Hardness measurements adjacent to eroded surfaces of ductile ferrous alloys

MATERIALS CHARACTERIZATION 24:115-134 (1990)

1 15

Hardness Measurements Adjacent to Eroded Surfaces of Ductile Ferrous Alloys

D. W. KOLLER* AND N. H. MACMILLANt

*400 Spring Meadow Road, South Windsor, CT 06074, and ?P.O. Box 1310, Albany, OR 97321

Any hardness indentation made closer to an unsupported edge than about the length of its diagonal tends to be distorted and to give an unreliable measure of hardness. Thus, to measure hardness close to the worn edge of a section through a worn surface, it is necessary to support that edge with a coating that has the same yield strength (hardness) and stiffness as the worn material. It is also important that this coating be coherently bonded to the specimen. It is shown that varying the coherence and/or the hardness of Ni coatings electroplated onto specimens of malleable cast iron affects the measurement of hardness close to the edges of cross-sections through those specimens in the expected manner. When the coating is properly tailored, it is possible to determine the effect of impact angle on the subsurface damage introduced into Armco iron during solid particle erosion. It is found that the hardness decreases monotonically with distance below the worn surface, and that there is no evidence of softening immediately below this surface.

Introduction One useful technique for study of tribological phenomena is to section the worn specimen in a plane perpendicular to its wear surface, to prepare one of the cut surfaces metallographically, and to measure the hardness of this surface as a function of distance from its worn edge. In practice, there are two difficulties. First, the formation of an indentation is a highly constrained process. In a ductile material, the elasticplastic boundary lies well beyond the periphery of the actual indentation, and the plastic processes occurring inside this boundary are constrained by the elastic stresses generated outside it. Far from the elastic-plastic boundary, these latter stresses decay only as the inverse of the square of * D. W. Koller is a former graduate student and t N. H. Macmillan is a former senior research associate, Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16801. © Elsevier Science Publishing Co., Inc., 1990 655 Avenue of the Americas, New York, NY 10010

1044-5803/90/$3.50

D. W. Koller and N. H. Macmillan

116 w

_ free ,u,oc.

~ J loss ofelastic and /_~ \ ~ .. plastic constraint ( ~ ~. / loss OTelastic ~.J ~/~constroint ~ ~" elastic-plastic boundary fully supported ~contour of elastic strain / I k

FIG. 1. Schematicdiagramshowingthe lesser constraintacting duringindentationclose to an unsupported edge. the distance from the indentation [1, 2]. Thus, as indentations are made closer and closer to an unsupported edge (Fig. 1), the constraints on their formation are gradually reduced. The second problem is that polishing the cut surface tends to round off and to damage further its worn edge. For both reasons, indentations made close to an unsupported edge can be highly distorted. As an example, Fig. 2 shows a Vickers indentation made close to such an edge of a polished cross-section oriented perpendicular to the axis of an extruded square bar of 2024-T35! AI alloy. The indented surface was prepared by successively grinding on wet 240,320, 400, and 600 grit SiC papers and polishing on Morris Alphalap PS (J. I. Morris Co., Southbridge, MA, USA) cloths with aqueous slurries of 3.0-, 1.0-, 0.3-, and 0.05-txm particles of A1203; the indentation was centered at a distance of about one-half the length of its diagonal from the unsupported edge, and the load and loading time were 0.1 N and 10 s, respectively. Figure 3 shows the apparent variation of Vickers hardness with distance from an unsupported edge of the same cross-section of the same bar of 2024-T351 AI alloy, as measured using the same loading time and indenter loads of 0.25, 0.5, and 1 N. The indenter was always oriented with its two diagonals respectively parallel and perpendicular to the unsupported edge; "distance from edge" was measured from the center of the indentation to the unsupported edge, along a line perpendicular to that edge; and the hardness H (Pa) was calculated as H -

4 x 1.854P (dj + d 2 ) 2

'

(1)

Hardness Measured Adjacent to Eroded Surfaces

117

F~6.2. Optical micrograph of a distorted Vickers hardness indentation near the unsupported edge of a polished specimen of 2024-T351 Al alloy.

where P (N) is the load applied to the indenter, and dl and dE (m) are the two diagonals of the indentation. Each datum point in Fig. 3 denotes a mean hardness calculated from three indentations, and the error bars indicate plus and minus one standard deviation about the mean. The lines indicating the lengths of the diagonals of "interior indentations"--i.e., indentations made far from the unsupported edge--represent mean values derived from five indentations. The data presented in Fig. 3 illustrate two important points. The first is that, in the absence of any real variation of hardness with position, the edge effects discussed above lead to an apparent decrease in hardness of about one-third as the center of the indentation is moved to within about one-half diagonal of the unsupported edge. The second point is that, regardless of the location of the indentation, the measured hardness increases as the load applied to the indenter decreases, even though the Vickers indenter preserves geometric similarity as the applied load varies. Possible explanations for this second observation have been reviewed elsewhere [3] and will not be discussed further here.

