The Influence of Silicon on the Mechanical Properties and Hardenability of PM Steels

Transcription

1 The Influence of Silicon on the Mechanical Properties and Hardenability of PM Steels Chris Schade & Tom Murphy Hoeganaes Corporation Cinnaminson, NJ Alan Lawley & Roger Doherty Drexel University Philadelphia, PA ABSTRACT The effects of silicon additions on the microstructures, mechanical properties and hardenability of powder metallurgy (PM) alloy systems have been investigated. It has been demonstrated that the addition of silicon can increase strength in both the sintered and heat treated conditions. In the sintered condition silicon strengthens the ferrite of the pearlitic microstructure by solid solution hardening. The heat treated properties of various silicon containing alloy systems were examined and the hardenability was compared for various alloys by means of microindentation hardness measurements. The effectiveness of accelerated cooling during sintering on sinter-hardening response of silicon-containing alloys was demonstrated. The addition of silicon to high carbon (>0.80 w/o) PM steels suppresses the formation of grain-boundary carbides which permits the use of carbon concentrations that are significantly higher than those commonly used for commercial PM applications. INTRODUCTION With the increasing use of high temperature sintering, manufacturers of alloy powders used in the PM industry are now evaluating the use of elements such as chromium, manganese, and vanadium in relation to alloy performance [1-3]. The higher sintering temperatures allow for the reduction of oxides from these elements in atmospheres that are predominately nitrogen with small amounts of hydrogen (90 v/o nitrogen / 10 v/o hydrogen). One alloying element that has not been examined significantly is silicon. Silicon is an abundant alloying element; it is the second most available element in the earth s crust [4]. In wrought steels, silicon is used primarily as a deoxidant in the steelmaking process and is typically added at levels between 0.15 w/o and 0.30 w/o. The high deoxdizing power of silicon allows for the production of steels with low sulfur levels (sulfur removal is favored by low dissolved oxygen in the liquid steel) and also aids in the recovery of highly oxidizable elements such as boron, manganese, titanium and zirconium. Since the cost of silicon is low compared with other alloying elements, it is used in a sacrificial manner to improve the recovery of other alloying elements. Higher levels of silicon (~1.60 w/o) are used in some special high strength low alloy steels

2 Apparent Hardness (HRA) (HSLA) but it s main use is in electrical steels where the silicon content can be as high as 5.0 w/o. Silicon is used in these materials because it leads to high magnetic permeability, high electrical resistivity and low core loss which are beneficial properties in the manufacture of electric motors. Steels containing this level of silicon are extremely brittle and require special processing to produce sheet steels for motor laminates. Although silicon has been utilized in a few PM products, an extensive study on the effects of silicon as an alloying element has not been undertaken [6]. In sintered PM steels, which have a predominately pearlitic microstructure, silicon can strengthen the ferrite by solid solution strengthening. Figure 1 shows the effect of the concentration of various alloying elements on the hardness of ferrite in various PM iron alloys with no carbon additions. The microstructure in each of the alloys was ferritic. The results indicate that silicon has the potential to significantly increase the strength of the ferrite in a pearlitic microstructure when compared with other alloying elements typically used in PM steels Si Mn Cu 30 Cr Ni 25 Mo Alloy Content (w/o) Figure 1: Effect of concentration of various alloying elements on apparent hardness of PM ferritic steels.

3 It has also been shown that factors such as the proportions of ferrite and pearlite, the interlamellar spacing, size of the pearlite nodules, and ferrite grain size, affect the mechanical properties [7-9]. Figure 2 shows that the interlamellar spacing controls the strength and hardness of wrought steels with pearlitic microstructures. Hyzak and Bernstein [8] found that the yield strength and hardness were controlled primarily by the interlamellar spacing and that impact energy was dependent on the prior austenite grain size, increasing with finer grain size, and to some extent on the pearlite colony size. The effect which silicon has on the pearlitic microstructure is unclear as Takahashi et al. have shown that silicon has only a small influence on the interlamellar spacing while Anya has found that silicon inhibits the prior austenite grain size, leading to finer pearlite which is beneficial to tensile and impact properties [10-11]. Figure 2: Hardness and yield strength of wrought steels as a function of interlamellar spacing in fully pearlitic microstructures [8]. The ability of an alloy to transform to martensite during heat treatment is affected by the various alloying elements. Hardenability is generally accepted as a qualitative measure describing the ease and depth to which steel is able to transform to martensite upon cooling from the austenitizing temperature. The mechanical properties of a heat treated steel depend primarily on its hardenability. Sokolowski et al. [12] have reviewed various factors influencing the hardenability of PM steels. Certain alloying elements, in particular chromium, manganese, molybdenum and nickel, have a strong influence on hardenability and have been utilized extensively in PM steels. It is also well known that elements such as molybdenum and nickel can have a synergistic effect leading to enhanced hardenability when alloyed together. In relation to hardenability, silicon has not been

