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Influence of sulfur content on the impact strength of electroformed nickel
Electrodeposition and Surface Treatment, 2 (1973/74) 165 - 176 0 Elsevier Sequoia S.A., Lausanne - Printed in Switzerland
INFLUENCE OF SULFUR CONTENT OF ELECTROFORMED NICKEL
J. W. DINI, H. R. JOHNSON
Metallurgy (Received
165
ON THE IMPACT STRENGTH
and H. J. SAXTON
Division I, 8312, Sandia Laboratories,
Livermore,
Calif. 94550
(U.S.A.)
May 8, 1973)
Summary The influence of sulfur content on the notch sensitivity of electroformed nickel was determined by Charpy impact tests. Specimens containing greater than 200 ppm sulfur exhibited severe embrittlement and therefore are highly notch sensitive. A modified Izod test specimen containing the same joint configuration as the plated part described in the report was designed and evaluated. Results showed that this test could serve as an indicator of the notch sensitivity of the joint and that it could be used for product certification. As additional benefits, the Izod specimen gives assurance that the plated bonds comprising the joint are sound and that no planes of weakness exist. Because hardness is a direct function of sulfur content, a rough approximation of the sulfur content in deposits can be made by checking hardness.
Introduction An earlier report [l] described a technique whereby thick nickel plating was used to make a structural joint between aluminium and stainless steel. The joint is shown schematically in Fig. 1. Subsequently, the technique was used to fabricate a number of full-scale parts. With one exception, all of these parts were plated in the nickel sulfamate solution described in Table 1. The one exception was plated in a solution containing a stress reducer (saccharin) which results in deposits with high sulfur contents [2] . The plating bath which contained no stress reducer produced sound structural joints. However, the part produced in the solution containing saccharin failed along the line where a plane of weakness would be expected (see Fig. 2). Postmortem analysis of the failed joint showed that the failure resulted from the high notch sensitivity (low impact strength*) of the nickel. Sulfur content of *In this report “high impact strength” and “low notch sensitivity” are used interchangeably. Impact strength is measured by determining the amount of energy absorbed in fracturing a sample at high velocity. Notch sensitivity is the reduction in tensile strength caused by the presence of a stress concentration such as a notch or other imperfection in the metal. Typically, specimens which have a high impact strength have a low notch sensitivity and vice versa.
166 39" DWMETER I
0.076"
Fig. 1. Schematic of electroplated joint.
Fig. 2. Electroplated joint which failed along a plane of weakness in the nickel.
the deposit in this case was 336 ppm, in contrast to sulfur contents of less than 50 ppm in deposits from baths containing no stress reducing additive. The failure of the joint plated in the solution containing saccharin prompted the study described in this paper. The study thus had a dual purpose: (1) to define impact strength as a function of the sulfur content of the nickel deposit; and (2) to specify techniques for checking plated parts for impact strength. In the process of achieving these aims, electroformed nickel was subjected to Charpy and Izod impact tests and to Knoop hardness tests. Literature review The literature was searched to determine the experience of others in dealing with the notch sensitivity of electroformed nickel and the importance of sulfur content. Sample and Knapp [3] found no notch sensitivity in nickel electroformed in a Watts-type solution which did not contain addition agents. Hooper [4, 51 reported a ratio of 1 or greater for notched to unnotched tensile strength in nickel electroformed in sulfamate baths containing little or no stress-reducing agent. As the sulfur content of the deposit was increased by the addition of a stress-reducing agent, the nickel became notch sensitive. However, his data for 62-mil-thick deposits showed considerable scatter. On analyzing Hooper’s work, Safranek [ 61 concluded that nickel containing more than 170 ppm sulfur was usually notch sensitive whereas nickel containing less than 170 ppm sulfur usually was not. In evaluating the effects of stress on the mechanical properties of electroformed nickel, Christian, Scheck and Cox [7] determined that too high a stress reducer content resulted in decreased elongation or toughness values and in lack of fusion weldability. Therefore, they recommended that organic stress reducers be added to the plating solution in amounts no greater than 0.24 oz/gal. The amount of sulfur in their deposits was not reported.
