SX-717 laser

This Cr-based laser coating exhibited very dense and crack-free dendritic (longer secondary arms than in Inconel 625) microstructure strongly bonded to the base material as illustrated in Figure 78. Compared with cellular columnar grains observed, for instance, in laser clad Inconel 625, such long secondary arms are claimed to be important for retarding the migration of sulphur ions into the alloy during the hot corrosion process [324]. In consequence of selective solidification and eutectic reaction, segregation of phases occurred as revealed by the high magnification BSE-image in Figure 78b. The average composition of Cr-rich dendritic phase, which was identified as bcc ordered α-Cr, was 35Ni-59Cr-3Mo-1Si-2Fe in wt.% on the basis of EDS point analyses. Interdendritic region, which consists of α-Cr and fcc ordered Ni-rich γ-phase, showed the composition of 47Ni-48Cr-2Mo-1Si-2Fe in wt.%.

Compositions of these areas were not detected to vary as a function of depth of the coating.

Obviously, the alloy exhibited extended solid solubility since Cr-rich α-phase dissolved much

more than just ~2 wt.% Ni as suggested by the eutectic binary Ni-Cr equilibrium phase diagram shown in Figure 79 [387]. Due to very fine-scale microstructure particularly in interdendritic regions, the volume fraction of the phases present was attempted to estimate using direct comparison method presented in Ref. [378]. The pairs of (200)γ and (110)α and (111)γ and (200)α reflections shown in Figure 80 were considered. On the basis of this calculation, as-laser-clad coating contained 47 vol.% of α-phase and 53 vol.% of γ-phase.

Compositional dilution was only 2%. Microhardness of the coating was 575–590 HV0.3 (552–

562 HV1). Small black dots irregular in shape shown in Figure 78b are probably shrinking cavities because they are found only in the interdendritic regions. Gas pores can be excluded since they would have been spherical in shape [166].

Figure 78. SX-717 laser coating; a) optical micrograph and b) BSE image. Light areas are rich in Cr and Mo compared to composition of initial powder, i.e. dendrites are α-Cr, whereas interdendritic regions consist of (α + γ) eutectic.

In hot corrosion tests, this Cr-based laser coating possessed the highest resistance among the tested alloys. The surface exposed to the salt deteriorated uniformly and the reaction product layer was rather dense in the vicinity of coating surface as illustrated in Figure 81. Elemental maps and EDS line scan analysis (Figures 81 and 82) revealed that the reaction product layer was rich in Cr in this area, whereas outermost layers were rich in Ni. High magnification BSE-images revealed that coating regions rich in Cr and Mo were more heavily attacked than Ni-rich regions. These Cr- and Mo-rich regions appear dark in Figures 83-84. It was also observed that during annealing at 650°C for 1000 h, Cr-rich phases enriched further with Cr as could be expected on the basis of equilibrium binary phase diagram. The average composition of the Cr-rich α-phase was now 32Ni-63Cr-3Mo-1Si-1Fe in wt.% compared with 35Ni-59Cr-3Mo-1Si-2Fe before the test. The average composition of the interdendritic region was 48Ni-47Cr-2Mo-1Si-2Fe in wt.% compared with 47Ni-48Cr-2Mo-1Si-2Fe before the test. It was also noted that the amount of α-phase increased during the test. If the volume fractions were 47α-53γ (%) before the test, they were 55α-45γ (%) after the test. Binary phase diagram suggests that the equilibrium chemical compositions (wt.%) of the α- and γ-phases at 650°C are 2Ni-98Cr and 68Ni-32Cr. With the initial composition of 44.3Ni-55.7Cr (wt.%) volume fractions of α- and γ-phases should have been 40% and 60%, respectively. This direct comparison method with the selected peaks gives obviously too high values for α-Cr and too low values for γ-Ni. It does not, however, change the observation that the amount of Cr-rich α-phase increased during annealing. In addition to EDS point analyses, another finding which may support the increase of Cr content in α-phase (and decrease of Cr in γ-phase) was the

change in lattice parameters (a). It was noted that the lattice parameter of γ-phase decreased 1.2% during the test from 0.3590 nm to 0.3548 nm. Lattice parameter of pure Ni is 0.3523 nm. Equivalent atomic radius of Ni is 0.1377 nm and Cr 0.1420 nm. In contrast, the lattice parameter of α-phase changed just 0.2%. Another factor noteworthy to point out is the amount of Fe. EDS analyses suggest that interdiffusion did not take place between coating and base material during annealing. Hardness profiles before and after the test are illustrated in Figure 85. It indicates that microhardness (HV1) decreased only on outer surface of coating layer.

Tomita et al. [388] annealed PTA overlay welded NiCr (50/50) clads at 500–750 °C. They also observed increase in α-phase but also the precipitation of fine Cr-rich α’-phases.

According to them, α-phases precipitated in γ-phases next to grain boundaries growing inside γ-phases. Fine Cr-rich α’-phases precipitated in γ-phases as well.

Figure 79. Eutectic binary equilibrium phase diagram for Ni-Cr. SX-717 is hypereutectic alloy having a composition to the right of the eutectic point. In equilibrium, liquid 44.3Ni-55.7Cr (wt.%) starts to solidify at 1371˚C. At this temperature solid α-Cr (32Ni-68Cr) starts to form (dendrites). Temperature drops and concentrations of phases follow liquidus and solidus. At eutectic temperature 1345˚C equilibrium exists between solid α-Cr (35Ni-65Cr) and liquid (47Ni-53Cr). According to lever rule, their weight fractions are 16% α-Cr and 84% liquid, i.e. the weight fraction of α-Cr is 16% and eutectic (α-Cr + γ-Ni) 84%. When cooling continues eutectic reaction takes place; liquid -> α-Cr and γ-Ni (this is interdendritic region). γ-Ni or α-Cr forms the matrix for interdendritic region. The one, which is not the matrix takes the form of, for example, plates (widmanstätten). At 650˚C eutectic matrix consists of 34% α-Cr and 66% γ-Ni in weight fractions. Overall, taking into account α-Cr primary dendrites, phase weight fractions in equilibrium at 650˚C are 37% α-Cr and 63% γ -Ni, which is in volume fractions 40% α-Cr and 60% γ-Ni.

Figure 80. X-ray diffraction patterns of SX-717: a) hot corrosion tested laser coating and b) as-laser-clad coating and c) powder. Distance from the coating/base material interface to the surface of the ground coating was 990 μm in both cases.