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The effect of particles in different sizes on the mechanical properties of spray formed steel composites
Materials Science and Engineering A326 (2002) 40 – 50 www.elsevier.com/locate/msea
The effect of particles in different sizes on the mechanical properties of spray formed steel composites Kenneth Petersen a,b,c,*, Allan Schrøder Pedersen b, Nini Pryds b, Knud Aage Thorsen c, Jan Laurberg List a a
The Danish Steel Works, 3300 Frederiks6aerk, Denmark b Risø National Laboratory, 4000 Roskilde, Denmark c Technical Uni6ersity of Denmark, 2800 Lyngby, Denmark
Abstract The main objective of the work was to investigate the effect of addition of ceramic particles with different size distributions on the mechanical properties, e.g. wear resistance and tensile strength, of spray formed materials. The experiments were carried out in a spray-forming unit at Risø National Laboratory, Denmark, where composites with a low alloyed boron steel (0.2 wt.% carbon) matrix containing alumina particles were produced. A comparison between cast hot-rolled material without particles, spray formed material without particles and the spray formed composites with an average ceramic particle size of 46 and 134 mm were carried out with respect to their mechanical properties e.g. wear resistance and tensile strength. It was found that the addition of Al2O3 particles to the steel improves its wear properties and reduces the elongation and tensile strength of the material, but the material still showed good ductility. The composite material was also forged and showed good forgeability. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Spray forming; Spray deposition; Composite materials; Mechanical properties
1. Introduction Metallic matrix composites (MMCs) have been subjected to intense research because of their potential in high-performance applications [1]. The aim of producing composite materials is to combine attractive properties from different materials. However, it is often difficult to achieve the potential benefits of MMCs using casting methods or a conventional powder metallurgy route because of inhomogeneous distribution and clustering of particles. Clustering is very often accompanied by porosity, which reduces the wear resistance and tensile strength of the composite [2,3]. Furthermore, if the bonding between particles and matrix is poor, the resulting mechanical properties can also end up being poor [4]. Production of MMC by the spray forming process provides a potential advantage because the microstructure free from macro segregations is combined with the * Corresponding author. E-mail address: [email protected] (K. Petersen).
reinforcement of a second phase, e.g. ceramic particles [5]. Compared to the traditional powder metallurgy route the spray forming process represents a near net shape process with a reduced number of process steps, which is appealing for industrial use. Another advantage is the possibility to produce larger composite parts, e.g. it is now possible to produce billets with a height of 2000 mm and a diameter of 500 mm [6]. In contrast to conventional powder metallurgy, the production of MMC by the spray forming process provides a good contact between melt and particle. This affects the spectrum of possible material combinations, because diffusion of elements in the melt is strongly increased compared to traditional powder metallurgy. A major challenge in using the spray forming process to produce MMC is to obtain a homogeneous particle distribution and a uniform particle concentration throughout the deposit. In cast materials segregation of the particles can be a problem because of the slow cooling rate. However, when dealing with the solidification of MMC materials the velocity of the solidification front has to exceed a
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Table 1 Composition of the steel matrix in wt.% before the spray forming Fe
C
Mn
Si
Cr
Al
N
Ti
B
98.1
0.198
0.97
0.21
0.4
0.038
0.0065
0.046
0.0039
critical value in order to ensure that the particles are engulfed and not pushed ahead of the solidification front [2,7,8]. In this way the distribution of the particles remains homogeneous. The critical velocity is dependent on the wetting conditions between melt and particles, the viscosity, the particle size and the thermal conductivities of the melt and the particle [9]. Poor wetting and small particles tend to increase the problem of particle segregation. The experiments have shown that the velocity of the solidification front in the spray formed MMC was sufficiently high to avoid problems with particle segregation. The work described in this paper has focused on low alloyed boron steel, which is an easily forgeable steel often used for heavy wear components. This type of steel was spray formed with addition of Al2O3 particles with an average size of 46 and 134 mm. The material composition, the microstructure and the particle/matrix interface of the spray formed composites were studied in relation to the tensile strength and wear resistance.
2. Experimental
ished to a 3 mm diamond-paste finish and etched with 2% nital.
2.3. Spray forming unit The spray forming unit consists of two chambers, a melting chamber and an atomisation chamber, see Fig. 2. Three ring shaped nozzles were placed around the outlet namely the primary, secondary and particle nozzle. The particles were mixed with nitrogen outside the spray forming container and carried into the spray forming chamber where they were injected into the spray cone at a low angle. The particles were injected in the as-delivered condition without any pre-treatment. Nitrogen was used as atomization gas for all three nozzles. The melt droplets were collected on a rotating substrate (not withdrawn) where they amalgamated and solidified. In Table 3 the process parameters for spray forming of the MMC material are shown. Compared to spray forming boron steel without particle injection, the atomisation pressure was reduced to compensate for the increased cooling from the particles and the particle gas.
2.1. Materials The experimental studies were conducted on boron steel with the composition listed in Table 1. The composition of the reference and the composite material was measured with an ARL 3460 Emission Spectrometer. The reinforcing particles used were sintered alumina (Hexagonal Al2O3). Fig. 1 shows a typical morphology of the particles before they were injected. As can be seen the particles have sharp edges and have an appearance like broken glass. The particle size distribution was measured by laser diffraction. The average particle diameter for the two powders was determined to be 46 and 134 mm. The composition of the alumina particles is shown in Table 2.
2.2. Microscopy The spray formed samples, the wear tracks and the tensile test bars were studied in light optical microscope (LOM) equipped with image analysis software and in Scanning Electron Microscope. The samples were pol-
Fig. 1. Morphology of Al2O3-particles (134 mm average size). Table 2 Composition of alumina particles in wt.% Al2O3
TiO2
SiO2
Fe2O3
CaO
MgO
97.88
1.60
0.30
0.08
0.04
0.10
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K. Petersen et al. / Materials Science and Engineering A326 (2002) 40–50
Fig. 2. The diagram of spray-forming plant at Risø.
2.4. Heat treatment
2.7. Tensile test
The spray formed MMC samples for tensile testing and wear test were subjected to the same heat treatment as recommended for the pure hot rolled boron steel. The samples were heated to 900 °C and then quenched in water. Afterwards the samples were tempered at 400 °C for 2 h and subsequently cooled in air. Only half of the wear samples were tempered. This was done to study the effect of increased hardness on the wear resistance for samples with and without particles.
