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Acta Metall. Sin. (Engl. Lett.) Vol.20 No.6 pp448-456 Dec. 2007

ACTA METALLURGICA SINICA (ENGLISH LETTERS)

www.arns,org.cn

STUDY ON DESIGN TECHNIQUES OF A LONG LIFE HOT FORGING DIE WITH MULTI-MATERIALS X . J . Liu*, H. C . Wang, and D.W. Li School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China Manuscript received 8 September 2006; in revised form 10 January 2007

A new design technique for the long life hot forging die has been proposed. By finite element

analvsis. the reason for the failure of hot forging die was analyzed and it was concluded that thermal stress is the main reason for the failure of hot forging die. Based on this conclusion, the whole hot forging die was divided into the substrate part and the heat-resistant part according to the thermal stress distribution. Moreover, the heat-resistant part was further subdivided into more zones and the material of each zone was reasonably selected to ensure that the hot forging die can work in an elastic state. When compared with the existing techniques, this design can greatly increase the service life because the use of multi-materials can alleviute the thermal stress in hot forging die. KEY WORDS hotforging die with multi-material;finite element analysis; thermal stress

1. Introduction The life problem of hot forging die has always been a hot topic in industry. People have increased the die life by optimizing the die forging process and die design[’.*].However, even though the die design was optimized to decrease the mechanical stress to the least, the service life of hot forging die can be increased by 30% utmost[’J.To hrther increase the service life of hot forging die, several experts have studied the failure mechanism of hot forging die to radically seek the measurements of increasing the life of hot forgsystematically studied the effects of heat and force on the die failing die. For example, Russian expertsL4] ure, and based on the research results, a bi-metal hot forging die was successfidly produced. They made a breakthrough in the hot forging die design and manufacture technology. Langlois et d.[’I and Sawamura et also investigated the failure mechanism of hot forging die and made bi-material hot forging tools by explosive cladding and sintering. They even presented the thermo-mechanical testing of the coatingsubstrate interface and found that the interface did not represent a weak point of the tool structure. Besides, some surface modification processes were used to increase the die These solutions are limited for increasing the service life of hot forging die, however, these methods provide a clue that multi-material ‘Corresponding author. Tel. : +86 27 63351540. t?’-rnnil uddress: [email protected] (X.J.Liu)

solution may be a solution for hot forging tool because it allows placing the material with more suitable thermal mechanical properties at the necessary place. In this article, the reason for the failure of hot forging die was analyzed by finite element analysis, and it was concluded that thermal stress is the main reason for the failure of hot forging die. Based on this conclusion and the analysis results, the whole hot forging die was divided into the substrate part and the heat-resistant part. The heat-resistant part was hrther subdivided into more zones, and the material of each part was reasonably selected to ensure that the hot forging die can work in an elastic state. When compared with the existing techniques, such a multi-material hot forging die can greatly increase the service life of the hot forging die as much as 10 times.

2. Forging Selected for the Design An experimental procedure was presented to analyze the design of a long life hot forging die. An axisymmetrical steel part was selected for the designing. In this study, the axisymmetrical operation for a common open-die forging of a cylinder will be examined for the purpose. According to Ref.[9], during the forging, the heat of the forging part transferred to the upper die is small and most of the heat is transferred to the lower die, and therefore, the working condition of the lower die is relatively worse than that of the upper die. Also, studies have proved that the lower die life is considerably shorter than the upper die life. Thus, only the lower die was selected for the designing. The section drawings of the round part shaped like a cake and the lower die are shown in Fig. 1. The forging piece was made of T10 steel. The initial temperature of the workpiece was 1300K and the die was preheated to 600K. The ambient temperature was 300K and the air convection coefficient was 25.961 1W/(m2.K).

0

m

Fig. 1 Section drawings (unit: mm)of the forging part (a) and the lower die (b).

3. Design of Hot Forging Die with Multi-Materials The advanced finite element method was used to determine the temperature distribution, thermal stress, mechanical stress, and resultant stress of the homogeneous hot forging die in a continuous working state. The results were then applied to the designing and manufacturing of the hot forging die with multi-materials.

3.1 Finite element analysis of the conventional homogeneous hot forging die in a continuous working state As for the selected forging, 4Cr5MoSiVl was commonly used as the lower die material. The mechanical and physical properties of the materials are listed in Table I .

