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Effects of Carbon Content and Microstructures on the Current Efficiency in Case of Electrochemical Machining of Fe-C Alloys
Content and Microstructures on the Current Efficiency in Case of Electrochemical Machining of Fe-C Alloys H. Fukunaga and M. Maruyama, Hiroshima University - Submitted by N. Taniguchi (1). Japan Summary: The prcscnt work was carried oat to investigate systematlcally the effect of the carbon content and microstructure on the current effj.ciency and surface roughness of wor!+ece in the case of the electrochemical machining of steel. The annealed samples have kept two contrastiny characteristics dlstrnyurshed by bcing hypereutectoid or hypoeutcctoid constitution. Onc cjroup of the annealed samples consisting of ferrite and pearlitc structure gives a higher current efficiency than 1001 and a rouoh surface finish. The current efficiency and surface roughness increaserlinearly with carbon content. Another group of annealed samples consistinq of networked cementite and pearlite structure gives a poor efficiency about 60 to 751 and also a rough surface finish. On the other hand, cvcry quenched sample, which consists of homogeneous martensite phase, glves almost the theoretlcal current efficiency and a good surface finish. When quenchi3g condition changed so as to give the networked cementite and the martensite together, the quenched sample shows the poor current efficiency. Further investigation of machined surfaces by E P M proves that these characteristics are caused by the appearance of networked cementite, by the increase in the carbide phase and by the carbon state in the iron phase.
1
Introduction
Electrochemical machining characteristics are strongly affected by the compositions and microstructures of work materials. A review of previous works dealing with the effects of carbon content on current efficiency of carbon steels shows results which are contradictory, although these steels are in common use in various machine parts. YcGeough') has reported that current efficiency decreases with carbon content, when the carbon steels are electrochemically machined with a 20 8 NaCl electrolyte at a range of current density from 7 . 8 to 46.5 A/cs'. Tanaka et al?)have carried out the electrochemical machining of pure iron, normalized medium carbon steel (S45C) and normalized tool steel (SK5)using pressurized flow of a 10% NaC1 solution at a current density ranging from 20 to 90 A/cm'. They have concluded that the pratical removal rate of these steols is always smaller than the theoretical removal rate, and that the removal rate decreases proportionally with the carbon content. They have claimed, however, that the spheroidized treatment of carbide has a removal rate increase greater than the theoretical
2
Work Materials and Heat Treatment
Ingots of Fe-C alloys with different carbon levels were cast by melting a certain amount of pure iron and carbon in a high frequency induction furnace on a laboratory scale. These were not-forgel and machined to a rectangular parallelepiped shape of a given size. The results of a check analysis of their chemical compositions are shown in Table 1. To give the microstructures suitable for the present prpose, all work materials were full-annealed in a vacuum furnace, and quenched from the heating temperatures at 50°C higher than the Ac3, Acm line, or of up to 50°C higher than Acl line. The microphotoqraphs of the heat-treated Fe-C alloys show well-known structures as presented in Fig.1; Fig.l(a) is of ferrite and lamellar pearlite,
removal rate. On the other hand, in hiaeda et al's experiment3), in which the annealed sample of plain carbon steels, low and high carbon alloyed steei and low alloy steels are machined in a 203 XaCl solution at a current density of about 3 0 A/cm', they have found that the current efficiency for evcry steel exceeds a theoretical value of loo%.. However they have reported that there is no clear dependency of carbon content on current efficiency. In another ECM, Freer et al?)have shown that normalized, as well a s quenched and tempered steels, with a carbon content of 0.991 or 1.26',, machined in 20:: NaC1 at 25 A/cm'and 15 V, have also indicated lwnr current efficiency and dull granular finishes. However Makino et al?' have suggested that the amounts of decrease in the weight of the anodes increase with an increase in carbon content when annealed steels with different carbon constituents are electrochcrnically dissolved in a stationary 5 9 SaNOZ solution at 5 A/cmL. From these studies, it is difficult to draw a conclusive tendency, as we often encounter a difference in the microstructure, alloying elements, initial condition of workpiecc surfaces and so on, in each of the reports. In the present WO:~, a systeaatic study was planned, using binary alloys, to discuss the fundamental e f i e c t s of csrbon cQntPnt on current efficiency.
