Hardness of WC-Co hard metals: Preparation, quantitative microstructure analysis, structure-property relationship and modelling

doi:10.1016/j.ijrmhm.2018.05.003

International Journal of Refractory Metals and Hard Materials

Volume 75, September 2018, Pages 287-293

$International Journal of Refractory Metals and Hard Materials$

https://doi.org/10.1016/j.ijrmhm.2018.05.003 Get rights and content

Highlights

•
Microstructural parameters were determined using semi-automated image analysis
•
A model for Vickers hardness based on these microstructural parameters is proposed
•
The model is in good agreement within the industrially relevant hardness range
•
The hardness is mainly determined by the WC grain size and the Co content

Abstract

Different commercial WC-Co hard metals with carbide grain sizes ranging from ultrafine to coarse and Co contents between 4.2 and 25 wt.% have been investigated with regard to their microstructural and mechanical properties. Therefore, novel preparation strategies – including specific etching reactions – and microscopic methods for the microstructural characterization were developed. Two sets of microscopic images were generated suitable for further semi-automatic determination of either the Co volume fraction φ_Co or the mean maximum Feret diameter d_Fer of the WC grains considering the irregular shape of the grains. Subsequently, the determined microstructural parameters and measured Vickers hardness values were used to develop a novel model calculating the hardness of WC-Co hard metals. The total hardness is mainly determined by the hard carbide whereby its influence is reduced by the soft Co binder phase. The current model is in good agreement with the measured values within almost the whole relevant hardness range of WC-Co hard metals (700 - 2300 HV 10) and do not require any statement of the existence of a carbide skeleton within the material and thus the Co binder mean free path which is essential for hardness models established so far but also hard to determined experimentally. So the current model is a significant simplification and improvement of the prediction of the hardness of WC-Co hard metals by means of microstructural parameters.

Graphical abstract

Introduction

Hard metals (also called cemented carbides) are composite materials containing a hard material phase (mostly carbide) embedded in a tough metallic binder matrix which leads to excellent mechanical properties. The most important system is WC-Co because of the combination of excellent toughness and wear resistance which makes it applicable for a broad range of industrial applications depending on its microstructure (see Fig. 1) [1]. Coarse-grained WC-Co hard metals with a high Co content are usually applied as material for mining tools because of their excellent toughness and impact resistance. WC-Co systems with finer microstructures and lower Co contents exhibit high hardness and wear resistances and are therefore, used for machining tools cutting various materials like steel, nickel-base alloys or carbon fiber reinforced plastics (CFRP) [2,3]. However, there is little current systematic research work on modern hard metal based cutting materials. Therefore, it is difficult for the applicants to identify the best material to specific machining task combination and therefore, the usage of a not optimally chosen cutting material leads to increased tool wear as well as machining induced damage in the work piece, like delamination and therefore, a decreased quality and finally benefit-cost ratio.

Hard metals used for cutting tools are classified by an international standard regarding their mechanical properties like hardness and toughness as well as the particular machined materials described by one non-quantitative figure [4]. However, the hardness and the toughness, which influence relevant process parameters like the cutting speed and the feed rate, respectively, are opposed, i.e. one property can only be improved by impairing the other one [5,6]. In the actual release of the standard, nowadays interesting machined materials like CFRP are not mentioned and therefore, selection recommendations are mainly provided by the hard metal manufacturers based on empirical findings. For a solid recommendation the mechanical behavior has to be measured or calculated from a model based on the microstructural features of the particular material, like e.g. the size d of the embedded particles and the mean free path λ of the binding matrix as well as the volume fraction φ_Co of the Co binder phase. In case of the hardness, first studies about the influence of microstructural parameters were established since the 1950s [7,8] and lead to various models calculating the hardness by means of these parameters with different assumptions [[9], [10], [11]].

The aims of this study were:

1.
Establishing a robust and quick preparation as well as characterization method for hard metals – especially for materials with finer microstructures – to determine the maximum Feret diameter d_Fer as a key figure for the WC grain size and its respective area-weighted lognormal distribution as well as the volume fraction φ_Co of the Co binder phase.
2.
Developing a new model to calculate the composite hardness of WC-Co hard metals which is applicable for the whole range of industrially applied WC-Co materials using their determined microstructural parameters mentioned above.

