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Plansee Seminar HM 1/1

Fabrication of Functionally Graded WC-Co Using a

Novel Carburizing Process

K.S. Hwang*, P. Fan*,**, H. Wang**, J. Guo*, X. Wang*,**

Z.Z. Fang*,**

* Department of Metallurgical Engineering, University of Utah,

135 S. 1460 E. Rm. 412, Salt Lake City, Utah, 84112, USA

** Heavystone Laboratory, LLC 1784 W. 2300 S. Unit 3,

Salt Lake City, UT 84119, USA

Abstract

Functionally graded cemented tungsten carbide (FG WC-Co) was fabricated using a novel and effective

process. The post-sintering carburization process transformed the conventional WC-Co into FG WC-Co with

a hard surface-tough core structure, especially the hardened surface due to lower cobalt content in that

region. The graded structure was created by liquid cobalt migration induced by surface carburization. The

new process can be applied to heat treat various WC-Co grades and tool geometries, allowing the

fabrication of FG WC-Co for a wide range of applications.

Keywords

Cemented carbide, Functionally graded materials, Gradient, WC-Co

Introduction

Cemented tungsten carbide (WC-Co) is an indispensable material used in many manufacturing sectors of

our economy, including metalworking, oil and gas drilling, mining, construction, high-wear components, and

other applications. The wide range of applications is primarily a consequence of cemented carbide’s

superior combination of high modulus, high hardness, wear resistance, and moderate fracture toughness.

The wear resistance and fracture toughness of WC-Co composites are, however, inversely related. Wear

resistance is often improved at the expense of the fracture toughness, and vice versa. This relationship

establishes the limit horizon for further expansion of industrial applications.

One method for improving the fracture toughness without sacrificing wear resistance of WC-Co materials is

to use functionally graded WC-Co composites (FG WC-Co) that have varying cobalt content from surface to

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1250

1300

1350

1400

5.2 5.3 5.4 5.5 5.6 5.7 5.8

C, wt%T

, oC

liquid Co

+ Co3W3C

+ WC

liquid Co

+ WC liquid Co

+

graphite

+ WC

solid Co

+ Co3W3C

+ WC

solid Co

+ WC solid Co + graphite

+ WC

solid Co

+ liquid Co

+ WC

solidusliquidus

eutectic

Fig. 1: Vertical section of the ternary phase diagram

of W-Co-C at constant 10wt% Co [10].

the interior. With such a cobalt gradient, the hardness and toughness of the material change

correspondingly with regional composition. The surface of the part with relatively low cobalt content has high

wear resistance, while the interior of the part with relatively high cobalt content exhibits higher toughness.

The functionally graded structure, thus, offers advantages by combining fracture toughness and wear

resistance, in comparison to conventional homogeneous WC-Co materials.

Manufacturing FG WC-Co does, however, present technical challenges. Cemented tungsten carbide is

typically processed via liquid phase sintering in vacuum.

When a green compact (i.e., WC-Co compact which has not

yet been sintered) with an initial cobalt gradient is subjected

to liquid phase sintering, the flow of liquid Co phase easily

occurs, and any gradient of cobalt content is essentially

eliminated [1-5]. One solution to this problem is to employ

high-pressure assisted sintering techniques, such as hot

isostatic pressing (HIP) and spark plasma sintering to

consolidate the graded WC-Co compact in its solid state.

However, these alternative high pressure processes have

limited industrial applications because they are generally

cost prohibitive.

Recently, a novel carburizing process for manufacturing FG

WC-Co was developed by the present authors [5-9]. The

new method, as described in the patent applications and

open literature, still has limitations. In this paper, the

process will be described along with its mechanism. The

cobalt gradients created in various grades of WC-Co

demonstrate the significant potential this process holds for

pushing the boundary of current applications.

The Process and Its Mechanism

The new treatment is a post-sintering carburizing heat

treatment process. In this process, the Co gradient is

formed by heat treating conventional liquid-phase-sintered

WC-Co in a carburizing atmosphere at temperatures and

compositions that allow equilibrium of three phases: solid

tungsten carbide (WC), solid cobalt and liquid cobalt.

The mechanism of Co gradient formation in the process is

based on the following two principles:

1) In a three-phase region, where liquid Co and solid Co coexist with solid WC, the volume fraction of

liquid Co is dependent on the carbon content.