4 Hardenability Factor used as extensively as these other elements in PM steels. Figure 3 shows the multiplying factors for wrought steels developed by Jatczak [13] for carbon contents in the range of 0.60 to 1.10 w/o and an austenitizing temperature of 926 o C (1700 o F). At these high carbon contents, which are typical for PM steels, silicon has a hardenability factor similar to that of chromium and manganese. It has also been found by the author that a synergistic hardenability effect exists between silicon and molybdenum, namely when silicon is used in the presence of molybdenum its hardenability is much higher than what is shown in Figure 3. 5 Mo Cr Si Mn Ni Alloy Addition (w/o) Figure 3: Hardenability factors for alloy elements in wrought steels. A number of authors studying high carbon pearlitic steels for rail applications have found that the addition of silicon suppresses the formation of continuous grain boundary carbides, allowing these materials to have a high strength while maintaining adequate ductility for cold drawing. The utilization of high carbon levels without embrittling grain boundary carbides is attractive in the PM industry since graphite is a relatively inexpensive alloying element. In the present study, a development program was undertaken in which silicon was added to several ferrous PM systems. Mechanical properties were measured and

5 microstructures characterized in the sintered and heat treated conditions to assess the effectiveness of silicon on improving the mechanical properties and hardenability of PM alloy systems. ALLOY PREPARATION AND TESTING Mixtures of base powders and master-alloys containing silicon were utilized to prepare test specimens. The mean particle size (d 50 ) of the additive was 8-15 m. Ancorsteel 1000B was used for the iron base alloy systems, and Ancorsteel 30HP, Ancorsteel 50HP and Ancorsteel 85HP were used for the iron-molybdenum alloys (nominal molybdenum levels of 0.30 w/o, 0.50 w/o and 0.85 w/o respectively). The powders were mixed with Acrawax C lubricant and graphite. Graphite additions were 0.70 w/o (unless otherwise noted) resulting in a sintered carbon level of approximately 0.65 w/o. Samples for transverse rupture (TR) and tensile testing were compacted uniaxially at a pressure of 690 MPa. The test pieces were sintered in a high temperature Abbott continuous-belt furnace at 1260 C (2300 F) for 30 min in an atmosphere of 90 v/o nitrogen / 10 v/o hydrogen. Sinter-hardening experiments were conducted using the same furnace but utilizing a gas quench to provide accelerated cooling at a rate of 1.9 o C/s. Test pieces that were sinter-hardened were tempered at 204 o C (400 o F) for 1 h. For heat treatment, samples were austenitized at 900 C (1650 F) for 60 min at temperature in a 75 v/o nitrogen / 25 v/o hydrogen atmosphere prior to quenching in oil. Prior to mechanical testing, green and sintered densities, dimensional change (DC), and apparent hardness were determined on the tensile and TR samples. Five tensile specimens and five TR specimens were evaluated for each composition. The densities of the green and sintered steels were determined in accordance with MPIF Standard 42. Tensile testing followed MPIF Standard 10 and apparent hardness measurements were made on the tensile and TR specimens, in accordance with MPIF Standard 43. The effect of silicon on quenched hardenability was also studied. 25 mm (1 in) dia x 25 mm high compacts were pressed to a green density of 6.95 g/cm 3 at a nominal pressure of 690 MPa (50 tsi). After sintering these samples were reheated to 900 C (1650 F) and oil quenched. The samples were cross-sectioned at mid-height and mounted for microstructural analysis. In addition, microindentation hardness traverses were made from the edge to the center of the compacts. In this way the hardenability could be measured in a similar fashion to the Jominy test. Specimens for microstructural characterization were prepared using standard metallographic procedures. Subsequently, they were examined by optical microscopy in the polished and etched (2 v/o nital / 4 w/o picral) conditions.

