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The effect of cryolite on grinding of stainless steel
Tribology International 143 (2020) 106021
Contents lists available at ScienceDirect
Tribology International journal homepage: http://www.elsevier.com/locate/triboint
The effect of cryolite on grinding of stainless steel J. Dellen a, *, L. Lynen a, A. Schwedt b, J. Mayer b, R. Telle a a b
RWTH Aachen, Institut für Gesteinshüttenkunde, Mauerstr. 5, D-52064, Aachen, Germany RWTH Aachen, Gemeinschaftslabors für Elektronenmikroskopie, Ahornstraße 55, D- 52074, Aachen, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Coated abrasive dry grinding Built-up edge Active filler
Active fillers play a major role in the dry grinding of stainless steel. However, the mechanism altering the process is still unknown. In this work, the efficiency of cryolite as filler was studied with a pin-on-disk tribometer. Simplified coated abrasives with and without active filler were used to reduce possible non-cryolite related in teractions to a minimum. Diffusion experiments with stainless steel pins and cryolite were performed and the change in efficiency during abrasion investigated. The analysis of the pins revealed a FexCr1-xF2 phase. Tensile tests were performed of heat-treated specimens to investigate changes in brittleness. Finally, dissolution ex periments were conducted, which proved a decrease in built-up edge volume in presence of cryolite, leading to a higher cutting efficiency.
1. Introduction In the process of dry grinding metals with coated abrasives, grinding efficiency is a major key point. Usually the cutting efficiency of the abrasive is guaranteed by a hard abrasive grain material and a strong anchoring of the grains in the support layer [1,2]. Nevertheless, metal adhesion during the grinding process can lead to built-up edges and therefore decrease the cutting efficiency [3]. Accordingly, it is necessary to use optimized materials capable of delivering new sharp cutting edges by means of grain fracture throughout the grinding process. Materials, such as Al2O3, SiC, etc. are widely used for that purpose [4]. However, some steels are difficult to process with abrasives. These hard-to-cut materials have either a high ductility, which can lead to smearing, or a high strength or hardness, which wears off the abrasive grains. Addi tionally, low thermal conductivities during the grinding process, which can also cause difficulties as well due to high temperatures [5], can increase thermal wear or lead to higher metal adhesion on the abrasive grain [6]. All these effects cause a decrease in cutting efficiency. Therefore, active fillers like FeS2, KBF4, Na3AlF6 etc. are used as mentioned in several patents [7–10]. It is known that these fillers foster the grinding process, but the mechanism of action is still unrevealed. Different attempts have been made to explain these phenomena. Without any chemical interaction only melting or decomposition of the fillers could lead to a lubricating [11] or cooling effect [12]. However, if chemical reactions are considered, interactions with the workpiece as well as the abrasive grain are possible. Two contrary theories address the
cryolite-grain interaction. Either fillers may protect the abrasive grain [12] or create new cutting edges by weakening the grain. This weak ening is likely to be caused by the eutectic of cryolite and corundum, which is well known due to the fused-salt electrolysis in aluminum production [13]. Interactions with the workpiece can be either chemical weakening as Rehbinder proved earlier with inorganic liquid additives [14] or a passivation of the created chip surfaces to avoid welding onto the abrasive [11,12]. However, no precise answer is available to that problem. This study investigates the interactions between the active filler cryolite and stainless steel during grinding and during a high-temperature corrosion based model test. 2. Materials and experimental method 2.1. Abrasives Simplified coated abrasives with a diameter of 125 mm were made by an established manufacturer of abrasive tools. The abrasives were made of a polyester finish cloth and a make coat of phenolic resole resin containing CaCO3. With this setup, two different top coatings were applied onto the abrasives, containing either Na3AlF6 as active filler (Cry) or CaCO3 as reference material (Ref). The abrasives were manu factured in one batch, which allows a better comparison. As grain ceramic α-Al2O3 size #60 was used. A cross-section of the above described abrasive is shown in Fig. 1. Additionally in some experiments, a commercially available abrasive (A) containing the same grain
* Corresponding author. E-mail address: [email protected] (J. Dellen). https://doi.org/10.1016/j.triboint.2019.106021 Received 2 August 2019; Received in revised form 7 October 2019; Accepted 14 October 2019 Available online 16 October 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
J. Dellen et al.
Tribology International 143 (2020) 106021
specification was used to check whether there are any effects besides that of cryolite. 2.2. Metal Austenitic stainless steel (1.4301) was studied in this work. The major alloying elements are Cr (~18%) and Ni (~10%). The C con centration is low with up to 0.07% [15]. Cold drawn round steel with a diameter of 15 mm, all of the same batch, was used in all grinding experiments. 2.3. Grinding experiments For the grinding experiments, a pin-on-disk tribometer was used. The tribometer was self-constructed in cooperation with Steinhauer GmbH. A technical drawing of the construction is presented in Fig. 2. The abrasive is mounted on a metal disk which can be accelerated up to 15000 rpm by a 7 kW engine. The pin was driven in a 90� angle onto the abrasive with a force of 15 N, which implied a pressure of 85 kPa. The circumferential velocity of the abrasive was set to 45.2 m/s, which led to an average velocity of 38.7 m/s in the contact area. During grinding, the pin was set to oscillate in radial direction on the disk. Grinding forces were measured with a K3D120 load cell by ME-Meßsysteme GmbH. As measurement amplifier, a Spider 8 was used in combination with the evaluation software Catman version 5, both by Hottinger Baldwin Messtechnik GmbH. All experiments were carried out with five to ten metal pins per abrasive disk. Each pin was driven onto the abrasive for 15 s, before the next pin was ground. This process was repeated until all cycles were completed. Each pairing of metal pins and disks was carried out at least three times to obtain results of higher precision. The grinding forces and time were measured for each grinding interval while mass reduction of the pins was determined by weighting the pins before and after each interval. The specific grinding energy was calculated with the average contact velocity and the above-mentioned three parameters as shown in equation (1). The specific energy was always calculated cumulatively from interval 1 to interval n. PIn R tfinal Fg υg dt I1 t0 � � spec: Wg ¼ (1) PIn ΔmgVA I1
Fig. 2. Technical drawing of the pin-on-disk tribometer. 1: Load Cell; 2: Pin; 3: Abrasive; 4: Engine.
