**4. Phase identification**

70 Tungsten Carbide – Processing and Applications

**Figure 15.** Microstructures of M3/2 HSS + 30%WC composites

**Figure 16.** SEM microstructures of M3/2 HSS based composites

shown in Figures 16 – 17.

linked to WC.

It can be seen that the microstructure of the M3/2 grade HSS based composites consists of a steel matrix with finely dispersed carbides and islands of copper. Figures 13 and 15 shows tungsten carbides located within the grains and on the grain boundaries as well. Microstructures show small porosity at both sintering temperatures, and carbides are larger at the pre-sintered skeleton infiltrated with copper, MC carbides being the white ones, while MC carbides are grey. Some free copper areas are also present. In some places, these added carbides are related with white MC carbides, but free copper dark grey is preferentially

green compact infiltrated with copper pre-sintered skeleton infiltrated with copper

SEM microstructures of a copper infiltrated green compact and pre-sintered skeleton are

green compact infiltrated with copper pre-sintered skeleton infiltrated with copper

Phase identification of the composites was performed by a Tur 62 X-ray diffraction (XRD) machine with Cu target (K, *λ* = 1.5406Å).

Figure 19 show the XRD patterns of samples M3/2 + 30%WC. They illustrate the existence of the main carbides M6C and MC as well as the existence of ferrite and austenite and the high intensity for the main Cu peak in sample green compact infiltrated with copper compared with sample pre-sintered skeleton infiltrated with copper as well as the higher intensity coming from higher volume of copper in as-infiltrated green compact. It should be noted that the intensity of the Fe3W3C peaks in sample pre-sintered skeleton infiltrated with copper can be explained by the chemical reaction between the tungsten monocarbide and HSS matrix.

The SEM and EDX analysis performed on the specimens containing M3/2 10 and 30% tungsten carbide have revealed the carbide phase evenly distributed within the copper-rich regions. As it is apparent from Figures 20 and 21, WC reacts with the surrounding HSS matrix and forms a tungsten and iron-rich M6C carbide grain boundary network during sintering of porous skeletons.

Tungsten Carbide as an Addition to High Speed Steel Based Composites 73

**Figure 20.** The microstructure of pre-sintered skeleton M3/2+30%WC infiltrated of copper and the

**Figure 21.** The microstructure of pre-sintered skeleton M3/2+30%WC infiltrated of copper and the

Intermediate carbides such as WC which include elements that are alloyed to high speed steel react with the steel matrix to produce new carbide phases with compositions similar to those of the normal primary carbides present in high speed steel, e.g M6C {Fe3W3C }. The EDS analysis was carried out on sample pre-sintered skeleton M3/2+30%WC infiltrated of

qualitative EDX analysis, 1 – steel matrix, 2 – carbide M6C type

qualitative EDX analysis, 1 – steel matrix, 2 – tungsten carbide WC, 3 – carbide M6C type

**Figure 18.** XRD pattern from M3/2 composites

**Figure 19.** XRD pattern from M3/2 +30% WC composites

**Figure 18.** XRD pattern from M3/2 composites

**Figure 19.** XRD pattern from M3/2 +30% WC composites

**Figure 20.** The microstructure of pre-sintered skeleton M3/2+30%WC infiltrated of copper and the qualitative EDX analysis, 1 – steel matrix, 2 – tungsten carbide WC, 3 – carbide M6C type

**Figure 21.** The microstructure of pre-sintered skeleton M3/2+30%WC infiltrated of copper and the qualitative EDX analysis, 1 – steel matrix, 2 – carbide M6C type

Intermediate carbides such as WC which include elements that are alloyed to high speed steel react with the steel matrix to produce new carbide phases with compositions similar to those of the normal primary carbides present in high speed steel, e.g M6C {Fe3W3C }. The EDS analysis was carried out on sample pre-sintered skeleton M3/2+30%WC infiltrated of

copper to illustrate the chemistry of this carbides. Figure 22 shows elemental intensity maps for the alloying elements Fe, W and Cu. Most Fe is found in the matrix and grey M6C carbides, while W is found in the whitish M6C carbides. From Figure 21 it also evident that copper diffuse to steel matrix.

Tungsten Carbide as an Addition to High Speed Steel Based Composites 75

**Figure 23.** Loss of mass of as infiltrated composites

**Figure 24.** Friction coefficient of as infiltrated composites

The carbide agglomerations observed are due to the non-assisted system used for mixing the powders; they could be avoided by using a more eficient mixing system such as a ball mill.

**Figure 22.** SEM micrograph and corresponding EDS maps of pre-sintered skeleton M3/2+30%WC infiltrated of copper

### **4.1. Tribological properties**

All the specimens were polished to an average roughness of *R*a = 1 µm. The tests were carried out at room temperature, keeping a relative humidity below 30%. The wear test results are given in Figures 23 and 24.

