Effect of Ekabor 2 powder on the mechanical properties of pure iron powder metal compacts

Ayberk Altintaş^a, Yetkin Sarigün^a, Uğur Çavdar^b,*

^aCelal Bayar University, Mechanical Engineering Department, Muradiye Campus, 45400 Manisa, Turkey

^bCelal Bayar University, Vocational School, Department of Machinery, Turgutlu Campus, 45400 Manisa, Turkey

*Corresponding author: ugur.cavdar@cbu.edu.tr

1. INTRODUCTIONTOP

Powder Metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. This process owe its popularies to its low energy consumption and high benefit with low production costs. Different grain sizes are generated for purpose of different applications, compressed for forming and heated up for resistance and finally their properties are checked. (Schatt and Wieters, 1997; German, 2005).

Sintering is the process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction. During sintering process, binder is burned, particles are interconnected and changing of size and grain growth are seen in microstructure (Çavdar and Atik, 2011).

Induction sintering is applied with variable current-carrying conductor coil. A source of high frequency electricity is used to drive a large alternating current through a coil. This causes tremendous currents to flow through the workpiece (German, 1996). Microwave sintering is mostly applied to ceramics. This method is carried out with 9 kW power source (Clark et al., 1993). Although Spark Plasma Sintering (SPS) is smilar to microwave sintering, SPS carried out fast sintering with its own thermal environment. Commonly, this method is performed on Al₂O₃, SiC and ZrO₂ (Upadhya, 1987; Kasai et al., 1989).

Cavdar and Atik used pure iron, 3 wt.% copper or 3 wt. % bronze mixed iron powders. Iron based PM compacts were sintered at 1120 ^oC by medium-frequency induction heat sintering (MFIHS) in 8.33 min (500 s). Results in MFIHS are compared with conventional sintering at 1120 °C in 30 min. Mechanical properties of these samples were investigated (Çavdar and Atik, 2014a).

Calık different used powder with different grain sizes (Ekabor 1, Ekabor 2, Ekabor 3, Ekabor HM) for boronizing. In this work, EN H320 LA steel sheets were used and boride layers were compared with each other. The boronizing was done at 700, 800, 900 °C for 3 h. The result obtained shows that, tensile properties were affected by the boronizing powders as temperature increased (Çalik, 2013).

In the study regarding boronizing, Taştan et al. (2009) who expect surface of hardness work on different compositon of materials. AISI 1020, AISI 1040, AISI 4140, H13 steels were used as materials for boronizing. The boronizing was carried out in 820 °C, 860 °C, 900 °C and 940 °C in 2, 4, 6, 8 h. Ekabor 2 was used for boronizing source. The thickness of the borize layer increased with increasing particle size of boronizing powders and with increasing boronizing temperature (Taştan et al., 2009).

Çelik et al. (2008) investigated the impact of thermal processing conditions on abrasive wear; and surface properties of borided AISI 1030 steel were analysed. Boroding was applied at 900, 950, 1000, 1050 °C, in 2, 4, 6 h by using Ekabor 2 powders. Result of the experimental studies show that borided layer thickness increases depending on boriding heat treatment temperature and time. It was discovered that the wear resistance in AISI 1030 steel increased much more than none treated samples according as borided layer properties (Çelik et al., 2008).

Barut et al. (2014) borided AISI 5140 steel and AISI 420 stainless steel at 850, 900 and 950 °C for dwell times of 2, 4 and 6 h. In XRD analysis, FeB, Fe₂B, CrB and MnB phases were seen on the boride layer. While the real hardness amounts of AISI 5140 and AISI 420 steels were approximately 265 HV and 340 HV. As result of boriding, they increased up to 1843 HV and 1972 HV. Activation energies (Q) of borided AISI 5140 steel and AISI 420 stainless steel used in the current study were determined as 194.951 and 206.161 kJ.mol⁻¹, respectively (Barut et al., 2014).

Turku, et al. (2014) made boronizing to low carbon microalloyed steels at 850 °C under various exposure times using Ekabor 2 commercial powders. It was concluded that boride layer thickness and hardness amounts increase with increasing boriding time due to hard boride phases and diffusion as a result of boronizing process (Turku et al., 2014).

Günen et al. (2013) used AISI 304 austenitic stainless steel for surface boriding. They made boronizing with elemental nanoboron powder of 10 to 50 nm in size hexagonal boron nitride powder of 50 to 100 nm in size and Ekabor 3 powder of 1400 µm in size at 900 °C in dwell time 3 h. Consequently, it has been identified that thickness, hardness and fracture toughness amounts increased with the decrease of boriding powder grain structure (Günen et al., 2013).