Annals of the ClRP Vol. 30/1/1981
a) d)
r):
Full-annealed
e): Quenchrd from the temperature of 5 l ) O C higher Lhan AcJ or .\cm line i ) : Quenched F r o m thc rempernture of i O ' C higbrr than A c l l i n t .
Fiq.1 Nicrophotoqraphs after heat-treatment.
117
Table 1 Chemical compositions of Fe-C alloys (wt.3)
( 1 ) work rnaterial consists of pure iron. ( 2 ) Iron dissolves as a
No.
C 0.003 0.13
1
2 3 4
0.31
5
0.38
6 7 8
0.44 0.45
0.32
nn
P
S
0.01 0.00
0.002
0.006 0.007 0.003 0.004 0.002 0.003 0.005 0.005 0.005 0.004
0.006
0.00 0.01 0.01 0.01
0.76
0.02 0.03
0.85 0.86
0.08 0.02
11 12 13
0.92
14 15 16
1.22 1.27
0.06 0.02 0.02 0.02 0.01 0.01-.-
9
10
0.04 1.18
1.30
A1
Si
<0.01 co.01 0.002 <3.01
co.01 0.002
0.002 0.002 <0.01 0.002
0.005
0.002
<0.01 co.01 0.002 <0.01 CO.01
0.003
0.005 0.003 0.002
0.009 0.004
0.004 0.005 0.007
0.004 0.004
D. 004 0.005 0.004 0.006 0.006 0.004 0.004 0.007 -~
tr 0.001 0.001
tr 0.002 0.001 tr tr tr 0.001
tr 0.001 0.001 tr 0.001 0.001
ferritic ion q f d i - v a l c ? n t :om. ( 3 ) Sub-reactions, such as the ycncration o f cizyqcn +ti - , d o not occur. gas, and I.'e+- * FC + c Looking at F i g . 3 , the results show that thc n ? . n e o l e d sample has two contrastinrj characteristics distinyished i s{ a hvpereutcctoid or hypoeutectoid constitution. Nasely, current efficiency sxceeds 1 0 0 % at a carbon content ranging from 0.003 to 0.459. However it drops steeply to a percentage ranqiny from 60 to 75: at a carbon content from 0.85 to l . . 3 G ? , . In each case, hown-~er,
Fig.l(b) is of lamellar pearlite, Fig. l(c) is of network cementite, Fig.l(d) and l(e) are of martensite, and Fig.l(f) is of network cementite and martensite.
3) Experimental Method for Electrochemical Machining An outline of the electrochemical machining appaA 10% NaCl solution at 20 ratus is shown in Fig.2. ? 1°C was forced through an electrode gap of 1 mm at a mean velocity of 10 m/sec. The applied potential fixed at 1 0 V . except when current density was changed for experimental purposes. An anode and cathode surface, 1Ox25mm and lOxlOmm wide, respectively, were smoothly The polished by buffing before every experiment. machining time was determined by preliminary experiments so as to give a stationary current efficiency.
5
0
0.2
0.4
0.6
o
0.8
1.0
1.2
1.4
w
Carbon content (Ye) Effect of carbon confenf on current efficiency for full annealed samples
Fig.3
Y
y1
:90
1
V
2,
70
\ 0.2 0.4 0.6 0 8 1.0 1.2 I4
Elect?yte reserviw
6o0
Fig.2 Outline of
ECM
Carbon content (7.)
apparatus Fg.4
The current efficiency was calculated from the weight difference before and after machining for both the workpiece and the cathode of the coulometer. 4
Results
The effects of carbon content on current effiency are illustrated in Fig.3 for annealed structures, and in Fiy.4 for quenched structures. The current efficiency, '1 , was calculated from the equaion:
'
=
Practical machined mass
x 100
($)
--[l)
Theoretical removal mass
The theoretical removal mass was derived by the following assumptions.
