Boronizing mechanism of cemented carbides and their wear resistance

Boronizing mechanism of cemented carbides and their wear resistance

Int. Journal of Refractory Metals and Hard Materials 41 (2013) 351–355 Contents lists available at ScienceDirect Int. Journal of Refractory Metals a...

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Int. Journal of Refractory Metals and Hard Materials 41 (2013) 351–355

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM

Boronizing mechanism of cemented carbides and their wear resistance Lin Guobiao a,⁎, Zhang Zhongjian b, Qiu Zhihai b, Luo Xiang a, Wang Jiahua a, Zhao Feifei a a b

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Zhuzhou Cemented Carbide Group Corp. Ltd., Zhuzhou, 412000, China

a r t i c l e

i n f o

Article history: Received 18 December 2012 Accepted 19 May 2013 Keywords: Cemented carbide Boronizing mechanism Microstructure Wear resistance

a b s t r a c t Embedded in the solid-state boronizing agent sealed in a container, WC–Co cemented carbides were boronized at an elevated temperature. With the help of microstructure investigation and X-ray diffraction examination of the agent and surface layers of the cemented carbides after boronization, it is determined that KBF4 comes into play to boronize the cemented carbides only on the existence of substance such as B4C, which is capable of providing boron. Induction-heating and hydrogen atmosphere are better for the boronization than resistance heating and argon atmosphere, and the presence of SiC and rare-earth elements is also favorable. The boronizing mechanism was studied based on experimental results. Abrasion test was applied with the result that the boronization made the wear resistance improve significantly. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Cemented tungsten carbides used in mining drilling should possess a unique combination of high wear resistance and good impact ductility to avoid the formation of top-surface checking and flake during their drilling applications. CVD (chemical vapor deposition) coatings, applied universally to the cemented tungsten carbides used as machining tools to increase wear resistance, are not suitable for rock drilling applications due to thin thickness (less than about 10 μm) and high local stress in the coatings [1,2]. Other coatings obtained by physical methods such as boron ion implantation [3–6] and pulse-plasma deposition [7] are also inappropriate because of inadequate thickness or inferior bonding strength with substrate. Boronizing has been used to the treatment of cemented carbides to form boride surface layer, which is well bonded with substrate owing to forming a favorable transition zone according to some papers [8,9,13] and patent literatures [10–12]. Nevertheless, there are fewer products about boronized cemented carbides in present market. So far, papers about the boronizing of cemented carbides mostly focus on the treatment of wire draw dies to increase their service durability [13] and boronization of Co phase of cemented carbide surface to promote the nucleation and growth of diamond film on the cemented carbide substrate [14–21]. According to the state of boronizing agent, methods about boronizing cemented carbides can be classified into three categories: gas-state boronization [14,15], such as using boron trichloride and hydrogen as boronizing media [14]; liquid-state boronization [16,17,22] and solid-state boronization [13,12–21]. In comparison, solid-state boronization is characterized by low cost, easy realization and easy ⁎ Corresponding author. Tel.: +86 010 62333152. E-mail address: [email protected] (L. Guobiao). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.05.008

cleaning. Generally, sintered billets of cemented carbides are used in the boronizing, and it is difficult for B element to bypass WC grains and diffuse deep into cemented carbides through Co phase. However, an adequate thickness of boride layer is important for boronized cemented carbides to apply to mining drill, and it is closely pertinent to boronizing agent composition. How boronizing agent acts with cemented carbides is rarely reported so far and it is demonstrated in this paper besides wear resistance of the boride layer.

2. Experimental materials and experimental methods The used cemented carbides were YG11 (WC–11 wt.% Co). The boronizing agents were selected from the groups consisting of B4C (b150 mesh), KBF4, SiC (b150 mesh), Al2O3 (b320 mesh), Mg powder (b60 mesh), graphite particles (b 1 mm) and rare earths. The testing process comprises cleaning up the cemented carbide surfaces by grinding to remove oxide layers, burying the cemented carbides in the boronizing agent in a graphite container, sealing the container and putting it into a furnace heating by resistance or electromagnetic induction. The furnace was pumped up to a vacuum level of 10 Pa and filled with argon or hydrogen as protective gas before heating to an elevated temperature for some hours. The boronizing agents produce gaseous substances at the temperature and promote boronizing. In order to observe the microstructure, the samples were made by cutting the boronized samples along their cross-sections, then grinding, polishing and eroding them in a water solution containing 10% KOH, 10% K3Fe(CN)6 and 80% H2O for about 2 minutes. The thicknesses of the boronized layers of the samples and their morphologies were examined by an optical microscope and a scanning electron microscope (LEO-1450) in back-scattered electron mode (SEM–BSE) or secondary electron image (SEM–SEI). X-ray diffraction (XRD) was

