Wear of advanced cemented carbides for metalforming tool materials

Wear of advanced cemented carbides for metalforming tool materials

Wear 256 (2004) 846–853 Wear of advanced cemented carbides for metalforming tool materials Heinrich Klaasen∗ , Jakob Kübarsepp Department of Material...

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Wear 256 (2004) 846–853

Wear of advanced cemented carbides for metalforming tool materials Heinrich Klaasen∗ , Jakob Kübarsepp Department of Materials Engineering, Tallinn Technical University, Ehitajate tee 5, 19086 Tallinn, Estonia Received 1 June 2003; received in revised form 7 August 2003; accepted 7 August 2003

Abstract Wear behavior of some advanced cemented carbides (TiC-base cermets and some WC-hardmetals) in a series of functional wear experiments (including blanking of electrotechnical sheet steel in production conditions) was analyzed. Comparative laboratory trials of related materials in abrasive-erosion and adhesive-wear conditions were conducted, complemented by SEM. A good correlation between adhesive-wear resistance of the alloys investigated and their resistance to side wear (flank wear of blanking tool) as well as similarity in the morphology of surface failure in both cases was found. With regard to blanking performance, TiC-base cermet with a Ni–steel binder proved to have advantages over conventional WC-base hardmetals (used in metalforming). © 2003 Elsevier B.V. All rights reserved. Keywords: Cemented carbide; Cermet; Wear; Blanking tool; Metalforming; Adhesive wear

1. Introduction Cemented carbides (hardmetals and cermets) allow the service life of tools and wearing machine parts to be prolonged. These materials are mainly used in service conditions where high wear resistance either in abrasive conditions or at elevated temperatures (high speed cutting) is required. Cemented carbides, particularly cermets, are not so widely used in non-cutting operations owing to complicated wear conditions in metalforming. In metalforming, primarily high alloyed tool steels or tungsten carbide-base hardmetals (with relatively high binder fraction) have been successfully used [1–3]. Cemented carbides are used to make coining (generally light coining of small pieces), wire-drawing, deep-drawing, blanking and bending dies in large industrial quantities. Of cemented carbides, straight tungsten carbide grades are most frequently used. Steel bonded carbides are also occasionally used, e.g. for deep-drawing tools [4,5]. This paper focuses on the tribological behavior of some advanced titanium carbide-base cermets with a steel binder, in particular, on their performance as tool materials for metalforming (blanking) applications and on their response as inserts in abrasive-erosion and adhesive-wear experiments. The performance of these cermets was compared to that of tungsten carbide hardmetals used for producing blanking tools (in blanking of electrotechnical sheet steel). Another ∗ Corresponding author. Tel.: +372-620-3359; fax: +372-620-3196. E-mail address: [email protected] (H. Klaasen).

0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2003.08.004

important aim was to identify any correlation that might exist between wear resistance and wear during blanking, on the one hand and adhesive wear and abrasion (abrasive erosion), on the other hand. This problem has mainly been discussed in terms of tool steels and ceramics for milling, turning and other cutting operations [2,6–9]. A good correlation between the performance of these materials in cutting operations and abrasive-erosion wear resistance has been reported. Fewer contributions can be found concerning the performance of tool materials for metalforming (blanking) applications, in particular, their response in different wear conditions [2,3,10,11].

2. Materials and experimental procedure 2.1. Materials Hardmetal dies used in stamping of laminations for electric motors were studied. It was found that WC-base hardmetals used in these dies differed in their carbide content (10, 15, 20 wt.%), grain size (1.8–3.1 ␮m), and production routes (carburization temperature). Structural and technological characteristics of alloys, their mechanical properties, as well as performance data (lifetime in blanking) are presented in Table 1. Table 1 shows some advanced WC-hardmetals (grade WC15-C) based on “high-temperature” WC powders (pow-

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Table 1 Structural and technological characteristics (WC-content, average grain size d of carbide phase, its carbidization technology—conventional and high-temperature carburization “C”), properties (Rockwell hardness HRA and Vickers hardness HV30 , transverse rupture strength RTZ ) and data of blanking performance (lifetime N, side wear ∆) of WC-base hardmetals exploited in production (in blanking) as tool (contour die) materials No. of die