Section snippets

Microstructural analysis

There are different strategies to determine microstructural key figures of WC-Co hard metals. Mathematic models established in several studies allow to calculate the microstructural parameter of the WC-Co hard metals by means of determining the magnetic saturation as well as the coercivity [8,[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. Additionally, the quantitative determination of magnetic properties allows proving the existence of additional phases within

Experimental

In this work 28 industrial WC-Co hard metal materials provided by various commercial manufacturers with grain grades in a range from ultrafine to coarse and Co contents between 4.2 and 25 wt.% have been investigated. So the materials investigated in this study cover nearly the whole spectrum of industrially applicable hard metals at present.

For each material small parts from the size of several mm had been cut off and embedded in epoxy resin before they were polished according to a defined

Results

The measured maximum Feret diameters d_Fer are typically lognormal distributed (see Fig. 2) according to the probability density function $f (d_{Fer}) = \frac{1}{\sqrt{2 π} σ d_{Fer}} exp (- \frac{{(ln d_{Fer} - ln μ)}^{2}}{2 σ^{2}})$ with the distribution parameters σ and μ. By means of fitting the frequency distribution with Eq. (7) delivering the parameters σ and μ, the arithmetic mean $<d_{Fer}> = \exp (μ + \frac{1}{2} σ)$ can be calculated for each investigated material as a key figure for the corresponding mean WC grain size. Table 1 shows the experimental results of this study.

Hardness model

For our model we assume that the hardness of the WC-Co hard metal is mainly determined by the carbide phase, especially for the submicron- and ultrafine-grained materials. At fixed Co contents, the in situ hardness H_WC of the WC phase only depends on the WC grain size and can be expressed by a modified Hall-Petch relation as assumed by Engqvist et al. [10] $H_{WC} = A + \frac{B}{\sqrt{d_{Fer} + C}}$

A, B and C are constants and d_Fer is the mean maximum Feret diameter of the WC grains. As shown for the investigated materials

Microstructural analysis

As mentioned above, the determination of the carbide grain size by means of microscopic images is mainly performed by using linear intercept method either in industrial standards and previous studies about modelling the Vickers hardness of WC-Co hard metals. However, the linear intercept method faces some metrological drawbacks. The method only yields a mean value without any information of the distribution. Furthermore, depending on the chosen distance between the lines a certain amount of WC

Conclusions

1.
By means of the semi-automatic analysis of the SEM images presented in this study the microstructural key figures, in particular the carbide grain size and the Co volume fraction, of various industrial WC-Co hard metals have been determined. The mean maximum Feret diameter is defined as an accurate parameter describing the mean WC grain size quantitatively with consideration of the non-spherical grain shape.
2.
In case of the Co volume fraction, the values measured by means of image analysis are

Acknowledgments

This work was funded generously by the Ministry of Science, Research and Art of Baden-Wuerttember, Germany, as part of the project ZAFH SPANTEC light (reference: 32-7545.24-10/1/2).

The authors express their deep and sincere gratitude to Dr. Lisa Weissmayer for measuring the microstructural values of different hard metals as well as for the fruitful discussions.

Thomas Kresse studied physics at the University of Goettingen, Germany, and graduated in 2013 at the Institute of Materials Physics at the same university on the influence of soluted carbon on the formation of vacancies in deformed steel. He then worked as a postdoctoral fellow at King Abdullah University of Science and Technology, Saudi Arabia, and at the Institute of Applied Materials at Karlsruhe Institute of Technology, Germany. Since 2016 he works as a research associate at the Materials

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Hardness of WC-Co hard metals: Preparation, quantitative microstructure analysis, structure-property relationship and modelling

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Microstructural analysis

Experimental

Results

Hardness model

Microstructural analysis

Conclusions

Acknowledgments

Int. J. Refract. Met. Hard Mater.

Compos. Struct.

Int. J. Refract. Met. Hard Mater.

Mater. Sci. Eng.

Wear

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Int. J. Refract. Met. Hard Mater.

Int. J. Refract. Met. Hard Mater.

Int. J. Refract. Met. Hard Mater.

Int. J. Refract. Met. Hard Mater.

Mater. Sci. Eng. A

Acta Metall.

Int. J. Refract. Met. Hard Mater.