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The investigation of the FM heavy forging process by the Moiré method part I: Single-reduction results
Journal of
J. Mater. Process. Technol. 43 (1994) 195-209
ELSEVIER
Materials Processing Technology
The investigation of the FM heavy forging process by the Moir6 method Part I: Single-reduction results ShunLong Wang *'a, QiXiang Cao b "Faculty of Mechanical Engineering, Eindhoven University of Technology, The Netherlands bDepartment of Mechanical Engineering, Tsinghua University, Beijing, China (Received January 4 1993; accepted in revised form November 19 1993)
Industrial Summary There exist some metallurgical defects, such as voids, microcracks, loose structure, etc., in the core of the heavy forging ingots. To insure a good quality of the finished product, these defects must be eliminated during the forging processes. In this paper, using the Moir~ method, a newly developed 1200 °C high-temperature Moir6 modelling technique and apparatus for measuring the critical closing reduction of artificial cavities, the single reduction of the FM heavy free-forging process has been investigated systematically, the critical dosing reduction rate curve of artificial cavities and the internal strain/stress distribution having been obtained. The Moir6 analysis shows that when the anvil width ratio (AWR) W/Ho = 0.6, the maximum deformation penetrates into the core of the specimen and, a nearly symmetrical deformation distribution is obtained in the use of an unsymmetric anvil arrangement, where W/Ho = 0.5 is the transition anvil width ratio of the central axial stress (from tension to compression). Experiments on porous material also show that W/Ho = 0.6 has the strongest forging effect. The author thus conclude that W/Ho = 0.6 is the optimum anvil width ratio for the FM forging process. The Moir6 patterns obtained after the 1200°C forgings process are very successful. The development of this technique affords a powerful tool in research into hot-forming processes, especially useful in the integration of plasticity theory with materials science. Finally, some suggestions are made for the improvement of the FM process.
Key words: FM process; Moir6 method; Heavy Forging
1. Introduction With the progress of technology, the metallurgical energy, chemical, shipbuilding, heavy machinery and aerospace industries all need high capacity, high performance
*Corresponding author. 0924-0136/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 3 ) E 0 1 2 9 - 5
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S. Wang, Q. Cao/Journal ~" Materials Processing Technology 43 (1994) 195 209
heavy equipment. There is the need for producing large forgings without any defects to serve the above-mentioned industries. Large forgings are usually the key components in the complete sets of equipment and their quality standards are rising. However, with the size of the large forgings becoming ever larger, there is an increasing difficulty in producing large forgings of high quality. The real production processes are very complex, so that systematic modelling experiments are necessary to ensure product quality. In the last three decades, a lot of progress have been made in the melting and casting processes of the heavy ingots. However, due to the solidification characteristics of the metal, it is inevitable that some metallurgical defects form in the core of the ingots, such as shrinkage cavities and porous structure. These defects must be eliminated during the forging process to ensure the service quality of the forgings. With the limitation of available press load capacity, the traditional upsetting process is limited. More attention is now being given to reduction processes (also referred to as blocking processes, which reduce the ingot cross section and increase its length by side pressing). Several reduction processes, such as FM, WHF, J.T.S, V-shaped anvil, etc, have been put forward and are used in forging practice [-1-6]. The FM (Free from Mannesmann effect) forging process is based on slip-line field analysis, using an unsymmetrical arrangement of the upper anvil and lower platform to avoid the generation of tensile stress, in the core of the billet (Fig. 1). In this paper, using a pneumatic apparatus for measuring the critical closing reduction ratio of artificial cavities, the Moir6 method and a high-temperature (H-T) Moir6 modelling technique, the critical closing reduction ratio curve is obtained, the internal strain and stress distribution is analyzed, the results for lead and steel specimens are compared and, finally, the optimum AWR is found.
Iy(v) /z(w)
Fig. 1. The schematic drawing of FM process.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
197
2. Experimental principle 2.1. The pneumatic apparatus
A pneumatic apparatus, shown in Fig. 2, has been designed for measuring the critical closing reduction ratio of artificial cavities. The critical closing reduction ratio and the closing sequences can be measured accurately, and several cavities can be measured simultaneously. This apparatus is very useful in searching for the optimal technical parameters. 2.2. MoirO method
The Moir6 method is used to measure the internal strain and stress distribution. Moir6 gratings are stuck (at room temperature (R-T)) and brush electroplated (at H-T) on the central section of the testpiece (Fig. 1). The Moir6 information is collected and processed with a "Photo-electric Scanning Digital Image Processing System". The strain is calculated using the Eulerian expression 19,10]:
( oy 1 - c~xJ + \ d x J '
er~ = 1 -
erE=l_
//(1 dv 8x
7x~ = arcsin-
av'~u
- ey/
+
(du~ 2,
+ \ay/
8u
dudu
dvdv
dy
dxdy
dxdy
The value of e~ is obtained from the incompressible condition 8Z E ~
__
E E E /3E ~ /3y - - ~x/3y E E E"
1 - e ~ - e r +exey
The stress deviator is obtained by the use of the Lev~ Mises plasticity theory with incremental loading, whilst the stress distribution is obtained by integrating the equilibrium equation [9-11].
1
2
3
4
5
6
Fig. 2. The schematic drawing of the pneumatic apparatus: 1. pump; 2. energy accumulator; 3. gas divider; 4. pipe connector; 5. testpiece; 6. water pool.