The tensile strengths of the samples were tested on an Instron 1115 tensile test machine. All the tensile test samples were prior to the tensile test heated to 900 °C and quenched in water. Afterwards the samples were tempered for 2 h at 400 °C. The cross section area of the samples were 12.6 mm2 and the drawing speed were kept constant at 0.5 mm min − 1. The elongation of the samples were measured with an extensometer with a gauge length of 10 mm which was able to measure up to 10% elongation. The tensile test experiments compare spray formed boron steel with and without the addition of particles.
2.5. Hardness measurement The hardnesses of the samples were measured according to the ASTM standard for measuring Vickers Hardness with a 50 kg load.
2.6. Forging To test the forgeability of the spray formed composites the material and a hot rolled reference material were drop forged at a temperature of 1200 °C. The samples were cylindrical with a diameter of 45 mm and a height of 40 mm. The samples were forged at Norsaenk Aalykke, Denmark.
Table 3 MMC spray forming parameters Gas pressure atomisation nozzle (bar) Gas pressure primary nozzle (bar) Gas pressure particle nozzle (bar) Gas/metal ratio (atomisation gas) Liquidus temperature (°C) Superheat temperature (°C) Mass spray formed (kg) Flight distance (mm)
11–13 2–2.5 7.5–10 1.2 1507 1620 10.5 440
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Fig. 3. Dry Sand/Rubber Wheel Abrasion Test with typical wear pattern shown.
Fig. 4. (a) Spray formed boron steel +particles, (b) spray formed boron steel without particles.
2.8. Wear test The abrasion wear resistance was tested in accordance with a modified ASTM G65 – 94 standard. The test is a Dry Sand/Rubber Wheel Abrasion test. A schematic diagram of the equipment is shown in Fig. 3 together with a picture of a typical wear pattern. All the samples were hardened by a quench from 900 °C in water and half of the samples were subsequently tempered at 400 °C for 2 h. The wear test is a three body wear test and the sand used is CEN standard sand with an average size of 0.6 mm. The feeding rate was 250 g min − 1. The samples were worn for 3 min. Before and after the test the samples were weighed and the wear losses were calculated in mm3 min − 1. The dimensions of the samples were 65×30 × 11 mm3. The contact area was :600 mm2 (small increase when the sample is worn). The experiments were conducted with a load of 0.22 N mm − 2. The sliding speed in the wear situation was 0.21 m s − 1. The machinery is composed of a steel disc mounted with a rubber rim. The sample is pressed against the rotating rubber disc while sand is poured into the space
between the disc and the sample. There is only contact between the rubber wheel and the sample in the start position. When the test starts the rubber disc drags the sand in between the sample and the wheel and the sample is worn. Because of small variations in the particle content of the spray formed deposit the actual particle content was measured in each wear and tensile test sample on basis of micrographs taken in a LOM.
3. Results
3.1. Microstructure The deposited material had a bell shaped geometry with a constant amount of particles in a cylindrical volume of the deposit. The spray formed samples were characterised as regarding the grain size, volume fraction of particles, particle behaviour in the deformed structure and the interfaces between ceramic and matrix.
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Table 4 Changes in composition of spray formed material compared to the reference material
Spray formed MMC Spray formed pure boron steel Reference material (hot-rolled)
Nitrogen (wt.%)
Carbon (wt.%)
0.0308 0.0388
0.144 0.167
0.0080
0.190
Fig. 4a and b shows typical micrographs of the microstructure of the as-sprayed material with and without particles. The microstructure of the sprayed material is found to be consisting of ferrite and perlite. No significant difference in grain size of the material with or without particles was found. Both materials had a grain size of 10 according to the ASTM standard. The important differences in the composition between the reference material, the pure spray formed boron steel and the spray formed composite materials are shown in Table 4. As can be seen the amount of nitrogen in the material increases when it is spray
formed. The carbon content shows the reverse tendency and decreases during the spray forming. A low magnification micrograph (Fig. 5b) shows the distribution of particles in the centre of the deposit with an average particle content of 13.5% by volume. As often observed and as seen from this figure a homogeneous distribution of the particles is obtained. In Fig. 5a the matrix, which contains ferrite/perlite phases, tightly surrounds the particles and the interface porosity between the particle and the matrix is therefore limited. In general, observations indicated that only a very limited number of particle–particle contacts were present.
3.2. Hardness/heat treatment As mentioned in the experimental procedure the samples were hardened by quenching the samples from 900 °C and followed by a tempering at 400 °C for 2 h. The hardness measurements of the samples are shown in Fig. 6. As can be seen in Fig. 6 the hardness of the spray formed material without particles in both the fully
Fig. 5. (a) High magnification of the interaction area between the alumina particle and the matrix, (b) low magnification.
Fig. 6. Effect of spray forming and particle injection on the hardenability.
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Table 5 Results from tensile test Boron steel
Tensile strength 1131 (N mm−2) Yield strength 1010 (N mm−2) Elongation (%) 10.6 a
MMC 8.9 (vol.%)a
MMC 6.2 (vol.%)
993
1031
880
–
4.0
–
Average of the two measurements.
Fig. 7. Drop forged samples without the flashing removed.
hardened and tempered state is higher than the hardness of the reference material despite the lower carbon content. The hardness of the composite materials are clearly affected by the presence of particles especially in the fully hardened state where the hardness is significantly lower than the hardness of the spray formed material without particles. After tempering the difference in hardness decreases. Further, there seems to be no correlation between the hardness of the composite materials and the particle size.
3.3. Forging On Fig. 7 the forged samples are shown. The composite material contains :6.5 vol.% of Al2O3 particles with an average size of 134 mm. During the forging a large amount of flashing is formed, which is also shown on the figure. The flashing has been heavily deformed and small cracks have initiated but there are no cracks on the surface of the actual part, which is a sign of sufficient ductility of the composite material. Fig. 8a and b shows the microstructure of the forged composite material in low and high magnification. The amount of damaged particles is very limited and the porosity is the same as in the as-sprayed condition. There is however a tendency to create porosities at particle–particle contacts. The particles have a preferred orientation parallel with the flow direction of the matrix material. Fig. 8b shows that there is still a good contact between matrix material and the particles and
no interface porosities have been created due to the forging with exception of contact between particles. Regarding the microstructure it is clear that the a ferrite zone has been created around the particle in the forged material. The same structure is also seen in the as-sprayed condition.