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140

Co

, w

t%

after treatment

as-sintered

Depth from surface, mm

Fig. 2: Co content profiles in WC-10wt%Co

specimens before and after carburizing treatment at

1300oC [6].

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0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160 180

Co

, w

t%

360 min

180 min

60 min

15 min

Depth from surface, mm

Fig. 3: Profiles of Co content in WC-10wt% Co

specimens heat-treated for various lengths of time via

the one-step process [6].

2) The liquid Co phase migrates from a region with more liquid Co phase towards a region with less liquid

Co phase within WC-Co.

The ternary phase diagram of the W-Co-C system [10], as shown in Fig. 1, is helpful to illustrate the

mechanism. The specific three phase region exists in the temperature range of 1275oC to 1325oC in the

central region of the diagram. Within this three-phase region, the amount of liquid Co phase increases

dramatically with increasing carbon content, at the expense of a corresponding decrease of the solid Co

phase. The solid Co phase transforms to liquid Co according to the phase diagram upon carburization in this

temperature range.

A conventional WC-Co specimen with a uniform cobalt distribution was carburized at a temperature in this

3-phase region, and resulted in the formation of a Co gradient, Fig. 2, with reduced Co content in the

surface region. The Co gradient was formed according to the following mechanistic sequence:

1) Surface carburized

2) Solid Co in surface region partially or totally transformed to liquid

3) Liquid Co in surface region increased

4) Balance of liquid Co distribution between surface and core regions destabilized

5) Liquid Co migrated from surface region to core region; and finally

6) Co gradient formed

One-step Process vs. Two-step Process

In the initial stage of the process development, the

cobalt gradients were created via a so-called one-step

process, in which the carburization was conducted at a

fixed temperature in the range of the 3-phase region.

However, it was found that the depth of the cobalt

gradient was very limited even after extended

treatment times. As shown in Fig. 3, in the initial stage,

the depth of the gradient increased significantly with

increased treatment time. However, as the treatment

time increased beyond 180 minutes, and the depth of

the Co gradient reached approximately 100μm, no

further increase in the depth was observed.

The limited depth of the gradient was attributed to the

rapid decrease of carbon diffusion flux, since the volume fraction of liquid Co near the surface layer

decreased with time. This, in turn, reduced the cross-section of available diffusion channels for carbon,

bearing in mind that carbon diffuses primarily through the Co phase in WC-Co. In the one-step process, C

diffusion and Co gradient formation occur simultaneously. In other words, C diffusion and Co gradient

formation are coupled in the one-step process, which results in a limited depth of the gradient formation.

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800

1000

1200

1400

1600

0 1 2 3 4 5

Time, h

Tem

pe

ratu

re,

°C

Two-step

processOne-step

process

3-phase-region temperature

1st step 2nd step

Fig. 4: Schematic of the temperature profile of the

two-step process in comparison to the one-step

process.

In order to create thicker gradients, a two-step process

has been developed [5,7]. In the two-step process, C

diffusion and Co gradient formation are de-coupled from

each other, such that they occur in different steps and at

different temperatures. The first step is to carburize the

specimen at a temperature higher than the three-phase-

region temperature, so that the depth of the C diffusion is

large without causing significant changes to the volume

fraction of liquid. The cobalt gradient is then formed in

the second step by cooling and holding the sample at a

temperature within the three-phase-region for a short

period of time. As soon as the temperature is lowered to

the three-phase-region temperature range, part of the

liquid Co in both the surface region and the core region transforms to solid. More liquid Co exists in the

surface region because the surface region has a higher C content. This breaks the balance of liquid Co

distribution between the surface region and the core region, causing the migration of liquid Co from the

surface towards the core, thus forming a Co gradient between the surface and the interior. In this way, the

final depth of the Co gradient was expected to be equal to the relatively large depth of C diffusion.

The temperature profile of the two-step process is

schematically drawn in Fig. 4. The temperature profile of

the one-step process is also plotted in the figure for

comparison. The most notable difference between the

two processes is that the temperature of the first step in

the two-step process is higher than the 3-phase-region

temperature.

The experimental results showed that the two-step

process created a graded WC-Co microstructure with

much thicker gradients than the one-step process. Figure

5 shows the hardness profiles of the WC-10wt%Co

specimens treated by the two-step process. (Note: the

hardness profile is a direct reflection of the cobalt content

profile; high hardness indicates low Co content). By

controlling the duration of the first step, FG WC-Co with a

wide range of thicknesses was successfully produced.