Other experiment about boronizing process was done by Usta and Ekmekciler. FeAl was used as material. The temperature of boriding was 875 °C, 940 °C, 1040 °C for 2, 4, 8 h. The thickness of boride layer was measured between 8 µm - 60 µm and hardness was found from 1290 HV to 1500 HV (Usta and Ekmekciler, 2010).

Sometimes, the boriding process can’t make better mechanical properties of metarial. In this study, pure cobalt (Co) was boronized at 900 °C and 1000 °C for 1, 3, 6 h by Çalik et al. (2014). In conclusion, despite the increase of temperature and time, a significant change wasn’t determined at the parameters (Çalik et al., 2014).

Boronizing process, salt bath and powder pack, was used martensitic stainless steel AISI 420 by Juijerm (2014). The temperature of process was 850 °C - 950 °C for 9 h. The empirical relationships of boride thickness was related between time and temperature in the study (Juijerm, 2014).

The other boronizing process was done by Jurci et al. (2012). Process was carried out at 1030 °C for 30 min, 45 min, 75 min, 150 min. H11 hot work tool steel was used as material. Better result was found at the intermediate region and they developed new carbides which were identified as M₂₃C₆. The hardness of boride layer is depended on time and temperature like the other studies (Jurci et al., 2012).

The last study which examined was done by Ptacinova et al. (2015). In this study, plain, low carbon steel S235JRG1 was boronized at 1000 °C for 45 min - 150 min by using a Durborid powder. The microstructure, phase constitution and oxidation behavior have been investigated. As result, FeB and Fe₂B phases were seen on the boride layer. A parabolic oxidation of the boride layer has been observed (Ptacinova et al., 2015).

In the literature, there are many surface treatment of boronizing studies on the effect of the process temperature and time on mechanical properties, chemical properties but no other studies have been found about composition of wt. % Ekabor 2-pure iron. Because of this, we investigated pure iron, 5 wt. % Ekabor 2, 10 wt. % Ekabor 2 and 30 wt. % Ekabor 2 powder mixtures. Sintered compacts were compared by their hardness of surface, density, wear abrasion and microstructure by each other. The primary goal of this study is to determine effect of composition quantity on the mechanical properties and wear abrasion.

2. MATERIALS AND METHODSTOP

The samples were compacted by pure iron powders. The compositon of pure iron powders is 99 wt. % pure iron 1 wt. % kenalube lubricant as a binder. Iron powders mixed with different ratios of Ekabor 2 powders. The code of samples connected with chemical composition is shown as Table 1. Iron powders were generated by the Hougenes Co. and Ekabor 2 powders were generated by the BorTec too. The particle size of iron powder were 45–106 µm and size of Ekabor 2 powder were <850 µm. Ekabor 2 powders have approximate chemical composition of 90 % SiC, 5 % B4C and 5 % KBF4. (Çalik, 2013). This chemical composition was used for calculating the theoretical density of pure iron powders.

All powder compositions which are given in Table 1 were mixed in a V-type mixer for 25 min. with 25 cycle/min to obtain homogeneous mixture. After mixing process, the powders of Ekabor 2 and iron were placed in a steel die (outside diameter, 30 mm; height, 80 mm). A uniaxial pressure of 100 bar was applied to mixtures. The size of the obtained cylindrical compact was Ø30 mm in diameter, and it approximately weighted 20 g. Height was different in each specimen depending on the intensity difference between Ekabor 2 and iron.

All compacts were sintered by conventional furnace. Heating for sintering were used a PROTERM PAF 120/7 furnace. In this sintering process compacts were sintered respectively at 900 °C, 950 °C for each composition of ratios in 4 h. Each compacts were cooled naturally. The image of the produced samples are shown as Fig. 1 after successful sintering process. Iron based PM samples for sintering is approximately between 1100 °C and 1200 °C under normal conditions, but we used 900 °C and 950 °C because of nearly 1149 °C is the melting temperature of the boron (Fichtl, 1981).

The relative densities of samples were measured by the Archimedes Method. The surface roughness was measured with Mitutoyo (2009) surface test device.

Ball on disc testing was used to evaluate the materials in a well-controlled multiple passage sliding contact. In this test Al₂O₃ ball was used and drown over surface several revolutions in the same circular track at a normal load of 10 N and a sliding speed of 25 cm.s⁻¹, tangential (friction) and normal forces were continuously measured with strain gauges, recorded by computer. The width of abrasion surfaces was evaluated with optic microscopy images. Nikon Eclipse LV150N optic microscopy was used for microstructure images.

The surface hardness was evaluated before taking cross-section for comparing with internal hardness values. BMS-201-R Analog Rockwell Hardness Tester at automatic load of 150 N and a dwell time of 15 s was used for equipment. The values found are given at Table 5.

