize surface crack

generation, the anvil edge shape which can minimize aspect ratio is effective.

Table3. Reduction and edge radius of the anvil.The edge radius of the anvil [mm] Stroke of the anvil [mm]

100 150, 200, 300

160 150, 320, 400

200 150, 300

300 150, 150

Fig. 9. (a) Definition of minor defect depth and (b) relationship between reduction and minor defect depth.

0

5

10

15

20

25

0 50 100 150 200 250 300 350 400 450 500

M i n o r d e

f e c t d e p t h / m m

R100

R160

R300

Reduction /mm

R200

0.4

0.0

0.2

Minor defect depth

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Fig. 10. (a) Definition of aspect ratio and (b) relationship between aspect ratio and minor defect depth.

6. Conclusion

Possibility of surface crack generation in cogging process was evaluated by Cockcroft and Latham ductile

fracture evaluation parameter (C-value) obtained from FE analysis. Maximum C-value is generated at minor defect parts. The result shows crack generation is easy to occur at minor defect part and it was suggested suppression of

minor defect leads to prevent surface crack generation. As a result of investigation minor defect generation in

cogging process, it was generated at anvil lap part of forged surface. In addition, it was found the minor defect

depth correlate with the edge shape of anvil. In order to minimize surface crack generation, the anvil edge shape

which can minimize aspect ratio is effective.

References

[1] Terasaki F., Yamanaka K, Ootani Y, Oda M., Yoshihara M., 1978. Tetsu-to-Hagane, 64, S719.

[2] Suzuki H., Nishimura S., Yamaguchi S., 1979. Tetsu-to-Hagane, 65, 2038.

[3] Suzuki H., Nishimura S., Imamura J., Nakamura Y., 1981. Tetsu-to-Hagane, 67, 1180 .

[4] Maki T., Nagamichi T., Abe N., Imamura I., 1985. Tetsu-to-Hagane, 71, 1367.

[5] Inoue T., Inazumi T., Hosoya Y., 2001. Tetsu-to-Hagane, 87, 552.[6] Kudo H., Aoi K., 1967. Journal of JSTP, 8, 72, 17.

[7] Nagumo M., Yamaguchi S., Takahashi T., Endo M., 1970. Tetsu-to-Hagane, 56, S205.

[8] Ishiguro N., Abe E., Ueno K., Yukawa N., Fujihara M., Yoshida H., Ishikawa T., Journal of the JSTP, 54, 634, 993-997.

[9] Morozumi F., Hirasaka M., 1970. J. Soc. Mater. Sci., Japan, 19, 255.

[10] Otsuka A., Maki T., Sakurai T., Iida H., 1984. Zairyo, 34, 381, 2-6.

[11] Okuno H., Koeda H., 2007. JSME annual meeting 2007(1), 613-614, 2007-09-07.

[12] Kakimoto H,. Arikawa T., Takahashi Y., Yoshida T., Matsuda M., Kagawa Y., 2009. JSTP spring conference 2009, 281-282.

[13] Arikawa T., Kakimoto H,. Takahashi Y., Matsuda M., Yoshida T., Yamabe D., 2009. The 60th JSTP Joint conference 2009, 91-92.

[14] Yamabe D., Ikegami T., Arikawa T., Takahashi Y., Matsuda M., Yoshida T., 2009. The 60th JSTP Joint conference 2009, 81-82.

[15] Yamabe D., Kagawa Y., 2011. 18th International Forge master Meeting, 275-279.

[16]Cockcroft,M. G., Latham,D. J., 1968. Inst. Met., 96, 33-39.

As ect ratio

M i n o r d e f e c t d e p t h / m m

0

5

10

15

20

25

0.1 0.2 0.3 0.4 0.5 0.6 0.7Axial direction

Height direction

a

h

Billet

Anvil

Aspect ratio =h/a

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