118
Effect of carbon content on current efficiency for quuuhed sampks
the current efficiency increases with the carbon content. 'Jpon inspection @f the machined surfaces, a thin black layer exists on the surface of the samples containing carbon of more than 0.1%. This layer is easily wiped off. Once wiped off, a surface with a metallic lustre is observed in samples of less than 0.455 C, but a black surface in samples of more than 0.861 C. Figure 4 shows the current efficiency for quenched samples.For workpieces quenched from a temperature over the Ac3 o r Acm line,the current efficiency remains at nearly 100% showing no relation to carbon content. Ilowever for workpieccs quenchcd from a temperature over the Rcl line, current efficiency drops to less than 90 5 , decresiny with carbon content. Upon inspection of
these samples, a homogeneous thin layer with mumerous small particles is found on the machined surfaces. When wiped off, a smooth surface appears in samples where current efficiency reaches nearly 100:. but a rough surface with numerous pitted holes in samples where current efficiency is less than 90%. The surface topography mentioned above shows a good corelation with the roughness of machined surfaces measured by a profile meter as shown in Fig. 5 . It is evident from the results that the surface roughness not only increases with carbon content, but also is greater on the annealed samples than on the quenched samples. Furtherinore, there seems to be sufficient corelation between dependencies of carbon content on surface roughness and on current efficiency, as can be seen from the fact that the upper and lower curves in Fig. 5
9 4
A 5
0 003%C 0 13%C 0 LL%C 08 6'l.C
Flow velocity 15mk Re L160 Turbulence
O*/.NaCI Z O W C
tv-+T---=.
Sap Irnrn
h uylc
I
10
20 Current
Fig 6
30
LO
density (Alcm')
Current efficiency as a function of current density for full annealed samples 0.003%C 013%C 0 44.I.C A 0867.C
3
Flow wlocily lSmls Re 4160 Turbulence IO*/&CI 2022.C Gap Irnm
A
-s 1oc Y
ZI
C
.-
U
I'
1 leXC I 1W.C
\
s
y
5c d
Quenched from 50'C o w ACI line
?2 I
0.6 0.8 1.0 Carbon content (x)
0
0 8
0.2 0 4
I
I
1.2
14
3 V
0
10
20 Current
Fig.5
Effect of carbon content on surfoce roughness
Fig.7
have an analogy with the curve in Fig.3 and the upper curve in Fig.4, respectively. The influence of current density on the current efficiency of annealed and quenched Fe-C alloys is shown in Fig.6 and 7, respectively. For both cases, it is apparent that current efficiency decreases with current density. This behaviour corresponds to Chin's reports6), in which he machined mild steel in a 4N solution of NaCl at 1 A/cm'to 100 A/cm!
30
40
density (Alcml)
Current efficiency as a function of current density tor quenched samples
5. Discussion
The varaeties of current efficiency, of the surface layer, and of the surface roughness for Fe-C alloys caused by the increase in carbon content and by the change of microstructure seems to depend on the chemical states of the carbon atoms, and also on the morphology of cementite. The ferrite and pearlite structure It is well known that the ferrite ?base solutes very small amounts of carbon a t o m and some workers3) say that the cementite phase is not dissoluble by means of dirrect current. So far as Faraday's law is applied, the theoretical current efficiency, y t , of the workpiece consisting of a ferrite and pearlite structure is given as a function of the carbon content by the following equation. 5.1
p t = (1+3MFe / Where, M
and Fe
MC)
1.1
C
X
C
4-
100 ( % )
---(2)
are the atomic weights of
(a) Srroiidary electron image C R I X-ray image ( e ) C 1 K j 'X-rAy imngc (c)
Fig.8
( b ) Absorbed current image (d) FeK't X-ray image ( f ) VaKIi
80 11m
X-rav imgr
An example of CP42A analysis on t h e machined surface. (Full-annealed sample of 0 . 1 3 4 C)
119
iron and carbon, respectively, and C is the wei& percent of carbon. The equation is illustrated by a thin solid linc in Fig.3. In this figurc, it can be seen that experimental plots seem to satisfy the equahave obtained similar tion. A few results. This may lead to justification of the assumptions which were necessary to derive the above equation, but such an ideal dissolution process is considered as not always being possible during ECM according to the analysis by EPMA on a machined surface. Figure 8 includes a typical example of the secondary electeon image, absorbed current image and the characteristic Xray images of Fe and C showing that.the ferrite is machined to a greater extent than the pearlite phase, and also that carbon rich particles are scattered on the machined surface. As these particles are about one tenth as small as the size of a pearlite grain, they appear to pearlite colonies, isolated by a selective dissolution of the ferrite phase. This leads to a saving in the quantity of electricity. Other characteristic X-ray images show a negative corelation between C and Na, the same as that between Na and C1, but a possitivf corelation hetween C and C1.This lead to the consumption of a quantity of electricity. In effect, this seems to be not a result of Faraday's law, but a balanced phenomencabetween a saving and consumption of electricity, and that the experimental value is nearly equal tp the theoretical one, as shown in Fig.3.
