Fracture toughness of carbides in tool steels evaluated by nanoindentation J. Caro a , D. Casellas a , S. Molas a , J. M. ...
INTRODUCTION Ultra high strength steels AHSS (tensile strength> 700 MPa) are increasingly used in automotive industry due ...
INTRODUCTION INTRODUCTION Premature fractures < 2000—5000 pieces <ul><ul><li>High yield stress High forming pressures ...
INTRODUCTION Cold work tool steels for forming AHSS must be: <ul><li>Tool steels are formed by a fine distribution </li></...
INTRODUCTION In order to develop high performance tool steels is necessary a detailed knowledge of the relationship betwee...
In brittle materials fracture toughness (K C ) can be determined by the Indentation Microfracture Method (IM) Types of fr...
Common expression used for K C determination (depending on indenter geometry and crack morphology): Anstis. : Limited to...
FRACTURE TOUGHNESS FRACTURE TOUGHNESS Berkovich indenters are generally used in small-scale determination of K C . The mo...
In this work, K C of micrometric primary carbides (10-20  m) in tool steels is determined using Berkovich indenters a...
Values of K C evaluated with expressions (1)-(3) for different primary carbides in different tool steels FRACTURE TOUGHNE...
K C values calculated by the expressions (1)-(3) at P=200 mN and a=2.8  m corresponding to the M 6 C carbides of 1.3202 ...
As postulated previously, Laugier’s expression seems to be more appropriate for nanoindentation tests using Berkovich in...
Nanoindentation on Si(100) plane at 200 mN Niihara (2) and Laugier (3) (using x V = 0.016) expressions give values of K C...
Niihara et al. set the applicability of equation (2) within the range 0.4<a/l<1 In our case, almost all the data for the a...
CONCLUSION: All these comments allow us to point out that the evaluation of K C by nanoindentation using Berkovich inde...
RESULTS <ul><li>High chromium-high carbon tool steel DIN 1.2379 (AISI D2) </li></ul><ul><li>High speed steel DIN 1.3202 (A...
RESULTS RESULTS Crystal structure was determined by XRD Samples were extracted with the same orientation, parallel to the ...
RESULTS RESULTS <ul><li>Variation in the chemical composition of carbides </li></ul><ul><li>High roundness of the surface ...
RESULTS RESULTS catastrophic carbide fracture 20  m 20  m catastrophic carbide fracture
RESULTS Hardness and Young modulus of primary carbides RESULTS <ul><li>Nanoindentation parameters: </li></ul><ul><li>CSM o...
RESULTS RESULTS 1.3202 Steel M 6 C M 6 C MC 7  m 10  m 10  m
RESULTS RESULTS MC-II Steel B 15  m MC-I Steel A 9  m 8  m MC-III Steel B
RESULTS Fracture anisotropy in M 7 C 3 carbides in 1.2379
RESULTS Primary carbides in 1.2379 are elongated as a consequence of forging steps during fabrication Cracks emanating fr...
RESULTS RESULTS Arrows for M 7 C 3 in 1.2379 steel indicate that K C may be even lower Hardness and fracture toughness o...
RESULTS RESULTS Toughness (K C ) vs. composition of MC carbides V-rich carbides exhibit high values of K C
RESULTS RESULTS <ul><li>High performance tool steels, with a high wear and fracture resistance, can be </li></ul><ul><li>o...
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Nanoindentation mts meeting madrid 2007

Published on: Mar 3, 2016
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Transcripts - Nanoindentation mts meeting madrid 2007

  • 1. Fracture toughness of carbides in tool steels evaluated by nanoindentation J. Caro a , D. Casellas a , S. Molas a , J. M. Prado a , I. Valls b a CTM Centre Tecnològic b ROVALMA S.A.
