2.3 | PSP treatment
One PSP treatment system in Figure 3(a) is established for turbine
blades. And the detailed parameters of the PSP treatment are listed in
Figure 3(b). The PSP treatment is conducted by two bucket-type enclosed
pneumatic machines and one air compressor, using the cast steel
particles with hardness from 45 to 48 HRC. These steel particles are
sprayed onto the serrations of turbine blades to produce the residual
compressive stress.
Before the CCF tests, the hardness and surfaces in the PSP treatment
area of the untreated and PSP treated blade are investigated. The
surfaces of turbine blades are observed by the scan electric microscope
(SEM). As shown in Figure 4(a), there are lots of machining traces left
on the surfaces of untreated blades. However, the machining traces are
eliminated and many surface pits form after the PSP treatment due to the
shooting of steel particles (Figure 4(b)).
The surface hardness (HV0.1) of the critical location is
examined by the Vickers hardness tester (Figure 4(c)). The result
indicates that the hardness is greatly improved by the PSP treatment
within the depth of 180um from the top surface. Besides, Figure 4(d)
shows the residual stress at the critical location examined by the
X-ray. After the PSP treatment, there is a high-level of compressive
residual stress and the maximum residual stress of which is about 60um
deep from the top surface. According to the curves in Figure 4(c) and
(d), the PSP treatment effect depth is about 180-190um.
The microstructure of K403 superalloy at the critical position before
the CCF tests is shown in Figure 4(e) and (f). The obvious plastic
deformation can be observed on the surface after the PSP treatment.
Moreover, a number of carbides with cracks form near the surface. It
indicates that the plastic deformation and large stress resulting from
the PSP treatment may cause the fragmentation of carbides due to the
brittleness.