198
S. Wang, Q. Cao/Journal o/ Materials Processing Technology 43 ~1994 ~ 195 209
3. The single reduction results at R-T
3.1. The critical closing reduction ratio curve Several researches I-1-5] have been carried out to search for the optimum AWR, but due to the limitation of the experimental technique and of the different judgement criterion, there is no commonly accepted optimum AWR. In this section, using the pneumatic apparatus, the critical closing reduction ratio curve for the closing of artificial cavities is obtained and the optimum AWR is found. Figure 3 shows the distribution of the artificial cavities. AWRs of W/Ho = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 were investigated. Here Wis the feed width of the upper anvil and Ho is the initial height of the testpiece. Five horizontal holes are made in the core of the 1/5Ho region as artificial defects. The experiment is carried out on a 300 kN test machine, the loading strain rate being ~ = 10 -4 s -1. Figure 4 presents the critical closing reduction ratio curve of the artificial cavities. It is found that for the closing of artificial cavities of either 1/40Ho (4~1.0 mm) or 1/50Ho (~b0.8 mm), the critical closing reduction ratio is the minimum when W/Ho = 0.6, the critical reduction ratios for the closing of the central cavities being AH/Ho = 14% and AH/Ho = 1 2 ° , respectively. Figure 5 is the forging load curve (AH/Ho = 20%). With increase of the AWR, the forging load increases. The forging load for W/Ho = 0.6 is 20% greater than that for W/Ho = 0.4. Fig. 6 is the deflection of central cavity. The unsymmetric anvil arrangement causes non-uniform material flow and the deflection of the central cavity, this being especially serious when W/Ho is less than W/Ho = 0.6. With the increase of the AWR, the deflection decreases: when W/Ho = 0.6, the deflection of the central cavity is about 3.5%. The critical reduction ratio reflects the comprehensive influence of the strain state, stress state, strain rate, etc., on the closing of the cavities, so that W/Ho = 0.6 is the optimum AWR of the FM forging process.
3.2. MoirO analysis of the stra& distribution F - F G Moir6 gratings (51/mm) were used, the two parts of the testpiece being connected together by low melting point alloy. Fig. 7 are the Moir6 patterns
Ho
V/
=
o
/ / / / / / / / / /
I
Fig. 3. The distribution of the artificial cavities. (Material: lead.)
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
x R--T(20*O
~, 20-
o
H-T(1200*C)
~
199
l/SHo re$iem * eernal cavity a
25-
~1515-
o 10
5
10,
,
wa-Io
0..3 0.4 0.'5 0.'6 0.7 0.'8 a)
5
,
w/ri0
0.,3 0.'4 0.'5 0.'6 0.7 0.'8 b)
Fig. 4. Critical closing reduction ratio curves: (a) ~b 1.0 mm (central cavity); (b) t# 0.8 mm.
/ 15105w/I 0 0.'4 0.'5 0.'6 0.'7 0.'8 0.3 Fig. 5. The forging load.
10- IX,fleeti~ of central cavity 8642 0.3 0.'4 0.'5 0.'6 0.'7 0.8 Fig. 6. The deflection of the central cavity.
corresponding to the critical closing reduction ratio, whilst Fig. 8 is the comparison of the strain distribution under the anvil center along the reduction direction. Table 1 presents the relationship between the AWR and the deformation features. Here H,rmax refers to the height from maximum *i to the bottom platform and *imaxis the maximum e~ in the cross section just under the anvil centre. It can be seen that when W/Ho < 0.5, the deformation zone is concentrated mainly on the upper part and the deformation is very non-uniform. With the increase of W/Ho, the zone of large deformation penetrates into the specimen centre and when W/Ho = 0.6 the maximum strain reaches the specimen centre, which is favourable both for closing of the cavity defects and for obtaining a refined and homogeneous internal structure. Figure 9 are the strain distributions in the central line. There is maximum deformation zone under the anvil centre, the deformation is small at the anvil corner, and with increase of the AWR, the zone of maximum deformation changes from a peak to a plateau: when W/Ho = 0.5 and W/Ho = 0.6, this plateau is at 0.3 and 0.5 W respectively. These strain distributions indicate that only the central part under the anvil is a valid closing zone, there existing unclosing zones at the anvil corners. These
200
S. Wang, Q. CaojJournal of Materials Processing Technology 43 (1994) 195-209
al
bl
el
S. Wang, Q. Cao/Journal o f Materials Processing Technology 43 (1994) 195-209
201
d)
el
f) Fig. 7. (continued).
unclosing zones must be covered by the valid closing zone in the later stages of reduction through an appropriate turn-over and feed procedure. The Moir6 analysis shows that when W/Ho = 0.6, using the unsymmetric anvil arrangement, a nearly symmetric deformation distribution is obtained and that there is also 0.5 W large-deformation plateau in the central line: this is favourable for the
Fig. 7. The Moir~ patterns for various A W R s (material: lead): (a) W/Ho--0.3, AH/Ho = 16%; (b) W/Ho ffi 0.4, AH/H o = 17%; (c) W/H o = 0.5, AH/Ho = 16.5%; (d) W/Ho--0.6, AH/Ho = 14%; (e) W/Ho = 0.7, AH/H o = 17%; (f) W/Ho = 0.8, AH/H o = 18.4%.
202
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195 209
1
0-1 ~ /~//~ / Jl,l//.I
o WJHo'0.4 >< v¢~..o.~
3t ~
* W/I-Io-0.8
]~ f / / /
0.2 0.0
0.2
0.4
~ wm,-o.6
0.6
0.8
Fig. 8. The strain distribution under the centre of the anvil. (Material: lead.)
Table I The features of the deformation
W/Ho
HEimax/H
elm~/(AH/Ho)
0.3 0.4 0.5 0.6 0.8
0.80 0.78 0.67 0.50 0.50
3.7 3.3 2.3 2.1 2.2
closing of the internal defects. When W/Ho = 0.5, there also exists a 0.3 W largedeformation plateau in the central line. When W/Ho < 0.5, the large-deformation zone is far from the central zone. From the viewpoint of an appropriate distribution of deformation, a value of 0.5 < = W/Ho < = 0.6 should be used in the main reduction processes.
4. Single reduction results at H - T
In Section 3, the investigation was carried out at R-T with lead as the model material. In order to include the influence of the temperature and material properties on the cavity closing law and to check the validity of the R-T results, H-T experiments were carried out. First, the temperature changes during the forging processes are measured; then the forging effect on the cavity defects and the porosity defects are examined; thirdly, using a newly developed H-T Moir6 modelling technique, the internal strain distribution is obtained.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
203
U'/,//.//////~
t//////////J
- ~ z ~ ' ~ "-'+''"'<~'";,'"~'"72 ~. -o.os I
_~I
l , , I , , , I ~,~,
, , r 41 , ~' , I , , , i
£y
Y~ - + o 3 ~
y
b
L//////////J
U'/////////J 0.4 7
.4
0.3
i
,
,
,
i
,
,
,
i
,
,Q.Q,
-
-
°-!-, -
~
[
~
........... 4
-0.4
8
l~re
-0.4 d
Fig. 9. The strain distribution of the centre line (material: lead): (a) W/Ho = 0.3; (b) W/Ho = 0.4; (c) W/Ho = 0.5; (d) W/Ho = 0.6.