Designation of die

Grade

WC (wt.%)

d (␮m)

HRA

HV30

RTZ (GPa)

N (×106 strokes)

∆ (mm)

1 2 3 4 5 6 7 8

117-6/3 117-5/1 103-6/2 103-6/1 117-4/1 117-6.1 117-6/1 103-6/3

WC20 WC20 WC20 WC20 WC15-C WC15-C WC15 WC10-KC

80 80 80 80 85 85 85 90

2.45 2.30 2.15 1.75 2.60 2.50 2.20 3.10

84.5 85.0 85.0 85.5 86.0 86.5 87.0 87.0

930 960 990 1020 1050 1070 1100 1120

2.7 2.8 2.9 2.7 2.9 3.0 2.6 2.6

3.0 5.0 9.6 24.0 3.4 6.5 12.0 7.5

0.11 0.12 0.12 0.13 0.09 0.10 0.09 0.09

Table 2 Structural characteristics (TiC-content, its average grain size d, binder composition and structure) and mechanical properties (hardness HRA, transverse rupture strength RTZ , proof stress Rco 1 and ultimate plastic strain εp —both in uniaxial compression, applied toughness εp RTZ ) of advanced carbide composites Position of die

Grade

Carbide and content (wt.%)

d (␮m)

Binder composition and structure

HRA

RTZ (GPa)

Rco 1 (GPa)

εp (%)

εp RTZ (J cm−3 )

1 2 3

WC15 T60/8 T70/14

WC, 85 TiC, 60 TiC, 70

2.15 2.10 2.05

Co (W) Fe + 8% Ni–steel, martensite–bainitea Fe + 14% Ni–steel, austenite–martensitea

87.5 88.0 88.2

2.8 2.3 2.4

2.3 2.1 2.2

1.4 1.5 1.7

39.0 35.5 40.0

a

Traces of bainite in Fe + 8% Ni binder and martensite in Fe + 14% Ni binder.

ders produced at enhanced carburization temperatures). These hardmetals are characterized by improved strength and toughness resulting from the enhanced strength of high-temperature carbides. They are used in some cutting and metalforming operations [11]. Advanced TiC-base cermets tested for their performance as tools for blanking applications (in relation to the conventional WC-hardmetal) are presented in Table 2. As can be seen, these alloys differ in their TiC content (60 and 70 wt.%), composition (8 and 14 wt.% Ni), and structure (martensite, austenite) of steel binder. Mechanical properties (durability characteristics) relevant in metalforming of the TiC-cermets selected corresponded to those of the conventional (optimal) of WC-base hardmetals used in blanking: hardness HRA ≥ 87, proof stress in uniaxial compression Rco 1 > 2 GPa, and applied toughness εp RTZ ≥ 28 J cm−3 (εp —ultimate plastic strain in uniaxial compression test and RTZ —transverse rupture strength) [3,10].

blanking performance, i.e. lifetime N in strokes, was determined by the increase (side wear ∆ measure) of diameter ∆D = 0.2 mm of contour die limiting the lifetime of the progressive die [2,3,12] (Fig. 1). The contour die diameter Do was 65 and 96 mm for rotor and stator laminations, respectively.

2.2. Experimental procedure Durability tests were conducted using production conditions, i.e. stamping from electrotechnical sheet steel (with hardness HV = 150, proof stress Rp 0.2 = 1600 N/mm2 , tensile strength Rm = 3200 N/mm2 , and thickness of t = 0.5 mm) laminations for electric motors in production conditions by means of progressive dies of high accuracy and rigidity (reinforced with the hardmetals investigated). The

Fig. 1. Progressive hardmetal blanking die for stamping of stator laminations; 1, 2—contour dies (Do = 96 mm) for examination of side wear ∆D.

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Fig. 2. Durability testing of cemented carbides in 3—position die mounted on the automatic mechanical press.

Fig. 3. The adhesive-wear testing conditions and geometry of specimen.