ratio of the specific grinding energy, as shown in equation (2). Here, Cry refers to cryolite and Ref to the CaCO3 reference. � � spec:WgCry Wg%Cry ¼ 1 *100% (2) spec:WgRef 2.4. Heat treatments In addition to the usual grinding experiments, heat treatments of 1.4301 stainless steel pins were accomplished. For this purpose, holes of 1.5 mm diameter were drilled into the pins as shown in Fig. 3 and either left blank (VAa) or filled with cryolite (VAaC) as active filler. The pre pared pins were placed into a tube furnace at 850 � C for 18 h. After heat treatment, the scale layer was removed with a non-woven abrasive and the holes were widened to a diameter of 1.6 mm. All experiments were carried out in argon atmosphere. The tube furnace was sealed with a silicon plug on each side and the remaining oxygen in the tube was washed out with argon for 1 h. During heat treatment, a constant flow of 2–3 l/min was applied. 2.5. Tensile experiments
ρVA
According to DIN EN ISO 6892–1, tensile tests were performed with an Instron Modell 1186 updated with a control unit 5500. The tensile specimens were manufactured from 1.4301 stainless steel. For manufacturing, the same batch of steel was used as for the grinding experiments. The specimens were produced with a diameter of 4 mm � 0.05 mm resulting in a characteristic length of 20 mm according to DIN 50125. Tensile specimens were tempered with and without cryolite at 850 � C for 18 h as the ones for grinding experiments described in paragraph 2.4. Due to the fact that the Instron was not equipped with an extensometer, tensile tests could not be used for determining the real strain-related values of the steel. However, comparison between refer ence and cryolite was possible due to the same sample geometry and apparatus.
Here, spec.Wg is the specific grinding energy, t the time, Fg the grinding Force, υg the grinding velocity, ΔmgVA the machined volume of stainless steel and ρVA the density of stainless steel. Fluctuations in single measurement intervals were avoided by this cumulative calculation, leading to better comparable data. Therefore, the percentage of energy savings (Wg%Cry) through active fillers can easily be determined by the
2.6. Dissolution experiments To investigate the built-up edge, dissolution experiments were per formed. In these experiments, the abrasives were placed in a defined volume of concentrated HCl acid for 24 h. The acid was analyzed af terwards via ICP-OES (Agilent Technologies 700 Series) to determine the concentrations of Fe, Cr and Ni. Fig. 1. SEM image of the unused abrasive. 1: Abrasive Grain; 2: Top Coat; 3: Make Coat; 4: Polyester Finish Cloth. 2
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Tribology International 143 (2020) 106021
Fig. 3. Schematic drawing of experimental set up for heat treatment and pin geometry.
3. Results & discussion Cryolite-containing abrasives were compared to reference ones. The total machined volume of 1.4301 stainless steel was measured. As shown in Fig. 4, the difference between reference and cryolite-containing coated abrasives is negligible. However, a commercially available and therefore optimized abrasive (A), containing grains of the same speci fication, makes a difference. This can be reasoned with better grain orientation and/or allocation, small amounts of other additives and an optimized resin. Pin-on-disk experiments were all run with a constant pressure; therefore, it was expected that the differences during the grinding ex periments would mainly occur in machined volume of the pins. How ever, due to the oscillation of the pins, the controller had to readjust constantly, so the grinding forces and therefore the pressing force varied periodically. Consequently, the pressing force was fluctuating around a certain value and the feed speed adjusted by the actuator became decisive, which led to a measurable change in force. The change in forces led to variations in energy consumption of the grinding process. The specific grinding energy was calculated by equation (1) and is plotted for the different abrasives in Fig. 5. The energy consumption of the reference abrasive is significantly higher than that of that abrasive containing cryolite. This behavior shows that there must be some posi tive influence on the grinding process caused by cryolite. However, it also shows that the difference in energy consumption can further be increased by optimizing the abrasive. It should be noted, however, that Cry- and Ref-abrasives were produced in one batch and therefore differ only regarding the used filler. Still, energy savings between these two are around 10%. Comparing Ref to the optimized abrasive A even more than 15% of energy can be saved. To understand this behavior, an Energy Dispersive Spectroscopy (EDS) X-ray mapping was made of a built-up edge from one of the simplified cryolite-containing abrasives. This mapping, shown in Fig. 6, indicates a small fluoride concentration in combination with chromium and iron but without any other cryolite components. Therefore, there might be the possibility of an interaction of the fluoride, coming from the cryolite, with Fe and Cr of the stainless steel. However, parallel determination of Fe and F can be inaccurate due to the nearby FK and FeL spectral lines [16]. A single determination of fluoride was not successful either due to the minimal concentrations available or the disturbing background of the iron.