4.1. The measurement of temperature The dimensions of the testpiece are 50 × 50 x 80 mm. Three holes, ~b3.5 × 20, were drilled (Fig. 10). An electric furnace of 600 W power is used as an additional heat source to compensate for heat loss. The testpiece is first heated to 1220 °C and, then transferred quickly to the test machine and pressed. The main results are as follows: (i) the temperature gradient (points 1 and 3) is 90 ~ 100 °C during the forging processes; (ii) the temperature of central point 1 is 1100 °C at the end of the forging process. These results show that the modelling condition is similar to that of the real production situation.
4.2. The forging effect at H-T First, the forging effect on the cavity defects is examined. A central cavity is made on the testpiece (Fig. 11), the cavity being sealed by welding. The testpiece is protected by an anti-oxide coating, then heated to 1220 °C and held for 30 min, after which it was pressed on a 300 MN test machine at = 3.5 x 10- 3 s- 1. Subsequently to being deformed and then cooled in air to R-T, it
204
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
~ 2
~ce
i
Fig. 10. The measurement of temperature. (Material: mild steel.) i
w t t
T
................. ~
.........
A
q i
a
b
Fig. 11. The artificial cavity (material: mild steel): (a) its position; (b) its dimensions. was separated at section A (Fig. l l(b)). The "critical closing reduction ratio" was found with the help of microscopic observation to ensure the total welding of the cavity defects. Figure 4 is the critical closing reduction ratio curve, only W/Ho = 0.4, 0.6 being examined. When W/Ho = 0.6, the critical closing reduction ratio is the smallest both at R-T and H-T, the critical closing reduction ratio of the artificial cavity at H - T being 2% lower than that at R-T. The R-T result can thus represent the H - T result well and it is safer using the R-T results in the real production process. The main reason for the difference of the result between H - T result and R-T result is that there exists an approximately 100 °C temperature gradient between the surface and the core of the testpiece. Second, the forging effect on porosity defects was examined. A sintered steel power rod with a relative density of Po = 0.7526 was put into the centre of the testpiece (Fig. 12). After reduction to a ratio AH/Ho = 20%, the testpiece was separated at section A-A, and then the area deformation and the hardness of the porous steel rod were examined. Figures 13 and 14 present the area deformation curve and the hardness curve respectively, from both of which curves it is seen that W/Ho = 0.6 is the optimum AWR for forging the porosity defects.
S. Wang, Q, Cao/Journal of Materh21s Processing Technology 43 (1994) 195-209
205
8Oo
~70(
hi
60-
+- ...... ,,- .... ,-,,'-0- ....
" ...... 1-i 'T ........
If
50W/Ho
4O
o.4
Fig. 12. The steel rod porosity specimen.
0.'5
0.'6
0.'7
Fig. 13. The area deformation, (Material: sintered steel powder rod.)
(Material: mild steel.)
200-~
180.
160140
0.4
o.g
o.~
o.~
Fig. 14. The hardness curve. (Material: sintered steel powder rod.)
Similar experiments on both cavity defects and on porosity defects show that at H-T, W/Ho = 0.6 is again the optimum AWR.
4.3. The Moir@ analysis at H-T The newly developed H-T Moir~ modelling technique is used to obtain the strain distribution after H-T deformation. It has been a long search to find an effective technique for the recording of H-T deformation information, but the Moir6 method is promising. The experiment is carried out in four steps. First, Moir6 gratings that can endure both a long period at H-T and large deformation are made on the central section of the testpiece by the brush plating method; second, a layer of anti-welding coating is put onto the gratings and the two parts of the testpiece are welded together and then deformed to the required reduction ratio; third, the testpiece is cooled to R-T in air and then separated and an oxidation process is used to raise the contrast of the Moir~ fringes; fourthly, the Moir6 information is collected and processed, the strain distribution then being obtained. Three AWRs of W/Ho = 0.4, 0.5, 0.6 were investigated, Fig. 15 being the Moir6 patterns for W/Ho = 0.4 and 0.6, whilst Figs. 16 and 17 are the strain distribution after H-T deformation. Comparing Fig. 16 with Fig. 9, the strain distribution at H-T at the centre line is more uniform than that at R-T. From Fig. 17 and Table 2, it is noted that with the increase of AWR, the maximum deformation penetrates into the testpiece centre, the
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195- 209
206
a)
b) Fig. 15. The Moir6 patterns after deformation at 1200°C (material: mild steel): (a) W/Ho=0.4,
AH/Ho = 15%; (b) W/Ho = 0.6, AH/Ho = 12%. (~ = 3.5 x 10 -3 s -1, 51/mm)
~//'///////3
U/////////)
0.4-
J
_
~
~
~
'
' '~' ' '~' ' '112' ' '1~
$
1 iiJ
Ir¢,l=,llr=,i,,OFO,
i1~
8
fl=
J~J
12 16 20
-0.4 J
-0.4
a)
i,i
4
b)
Fig. 16. The strain distribution in the centre line (material: mild steel): (a) W/Ho = 0.4; (b) W/Ho = 0.6.