Additional durability tests were conducted in blanking of grooves into a sheet electrotechnical steel (HV = 140, t = 0.5 mm) by means of a 3-position (reinforced with three different cemented carbides) experimental die mounted on the automatic mechanical press (Fig. 2). Their performance was evaluated by the side wear (∆ = ∆D/2) after intermediate service time N = 0.5 × 106 strokes as N/∆ (lifetime in strokes if side wear ∆ exceeds 1 mm). The intermediate service time 0.5 × 106 corresponded to the time between two consecutive sharpenings used for blanking dies [3,10]. The side wear measure 2∆ = ∆D was determined by means of the measuring machine WMM500 at constant environmental conditions (at constant room temperature of 20 ◦ C and relative humidity of 50%) as an average of 4–6 measurements. Abrasive-erosion tests were conducted by means of a centrifugal accelerator [13]. The conditions were as follows: abrasive—quartz sand with particle size 0.1–0.2 mm, jet velocity 80 ms−1 and attack angle α = 30◦ . The abrasive-erosion rate K (the specific volumetric wear, mm3 kg−1 abrasive) was determined as K = ∆G/ρGa , where ∆G—the mass wear of specimen, ρ—the density of alloy, and Ga —the mass of abrasive. The relative wear resistance X was determined as the ratio of the abrasive-erosion rate of the primary standard material Kstandard (normalized steel 0.45% C with Vickers hardness = 200) to the wear rate of the investigated alloy Kalloy . For the probability factor of 95%, the wear rate confidence interval did not exceed 10% when the number of test pieces was 4. A special method was used for the studies of adhesive wear [10,13,14]—which consisted turning of mild steel (HV = 175) at low speed ν< 12 m min−1 when adhesive failure prevails. The wear rate was determined as the length of the wear land h at the tool (specimen) nose after specific length of the cutting path L (see Fig. 3). The adhesive-wear resistance L/h was evaluated as the length of the cutting

path L per 1 mm length of the wear land h [13]. Confidence interval of wear resistance did not exceed 10% when the number of test pieces was at least 3.

3. Results and discussion The results of production tests are presented in Table 1 and Fig. 4, where the dies are arranged according to their hardness. It must be emphasized that during use of dies (eight contour dies) neither fracture nor brittle chipping of their cutting edges could be detected. Cutting edges became blunt as a result of uniform wear. In terms of hardmetal tool performance (side wear), the results obtained refer to the influence of standard properties, in particular, hardness on blanking performance— specific durability N/∆—being uncertain. Similarly to abrasive-erosion, in particular, to adhesive wear, the conventional measure of wear resistance—hardness enables one to estimate wear resistance in blanking (blanking performance) only as the first approximation [13,15]. Lack of correlation between blanking performance N/∆ and hardness may be due to high structure sensitivity of hardmetal wear resistance [16]. High structural sensitivity means that imperceptible changes in microstructure not affecting mechanical properties cause significant alterations of wear. In Fig. 5 it can be seen that imperceptible change in carbide grain size (∆dWC = 0.1–0.2 ␮m) not affecting hardness, results in remarkable change (up to two times) of blanking performance. The results presented in Fig. 5 demonstrate that hardmetal blanking performance depends strongly on its microstructure, i.e. the carbide grain size. Similar dependence of hardmetal wear resistance on the average size of carbide grains was proved in adhesive-wear tests [16].

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200

WC (80 wt% Co) WC-C(75 wt% Co) x 106, strokes/mm Blanking performance N/

20

Blanking performance N x 10 6, strokes

WC (80 wt% Co)

15

WC (75 wt% Co) 150

100

50

10

1.9 2.1 2.3 2.5 2.7 Average carbide grain size dwc,,µm

5

Fig. 5. Blanking performance N/∆ of WC-base hardmetals vs. their average carbide grain size dWC : 䊉—series of hardmetals with 80 wt.% Co; 䊏—series of hardmetals with 75 wt.% Co.

84

85

86

87

Hardness HRA Fig. 4. Performance of hardmetals as tool materials for blanking vs. Rockwell-hardness number HRA: 䊉—series of WC-hardmetals with 80 wt.% Co; 䊏—series of hardmetals with 75 wt.% Co.