167 TABLE 1 Composition of and operating conditions for nickel sulfamate solution Composition Nickel Nickel sulfamate Nickel chloride Boric acid Operating
76 - 82 450 < 1.1 41
g/l g/l g/l g/l
conditions
Surface tension PH Temperature Current density’ Anodes Filtration
34 - 38 dynes/cm 3.8 - 4.0 46-49°C 27 A/dm2 SD nickel continuous
Brenner et al. [8] mentioned that the presence of high amounts of sulfur in electroplated nickel deposits is partly responsible for brittleness of the material. They stated that this brittleness was similar to the effects of sulfur on nickel prepared by conventional metallurgical methods. In both instances, the sulfur combines as Ni3S2, a sulfide which collects in the form of thin brittle films around the grains and impairs their cohesion. This sulfide also melts at a low temperature, a characteristicwhich explains the loss of strength (the so-called “embrittling effect”) of sulfur-containing deposits at elevated temperatures. Safranek [6] reported sulfur embrittlement of nickel deposits at temperatures as low as 260 “C. Preparation
of electroformed
test samples
Plating was done in a lo-liter nickel sulfamate solution containing a saccharin additive. Composition was the same as that in Table 1, except for the additive. Thick nickel sheets of relatively uniform thickness were made by electroplating onto a 3 inch by 6 inch aluminium substrate in a Mohler Schaefer box [9]. The aluminium substrate was then dissolved in a caustic solution, and the sulfur content of the deposits was determined as NisSs using the procedure described by Luke [lo]. Charpy impact tests Procedure The Charpy test is an impact test in which a center-notched specimen supported at both ends as a simple beam is broken by the impact of a rigid, falling pendulum. The energy absorbed in breaking the specimen, as determined by the decreased rise of the pendulum, is a measure of the impact strength of the metal. Standard Charpy bars are 394 mils thick [ll] . However, since it takes quite a while to plate to this thickness, modified Charpy
1.082 t 0.005”
Fig. 3. Modified
01
0
Charpy
NO
SCALE
140
160
test specimen.
. 20
40
60
80
100
120 SULFUR
Fig. 4. Influence
of sulfur content
CONTENT
on impact
180
200
220
240
260
280
ml
I PPII 1
strength
of electroformed
nickel.
bars of two lesser thicknesses, 75 mils (corresponding to the thickness of the plated joint) and 200 mils, were machined from the electroformed nickel sheets (see Fig. 3). Following testing on a 128 ftrlb capacity, rigidpendulum Charpy machine, the fracture surfaces of selected specimens were observed with a scanning electron microscope.
169
Results To provide a better comparison between fracture resistance and sulfur content than that provided using absolute values of impact energy, the total impact energy for each specimen was divided by the net cross section of the modified Charpy specimen below the notch root. These absolute values of impact energy were then plotted as a function of sulfur content (Fig. 4) for both the 75 mil and 200 mil thick modified Charpy specimens. The most outstanding feature of the test results shown in Fig. 4 is that increasing the sulfur content reduces the fracture resistance of electroformed nickel; above 280 ppm sulfur, both sets of specimens displayed virtually no impact resistance. Whereas the thicker specimens displayed a steady decrease of impact energy with sulfur content, the thinner specimens maintained roughly constant impact energy values below 200 ppm. Below 200 ppm, the thinner specimens were in a plane stress condition typified by shear fracture surfaces and relative insensitivity to sulfur content. In contrast, the plane strain conditions existing in thicker specimens led to higher triaxial tensile states and a significant sensitivity to sulfur content*. The macroscopic fracture surfaces of the Charpy specimens containing about 200 ppm sulfur are shown in Fig. 5 because they bracket the large change in fracture resistance which occurred in this composition region. As indicated by an 80% reduction in area and total absence of flat fracture surfaces, the 75 mil thick specimen containing 157 ppm sulfur (Fig. 5(a)) apparently fractured under plane stress conditions. In contrast, the smaller reduction in area displayed by the 200 mil thick specimen containing 157 ppm sulfur (Fig. 5(b)) suggests that, although the specimen was still very ductile, conditions of plane strain were approximated in the specimen interior. The two types of resulting fracture surfaces are shown on a microscopic scale in Fig. 6. In the thin sample (Fig. 6(a)), the ductile fracture surface is composed mostly of shear dimples, whereas in the thick sample (Fig. 6(b)), the ductile fracture surface consists mostly of normal rupture dimples. These observations of the ductile fracture surfaces are in accord with classical plane stress and plane strain fracture processes [ 121. For a sulfur content greater than 200 ppm, a.more brittle fracture process was observed. The inherent toughness was so decreased in both thin and thick specimens that they failed with virtually no reduction in area (Fig. 5(c) and (d)). Furthermore, flat fracture appeared for the first time in the thin specimen containing 221 ppm sulfur (Fig. 5(c)). On the thick specimen containing 291 ppm sulfur, the shear lips disappeared entirely (Fig. 5(d)). On the microscopic scale, the fracture surfaces of both thick and thin specimens appeared identical (Figs. 7(a) and (b)). The very flat nature of these fracture *Plane strain is a stress state that is characteristic of thick parts for which the stress adjacent to a flaw is triaxial tension. Plane stress is characteristic of thin parts for which only biaxial stresses can be sustained. For more detail on the basics of fracture mechanics, see ref. 12.