D. W. Koller and N. H. Macmillan

118

1.70

I i diagonal

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FIG. 3. Vickers hardness as a function of indentation load and distance from the unsupported edge of a polished specimen of 2024-T351 Al alloy. T o t r y to o v e r c o m e e d g e e f f e c t s w h e n o n e is u s i n g h a r d n e s s as a n i n d i c a t o r o f s u b s u r f a c e w e a r d a m a g e , the u s u a l p r o c e d u r e is to c o a t the w e a r s u r f a c e w i t h a t h i c k l a y e r o f s o m e p r o t e c t i v e m a t e r i a l p r i o r to sect i o n i n g a n d m e t a l l o g r a p h i c p r e p a r a t i o n . W h e n this is d o n e , t h e m e a s u r e d v a r i a t i o n o f h a r d n e s s w i t h d i s t a n c e f r o m the w o r n e d g e d e p e n d s n o t o n l y o n t h e t r u e v a r i a t i o n arising f r o m the w e a r d a m a g e , b u t a l s o o n the stiffn e s s a n d the h a r d n e s s , o r y i e l d s t r e n g t h , o f the c o a t i n g a n d o n the coh e r e n c e o f t h e b o n d b e t w e e n t h e c o a t i n g a n d the (worn) s u b s t r a t e . This TABLE 1 Compositions of Armco Iron a and Ferritic Malleable Cast Iron b

Armco iron Ferritic malleable cast iron

C

Si

P

Mn

S

Cr

Fe

0.043 2.65

0.003 1.50

0.005 0.02

0.054 0.35

0.008 0.10

0.005 <0.05

Balance Balance

a Armco Inc., Middletown, OH, USA. b Buck Iron Co. Inc., Quarryville, PA, USA.

Hardness Measured Adjacent to Eroded Surfaces

119

article begins by exploring the effects of the last two of these three variables on the measurement of hardness versus depth below pristine and variously eroded surfaces on a ferritic malleable cast iron when these surfaces are protected by electroplated coatings of Ni. Thereafter, the article demonstrates how such coatings may be used to obtain reliable information about the effect of impact angle on the subsurface damage generated immediately below the wear surface during the erosion of Armco iron.

Experimental Procedures As part of a larger study of the effects of microstructure on the resistance of plain carbon irons and steels to solid particle erosion, specimens of Armco iron and of a ferritic malleable (annealed white) cast iron were eroded with 1.69-mm diameter spheres made of WC-14 wt % Co at an impact velocity of 80 m/s and various impact angles. The compositions of the two materials are listed in Table 1, and their microstructures are shown in Figs. 4 and 5. Both figures show specimens prepared in the same

FIG. 4.

Optical micrograph showing the microstructure of Armco iron.

120

FIG. 5.

D. W. Koller and N. H. Macmillan

Optical micrograph showing the microstructure of ferritic malleable cast iron.

fashion as the 2024-T351 AI bar discussed above and then etched in 2% nital (2 vol.% HNO3 in ethanol) for 3-5 s. The Armco iron is 100% ferrite, with an average grain "diameter" o f - 2 ~m, while the malleable cast iron consists of a matrix of ferrite containing a random dispersion of ASTM type 3 temper carbon nodules. Both these nodules and the grains of ferrite have average "diameters" of -35 Ixm. The specimens used in the erosion study were right cylinders 12.7 mm in diameter and either 6.35 or 12.7 mm in length. Those eroded at normal incidence (an impact angle of 90°) developed a "mogul-like" topography with an ill-defined amplitude and wavelength of 0.1-0.5 mm and I-2 mm, respectively; those eroded at oblique angles of incidence developed a "ridge and valley" or "breaking wave" topography, with the ridges and valleys or waves running perpendicular to the tangential component of the impact velocity vector. The amplitude of these surface features decreased and their wavelength increased as the impact angle decreased toward zero (glancing incidence). In every case, these features are about an order of magnitude larger than any individual impact crater, showing that they result from the cumulative effect of many impacts.