Fig. 5. Spec. grinding energy in pin-on-disk experiments.
To find out whether fluoride ions reacted with the steel surface or diffused into the matrix, heat treatments of the pins were carried out. Holes of 1.5 mm diameter were drilled into the pins and filled with cryolite or left empty and afterwards tempered under argon atmosphere at 850 � C for 18 h. Resulting scale layers were removed and pin-on-disk experiments performed with the following pairings: Ref&VAa, Ref&VAaC and Cry&VAa, as shown in Fig. 7. As soon as cryolite enters the system, the specific grinding energy is reduced. The difference in specific grinding energy between cryolite-containing abrasives or tempered pins is nearly the same. However, by thermal treatment of the pins with cryolite, fluoride can diffuse into quite a large volume of the pin. There is no literature dealing with the diffusion of fluorine in the γ-iron lattice; therefore, only an estimation based on a similar sized ion like O2 is possible [17]. However, due to the lower charge of F -Ions, at least the same diffusion rate can be expected. The diffusion coefficient of oxygen in the polycrystalline γ-iron lattice at 850 � C is approx. 2, 5 � 10 8 cm2/s [18]. Therefore, one can calculate by means of equation (3) that approx. 90% of the steel volume used in the pin-on-disk-tribometer experiments was affected by fluoride. The in clusion of cryolite through tempered Cry-pins led to higher energy savings in the first 300 s of the experiment compared to the inclusion of cryolite via top coating as seen in Fig. 7. For the whole grinding process of 600 s the energy savings are similar for both set-ups. This can be explained by the diffusion of fluorine as the time-controlling step in the interaction between cryolite and steel while grinding. Therefore, the diffusion which took place in the previous heat treatments enhanced this interaction. pffiffiffiffiffiffiffiffiffiffiffiffi x ¼ 6*D*t (3) here, x describes the diffusion depths, D is the diffusion coefficient and t the time. The tempered pins were analyzed via Scanning Electron Microscopy (SEM), shown in Fig. 8, and EDS X-ray mapping as shown in Fig. 9. Different phases occur around the drilled holes. On the left hand side, one can see the drilled hole. Moving further to the right, an oxide phase occurs at the boundary, followed by a large fluoride-containing phase and finally reaching the original stainless steel composition. Fluoride diffuses into the stainless steel deeper than any other components of the cryolite. EDS data shows a fluoride concentration in deeper diffusion
Fig. 4. Machined stainless steel volume in pin-on-disk experiments. 3
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Tribology International 143 (2020) 106021
Fig. 6. SEM overview and EDS X-ray mapping of a cross-section of a cryolite-containing abrasive after machining.
Fig. 7. Spec. grinding energy in pin-on-disk experiments using heattreated samples.
layers of more than 60 at% leading to an empirically calculated molec ular formula of approx. FexCr1-xF2 as shown in Table 1. To prove this result, a FIB lamella was investigated by Transmission Electron Micro scopy (TEM). The resulting lattice parameters verify the earlier calcu lated phase and are in good agreement with lattice parameters from literature as shown in Table 2, especially when considering that Cr-ions cause a distortion of the lattice. To find out if this phase has an influence on the mechanical prop erties of the steel like brittleness, tensile tests were performed as described in paragraph 2.5 and investigated by SEM and EDS after wards. Results show that samples treated with cryolite have a lower tensile strength of 600 MPa compared to the reference values of 630 MPa. However, the strain at rupture is 3% higher for the reference samples as shown in Fig. 10. SEM investigations showed that the
Fig. 8. SEM image of the cross section of a pin after heat-treating with cryolite.
cryolite-treated samples have a coarse surface and a smaller diameter after tensile tests, which implies flaking of a brittle phase. Therefore, the tensile tests of the cryolite specimens were recalculated with a 2% smaller diameter, according to the SEM pictures. This approximation delivers a similar mechanical behavior except small differences in yield stress and rupture strain. The higher yield stress obtained with cryolitetreated samples is in good agreement with the findings by Harper et al. [20], who showed that corrosion of surface layers has an impact on plasticity. Due to the fact that heat-treated pins and tensile specimens showed the occurrence of a metal fluoride phase which seemed to be more 4
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Tribology International 143 (2020) 106021
Fig. 9. EDS X-ray mapping of a cross-section of a heat-treated pin filled with cryolite. Table 1 EDS measurements in atom %. (For clarity small amounts of Si, Al, Mn, K and Mo are not listed). Element Fe Cr Ni Na F O
Measurement positions as shown in Fig. 6 1
2
3
4
Ø 5-7
8
22.7 31.3 2.4 – 2.0 41.1
20.3 – 79.8 – – –
20.9 19.4 13.1 8.7 30.4 –
31.7 7.3 3.0 – 56.4 –
28.3 7.8 – – 62.9 –
67.0 18.8 9.0 – – –
Table 2 Lattice parameter of the FexCr1-xF2 phase found after heat treatment via TEM investigations and values from literature. Lattice parameter
a [Å]
c [Å]
Measured (FexCr1-xF2) Literature (FeF2) [19]
4.693 4.6945
3.319 3.3097
Fig. 11. Quantification of built-up edge components via ICP-OES after partial dissolution with HCl.