(a) (b)
Cd) Fig. 5. Macroscopic fracture surfaces (X 4.5) of Charpy impact specimens. (a) 75 mil thick specimen containing 157 ppm sulfur (Specimen 3 from Fig. 4); (b) 200 mil thick specimen containing 157 ppm sulfur (Specimen 1 from Fig. 4); (c) 75 mil thick specimen containing 221 ppm sulfur (Specimen 4 from Fig. 4); (d) 200 mil thick specimen containing 291 ppm sulfur (Specimen 2 from Fig. 4).
(a)
(b)
Fig. 6. Ductile fracture surfaces (X 800) of specimens sulfur. (a) 75 mil thick specimen containing 157 ppm exhibiting large number of shear dimples; (b) 200 mil sulfur with fracture surface exhibiting normal rupture
containing less than 200 ppm sulfur with fracture surface thick specimen containing 157 ppm dimples.
surfaces reflects the brittle response of the specimens during impact testing, but the fracture mode is essentially unresolvable at 800 x . At 8000 x (Fig. 7(c)), the fine structure of the fracture is barely resolvable. Although the exact mode cannot be identified, the average size of the fine cleavage facets or ductile dimples is about equal to the extremely fine grain size of the electroplated nickel (about 2000 A). This apparent relationship between the structure of the fracture surface and the grain size supports previously disclosed reports that high sulfur content promotes intergranular failure and grain boundary embrittlement. This analysis of Charpy test specimens indicates that parts containing greater than about 200 ppm sulfur would exhibit very little impact strength and would therefore be notch sensitive, a conclusion in good agreement with the work of Hooper [4 - 61 alluded to earlier. Hooper’s deposits were notch sensitive when the sulfur content was greater than 170 ppm. Izod impact tests The results obtained from the Charpy tests suggested that an Izod impact test might be one beneficial way to verify the integrity of a plated joint. Izod tests would serve two purposes: (1) The impact strength of the nickel could be determined (as in the Charpy tests) and could serve as an indicator of the notch sensitivity of the nickel; and
(c) Fig. 7. Brittle fracture surfaces ((a) and (b) x 800) of specimens containing more than 200 ppm sulfur. (a) 75 mil thick specimen containing 221 ppm sulfur exhibiting a very flat brittle fracture surface; (b) 200 mil thick specimen containing 291 ppm sulfur exhibi(X 8000), fracture ting a very flat brittle fracture surface; (c) at higher magnification surface shown in (b) reveals cleavage facets or fine dimples about equal in size to the average grain size.
173
==riJ&% .. NOTE:
SAMPLE IMPACTED AT A
Fig. 8. Modified Izod test specimen. TABLE 2 Influence of sulfur content of electroformed
nickel on modified Izod impact strength
No. of samples tested
Average sulfur content (ppm)
Impact strength (ft-lb)
Remarks
2 2 5 2 4 4* 1**
12 14 23 43 104 110 336
24.7 17.7 16.8 16.9 19.5 6.8 7.2
Failed in Ni Failed in Ni Failed in Ni Failed in Ni Failed in Ni Failed in Ni Failed in nickel at crystal growth interface.
* Sulfur content in these samples varied from 210 ppm on the side where plating was initiated to 88 ppm on the side where plating was terminated. **Five out of six of the specimens with this sulfur content failed during machining.
(2) The test would give assurance that the plated bonds between the nickel and stainless steel were sound. Procedure The Izod test is a pendulum-impact test in which the specimen is sup-
174
ported at one end as a cantilever beam, and the energy required to break off the free end is used as a measure of impact strength. A test specimen would be available because, concurrently with the plating of each full-scale part, a 3-inch diameter part with a full-size joint is also plated under as near identical conditions as possible [l] . This small part is later cut into test coupons which are used in certifying the full-scale part. Because of the joint design, it is impossible to obtain a Charpy-type test specimen. However, a modified Izod test specimen could be machined from the part (see Fig. 8), with a notch being machined at the location where a plane of weakness would be if one existed. As specimens from either full-size or smaller diameter joints became available, they were machined and tested by the modified Izod technique. The sulfur content of the parts varied from 12 to 336 ppm.