Table 1 Parameters of mechanical and physical properties of the materials ~

~~

Material

Coefficient of thermal conductivity

Coefficient of thermal expansion

Elastic modulus

Specific heat

Poisson's ratio

10

1O'MPa

TI0

W/(m-K) 30.53

13.5

210

J/(kg*K) 488

0.28

4CrSMoSiVI

(600K) 33.91

( 3 0 0 - 1300K) 32.4

200

488

0.3

( 1 300K)

(300-1000K)

3.1.1 Analysis method To obtain the temperature distribution, thermal stress, and mechanical stress in the lower die, the advanced finite element method was used. Fig.2 shows the block diagram of the analysis method. The forging operation consists of two processes: heating and cooling. In the continuous working state, the working pace can greatly influence the stress distribution in the lower die. Therefore, before calculating the stress in the lower die, the reasonable working pace of the lower die must be determined. According to the actual process condition, the heating time was found to be 0.5s. To determine the cooling time, herein a tentative calculation method has been put forward; its procedure chart is shown in Fig.3, where N is set to be 20 times. In the tentative calculation method, a heat exchange balance theory has been used[''], that is, the heat absorbed by the die is equal to the heat dissipated from the die, and the theory has been proved to be a necessary condition for the hot forging die to continuously work effectively. In the thermal analysis, halves of the forging part and the lower die are taken as the analysis object. The element chosen for the thermal analysis is the 2-dimensional axial symmetry thermal analysis solid element Plane55. After completing the calculation for the heating process, all data about the temperature distribution in the tool were stored in the database. Next, these data were taken as a premise for the cooling process. Such procedure was repeated for each sequent forging. In this way, the distribution of the temperature field at any time can be obtained.

Obtaining the temparature

Calculating the force distribution on

distribution by thermal analysis

die cavity by main stress method

w

w

Obtaining the thermal stress by

Obtaining the mechanical stress by

thermal-structuralanalysis

structural analysis

Fig.2 Block diagram of the analysis method.

1

T = 1,. (t,. , c 1,)

T = t , . (t,.,> f,)

Ncycles forging I

I

Fig.3 Procedure chart of the tentative calculation method.

In the thermal-structural analysis, the analysis model was obtained by rotating the lower die in the thermal analysis model for 360" and the analysis element used was the 3-dimensional structural solid element Solid45. The calculation method for the mechanical stress in the lower die is as follows: according to the initial temperature of the workpiece, after determining the deformation resistance of T10 steel, a,, the force distribution on the die cavity at the end of the loading process was calculated using the main stress method["]. Next, in structural analysis, the force distribution calculated was exerted on the model of the structural analysis as boundary condition to calculate the mechanical stress at each node at the end of the loading process, which is the maximum mechanical stress introduced by the maximum mechanical load. In terms of material mechanics, the ratio of the force to the stress introduced by the force is constant in elastic body. Therefore, after calculating the maximum mechanical stress by structural analysis, the mechanical stress in the lower die at any time was obtained by the change regularity of the mechanical load exerted on it. 3.1.2 Results of calculations In this example, the optimal cooling time is 50.2s by the tentative calculation method, therefore, the optimal working pace of the lower die, i.e. time for heating th=0.5sand time for cooling t, = 50.2s. The graph of temperature change at each node of the lower die of continuous forgings was then Balancearea l3 obtained. According to the results of the thermal analysis, the whole lower die can be divided into three areas in terms of the extent of the temperature change: fluctuating area where the temperature changes intensely, balance area where the temperature remains almost the same, and descending area Fig.4 Temperature division. where the temperature tends to decrease (shown in Fig.4).

1

/

i

452

*

i ' r , 800

7W

600

600

500

500

0

msmo Time, s

0

2000

4000

m

Time, s

FigS Temperature change graph of the lower of 100 continuous forgings: (a) fluctuating area (point I in Fig.4); (b) balance area (point 10 in Fig.4); (c) descending area (point 15 in Fig.4).