brittle, dissolution experiments of the used abrasives have been per formed. The abrasives were placed in a bath of concentrated hydro chloric acid and were leached for 24 h. Afterwards the HCl solution was analyzed by ICP-OES and the concentrations of the metal ions Fe, Ni and Cr were determined. However, because the abrasive layer contained iron only, Cr and Ni deliver a meaningful value and are shown in Fig. 11. The occurrence of cryolite in the system reduces the amount of these alloying elements. This reduction in built-up edges is in good agreement with the findings of Vernhet et al. [21]. 4. Conclusion It was possible to show by pin-on-disk experiments that cryolite has a positive effect on the abrasion process of stainless steel. In this work, the effect resulted in energy savings of 10%. In comparison, commercially available abrasives yielded even higher energy savings. Therefore, the before-mentioned cooling hypothesis might not be an active cooling process but a reduction of the heat input. It was also possible to repro duce the energy saving effect by the diffusion of fluoride into the steel lattice via heat treatments. Our experiments have shown that one of the benefits of cryolite must be the interaction between the fluoride and the
Fig. 10. Influence of cryolite on the stress-strain curves of 1.4301.
5
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Tribology International 143 (2020) 106021
metal, which was not known yet. Therefore, hypotheses like the chem ical weakening of the material as well as the passivation of nascent metallic surfaces, which reduces metal adhesion, seem to be likely. Still, one cannot exclude further interactions in system. A new phase, un known to the literature, has been found in the steel, which reacted with cryolite during the heat treatments. This phase is more brittle as could be proven by tensile experiments. It was shown by dilution experiments of the used abrasives that less metal is attached to its surface, which leads to smaller clogging of the abrasive grains and; therefore, a higher cutting efficiency. In future experiments, it would be desirable to detect this new phase in build-up edges or on workpiece surfaces. However, this will be challenging due to its brittleness as well as the low concentration and additionally difficult detection of F in presence of Fe.
[2] Zum Gahr K-H. Wear by hard particles. Tribol Int 1998;31(No. 10):587–96. [3] Czichos H, Habig K-H. Tribologie-Handbuch. Springer; 2015. ch. 15.2. [4] Klocke F, K€ onig W. fertigungsverfahren schleifen, honen, l€ appen. Springer; 2005. ch. 3 pp. 19ff. [5] Reza Bateni M, et al. Wear and corrosion wear of medium carbon steel and 304 stainless steel. Wear 2006;260:116–22. [6] Nadolny K, et al. Effects of sulfurization of grinding wheels on internal cylindrical grinding of Titanium Grade 2. Indian J Eng Mater Sci 2013;20:108–24. [7] Kistler S. S. Abrasive Articles. U.S. Patent 2,308,981 (1943). [8] Kistler S. S. Bonded abrasive articles containing fillers. U.S. Patent 2,308,983 1943. [9] Kistler S. S., Abrasive Articles, U.S. Patent 2,408,319 (1946). [10] Chuda K. et al. Abrasive products including active fillers. U.S. Patent 8,491,681 (2013). [11] Coes L. Knowledge of the scientific principles of grinding is basis of recent progress in abrasives. Ind Eng Chem 1955;47(12):2493–4. [12] Jamrozik A, et al. Additives for abrasive materials. Abrasive TechnologyCharacteristics and Applications. IntechOpen; 2018. [13] Yurkov A. Refractories for aluminium. Springer Cham; 2015. ch. 2 pp. 65ff. [14] Rehbinder P. Verminderung der Ritzh€ arte bei Adsorption grenzfl€ achenaktiver Stoffe. Z Phys 1931;72:3–4. 191-205. [15] Edelstahlwerke Deutsche. Werkstoffdatenblatt X5CrNi18-10 1.4301. 2015-0046. 2015. [16] Eggert F. Standardfreie Elektronenstrahl-Mikroanalyse mit dem EDX im Rasterelektronenmikroskop ein Handbuch für die Praxis. BoD–Books on Demand; 2005. [17] Whittaker EJW, Muntus R. Ionic radii for use in geochemistry. Geochem Cosmochim Acta 1970;34(9):945–56. [18] Takada J, et al. Determination of diffusion coefficient of oxygen in γ-iron from measurements of internal oxidation in Fe-Al alloys. Metal Trans A 1986;17(2): 221–9. [19] Baur WH, Khan AA. Rutile-type compounds. IV. SiO2, GeO2 and a comparison with other rutile-type structures. Acta Crystallogr B 1971;27(11):2133–9. [20] Harper S, Cottrell AH. Surface effects and the plasticity of zinc crystals. Proc Phys Soc Sect B 1950;63(5):331–8. [21] Vernhet L, et al. Metal adhesion issues in dry grinding: the role of active fillers. Wear 2016;346:46–55.
Acknowledgements We would like to thank everyone from the Institute of Mineral En gineering (GHI) for their help and support. Special thanks go to D. Pletz and his team of the technical workshop who spent a lot of time on manufacturing all the stainless steel pins. The industrial partners and the FGS gave substantial support through knowledge and materials. The authors like to thank Arbeitsgemeinschaft industrieller For schungsvereinigungen “Otto von Guericke” e.V. for financial support (AiF intention no. 18282/N). References [1] Pintaude G, et al. Mild and severe wear of steels and cast irons in sliding abrasion. Wear 2009;267:19–25.