3.4. Tensile test The tensile strength was measured for two different materials: (1) Spray formed boron steel without particles and (2) Spray formed boron steel with alumina particles with a mean size of 134 mm. The material was tested on I-shaped cylindrical bars narrowed to a diameter of 4 mm at the midsection. In Table 5 the results from the tensile tests are shown. Because of the small cross section area the effect from crack initiating irregularities at the surface may have an influence on the apparent strength. The measured values should therefore be taken as minimum values. The values of the tensile strength, the yield strength and the elongation were reduced by the addition of particles. Because of the low number of measurements the values should be taken as indications. Though subsequent experiments with a boron steel with 0.33 wt.% carbon have shown the same tendency. Fig. 9 shows the stress–elongation curves of three samples (two identical samples with the same particle content were tested). The elongation of the pure spray formed boron steel sample exceeded the limit of the
Fig. 8. (a) Microstructure of the forged composite; (b) large magnification of a particle in the forged material.
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extensometer (10% in elongation) which is the explanation for the sudden fall of the stress– strain curve.
3.5. Wear test The wear test compared four different materials: (1 – 2) Spray formed boron steel with alumina particles with an average particle size of 134 and 46 mm, respec-
tively, (3) Spray formed boron steel and (4) Conventionally hot-rolled boron steel. All the samples had a surface roughness equivalent to grinding on P320 SiC paper. The results from the wear test are shown in Table 6 and in Fig. 10 the values are graphically compared. For the materials without particles the cast material has higher wear resistance than the spray formed mate-
Fig. 9. Stress – elongation curve for spray formed boron steel and MMC. Table 6 Wear test results for boron steel materials Sample
Hardness (HV)
Part. Cont. (vol.%)
Wear (mm3 min−1)
Cast Cast Spray Spray Spray Spray Spray Spray
376 524 400 568 379 494 525 375
– – – – 6.5 6.5 5.0 5.0
3.15 4.07 3.39 4.48 2.50 1.60 2.06 1.13
formed formed formed+134 mm formed+134 mm formed+46 mm formed+46 mm
Fig. 10. Wear rates for the composite material, spray formed and cast reference materials.
K. Petersen et al. / Materials Science and Engineering A326 (2002) 40–50
rial and surprisingly the wear resistance is higher in the hardened and tempered state than in the fully hardened state. For the composite materials the dependency on the hardness is the reverse. The fully hardened materials clearly have the best wear resistance and in both states the material reinforced with the small 46 mm particles has the highest wear resistance.
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4. Discussion
caused by differences in thermal expansion coefficient (hFe = 17.2× 10 − 6 K − 1, hAl2O3 = 8.3× 10 − 6 K − 1). An approximation to the stresses caused by the difference in thermal expansion can be estimated by Eshelbys modified model [12]. When the particles are injected into the material they are probably quickly heated to a common temperature with the matrix. During the cooling the difference in thermal contraction between the matrix and a particle results in a strain which can be expressed by Eq. (1) [13]:
4.1. Composition of spray formed material
m= Dh·DT,
During the spray forming the nitrogen content was expectedly increased in the metal phase because of prolonged contact between melt and nitrogen gas in the process (Table 4). The increased amount of nitrogen might affect the ageing behaviour of the steel. Ageing is caused by the diffusion of nitrogen to dislocations in the steel thereby reducing the ductility of the steel [10]. Increasing the content of aluminium or titanium however can minimise this effect. The aluminium and titanium will react with the nitrogen and form nitrides and thereby prevent the diffusion. Through TEM investigations an increased dislocation density has been observed in spray formed steel composites with the addition of Al2O3 particles which led to an increased ageing of the steel [11]. During the spray forming process the carbon content was reduced. The reduction of carbon is probably caused by small oxygen content in the nitrogen gas, which reacts with carbon under the formation of CO. The total reduction for carbon is therefore probably dependent on the heating time. Furthermore reactions with the refractory material may also lead to a reduced carbon content. The loss of carbon can easily be compensated for.
4.2. Microstructure One of the limiting factors for obtaining good mechanical properties is the amount of particle–particle contacts due to agglomeration. The lower mechanical properties are due to the fact that such particle–particle contacts often create porosities between the particles which prevents a proper fixation in a wear situation [2]. The injection method used in the current work gives a good dispersion of the particles and the matrix material tightly surrounds the particles with very limited contact between the particles. The porosities seen inside the alumina particles on Figs. 4a and 5a probably originate from spalling of the particles during the metallographic preparation. The low amount of porosity observed at the interface indicates that there is no problem with neither degassing of the particles nor cracking of the particles
(1)
which through a cooling interval DT =400 °C (estimated temperature interval where stresses are build up) leads to a strain of mDh = 3.56× 10 − 3. The strain is absorbed by the materials, which, with a particle content of 10 vol.%, leads to an average stress of :685 N mm − 2 in the matrix material adjacent to the particle according to Eshelbys modified model [12]. In spite of the pressure on the particles exerted by the matrix very few particles were cracked. As can be seen in Fig. 4a the grain size of the microstructure close to the particles is not affected by the increased cooling rate as might be expected because the particles are at room temperature when they are injected. The explanation for that is probably that the heat content of the melt is so high, that the conduction of heat from the melt to the particles does not significantly affect the solidification of the melt around the particles. It is not possible to detect any reaction zones at the matrix/particle interface on the micrographs but around some of the particles a layer of ferrite is formed. Other studies with the same materials have shown that titanium segregates to the interface because of its large affinity to oxygen and they also show that the effect increases by the use of nitrogen. Titanium stabilises ferrite, which is possibly the explanation why a ferrite zone is created around some of the particles. The reason why this is only observed for some of the particles is probably due to a complex cooling pattern around the particles depending of particle size and the location in the deposited material. In Fig. 11 the effect is clearly shown.
4.3. Hardness The lower carbon content of the spray formed steels affects the amount of perlite in the structure thereby lowering the overall hardness. The lower carbon content also affects the hardenability of the steel because the hardness of the martensite created by the quench is proportional to the carbon content for these levels of carbon content. But in spite of the lower carbon content the spray formed material still show the highest hardness, which must be due to a finer microstructure.
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Al2O3 particles with an average size of 134 mm. The material showed good forgeability. However a higher particle content will result in a decreased ductility and the number of particle–particle contacts will increase probably leading to reduced forgeability.
4.5. Tensile test
Fig. 11. Ferrite zone created around an alumina particle.