I I
Ductile (-
fracture
100 ppm sulfur)
Brittle fracture (- 200 ppm sulfur)
Fig. 9. Fracture
surface
of part with sulfur content
varying
from 88 to 210 ppm (X
1000)
175
Results The results of the Izod tests are summarized in Table 2. Specimens containing 104 ppm sulfur and less exhibited impact strengths varying from 16.9 to 24.7 f&lb. By contrast, a sample containing 336 ppm sulfur failed at 7.2 ft-lb, and five other samples containing this amount of sulfur failed during machining. The sulfur content in one sample varied from 210 ppm on the side where plating was initiated to 88 ppm on the side where plating was terminated. When tested, the specimens from this part failed at 6.8 ft-lb, a value indicative of a low-strength joint. The fracture surface of these specimens (Fig. 9) showed that about l/3 of the area failed in an extremely brittle fashion typical of parts containing greater than 200 ppm sulfur, whereas the rest of the area failed in the ductile fashion typical of parts containing less than 100 ppm sulfur. The average sulfur content of this part was 110 ppm, and sulfur analysis by itself would not have predicted a low-strength part. This particular part showed the importance of running the Izod test, since average sulfur content is not necessarily an exact barometer of the graduation of sulfur through the part thickness. These data support conclusions drawn from the Charpy work where specimens containing greater than 200 ppm sulfur were shown to exhibit low impact resistance. This information also shows that the modified Izod test provides a reliable indication of the notch sensitivity of the plated joint configuration. For all of these tests, the bond between the nickel deposit and stainless steel was suitable since all failures occurred in the nickel. This lends further credence to use this test for product certification. Knoop hardness tests Procedures
Knoop hardness tests were made to correlate hardness and sulfur content in electroformed nickel. Samples were obtained by making metallographic cross sections of some of the electroformed panels. Results
Results of the Knoop hardness tests are shown in Fig. 10. As sulfur content increased, impact strength decreased and hardness increased. The results indicated that hardness measurements could provide an estimate of sulfur content. References 1 J. W. Dini, H. R. Johnson and J. R. Helms, Joining aluminium to stainless steel by electroplating. In Symposium on Properties of Electrodeposited Metals and Alloys as Materials for Selected Applications, Battelle Columbus Laboratories, Metals and Ceramics Information Center MCIC Report 72-05, Jan. 1972; or AD 738272. 2 J. L. Marti and G. P. Lanza, Hardness of sulfamate nickel deposits, Plating, 56 (1969) 377. 3 C. H. Sample and B. B. Knapp, Physical and mechanical properties of electroformed nickel at elevated and subzero temperatures. In Symposium on Electroforming, ASTM Spec. Publ. No. 318, July 1962, pp 32 - 42.
176
0
40
80
120
160
200
SULFUR CONTENT
Fig. 10. Influence
of sulfur content
240
280
320
360
IPPMI
on hardness
of electroformed
nickel.
4 A. F. Hooper, Compilation of Materials Research Data, Genera1 Dynamics, First Quarterly Report -Phase I, Contract AF 33(616) - 7984, MRG 288, Sept. 1, 1962. 5 A. F. Hooper, Genera1 Dynamics, Second Quarterly Progress Report -Phase I, Contract AF 33(616) - 7984, MRG 319 (Dec. 1,1962) and MRG 316 (Sept. 1 Dec. 1, 1962). 6 W. H. Safranek, A Survey of Electroforming for Fabricating Structures, Battelle Memorial Institute, RSIC 210 (AD46480), (Oct. 1964). 7 J. L. Christian, W. G. Scheck and J. D. Cox, Mechanical properties of electroformed nickel at room and cryogenic temperatures, presented at The 1965 Cryogenic Engineering Conference, Rice Univ., Houston, Texas, August 1965. 8 A. Brenner, V. Zentner and C. W. Jennings, Physical properties of electroformed metals, Plating, 39 (1952) 864. 9 J. B. Mohler and R. A. Schaefer, Laboratory apparatus for controlled current distribution on small flat specimens, Monthly Rev. Am. Electroplaters’ Sot., 34 (1947) 1361. 10 C. L. Luke, Determination of sulfur in nickel by the evolution method, Analyt. Chem., 29 (8) (1957) 1227. 11 12
Standard Method of Notched Bar Impact Testing of Metallic E23-66, 1966. A. S. Tetelman and A. J. McEvily, Jr., Fracture of Structural New York, 1967.