(a) Secondarv electron tmagr
( c ) CK? (c)
X-ray image S-ray inap,e
C1K.k
Tiq.9
(b) Absorbed current image (d) FeK. X-ray image ( f ) SnKn X - r a y
U
80,,,m
imay
An example of EPMA analysis on the machined surface. (Full-annealed sample of 1.273 C)
5.2 The network cementite and Bearlite structure The analysis by EPNA presented in Fig.9 shows that the machined surface is covered with cementite which protrudes in the form of a network, and that small holes exist here and there on the surface. Observation of the characteristic images tells us that the C1 element is rich on the network cementite and also that there is a negative corelation between C1 and Na. The severe decrease in current efficiency of hypereutectoid Fe-C alloys is explainable as follows: The cementites, being passive, are exposed over the entire machined surface. Thus the anode overpotential and current density rise locally, resulting in harmful secondary reactions which become predeominant.
The martensite structure An example of the EPElA analysis is illustrated in Fig.10. Numerous raised particles can be seen on the machined surface, which consist of carbon. Chlorine can be identified, but this corclates neither possitively nor negatively with the carbon. As the workpiece is free from cementite and as it dissolutes carbon to an atomic state, the electrochemical dissolution of the martensite seems to advance as follows: The dissolution of di-valent iron atoms results in the isolation of solute carbon atoms without electric charge. The isolated carbon atoms are so active that they cohere to each other as particles on the surface. Theafore, the honoge5.3
120
(a) Secondarv e l e c t r o n Image
(h) FeKr X-ray
( c ) C 1 K i S-rav image
(d) CK7
~lq.10
An
image
U 20/im
X-ray image
example of EPMk analysis on the machlned surface.
(Quenched sample from 5 0 ° C over Acm, of 1.303 C)
neous martensite phase gives a current efficiency which is close to the theoretical potential and a good surface finish. The network Cementite and martensite structure The results in Fig.4 have shown that the current efficiency lies between those of the network cementite and pcarlite on the one hand, the martensite on the 5.4
othcr, and that it is significantly decreased with the growth of network ccmentitc. This behaviour is explained a s being due to the proposed dissolutlon prucesses if the two structures arc bcinq clectrochemically machined at the same time. These results could add a soundess to the proposed dissolution processes.
3) The EPMA analysis shows a close relation between the cementite and chlorine on machined surfaces of annealed samples, but no relationshi? between the carbon particles and chlorine on machined surfaces of quenched samples.
References
6. Conclusions
The results obtained by the electrochemical machining of Fe-C alloy in NaCl solution at a comparatively low current density are summarized as follows: 1) For the ferrite and lamellar pearlitc structures, a ferrite phase attack, accompanied by sub-reactions, results in current efficiencies in excess of 100%. and increasing current efficiency with increase of the carbon content. The primary network cementite brings current efficiencies down remarkably, as if it were a passive film. The current efficiency of martensite is nearly 100% regardless of the carbon content up to a certain current density. 2 ) Surface roughnesses of the ferrite and lamellar pearlite structures are increased with the carbon content due to the selective dissolution of ferrite phases. For the martensite structures, the surface roughness is small because of comparatively homogeneous dissolution.