Applicability of the Process to Various Grades of WC-Co

For various applications of WC-Co, different grades of WC-Co are chosen to achieve optimum combinations

of mechanical properties including wear resistance and fracture toughness. The grades of WC-Co are

Fig. 5: Hardness profiles of FG WC-Co obtained by

the two-step process. The time in the figure is the

time length of the first step, that of C diffusion into the

sample.

1000

1050

1100

1150

1200

0 500 1000 1500 2000 2500

har

dn

ess

, HV

2

depth, microns

20 min

40 min80 min 120 min

core hardness

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primarily categorized based on WC grain size and Co content, since these are the primary two parameters

that determine the mechanical properties in the WC-Co system. In order to evaluate the applicability of the

new process to a large variety of WC-Co products and applications, various grades of WC-Co samples have

been treated and tested for performance. It has been confirmed that a wide range of WC-Co grades can be

processed to achieve desired gradients. These grades include WC grain sizes from superfine to coarse

grain sizes, and Co contents from 6 wt% to 16 wt%, as shown in Fig. 6. It is worth pointing out that even the

superfine WC grade of Fig. 6 (d), which contained VC to inhibit grain growth, posed no difficulty to the

creation of Co gradients through this process. Metal cutting inserts that contain TiC, NbC, CrC and other

carbides have also been successfully treated to get desired gradients.

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Fig.6 (a) to (e) Hardness profiles of FG WC-Co after treating grades of WC-Co with various cobalt contents

and WC grain sizes. (f) Cross-sectional image of a typical FG WC-Co rock drilling insert, where the darker

band is the surface gradient layer with lower Co content and, thus, higher hardness. The white dotted line in

the center indicates the location of indentation marks from hardness measurements.

1000

1100

1200

1300

0 1000 2000 3000

har

dn

ess

, HV

2

depth, microns

fine WC grain, 16% Co

1600

1700

1800

1900

2000

0 200 400 600 800 1000

har

dn

ess

, HV

2

depth, microns

superfine WC grain, 10% Co

1100

1200

1300

1400

1500

0 1000 2000 3000 4000

har

dn

ess

, HV

2

depth, microns


1000

1100

1200

1300

0 1000 2000 3000

har

dn

ess

, HV

2

depth, microns

coarse WC grain, 10% Co

1300

1400

1500

1600

0 1000 2000 3000

har

dn

ess

, HV

2

depth, microns


(a) (b)

(c) (d)

(e) (f)

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Mechanical properties of FG WC-Co by the new process have been evaluated, demonstrating that FG WC-

Co has improved wear resistance, impact resistance and fatigue strength in comparison to conventional

WC-Co, as previously reported [11]. A comprehensive study of the mechanical properties of FG WC-Co of

various compositions is also underway at Heavystone Laboratory LLC.

Summary

The new post-sintering carburization process is effective for making FG WC-Co. The mechanism of the

gradient formation in the process is the liquid cobalt migration induced by surface carburization. The

process can be used to treat a wide range of WC-Co grades in terms of WC grain sizes, cobalt contents and

geometries, and is, thus, applicable to various high performance applications of WC-Co materials.

References

1. A.F. Lisovsky, Int J Heat Mass Transfer 33,1599-1603, (1990)

2. C. Colin, L. Durant, N. Favrot, J. Besson, G. Barbier, F. Delannay, Int J Refract Metals Hard Materials

12, 145-152, (1993-1994)

3. Z. Fang, O. Eso, Scripta Mater 52, 785-791, (2005)

4. P. Fan, Z. Fang, H.Y. Sohn, Acta Mater 55, 3111-9, (2007)

5. P. Fan, Z. Fang, J. Guo, Int J Refract Metals Hard Materials 36, 2-9, (2013)

6. Z. Fang, P. Fan, J. Guo, US Patent 8163232.

7. Z. Fang, P. Fan, J. Guo, US Patent Application Publication No. 2011/0116963.

8. J. Guo, P. Fan, X. Wang, Z. Fang, Int J of Powder Metallurgy 47, 55-62, (2011)

9. J. Guo, Z. Fang, P. Fan, X. Wang, Acta Materialia 59, 4719-4731, (2011)

10. A.E. Mahale, Phase Diagrams for Ceramists, vol. X. American Ceramic Society; 1994.

11. X. Wang, K.S. Hwang, M. Koopman, Z. Fang, L. Zhang, Int J Refract Metals Hard Materials 36, 46,

(2013)

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