**2.1. The powders**

60 Tungsten Carbide – Processing and Applications

or that forms a eutectic,

multiple contact arrangements.

3. by infiltration technique [20, 21].

temperature of the material to be sintered,

1. by addition of a compound with a lower melting point than the material to be sintered,

2. by supersolidus sintering, which consists of heating to temperatures above the solidus

Infiltration is basically defined as "a process of filling the pores of a sintered or unsintered compact with a metal or alloy of a lower melting point." In the particular case of copper infiltrated iron and steel compacts, the base steel matrix, or skeleton, is heated in contact with the copper alloy to a temperature above the melting point of the copper, normally within the range of 1095 to 1150C [22, 23]. Through capillary action, the molten copper alloy is drawn into the interconnected pores of the skeleton and ideally fills the entire pore volume. Filling of the pores with higher density copper can result in final densities in excess of 95% of the composite theoretical value. Completely filled skeletons also allow for secondary operation such as pickling and plating without damaging the structure through internal corrosion. Pressure tight infiltrated components are also possible for specific applications that demand the absence of interconnected porosity. The infiltration process is generally subdivided into two fundamental methods: single step or double step. The single step or single pass is presently the preferred infiltration method that consists of one run or passes through the furnace. In this process, the unsintered (green) steel and copper alloy compacts are placed in contact prior to furnace entry. The typical arrangement is to place the copper alloy infiltrant compact on the top surface of the steel compact. In some cases, it is preferred to place the steel compact on top of the infiltrant compact, or, infiltrate from top and bottom simultaneously. During the full furnace cycle, the steel base compact is ideally partially sintered prior to attaining the melting points of the infiltrant composition. Preferably, multi-independent zone furnaces are employed allowing for preheat, or lubricant burn-off, followed by pre-sintering (graphite solution) and finally infiltration. The double step or double pass infiltration method consists of presintering or full sintering of only the steel compact in one pass through the furnace. After the first sintering pass, the unsintered (green) infiltrant compact is placed in contact with the sintered steel part, and the full furnace cycle is repeated. The infiltrating powders available may be used for both the single and double step processes. Most, if not all, infiltrating powders are prepared as a pre-blended and/or a pre-lubricated lot or batch and are designed for typical compacting operations. Shapes of infiltrant compact forms vary substantially depending upon the amount required and the configuration of the steel skeleton. Usually, simple infiltrant shapes, such as bars, cylindrical slugs, or annuli are compacted to a specific weight and are placed on the iron components in single or

The M3/2 high speed steel reinforced with tungsten carbide and infiltrated with copper was chosen for this investigation. The present paper describes and discusses the microstructural

characteristics and mechanical behaviour of the composite system M3/2-WC-Cu.

Water atomised M3 grade 2 powder of - 160m were obtained from POWDREX SA in the annealed condition. This powder was chosen, in preference to the more commonly used M2, for its better compressibility and sinterability. Chemical composition of this powder is given in the Table 1. As reinforcement's commercial tungsten carbides of - 3m were used. The powder properties, including electrolytic copper are given in Table 2.


**Table 1.** Chemical composition of M3/2 HSS powders, wt-%


**Table 2.** Properties of the used powders

The powders morphology is shown in fig 1.

**Figure 1.** Scanning electron microscopy (SEM) morphology of powder particles: a) high speed steel M3/2 class, b) tungsten carbides WC

It can be seen that the microstructure of the M3/2 grade HSS powders consists of a thin carbides in a martensitic-bainitic matrix. MC carbides being the white ones, Chile M C carbides are grey. Typical microhardness values for a powder is HV0,065 = 284 ± 17.

Tungsten Carbide as an Addition to High Speed Steel Based Composites 63

t a ab b r (r X r X ) (1)

values to obtain relative densities (t). The theoretical densities for the composites were

where a is carbide density, Xa the volumetric fraction of WC carbide, b the density of high

The MMC composites materials were characterized using various techniques. The infiltrated specimens were subsequently tested for mechanical properties. Therefore, mechanical properties were characterized by Brinell hardness, wear resistance test and three points bend tests in order to determine the influence of tungsten carbide. The microstructure of the composites was examined by means of both light microscopy (LM) and scanning electron microscopy (SEM). Characterization of microstructures and the identification of phases present were performed by both optical and scanning electron microscopy, assisted by the use of X-ray energy dispersive analysis (EDX), backscattered electron image contrast, and some X-ray diffraction data. Reaction temperatures were determined by dilatometric study.

During the test a rectangular wear sample (1) was mounted in a sample holder (4) equipped with a hemispherical insert (3) ensuring proper contact between the test sample and a steel ring (2) rotating at a constant speed. The wear surface of the sample was perpendicular to the loading direction. Double lever system was used to force the sample towards the ring

calculated according to the expression (1):

**Figure 3.** Tribosystem T05 - wear test principle

test sample dimensions: 20 x 4 x 4 mm,

rotating ring: heat treated steel 100Cr6, 55 HRC, ø49,5 x 8 mm,

friction force *F* (used to calculate the coefficient of friction).

with the load accuracy of ±1%. The wear test conditions were:

rotational speed: 500 rpm,

sliding distance: 1000 m.

loss of sample mass,

The measured parameters were:

load: 165 N,

speed steel M3/2, and Xb the volumetric fraction of the HSS.