.YcCeough, J.A., “Some effects of carbon content on the efficiency of electrochemically machined carbon steels and cast iron”, Int. J. Production Research, 9-2, 311 (1971). Tanaka, U.,Kikuchi, C.,”Effect of work-metal micro structures on the machining characteristics in electrochemical machining”, Technical Reports of Huroran Tech. College, 1-2, 589(1971). Maeda, S., Saito, 3 . and Hapma, Y., “Principle and characteristics of electrochemical machining”, Tech. Rep. Mitsubishi Electrics, 41-10, 1267f1967). Freer, H.E., Hanley, J.B. and Maclellan G . D . S . , Electrochemical Society Softbound Symposium Series, Princeton, 103(1971). :.lakino, H. and Kawakatsu, K., “Effect of carbon content on electrochemical grinding of carbon Steels, ?art 1. Fundamentals“, JSPE ?reprint, 121 (1970). Chin, D.T. and Wallace, A.J.,“Anodic current efficiency and dimensional control in electrochemical machining“, Electrochem. Society, 120-11, 1487 (1973).
121
1
Introduction
Electrochemical machining characteristics are strongly affected by the compositions and microstructures of work materials. A review of previous works dealing with the effects of carbon content on current efficiency of carbon steels shows results which are contradictory, although these steels are in common use in various machine parts. YcGeough') has reported that current efficiency decreases with carbon content, when the carbon steels are electrochemically machined with a 20 8 NaCl electrolyte at a range of current density from 7 . 8 to 46.5 A/cs'. Tanaka et al?)have carried out the electrochemical machining of pure iron, normalized medium carbon steel (S45C) and normalized tool steel (SK5)using pressurized flow of a 10% NaC1 solution at a current density ranging from 20 to 90 A/cm'. They have concluded that the pratical removal rate of these steols is always smaller than the theoretical removal rate, and that the removal rate decreases proportionally with the carbon content. They have claimed, however, that the spheroidized treatment of carbide has a removal rate increase greater than the theoretical
2
Work Materials and Heat Treatment
Ingots of Fe-C alloys with different carbon levels were cast by melting a certain amount of pure iron and carbon in a high frequency induction furnace on a laboratory scale. These were not-forgel and machined to a rectangular parallelepiped shape of a given size. The results of a check analysis of their chemical compositions are shown in Table 1. To give the microstructures suitable for the present prpose, all work materials were full-annealed in a vacuum furnace, and quenched from the heating temperatures at 50°C higher than the Ac3, Acm line, or of up to 50°C higher than Acl line. The microphotoqraphs of the heat-treated Fe-C alloys show well-known structures as presented in Fig.1; Fig.l(a) is of ferrite and lamellar pearlite,
removal rate. On the other hand, in hiaeda et al's experiment3), in which the annealed sample of plain carbon steels, low and high carbon alloyed steei and low alloy steels are machined in a 203 XaCl solution at a current density of about 3 0 A/cm', they have found that the current efficiency for evcry steel exceeds a theoretical value of loo%.. However they have reported that there is no clear dependency of carbon content on current efficiency. In another ECM, Freer et al?)have shown that normalized, as well a s quenched and tempered steels, with a carbon content of 0.991 or 1.26',, machined in 20:: NaC1 at 25 A/cm'and 15 V, have also indicated lwnr current efficiency and dull granular finishes. However Makino et al?' have suggested that the amounts of decrease in the weight of the anodes increase with an increase in carbon content when annealed steels with different carbon constituents are electrochcrnically dissolved in a stationary 5 9 SaNOZ solution at 5 A/cmL. From these studies, it is difficult to draw a conclusive tendency, as we often encounter a difference in the microstructure, alloying elements, initial condition of workpiecc surfaces and so on, in each of the reports. In the present WO:~, a systeaatic study was planned, using binary alloys, to discuss the fundamental e f i e c t s of csrbon cQntPnt on current efficiency.
Annals of the ClRP Vol. 30/1/1981
a) d)
r):
Full-annealed
e): Quenchrd from the temperature of 5 l ) O C higher Lhan AcJ or .\cm line i ) : Quenched F r o m thc rempernture of i O ' C higbrr than A c l l i n t .
Fiq.1 Nicrophotoqraphs after heat-treatment.
117
Table 1 Chemical compositions of Fe-C alloys (wt.3)
( 1 ) work rnaterial consists of pure iron. ( 2 ) Iron dissolves as a
No.
