Transactions of Materials Processing (소성∙가공)

The Korean Society for Technology of Plasticity and materials processing (한국소성∙가공학회)
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Development of Uniaxial Tensile Test Method to Evaluate Material Property of Tungsten Carbide-Cobalt Alloys for Cold Forging Dies

냉간단조 금형 WC-Co합금의 인장시험방법 개발 및 물성평가

  • 권인우 (한국생산기술연구원 뿌리산업기술연구소 성형기술그룹) ;
  • 서영호 (포스코(주) 선재마케팅실 선재솔루션그룹) ;
  • 정기호 (한국생산기술연구원 뿌리산업기술연구소 성형기술그룹)
  • Received : 2017.12.13
  • Accepted : 2018.10.15
  • Published : 2018.12.01
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Abstract

Cold forging, carried out at room temperature, leads to high dimensional accuracy and excellent surface integrity as compared to other forging methods such as warm and hot forgings. In the cold forging process, WC-Co (Tungsten Carbide-Cobalt) alloy is the mainly used material as a core dies because of its superior hardness and strength as compared to other structural materials. For cold forging, die life is the most significant factor because it is directly related to the manufacturing cost due to periodic die replacement in mass production. To investigate die life of WC-Co alloy for cold forging, mechanical properties such as strength and fatigue are essentially necessary. Generally, uniaxial tensile test and fatigue test are the most efficient and simplest testing method. However, uniaxial tension is not efficiently application to WC-Co alloy because of its sensitivity to alignment of the specimen due to its brittleness and difficulty in thread machining. In this study, shape of specimen, tools, and testing methods, which are appropriate for uniaxial tensile test for WC-Co alloy, are proposed. The test results such as Young's modulus, tensile strength and stress-strain curves are compared to those in previous literature to validate the proposed testing methods. Based on the validation of test results it was concluded that the newly developed testing method is applicable to other cemented carbides like Titanium carbides with high strength and brittleness, and also can be utilized to carry out fatigue tests for further investigation on die life of cold forging.

Keywords

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Fig. 1 Assembled jig and specimen

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Fig. 3 Initially designed WC-Co alloy tensile specimen

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Fig. 4 Tensile test, conducted with the initially designed jig and specimen

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Fig. 12 Comparison of elastic range and non-linear deforming behavior depending on wt. Co contents (a) Co 20% (b) Co 14% (c) Co 6%

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Fig. 2 (a) Assembly drawing of the initially designed jig for tensile test (b) Front and right side views (c) Bottom side view

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Fig. 5 (a) Fractured specimen after tensile test with initially designed jig (b) Measured strain values at two different points within gage length

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Fig. 6 (a) Asymmetry of initially designed jig (b) Strain distribution of jig (c) Strain distribution of specimen

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Fig. 7 (a) Strain distribution of modified jig (b) Strain distribution of specimen under tension

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Fig. 8 (a) Initially designed specimen (b) Modified specimen with the head of Ø31 mm

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Fig. 9 (a) Modified jig with symmetry (b) Pin pressing the specimen for fatigue test

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Fig. 10 (a) Fractured specimen after tensile test with modified jig and specimen (b) Measured strain values before and after modification

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Fig. 11 (a) Stress-strain curve of WC-Co alloy (wt. Co 20%) based on tensile test result (b) Comparison to the test results in the literature (wt. Co 6% & 14%)

Table. 1 Condition of finite element analysis

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Table 2. Comparison of elastic modulus and tensile strengths depending on Co content

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References

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