  • 2. INTRODUCTION Ultra high strength steels AHSS (tensile strength> 700 MPa) are increasingly used in automotive industry due to: <ul><ul><li>Reduction in car weight and fuel consumption </li></ul></ul><ul><ul><li>High mechanical strength and high energy absorption in crash </li></ul></ul><ul><ul><li>Good formability </li></ul></ul>INTRODUCTION
  • 3. INTRODUCTION INTRODUCTION Premature fractures < 2000—5000 pieces <ul><ul><li>High yield stress High forming pressures </li></ul></ul>Forming of AHSS Accelerated wear and early fractures of forming tools (dies and punches)
  • 4. INTRODUCTION Cold work tool steels for forming AHSS must be: <ul><li>Tool steels are formed by a fine distribution </li></ul><ul><li>of micrometric hard carbides dispersed in a </li></ul><ul><li>metallic matrix. The macroscopic hardness and </li></ul><ul><li>toughness of the material is highly dependent </li></ul><ul><li>on the properties of primary carbides: </li></ul><ul><li>Morphology, size and orientation </li></ul><ul><li>Chemical composition </li></ul><ul><li>Hardness: determines the wear resistance </li></ul><ul><li>Fracture toughness: determines the fracture </li></ul><ul><li>resistance (fracture initiation sites) </li></ul>Hardness-toughness relationship in tool steels must be optimised INTRODUCTION Wear has traditionally been overcome by increasing hardness, which inevitably leads to a decrease in toughness high wear resistant  high hardness high fracture resistant  high toughness
  • 5. INTRODUCTION In order to develop high performance tool steels is necessary a detailed knowledge of the relationship between the hardness and fracture toughness of carbides and the macroscopic wear and fracture resistance of the bulk material The micrometric size of primary carbides to be analyzed forces the use of nanoindentation techniques Hardness and Young modulus : determined by confined indentation on carbides Fracture toughness : determined by the generation of cracks at the corner of the indentation This work has been recently published in D. Casellas et al., Acta Materialia 55 (2007) 4277 INTRODUCTION
  • 6. In brittle materials fracture toughness (K C ) can be determined by the Indentation Microfracture Method (IM) Types of fracture generated by sharp indenters Halfpenny or radial cracks Connected underneath the hardness impression. The most common crack configuration for brittle materials Palmqvist cracks Developed at low indentation loads or in high-toughness materials (i.e. hard metals or zirconia ceramics) Berkovich cracks FRACTURE TOUGHNESS FRACTURE TOUGHNESS
  • 7. Common expression used for K C determination (depending on indenter geometry and crack morphology): Anstis. : Limited to radial cracks. Applied to the symmetric indenters (i.e. Vickers) P indentation load, c crack length A = 0.016 and n =0.5 Laugier : Used for Palmqvist cracks x V = 0.015 Niihara : Used for Palmqvist cracks a : half-diagonal of the impression c = a + l  = 0.0089 (for WC-Co) [1] [2] [3] FRACTURE TOUGHNESS FRACTURE TOUGHNESS
  • 8. FRACTURE TOUGHNESS FRACTURE TOUGHNESS Berkovich indenters are generally used in small-scale determination of K C . The most commonly used expression is Anstis equation. However, the nonsymmetrical nature of Berkovich indenters does not permit the radial cracks to be joined beneath the hardness impression The application of Anstis equation in this case is doubtful Dukino and Swain determined that good K c values using Berkovich indenters are obtained using Laugier expression with a little modification (x V = 0.016)
  • 9. In this work, K C of micrometric primary carbides (10-20  m) in tool steels is determined using Berkovich indenters at low indentation loads (200-500 mN). It is expected to generate Palmqvist-type cracks (equations 2-3 are expected to give better results than equation 1). It is necessary to generate small cracks (shorten than 5-10  m) within the carbide. At this crack extension application of equations (1)-(3) could give rise to considerable differences in the calculated K C value. FRACTURE TOUGHNESS FRACTURE TOUGHNESS 9  m
  • 10. Values of K C evaluated with expressions (1)-(3) for different primary carbides in different tool steels FRACTURE TOUGHNESS FRACTURE TOUGHNESS
  • 11. K C values calculated by the expressions (1)-(3) at P=200 mN and a=2.8  m corresponding to the M 6 C carbides of 1.3202 steel. At short crack lengths Laugier’s expression (3) gives the highest values of K C , 2.4 times larger than Anstis’s expression (1) FRACTURE TOUGHNESS FRACTURE TOUGHNESS