p e n e t r a t i n g speed at H - T is h i g h e r t h a n t h a t at R-T, a n d o n l y w h e n W/Ho --- 0.6 does the m a x i m u m d e f o r m a t i o n reach the centre, t h e r e a s o n for w h i c h is t h a t there exists a t e m p e r a t u r e g r a d i e n t b e t w e e n the surface a n d the c e n t r e z o n e at H - T . T h u s , the R - T results for l e a d c a n be used with safety in p r o d u c t i o n processes.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195- 209
YllI
207
~.4 (t-T) • W/Ho-O.6 ~-T) o w/tk-0.4 0t-T)
, w/no-o.6~-T) Ei
I
'
I
'
[
0.6 0.4- 0.2
'
I
'
I
'
I
0 0.2 0.4 0.6
Fig. 17. The distribution of effectivestrain under the anvil (left: lead, right: steel). Table 2 The relationship between the deformation zone and the AWR W/Ho
0.4 0.5 0.6
Hsim,jH
e,~/(AH/Ho)
R-T
H-T
R-T
H-T
0.80 0.67 0.50
0.68 0.60 0.50
3.3 2.3 2.1
3.77 3.25 3.0
The H - T Moir6 technique showed that the Moir6 patterns are sufficiently clear and that the technique can be used in the investigation of hot forming processes to search for the o p t i m u m technological parameters. It is especially useful in combining plasticity analysis with engineering materials science to construct a quality control system for a forming processes.
5. T h e i n t e r n a l s t r e s s a n a l y s i s
Based on plasticity theory for porous material, the critical closing criterion of internal cavities was derived by Jin [7] as:
where ei is the effective strain; am/a, is the triaxility; and Po is the initial porosity ratio. Thus, the o p t i m u m technological parameters can only be achieved when both the stress state and the strain state are favourable for closing the internal cavities. Using the Moir6 method at R-T, four AWRs of W/Ho = 0.4, 0.5, 0.6, 0.8 are analyzed.
208
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195- 209 0.2
axlas
-
*
W/Ho=0.8
Fig. 18. The axial stress ax in centre line. y15-
" * W/Ho=0.8
,
,
,
,
i~ ado,
, - .
- 1 . 2 - 0 . 8 - 0 . 4 00.
,
0.4
0.8
",1 "% \o,./a, I
-1.2
I
d,Sl
f
II
I~
- 0 . 4 0.0
~-- I
0.4
"T
I
0.8
Fig. 19. The stress distribution under the anvil central section (left. ax; right, am) F r o m Fig. 18, when W/Ho = 0.5, the axial stress a~ realized the transition from tension to compression. W h e n W/Ho = 0.6, the distribution of a~ in the whole central line is compressive and is m u c h better than that for W/Ho = 0.8. F r o m Fig. 19, for every A W R , the value of ~r~ is compressive at the upper part and tensile at the lower part. The triaxiality distribution shows that W/Ho = 0.4 is not an appropriate A W R , reasonable A W R s being W/Ho = 0.5 and 0.6. W h e n W/Ho = 0.8, the triaxiality is fairly uniform and in the central zone is smaller than that when W/Ho = 0.5 and 0.6. F o r an appropriate stress distribution, therefore, W/Ho = 0.5 and 0.6 are recommended, where W/Ho = 0.6 is the o p t i m u m AWR.
6. Conclusions Based on the above investigation on the F M forging process (single reduction), the following conclusions can be drawn: (1) The "critical closing reduction ratio" under R-T with lead as the model material has been obtained, showing that W/Ho = 0.6 is the o p t i m u m A W R for the F M process.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
209
(2) The H-T experiment verified that for both cavity defects and porosity defects,
W/Ho = 0.6 is the optimum AWR for the FM process. (3) The Moir6 analysis of the internal distribution of strain and stress showed that
W/Ho = 0.4 should not be used in the main reduction process. W/Ho = 0.5 can be used as the initiation AWR for the main reduction process. The distribution of both strain and stress distribution at W/Ho = 0.6 is favourable for closing the internal defects, a nearly symmetrical strain distribution being obtained. (4) The H-T experiments showed that when the temperature gradient is not large ( < 100 °C), an experiment into the forging effect with lead at R-T can simulate that with steel at H-T and the results can be used in production with safety. The difference between the results for lead and those for steel is caused mainly by the existence of a temperature gradient between the surface and the core: this showed also that the temperature gradient has an influence on the forging effect. (5) The newly developed high-temperature Moir~ modelling technique is a powerful tool in research into hot forming processes.
References [1] K. Nakajima, K. Watanabe, S. Watanabe and I. Tamura, Study on the closing and consolidation of internal cavities in heavy ingots by hot free forging, Proc. 4th ICPE, Tokyo, 1980, p. 166. [2J I. Tamura, S. Watanabe, K. Watanabe and K. Nakajima, Development of new processes for control of internal deformation and internal stress in hot free forging of heavy ingots, Trans. Iron Steel Inst. Jpn., 24 (1984) p. 101. I-3] K.D. Haverkamp and H.P. Heil, Deformation conditions for close forging of the core zone by drawing down, Stahl Eisen, 105 (22) (1985) p. 1214. 1'4] S.P. Dudra and Y.-T. Im; Investigation of metal flow in open-die forging with different die and billet geometries, J. Mater. Process. Technol., 21 (1990) 143-153. I-5] E. Erman, N.M. Medei, A.R. Roesch and D.C. Shah, Physical modelling of the blocking process in open-die press forging, J. Mech. Working Technol., 19 (1989) p. 165-194. 1-6] QX. Cao, S.Y. Ye, B. Xie and Y.X. Zhong, Modelling of free large forging processes by photo-electric scanning Moir6 method, Proc. 2nd ICTP, Sttuttgart, 1987. 1'7] N. Jin, An investigation on criterion for collapsing cavities and porous defects in large Forgings, to be published. 1'8] S.L. Wang, Modelling FM (Free from Mannesmann effect) heavy forging process by Moir6 method, Ph.D. Thesis, Tsinghua University, January 1991 (in Chinese). I-9] S.L. Wang, An introduction to Moir6 method, TUE Internal Report, WPA 1278, March 1992. [10] Q.X. Cao, S.Y. Ye, B. Xie and X.T. Ma, The Principle and Application of Moirb Method, Tsinghua University Press, China, 1983 (in Chinese). 1'11] Q.X. Cao, B. Xie and Y.X. Zhong, The Engineering Application and Automatic fringes Processing of Moirb Method, China Railway Press, China 1990 (in Chinese).
