Stellite 21

The microstructure of low diluted solid solution strengthened Stellite 21 laser coating (1.3 wt.% Fe) consisted of fcc ordered directionally solidified dendritic γ-Co as revealed by metallographic and XRD studies. The secondary dendrite arm spacing was approximately 3 µm. The average hardness of the coating was approximately 400 HV1 in as-laser-clad condition. According to microhardness measurements and XRD studies, work hardening or allotropic crystal structure transformation from γ-Co (fcc) to ε-Co (hcp) did not take place in surface to be tested during the preparation of the wear test blocks. In sliding wear tests against

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Figure 139. Block temperatures measured during the period of 90–120 min against 42CrMo4 (30 HRC) ring. Recording interval was 1 s.

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Figure 140. Mass losses of the blocks as a function of time (= sliding distance) against 42CrMo4 ring. Despite decreasing nominal contact pressure over the contact region during the test, Stellite 21 showed constant wear rate. Stellite 6 exhibited strongly decreasing and the rest slightly decreasing wear rates.

42CrMo4 (30 HRC), the average wear volume of 47.5 mm³ was the third highest among the tested materials leaving only the laser hardened 42CrMo4 and heavily diluted PTA overlay welded Stellite 21 (24.0 wt.% Fe) behind. Despite decreasing nominal contact pressure over the contact region during the test, this coating exhibited the constant wear rate between 0–90 minutes as shown in Figure 140. During this time period, no visible oxide layer (by naked eye) formed either on the block or ring. Simultaneously, the temperatures measured from the block increased to the level of 110°C, which was the highest among the tested alloys. In additional test between 90-120 minutes, the oxide layer, however, formed. Simultaneously, wear mechanism changed from severe to mild due to decreased nominal contact pressure on

the surface and reduced plastic deformation, which allowed oxide layer to form [394]. For that reason, temperature remained rather low (80-90°C) during the period of 90–120 minutes as displayed in Figure 139. In other alloys this last period showed always higher temperatures compared with previous ones due to better weld joint between the thermocouple and the block. Block temperature, however, fluctuated strongly as a function of time during that last period. This indicated occasional peeling off and reformation of oxide films.

Transverse cross-section prepared from the 90 min tested coating revealed slight subsurface deformation as illustrated in Figure 141. That is, the grains at the very top of the tested surface aligned towards the sliding direction of the ring. It was also noted that the microhardness of the coating increased from initial 400 to 460–480 HV1. This work-hardened zone reached the depth of 0.3 mm. At the depth of 40 μm, microhardness was as high as 510-560 HV0.3. Nevertheless, XRD measurements conducted on the tested surface did not reveal, at least severe allotropic fcc -> hcp transformation. There is, however, some uncertainty because diffraction peak of hcp ordered Co (0002) overlaps with the fcc Co peak (111) since their 2θ values are 44.8° and 44.2°, respectively. The highest peak of randomly oriented hcp ordered Co should have arisen at 2θ = 47.6°. This peak was absent for sure. Large scale fcc ->

hcp transformation may have, however, occurred in the beginning of the test where the initial Hertzian stress was at maximum. Due to progressive wear, allotropically transformed alloy may have worn away and was not clearly revealed by XRD. In fact, this could also explain the constant wear rates throughout the test assuming that fully hcp ordered Co possess better sliding wear resistance.

Figure 141. Optical micrographs showing slight subsurface deformation in etched Stellite 21 laser coating after severe dry sliding wear against 42CrMo4 steel. Transversal cross-section is parallel to cladding direction. White arrow indicates the sliding direction. Work-hardened zone reached the depth of 0.3 mm. Within that zone microhardness was 460–480 HV₁.

The SEM examination of the worn surfaces (90 minutes test) showed that significant transfer of material and cold welding or galling took place when Stellite 21 laser coating slid against 42CrMo4 steel. Several patches and stripes of material, which contained Fe up to 20 wt.%, were discovered on the surface of coating. One typical example is shown in Figure 142a. The surface was also plastically deformed. Oxygen was detected, but it was obvious that continuous and thick oxide layer was absent. The worn ring surface shown in Figure 142b contained adhered coating alloy, which was partly fractured and worn away together with ring material. This fractured wear debris may have promoted some abrasive wear even if deep

a) b)

Figure 142. SEM micrographs of the sliding wear tested surfaces of a) Stellite 21 laser coating and its b) 42CrMo4 ring counterpart.

grooves were missing. The amount of oxygen detected on the ring surface was noticeably smaller than, for instance, on the surface of ring against T-800 as will be described later.

Large amount of metallic wear debris was collected under the ring, which slid against the Stellite 21 laser coating. They consisted exclusively of thick Fe-based metallic platelets varying 20–200 μm in length as presented in Figure 143. Their surface was covered with plastically deformed Co-based “tongues” indicating strong galling before detachment.

Basically, the platelet detaches from the ring when the adhesive forces at the junctions between the block and the ring are stronger than the cohesive strength of the ring alloy.

However, according to delamination theory there is also fatigue cycling involved in the formation of such plate-like wear particles, which are typical in severe wear regime [394, 395]. Rotating ring inevitably underwent such fatigue cycling.

Compared with more heavily diluted and considerably softer PTA overlay welded Stellite 21, laser coating exhibited 1.4 times better sliding wear resistance. Besides lower microhardness, this could be attributed to the weaker ability to work-harden since Fe is known to increase the

a) b)

Figure 143. Wear debris of Stellite 21 laser coating against 42CrMo4 (load, 57 N; sliding velocity, 140 m/min); a) thick platelets and b) detail of plastically deformed Co-based

“tongues” on the surface of Fe-based platelet.

stacking fault energy and to stabilize the fcc ordered structure. According to microhardness measurements, work-hardened zone exhibited, however, the hardness of ~390 HV0.3. Hardness increase from 300 to 390 HV0.3 was at same level as that measured from the less diluted laser coating (from 400 to 510-560 HV0.3). Yet, it should be kept in mind that nominal contact pressure was somewhat lower at the end of the test due to higher wear volume compared with corresponding laser coating. On the basis of this, its ability to work-harden was not diminished. Analogous with laser coating, PTA overlay welded one exhibited constant wear rate throughout the test and oxide layer did not form at any stage. Wear block temperature was not monitored.