6 RESULTS AND DISCUSSION Sintered Alloys It is recommended that silicon-containing PM alloys be sintered at high temperatures in order to reduce the oxides that form at the grain boundaries. Figure 4 shows a 0.50 w/o Si pre-alloy with the addition of 0.70 w/o graphite sintered at various temperatures. The light optical photomicrographs shows the oxide located at particle boundaries for a range of sintering temperatures from 1120 o C to 1260 o C (2050 o F 2300 o F). The amount of oxide at prior particle boundaries is still significant up to 1204 o C (2220 o F). The corresponding TR strength properties do not exhibit any significant increase nor was a decrease in oxygen content seen until the powder mix is sintered > 1163 o C (2125 o F). Oxygen levels approaching those in traditional PM alloys are not achieved until the sintering temperature reaches ~ 1260 o C (2300 o F). Therefore property data was developed at 1260 o C (2300 o F) in the rest of this study. (a) 1120 ( o C) (b) 1163 ( o C) (c) 1204 ( o C) (d) 1260 ( o C) Figure 4: Oxides at particle boundaries in a 0.50 w/o Si alloy as a function of sintering temperature.

7 The effect of silicon level on mechanical properties is shown in Table I. A mixture of Ancorsteel 1000B and a silicon master-alloy and 0.70 w/o graphite was used to determine the effects of silicon concentration on mechanical properties. As the silicon content increased, the apparent hardness, yield strength and ultimate tensile strength all increase with only a slight decrease in the elongation. The impact energy increases with increasing silicon content notwithstanding the increase in hardness and strength. It is clear from microindentation hardness of the pearlite (Figure 5) that silicon has increased the strength of the pearlite through solid solution strengthening of the ferrite. Table I: Mechanical Properties of Sintered PM Steels with Silicon Additions Sintered Density Impact Energy Apparent Hardness Elongation Alloy (g/cm 3 ) (J) (ft.lbf) (HRA) (10 3 psi) (MPa) (10 3 psi) (MPa) (%) 0.0 w/o Si w/o Si w/o Si w/o Si w/o Si UTS 0.20% Offset Yield Figure 5 also shows that the pearlite spacing decreased as the silicon content increased. The increase in impact energy is presumably due to the reduction in grain size as a result of the silicon addition. Table II shows the results of 0.60 w/o addition of silicon to a number of common ferrous PM material systems. In the copper system the silicon addition leads to only slight increases in strength while systems such as the Fe-C and Fe- Mo-C lead to more significant increases in strength and hardness (~ 15 to 20%). The iron-copper-carbon alloy was the only system investigated which did not show a significant increase (>10%) in tensile strength with the addition of silicon. Since copper is an austenite stabilizer, the amount of austenite that is available to transform to pearlite is increased. It may also be possible that, at this level of copper, the amount of pearlite is already maximized and the pearlite spacing is already fine so that the use of silicon does not contribute to an increase in properties. Since copper does not form carbides, the copper is in solution in ferrite and Figure 1 shows that copper additions also harden the ferrite to a similar level as silicon.

8 (a) 0.0 w/o Si (b) 0.30 w/o Si (c) 0.60 w/o Si (d) 0.90 w/o Si (e) 1.20 w/o Si (f) Microindentation Hardness of Pearlite Figure 5: Microstructure of Ancorsteel 1000B w/o Graphite at various silicon levels and microindentation hardness of the pearlite. Light optical microscopy.

9 Table II: Mechanical Properties of Sintered PM Steels With and Without Silicon Additions. Sintered Density Impact Energy Apparent Hardness Elongation Alloy (g/cm 3 ) (J) (ft.lbf) (HRA) (10 3 psi) (MPa) (10 3 psi) (MPa) (%) Fe-C No Si w/o Si Fe-2 w/o Ni-C No Si w/o Si Fe-2 w/o Cu-C No Si w/o Si Fe-0.30 w/o Mo-C No Si w/o Si UTS 0.20% Offset Yield Heat Treated Alloys The hardenability of PM steels is an important measure of how readily certain alloy systems can be heat treated. One of the most widely used tests for hardenability is the Jominy end-quench test, where samples are heated into the austenite range and water quenched on one end of the sample, producing a wide range in cooling rate within one sample. This technique requires the compaction of a large specimen and multiple machining steps to produce the final test specimen. Lindsley et al have developed a simplified technique in which 25 mm dia x 25 mm high compacts, were austenitized and oil quenched. Subsequently the microindentation hardness was measured through the thickness of the specimen [14]. This technique gives a measure of hardenability similar to that derived from the Jominy end quench test.