C 0.003 0.13
1
2 3 4
0.31
5
0.38
6 7 8
0.44 0.45
0.32
nn
P
S
0.01 0.00
0.002
0.006 0.007 0.003 0.004 0.002 0.003 0.005 0.005 0.005 0.004
0.006
0.00 0.01 0.01 0.01
0.76
0.02 0.03
0.85 0.86
0.08 0.02
11 12 13
0.92
14 15 16
1.22 1.27
0.06 0.02 0.02 0.02 0.01 0.01-.-
9
10
0.04 1.18
1.30
A1
Si
<0.01 co.01 0.002 <3.01
co.01 0.002
0.002 0.002 <0.01 0.002
0.005
0.002
<0.01 co.01 0.002 <0.01 CO.01
0.003
0.005 0.003 0.002
0.009 0.004
0.004 0.005 0.007
0.004 0.004
D. 004 0.005 0.004 0.006 0.006 0.004 0.004 0.007 -~
tr 0.001 0.001
tr 0.002 0.001 tr tr tr 0.001
tr 0.001 0.001 tr 0.001 0.001
ferritic ion q f d i - v a l c ? n t :om. ( 3 ) Sub-reactions, such as the ycncration o f cizyqcn +ti - , d o not occur. gas, and I.'e+- * FC + c Looking at F i g . 3 , the results show that thc n ? . n e o l e d sample has two contrastinrj characteristics distinyished i s{ a hvpereutcctoid or hypoeutectoid constitution. Nasely, current efficiency sxceeds 1 0 0 % at a carbon content ranging from 0.003 to 0.459. However it drops steeply to a percentage ranqiny from 60 to 75: at a carbon content from 0.85 to l . . 3 G ? , . In each case, hown-~er,
Fig.l(b) is of lamellar pearlite, Fig. l(c) is of network cementite, Fig.l(d) and l(e) are of martensite, and Fig.l(f) is of network cementite and martensite.
3) Experimental Method for Electrochemical Machining An outline of the electrochemical machining appaA 10% NaCl solution at 20 ratus is shown in Fig.2. ? 1°C was forced through an electrode gap of 1 mm at a mean velocity of 10 m/sec. The applied potential fixed at 1 0 V . except when current density was changed for experimental purposes. An anode and cathode surface, 1Ox25mm and lOxlOmm wide, respectively, were smoothly The polished by buffing before every experiment. machining time was determined by preliminary experiments so as to give a stationary current efficiency.
5
0
0.2
0.4
0.6
o
0.8
1.0
1.2
1.4
w
Carbon content (Ye) Effect of carbon confenf on current efficiency for full annealed samples
Fig.3
Y
y1
:90
1
V
2,
70
\ 0.2 0.4 0.6 0 8 1.0 1.2 I4
Elect?yte reserviw
6o0
Fig.2 Outline of
ECM
Carbon content (7.)
apparatus Fg.4
The current efficiency was calculated from the weight difference before and after machining for both the workpiece and the cathode of the coulometer. 4
Results
The effects of carbon content on current effiency are illustrated in Fig.3 for annealed structures, and in Fiy.4 for quenched structures. The current efficiency, '1 , was calculated from the equaion:
'
=
Practical machined mass
x 100
($)
--[l)
Theoretical removal mass
The theoretical removal mass was derived by the following assumptions.
118
Effect of carbon content on current efficiency for quuuhed sampks
the current efficiency increases with the carbon content. 'Jpon inspection @f the machined surfaces, a thin black layer exists on the surface of the samples containing carbon of more than 0.1%. This layer is easily wiped off. Once wiped off, a surface with a metallic lustre is observed in samples of less than 0.455 C, but a black surface in samples of more than 0.861 C. Figure 4 shows the current efficiency for quenched samples.For workpieces quenched from a temperature over the Ac3 o r Acm line,the current efficiency remains at nearly 100% showing no relation to carbon content. Ilowever for workpieccs quenchcd from a temperature over the Rcl line, current efficiency drops to less than 90 5 , decresiny with carbon content. Upon inspection of
these samples, a homogeneous thin layer with mumerous small particles is found on the machined surfaces. When wiped off, a smooth surface appears in samples where current efficiency reaches nearly 100:. but a rough surface with numerous pitted holes in samples where current efficiency is less than 90%. The surface topography mentioned above shows a good corelation with the roughness of machined surfaces measured by a profile meter as shown in Fig. 5 . It is evident from the results that the surface roughness not only increases with carbon content, but also is greater on the annealed samples than on the quenched samples. Furtherinore, there seems to be sufficient corelation between dependencies of carbon content on surface roughness and on current efficiency, as can be seen from the fact that the upper and lower curves in Fig. 5
9 4
A 5
0 003%C 0 13%C 0 LL%C 08 6'l.C
Flow velocity 15mk Re L160 Turbulence
O*/.NaCI Z O W C
tv-+T---=.