The samples were then sectioned from bisect prepared metallographically up to 1200 grid emery paper and then polished using 1 µm alumina paste. The Metkon Forcipol IV was used for etching and polishing. Microstructural characterization was made on product was made on product samples which had been polished and etched using a solution of C₂H₆O (97 vol. %), HNO₃ (3 vol. %). Microstructures were observed with Nikon Eclipse LV150N optical microscope and obtained image is shown at Fig. 2.

3. RESULTS AND DISCUSSIONSTOP

The samples of weight and extension measurements were evaluated before sintering process and rate of change after process was shown at Table 2. Volume is maintained during sintering process. Therefore, while diameters of specimens were increasing, height of specimens decreased. Change in height and in diameter is due to gravity. Binder moved away during sintering process thus, weight decreased.

In a study of Çalik (2013), it was explained that composition of Ekabor 2 has obtained 90 wt. % SiC; 5 wt. % B₄C; 5 wt. % KBF₄ (Çalik, 2013). Densities of Ekabor 2 and pure iron were measured and percentage of density values were estimated. Results of experiment, it was found that density values changed between 6.7331 g.cm⁻³ and 3.9647 g.cm⁻³, percentage of density values changed from 86% to 72% (*Error range is ± 4%). It’s indicated that densities samples were decreased gradually from pure iron sample to pure iron-50 wt. % Ekabor 2 samples. After Archimedes experiment, surface roughness test was applied to specimen and results are shown in the Table 3. Three different values were taken from samples and the average of results were received. The table below, R_a means arithmetic average of absolute values, R_y is the maximum peak to lowest valley vertical distance within a single sample length and R_z is a parameter that averages the height of the five highest peaks plus the depth of the five deepest valleys over the evaluation length (Mitutoyo, 2009). As shown below, when the amount of Ekabor 2 powder were increased, values of surface roughness were increased. Due to increase in temperature, surface roughness decreased and surface roughness occurred due to a portion of boron layer diffuse into the iron.

The abrasion test was applied to specimens. At the beginning and at the end of test their weights were measured. Then samples weight loss were calculated and it was given at Table 4. As expected, rate of Ekabor 2 increased by temperature and percent weight loss decreased. Because when rate of Ekabor 2 and temperature increased, wear resistance increased too. Also, after abrasion testing, the width of average abrasion surface was found between 1327 µm and 904 µm for 900 °C, 1616 µm and 822 µm for 950 °C (*Error range is ±2%)

Average values of internal and surface hardness were taken from three different spots evacuated. It is shown that at Table 5. Sintering results of materials without boron, the average surface and internal hardness were obtained approximately 28 and 32 HRC or 286 HV and 323 HV. When amount of boron increased, values of hardness increased and the average surface and internal hardness were obtained 73 HRC and 66 HRC or 1000 HV and 866 HV in the mixture containing 10 wt. % Ekabor 2. The hardness of other samples couldn’t be measured. Because during preliminary load of measurement the crack was occurred in specimens and values of hardness weren’t shown at the Table 5. The study results were compared with Çavdar and Atik (2014b) and Çavdar et al. (2015). The boronizing process was applied to iron based powders, iron powder, 0-1 wt. % Al, 2-3 wt. % and 0,2-0,8 wt. % graphite (Çavdar et al., 2015), 1-6 wt. % Cu; 0,2-0,8 wt. % (Çavdar and Atik, 2014b) at 950 °C. The average of hardness was found approximately 1000 HV and in our study, the average of hardness was found 48 HRC, 480 HV, at same temperature.

Morphology and microstructure of grain boundaries of PM compacts observed by optic microscopy are given in Fig. 2. Microscopy images showed that Ekabor 2 with gap was present in the iron layer in the form of fragments. Different phases are observed in images Fig 2e and Fig 2f. It is estimated that Fe₂B and FeB phases occurred in 10 wt.% Ekabor 2 and the best hardness values were taken from these specimens.

30 wt. % by weight Ekabor 2 containing iron PM samples were taken from near to broken surface of the microscopy image (Fig. 2g, h). It is shown that rise of Ekabor 2 densities are obtained porosity which increase possibility of cracking. Phases of Fe₂B and FeB are shown at Fig. 2g. It is supposed that these phases make more brittle for the specimen. Ekabor 2 was seen as gray in the microstructure. Gray layer couldn’t be obtained in the microstructure without Ekabor 2.

4. CONCLUSIONSTOP

In this work, Ekabor 2 powder of different weights was mixed with pure iron powder specimens sintered at 900 °C and 950 °C for 4 h and the mechanical properties were investigated. These conclusions were reached:

ACKNOWLEDGMENTSTOP

The authors would like to acknowledge responsible for laboratory Mr. Gökhan Eyici for his assistance with optic microscopy images.

REFERENCESTOP