Materials,
ASTM
Materials,
Wiley,
INFLUENCE OF SULFUR CONTENT OF ELECTROFORMED NICKEL
J. W. DINI, H. R. JOHNSON
Metallurgy (Received
165
ON THE IMPACT STRENGTH
and H. J. SAXTON
Division I, 8312, Sandia Laboratories,
Livermore,
Calif. 94550
(U.S.A.)
May 8, 1973)
Summary The influence of sulfur content on the notch sensitivity of electroformed nickel was determined by Charpy impact tests. Specimens containing greater than 200 ppm sulfur exhibited severe embrittlement and therefore are highly notch sensitive. A modified Izod test specimen containing the same joint configuration as the plated part described in the report was designed and evaluated. Results showed that this test could serve as an indicator of the notch sensitivity of the joint and that it could be used for product certification. As additional benefits, the Izod specimen gives assurance that the plated bonds comprising the joint are sound and that no planes of weakness exist. Because hardness is a direct function of sulfur content, a rough approximation of the sulfur content in deposits can be made by checking hardness.
Introduction An earlier report [l] described a technique whereby thick nickel plating was used to make a structural joint between aluminium and stainless steel. The joint is shown schematically in Fig. 1. Subsequently, the technique was used to fabricate a number of full-scale parts. With one exception, all of these parts were plated in the nickel sulfamate solution described in Table 1. The one exception was plated in a solution containing a stress reducer (saccharin) which results in deposits with high sulfur contents [2] . The plating bath which contained no stress reducer produced sound structural joints. However, the part produced in the solution containing saccharin failed along the line where a plane of weakness would be expected (see Fig. 2). Postmortem analysis of the failed joint showed that the failure resulted from the high notch sensitivity (low impact strength*) of the nickel. Sulfur content of *In this report “high impact strength” and “low notch sensitivity” are used interchangeably. Impact strength is measured by determining the amount of energy absorbed in fracturing a sample at high velocity. Notch sensitivity is the reduction in tensile strength caused by the presence of a stress concentration such as a notch or other imperfection in the metal. Typically, specimens which have a high impact strength have a low notch sensitivity and vice versa.
166 39" DWMETER I
0.076"
Fig. 1. Schematic of electroplated joint.
Fig. 2. Electroplated joint which failed along a plane of weakness in the nickel.
the deposit in this case was 336 ppm, in contrast to sulfur contents of less than 50 ppm in deposits from baths containing no stress reducing additive. The failure of the joint plated in the solution containing saccharin prompted the study described in this paper. The study thus had a dual purpose: (1) to define impact strength as a function of the sulfur content of the nickel deposit; and (2) to specify techniques for checking plated parts for impact strength. In the process of achieving these aims, electroformed nickel was subjected to Charpy and Izod impact tests and to Knoop hardness tests. Literature review The literature was searched to determine the experience of others in dealing with the notch sensitivity of electroformed nickel and the importance of sulfur content. Sample and Knapp [3] found no notch sensitivity in nickel electroformed in a Watts-type solution which did not contain addition agents. Hooper [4, 51 reported a ratio of 1 or greater for notched to unnotched tensile strength in nickel electroformed in sulfamate baths containing little or no stress-reducing agent. As the sulfur content of the deposit was increased by the addition of a stress-reducing agent, the nickel became notch sensitive. However, his data for 62-mil-thick deposits showed considerable scatter. On analyzing Hooper’s work, Safranek [ 61 concluded that nickel containing more than 170 ppm sulfur was usually notch sensitive whereas nickel containing less than 170 ppm sulfur usually was not. In evaluating the effects of stress on the mechanical properties of electroformed nickel, Christian, Scheck and Cox [7] determined that too high a stress reducer content resulted in decreased elongation or toughness values and in lack of fusion weldability. Therefore, they recommended that organic stress reducers be added to the plating solution in amounts no greater than 0.24 oz/gal. The amount of sulfur in their deposits was not reported.