6
Contents lists available at ScienceDirect
Tribology International journal homepage: http://www.elsevier.com/locate/triboint
The effect of cryolite on grinding of stainless steel J. Dellen a, *, L. Lynen a, A. Schwedt b, J. Mayer b, R. Telle a a b
RWTH Aachen, Institut für Gesteinshüttenkunde, Mauerstr. 5, D-52064, Aachen, Germany RWTH Aachen, Gemeinschaftslabors für Elektronenmikroskopie, Ahornstraße 55, D- 52074, Aachen, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Coated abrasive dry grinding Built-up edge Active filler
Active fillers play a major role in the dry grinding of stainless steel. However, the mechanism altering the process is still unknown. In this work, the efficiency of cryolite as filler was studied with a pin-on-disk tribometer. Simplified coated abrasives with and without active filler were used to reduce possible non-cryolite related in teractions to a minimum. Diffusion experiments with stainless steel pins and cryolite were performed and the change in efficiency during abrasion investigated. The analysis of the pins revealed a FexCr1-xF2 phase. Tensile tests were performed of heat-treated specimens to investigate changes in brittleness. Finally, dissolution ex periments were conducted, which proved a decrease in built-up edge volume in presence of cryolite, leading to a higher cutting efficiency.
1. Introduction In the process of dry grinding metals with coated abrasives, grinding efficiency is a major key point. Usually the cutting efficiency of the abrasive is guaranteed by a hard abrasive grain material and a strong anchoring of the grains in the support layer [1,2]. Nevertheless, metal adhesion during the grinding process can lead to built-up edges and therefore decrease the cutting efficiency [3]. Accordingly, it is necessary to use optimized materials capable of delivering new sharp cutting edges by means of grain fracture throughout the grinding process. Materials, such as Al2O3, SiC, etc. are widely used for that purpose [4]. However, some steels are difficult to process with abrasives. These hard-to-cut materials have either a high ductility, which can lead to smearing, or a high strength or hardness, which wears off the abrasive grains. Addi tionally, low thermal conductivities during the grinding process, which can also cause difficulties as well due to high temperatures [5], can increase thermal wear or lead to higher metal adhesion on the abrasive grain [6]. All these effects cause a decrease in cutting efficiency. Therefore, active fillers like FeS2, KBF4, Na3AlF6 etc. are used as mentioned in several patents [7–10]. It is known that these fillers foster the grinding process, but the mechanism of action is still unrevealed. Different attempts have been made to explain these phenomena. Without any chemical interaction only melting or decomposition of the fillers could lead to a lubricating [11] or cooling effect [12]. However, if chemical reactions are considered, interactions with the workpiece as well as the abrasive grain are possible. Two contrary theories address the
cryolite-grain interaction. Either fillers may protect the abrasive grain [12] or create new cutting edges by weakening the grain. This weak ening is likely to be caused by the eutectic of cryolite and corundum, which is well known due to the fused-salt electrolysis in aluminum production [13]. Interactions with the workpiece can be either chemical weakening as Rehbinder proved earlier with inorganic liquid additives [14] or a passivation of the created chip surfaces to avoid welding onto the abrasive [11,12]. However, no precise answer is available to that problem. This study investigates the interactions between the active filler cryolite and stainless steel during grinding and during a high-temperature corrosion based model test. 2. Materials and experimental method 2.1. Abrasives Simplified coated abrasives with a diameter of 125 mm were made by an established manufacturer of abrasive tools. The abrasives were made of a polyester finish cloth and a make coat of phenolic resole resin containing CaCO3. With this setup, two different top coatings were applied onto the abrasives, containing either Na3AlF6 as active filler (Cry) or CaCO3 as reference material (Ref). The abrasives were manu factured in one batch, which allows a better comparison. As grain ceramic α-Al2O3 size #60 was used. A cross-section of the above described abrasive is shown in Fig. 1. Additionally in some experiments, a commercially available abrasive (A) containing the same grain
* Corresponding author. E-mail address: [email protected] (J. Dellen). https://doi.org/10.1016/j.triboint.2019.106021 Received 2 August 2019; Received in revised form 7 October 2019; Accepted 14 October 2019 Available online 16 October 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
J. Dellen et al.
Tribology International 143 (2020) 106021
specification was used to check whether there are any effects besides that of cryolite. 2.2. Metal Austenitic stainless steel (1.4301) was studied in this work. The major alloying elements are Cr (~18%) and Ni (~10%). The C con centration is low with up to 0.07% [15]. Cold drawn round steel with a diameter of 15 mm, all of the same batch, was used in all grinding experiments. 2.3. Grinding experiments For the grinding experiments, a pin-on-disk tribometer was used. The tribometer was self-constructed in cooperation with Steinhauer GmbH. A technical drawing of the construction is presented in Fig. 2. The abrasive is mounted on a metal disk which can be accelerated up to 15000 rpm by a 7 kW engine. The pin was driven in a 90� angle onto the abrasive with a force of 15 N, which implied a pressure of 85 kPa. The circumferential velocity of the abrasive was set to 45.2 m/s, which led to an average velocity of 38.7 m/s in the contact area. During grinding, the pin was set to oscillate in radial direction on the disk. Grinding forces were measured with a K3D120 load cell by ME-Meßsysteme GmbH. As measurement amplifier, a Spider 8 was used in combination with the evaluation software Catman version 5, both by Hottinger Baldwin Messtechnik GmbH. All experiments were carried out with five to ten metal pins per abrasive disk. Each pin was driven onto the abrasive for 15 s, before the next pin was ground. This process was repeated until all cycles were completed. Each pairing of metal pins and disks was carried out at least three times to obtain results of higher precision. The grinding forces and time were measured for each grinding interval while mass reduction of the pins was determined by weighting the pins before and after each interval. The specific grinding energy was calculated with the average contact velocity and the above-mentioned three parameters as shown in equation (1). The specific energy was always calculated cumulatively from interval 1 to interval n. PIn R tfinal Fg υg dt I1 t0 � � spec: Wg ¼ (1) PIn ΔmgVA I1
Fig. 2. Technical drawing of the pin-on-disk tribometer. 1: Load Cell; 2: Pin; 3: Abrasive; 4: Engine.