10 Microindentation Hardness (HV 50 gf) Microindentation Hardness (HV 50 gf) Microindentation Hardness (HV 50 gf) Microindentation Hardness (HV 50 gf) w/o Si w/o Si Distance (mm) (a) Distance (mm) (b) w/o Si w/o Si 700 Silicon rich area's 700 Silicon rich area's Distance (mm) (c) Distance (mm) (d) Figure 6: Microindentation hardness profiles from the surface (distance = 0) toward the center of oil quenched 25 mm dia compacts. Ancorsteel 1000B with 0.70 w/o graphite: (a) 0.0 w/o Si; (b) 0.30 w/o Si; (c) 0.60 w/o Si; (d) 0.90 w/o Si. A series of 25 mm dia compacts were produced with increasing levels of silicon followed by austenitizing and oil quenching. Microindentation hardness profiles from the surface to the center of the compacts were then measured. Figure 6 shows the results of these hardenability traces. As the silicon level increases the hardenability (distance towards the center of the compact at which the surface hardness is maintained) increases. One point becomes obvious, namely that since the silicon is not prealloyed, hardness level depends on the diffusion of the silicon from the additive to the base powder. Maxima and minima in the hardness measurements occur over short distances corresponding to areas where silicon has and has not diffused into the base powder. However, it is evident that as

11 Microindentation Hardness (HV 50 gf) silicon is increased in the matrix the hardenability is increased. Figure 3 indicates that in high carbon steels (carbon levels typically used in PM) silicon has a hardenability factor similar to that of chromium at concentrations <1.0 w/o. In his work on the hardenability of wrought high carbon steels (0.60 to 1.0 w/o carbon), Jatczak discovered that the hardenability factor for silicon was higher when used in the presence of molybdenum, particularly when austenitized at 927 o C. When used in conjunction with molybdenum the hardenability of silicon exceeded that of chromium Without Silicon With Silicon Distance (mm) Figure 7: Microindentation hardness profiles from the surface (distance = 0) toward the center of oil quenched 25 mm diameter compacts in an Ancorsteel 30HP (~ 0.30 w/o Mo) with 0.70% graphite: Solid line w/o Si; Dashed Line w/o Si. Figure 7 shows the hardenability traces of Ancorsteel 30HP with and without the addition of silicon. With no silicon present the hardness drops off as the center of the compact is approached. With the addition of silicon the compact through hardens (i.e. the hardness level is maintained from the surface to the core of the 25 mm dia compact). Unlike the iron based alloy the hardness in the molybdenum-containing alloy does not exhibit the variation in hardness (the minima s and maxima s) seen with the iron based system. The reason for this is unclear, but may be related to enhanced diffusion of the silicon in combination with molybdenum, or the overall increase in hardenability masks the effect.

12 Microindentation Hardness (HV 50 gf) Microindentation Hardness (HV 50 gf) Microindentation Hardness (HV 50 gf) Microindentation Hardness (HV 50 gf) Cr Only Cr+Si Distance (mm) (a) Distance (mm) (b) Mn Only Mn+Si Distance (mm) (c) Distance (mm) (d) Figure 8: Microindentation hardness profiles from the surface (distance = 0) toward the center of oil quenched 25 mm dia compacts with Ancorsteel 1000B % graphite with: (a) 0.35 w/o Cr; (b) 0.35 w/o Cr w/o Si; (c) 0.75 w/o Mn; (d) 0.75 w/o Mn w/o Si. Figure 8 shows the effects of adding silicon to a 0.35 w/o Cr alloy and a 0.75% Mn alloy. These alloys were produced by the addition of high carbon ferrochromium and high carbon ferromanganese to Ancorsteel 1000B. The mean particle size of the ferroalloy additives was 10 um. In each case the addition of the alloying element by itself had little effect on hardenability. When silicon was added at a concentration of 0.60 w/o the hardenability of the alloy systems increased dramatically. In both cases the large maxima