  • 12. As postulated previously, Laugier’s expression seems to be more appropriate for nanoindentation tests using Berkovich indenters. This expression has been successfully proved in a wide range of ceramic materials using Berkovich indenters. However, the differences between the results obtained with different expressions should be assessed. Consequently, the IM method was applied to a material whose K C is well known using low loads. The results were compared with the K C values obtained with macroscopic methods based on the propagation and fracture of large cracks reported in the literature. Chosen material  Silicon single crystal Si (100) FRACTURE TOUGHNESS FRACTURE TOUGHNESS
  • 13. Nanoindentation on Si(100) plane at 200 mN Niihara (2) and Laugier (3) (using x V = 0.016) expressions give values of K C within the range of the values of Si reported with large crack testing methods FRACTURE TOUGHNESS FRACTURE TOUGHNESS large crack testing methods
  • 14. Niihara et al. set the applicability of equation (2) within the range 0.4<a/l<1 In our case, almost all the data for the analyzed carbides lie outside this range FRACTURE TOUGHNESS FRACTURE TOUGHNESS
  • 15. CONCLUSION: All these comments allow us to point out that the evaluation of K C by nanoindentation using Berkovich indenters should be conducted using Laugier expression (with x V = 0.016): This assertion is specially relevant at low applied loads, when cracks are shorten than 10  m, which usually occurs in nanoindentation of small particles such as in primary carbides of commercial cold work tool steels FRACTURE TOUGHNESS FRACTURE TOUGHNESS
  • 16. RESULTS <ul><li>High chromium-high carbon tool steel DIN 1.2379 (AISI D2) </li></ul><ul><li>High speed steel DIN 1.3202 (AISI T15) </li></ul><ul><li>Experimental tool steels A and B developed by ROVALMA company </li></ul>Analyzed tool steels RESULTS
  • 17. RESULTS RESULTS Crystal structure was determined by XRD Samples were extracted with the same orientation, parallel to the rolling direction
  • 18. RESULTS RESULTS <ul><li>Variation in the chemical composition of carbides </li></ul><ul><li>High roundness of the surface in hard carbides </li></ul><ul><li>Mechanical influence of metallic matrix </li></ul><ul><li>Boundary effects </li></ul><ul><li>Carbides can be under compressive stress </li></ul>Scattering in H, E, K C values can be due to: Confocal laser microscopy image 0 4.5  m 80  m 60  m
  • 19. RESULTS RESULTS catastrophic carbide fracture 20  m 20  m catastrophic carbide fracture
  • 20. RESULTS Hardness and Young modulus of primary carbides RESULTS <ul><li>Nanoindentation parameters: </li></ul><ul><li>CSM operation mode (strain rate: 0.050 s -1 , 2nm harmonic displacement at 45 Hz,  =0.25) </li></ul><ul><li>Hardness and Young modulus measurements at 400 mN </li></ul><ul><li>Fracture toughness measurements at an indentation load range from 200 to 500 mN </li></ul>
  • 21. RESULTS RESULTS 1.3202 Steel M 6 C M 6 C MC 7  m 10  m 10  m
  • 22. RESULTS RESULTS MC-II Steel B 15  m MC-I Steel A 9  m 8  m MC-III Steel B
  • 23. RESULTS Fracture anisotropy in M 7 C 3 carbides in 1.2379
  • 24. RESULTS Primary carbides in 1.2379 are elongated as a consequence of forging steps during fabrication Cracks emanating from the indenter corners propagate preferentially in the direction parallel to the larger edge. XRD study reveal that M 7 C 3 carbides are strongly textured. The observed fracture anisotropy is independent of the tip-carbide orientation and can be attributed to crystallographic anisotropy crystallographic directions <ul><li>Two sets of K C values were found: </li></ul><ul><li>K C < 1 MPa m 1/2 // larger edge </li></ul><ul><li>1.3 < K C < 4.5 MPa m 1/2 ┴ larger edge </li></ul>Fracture anisotropy in M 7 C 3 carbides in 1.2379
  • 25. RESULTS RESULTS Arrows for M 7 C 3 in 1.2379 steel indicate that K C may be even lower Hardness and fracture toughness of different carbides Hard and tough carbides Potential candidates for incorporating in tool steels aimed at optimizing the relationship between wear and fracture resistance
  • 26. RESULTS RESULTS Toughness (K C ) vs. composition of MC carbides V-rich carbides exhibit high values of K C
  • 27. RESULTS RESULTS <ul><li>High performance tool steels, with a high wear and fracture resistance, can be </li></ul><ul><li>obtained through an exhaustive study of the influence of the microstructure </li></ul><ul><li>on the global mechanical response of the material. </li></ul><ul><li>Nanoindentation is a powerful technique for the mechanical characterization </li></ul><ul><li>of micrometric primary carbides in tool steels. The relationship between H </li></ul><ul><li>and K C can be used as a guideline for the selection and design of the most </li></ul><ul><li>appropriate type of carbide to a given mechanical requirement. </li></ul>

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