ratio of the specific grinding energy, as shown in equation (2). Here, Cry refers to cryolite and Ref to the CaCO3 reference. � � spec:WgCry Wg%Cry ¼ 1 *100% (2) spec:WgRef 2.4. Heat treatments In addition to the usual grinding experiments, heat treatments of 1.4301 stainless steel pins were accomplished. For this purpose, holes of 1.5 mm diameter were drilled into the pins as shown in Fig. 3 and either left blank (VAa) or filled with cryolite (VAaC) as active filler. The pre pared pins were placed into a tube furnace at 850 � C for 18 h. After heat treatment, the scale layer was removed with a non-woven abrasive and the holes were widened to a diameter of 1.6 mm. All experiments were carried out in argon atmosphere. The tube furnace was sealed with a silicon plug on each side and the remaining oxygen in the tube was washed out with argon for 1 h. During heat treatment, a constant flow of 2–3 l/min was applied. 2.5. Tensile experiments
ρVA
According to DIN EN ISO 6892–1, tensile tests were performed with an Instron Modell 1186 updated with a control unit 5500. The tensile specimens were manufactured from 1.4301 stainless steel. For manufacturing, the same batch of steel was used as for the grinding experiments. The specimens were produced with a diameter of 4 mm � 0.05 mm resulting in a characteristic length of 20 mm according to DIN 50125. Tensile specimens were tempered with and without cryolite at 850 � C for 18 h as the ones for grinding experiments described in paragraph 2.4. Due to the fact that the Instron was not equipped with an extensometer, tensile tests could not be used for determining the real strain-related values of the steel. However, comparison between refer ence and cryolite was possible due to the same sample geometry and apparatus.
Here, spec.Wg is the specific grinding energy, t the time, Fg the grinding Force, υg the grinding velocity, ΔmgVA the machined volume of stainless steel and ρVA the density of stainless steel. Fluctuations in single measurement intervals were avoided by this cumulative calculation, leading to better comparable data. Therefore, the percentage of energy savings (Wg%Cry) through active fillers can easily be determined by the
2.6. Dissolution experiments To investigate the built-up edge, dissolution experiments were per formed. In these experiments, the abrasives were placed in a defined volume of concentrated HCl acid for 24 h. The acid was analyzed af terwards via ICP-OES (Agilent Technologies 700 Series) to determine the concentrations of Fe, Cr and Ni. Fig. 1. SEM image of the unused abrasive. 1: Abrasive Grain; 2: Top Coat; 3: Make Coat; 4: Polyester Finish Cloth. 2
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Fig. 3. Schematic drawing of experimental set up for heat treatment and pin geometry.
3. Results & discussion Cryolite-containing abrasives were compared to reference ones. The total machined volume of 1.4301 stainless steel was measured. As shown in Fig. 4, the difference between reference and cryolite-containing coated abrasives is negligible. However, a commercially available and therefore optimized abrasive (A), containing grains of the same speci fication, makes a difference. This can be reasoned with better grain orientation and/or allocation, small amounts of other additives and an optimized resin. Pin-on-disk experiments were all run with a constant pressure; therefore, it was expected that the differences during the grinding ex periments would mainly occur in machined volume of the pins. How ever, due to the oscillation of the pins, the controller had to readjust constantly, so the grinding forces and therefore the pressing force varied periodically. Consequently, the pressing force was fluctuating around a certain value and the feed speed adjusted by the actuator became decisive, which led to a measurable change in force. The change in forces led to variations in energy consumption of the grinding process. The specific grinding energy was calculated by equation (1) and is plotted for the different abrasives in Fig. 5. The energy consumption of the reference abrasive is significantly higher than that of that abrasive containing cryolite. This behavior shows that there must be some posi tive influence on the grinding process caused by cryolite. However, it also shows that the difference in energy consumption can further be increased by optimizing the abrasive. It should be noted, however, that Cry- and Ref-abrasives were produced in one batch and therefore differ only regarding the used filler. Still, energy savings between these two are around 10%. Comparing Ref to the optimized abrasive A even more than 15% of energy can be saved. To understand this behavior, an Energy Dispersive Spectroscopy (EDS) X-ray mapping was made of a built-up edge from one of the simplified cryolite-containing abrasives. This mapping, shown in Fig. 6, indicates a small fluoride concentration in combination with chromium and iron but without any other cryolite components. Therefore, there might be the possibility of an interaction of the fluoride, coming from the cryolite, with Fe and Cr of the stainless steel. However, parallel determination of Fe and F can be inaccurate due to the nearby FK and FeL spectral lines [16]. A single determination of fluoride was not successful either due to the minimal concentrations available or the disturbing background of the iron.