Further it is possible that a large number of fine dispersed nitrides have been created during the spray forming, which results in dispersion hardening. The addition of particles to the material results in a significant decrease of the hardness in the fully hardened state. A possible explanation for this could be that the high hardness of the matrix results in a higher load on the particles during the hardness indent. This might lead to particle cracking and interface sliding between particles and matrix, which can lead to a lower measured hardness of the composites. By tempering the composite material the load on the particles is smaller during hardness indent and the amount of particle damage is reduced leading to only small differences in the hardness compared to the material without particles.
4.4. Forging The results show that forging is a possible shaping process for the composites which minimises the amount of cutting operations which are very difficult for these materials. The forged material contained 6.5 vol.%
By adding 8.9 vol.% Al2O3 particles to the steel the yield strength and ultimate strength is reduced by : 12.5% and elongation of the samples has decreased from 11 to : 4%. Although the number of measurements is limited, it is clear that the tendency is toward reduction of the yield strength and ultimate strength when particles are added to the matrix. The yield strength and the ultimate strength of the specimen is probably reduced because there is no bonding between the particles and the matrix at the high tensile stresses which reduces the load carrying area of the material. Stress concentrations are generated around the particles leading to an earlier locally yield at relatively low loads. This is also the reason why it is difficult to determine the yield strength of particle reinforced materials [14]. The reduced load carrying area and the stress concentrations around the particles are also the explanation for the decrease in elongation. At earlier stages in the tensile test the matrix material around the particles compensate for the higher stresses by work hardening. But when the stretch limit for the material is reached this is no longer an option and a crack is initiated that eventually leads to failure of the material. To overcome this problem the use of very ductile steel with good work hardening abilities can prevent this to a certain degree [15]. The fact that the material is stretched during the deformation can clearly be seen in Fig. 12a and b. The specimen is cut parallel to the pull direction and it is clear that porosities have developed on the sides of the particles during yield. Some of the particles have fractured probably because of the transverse contraction of the material.
Fig. 12. (a) Cross section of tensile test specimen; (b) particle cracked during tensile test.
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Fig. 13. (a) Fracture surface of boron steel; (b) fracture surface of boron steel +Al2O3.
On Fig. 13 the fracture surface of the material with and without particles are shown. From the particle imprints and the particle appearance it is believed the crack has propagated along the particle– matrix interface. The fracture surface without particles (Fig. 13a) has a cup and cone surface with more and smaller dimples compared to the one with particles, which is a clear indication of the crack initiating effect of the particles. An EDX analysis was conducted on the particle imprint seen in Fig. 13b. The result showed a drastic increase of the titanium concentration at the surface of the particle imprint compared to the average composition of the matrix which might suggest some kind of a chemical bond between the matrix and the particles. Based on the results from the tensile test it is not possible to determine whether there is a bonding between the matrix and the particles which is broken at higher loads. If there was a bonding the shear lag theory [15] predicts a transfer of stresses between the matrix and the particles resulting in a higher E-modulus of the material because of the higher modulus of alumina. If there on the other hand was no bonding the E-modulus would decrease because the area carrying the load is smaller than the apparent area. But because the particle content is only 8–9 vol.% the difference between the two situations is not big enough to conclude whether there is a bond or not. Furthermore stress concentrations around the particles causes the nearby material to reach the yield point at a significantly lower load than the overall yield strength of the material [14,16]. This leads to a decrease of the average E-modulus and it is not possible, based on the results from the tensile test, to determine whether there is a bonding between particles and matrix that break at higher loads.
4.6. Wear For the pure cast hot-rolled steel and spray formed steel the wear resistance was better in the hardened and tempered state than in the fully hardened state. The reason for this is probably that the deformation of the surface during the wear exceeds the stretch limit for the
material leading to crack formation and propagation. This wear mechanism has a very high wear rate [17]. The wear is lowered when the material is tempered and the ductility of the material is increased. The results also show that the cast material has a higher wear resistance than the spray formed material despite a lower overall hardness. A reason for this could be that the hardness of the martensite in the spray formed material is lower than in the cast material due to the lower carbon content. A possible conclusion could, therefore, be that the hardness of the martensite and not the overall hardness is decisive for the wear resistance of the boron steel. Other authors have also found that the effect of precipitation hardening on the wear resistance of steel is limited [18]. For the composite materials the dependency of hardness on the wear resistance is reverse: The wear resistance increases with matrix hardness even though the ductility is reduced further by the presence of particles. A possible explanation of the result could be that the presence of particles in the material in a wear situation reduces the deformation of the surface. Further the particles get a better support by the higher matrix hardness and are less prone to get pulled out of the matrix. It is also more difficult for the abrasives to penetrate the surface and dig out the alumina particles when the hardness of the matrix is increased. Axen and Zum Gahr [19] have reported that the hardness of the matrix material should be as high as possible to obtain the best wear resistance for a composite material under the assumption that it has a sufficient ductility. Richardson [20] has reported that the best wear resistance for an MMC is obtained if the hard particles are larger than the wearing particles (assuming that the particle hardness is higher than hardness of the wearing particle). The abrasive particles had an average size of 0.6 mm but still the best wear resistance is obtained with the small particles. The reason for this is probably the shorter mean free path between the particles [19,21]. When the distance between the particles is reduced the surface will give more resistance against penetration of a wearing particle since the radius of the wearing particle edge is bigger than the mean free path [21]. It
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has also been found that larger particles need a better support than small particles to have the same effect on the wear resistance [21].
technical support and discussions. Further we would like to thank F.L. Smidth A/S for the use of wear test equipment and Norsaenk Aalykke for forging the materials.
5. Conclusion References The particle injection method used in the current spray forming process results in a homogeneous distribution of the particles in the matrix, which results in a very limited particle– particle contact, which is beneficial for the mechanical properties. The matrix tightly surrounds the particle, which gives a good particle support in a wear situation. The tensile strength and elongation of the boron steel is reduced by the addition of Al2O3-particles. The decrease in tensile strength and elongation is due to lack of bonding between the particles and the matrix that reduces the load carrying area of the test sample. This results in an earlier failure of the materials. It is not determined whether there is a bonding between particles and matrix that cracks during the tensile test, but the fact that titanium was observed at the imprint surface suggests a possible bonding reaction. The forging experiments results suggest that it is possible to shape the composite material and by that to reduce the number of cutting operations which is otherwise needed. The wear resistance of the tested materials against abrasive wear showed excellent results. The wear tests indicate that further improvement is possible by choosing a steel matrix with a higher hardness, which can provide a better support for the embedded particles during a wear situation. Further it is possible that higher particle contents could increase the wear resistance of the composite materials.