Fig.5 exemplifies the temperature change at the selected point 1 in fluctuating area, point 10 in balance area, and point 15 in descending area (shown in Fig.4) of the lower die of 100 continuous forgings under the given working pace. Fig.5a shows that the temperature in fluctuating area changes like serrated waves and after many cycles the temperature at each node in the fluctuating area tends to be stable change. To analyze quantitatively the range of the fluctuating area, a new concept, which is used to indicate the extent of temperature fluctuation at each node, has been introduced, and it is defined as

where, T , and T, are the maximum temperature and average temperature at each node after temperature balance, respectively. Table 2 exemplifies the values of u at several special points (shown in Fig.4). According to Table 2 and Fig.4, it can be concluded that the fluctuating area mainly focuses on a thin layer under the die cavity surface. As for the thermal stress in the lower die of 100 continuous forgings under the given working pace, it was calculated by the above mentioned thermal-structural analysis. By comparing the temperature field with the thermal stress field calculated, the results obtained indicated that the thermal stress distribution almost corresponds with the temperature change, that is, the fluctuating area where the temperature intensively changes presents higher thermal stress, and the balance area and descending area present relatively lower thermal stress. Table 3 exemplifies the maximal thermal stress at several special points (shown in Fig.4) of the lower die at different forging times. Thus, in each forging, the maximal thermal stress occurs at the surface of the die cavity. The deformation force distribution on the lower die cavity surface at the end of the loading process calculated using the main stress method is shown in Fig.6. Here, F , is the projection area, h and b are the height and width of the flash slot, respectively, D is the diameter of the forging part, and p, is the friction coefficient. Then, by means of the structural analysis of Ansys, the mechanical stress in the lower die at the end of the loading process, which is the maximal mechanical stress of each node in a single forging, was calculated. And the maximal mechanical stresses at several special points 1- 15 (shown in Fig.4) are exemplified as follows: 95.96, 72.34, 76.21, Table 2 The values of u at several special points 78.17, 72.73, 67.48, 64.91, 53.13, 26.52, 47.63, No. of points 1 2 3 4 5 6 7 35.38, 44.8, 45.58, 41.20, and 33.12MPa. The Depth, mm 90 80 70 SO 30 I S 0 maximal mechanical stress occurs at the center fT 7 0 12.14 1.24 0 0 0 0 0 of the die cavity.

Table 3 The maximal thermal stress at several special points (MPa) Point

Forging times

mark

1

6

10

20

50

70

I00

1

1015.2

998.53

996.21

994.08

991.38

991.06

988.80

2

27.089

27.041

26.994

26.975

26.950

26.941

26.930

3

10.521

10.066

10.058

10.053

10.045

10.039

10.030

4

8.6339

8.7414

8.7730

8.7689

8.7210

8.7087

8.6968

5

6.0591

6.0742

6.0523

6.0116

5.9818

5.9795

5.9771

6

11.186

11.489

11.436

11.397

11.329

11.316

11.297

7

12.823

12.102

12.054

12.019

11.898

11.852

11.792

8

571.10

562.74

561.28

559.47

557.20

556.98

556.27

9

339.47

338.11

337.82

337.45

336.46

335.18

334.14

10

38.142

37.761

37.728

37.657

37.518

37.473

37.334

11

13.516

13.826

13.814

13.778

13.676

13.625

13.573

12

101.83

100.81

100.69

100.53

99.736

99.34 I

99.659

13

117.42

116.07

115.77

115.48

114.69

114.22

113.75

14

1 8.459

18.428

18.396

18.287

18.279

18.227

I 8.262

15

6.8154

6.1956

6.1911

6.1704

6.0727

6.0421

6.0126

Moreover, the thermal-structural analysis indicated that the maximal thermal stress in the lower die occurs at the end of the heating process. Similarly, the maximal mechanical stress at each node occurs at the end of the loading process, and the time A g!o. for the heating process to complete is the time for L the loading process to end. Thus, the maximal thermal stress and the maximal mechanical stress apFig.6 Procedure chart of the tentative calculation method. pear simultaneously. Besides, the above analysis results show that the ratio of thermal stress in the lower die to mechanical stress is within 8 and 10. Thus, thermal stress is the main factor for hot forging die failures, and decreasing the thermal stress in hot forging die is the key to increase the die life.