Hardness Measured Adjacent to Eroded Surfaces

121

Once steady state erosion was achievedmi.e., the average mass loss per impact reached a constant value characteristic of the particular target material and impact angle involved--each specimen was electroplated with a layer of Ni 100-300 Cm thick. Ni was chosen for two reasons. First, it has an elastic modulus (207 GPa [4]) very close to that of Fe, thereby ensuring minimum variation in elastic constraint as indentations are made closer and closer to the worn edge; and second, it can be deposited in a controlled manner with widely varying hardness, thereby permitting the properties of the coatings to be tailored to those of the eroded specimens [5, 6]. The electroplating was done from a Watts bath containing 225 g/L NiSO4, 60 g/L NiCI~, and 30 g/L H3BO3, plus a daily addition of 0.5 mL/ L of 30 voi.% H~O2. The bath was prepared as follows: 1. Appropriate quantities of NiSO4 and NiCl2 were dissolved at 318 K in sufficient water to make up 80% of the requisite volume. 2. 0.5 mL/L of 30 vol.% H202 was stirred into the bath, which was then allowed to stand for 1 h. 3. The bath was heated to 339 K, after which 1.8 g/L of NiCO3 were stirred in, and the bath was left to stand for 8-16 h. 4. The bath was filtered, and sufficient H3BO3 and water were then added to the filtrate to bring its volume up to the requisite final value. With the use of this bath, the hardness of the Ni layer electroplated onto the eroded specimens could be varied between 5.40 and 2.95 GPa by adjustment of the purity, temperature, pH, and/or current density. In addition, it was found that softer coatings could be obtained by using a Ni-plated steel cathode to purify the bath electrolytically at a current density of 500 A/m 2 until - A h / L had passed. Their hardnesses could be varied between 1.96 and 2.45 GPa by maintenance of the bath temperature at 328 K, use of a current density of 500 A/m 2, and variation of the pH between 3 and 4. After they were plated, the eroded specimens were sectioned diametrically along their lengths using a carbide cut-off wheel. In the case of a specimen eroded at oblique incidence, the cut was always made parallel to the tangential component of the impact velocity vector. The cut surfaces were then polished in the same fashion as the 2024-T351 AI alloy specimen discussed above, etched in 2% nital for 3-5 s, and indented using a Vickers indenter, a loading time of 10 s, and loads ranging between 0.5 and 5 N. The indenter was always oriented with its two diagonals respectively parallel and perpendicular to the axis of the (cylindrical) specimen; and each datum point was calculated by means of Eq. (1) from three

122

D. W. Koller and N. H. Macmillan

front

side

bottom

Fic. 6. Front, side, and bottom views of the indenter used for scratch testing. The arrows indicate the direction of motion of the indenter.

to six indentations and plotted as the mean value plus and minus one standard deviation. For purposes of comparison, a few uneroded specimens were also subjected to the same experimental protocol. In addition, several qualitative scratch hardness tests were carried out, using loads in the same range and translating the indenter at a few tens of ixrn/s by manual operation of the specimen-positioning micrometers. The indenter was the 120° diamond wedge illustrated in Fig. 6, and it was always moved as indicated therein.

Results and Discussion THE INFLUENCES OF THE HARDNESS AND THE COHERENCE OF THE COATING Figure 7 shows that far from any edge the Vickers hardness of an uneroded, polished, and etched malleable cast iron specimen increases as the load applied to the indenter decreases. Only at loads ->2 N is the interior (bulk, as opposed to edge or surface) hardness independent of load. Figure 8 presents plots of hardness versus distance from the edge of another uneroded, polished, and etched malleable cast iron specimen before and after it has been plated with a very hard and fully coherent layer of Ni. These measurements were made using a load of 5 N. Just as in the case of 2024-T351 A1 alloy (Fig. 3), the measured hardness decreases when

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the center of the indentation is placed closer to an unsupported edge than about the length of its diagonal. However, when the edge is supported by a layer of Ni with a hardness--and therefore a yield strength--more than three times that of the cast iron, the additional plastic constraint converts the decrease in measured hardness into a sharp increase. The slight increase in interior hardness following plating is tentatively attributed to diffusion of hydrogen into the cast iron during that operation. Figure 9(a,b) shows some of the indentations from which the data in Fig. 8 were derived, together with grooves from scratch hardness tests. In Fig. 9(b), note the coherent interface between the Ni and the cast iron, and also the much smaller indentations and narrower groove formed in the harder Ni. Note also in both figures the variation of indentation size and shape and of scratch width and symmetry from one location to another in the cast iron. This is the result of two factors--anisotropy and proximity to grain or phase boundaries--and is reflected in the error bars in Fig. 8. Figures 10 and 11 show that, when the Ni plating matches both the stiffness and the hardness of the specimen, it is possible to obtain meaningful hardness data right up to a worn edge. In this instance, the specimen is malleable cast iron that has been eroded at an impact angle of 15°. The