167 TABLE 1 Composition of and operating conditions for nickel sulfamate solution Composition Nickel Nickel sulfamate Nickel chloride Boric acid Operating
76 - 82 450 < 1.1 41
g/l g/l g/l g/l
conditions
Surface tension PH Temperature Current density’ Anodes Filtration
34 - 38 dynes/cm 3.8 - 4.0 46-49°C 27 A/dm2 SD nickel continuous
Brenner et al. [8] mentioned that the presence of high amounts of sulfur in electroplated nickel deposits is partly responsible for brittleness of the material. They stated that this brittleness was similar to the effects of sulfur on nickel prepared by conventional metallurgical methods. In both instances, the sulfur combines as Ni3S2, a sulfide which collects in the form of thin brittle films around the grains and impairs their cohesion. This sulfide also melts at a low temperature, a characteristicwhich explains the loss of strength (the so-called “embrittling effect”) of sulfur-containing deposits at elevated temperatures. Safranek [6] reported sulfur embrittlement of nickel deposits at temperatures as low as 260 “C. Preparation
of electroformed
test samples
Plating was done in a lo-liter nickel sulfamate solution containing a saccharin additive. Composition was the same as that in Table 1, except for the additive. Thick nickel sheets of relatively uniform thickness were made by electroplating onto a 3 inch by 6 inch aluminium substrate in a Mohler Schaefer box [9]. The aluminium substrate was then dissolved in a caustic solution, and the sulfur content of the deposits was determined as NisSs using the procedure described by Luke [lo]. Charpy impact tests Procedure The Charpy test is an impact test in which a center-notched specimen supported at both ends as a simple beam is broken by the impact of a rigid, falling pendulum. The energy absorbed in breaking the specimen, as determined by the decreased rise of the pendulum, is a measure of the impact strength of the metal. Standard Charpy bars are 394 mils thick [ll] . However, since it takes quite a while to plate to this thickness, modified Charpy
1.082 t 0.005”
Fig. 3. Modified
01
0
Charpy
NO
SCALE
140
160
test specimen.
. 20
40
60
80
100
120 SULFUR
Fig. 4. Influence
of sulfur content
CONTENT
on impact
180
200
220
240
260
280
ml
I PPII 1
strength
of electroformed
nickel.
bars of two lesser thicknesses, 75 mils (corresponding to the thickness of the plated joint) and 200 mils, were machined from the electroformed nickel sheets (see Fig. 3). Following testing on a 128 ftrlb capacity, rigidpendulum Charpy machine, the fracture surfaces of selected specimens were observed with a scanning electron microscope.
169
Results To provide a better comparison between fracture resistance and sulfur content than that provided using absolute values of impact energy, the total impact energy for each specimen was divided by the net cross section of the modified Charpy specimen below the notch root. These absolute values of impact energy were then plotted as a function of sulfur content (Fig. 4) for both the 75 mil and 200 mil thick modified Charpy specimens. The most outstanding feature of the test results shown in Fig. 4 is that increasing the sulfur content reduces the fracture resistance of electroformed nickel; above 280 ppm sulfur, both sets of specimens displayed virtually no impact resistance. Whereas the thicker specimens displayed a steady decrease of impact energy with sulfur content, the thinner specimens maintained roughly constant impact energy values below 200 ppm. Below 200 ppm, the thinner specimens were in a plane stress condition typified by shear fracture surfaces and relative insensitivity to sulfur content. In contrast, the plane strain conditions existing in thicker specimens led to higher triaxial tensile states and a significant sensitivity to sulfur content*. The macroscopic fracture surfaces of the Charpy specimens containing about 200 ppm sulfur are shown in Fig. 5 because they bracket the large change in fracture resistance which occurred in this composition region. As indicated by an 80% reduction in area and total absence of flat fracture surfaces, the 75 mil thick specimen containing 157 ppm sulfur (Fig. 5(a)) apparently fractured under plane stress conditions. In contrast, the smaller reduction in area displayed by the 200 mil thick specimen containing 157 ppm sulfur (Fig. 5(b)) suggests that, although the specimen was still very ductile, conditions of plane strain were approximated in the specimen interior. The two types of resulting fracture surfaces are shown on a microscopic scale in Fig. 6. In the thin sample (Fig. 6(a)), the ductile fracture surface is composed mostly of shear dimples, whereas in the thick sample (Fig. 6(b)), the ductile fracture surface consists mostly of normal rupture dimples. These observations of the ductile fracture surfaces are in accord with classical plane stress and plane strain fracture processes [ 121. For a sulfur content greater than 200 ppm, a.more brittle fracture process was observed. The inherent toughness was so decreased in both thin and thick specimens that they failed with virtually no reduction in area (Fig. 5(c) and (d)). Furthermore, flat fracture appeared for the first time in the thin specimen containing 221 ppm sulfur (Fig. 5(c)). On the thick specimen containing 291 ppm sulfur, the shear lips disappeared entirely (Fig. 5(d)). On the microscopic scale, the fracture surfaces of both thick and thin specimens appeared identical (Figs. 7(a) and (b)). The very flat nature of these fracture *Plane strain is a stress state that is characteristic of thick parts for which the stress adjacent to a flaw is triaxial tension. Plane stress is characteristic of thin parts for which only biaxial stresses can be sustained. For more detail on the basics of fracture mechanics, see ref. 12.