13 and minima in microindentation hardness were not significant indicating that all the elements were in solution at 1260 o C. Sinter-hardening Sinter-hardening is a process in which hardening of a component is accomplished during an accelerated cooling from the sintering temperature. Traditional sinter-hardening PM steel compositions utilize high levels of copper, molybdenum and nickel along with high carbon contents to achieve martensitic microstructures in the as-sintered condition. The preceding discussion has shown that the use of silicon imparts an increased level of hardenability when used in combination with chromium, manganese and molybdenum. To investigate the effectiveness of silicon in relation to sinter-hardening a series of molybdenum prealloys were alloyed with the silicon master alloy. The alloys were sintered conventionally (no accelerated cooling) and with accelerated cooling and then tempered at 204 o C (400 o F) for 1 h. Tempering after sinter-hardening is recommended to increase strength through stress reduction and modification of the microstructure. Table III: Mechanical Properties of Molybdenum (With and Without silicon) Containing Alloys: Conventional Cooling versus Accelerated Cooling. Silicon Addition Sintered Density Impact Energy Apparent Hardness Elongation Alloy (w/o) (g/cm 3 ) (J) (ft.lbf) (HRA) (10 3 psi) (MPa) (10 3 psi) (MPa) (%) 0.30 w/o Mo Conventional Cooling Conventional Cooling Accelerated Cooling Accelerated Cooling w/o Mo Conventional Cooling Conventional Cooling UTS 0.20% Offset Yield Accelerated Cooling Accelerated Cooling w/o Mo Conventional Cooling Conventional Cooling Accelerated Cooling Accelerated Cooling When examining the data in Table III, for conventional cooling, it is obvious that the addition of silicon increases the mechanical properties (ultimate tensile strength, yield strength and apparent hardness). As previously discussed, this is a result of stengthening the ferrite and refining the pearlite. But the role of silicon in hardenability can be seen

14 when comparing the conventionally cooled material to the material in which accelerated cooling was utilized. In the case where no silicon was added there was only small increases in mechanical properties between the conventionally sintered material and the material that was sinter-hardened (accelerated cooling). However, the molybdenumcontaining alloys with silicon show a significant increase in ultimate tensile, yield strength and hardness; while surprisingly the ductility and elongation also increase. The microstructure s in Figure 9 (for the 0.50 w/o Mo alloy) show the reason for this increase in mechanical properties. The addition of silicon has increased the hardenability utilizing accelerated cooling resulting in a mixture of bainite and martensite whereas the accelerated cooled material with no silicon added has a pearlitic microstructure. At the lower molybdenum levels (0.30 w/o Mo), where pearlite is predominant in the microstructure, the accelerated cooling in combination with silicon leads to a more refined microstructure and the impact energy is increased notwithstanding an increase in hardness. As the structure transforms more to marteniste and bainite, due to the higher molybdenum levels, there is no increase in the impact energy and elongation but the levels of each are still maintained despite the attendant increase in strength and hardness. (a) (b) Figure 9: Microstructure of Ancorsteel 50HP utilizing accelerated cooling: (a) with no silicon addition, divorced pearlite and (b) with silicon addition, bainite/martensite. Suppression of Carbides Research on wrought steels used for rails and wire has shown that when silicon is added to high carbon steels ( >0.80 w/o) it retards the formation of grain boundary carbides [15-17 ]. The cost effectiveness of carbon has been exploited in PM steels for years with levels of carbon typically higher than those in wrought steels. However, above 0.80 w/o C a near continuous film of cementite precipitates tends to form on the grain boundaries in PM steels leading to embrittlement and a decrease in mechanical properties. It has been found that in wrought steels that silicon slows the diffusion of carbon to the cementite, thereby surpressing the formation of the grain boundary cementite. These two effects have resulted in wrought steels with carbon levels between 0.90 w/o C and 1.10 w/o C, with excellent mechanical properties.