124

D. W. Koller and N. H. Macmillan •

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FIG. 8. Vickershardness of polished cast iron as a functionof distance from unsupported and Ni-plated edges. erosive particles travelled along the edge shown in Fig. 10 from right to left, introducing extensive shear deformation into the material as they ricocheted off its surface. The distribution of shear strain with both distance parallel to and distance below the target surface is shown clearly by the distortion of both the ferrite grains and the nodules of temper carbon. Of these two position variables, the latter is much more important. Increase in distance below any given point on the target surface invariably produces changes in hardness significantly larger than those arising from anisotropy and proximity to grain or phase boundaries. In contrast, any variation in hardness parallel to the surface at constant depth is small enough that it is masked by the variation arising from these latter influences. Thus, hardness values measured at the same depth below different points on the target surface have been averaged together in Fig. ll. The resultant error bars do not differ significantly from those shown in Fig. 8. Figure l0 also shows that in this case the Ni layer is at once coherently bonded to, and comparable in hardness with, the cast iron. Consequently, the hardness data obtained very close to the worn edge after plating with Ni lie on an extension of the plot of the data obtained further from this edge prior to plating (Fig. I I). The decrease of hardness with increase in

FIG. 9. Optical micrographs of Vickers hardness indentations in, and grooves from scratch hardness tests on, polished cast iron: (a) unsupported edge; (b) Ni-plated edge. 125

126

D. W. Koller and N. H. Macmillan

FIG. 10. Optical micrograph of Vickers hardness indentations in, and a groove from a scratch hardness test on, Ni-plated cast iron eroded at an impact angle of 15". The erosive particles traveled from right to left.

depth below the wear surface is monotonic, in accord with the qualitative microstructural observations. Again, there is evidence of a slight increase in interior hardness as a result of the plating operation. The situation illustrated in Figs. 10 and 11 persists only as long as the Ni layer is coherent with the substrate. Figure 12 shows another specimen of malleable cast iron that was eroded at normal incidence prior to being plated with Ni. The microstructure reveals clearly the complex distribution of strain introduced by repeated particle impacts at random locations. The Ni layer is again comparable in hardness with the heavily worked surface of the cast iron, but in this instance it is debonded from the cast iron. Consequently, the hardness does not rise monotonically all the way to the worn edge, but decreases once the center of each indentation lies closer to this edge than about the length of its diagonal (Fig. 13). This local decrease in hardness, which is hard to reconcile with the microstructural observations, is logically explained as a consequence of the debonding of the Ni coating from the cast iron specimen.

Hardness Measured Adjacent to Eroded Surfaces I

4.00

//

I

Ni

127

I

I

cast iron (eroded at 15 ° )

~. 3.25

n

(.9 ...

o = unplated

= plated

-

C

z.5o-

U

1.75 -

1.00

-

- I I-0 I/

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I I I0 I00 distance from edge/p.m

I I000

FIG. 11. Spatialvariation of Vickers hardness for cast iron eroded at an impact angle of 15°, showing consistency between measurements made far from an unsupported edge and measurements made right up to that edge after plating with Ni. T H E I N F L U E N C E OF IMPACT A N G L E ON T H E S U B S U R F A C E P E N E T R A T I O N OF E R O S I O N D A M A G E The mosaics presented in Figs. 14-16 show diametral cross-sections of specimens of Armco iron eroded at impact angles of 15°, 45 °, and 90 °, respectively. The arrows indicate the direction of travel of the erosive particles. Each wear surface is protected by a coherent layer of Ni having a Vickers hardness in the range 2.05-2.25 GPa, and each has been metallographically polished and etched. Note the greater amplitude and shorter wavelength of the topography developed at steeper (higher) angles of incidence, the different patterns of distortion of the grain shape at different such angles, and the indentations made at various distances from the wear surfaces. The hardness profiles derived from these (and other) indentations are presented in Figs. 17-19. As in Fig. 11, data obtained at constant depth below different points on the target surface have been averaged together. Also included in each of Figs. 17-19 is a profile obtained from an un-

128

D. W. Koller and N. H. Macmillan

FIG. 12. Optical micrograph of Vickers hardness indentations in, and a groove from a scratch hardness test on, Ni-plated cast iron eroded at an impact angle of 90°. eroded, polished, and etched specimen of Armco iron coated with a coherent layer of Ni having a hardness of 2.35 GPa. This last coating proved to be somewhat harder than intended, and so caused an apparent slight increase in hardness close to the edge of the specimen. Although it can be argued that the Ni coatings applied to the eroded specimens ideally should be a little harder, it is probable that they provide the worn edge with enough support to enable meaningful conclusions to be drawn from a comparison of the three profiles presented in Figs. 1719. It is apparent from these profiles that hardness decreases monotonically with distance from the worn edge at all impact angles. This is contrary to the claim made by Bellman and L e v y [8] on the basis of measurements on uncoated specimens. In addition, it is seen that both the extent of the erosion-induced hardening at any depth below the eroded surface, and the depth to which measurable such hardening extends, increase slightly with increase in impact angle.