Fig. 5. Spec. grinding energy in pin-on-disk experiments.
To find out whether fluoride ions reacted with the steel surface or diffused into the matrix, heat treatments of the pins were carried out. Holes of 1.5 mm diameter were drilled into the pins and filled with cryolite or left empty and afterwards tempered under argon atmosphere at 850 � C for 18 h. Resulting scale layers were removed and pin-on-disk experiments performed with the following pairings: Ref&VAa, Ref&VAaC and Cry&VAa, as shown in Fig. 7. As soon as cryolite enters the system, the specific grinding energy is reduced. The difference in specific grinding energy between cryolite-containing abrasives or tempered pins is nearly the same. However, by thermal treatment of the pins with cryolite, fluoride can diffuse into quite a large volume of the pin. There is no literature dealing with the diffusion of fluorine in the γ-iron lattice; therefore, only an estimation based on a similar sized ion like O2 is possible [17]. However, due to the lower charge of F -Ions, at least the same diffusion rate can be expected. The diffusion coefficient of oxygen in the polycrystalline γ-iron lattice at 850 � C is approx. 2, 5 � 10 8 cm2/s [18]. Therefore, one can calculate by means of equation (3) that approx. 90% of the steel volume used in the pin-on-disk-tribometer experiments was affected by fluoride. The in clusion of cryolite through tempered Cry-pins led to higher energy savings in the first 300 s of the experiment compared to the inclusion of cryolite via top coating as seen in Fig. 7. For the whole grinding process of 600 s the energy savings are similar for both set-ups. This can be explained by the diffusion of fluorine as the time-controlling step in the interaction between cryolite and steel while grinding. Therefore, the diffusion which took place in the previous heat treatments enhanced this interaction. pffiffiffiffiffiffiffiffiffiffiffiffi x ¼ 6*D*t (3) here, x describes the diffusion depths, D is the diffusion coefficient and t the time. The tempered pins were analyzed via Scanning Electron Microscopy (SEM), shown in Fig. 8, and EDS X-ray mapping as shown in Fig. 9. Different phases occur around the drilled holes. On the left hand side, one can see the drilled hole. Moving further to the right, an oxide phase occurs at the boundary, followed by a large fluoride-containing phase and finally reaching the original stainless steel composition. Fluoride diffuses into the stainless steel deeper than any other components of the cryolite. EDS data shows a fluoride concentration in deeper diffusion
Fig. 4. Machined stainless steel volume in pin-on-disk experiments. 3
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Fig. 6. SEM overview and EDS X-ray mapping of a cross-section of a cryolite-containing abrasive after machining.
Fig. 7. Spec. grinding energy in pin-on-disk experiments using heattreated samples.
layers of more than 60 at% leading to an empirically calculated molec ular formula of approx. FexCr1-xF2 as shown in Table 1. To prove this result, a FIB lamella was investigated by Transmission Electron Micro scopy (TEM). The resulting lattice parameters verify the earlier calcu lated phase and are in good agreement with lattice parameters from literature as shown in Table 2, especially when considering that Cr-ions cause a distortion of the lattice. To find out if this phase has an influence on the mechanical prop erties of the steel like brittleness, tensile tests were performed as described in paragraph 2.5 and investigated by SEM and EDS after wards. Results show that samples treated with cryolite have a lower tensile strength of 600 MPa compared to the reference values of 630 MPa. However, the strain at rupture is 3% higher for the reference samples as shown in Fig. 10. SEM investigations showed that the
Fig. 8. SEM image of the cross section of a pin after heat-treating with cryolite.
cryolite-treated samples have a coarse surface and a smaller diameter after tensile tests, which implies flaking of a brittle phase. Therefore, the tensile tests of the cryolite specimens were recalculated with a 2% smaller diameter, according to the SEM pictures. This approximation delivers a similar mechanical behavior except small differences in yield stress and rupture strain. The higher yield stress obtained with cryolitetreated samples is in good agreement with the findings by Harper et al. [20], who showed that corrosion of surface layers has an impact on plasticity. Due to the fact that heat-treated pins and tensile specimens showed the occurrence of a metal fluoride phase which seemed to be more 4
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Fig. 9. EDS X-ray mapping of a cross-section of a heat-treated pin filled with cryolite. Table 1 EDS measurements in atom %. (For clarity small amounts of Si, Al, Mn, K and Mo are not listed). Element Fe Cr Ni Na F O
Measurement positions as shown in Fig. 6 1
2
3
4
Ø 5-7
8
22.7 31.3 2.4 – 2.0 41.1
20.3 – 79.8 – – –
20.9 19.4 13.1 8.7 30.4 –
31.7 7.3 3.0 – 56.4 –
28.3 7.8 – – 62.9 –
67.0 18.8 9.0 – – –
Table 2 Lattice parameter of the FexCr1-xF2 phase found after heat treatment via TEM investigations and values from literature. Lattice parameter
a [Å]
c [Å]
Measured (FexCr1-xF2) Literature (FeF2) [19]
4.693 4.6945
3.319 3.3097
Fig. 11. Quantification of built-up edge components via ICP-OES after partial dissolution with HCl.