15 Figure 10 shows the microstructure of a Fe-1.1 w/o C alloy with and without silicon additions. The alloy without a silicon addition (Figure 10a) has an extensive amount of iron-carbide located at grain boundaries. This grain boundary cementite network is brittle and provides both crack nucleation sites and serves as an easy fracture path and it is the reason why PM steels with this high carbon content are normally too brittle for commercial use. Figure 10b shows the same alloy system but with the addition of 0.60 w/o Si. There is minimal embrittling grain boundary carbides and those that exist are thinner and not continuous; consequently this leads to less deleterious effects on mechanical properties as cited in Table III. (a) (b) Figure 10: Appearance of grain boundary carbides (light etched areas) in a Fe-1.1 w/o C system (a) with no Si and (b) with 0.60 w/o Si. The mechanism by which silicon suppresses carbide formation is related its diffusion. As the carbides form the silicon is rejected from the carbide into the ferrite. This raises the silicon level outside the carbides and slows the rate of silicon diffusion which, in turn slows, the growth rate of the carbides. Figure 11 shows EDX analysis of the silicon levels in the carbide (yellow) and ferrite (red) of the material from a Fe-1.1 w/o C w/o Si based alloy.

16 Figure 11: EDX analysis of carbide in Fe-1.1 w/o C w/o Si. The red line indicates that the silicon level is higher in the ferrite than in the carbide that forms at the grain boundaries. Semi-quantitative estimates indicate that the silicon level in the ferrite is approximately 1.0 w/o while the carbide contains 0.20 w/o silicon. This increase in the silicon level in the ferrite leads to a gradient slowing the growth of the carbide and therefore leading to decreased carbide formation in the alloy containing silicon. As a result the high carbon alloy containing silicon exhibits less carbide formation then a similar alloy without the silicon. Table III shows data for a Fe-C system with a range of carbon levels from 0.70 w/o to 1.3 w/o graphite. One composition has no silicon while the other has 0.60 w/o silicon. Not only does the silicon addition lead to higher strength (both yield and ultimate tensile) and hardness, but the impact energy is higher, particularly at graphite levels >1.1 w/o in the alloy containing silicon. Table III. Effect of Silicon and Carbon on Properties of PM Steels. Sintered Density Impact Energy Apparent Hardness Elongation Alloy (g/cm 3 ) (J) (ft.lbf) (HRA) (10 3 psi) (MPa) (10 3 psi) (MPa) (%) 0.70 w/o Carbon No Si w/o Si w/o Carbon No Si w/o Si w/o Carbon No Si w/o Si w/o Carbon No Si w/o Si UTS 0.20% Offset Yield

17 The use of silicon in combination with high carbon levels (> 1.0 w/o) has been shown to be effective in a number of alloy systems including iron-carbon-molybdenum, ironcarbon-nickel and iron-carbon-copper. Economics of Alloying with Silicon It is obvious from Table II that silicon additions increase the mechanical properties in a number of different ferrous alloy systems. The cost savings are difficult to quantify. However the use of silicon in combination with high graphite levels can lead to direct replacement of certain alloying elements. In addition, if these elements are used in combination with sinter-hardening, considerable savings in alloy cost can be realized. These cost savings can be balanced against the use of high temperature sintering. Table IV shows typical property data for sinterhardening grades contained in MPIF Standard 35. Most of these alloys use combinations of chromium, copper, molybdenum, manganese and nickel in excess of 3 w/o. In Table IV the mechanical properties of an alloy comprised of 0.50 w/o Mo with 0.75 w/o Si is compared with the sinter-hardened steels at a density of 7.0 g/cm 3. In addition to matching the strength level of most of the sinter-hardening grades, with higher alloy content, the silicon containing alloy maintains some ductility. Table IV. Comparison of Mechanical of Experimental Alloy with MPIF Sinterhardening Grades (density = 7.0 g/cm 3 ). Apparent TRS TRS Impact Energy UTS 0.20% Offset Yield Elongation Hardness Material Designation Code (10 3 psi) (MPa) (J) (ft.lbf) (HRC) (10 3 psi) (MPa) (10 3 psi) (MPa) (%) FLNC HT D D <1 FLC HT D D <1 FLC HT D D <1 FLC HT D D <1 FLC HT <1 FL HT D D <1 Ancorsteel 50HP w/o Si w/o Graphite CONCLUSIONS Silicon solid solution strengthens the ferrite of pearlitic PM steels resulting in an increase in strength. Increasing the silicon content leads to a refined pearlitic microstructure resulting in an increase in strength and hardness accompanied by an increase in impact energy. Silicon can be added to a number of common ferrous PM alloy systems leading to improved mechanical properties.

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