The steam inlet edge of the last stage blade of the steam turbine is very susceptible to cavitation damage during service, resulting in size loss and affecting the overall operation of the unit. In order to repair the size of the turbine blade and increase its service life, the laser cladding technology was used to prepare a cladding layer of the same material on 17-4PH steel, and post-heat treatment was used to improve its structure and improve its cavitation resistance. The 17-4PH cladding layer was directly aged at 530, 580 and 630 ℃ in a high-temperature box furnace, and the structure was characterized and analyzed using OM, XRD, SEM and matching EBSD, and the cavitation resistance of the sample was tested by an ultrasonic system. The results show that after direct aging treatment, the residual stress in the cladding layer is released, the austenite content in the structure increases, and a large amount of precipitation phase is precipitated. The cavitation cumulative mass loss of the cladding layer first increases and then decreases with the increase of aging temperature: after treatment at 530℃, the cavitation cumulative mass loss of the sample is the lowest, only 60.6 mg, which is about 58% less than the deposited state 145.4 mg. In addition, the cavitation surface roughness is well improved after aging treatment, and the best performance is 180 μm in the 580 ℃ sample, showing good cavitation resistance. By using the appropriate temperature direct aging treatment for the 17-4PH cladding layer, it can effectively promote the homogenization of the organization and improve the cavitation resistance, providing important engineering guidance for the cavitation damage repair of the last stage blade of the turbine.

17-4PH (0Cr17Ni4Cu4Nb) is a martensitic precipitation aging steel. After solution aging treatment, it can form small and uniformly distributed second phase particles in the matrix, thereby giving it excellent plasticity, toughness and corrosion resistance. This makes 17-4PH alloy widely used in many fields such as gas turbines and steam turbines [1]. In steam turbine units, 17-4PH is usually selected to manufacture the last stage blades of steam turbines. These last stage blades are exposed to the harsh environment of high temperature and humidity for a long time during operation and are prone to cavitation damage [2-3]. When the damage reaches a certain extent, it may cause failure problems such as blade deformation and fracture, causing them to be scrapped and replaced, which in turn causes significant economic losses. Therefore, taking appropriate repair measures to extend the service life of the blades and prevent further fatigue fracture is crucial to ensure the long-term stable operation and environmental protection of the blades.

At present, many scholars at home and abroad have studied various surface repair technologies to extend the service life of blades, such as brazing, thermal spraying [4], arc welding [5] and laser cladding [6-7]. Among them, laser cladding technology is highly favored in the repair field due to its high precision, small heat affected zone, low thermal deformation and strong metallurgical bonding with the substrate. To this end, this paper will use laser cladding technology and use 17-4PH powder with the same composition as the blade as the cladding material, aiming to restore the size of the damaged turbine blade, avoid direct scrapping, and realize the recycling of the blade.

However, due to the unique rapid heating and cooling characteristics of laser cladding technology, it is easy to cause uneven internal structure of the cladding layer, and defects such as pores, inclusions and residual stress may appear, which will significantly reduce the mechanical properties of the coating. Existing studies have shown [8-9] that subsequent heat treatment can effectively improve the microstructure of the cladding layer and optimize its organizational properties. However, at present, domestic and foreign research on the heat treatment of additively manufactured 17-4PH stainless steel is still insufficient, and the main research focuses on solid solution + aging heat treatment [10]. Li Yangyang [11] conducted an in-depth study on the organization and properties of the 17-4PH cladding layer before and after solid solution aging treatment. The research results show that subsequent heat treatment can make the cladding layer structure better homogenized and precipitate dispersed precipitation particles, thereby effectively improving the tensile strength of the cladding layer. However, this treatment has limited effect on improving the plasticity and toughness of the cladding layer, and even caused a decrease in elongation. This result is consistent with the conclusion drawn by LASHGARI et al. [12] when the 17-4PH additive sample was subjected to solution aging. However, in the research of ESKANDARI and SABOONI et al. [13-14], it was found that compared with the 17-4PH additive parts treated with traditional solution + aging, the direct aging treatment caused the cladding layer to retain a higher volume fraction of reversed austenite. This can not only improve the strength of the sample, but also provide better plasticity and toughness, forming a strong and tough cladding layer. Related research shows [15] that this reversed austenite strong and tough sample will absorb the impact energy during the cavitation process to induce martensitic phase transformation, which helps to reduce the brittle fracture phenomenon during the cavitation process and improve the cavitation resistance. However, there is no specific research on the effect of direct aging treatment on the cavitation resistance of 17-4PH additive samples. In view of this, this paper will study the influence of direct aging treatment on the microstructure and cavitation resistance of the 17-4PH cladding layer, and further explore the appropriate aging temperature for improving the cavitation resistance of the cladding layer, providing a reference for the repair of the low-pressure last stage blade of the turbine.