J. Mater. Process. Technol. 43 (1994) 195-209
ELSEVIER
Materials Processing Technology
The investigation of the FM heavy forging process by the Moir6 method Part I: Single-reduction results ShunLong Wang *'a, QiXiang Cao b "Faculty of Mechanical Engineering, Eindhoven University of Technology, The Netherlands bDepartment of Mechanical Engineering, Tsinghua University, Beijing, China (Received January 4 1993; accepted in revised form November 19 1993)
Industrial Summary There exist some metallurgical defects, such as voids, microcracks, loose structure, etc., in the core of the heavy forging ingots. To insure a good quality of the finished product, these defects must be eliminated during the forging processes. In this paper, using the Moir~ method, a newly developed 1200 °C high-temperature Moir6 modelling technique and apparatus for measuring the critical closing reduction of artificial cavities, the single reduction of the FM heavy free-forging process has been investigated systematically, the critical dosing reduction rate curve of artificial cavities and the internal strain/stress distribution having been obtained. The Moir6 analysis shows that when the anvil width ratio (AWR) W/Ho = 0.6, the maximum deformation penetrates into the core of the specimen and, a nearly symmetrical deformation distribution is obtained in the use of an unsymmetric anvil arrangement, where W/Ho = 0.5 is the transition anvil width ratio of the central axial stress (from tension to compression). Experiments on porous material also show that W/Ho = 0.6 has the strongest forging effect. The author thus conclude that W/Ho = 0.6 is the optimum anvil width ratio for the FM forging process. The Moir6 patterns obtained after the 1200°C forgings process are very successful. The development of this technique affords a powerful tool in research into hot-forming processes, especially useful in the integration of plasticity theory with materials science. Finally, some suggestions are made for the improvement of the FM process.
Key words: FM process; Moir6 method; Heavy Forging
1. Introduction With the progress of technology, the metallurgical energy, chemical, shipbuilding, heavy machinery and aerospace industries all need high capacity, high performance
*Corresponding author. 0924-0136/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 3 ) E 0 1 2 9 - 5
196
S. Wang, Q. Cao/Journal ~" Materials Processing Technology 43 (1994) 195 209
heavy equipment. There is the need for producing large forgings without any defects to serve the above-mentioned industries. Large forgings are usually the key components in the complete sets of equipment and their quality standards are rising. However, with the size of the large forgings becoming ever larger, there is an increasing difficulty in producing large forgings of high quality. The real production processes are very complex, so that systematic modelling experiments are necessary to ensure product quality. In the last three decades, a lot of progress have been made in the melting and casting processes of the heavy ingots. However, due to the solidification characteristics of the metal, it is inevitable that some metallurgical defects form in the core of the ingots, such as shrinkage cavities and porous structure. These defects must be eliminated during the forging process to ensure the service quality of the forgings. With the limitation of available press load capacity, the traditional upsetting process is limited. More attention is now being given to reduction processes (also referred to as blocking processes, which reduce the ingot cross section and increase its length by side pressing). Several reduction processes, such as FM, WHF, J.T.S, V-shaped anvil, etc, have been put forward and are used in forging practice [-1-6]. The FM (Free from Mannesmann effect) forging process is based on slip-line field analysis, using an unsymmetrical arrangement of the upper anvil and lower platform to avoid the generation of tensile stress, in the core of the billet (Fig. 1). In this paper, using a pneumatic apparatus for measuring the critical closing reduction ratio of artificial cavities, the Moir6 method and a high-temperature (H-T) Moir6 modelling technique, the critical closing reduction ratio curve is obtained, the internal strain and stress distribution is analyzed, the results for lead and steel specimens are compared and, finally, the optimum AWR is found.
Iy(v) /z(w)
Fig. 1. The schematic drawing of FM process.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
197
2. Experimental principle 2.1. The pneumatic apparatus
A pneumatic apparatus, shown in Fig. 2, has been designed for measuring the critical closing reduction ratio of artificial cavities. The critical closing reduction ratio and the closing sequences can be measured accurately, and several cavities can be measured simultaneously. This apparatus is very useful in searching for the optimal technical parameters. 2.2. MoirO method
The Moir6 method is used to measure the internal strain and stress distribution. Moir6 gratings are stuck (at room temperature (R-T)) and brush electroplated (at H-T) on the central section of the testpiece (Fig. 1). The Moir6 information is collected and processed with a "Photo-electric Scanning Digital Image Processing System". The strain is calculated using the Eulerian expression 19,10]:
( oy 1 - c~xJ + \ d x J '
er~ = 1 -
erE=l_
//(1 dv 8x
7x~ = arcsin-
av'~u
- ey/
+
(du~ 2,
+ \ay/
8u
dudu
dvdv
dy
dxdy
dxdy
The value of e~ is obtained from the incompressible condition 8Z E ~
__
E E E /3E ~ /3y - - ~x/3y E E E"
1 - e ~ - e r +exey
The stress deviator is obtained by the use of the Lev~ Mises plasticity theory with incremental loading, whilst the stress distribution is obtained by integrating the equilibrium equation [9-11].
1
2
3
4
5
6
Fig. 2. The schematic drawing of the pneumatic apparatus: 1. pump; 2. energy accumulator; 3. gas divider; 4. pipe connector; 5. testpiece; 6. water pool.