brittle, dissolution experiments of the used abrasives have been per formed. The abrasives were placed in a bath of concentrated hydro chloric acid and were leached for 24 h. Afterwards the HCl solution was analyzed by ICP-OES and the concentrations of the metal ions Fe, Ni and Cr were determined. However, because the abrasive layer contained iron only, Cr and Ni deliver a meaningful value and are shown in Fig. 11. The occurrence of cryolite in the system reduces the amount of these alloying elements. This reduction in built-up edges is in good agreement with the findings of Vernhet et al. [21]. 4. Conclusion It was possible to show by pin-on-disk experiments that cryolite has a positive effect on the abrasion process of stainless steel. In this work, the effect resulted in energy savings of 10%. In comparison, commercially available abrasives yielded even higher energy savings. Therefore, the before-mentioned cooling hypothesis might not be an active cooling process but a reduction of the heat input. It was also possible to repro duce the energy saving effect by the diffusion of fluoride into the steel lattice via heat treatments. Our experiments have shown that one of the benefits of cryolite must be the interaction between the fluoride and the
Fig. 10. Influence of cryolite on the stress-strain curves of 1.4301.
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metal, which was not known yet. Therefore, hypotheses like the chem ical weakening of the material as well as the passivation of nascent metallic surfaces, which reduces metal adhesion, seem to be likely. Still, one cannot exclude further interactions in system. A new phase, un known to the literature, has been found in the steel, which reacted with cryolite during the heat treatments. This phase is more brittle as could be proven by tensile experiments. It was shown by dilution experiments of the used abrasives that less metal is attached to its surface, which leads to smaller clogging of the abrasive grains and; therefore, a higher cutting efficiency. In future experiments, it would be desirable to detect this new phase in build-up edges or on workpiece surfaces. However, this will be challenging due to its brittleness as well as the low concentration and additionally difficult detection of F in presence of Fe.
[2] Zum Gahr K-H. Wear by hard particles. Tribol Int 1998;31(No. 10):587–96. [3] Czichos H, Habig K-H. Tribologie-Handbuch. Springer; 2015. ch. 15.2. [4] Klocke F, K€ onig W. fertigungsverfahren schleifen, honen, l€ appen. Springer; 2005. ch. 3 pp. 19ff. [5] Reza Bateni M, et al. Wear and corrosion wear of medium carbon steel and 304 stainless steel. Wear 2006;260:116–22. [6] Nadolny K, et al. Effects of sulfurization of grinding wheels on internal cylindrical grinding of Titanium Grade 2. Indian J Eng Mater Sci 2013;20:108–24. [7] Kistler S. S. Abrasive Articles. U.S. Patent 2,308,981 (1943). [8] Kistler S. S. Bonded abrasive articles containing fillers. U.S. Patent 2,308,983 1943. [9] Kistler S. S., Abrasive Articles, U.S. Patent 2,408,319 (1946). [10] Chuda K. et al. Abrasive products including active fillers. U.S. Patent 8,491,681 (2013). [11] Coes L. Knowledge of the scientific principles of grinding is basis of recent progress in abrasives. Ind Eng Chem 1955;47(12):2493–4. [12] Jamrozik A, et al. Additives for abrasive materials. Abrasive TechnologyCharacteristics and Applications. IntechOpen; 2018. [13] Yurkov A. Refractories for aluminium. Springer Cham; 2015. ch. 2 pp. 65ff. [14] Rehbinder P. Verminderung der Ritzh€ arte bei Adsorption grenzfl€ achenaktiver Stoffe. Z Phys 1931;72:3–4. 191-205. [15] Edelstahlwerke Deutsche. Werkstoffdatenblatt X5CrNi18-10 1.4301. 2015-0046. 2015. [16] Eggert F. Standardfreie Elektronenstrahl-Mikroanalyse mit dem EDX im Rasterelektronenmikroskop ein Handbuch für die Praxis. BoD–Books on Demand; 2005. [17] Whittaker EJW, Muntus R. Ionic radii for use in geochemistry. Geochem Cosmochim Acta 1970;34(9):945–56. [18] Takada J, et al. Determination of diffusion coefficient of oxygen in γ-iron from measurements of internal oxidation in Fe-Al alloys. Metal Trans A 1986;17(2): 221–9. [19] Baur WH, Khan AA. Rutile-type compounds. IV. SiO2, GeO2 and a comparison with other rutile-type structures. Acta Crystallogr B 1971;27(11):2133–9. [20] Harper S, Cottrell AH. Surface effects and the plasticity of zinc crystals. Proc Phys Soc Sect B 1950;63(5):331–8. [21] Vernhet L, et al. Metal adhesion issues in dry grinding: the role of active fillers. Wear 2016;346:46–55.
Acknowledgements We would like to thank everyone from the Institute of Mineral En gineering (GHI) for their help and support. Special thanks go to D. Pletz and his team of the technical workshop who spent a lot of time on manufacturing all the stainless steel pins. The industrial partners and the FGS gave substantial support through knowledge and materials. The authors like to thank Arbeitsgemeinschaft industrieller For schungsvereinigungen “Otto von Guericke” e.V. for financial support (AiF intention no. 18282/N). References [1] Pintaude G, et al. Mild and severe wear of steels and cast irons in sliding abrasion. Wear 2009;267:19–25.
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