Acknowledgements We would like to thank the technicians involved especially Bent Larsen at Risø National Laboratory for
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The effect of particles in different sizes on the mechanical properties of spray formed steel composites Kenneth Petersen a,b,c,*, Allan Schrøder Pedersen b, Nini Pryds b, Knud Aage Thorsen c, Jan Laurberg List a a
The Danish Steel Works, 3300 Frederiks6aerk, Denmark b Risø National Laboratory, 4000 Roskilde, Denmark c Technical Uni6ersity of Denmark, 2800 Lyngby, Denmark
Abstract The main objective of the work was to investigate the effect of addition of ceramic particles with different size distributions on the mechanical properties, e.g. wear resistance and tensile strength, of spray formed materials. The experiments were carried out in a spray-forming unit at Risø National Laboratory, Denmark, where composites with a low alloyed boron steel (0.2 wt.% carbon) matrix containing alumina particles were produced. A comparison between cast hot-rolled material without particles, spray formed material without particles and the spray formed composites with an average ceramic particle size of 46 and 134 mm were carried out with respect to their mechanical properties e.g. wear resistance and tensile strength. It was found that the addition of Al2O3 particles to the steel improves its wear properties and reduces the elongation and tensile strength of the material, but the material still showed good ductility. The composite material was also forged and showed good forgeability. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Spray forming; Spray deposition; Composite materials; Mechanical properties
1. Introduction Metallic matrix composites (MMCs) have been subjected to intense research because of their potential in high-performance applications [1]. The aim of producing composite materials is to combine attractive properties from different materials. However, it is often difficult to achieve the potential benefits of MMCs using casting methods or a conventional powder metallurgy route because of inhomogeneous distribution and clustering of particles. Clustering is very often accompanied by porosity, which reduces the wear resistance and tensile strength of the composite [2,3]. Furthermore, if the bonding between particles and matrix is poor, the resulting mechanical properties can also end up being poor [4]. Production of MMC by the spray forming process provides a potential advantage because the microstructure free from macro segregations is combined with the * Corresponding author. E-mail address: [email protected] (K. Petersen).
reinforcement of a second phase, e.g. ceramic particles [5]. Compared to the traditional powder metallurgy route the spray forming process represents a near net shape process with a reduced number of process steps, which is appealing for industrial use. Another advantage is the possibility to produce larger composite parts, e.g. it is now possible to produce billets with a height of 2000 mm and a diameter of 500 mm [6]. In contrast to conventional powder metallurgy, the production of MMC by the spray forming process provides a good contact between melt and particle. This affects the spectrum of possible material combinations, because diffusion of elements in the melt is strongly increased compared to traditional powder metallurgy. A major challenge in using the spray forming process to produce MMC is to obtain a homogeneous particle distribution and a uniform particle concentration throughout the deposit. In cast materials segregation of the particles can be a problem because of the slow cooling rate. However, when dealing with the solidification of MMC materials the velocity of the solidification front has to exceed a
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Table 1 Composition of the steel matrix in wt.% before the spray forming Fe
C
Mn
Si
Cr
Al
N
Ti
B
98.1
0.198
0.97
0.21
0.4
0.038
0.0065
0.046
0.0039
critical value in order to ensure that the particles are engulfed and not pushed ahead of the solidification front [2,7,8]. In this way the distribution of the particles remains homogeneous. The critical velocity is dependent on the wetting conditions between melt and particles, the viscosity, the particle size and the thermal conductivities of the melt and the particle [9]. Poor wetting and small particles tend to increase the problem of particle segregation. The experiments have shown that the velocity of the solidification front in the spray formed MMC was sufficiently high to avoid problems with particle segregation. The work described in this paper has focused on low alloyed boron steel, which is an easily forgeable steel often used for heavy wear components. This type of steel was spray formed with addition of Al2O3 particles with an average size of 46 and 134 mm. The material composition, the microstructure and the particle/matrix interface of the spray formed composites were studied in relation to the tensile strength and wear resistance.
2. Experimental
ished to a 3 mm diamond-paste finish and etched with 2% nital.
2.3. Spray forming unit The spray forming unit consists of two chambers, a melting chamber and an atomisation chamber, see Fig. 2. Three ring shaped nozzles were placed around the outlet namely the primary, secondary and particle nozzle. The particles were mixed with nitrogen outside the spray forming container and carried into the spray forming chamber where they were injected into the spray cone at a low angle. The particles were injected in the as-delivered condition without any pre-treatment. Nitrogen was used as atomization gas for all three nozzles. The melt droplets were collected on a rotating substrate (not withdrawn) where they amalgamated and solidified. In Table 3 the process parameters for spray forming of the MMC material are shown. Compared to spray forming boron steel without particle injection, the atomisation pressure was reduced to compensate for the increased cooling from the particles and the particle gas.
2.1. Materials The experimental studies were conducted on boron steel with the composition listed in Table 1. The composition of the reference and the composite material was measured with an ARL 3460 Emission Spectrometer. The reinforcing particles used were sintered alumina (Hexagonal Al2O3). Fig. 1 shows a typical morphology of the particles before they were injected. As can be seen the particles have sharp edges and have an appearance like broken glass. The particle size distribution was measured by laser diffraction. The average particle diameter for the two powders was determined to be 46 and 134 mm. The composition of the alumina particles is shown in Table 2.
2.2. Microscopy The spray formed samples, the wear tracks and the tensile test bars were studied in light optical microscope (LOM) equipped with image analysis software and in Scanning Electron Microscope. The samples were pol-
Fig. 1. Morphology of Al2O3-particles (134 mm average size). Table 2 Composition of alumina particles in wt.% Al2O3
TiO2
SiO2
Fe2O3
CaO
MgO
97.88
1.60
0.30
0.08
0.04
0.10
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K. Petersen et al. / Materials Science and Engineering A326 (2002) 40–50
Fig. 2. The diagram of spray-forming plant at Risø.
2.4. Heat treatment
2.7. Tensile test
The spray formed MMC samples for tensile testing and wear test were subjected to the same heat treatment as recommended for the pure hot rolled boron steel. The samples were heated to 900 °C and then quenched in water. Afterwards the samples were tempered at 400 °C for 2 h and subsequently cooled in air. Only half of the wear samples were tempered. This was done to study the effect of increased hardness on the wear resistance for samples with and without particles.