198
S. Wang, Q. Cao/Journal o/ Materials Processing Technology 43 ~1994 ~ 195 209
3. The single reduction results at R-T
3.1. The critical closing reduction ratio curve Several researches I-1-5] have been carried out to search for the optimum AWR, but due to the limitation of the experimental technique and of the different judgement criterion, there is no commonly accepted optimum AWR. In this section, using the pneumatic apparatus, the critical closing reduction ratio curve for the closing of artificial cavities is obtained and the optimum AWR is found. Figure 3 shows the distribution of the artificial cavities. AWRs of W/Ho = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 were investigated. Here Wis the feed width of the upper anvil and Ho is the initial height of the testpiece. Five horizontal holes are made in the core of the 1/5Ho region as artificial defects. The experiment is carried out on a 300 kN test machine, the loading strain rate being ~ = 10 -4 s -1. Figure 4 presents the critical closing reduction ratio curve of the artificial cavities. It is found that for the closing of artificial cavities of either 1/40Ho (4~1.0 mm) or 1/50Ho (~b0.8 mm), the critical closing reduction ratio is the minimum when W/Ho = 0.6, the critical reduction ratios for the closing of the central cavities being AH/Ho = 14% and AH/Ho = 1 2 ° , respectively. Figure 5 is the forging load curve (AH/Ho = 20%). With increase of the AWR, the forging load increases. The forging load for W/Ho = 0.6 is 20% greater than that for W/Ho = 0.4. Fig. 6 is the deflection of central cavity. The unsymmetric anvil arrangement causes non-uniform material flow and the deflection of the central cavity, this being especially serious when W/Ho is less than W/Ho = 0.6. With the increase of the AWR, the deflection decreases: when W/Ho = 0.6, the deflection of the central cavity is about 3.5%. The critical reduction ratio reflects the comprehensive influence of the strain state, stress state, strain rate, etc., on the closing of the cavities, so that W/Ho = 0.6 is the optimum AWR of the FM forging process.
3.2. MoirO analysis of the stra& distribution F - F G Moir6 gratings (51/mm) were used, the two parts of the testpiece being connected together by low melting point alloy. Fig. 7 are the Moir6 patterns
Ho
V/
=
o
/ / / / / / / / / /
I
Fig. 3. The distribution of the artificial cavities. (Material: lead.)
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
x R--T(20*O
~, 20-
o
H-T(1200*C)
~
199
l/SHo re$iem * eernal cavity a
25-
~1515-
o 10
5
10,
,
wa-Io
0..3 0.4 0.'5 0.'6 0.7 0.'8 a)
5
,
w/ri0
0.,3 0.'4 0.'5 0.'6 0.7 0.'8 b)
Fig. 4. Critical closing reduction ratio curves: (a) ~b 1.0 mm (central cavity); (b) t# 0.8 mm.
/ 15105w/I 0 0.'4 0.'5 0.'6 0.'7 0.'8 0.3 Fig. 5. The forging load.
10- IX,fleeti~ of central cavity 8642 0.3 0.'4 0.'5 0.'6 0.'7 0.8 Fig. 6. The deflection of the central cavity.
corresponding to the critical closing reduction ratio, whilst Fig. 8 is the comparison of the strain distribution under the anvil center along the reduction direction. Table 1 presents the relationship between the AWR and the deformation features. Here H,rmax refers to the height from maximum *i to the bottom platform and *imaxis the maximum e~ in the cross section just under the anvil centre. It can be seen that when W/Ho < 0.5, the deformation zone is concentrated mainly on the upper part and the deformation is very non-uniform. With the increase of W/Ho, the zone of large deformation penetrates into the specimen centre and when W/Ho = 0.6 the maximum strain reaches the specimen centre, which is favourable both for closing of the cavity defects and for obtaining a refined and homogeneous internal structure. Figure 9 are the strain distributions in the central line. There is maximum deformation zone under the anvil centre, the deformation is small at the anvil corner, and with increase of the AWR, the zone of maximum deformation changes from a peak to a plateau: when W/Ho = 0.5 and W/Ho = 0.6, this plateau is at 0.3 and 0.5 W respectively. These strain distributions indicate that only the central part under the anvil is a valid closing zone, there existing unclosing zones at the anvil corners. These
200
S. Wang, Q. CaojJournal of Materials Processing Technology 43 (1994) 195-209
al
bl
el
S. Wang, Q. Cao/Journal o f Materials Processing Technology 43 (1994) 195-209
201
d)
el
f) Fig. 7. (continued).
unclosing zones must be covered by the valid closing zone in the later stages of reduction through an appropriate turn-over and feed procedure. The Moir6 analysis shows that when W/Ho = 0.6, using the unsymmetric anvil arrangement, a nearly symmetric deformation distribution is obtained and that there is also 0.5 W large-deformation plateau in the central line: this is favourable for the
Fig. 7. The Moir~ patterns for various A W R s (material: lead): (a) W/Ho--0.3, AH/Ho = 16%; (b) W/Ho ffi 0.4, AH/H o = 17%; (c) W/H o = 0.5, AH/Ho = 16.5%; (d) W/Ho--0.6, AH/Ho = 14%; (e) W/Ho = 0.7, AH/H o = 17%; (f) W/Ho = 0.8, AH/H o = 18.4%.
202
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195 209
1
0-1 ~ /~//~ / Jl,l//.I
o WJHo'0.4 >< v¢~..o.~
3t ~
* W/I-Io-0.8
]~ f / / /
0.2 0.0
0.2
0.4
~ wm,-o.6
0.6
0.8
Fig. 8. The strain distribution under the centre of the anvil. (Material: lead.)
Table I The features of the deformation
W/Ho
HEimax/H
elm~/(AH/Ho)
0.3 0.4 0.5 0.6 0.8
0.80 0.78 0.67 0.50 0.50
3.7 3.3 2.3 2.1 2.2
closing of the internal defects. When W/Ho = 0.5, there also exists a 0.3 W largedeformation plateau in the central line. When W/Ho < 0.5, the large-deformation zone is far from the central zone. From the viewpoint of an appropriate distribution of deformation, a value of 0.5 < = W/Ho < = 0.6 should be used in the main reduction processes.
4. Single reduction results at H - T
In Section 3, the investigation was carried out at R-T with lead as the model material. In order to include the influence of the temperature and material properties on the cavity closing law and to check the validity of the R-T results, H-T experiments were carried out. First, the temperature changes during the forging processes are measured; then the forging effect on the cavity defects and the porosity defects are examined; thirdly, using a newly developed H-T Moir6 modelling technique, the internal strain distribution is obtained.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
203
U'/,//.//////~
t//////////J
- ~ z ~ ' ~ "-'+''"'<~'";,'"~'"72 ~. -o.os I
_~I
l , , I , , , I ~,~,
, , r 41 , ~' , I , , , i
£y
Y~ - + o 3 ~
y
b
L//////////J
U'/////////J 0.4 7
.4
0.3
i
,
,
,
i
,
,
,
i
,
,Q.Q,
-
-
°-!-, -
~
[
~
........... 4
-0.4
8
l~re
-0.4 d
Fig. 9. The strain distribution of the centre line (material: lead): (a) W/Ho = 0.3; (b) W/Ho = 0.4; (c) W/Ho = 0.5; (d) W/Ho = 0.6.
