Residual stress measurements were conducted on Tribaloy T-800 and Stellite 21 laser coatings on different base materials. T-800 was laser clad on mild steel, martensitic and austenitic stainless steels 20 mm in thickness using preheat of ~500°C. Stellite 21 was laser clad on martensitic and austenitic stainless steels without preheat. To get the residual stress profiles by XRD method, material was removed by electrolytic polishing layer by layer until the base material was met and further. To verify the results obtained with XRD, hole-drilling method was applied on T-800 coating on martensitic and Stellite 21 on martensitic and austenitic stainless steels.
3.6.1 Tribaloy T-800 and Stellite 21
Residual stress profiles measured from T-800 on mild steel by XRD method using Laves phase peaks are illustrated in Figure 153. Owing to the manual surface grinding (~0.2 mm was removed from the as-laser-clad surface before measurement), large compressive residual stresses generated to a depth of 0.1 mm on the surface. Otherwise, the coating was slightly in tension (<+100 MPa). For instance, at the depth of 0.4 mm, average longitudinal residual stress of 5 measurements was +74 MPa (STDEV 31 MPa). The magnitude of tensile stress was not dependent on measuring direction in relation to cladding direction.
-1500
Figure 153. Residual stress profiles along a) longitudinal and b) transverse directions in relation to cladding direction measured from T-800 laser coating on mild steel.
Measurements were conducted with XRD method. The layer thickness removed by electrolytic polishing varied from 70 to 130 μm. Average removed thickness was 100 μm.
Similar to T-800 on mild steel, T-800 on martensitic SS was in tension except for the surface where the large compressive residual stresses (0.4 mm ground away) existed as shown in Figure 154. The magnitude of tensile stress is noticeably higher than in coating on mild steel.
It is also noteworthy to point out that hardened HAZ beneath the coating was in compression.
Base material hardness before cladding was ~340 HV1 and after ~750 HV1 in HAZ.
Volumetric expansion associated with hardening of HAZ may have increased the tensile stress in coating compared with coating on mild steel. There is also larger difference in CTEs between coating and base material in this case compared with mild steel. Error of measurement is clearly larger (larger difference between minimum and maximum values) compared with T-800 on mild steel. This can be attributed to the quality of etched/polished surface. Overlapped areas were more heavily etched in this case, which caused uneven surface. This can be, in turn, attributed perhaps to the more unevenly distributed Fe. The average Fe content of the coating on martensitic SS was 1.7% wt.%. Fe content was not measured from the coating on mild steel but microhardness values suggest that coating on mild steel was more heavily diluted since its average hardness was 680 HV1 compared with 820 HV1 in coating on martensitic SS. Low error of measurement obtained from the base material verifies this influence of surface condition on error since the etched base material surface was very smooth.
Figure 154. Residual stress profile along longitudinal direction measured from T-800 laser coating on martensitic stainless steel. Measurements were conducted with XRD method. The layer thickness removed by electrolytic polishing varied from 110 to 230 μm. Average removed thickness was 150 μm.
Residual stress profiles of T-800 on martensitic SS obtained via hole-drilling method showed also compressive stresses on the ground surface and tensile stresses deeper in the coating (Figure 155). Magnitude of tensile stresses is in accordance with the results obtained by XRD method. High tensile peaks under compressive surface result from counterbalancing stresses.
This zone was probably etched away in XRD measurements and not shown in profiles (Figures 153 and 154) due to larger steps between measurements (~100-150 μm vs. ~20-50 μm). Rotational angles (αr) between maximum principal stresses and stresses along cladding (longitudinal) direction varied from 20 to 30° depending on the depth.
Figure 155. Residual stress profiles measured from T-800 laser coating on martensitic SS.
Measurements were conducted using hole-drilling method.
Contrary to previous results, T-800 on austenitic SS was predominantly in compression as shown in Figure 156, whereas the base material beneath the coating was in tension. This can be attributed to the difference in CTEs between coating and base material. During cooling the base material with higher CTE underwent larger shrinking than coating causing compressive stress in it.
Figure 156. Residual stress profile along transverse direction measured from T-800 laser coating on austenitic SS. Measurements were conducted with XRD method. The layer thickness removed by electrolytic polishing varied from 80 to 240 μm. Average removed thickness was 150 μm.
For the sake of comparison average strain values (average of three measuring grids) obtained during hole-drilling for T-800 and Stellite 21 laser coatings are illustrated in Figure 157. They confirm the influence of CTE difference between coating and base material. Stress values calculated with Kockelmann method for the hole-drilled Stellite 21 gave values of 200-800 and 400-900 MPa on austenitic and martensitic base materials, respectively. Values are unrealistically high since yield strength of Stellite 21 is 517 MPa [400]. Calculation method
used is known to give too high values if stresses are higher than 60% of the yield strength.
According to XRD method, residual stresses on the surface of as-laser-clad Stellite 21 on martensitic and on austenitic stainless steels were ~ +500 MPa and ~ +100 MPa, respectively.
These values are in good agreement with values found from the literature and reported earlier in section 1.5.5. Average Fe content in Stellite 21 on martensitic SS was 1.2 wt.% and on austenitic SS 1.9 wt.%.
-100 0 100 200 300 400 500 600
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Depth (mm) Strain (10-6 )
St 21 on mart.
St 21 on aust.
T-800 on mart.
Figure 157. Average strain values recorded during hole-drilling process for different coating/base material pairs.