The tensile strengths of the samples were tested on an Instron 1115 tensile test machine. All the tensile test samples were prior to the tensile test heated to 900 °C and quenched in water. Afterwards the samples were tempered for 2 h at 400 °C. The cross section area of the samples were 12.6 mm2 and the drawing speed were kept constant at 0.5 mm min − 1. The elongation of the samples were measured with an extensometer with a gauge length of 10 mm which was able to measure up to 10% elongation. The tensile test experiments compare spray formed boron steel with and without the addition of particles.
2.5. Hardness measurement The hardnesses of the samples were measured according to the ASTM standard for measuring Vickers Hardness with a 50 kg load.
2.6. Forging To test the forgeability of the spray formed composites the material and a hot rolled reference material were drop forged at a temperature of 1200 °C. The samples were cylindrical with a diameter of 45 mm and a height of 40 mm. The samples were forged at Norsaenk Aalykke, Denmark.
Table 3 MMC spray forming parameters Gas pressure atomisation nozzle (bar) Gas pressure primary nozzle (bar) Gas pressure particle nozzle (bar) Gas/metal ratio (atomisation gas) Liquidus temperature (°C) Superheat temperature (°C) Mass spray formed (kg) Flight distance (mm)
11–13 2–2.5 7.5–10 1.2 1507 1620 10.5 440
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Fig. 3. Dry Sand/Rubber Wheel Abrasion Test with typical wear pattern shown.
Fig. 4. (a) Spray formed boron steel +particles, (b) spray formed boron steel without particles.
2.8. Wear test The abrasion wear resistance was tested in accordance with a modified ASTM G65 – 94 standard. The test is a Dry Sand/Rubber Wheel Abrasion test. A schematic diagram of the equipment is shown in Fig. 3 together with a picture of a typical wear pattern. All the samples were hardened by a quench from 900 °C in water and half of the samples were subsequently tempered at 400 °C for 2 h. The wear test is a three body wear test and the sand used is CEN standard sand with an average size of 0.6 mm. The feeding rate was 250 g min − 1. The samples were worn for 3 min. Before and after the test the samples were weighed and the wear losses were calculated in mm3 min − 1. The dimensions of the samples were 65×30 × 11 mm3. The contact area was :600 mm2 (small increase when the sample is worn). The experiments were conducted with a load of 0.22 N mm − 2. The sliding speed in the wear situation was 0.21 m s − 1. The machinery is composed of a steel disc mounted with a rubber rim. The sample is pressed against the rotating rubber disc while sand is poured into the space
between the disc and the sample. There is only contact between the rubber wheel and the sample in the start position. When the test starts the rubber disc drags the sand in between the sample and the wheel and the sample is worn. Because of small variations in the particle content of the spray formed deposit the actual particle content was measured in each wear and tensile test sample on basis of micrographs taken in a LOM.
3. Results
3.1. Microstructure The deposited material had a bell shaped geometry with a constant amount of particles in a cylindrical volume of the deposit. The spray formed samples were characterised as regarding the grain size, volume fraction of particles, particle behaviour in the deformed structure and the interfaces between ceramic and matrix.
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Table 4 Changes in composition of spray formed material compared to the reference material
Spray formed MMC Spray formed pure boron steel Reference material (hot-rolled)
Nitrogen (wt.%)
Carbon (wt.%)
0.0308 0.0388
0.144 0.167
0.0080
0.190
Fig. 4a and b shows typical micrographs of the microstructure of the as-sprayed material with and without particles. The microstructure of the sprayed material is found to be consisting of ferrite and perlite. No significant difference in grain size of the material with or without particles was found. Both materials had a grain size of 10 according to the ASTM standard. The important differences in the composition between the reference material, the pure spray formed boron steel and the spray formed composite materials are shown in Table 4. As can be seen the amount of nitrogen in the material increases when it is spray
formed. The carbon content shows the reverse tendency and decreases during the spray forming. A low magnification micrograph (Fig. 5b) shows the distribution of particles in the centre of the deposit with an average particle content of 13.5% by volume. As often observed and as seen from this figure a homogeneous distribution of the particles is obtained. In Fig. 5a the matrix, which contains ferrite/perlite phases, tightly surrounds the particles and the interface porosity between the particle and the matrix is therefore limited. In general, observations indicated that only a very limited number of particle–particle contacts were present.
3.2. Hardness/heat treatment As mentioned in the experimental procedure the samples were hardened by quenching the samples from 900 °C and followed by a tempering at 400 °C for 2 h. The hardness measurements of the samples are shown in Fig. 6. As can be seen in Fig. 6 the hardness of the spray formed material without particles in both the fully
Fig. 5. (a) High magnification of the interaction area between the alumina particle and the matrix, (b) low magnification.
Fig. 6. Effect of spray forming and particle injection on the hardenability.
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Table 5 Results from tensile test Boron steel
Tensile strength 1131 (N mm−2) Yield strength 1010 (N mm−2) Elongation (%) 10.6 a
MMC 8.9 (vol.%)a
MMC 6.2 (vol.%)
993
1031
880
–
4.0
–
Average of the two measurements.
Fig. 7. Drop forged samples without the flashing removed.
hardened and tempered state is higher than the hardness of the reference material despite the lower carbon content. The hardness of the composite materials are clearly affected by the presence of particles especially in the fully hardened state where the hardness is significantly lower than the hardness of the spray formed material without particles. After tempering the difference in hardness decreases. Further, there seems to be no correlation between the hardness of the composite materials and the particle size.
3.3. Forging On Fig. 7 the forged samples are shown. The composite material contains :6.5 vol.% of Al2O3 particles with an average size of 134 mm. During the forging a large amount of flashing is formed, which is also shown on the figure. The flashing has been heavily deformed and small cracks have initiated but there are no cracks on the surface of the actual part, which is a sign of sufficient ductility of the composite material. Fig. 8a and b shows the microstructure of the forged composite material in low and high magnification. The amount of damaged particles is very limited and the porosity is the same as in the as-sprayed condition. There is however a tendency to create porosities at particle–particle contacts. The particles have a preferred orientation parallel with the flow direction of the matrix material. Fig. 8b shows that there is still a good contact between matrix material and the particles and
no interface porosities have been created due to the forging with exception of contact between particles. Regarding the microstructure it is clear that the a ferrite zone has been created around the particle in the forged material. The same structure is also seen in the as-sprayed condition.