4.1. The measurement of temperature The dimensions of the testpiece are 50 × 50 x 80 mm. Three holes, ~b3.5 × 20, were drilled (Fig. 10). An electric furnace of 600 W power is used as an additional heat source to compensate for heat loss. The testpiece is first heated to 1220 °C and, then transferred quickly to the test machine and pressed. The main results are as follows: (i) the temperature gradient (points 1 and 3) is 90 ~ 100 °C during the forging processes; (ii) the temperature of central point 1 is 1100 °C at the end of the forging process. These results show that the modelling condition is similar to that of the real production situation.
4.2. The forging effect at H-T First, the forging effect on the cavity defects is examined. A central cavity is made on the testpiece (Fig. 11), the cavity being sealed by welding. The testpiece is protected by an anti-oxide coating, then heated to 1220 °C and held for 30 min, after which it was pressed on a 300 MN test machine at = 3.5 x 10- 3 s- 1. Subsequently to being deformed and then cooled in air to R-T, it
204
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
~ 2
~ce
i
Fig. 10. The measurement of temperature. (Material: mild steel.) i
w t t
T
................. ~
.........
A
q i
a
b
Fig. 11. The artificial cavity (material: mild steel): (a) its position; (b) its dimensions. was separated at section A (Fig. l l(b)). The "critical closing reduction ratio" was found with the help of microscopic observation to ensure the total welding of the cavity defects. Figure 4 is the critical closing reduction ratio curve, only W/Ho = 0.4, 0.6 being examined. When W/Ho = 0.6, the critical closing reduction ratio is the smallest both at R-T and H-T, the critical closing reduction ratio of the artificial cavity at H - T being 2% lower than that at R-T. The R-T result can thus represent the H - T result well and it is safer using the R-T results in the real production process. The main reason for the difference of the result between H - T result and R-T result is that there exists an approximately 100 °C temperature gradient between the surface and the core of the testpiece. Second, the forging effect on porosity defects was examined. A sintered steel power rod with a relative density of Po = 0.7526 was put into the centre of the testpiece (Fig. 12). After reduction to a ratio AH/Ho = 20%, the testpiece was separated at section A-A, and then the area deformation and the hardness of the porous steel rod were examined. Figures 13 and 14 present the area deformation curve and the hardness curve respectively, from both of which curves it is seen that W/Ho = 0.6 is the optimum AWR for forging the porosity defects.
S. Wang, Q, Cao/Journal of Materh21s Processing Technology 43 (1994) 195-209
205
8Oo
~70(
hi
60-
+- ...... ,,- .... ,-,,'-0- ....
" ...... 1-i 'T ........
If
50W/Ho
4O
o.4
Fig. 12. The steel rod porosity specimen.
0.'5
0.'6
0.'7
Fig. 13. The area deformation, (Material: sintered steel powder rod.)
(Material: mild steel.)
200-~
180.
160140
0.4
o.g
o.~
o.~
Fig. 14. The hardness curve. (Material: sintered steel powder rod.)
Similar experiments on both cavity defects and on porosity defects show that at H-T, W/Ho = 0.6 is again the optimum AWR.
4.3. The Moir@ analysis at H-T The newly developed H-T Moir~ modelling technique is used to obtain the strain distribution after H-T deformation. It has been a long search to find an effective technique for the recording of H-T deformation information, but the Moir6 method is promising. The experiment is carried out in four steps. First, Moir6 gratings that can endure both a long period at H-T and large deformation are made on the central section of the testpiece by the brush plating method; second, a layer of anti-welding coating is put onto the gratings and the two parts of the testpiece are welded together and then deformed to the required reduction ratio; third, the testpiece is cooled to R-T in air and then separated and an oxidation process is used to raise the contrast of the Moir~ fringes; fourthly, the Moir6 information is collected and processed, the strain distribution then being obtained. Three AWRs of W/Ho = 0.4, 0.5, 0.6 were investigated, Fig. 15 being the Moir6 patterns for W/Ho = 0.4 and 0.6, whilst Figs. 16 and 17 are the strain distribution after H-T deformation. Comparing Fig. 16 with Fig. 9, the strain distribution at H-T at the centre line is more uniform than that at R-T. From Fig. 17 and Table 2, it is noted that with the increase of AWR, the maximum deformation penetrates into the testpiece centre, the
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195- 209
206
a)
b) Fig. 15. The Moir6 patterns after deformation at 1200°C (material: mild steel): (a) W/Ho=0.4,
AH/Ho = 15%; (b) W/Ho = 0.6, AH/Ho = 12%. (~ = 3.5 x 10 -3 s -1, 51/mm)
~//'///////3
U/////////)
0.4-
J
_
~
~
~
'
' '~' ' '~' ' '112' ' '1~
$
1 iiJ
Ir¢,l=,llr=,i,,OFO,
i1~
8
fl=
J~J
12 16 20
-0.4 J
-0.4
a)
i,i
4
b)
Fig. 16. The strain distribution in the centre line (material: mild steel): (a) W/Ho = 0.4; (b) W/Ho = 0.6.
p e n e t r a t i n g speed at H - T is h i g h e r t h a n t h a t at R-T, a n d o n l y w h e n W/Ho --- 0.6 does the m a x i m u m d e f o r m a t i o n reach the centre, t h e r e a s o n for w h i c h is t h a t there exists a t e m p e r a t u r e g r a d i e n t b e t w e e n the surface a n d the c e n t r e z o n e at H - T . T h u s , the R - T results for l e a d c a n be used with safety in p r o d u c t i o n processes.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195- 209
YllI
207
~.4 (t-T) • W/Ho-O.6 ~-T) o w/tk-0.4 0t-T)
, w/no-o.6~-T) Ei
I
'
I
'
[
0.6 0.4- 0.2
'
I
'
I
'
I
0 0.2 0.4 0.6
Fig. 17. The distribution of effectivestrain under the anvil (left: lead, right: steel). Table 2 The relationship between the deformation zone and the AWR W/Ho
0.4 0.5 0.6
Hsim,jH
e,~/(AH/Ho)
R-T
H-T
R-T
H-T
0.80 0.67 0.50
0.68 0.60 0.50
3.3 2.3 2.1
3.77 3.25 3.0
The H - T Moir6 technique showed that the Moir6 patterns are sufficiently clear and that the technique can be used in the investigation of hot forming processes to search for the o p t i m u m technological parameters. It is especially useful in combining plasticity analysis with engineering materials science to construct a quality control system for a forming processes.
