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.