The wear tests were carried out using the block-on-ring tester (Figure 3).

**Figure 2.** The microstructure of M3/2 grade HSS powder, SEM

## **2.2. Experimental technique**

The compositions of powder mixtures are:


Composite mixtures were blended in a Turbula® T2F blender for 30 min. The M3/2 powder and composite powders were uniaxially cold compacted in a cylindrical die at 800 MPa.

The infiltration process is subdivided into two fundamental methods: single step or double step. In the single step the unsintered (green) high speed steels or composite mixtures and copper alloy compacts were placed in contact prior to vacuum furnace entry. The copper alloy infiltrant compacts were placed on the top surface of the green compacts. The double step infiltration method consists of pre-sintering of only the green compacts. After the first sintering process, the infiltrant compacts (specify weight copper green compacts) were placed on the top surface of the sintered composites and were placed to vacuum furnace entry.

Half of green compacts were sintered in vacuum at 1150C for 60 minutes. Sintered specimens and green compacts were analysed before infiltration. Density measurements via the Archimedes method were used to define the level of porosity.

Thereby obtained skeletons were subsequently infiltrated with copper, by gravity method, in vacuum furnace at 1150C for 15 minutes. The infiltrated composites were cooled as fast as vacuum furnace.

The sintering and infiltration process was carried out in vacuum better than 10-2 Pa.

Densities of sintered materials were evaluated by a method based on the Archimedes principle, according to MPIF standard 42. Measured values were compared with theoretical values to obtain relative densities (t). The theoretical densities for the composites were calculated according to the expression (1):

$$\mathbf{r\_t = (r\_a \times X\_a + r\_b \times X\_b)} \tag{1}$$

where a is carbide density, Xa the volumetric fraction of WC carbide, b the density of high speed steel M3/2, and Xb the volumetric fraction of the HSS.

The MMC composites materials were characterized using various techniques. The infiltrated specimens were subsequently tested for mechanical properties. Therefore, mechanical properties were characterized by Brinell hardness, wear resistance test and three points bend tests in order to determine the influence of tungsten carbide. The microstructure of the composites was examined by means of both light microscopy (LM) and scanning electron microscopy (SEM). Characterization of microstructures and the identification of phases present were performed by both optical and scanning electron microscopy, assisted by the use of X-ray energy dispersive analysis (EDX), backscattered electron image contrast, and some X-ray diffraction data. Reaction temperatures were determined by dilatometric study.

The wear tests were carried out using the block-on-ring tester (Figure 3).

**Figure 3.** Tribosystem T05 - wear test principle

During the test a rectangular wear sample (1) was mounted in a sample holder (4) equipped with a hemispherical insert (3) ensuring proper contact between the test sample and a steel ring (2) rotating at a constant speed. The wear surface of the sample was perpendicular to the loading direction. Double lever system was used to force the sample towards the ring with the load accuracy of ±1%.

The wear test conditions were:


62 Tungsten Carbide – Processing and Applications

**Figure 2.** The microstructure of M3/2 grade HSS powder, SEM

Composite mixtures were blended in a Turbula® T2F blender for 30 min. The M3/2 powder and composite powders were uniaxially cold compacted in a cylindrical die at 800 MPa.

The infiltration process is subdivided into two fundamental methods: single step or double step. In the single step the unsintered (green) high speed steels or composite mixtures and copper alloy compacts were placed in contact prior to vacuum furnace entry. The copper alloy infiltrant compacts were placed on the top surface of the green compacts. The double step infiltration method consists of pre-sintering of only the green compacts. After the first sintering process, the infiltrant compacts (specify weight copper green compacts) were placed on the top surface of the sintered composites and were placed to vacuum furnace entry.

Half of green compacts were sintered in vacuum at 1150C for 60 minutes. Sintered specimens and green compacts were analysed before infiltration. Density measurements via

Thereby obtained skeletons were subsequently infiltrated with copper, by gravity method, in vacuum furnace at 1150C for 15 minutes. The infiltrated composites were cooled as fast

Densities of sintered materials were evaluated by a method based on the Archimedes principle, according to MPIF standard 42. Measured values were compared with theoretical

The sintering and infiltration process was carried out in vacuum better than 10-2 Pa.

the Archimedes method were used to define the level of porosity.

**2.2. Experimental technique** 

1. 100% M3/2, 2. M3/2 + 10% WC, 3. M3/2 + 30%WC.

as vacuum furnace.

The compositions of powder mixtures are:

sliding distance: 1000 m.

The measured parameters were:


The friction coefficient was measured continuously during the test, and the wear coefficient was calculated by means of the following expression (2):

$$F = \frac{friction \cdot force \text{[N]}}{load \text{[N]}} \tag{2}$$

Tungsten Carbide as an Addition to High Speed Steel Based Composites 65

resulting from a chemical reaction occurring between the HSS matrix and tungsten carbide particles. As exemplified in Fig. 6, marked specimen expansion followed by its rapid contraction has indicated that the chemical reaction takes place at temperatures between

**Figure 5.** Shrinkage of compacts during sintering as a function of tungsten carbide WC content

**Figure 6.** Dilatometric curves recorded on heating the M3/2 and HSS M3/2 + 30% WC material to the

1080 and 1110C.

sintering temperature

Wear tracks were analyzed by LM to clarify wear mechanisms.