The global presentation plotted in Figs. 6 and 7 refers to the existence of a good correlation between blanking performance N/∆ and adhesive-wear resistance L/h of hardmetals. The dependence of blanking performance on the abrasive-erosion wear resistance X is less obvious. Thus, the results obtained demonstrate that the side wear ∆, limiting hardmetal blanking die lifetime, is controlled by the adhesive-wear resistance of the cemented carbide used. The results obtained enable one to conclude that in wear conditions under prevailing adhesion-failure (blanking of up-to-date electrotechnical sheet steels), a decrease in carbide grain size is more effective to improve hardmetal blanking performance than an increase in carbide strength (achievable through “high-temperature” carbide use) and its fraction. In this sense, advanced hardmetal

use, i.e. hardmetals on the basis of high-temperature carbides in blanking of electrotechnical sheet steels, is not justified. Our results are also confirmed by the results of the SEM analysis of worn surfaces of hardmetal blanking tools. Scanning electron micrographs of both worn surfaces, i.e. that of blanking tool and adhesion-test specimen, were compared. As a result, these were found similar: in both cases, the failure (removal of material) started selectively in the binder phase (Fig. 8a and b). After removal of the binder the worn areas in the microstructure act as failure initiating stress concentrators inducing acceleration of wear—cracking of carbide grains and their removal by extraction in the process of losing the binder layer. The binder generates favorable volumetric compression stresses in carbides resulting in increase of their resistance to brittle fracture. In the last step, the carbides removed may act as abrasive particles. Briefly, the hardmetal wear resistance in blanking (blanking performance) is controlled by the adhesive-wear resistance of the binder (by its resistance to adhesion interaction and removal) and its dimensions in the structure. The dimensions of the binder, i.e. its “mean free path”, decrease most effectively with a decrease in carbide grain size at constant volume fraction of carbides [1,17]. The second way to improve adhesive-wear resistance of a binder is to increase its strength—resistance to extraction

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Fig. 6. A global representation of the results of hardmetals performance in blanking, abrasive-erosion and adhesion wear tests.

WC (80 wt% Co)

Blanking performance N/ strokes/mm

x 10 6,

WC (75 wt% Co)

Fig. 8. SEM image of hardmetal surface microstructure after blanking (a) and adhesive-wear (b) testing.

150

100

50

50

100

150

Resistance to adhesive wear L/h, m/mm Fig. 7. Relationship between hardmetal blanking performance N/∆ and adhesive-wear resistance L/h: 䊉—series of hardmetals with 80 wt.% Co; 䊏—series of hardmetals with 75 wt.% Co.

and adhesive interaction. The adhesive interaction between contacting wearing surfaces (local plastic strains, resulting in frustration and removal of oxide films, origination of physical contact and adhesive junctions) is a precondition for adhesive failure (material removal) [16,18–22]. Figs. 9–11 show the test results for the advanced titanium carbide-base cermets (Table 2) elaborated for metalforming applications [10]. The values of durability characteristics relevant to metalforming of these cermets, i.e. hardness, proof stress Rco 1 , applied toughness εp RTZ , were close to those of the conventional WC–15Co hardmetal used as relevant to blanking and other metalforming operations. In terms of abrasive-erosion wear resistance, as expected, both TiC-base cermets are in considerable disadvantage over the WC-hardmetal. As stated previously, the abrasive-erosion wear resistance of carbide composites de-

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Erosive wear resistance X

10

5

Adhesive wear resistance L/h, m/mm

200

Fig. 9. Wear of blanking tool cutting edges—side wear ∆ after blanking of 5 × 105 strokes. H—depth of groove in die, measured from its cutting edge (frontal area).