5. T h e i n t e r n a l s t r e s s a n a l y s i s
Based on plasticity theory for porous material, the critical closing criterion of internal cavities was derived by Jin [7] as:
where ei is the effective strain; am/a, is the triaxility; and Po is the initial porosity ratio. Thus, the o p t i m u m technological parameters can only be achieved when both the stress state and the strain state are favourable for closing the internal cavities. Using the Moir6 method at R-T, four AWRs of W/Ho = 0.4, 0.5, 0.6, 0.8 are analyzed.
208
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195- 209 0.2
axlas
-
*
W/Ho=0.8
Fig. 18. The axial stress ax in centre line. y15-
" * W/Ho=0.8
,
,
,
,
i~ ado,
, - .
- 1 . 2 - 0 . 8 - 0 . 4 00.
,
0.4
0.8
",1 "% \o,./a, I
-1.2
I
d,Sl
f
II
I~
- 0 . 4 0.0
~-- I
0.4
"T
I
0.8
Fig. 19. The stress distribution under the anvil central section (left. ax; right, am) F r o m Fig. 18, when W/Ho = 0.5, the axial stress a~ realized the transition from tension to compression. W h e n W/Ho = 0.6, the distribution of a~ in the whole central line is compressive and is m u c h better than that for W/Ho = 0.8. F r o m Fig. 19, for every A W R , the value of ~r~ is compressive at the upper part and tensile at the lower part. The triaxiality distribution shows that W/Ho = 0.4 is not an appropriate A W R , reasonable A W R s being W/Ho = 0.5 and 0.6. W h e n W/Ho = 0.8, the triaxiality is fairly uniform and in the central zone is smaller than that when W/Ho = 0.5 and 0.6. F o r an appropriate stress distribution, therefore, W/Ho = 0.5 and 0.6 are recommended, where W/Ho = 0.6 is the o p t i m u m AWR.
6. Conclusions Based on the above investigation on the F M forging process (single reduction), the following conclusions can be drawn: (1) The "critical closing reduction ratio" under R-T with lead as the model material has been obtained, showing that W/Ho = 0.6 is the o p t i m u m A W R for the F M process.
S. Wang, Q. Cao/Journal of Materials Processing Technology 43 (1994) 195-209
209
(2) The H-T experiment verified that for both cavity defects and porosity defects,
W/Ho = 0.6 is the optimum AWR for the FM process. (3) The Moir6 analysis of the internal distribution of strain and stress showed that
W/Ho = 0.4 should not be used in the main reduction process. W/Ho = 0.5 can be used as the initiation AWR for the main reduction process. The distribution of both strain and stress distribution at W/Ho = 0.6 is favourable for closing the internal defects, a nearly symmetrical strain distribution being obtained. (4) The H-T experiments showed that when the temperature gradient is not large ( < 100 °C), an experiment into the forging effect with lead at R-T can simulate that with steel at H-T and the results can be used in production with safety. The difference between the results for lead and those for steel is caused mainly by the existence of a temperature gradient between the surface and the core: this showed also that the temperature gradient has an influence on the forging effect. (5) The newly developed high-temperature Moir~ modelling technique is a powerful tool in research into hot forming processes.
References [1] K. Nakajima, K. Watanabe, S. Watanabe and I. Tamura, Study on the closing and consolidation of internal cavities in heavy ingots by hot free forging, Proc. 4th ICPE, Tokyo, 1980, p. 166. [2J I. Tamura, S. Watanabe, K. Watanabe and K. Nakajima, Development of new processes for control of internal deformation and internal stress in hot free forging of heavy ingots, Trans. Iron Steel Inst. Jpn., 24 (1984) p. 101. I-3] K.D. Haverkamp and H.P. Heil, Deformation conditions for close forging of the core zone by drawing down, Stahl Eisen, 105 (22) (1985) p. 1214. 1'4] S.P. Dudra and Y.-T. Im; Investigation of metal flow in open-die forging with different die and billet geometries, J. Mater. Process. Technol., 21 (1990) 143-153. I-5] E. Erman, N.M. Medei, A.R. Roesch and D.C. Shah, Physical modelling of the blocking process in open-die press forging, J. Mech. Working Technol., 19 (1989) p. 165-194. 1-6] QX. Cao, S.Y. Ye, B. Xie and Y.X. Zhong, Modelling of free large forging processes by photo-electric scanning Moir6 method, Proc. 2nd ICTP, Sttuttgart, 1987. 1'7] N. Jin, An investigation on criterion for collapsing cavities and porous defects in large Forgings, to be published. 1'8] S.L. Wang, Modelling FM (Free from Mannesmann effect) heavy forging process by Moir6 method, Ph.D. Thesis, Tsinghua University, January 1991 (in Chinese). I-9] S.L. Wang, An introduction to Moir6 method, TUE Internal Report, WPA 1278, March 1992. [10] Q.X. Cao, S.Y. Ye, B. Xie and X.T. Ma, The Principle and Application of Moirb Method, Tsinghua University Press, China, 1983 (in Chinese). 1'11] Q.X. Cao, B. Xie and Y.X. Zhong, The Engineering Application and Automatic fringes Processing of Moirb Method, China Railway Press, China 1990 (in Chinese).