3.4. Tensile test The tensile strength was measured for two different materials: (1) Spray formed boron steel without particles and (2) Spray formed boron steel with alumina particles with a mean size of 134 mm. The material was tested on I-shaped cylindrical bars narrowed to a diameter of 4 mm at the midsection. In Table 5 the results from the tensile tests are shown. Because of the small cross section area the effect from crack initiating irregularities at the surface may have an influence on the apparent strength. The measured values should therefore be taken as minimum values. The values of the tensile strength, the yield strength and the elongation were reduced by the addition of particles. Because of the low number of measurements the values should be taken as indications. Though subsequent experiments with a boron steel with 0.33 wt.% carbon have shown the same tendency. Fig. 9 shows the stress–elongation curves of three samples (two identical samples with the same particle content were tested). The elongation of the pure spray formed boron steel sample exceeded the limit of the
Fig. 8. (a) Microstructure of the forged composite; (b) large magnification of a particle in the forged material.
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extensometer (10% in elongation) which is the explanation for the sudden fall of the stress– strain curve.
3.5. Wear test The wear test compared four different materials: (1 – 2) Spray formed boron steel with alumina particles with an average particle size of 134 and 46 mm, respec-
tively, (3) Spray formed boron steel and (4) Conventionally hot-rolled boron steel. All the samples had a surface roughness equivalent to grinding on P320 SiC paper. The results from the wear test are shown in Table 6 and in Fig. 10 the values are graphically compared. For the materials without particles the cast material has higher wear resistance than the spray formed mate-
Fig. 9. Stress – elongation curve for spray formed boron steel and MMC. Table 6 Wear test results for boron steel materials Sample
Hardness (HV)
Part. Cont. (vol.%)
Wear (mm3 min−1)
Cast Cast Spray Spray Spray Spray Spray Spray
376 524 400 568 379 494 525 375
– – – – 6.5 6.5 5.0 5.0
3.15 4.07 3.39 4.48 2.50 1.60 2.06 1.13
formed formed formed+134 mm formed+134 mm formed+46 mm formed+46 mm
Fig. 10. Wear rates for the composite material, spray formed and cast reference materials.
K. Petersen et al. / Materials Science and Engineering A326 (2002) 40–50
rial and surprisingly the wear resistance is higher in the hardened and tempered state than in the fully hardened state. For the composite materials the dependency on the hardness is the reverse. The fully hardened materials clearly have the best wear resistance and in both states the material reinforced with the small 46 mm particles has the highest wear resistance.
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4. Discussion
caused by differences in thermal expansion coefficient (hFe = 17.2× 10 − 6 K − 1, hAl2O3 = 8.3× 10 − 6 K − 1). An approximation to the stresses caused by the difference in thermal expansion can be estimated by Eshelbys modified model [12]. When the particles are injected into the material they are probably quickly heated to a common temperature with the matrix. During the cooling the difference in thermal contraction between the matrix and a particle results in a strain which can be expressed by Eq. (1) [13]:
4.1. Composition of spray formed material
m= Dh·DT,
During the spray forming the nitrogen content was expectedly increased in the metal phase because of prolonged contact between melt and nitrogen gas in the process (Table 4). The increased amount of nitrogen might affect the ageing behaviour of the steel. Ageing is caused by the diffusion of nitrogen to dislocations in the steel thereby reducing the ductility of the steel [10]. Increasing the content of aluminium or titanium however can minimise this effect. The aluminium and titanium will react with the nitrogen and form nitrides and thereby prevent the diffusion. Through TEM investigations an increased dislocation density has been observed in spray formed steel composites with the addition of Al2O3 particles which led to an increased ageing of the steel [11]. During the spray forming process the carbon content was reduced. The reduction of carbon is probably caused by small oxygen content in the nitrogen gas, which reacts with carbon under the formation of CO. The total reduction for carbon is therefore probably dependent on the heating time. Furthermore reactions with the refractory material may also lead to a reduced carbon content. The loss of carbon can easily be compensated for.
4.2. Microstructure One of the limiting factors for obtaining good mechanical properties is the amount of particle–particle contacts due to agglomeration. The lower mechanical properties are due to the fact that such particle–particle contacts often create porosities between the particles which prevents a proper fixation in a wear situation [2]. The injection method used in the current work gives a good dispersion of the particles and the matrix material tightly surrounds the particles with very limited contact between the particles. The porosities seen inside the alumina particles on Figs. 4a and 5a probably originate from spalling of the particles during the metallographic preparation. The low amount of porosity observed at the interface indicates that there is no problem with neither degassing of the particles nor cracking of the particles
(1)
which through a cooling interval DT =400 °C (estimated temperature interval where stresses are build up) leads to a strain of mDh = 3.56× 10 − 3. The strain is absorbed by the materials, which, with a particle content of 10 vol.%, leads to an average stress of :685 N mm − 2 in the matrix material adjacent to the particle according to Eshelbys modified model [12]. In spite of the pressure on the particles exerted by the matrix very few particles were cracked. As can be seen in Fig. 4a the grain size of the microstructure close to the particles is not affected by the increased cooling rate as might be expected because the particles are at room temperature when they are injected. The explanation for that is probably that the heat content of the melt is so high, that the conduction of heat from the melt to the particles does not significantly affect the solidification of the melt around the particles. It is not possible to detect any reaction zones at the matrix/particle interface on the micrographs but around some of the particles a layer of ferrite is formed. Other studies with the same materials have shown that titanium segregates to the interface because of its large affinity to oxygen and they also show that the effect increases by the use of nitrogen. Titanium stabilises ferrite, which is possibly the explanation why a ferrite zone is created around some of the particles. The reason why this is only observed for some of the particles is probably due to a complex cooling pattern around the particles depending of particle size and the location in the deposited material. In Fig. 11 the effect is clearly shown.
4.3. Hardness The lower carbon content of the spray formed steels affects the amount of perlite in the structure thereby lowering the overall hardness. The lower carbon content also affects the hardenability of the steel because the hardness of the martensite created by the quench is proportional to the carbon content for these levels of carbon content. But in spite of the lower carbon content the spray formed material still show the highest hardness, which must be due to a finer microstructure.
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Al2O3 particles with an average size of 134 mm. The material showed good forgeability. However a higher particle content will result in a decreased ductility and the number of particle–particle contacts will increase probably leading to reduced forgeability.
4.5. Tensile test
Fig. 11. Ferrite zone created around an alumina particle.
Further it is possible that a large number of fine dispersed nitrides have been created during the spray forming, which results in dispersion hardening. The addition of particles to the material results in a significant decrease of the hardness in the fully hardened state. A possible explanation for this could be that the high hardness of the matrix results in a higher load on the particles during the hardness indent. This might lead to particle cracking and interface sliding between particles and matrix, which can lead to a lower measured hardness of the composites. By tempering the composite material the load on the particles is smaller during hardness indent and the amount of particle damage is reduced leading to only small differences in the hardness compared to the material without particles.