100

200

50

1

T 60/8

100

2 Position ¹ of die

T 70/14

150

WC 15

Blanking performance N/ x 106, strokes/mm

pends primarily on the properties (strength and rigidity) of the carbide phase and then on those of the binder [13]. Fig. 10 demonstrates the superiority of titanium carbide-base cermets over the WC-hardmetal in relation to adhesive-wear resistance L/h. This confirms the above mentioned consideration—adhesive-wear resistance of carbide composites (in contrast to abrasion wear) depends primarily on the properties of the binder, rather than on those of the carbide phase. Concerning the blanking performance of carbide composites (tested as tools in blanking operations), a good correlation between the blanking performance of alloys and their adhesive-wear resistance may be stated (Figs. 9 and 10). The highest blanking performance (exceeding that of WC-hardmetal by the factor of 2) was demonstrated by the TiC-base cermet T70/14 (consisting of 70 wt.% TiC and austenite–martensite Ni14–steel binder). The blanking performance of the cermet T60/8 (consisting of 60 wt.% TiC and a Ni8–steel binder with martensite–bainite microstructure), although characterized by adhesive-wear resistance close to that of cermet T70/14, was markedly lower—it did not exceed that of the conventional WC-hardmetal. Studies of worn surfaces (near the cutting edges of blanking tools) by SEM revealed some differences in the micromechanism of wear (Fig. 11a and b). The SEM image of the worn alloy T70/14 showed that uniform wear had prevailed. Microcracks were not observed. The microstructure of a conventional WC-hardmetal, as well as that of TiC-cermet T60/8, showed microcracks near the cutting edges. Microcracks may result from fatigue—local microfailure caused by repeated blanking (shearing) forces. Local mi-

150

3

Fig. 10. Relationships between blanking performance of different cemented carbides and their anti-wear behavior in abrasive-erosion and adhesive-wear conditions.

crofailure results in microchipping of cutting edges during blanking, causing an increase in shearing forces, which results in acceleration of side wear [10,12]. Thus, the advantage of TiC-base cermet T70/14—its higher blanking performance—over the conventional WChardmetal may be related first to the higher adhesive-wear resistance of its Ni–steel binder (compared with Co-binder) and second, to its higher fatigue endurance (resistance to microcrack formation during repeating loads). In relation to titanium carbide-cermet T60/8 with martensitic–steel binder, cermet T70/14 is superior, which may be explained by its higher fatigue endurance. In a recent study of hardmetals and cermets [23] a good fatigue endurance of TiC-base cermet T70/14 was observed. It was found that fatigue sensitivity of TiC-base cermet T70/14 (represented in less sharp slope of Wöhler “stress—number of cycles to failure” curves) was lower than that of the conventional WC-base hardmetal WC15.

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• The adhesive failure of cemented carbides starts preferably in the binder phase and therefore the adhesive-wear resistance of such materials depends primarily on properties (strength, resistance to adhesive interaction) as well as on the dimensions of the binder (mean free path between carbides), less on the properties (strength, rigidity) and fraction of the carbide phase. • The blanking performance of the cemented carbides investigated is controlled, first, by their adhesive-wear resistance (in particular of the binder) and second, by fatigue endurance, i.e. its resistance to origin and propagation of microcracks in structure during repeated loads. • The blanking performance of advanced TiC-base cermets is superior over conventional WC-hardmetals due to higher adhesive-wear resistance of its Ni–steel binder as compared to Co-one and its relatively high fatigue endurance.

Acknowledgements We are thankful to R. Eigi and E. Narusk from Zircon Tools Ltd., A. Kahar, Ph.D., from ABF-Baltic and A. Talkop, V. Kull from AMP Ltd., for their contribution to the performance of production-functional tests and design-production of testing dies. This work was supported by the Estonian Science Foundation. References

Fig. 11. SEM image of WC-hardmetal (a) and TiC-cermet T60/8 (b) surface microstructures representing origin of microcracks during blanking (N = 0.5 × 106 strokes).

4. Conclusions Based on the results of blanking, erosion and adhesive-wear tests, the following conclusions can be drawn: • The dependence of the global anti-wear performance of the cemented carbides investigated on hardness cannot be defined distinctly. Neither hardness nor abrasive-erosion resistance enable satisfactory prognostication of cemented carbide blanking performance (its wear resistance as a material for blanking tools). • A good correlation between cemented carbides blanking performance (their resistance to side wear limiting the lifetime of blanking tool) and their adhesive-wear resistance as well as similarity in surface failure morphology during wear were found.

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