Sap Irnrn
h uylc
I
10
20 Current
Fig 6
30
LO
density (Alcm')
Current efficiency as a function of current density for full annealed samples 0.003%C 013%C 0 44.I.C A 0867.C
3
Flow wlocily lSmls Re 4160 Turbulence IO*/&CI 2022.C Gap Irnm
A
-s 1oc Y
ZI
C
.-
U
I'
1 leXC I 1W.C
\
s
y
5c d
Quenched from 50'C o w ACI line
?2 I
0.6 0.8 1.0 Carbon content (x)
0
0 8
0.2 0 4
I
I
1.2
14
3 V
0
10
20 Current
Fig.5
Effect of carbon content on surfoce roughness
Fig.7
have an analogy with the curve in Fig.3 and the upper curve in Fig.4, respectively. The influence of current density on the current efficiency of annealed and quenched Fe-C alloys is shown in Fig.6 and 7, respectively. For both cases, it is apparent that current efficiency decreases with current density. This behaviour corresponds to Chin's reports6), in which he machined mild steel in a 4N solution of NaCl at 1 A/cm'to 100 A/cm!
30
40
density (Alcml)
Current efficiency as a function of current density tor quenched samples
5. Discussion
The varaeties of current efficiency, of the surface layer, and of the surface roughness for Fe-C alloys caused by the increase in carbon content and by the change of microstructure seems to depend on the chemical states of the carbon atoms, and also on the morphology of cementite. The ferrite and pearlite structure It is well known that the ferrite ?base solutes very small amounts of carbon a t o m and some workers3) say that the cementite phase is not dissoluble by means of dirrect current. So far as Faraday's law is applied, the theoretical current efficiency, y t , of the workpiece consisting of a ferrite and pearlite structure is given as a function of the carbon content by the following equation. 5.1
p t = (1+3MFe / Where, M
and Fe
MC)
1.1
C
X
C
4-
100 ( % )
---(2)
are the atomic weights of
(a) Srroiidary electron image C R I X-ray image ( e ) C 1 K j 'X-rAy imngc (c)
Fig.8
( b ) Absorbed current image (d) FeK't X-ray image ( f ) VaKIi
80 11m
X-rav imgr
An example of CP42A analysis on t h e machined surface. (Full-annealed sample of 0 . 1 3 4 C)
119
iron and carbon, respectively, and C is the wei& percent of carbon. The equation is illustrated by a thin solid linc in Fig.3. In this figurc, it can be seen that experimental plots seem to satisfy the equahave obtained similar tion. A few results. This may lead to justification of the assumptions which were necessary to derive the above equation, but such an ideal dissolution process is considered as not always being possible during ECM according to the analysis by EPMA on a machined surface. Figure 8 includes a typical example of the secondary electeon image, absorbed current image and the characteristic Xray images of Fe and C showing that.the ferrite is machined to a greater extent than the pearlite phase, and also that carbon rich particles are scattered on the machined surface. As these particles are about one tenth as small as the size of a pearlite grain, they appear to pearlite colonies, isolated by a selective dissolution of the ferrite phase. This leads to a saving in the quantity of electricity. Other characteristic X-ray images show a negative corelation between C and Na, the same as that between Na and C1, but a possitivf corelation hetween C and C1.This lead to the consumption of a quantity of electricity. In effect, this seems to be not a result of Faraday's law, but a balanced phenomencabetween a saving and consumption of electricity, and that the experimental value is nearly equal tp the theoretical one, as shown in Fig.3.