,?.2 Division of hot forging die Thermal stress is introduced by the temperature gradient. In Ref. [ 121, the author indicates that the thermal stress is influenced by the temperature gradient AT and some thermal parameters such as elastic modulus E, thermal expansion coefficient p, thermal conductivity coefiicient A,,,, and thermal specific heat q; therefore, by dividing the whole die into several layers and by reasonably arranging the material of each layer, the temperature gradient in each layer can be greatly decreased and the purpose of alleviating the thermal stress in the heat-resistant part can be achieved. The above analysis results showed that the temperature and stress in the balance area and descending area are at a considerably lower level than that in the fluctuating area; thus, the lower die can first be

454 divided into two parts: the substrate part (including the balance area and descending area), which accounts for about 90%-95% of the whole lower die, and the heat-resistant part (including the fluctuating area). Moreover, the substrate part is composed of the commonly used die steels. However, the commonly used die steels cannot meet the performance demands of the fluctuating area, where the thermal stress is rather high and die failure occurs easily, and therefore the heat-resistant part must be divided inI to more layers to alleviate the thermal stress. In the example, the above analysis shows that the peak value Fig.7 Division of the lower die. of the temperature at the dangerous point on the lower die cavity surface is 750°C and the temperature fluctuating amplitude is 400°C. Therefore, according to the isotherm of the lower die at the end of the heating process, the lower die was divided into 4 layers in terms of the 100°Ctemperature difference in Fig.7. Thus, the working temperature zone of the first layer is 650-750°C and the working temperature zone of the second layer is 550-650°C ; similarly, the working temperature zone of the third layer is 450-550°C and the working temperature zone of the fourth layer is.350450°C.

3.3 Material selection for hot forging die Referring to Ref.[ 131, the peak value function of the thermal stress o;,along the depth direction (z direction) of a hot forging die is expressed as

where u is the linear thermal expansion coefficient, w is the harmonic frequency of the thermal stress function. Thus, thermal stress in hot forging die is in direct ratio with relation to the elastic modulus E, the linear thermal expansion coefficient p, and the temperature gradient AT. In addition, the yield strength of the die material is a function of temperature and the relation between the yield strength and temperature is approximately calculated by the interpolation method as follows 0fT) = ~

0.17T0.02T2

0

(3)

To ensure that all layers of the die maintain entirely elastic state under the peak value of temperature in continuous working state, the die material of all layers must be chosen to meet the following condition ~

o

n

n

4T) x ~

(4)

Moreover, the maximal use temperature of hot forging die material is influenced by the thermal strength performance of the material[I3].A concept of thermal strength coefficient Q, which is used to weigh the enduring thermal load capability of a die material, was introduced and is given

where, Eo is the elastic modulus at room temperature, T is the temperature variable. Thus, to ensure that all layers of the die work effectively, the maximal use temperature of the selected material for each layer must be higher than the temperature peak value of the corresponding layer, that is, the following critical condition must be met T=T-

Q>O

(6)

Eqs. (4) and (6) are the criterions for the material selection of each layer. However, after the possibly reasonable materials were found according to the criterions, the temperature and stress fields of multimaterial hot forging die must be simulated again in the same way as mentioned above. If the heat-resistant part material meets the demands of temperature and strength, the selected material is reasonable, otherwise, the materials must be re-chosen and the above steps must be repeated.

4. ManufactwingTechniques The substrate part made of conventional die steels is manufactured by conventional technology. As for the heat-resistant part of the long life hot forging die, two methods are proposed for its manufacture. One is deposited welding. In this method, after the substrate part composed of conventional hot forging die materials is made by traditional manufacture techniques, the materials forming the heat-resistant part are deposited on the substrate part layer by layer using the overlay welding process. The materials of each layer are chosen among the existing materials as much as possible. Otherwise, the materials can be obtained by smelting in vacuum-induction melting furnace and can be made into a welding rod. To control the shape and size of each layer, it must be machined by an electrical discharge machine tool after depositing a layer. The other method is powder metallurgy as shown in Fig.8. After the forging die is made, for eliminating the residual stress, the die must be processed by appropriate heat treatment. Manufacturingthe substrate part by conventional technology

each layer of the heat-resistantpart

Preforming billets are superposed on the substrate part layer by layer

I The whole die is sintered on sintering hot isostatic press machine

The whole die is heated in vacuum furnace

The whole die is plastically deformed on Hydraulic-Machine

Fig.8 Scheme of the powder metallurgy method for the manufacture.

6. Conclusions Thermal stress is the main factor for the failure of hot forging die. Using the finite element method, the unsafe area in hot forging die in a continuous working state can be determined and can then be divided into more layers, which are composed of various materials. The use of multi-materials can decrease the thermal stress and can greatly increase the die life.

Acknowledgements-This

work was supported b y the National Natural Science Foundation of China (No. ,50675165).

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