1 Materials and methods
The test substrate is forged steel used for 17-4PH blades, with a size of 120 mm×60 mm×20 mm. The cladding powder is 17-4PH powder prepared by water atomization, with a particle size of 50-150 μm and its chemical composition as shown in Table 1. The test specimens were prepared by laser cladding using the LDF400-2000 fiber-coupled semiconductor laser generated by Laserline, Germany, with a spot diameter of 4 mm. The optimized preparation process parameters were: laser power P=1800 W, scanning speed v=6 mm/s, powder feeding rate of 8 g/min, and protective gas argon flow rate of 15 L/min. In addition, in the process of preparing the cladding coating, the laser scanning method adopted unidirectional parallel scanning with an overlap rate of 40%. The specific scanning strategy is shown in Figure 1a.

The deposited samples were directly aged in a high-temperature box furnace. The temperature range of the direct aging treatment in this study was referenced to the relevant literature [16-17]. The specific process parameters are shown in Table 2.

The deposited and heat-treated samples were coarsely ground and polished with water-abrasive sandpaper to obtain a smooth surface. After grinding and polishing, FeCl3 reagent (10 g FeCl3 + 20 mL HCl + 100 mL CH3CH2O) was used for corrosion treatment for 30 to 45 s. The microstructure morphology of each group of samples was observed using a Sunny Optical Metallographic Microscope (OM) and a German Zeiss EVO 18 multi-function scanning electron microscope (SEM). The X-ray diffractometer (XRD) used was the Empyrean series manufactured by PANalytical of the Netherlands, equipped with a PIXcel 1D array detector (Cu Kα, λ = 0.1541 nm). The working voltage and current were set to 40 kV and 40 mA respectively, the scanning range was set between 20° and 90°, and the scanning step length was 0.02°. Electron Backscattered Diffraction (EBSD) test specimens were prepared by electropolishing. The electropolishing equipment was produced by Shanghai Mainuo Metal Surface Treatment Technology Co., Ltd. The electropolishing parameters included: the electrolyte was 3.5wt.% perchloric acid alcohol solution, the voltage range was 20-23 V, and the polishing time was 20-30 s. After the sample was prepared, the matching electron backscatter diffraction probe was used for image acquisition. The XOQS-2500 vibration ultrasonic cavitation system was used. The specific composition of the system is shown in Figure 2. The sample was placed in a 3.5wt.% NaCl solution at a constant temperature of 25 °C to test the cavitation resistance of the coating. The mass loss of the sample was measured by a Sartorius BT25S electronic balance to evaluate the cumulative effect of cavitation on the sample. In addition, the VK-1100 laser confocal 3D morphology measurement microscope produced by Keyence Corporation of Japan was used to measure the surface roughness of the sample after cavitation. The specific sampling position and preparation scheme of the cavitation test sample are shown in Figures 1b and 1c.

2 Results and Analysis
2.1 Microstructure
Figure 3 shows the microstructure of the 17-4PH cladding layer after direct aging treatment. In Figure 3a, an obvious columnar structure can be seen in the deposited structure, and a large number of carbide precipitation phases are distributed in the columnar crystals. Similar organizational characteristics were also observed in the 17-4PH deposited state after laser selective melting and laser solution treatment [18-19]. Figures 3b~3d show the OM microstructure of the cladding layer after direct aging treatment. It can be seen that the number of precipitated particles in the structure after aging at 530 °C has increased. However, since the heat treatment temperature is low at this time, it is not enough to provide sufficient energy to achieve complete segregation diffusion [20]. As a result, the columnar substructure formed in the cladding process can be retained to a certain extent. In contrast, when the aging temperature rises to 580 °C, the atomic diffusion capacity is enhanced, and the columnar crystal structure inside the cladding layer is thermally decomposed to form obvious lath martensite. Further, when the aging temperature increases to 630 °C, the lath martensite inside the organization coarsens, accompanied by an increase in the content of white tempered martensite. This phenomenon is mainly caused by the high aging temperature that prompts the transformation of martensite to austenite, and in the subsequent cooling process to room temperature, part of the austenite is transformed into martensite again, causing the content of tempered martensite to increase [21]. Figures 3e to 3h are SEM high-magnification microstructures of the 17-4PH cladding layer as deposited and after direct aging treatment. It can be seen that a large amount of white precipitate phase is precipitated on the surface of the sample after direct aging treatment. According to relevant studies [22-23], these dispersed fine precipitates can stably absorb the energy released when the cavitation bubble collapses, thereby effectively slowing down the material loss and improving the material’s anti-cavitation performance.