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used to identify formed phases in the surfaces of the boronized samples and the boronizing agents after boronization. The wear resistance was measured on an MPV-1000 computercontrolled abrasion tester according to ASTM B611 (i.e., the highstress abrasion wear test). 3. Results and discussion 3.1. Function of main constituents in boronizing agents Generally, boronizing agent consists of boron-yielding materials (B4C, etc.), activators (KBF4, rare earth, etc.) and fillers (Al2O3, SiC, etc.). In order to analyze the function of main constituents in the boronizing agents during boronization, a series of experiments were designed and some of them are displayed in Table 1. By comparing experiments 2, 3 and 4 with experiment 1 respectively, we aimed to determine the effect of B4C, KBF4 and graphite particles accordingly. Experiments 5 and 6 were designed to find out which was better between Al2O3 and SiC powder as a filler. Mg is also an active substance and vaporizes easily at a temperature higher than its melting point of 649 °C; thus it was selected as a catalyst in some of our experiments. These experiments were conducted in a resistance heating furnace with the process of 950 °C × 5 h and protective gas was argon. The treated sample surface of experiment 1 was analyzed by XRD, showing the formation of CoB phase on the surface. Its boronized layer was also investigated with the results displayed in Fig. 1. It could be inferred that B element was produced in the boronizing agent of B4C + KBF4 + rare earths during boronization, and B element diffused into and reacted with cemented carbides, so as to form boronizing layer and tiny boride particles distributed in Co phase among regular tungsten carbides, as shown in Fig. 1(a) and (b). The boronizing agent of KBF4 + rare earths in experiment 2 did not make the cemented carbides boronized but produced bores in them owing to removal of Co phase as shown in Fig. 2 and XRD also revealed no presence of boride on their surfaces. It is speculated that the reason why Co is removed is that Co reacts with gaseous BF3 produced by decomposition of KBF4, forming other gaseous substance, but it needs further investigation. As for experiment 3, the reaction of B4C powder and a small amount of rare earths should lead to boronization according to the report [23]. Actually, maybe the amount of boride was so small that boride was not detected by XRD on the surface of the treated sample in experiment 3, but on the observation with magnification of 4000 fold, there seemed to be measly boride in 10–20 μm thickness range beneath surface with appearance analogous to Fig. 1(b). Compared with experiment 1, the boronizing agent of experiment 4 contained about 40% graphite particles added as filler and the surface of its boronized sample was verified to have the same phases as those in experiment 1 by XRD analyses; its boride layer also has a similar thickness to that in experiment 1, but has cracks in itself as shown in Fig. 3. This reflects that carbon does not participate in generation reaction of [B] and excessive graphite powder can bring about brittleness of the boride layer due to diffusion of mass C element into the boride layer. When Al2O3 powder was used

Fig. 1. Microstructure of boride layer of the sample in experiment 1: (a) optical micrograph; (b) SEM–SEI micrograph of boride layer.

as a filler to add in the boronizing agent, the result represented by experiment 5 demonstrated no formation of borides but generation of oxidation as indicated in Fig. 4, which is XRD pattern of the sample

Table 1 Experimental scheme to investigate boronization mechanism. Experiment number

Composition of boronizing agent

1 2 3 4

(25B4C: 35KBF4) + additional 3% (Re2O3 + Re) 100% KBF4 + additional 3%(Re2O3 + Re) 100% B4C + additional 3%(Re2O3 + Re) 25% B4C + 35% KBF4 + 40% graphite + additional 3%(Re2O3 + Re) 25% B4C + 35% KBF4 + 40% Al2O3 + additional 1% Mg 25% B4C + 35% KBF4 + 10% graphite + 30% SiC + additional 1% Mg

5 6

Fig. 2. SEM magnification image of the surface layer of the sample in experiment 2.

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Fig. 3. Optical micrograph of the boride layer of the sample in experiment 4.

surface. This may be because Al2O3 powder particles are too fine to keep good permeability for the gas and using Mg instead of rare earths, the reducing atmosphere is not good. The powder of graphite and SiC powder in the ratio of 10: 30 was used in experiment 6 to replace Al2O3 powder in the boronizing agent of experiment 5; B element was detected on the surface of the sample and formed BCo2 phase by XRD (seen in Fig. 5(a)). Fig. 5(b) is the optical micrograph of cross-section of the sample, exhibiting the presence of an obvious good boride layer, which validates that SiC powder is better than Al2O3 as a filler. This is consistent with the report that SiC promotes boronizing [18]. The different kinds of boronizing agents after using were analyzed by XRD; it could be summarized from these XRD results that there were residual SiC, B4C, Al2O3, graphite after boronization reaction though in the original boronizing agents everyone of them had the content of only 7% (by weight), but KBF4 disappeared and KF phase formed. Comparing the experiment results above with experiment 1, it can be drawn that for a better effect of boronizing, the co-action of KBF4 and B4C is essential. Though SiC and rare earths promote boronizing, by comparison, it is KBF4 and B4C that play a major role in boronizing.

Fig. 4. X-ray diffraction pattern of the surface of the sample in experiment 5.

Fig. 5. Investigation of the microstructure of the boronized sample in experiment 6: (a) XRD pattern of the sample surface; (b) optical micrograph.