**Figure 23.** Loss of mass of as infiltrated composites

copper diffuse to steel matrix.

mill.

infiltrated of copper

**4.1. Tribological properties** 

results are given in Figures 23 and 24.

copper to illustrate the chemistry of this carbides. Figure 22 shows elemental intensity maps for the alloying elements Fe, W and Cu. Most Fe is found in the matrix and grey M6C carbides, while W is found in the whitish M6C carbides. From Figure 21 it also evident that

The carbide agglomerations observed are due to the non-assisted system used for mixing the powders; they could be avoided by using a more eficient mixing system such as a ball

**Figure 22.** SEM micrograph and corresponding EDS maps of pre-sintered skeleton M3/2+30%WC

All the specimens were polished to an average roughness of *R*a = 1 µm. The tests were carried out at room temperature, keeping a relative humidity below 30%. The wear test

**Figure 24.** Friction coefficient of as infiltrated composites

The measurements of the wear resistance and friction coefficient permit classification of the as-infiltrated composites with respect to their tribological properties. Direct infiltration of green compacts with copper results in the highest wear resistance and almost the same friction coefficient of the as-infiltrated M3/2 and M3/2+30% WC composites. By comparing the wear resistance of composites received through direct infiltration of green compacts and infiltration of pre-sintered skeletons it is evident that the green compacts M3/2 and M3/2+30%WC compositions show 2÷3 times higher loss of mass than the tungsten carbide containing as-sintered materials infiltrated with copper. This can be explained by the diffusion of carbon and alloying of iron particles during sintering and chemical reaction between the tungsten monocarbide and HSS matrix. Friction coefficients are not highly influenced by the tungsten carbide additions, but the additions of 30% WC to high speed steel and infiltration with copper increase the wear resistance of these composites comparing to base material (M3/2 HSS infiltrated with copper). Wear tracks were analyzed by SEM to clarify wear mechanisms. Characteristic surface topographies after the wear test are exemplified in Figures 25 and 26.

Tungsten Carbide as an Addition to High Speed Steel Based Composites 77

composite which implies marked contribution of adhesive wear, whereas the extensive formation of iron oxides may account for the higher friction coefficients. In MMCs, the level of oxidation is lower than in plain steel. The best behaviour is observed for composite green compact M3/2+30%WC infiltrated of copper. These carbides are well linked to the matrix and cannot be easily detached. M6C formed as a result of chemical reaction between additions of WC and steel matrix are affected by abrasion, while MC carbides remain in the matrix withstanding wear and creating barriers where oxides from matrix debris are accumulated. WC forms large size agglomerates of small particles, which are detached when abraded and spread across the wear track. At first, these particles act as abrasives, promoting three-body wear behavior. The infiltration of green compacts has two effects: on one hand, fewer particles are detached from the carbide clusters, and on the other hand, these particles are not encrusted in the matrix, producing three-body abrasion in all

**Figure 26.** The surface of the as-infiltrated M3/2+30%WC composites after examining the wear

reinforcement is the optimal composition for this type of composites.

The best materials from the viewpoint of mechanical properties were tested for wear properties is green compact M3/2 +30% tungsten carbide WC infiltrated with copper. Figures 23 and 24 suggest that, from the viewpoint of wear behaviour, the 30%

1. Infiltration of porous HSS skeleton with liquid copper has proved to be a suitable technique whereby fully dense HSS based materials are produced at low cost.

compositions.

resistance

**5. Conclusions** 

**Figure 25.** The surface of the as-infiltrated M3/2 composites after examining the wear resistance

The surface topographies of M3/2 and M3/2+30%WC specimens indicate occurrence of different wear mechanisms (Figure 25 and 26). In Fig. 25, typical abrasion scratches are seen in the base material. As a result of abrasion, ferrous oxides are generated and then dispersed through the wear track. The carbides seen on the wear-surfaces are being crushed and pulled out of the matrix to act as abrasive particles which increase the coefficient of friction. Figure 25 provide evidence of ploughing and sideways displacement of material in M3/2. Figure 26 shows smearing of iron oxides over the surface of the as-infiltrated M3/2+30%WC composite which implies marked contribution of adhesive wear, whereas the extensive formation of iron oxides may account for the higher friction coefficients. In MMCs, the level of oxidation is lower than in plain steel. The best behaviour is observed for composite green compact M3/2+30%WC infiltrated of copper. These carbides are well linked to the matrix and cannot be easily detached. M6C formed as a result of chemical reaction between additions of WC and steel matrix are affected by abrasion, while MC carbides remain in the matrix withstanding wear and creating barriers where oxides from matrix debris are accumulated. WC forms large size agglomerates of small particles, which are detached when abraded and spread across the wear track. At first, these particles act as abrasives, promoting three-body wear behavior. The infiltration of green compacts has two effects: on one hand, fewer particles are detached from the carbide clusters, and on the other hand, these particles are not encrusted in the matrix, producing three-body abrasion in all compositions.

**Figure 26.** The surface of the as-infiltrated M3/2+30%WC composites after examining the wear resistance

The best materials from the viewpoint of mechanical properties were tested for wear properties is green compact M3/2 +30% tungsten carbide WC infiltrated with copper. Figures 23 and 24 suggest that, from the viewpoint of wear behaviour, the 30% reinforcement is the optimal composition for this type of composites.
