Tribaloy T-800

Figure 145. XRD patterns measured from the manually ground a) laser clad and b) HIPped Stellite 6 alloys.

3.5.3 Tribaloy T-800

This pore and crack-free hypereutectic intermetallic type laser coating consisted of light and dark regions as illustrated in BSE images taken from the transverse cross-section (perpendicular to cladding direction) in Figure 146. Light regions, which were rich in Mo and Si compared to the dark matrix, were identified as hcp ordered Co3Mo2Si and/or CoMoSi Laves phases, which are the type MgZn2 (C14 or hP12) [397]. In the central regions of the beads, these intermetallics were nearly spherical approximately just 5 µm or less in diameter.

In overlapped regions, they were feather-like and distinctively coarser than spherical colonies in central regions as seen in Figure 147. For the sake of comparison, mean sizes of primary Laves phases in as-cast Tribaloys were reported to be 17-25 µm and with higher solidification rates in casting 9 µm [398]. Despite this difference in microstructure, laser coating exhibited uniform microhardness in both directions; parallel and perpendicular to coating/base material interface. According to elemental maps taken from the transverse cross-sections, Cr and intermixed Fe from the base material were mainly found in the dark matrix, which consisted of fcc ordered Co-based solid solution and Laves phases particularly in overlapped regions.

According to Halstead and Rawlings [397] and Gnanamuthu [398], Cr is typically partitioned about one-third in the Laves phase and two-thirds in the solid solution. XRD patterns showed that the highest peak intensity in laser coating was measured from the Laves phases, whereas the Co-based matrix gave the highest peak intensity in T-800 powder. Halstead and Rawlings [397] reported that the faster the solidification in casting, the higher the volume fraction of primary Laves phases. According to image analysis the amount of Laves phases in laser coating was around 54–57 vol.%, which is in good agreement with the results concerning as-cast T-800 reported in Refs. [397, 399].

In sliding wear tests, intermetallic type hardfacing alloys including T-800 responded differently to dry sliding conditions than solid solution strengthened Stellite 21 and carbide type Stellite 6 discussed earlier. Due to rapid formation and rather tightly bonded oxide layers between mating surfaces and particularly on top of coating, low diluted T-800 laser coating exhibited one of the lowest wear volumes among the studied monolithic alloys and one of the lowest frictional heats. These oxide layers, which formed already during the first 4 minutes of the test, on the top of T-800 laser coating are illustrated in Figure 148. As can be seen in Figure 148a, there are, however, some signs of oxide layer spalling. According to EDS

Figure 146. BSE images of transverse cross-section (perpendicular to cladding direction) of T-800 laser coating. Higher magnification is taken from the central region of the bead.

Figure 147. Low and high magnification BSE images of overlapped zones in T-800 laser coating.

analyses, these oxides contained elements of Fe, Co, Mo, Cr and Si. The regions between the oxide layers consisted of elements of T-800 with traces of O, but not adhered Fe from the ring. Some occasional iron splats were, however, detected as one shown in Figure 149, but this was very rare. Oxide layers were also observed on the top of the worn ring surface, which was relatively smooth. EDS analyses taken from the worn ring showed Fe and O and traces of Mo and Cr. Co was not detected from the ring surface. This surface examination proved that the oxide layers separated the sliding surfaces and only occasional direct metallic contact took place.

The amount of wear debris collected under the ring, which slid against T-800 laser coating, was very small. This is consistent with the wear volume results. Wear debris consisted predominantly of finely divided dust-like oxides including elements of Fe (52 wt.%) and smaller amounts of O (16%), Co (16%), Mo (9%), Cr (5%) and Si (1%). This elemental analysis indicated that it originated mainly from the ring, which is in accordance with the volume loss results obtained for T-800 and T-400 laser coatings. Large metallic particles were not detected. This kind of fine wear debris is typical for mild wear regime.

a) b)

Figure 148. SEM micrographs of the tested surfaces of a) T-800 laser coating and b) 42CrMo4 ring counterpart.

Figure 149. BSE image of the tested surface of T-800 laser coating. The lightest regions are Laves phases, plastically deformed gray splat is adhered Fe from the ring and the darkest regions are oxide layers.

More heavily diluted (0.4 vs. 16.0 wt.% Fe) PTA overlay welded T-800 coating exhibited 1.3 times higher volume loss than corresponding laser coating. Figure 150 shows the microstructure of the PTA overlay welded coating. Owing to higher heat input and slower solidification and cooling rates, the resulting microstructure of the weld overlay was much coarser than that of laser coating. The hardness values of PTA welded deposit (800 vs. 640 HV1) were generally lower than those of laser coating owing to higher dilution (16 wt.% Fe).

According to image analysis, the amount of Laves phases in PTA coating was around 57–64 vol.%, which was slightly higher than that in laser coating. XRD patterns of PTA and laser coatings are shown in Figure 151. They suggest that PTA coating contains less Laves phases than laser coating. Due to higher dilution the amount of Laves phases in PTA coating could be expected to be lower. As XRD pattern was taken from the larger surface area than image analysis, its results are considered more reliable.

Wear debris collected under the ring, which slid against T-800 PTA was similar to wear debris of laser coating except for few large fractured metallic platelets 80-350 μm in length and 50-160 μm in width shown in Figure 152a. EDS analysis taken from the platelet showed mainly Fe (42 wt.%) and smaller amounts of O (19%), Mo (15%), Co (12%), Cr (11%) and Si

Figure 150. BSE images of transverse cross-section (parallel to oscillation direction) of T-800 PTA coating.

b) CoMoSi, Co₃Mo₂Si

+ Co-based

a) CoMoSi, Co₃Mo₂Si

+ Co-based

20 30 40 50 60 70 8 200 30 40 50 60 70 80

2θ 2θ

Figure 151. XRD patterns measured from T-800 a) laser and b) PTA coatings.

(2%). These platelets were obviously detached from the ring. On the basis of elemental mapping, light areas in Figure 152b were oxygen-free and rich in cobalt. Other areas were covered with oxides of Co, Mo, Fe and Si. This indicates that unlike in T-800 laser coating there was some Co transfer from the matrix of PTA coating to the ring.

a)

b)

Figure 152. Wear debris of T-800 PTA coating against 42CrMo4 ring (load 57 N; sliding velocity, 140 m/min); a) large platelet and dust-like debris; b) detail of plastically deformed platelet surface, light regions are oxygen free and rich in Co.