Summary and Conclusions It is demonstrated that reliable hardness data can only be obtained close to the edge of a specimen when the edge is supported by a coating that

Hardness Measured Adjacent to Eroded Surfaces I

¢-

1

I

Ni

3.0

g 2.5

//

129 I

cast iron

t

'13

o 2.0 ~D U

>

1.5-

1.0 ~ /

I

I

I

I0 distance

I00 from

I

I000

edqe//~m

FIG. 13. Spatialvariation of Vickers hardness for cast iron eroded at an impact angle of 90° and then plated with a layer of Ni that did not bond coherently to the iron. I. is well bonded to the specimen and 2. has an elastic modulus and a yield strength (hardness) closely matching those of the specimen. The distance from the edge over which the influence of the coating extends is about equal to the length of the diagonal of the hardness indentation involved. The present research further shows that electroplating can be used to bond Ni coatings of appropriate hardness to both eroded and uneroded specimens of Armco iron and of a ferritic malleable cast iron in a fully coherent manner. When such coatings are applied, it is found that hardness decreases monotonically with distance below the worn target surfaces of eroded specimens of both materials. For Armco iron, it is also found that increasing the impact angle, so as to produce normal rather than tangential impacts, increases both the extent of erosion-induced hardening at any given depth and the total depth to which such hardening is detectable.

This work was supported at The Pennsylvania State University by The American Iron and Steel Institute under Project No. 53-458.

Fro. 14. Mosaic of optical micrographs showing a diametral section of a specimen of Armco iron eroded at an impact angle of 15° and then plated with Ni.

FIG. 15. Mosaic of optical micrographs s h o w i n g a diametral section of a s p e c i m e n of A r m c o iron eroded at an impact angle of 45 ° and t h e n plated with Ni.

~.a

F~6. 16. Mosaic of optical micrographs showing a diametral section of a specimen of Armco iron eroded at an impact angle of 90 ° and then plated with Ni.

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distance from edge//=m F]o. 17. Spatial variation of Vickers hardness for Ni-plated specimens of polished and eroded Armco iron. Impact angle 15°.

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°

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FIG. 18. Spatial variation of Vickers hardness for Ni-plated specimens of polished and eroded Armco iron. Impact angle 45°. 133

134

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D. W. Koller and N. H. Macmillan

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Armco iron

2.5 - ~.

~.%

roded at 9 0 °)

¢,"0

h.

2.0if)

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1.5-

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FIG. 19. Spatial variation of Vickers hardness for Ni-plated specimens of polished and eroded Armco iron. Impact angle 90°.

References 1. Sir W. Thomson (Lord Kelvin), Note on the integration of the equations of equilibrium of an elastic solid, Cambridge and Dublin Math. J. 111:87-89 (1848). [See also Sir W. Thomson, Mathematical and Physical Papers, Cambridge University Press, Cambridge (1882), Vol. 1, pp. 97-99]. 2. J. Boussinesq, Applications des Potentials ~ I'l~tude de I'l~quilibre et du Mouvement des Solides Elastiques, Lille Soc. Mem. 13:1-722 (1885). 3. A. Kelly and N. H. Macmillan, Strong Solids, 3rd Ed., Oxford University Press, Oxford (1986), pp. 144-145. 4. G. Simmons and H. Wang, Single Crystal Elastic Constants and Calculated Aggregate Properties: A Handbook, 2nd Ed., M.I.T. Press, Cambridge, Massachusetts (1971), pp. 153-294. 5. F. A. Lowenheim, Electroplating, McGraw-Hill, New York (1978), pp. 205-225. 6. V. Zentner, A. Brenner, and C. W. Jennings, Physical Properties of Electrodeposited Metals. I. Nickel. 3. The Effect of Plating Variables on the Structure and Properties of Electrodeposited Nickel, A.E.S. Research Project No. 9, Plating 19:865-927 (1952). 7. W. G. Wood (Coordinator), Nickel Plating, Metals Handbook, 9th Ed., ASM, Metals Park, Ohio (1982), Vol. 5, pp. 199-243. 8. R. Bellman and A. V. Levy, Erosion mechanism in ductile metals, Wear 70:1-27 (1981). Received July 1988; accepted June 1989.