(a) (b)
Cd) Fig. 5. Macroscopic fracture surfaces (X 4.5) of Charpy impact specimens. (a) 75 mil thick specimen containing 157 ppm sulfur (Specimen 3 from Fig. 4); (b) 200 mil thick specimen containing 157 ppm sulfur (Specimen 1 from Fig. 4); (c) 75 mil thick specimen containing 221 ppm sulfur (Specimen 4 from Fig. 4); (d) 200 mil thick specimen containing 291 ppm sulfur (Specimen 2 from Fig. 4).
(a)
(b)
Fig. 6. Ductile fracture surfaces (X 800) of specimens sulfur. (a) 75 mil thick specimen containing 157 ppm exhibiting large number of shear dimples; (b) 200 mil sulfur with fracture surface exhibiting normal rupture
containing less than 200 ppm sulfur with fracture surface thick specimen containing 157 ppm dimples.
surfaces reflects the brittle response of the specimens during impact testing, but the fracture mode is essentially unresolvable at 800 x . At 8000 x (Fig. 7(c)), the fine structure of the fracture is barely resolvable. Although the exact mode cannot be identified, the average size of the fine cleavage facets or ductile dimples is about equal to the extremely fine grain size of the electroplated nickel (about 2000 A). This apparent relationship between the structure of the fracture surface and the grain size supports previously disclosed reports that high sulfur content promotes intergranular failure and grain boundary embrittlement. This analysis of Charpy test specimens indicates that parts containing greater than about 200 ppm sulfur would exhibit very little impact strength and would therefore be notch sensitive, a conclusion in good agreement with the work of Hooper [4 - 61 alluded to earlier. Hooper’s deposits were notch sensitive when the sulfur content was greater than 170 ppm. Izod impact tests The results obtained from the Charpy tests suggested that an Izod impact test might be one beneficial way to verify the integrity of a plated joint. Izod tests would serve two purposes: (1) The impact strength of the nickel could be determined (as in the Charpy tests) and could serve as an indicator of the notch sensitivity of the nickel; and
(c) Fig. 7. Brittle fracture surfaces ((a) and (b) x 800) of specimens containing more than 200 ppm sulfur. (a) 75 mil thick specimen containing 221 ppm sulfur exhibiting a very flat brittle fracture surface; (b) 200 mil thick specimen containing 291 ppm sulfur exhibi(X 8000), fracture ting a very flat brittle fracture surface; (c) at higher magnification surface shown in (b) reveals cleavage facets or fine dimples about equal in size to the average grain size.
173
==riJ&% .. NOTE:
SAMPLE IMPACTED AT A
Fig. 8. Modified Izod test specimen. TABLE 2 Influence of sulfur content of electroformed
nickel on modified Izod impact strength
No. of samples tested
Average sulfur content (ppm)
Impact strength (ft-lb)
Remarks
2 2 5 2 4 4* 1**
12 14 23 43 104 110 336
24.7 17.7 16.8 16.9 19.5 6.8 7.2
Failed in Ni Failed in Ni Failed in Ni Failed in Ni Failed in Ni Failed in Ni Failed in nickel at crystal growth interface.
* Sulfur content in these samples varied from 210 ppm on the side where plating was initiated to 88 ppm on the side where plating was terminated. **Five out of six of the specimens with this sulfur content failed during machining.
(2) The test would give assurance that the plated bonds between the nickel and stainless steel were sound. Procedure The Izod test is a pendulum-impact test in which the specimen is sup-
174
ported at one end as a cantilever beam, and the energy required to break off the free end is used as a measure of impact strength. A test specimen would be available because, concurrently with the plating of each full-scale part, a 3-inch diameter part with a full-size joint is also plated under as near identical conditions as possible [l] . This small part is later cut into test coupons which are used in certifying the full-scale part. Because of the joint design, it is impossible to obtain a Charpy-type test specimen. However, a modified Izod test specimen could be machined from the part (see Fig. 8), with a notch being machined at the location where a plane of weakness would be if one existed. As specimens from either full-size or smaller diameter joints became available, they were machined and tested by the modified Izod technique. The sulfur content of the parts varied from 12 to 336 ppm.