4.4. Forging The results show that forging is a possible shaping process for the composites which minimises the amount of cutting operations which are very difficult for these materials. The forged material contained 6.5 vol.%
By adding 8.9 vol.% Al2O3 particles to the steel the yield strength and ultimate strength is reduced by : 12.5% and elongation of the samples has decreased from 11 to : 4%. Although the number of measurements is limited, it is clear that the tendency is toward reduction of the yield strength and ultimate strength when particles are added to the matrix. The yield strength and the ultimate strength of the specimen is probably reduced because there is no bonding between the particles and the matrix at the high tensile stresses which reduces the load carrying area of the material. Stress concentrations are generated around the particles leading to an earlier locally yield at relatively low loads. This is also the reason why it is difficult to determine the yield strength of particle reinforced materials [14]. The reduced load carrying area and the stress concentrations around the particles are also the explanation for the decrease in elongation. At earlier stages in the tensile test the matrix material around the particles compensate for the higher stresses by work hardening. But when the stretch limit for the material is reached this is no longer an option and a crack is initiated that eventually leads to failure of the material. To overcome this problem the use of very ductile steel with good work hardening abilities can prevent this to a certain degree [15]. The fact that the material is stretched during the deformation can clearly be seen in Fig. 12a and b. The specimen is cut parallel to the pull direction and it is clear that porosities have developed on the sides of the particles during yield. Some of the particles have fractured probably because of the transverse contraction of the material.
Fig. 12. (a) Cross section of tensile test specimen; (b) particle cracked during tensile test.
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Fig. 13. (a) Fracture surface of boron steel; (b) fracture surface of boron steel +Al2O3.
On Fig. 13 the fracture surface of the material with and without particles are shown. From the particle imprints and the particle appearance it is believed the crack has propagated along the particle– matrix interface. The fracture surface without particles (Fig. 13a) has a cup and cone surface with more and smaller dimples compared to the one with particles, which is a clear indication of the crack initiating effect of the particles. An EDX analysis was conducted on the particle imprint seen in Fig. 13b. The result showed a drastic increase of the titanium concentration at the surface of the particle imprint compared to the average composition of the matrix which might suggest some kind of a chemical bond between the matrix and the particles. Based on the results from the tensile test it is not possible to determine whether there is a bonding between the matrix and the particles which is broken at higher loads. If there was a bonding the shear lag theory [15] predicts a transfer of stresses between the matrix and the particles resulting in a higher E-modulus of the material because of the higher modulus of alumina. If there on the other hand was no bonding the E-modulus would decrease because the area carrying the load is smaller than the apparent area. But because the particle content is only 8–9 vol.% the difference between the two situations is not big enough to conclude whether there is a bond or not. Furthermore stress concentrations around the particles causes the nearby material to reach the yield point at a significantly lower load than the overall yield strength of the material [14,16]. This leads to a decrease of the average E-modulus and it is not possible, based on the results from the tensile test, to determine whether there is a bonding between particles and matrix that break at higher loads.
4.6. Wear For the pure cast hot-rolled steel and spray formed steel the wear resistance was better in the hardened and tempered state than in the fully hardened state. The reason for this is probably that the deformation of the surface during the wear exceeds the stretch limit for the
material leading to crack formation and propagation. This wear mechanism has a very high wear rate [17]. The wear is lowered when the material is tempered and the ductility of the material is increased. The results also show that the cast material has a higher wear resistance than the spray formed material despite a lower overall hardness. A reason for this could be that the hardness of the martensite in the spray formed material is lower than in the cast material due to the lower carbon content. A possible conclusion could, therefore, be that the hardness of the martensite and not the overall hardness is decisive for the wear resistance of the boron steel. Other authors have also found that the effect of precipitation hardening on the wear resistance of steel is limited [18]. For the composite materials the dependency of hardness on the wear resistance is reverse: The wear resistance increases with matrix hardness even though the ductility is reduced further by the presence of particles. A possible explanation of the result could be that the presence of particles in the material in a wear situation reduces the deformation of the surface. Further the particles get a better support by the higher matrix hardness and are less prone to get pulled out of the matrix. It is also more difficult for the abrasives to penetrate the surface and dig out the alumina particles when the hardness of the matrix is increased. Axen and Zum Gahr [19] have reported that the hardness of the matrix material should be as high as possible to obtain the best wear resistance for a composite material under the assumption that it has a sufficient ductility. Richardson [20] has reported that the best wear resistance for an MMC is obtained if the hard particles are larger than the wearing particles (assuming that the particle hardness is higher than hardness of the wearing particle). The abrasive particles had an average size of 0.6 mm but still the best wear resistance is obtained with the small particles. The reason for this is probably the shorter mean free path between the particles [19,21]. When the distance between the particles is reduced the surface will give more resistance against penetration of a wearing particle since the radius of the wearing particle edge is bigger than the mean free path [21]. It
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has also been found that larger particles need a better support than small particles to have the same effect on the wear resistance [21].
technical support and discussions. Further we would like to thank F.L. Smidth A/S for the use of wear test equipment and Norsaenk Aalykke for forging the materials.
5. Conclusion References The particle injection method used in the current spray forming process results in a homogeneous distribution of the particles in the matrix, which results in a very limited particle– particle contact, which is beneficial for the mechanical properties. The matrix tightly surrounds the particle, which gives a good particle support in a wear situation. The tensile strength and elongation of the boron steel is reduced by the addition of Al2O3-particles. The decrease in tensile strength and elongation is due to lack of bonding between the particles and the matrix that reduces the load carrying area of the test sample. This results in an earlier failure of the materials. It is not determined whether there is a bonding between particles and matrix that cracks during the tensile test, but the fact that titanium was observed at the imprint surface suggests a possible bonding reaction. The forging experiments results suggest that it is possible to shape the composite material and by that to reduce the number of cutting operations which is otherwise needed. The wear resistance of the tested materials against abrasive wear showed excellent results. The wear tests indicate that further improvement is possible by choosing a steel matrix with a higher hardness, which can provide a better support for the embedded particles during a wear situation. Further it is possible that higher particle contents could increase the wear resistance of the composite materials.
Acknowledgements We would like to thank the technicians involved especially Bent Larsen at Risø National Laboratory for
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