3.2. The effects of heating mode and hydrogen during boronization From investigation above, it can be figured out that using B4C as boron-yielding material, KBF4 plus a small amount of rare earths as activators and SiC plus small amount of graphite particle as fillers has good effects on boronizing cemented carbides. Further investigation indicates that induction heating can result in thicker boride layer than resistance heating. For example, under the condition of the boronizing agents being 35% B4C + 5% KBF4 + 35% graphite + 20% SiC + 4% Re2O3 + 1% Mg and boronizing process being 900 °C × 5 h, the thickness of boride layer by resistance heating was 32.5–37.8 μm, while by induction-heating it was 60.7–67.5 μm. This maybe originates from the fact that WC phase in cemented carbides has inferior conductibility but Co phase has good conductibility. This can be just utilized by induction heating to activate Co phase, which is different from WC phase and can serve as the passage for diffusion of B element. In this way, long diffusion distance of B can be achieved in the cemented carbides. If the graphite containers occupied by the boronizing agents and cemented carbides are sealed with screw thread and heated in an induction furnace filled with hydrogen atmosphere, the thickness of the boride layer will be further remarkably increased because of hydrogen effect. For example, using the same agents (35% B4C + 5% KBF4 + 35% graphite + 20% SiC + 4% Re2O3 + 1% Mg) and boronizing process (900 °C × 5 h) as the above, the thickness of boride layer in

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hydrogen atmosphere could reach 119.3–231.8 μm as shown in Fig. 6(a). By means of SEM–BSE, the microstructure of the boride layer was magnified as shown in Fig. 6(b), in which the light gray substance around WC grains is CoW2B2 phase according to electronic spectra analyses and the fine feather-shaped phase in the Co binder was testified to contain Co, W, C and B, which is in accordance with the reports [11,24]. 3.3. Analysis of the boronizing mechanism for cemented carbides According to the above-mentioned, hydrogen should be capable of promoting boronization reactions; single KBF4 cannot have the effect of boronization and it works with B4C to take primary boriding action. During boronization process, KBF4 will decompose above 530 °C: KBF4 → KF + BF3↑ and produce gaseous BF3 to participate in the reactions. Referring to some related information [12,18], the following reaction process seems to be reasonable.

agent is, the higher the concentration of resultant B element should be. Hydrogen not only takes part in the generation reaction of B element, but also can act as a medium to transport B element to the surfaces of cemented carbides, additionally avoiding the oxidation of cemented carbides and creative B element in the reaction. Therefore hydrogen is very helpful. Rare earths actually act as a catalyst, since rare earths can accelerate Reaction (3) to produce a large amount of B element and they have a strong oxygen combining power to avoid partly the oxidation of B element. SiC mainly acts as a filler, but it can play some reduction role and produce B element by reacting with the remaining oxygen and BF3 in the atmosphere [18], so it behaves to be in favor of boronization. Graphite particles, conducting electricity, thus can operate as heating units during induction heating besides being a filler. The generated B atoms diffuse into the inner part of the cemented carbides, and the following reactions take place: Co þ ½B→CoB or Co2 B ðdistributed mostly on the surface of the boronized sampleÞ

KBF4 ¼ KF þ BF3

ð1Þ

2BF3 þ 2B4 C þ 4H2 ¼ 3BF2 þ 7½B þ 2CH4

ð2Þ

Co þ WC þ ½B→CoW2 B2 or CoWB ðaround the WC particlesÞ þCo−B−W−C ðbeing fine in Co phaseÞ

3BF2 ¼ ½B þ 2BF3

ð3Þ

3.4. Wear resistance of boronized cemented carbides

BF3, BF2 and CH4 are in gaseous state. From these reaction equations, it is inferred that the higher the content of KBF4 in boronizing

The carbide coupons boronized using the above-mentioned process (35% B4C + 5% KBF4 + 35% graphite + 20% SiC + 4% Re2O3 + 1% Mg, 900 °C × 5 h, H2) were subjected to the high stress abrasion test. During the test, the coupon was pressed against the rotating wheel of 100 r/min with a 20 kg force for 1000 revolutions. The weight loss of the coupon was measured by weighting the sample before and after the test and then converted as the reciprocal of the volume loss (in cubic centimeters) to denote its wear resistance. The larger this number is, the better the wear resistance is. The wear-resistance number of resulting boronized carbide coupons is 4.0772, which increases by 30% in comparison with 3.1379 of the carbide coupons before boronizing. 4. Conclusions Single KBF4 in a boronizing agent, can not play the role of boronizing cemented carbides; on the contrary, it brings about pores in cemented carbides. The boronizing mechanism of cemented carbides is mainly the reaction of KBF4 and B4C at an elevated temperature to produce B element, which diffuses into the Co phase of the cemented carbide and creates a series of compounds. Hydrogen atmosphere and the presence of SiC or rare-earths all promote B generation reaction in the boronizing agent. The higher the content of KBF4 in boronizing agent is, the higher the concentration of resultant B should be. Inductionheating mode should be favorable to the diffusion of B element in Co phase of the cemented carbide. Abrasion test indicates that the boronization makes the wear resistance of cemented carbide improve significantly. References

Fig. 6. Microstructure of the boronized sample in H2 atmosphere: (a) optical micrograph; (b) SEM–BSE magnification image of the interior of the boride layer.

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