(a) Secondarv electron tmagr
( c ) CK? (c)
X-ray image S-ray inap,e
C1K.k
Tiq.9
(b) Absorbed current image (d) FeK. X-ray image ( f ) SnKn X - r a y
U
80,,,m
imay
An example of EPMA analysis on the machined surface. (Full-annealed sample of 1.273 C)
5.2 The network cementite and Bearlite structure The analysis by EPNA presented in Fig.9 shows that the machined surface is covered with cementite which protrudes in the form of a network, and that small holes exist here and there on the surface. Observation of the characteristic images tells us that the C1 element is rich on the network cementite and also that there is a negative corelation between C1 and Na. The severe decrease in current efficiency of hypereutectoid Fe-C alloys is explainable as follows: The cementites, being passive, are exposed over the entire machined surface. Thus the anode overpotential and current density rise locally, resulting in harmful secondary reactions which become predeominant.
The martensite structure An example of the EPElA analysis is illustrated in Fig.10. Numerous raised particles can be seen on the machined surface, which consist of carbon. Chlorine can be identified, but this corclates neither possitively nor negatively with the carbon. As the workpiece is free from cementite and as it dissolutes carbon to an atomic state, the electrochemical dissolution of the martensite seems to advance as follows: The dissolution of di-valent iron atoms results in the isolation of solute carbon atoms without electric charge. The isolated carbon atoms are so active that they cohere to each other as particles on the surface. Theafore, the honoge5.3
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(a) Secondarv e l e c t r o n Image
(h) FeKr X-ray
( c ) C 1 K i S-rav image
(d) CK7
~lq.10
An
image
U 20/im
X-ray image
example of EPMk analysis on the machlned surface.
(Quenched sample from 5 0 ° C over Acm, of 1.303 C)
neous martensite phase gives a current efficiency which is close to the theoretical potential and a good surface finish. The network Cementite and martensite structure The results in Fig.4 have shown that the current efficiency lies between those of the network cementite and pcarlite on the one hand, the martensite on the 5.4
othcr, and that it is significantly decreased with the growth of network ccmentitc. This behaviour is explained a s being due to the proposed dissolutlon prucesses if the two structures arc bcinq clectrochemically machined at the same time. These results could add a soundess to the proposed dissolution processes.
3) The EPMA analysis shows a close relation between the cementite and chlorine on machined surfaces of annealed samples, but no relationshi? between the carbon particles and chlorine on machined surfaces of quenched samples.
References
6. Conclusions
The results obtained by the electrochemical machining of Fe-C alloy in NaCl solution at a comparatively low current density are summarized as follows: 1) For the ferrite and lamellar pearlitc structures, a ferrite phase attack, accompanied by sub-reactions, results in current efficiencies in excess of 100%. and increasing current efficiency with increase of the carbon content. The primary network cementite brings current efficiencies down remarkably, as if it were a passive film. The current efficiency of martensite is nearly 100% regardless of the carbon content up to a certain current density. 2 ) Surface roughnesses of the ferrite and lamellar pearlite structures are increased with the carbon content due to the selective dissolution of ferrite phases. For the martensite structures, the surface roughness is small because of comparatively homogeneous dissolution.
.YcCeough, J.A., “Some effects of carbon content on the efficiency of electrochemically machined carbon steels and cast iron”, Int. J. Production Research, 9-2, 311 (1971). Tanaka, U.,Kikuchi, C.,”Effect of work-metal micro structures on the machining characteristics in electrochemical machining”, Technical Reports of Huroran Tech. College, 1-2, 589(1971). Maeda, S., Saito, 3 . and Hapma, Y., “Principle and characteristics of electrochemical machining”, Tech. Rep. Mitsubishi Electrics, 41-10, 1267f1967). Freer, H.E., Hanley, J.B. and Maclellan G . D . S . , Electrochemical Society Softbound Symposium Series, Princeton, 103(1971). :.lakino, H. and Kawakatsu, K., “Effect of carbon content on electrochemical grinding of carbon Steels, ?art 1. Fundamentals“, JSPE ?reprint, 121 (1970). Chin, D.T. and Wallace, A.J.,“Anodic current efficiency and dimensional control in electrochemical machining“, Electrochem. Society, 120-11, 1487 (1973).
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