I I
Ductile (-
fracture
100 ppm sulfur)
Brittle fracture (- 200 ppm sulfur)
Fig. 9. Fracture
surface
of part with sulfur content
varying
from 88 to 210 ppm (X
1000)
175
Results The results of the Izod tests are summarized in Table 2. Specimens containing 104 ppm sulfur and less exhibited impact strengths varying from 16.9 to 24.7 f&lb. By contrast, a sample containing 336 ppm sulfur failed at 7.2 ft-lb, and five other samples containing this amount of sulfur failed during machining. The sulfur content in one sample varied from 210 ppm on the side where plating was initiated to 88 ppm on the side where plating was terminated. When tested, the specimens from this part failed at 6.8 ft-lb, a value indicative of a low-strength joint. The fracture surface of these specimens (Fig. 9) showed that about l/3 of the area failed in an extremely brittle fashion typical of parts containing greater than 200 ppm sulfur, whereas the rest of the area failed in the ductile fashion typical of parts containing less than 100 ppm sulfur. The average sulfur content of this part was 110 ppm, and sulfur analysis by itself would not have predicted a low-strength part. This particular part showed the importance of running the Izod test, since average sulfur content is not necessarily an exact barometer of the graduation of sulfur through the part thickness. These data support conclusions drawn from the Charpy work where specimens containing greater than 200 ppm sulfur were shown to exhibit low impact resistance. This information also shows that the modified Izod test provides a reliable indication of the notch sensitivity of the plated joint configuration. For all of these tests, the bond between the nickel deposit and stainless steel was suitable since all failures occurred in the nickel. This lends further credence to use this test for product certification. Knoop hardness tests Procedures
Knoop hardness tests were made to correlate hardness and sulfur content in electroformed nickel. Samples were obtained by making metallographic cross sections of some of the electroformed panels. Results
Results of the Knoop hardness tests are shown in Fig. 10. As sulfur content increased, impact strength decreased and hardness increased. The results indicated that hardness measurements could provide an estimate of sulfur content. References 1 J. W. Dini, H. R. Johnson and J. R. Helms, Joining aluminium to stainless steel by electroplating. In Symposium on Properties of Electrodeposited Metals and Alloys as Materials for Selected Applications, Battelle Columbus Laboratories, Metals and Ceramics Information Center MCIC Report 72-05, Jan. 1972; or AD 738272. 2 J. L. Marti and G. P. Lanza, Hardness of sulfamate nickel deposits, Plating, 56 (1969) 377. 3 C. H. Sample and B. B. Knapp, Physical and mechanical properties of electroformed nickel at elevated and subzero temperatures. In Symposium on Electroforming, ASTM Spec. Publ. No. 318, July 1962, pp 32 - 42.
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SULFUR CONTENT
Fig. 10. Influence
of sulfur content
240
280
320
360
IPPMI
on hardness
of electroformed
nickel.
4 A. F. Hooper, Compilation of Materials Research Data, Genera1 Dynamics, First Quarterly Report -Phase I, Contract AF 33(616) - 7984, MRG 288, Sept. 1, 1962. 5 A. F. Hooper, Genera1 Dynamics, Second Quarterly Progress Report -Phase I, Contract AF 33(616) - 7984, MRG 319 (Dec. 1,1962) and MRG 316 (Sept. 1 Dec. 1, 1962). 6 W. H. Safranek, A Survey of Electroforming for Fabricating Structures, Battelle Memorial Institute, RSIC 210 (AD46480), (Oct. 1964). 7 J. L. Christian, W. G. Scheck and J. D. Cox, Mechanical properties of electroformed nickel at room and cryogenic temperatures, presented at The 1965 Cryogenic Engineering Conference, Rice Univ., Houston, Texas, August 1965. 8 A. Brenner, V. Zentner and C. W. Jennings, Physical properties of electroformed metals, Plating, 39 (1952) 864. 9 J. B. Mohler and R. A. Schaefer, Laboratory apparatus for controlled current distribution on small flat specimens, Monthly Rev. Am. Electroplaters’ Sot., 34 (1947) 1361. 10 C. L. Luke, Determination of sulfur in nickel by the evolution method, Analyt. Chem., 29 (8) (1957) 1227. 11 12
Standard Method of Notched Bar Impact Testing of Metallic E23-66, 1966. A. S. Tetelman and A. J. McEvily, Jr., Fracture of Structural New York, 1967.
Materials,
ASTM
Materials,
Wiley,