In order to further explore the chemical composition of the precipitates in the sample, this study used EDS point scanning technology to conduct a detailed analysis of the white precipitates in the sample. The specific scanning point position is shown in Figure 4a, and the corresponding EDS energy spectrum analysis results are shown in Figure 4b. The results show that these white precipitates are mainly precipitates containing Nb. In view of the precipitation hardening characteristics of 17-4PH martensitic stainless steel, it can be seen that in the subsequent aging process, the Nb element originally dissolved in the matrix often precipitates in the form of NbC [19]. Based on this, it can be inferred that these white precipitates are NbC particles. In addition, the aging process will also cause a certain amount of ε-Cu precipitation inside the 17-4PH organization, but because the size of the ε-Cu precipitates is generally at the nanometer level, these precipitates are difficult to be directly observed.

2.2 Phase analysis
The XRD spectra of the samples treated at different aging temperatures are shown in Figure 5. The main phase of each sample in the spectrum is α-phase martensite structure. When the aging temperature rises to 580 and 630 °C, the diffraction peak of austenite (γ-Fe) in the cladding layer shows a significant enhancement trend. In addition, in this temperature range, the diffusion rate of elements increases, resulting in changes in crystal orientation. According to the corresponding spectrum, it can be clearly seen that the peak of the α phase of the cladding layer shifts from (110) to (200). Combined with the above microstructure analysis, it can be seen that this is caused by the tempering martensitic transformation inside the structure. In addition, after direct aging treatment, the sample showed strong copper and austenite diffraction peaks. This is due to the agglomeration and precipitation of the copper-rich phase, which is desolvated from the cladding layer matrix. After aging treatment, a strong diffraction peak of the copper-rich phase appears [19].

In order to further analyze the phase composition inside the structure, EBSD test was carried out on the sample. Figures 6a to 6d are the inverse pole figures (IPF) of the five groups of samples. It can be seen that most of the grain orientations in the deposited samples tend to grow in the <001> direction. This is because during the solidification of the molten pool, the grain growth direction preferentially grows along the direction of the molten pool temperature gradient [21], and the <001> direction is consistent with the molten pool temperature gradient, has a lower surface energy and a higher growth rate, so the grains are more likely to grow in this direction. Such a growth tendency will cause the cladding layer to show anisotropy in performance, and the growth of grains in the same direction will cause inconsistent thermal expansion and contraction inside the material, forming residual stress [24]. In contrast, the grain orientation of the samples treated with aging does not have an obvious preferential orientation. This phenomenon shows that direct aging heat treatment can effectively reduce the internal heterogeneity of the cladding layer and reduce the internal residual stress. Kernel average misorientation (KAM) is often used to measure the degree of orientation mismatch between grains inside the material. Its value can usually reflect the distribution of stress and strain inside the organization. The average local orientation differences of the four groups of samples shown in Figures 6e to 6h are 2.06°, 1.70°, 1.72°, and 1.58°, respectively. This difference is due to the significant thermal stress usually introduced during the laser cladding process, resulting in a large lattice distortion in the deposited state as shown in Figure 6e. However, in the subsequent heat treatment process, the lattice mismatch is significantly improved, which further indicates that the residual stress inside the cladding layer is effectively released after direct aging treatment. Figures 6i to 6l are the corresponding phase distribution diagrams. It can be seen that in the 17-4PH cladding deposited state, the austenite content is low, only about 1%. As the aging temperature rises to 530, 580, and 630 °C, the austenite content increases to 4.7%, 5.4%, and 4%, respectively. This increase in content is mainly due to the formation of reversed austenite in the structure, which is related to the segregation of austenitizing elements during the cladding process [25]. Since the rapid cooling process of cladding usually causes element segregation, the segregation of austenite stabilizing elements (such as Ni) will reduce the martensite start temperature (Ms) of the local area, thereby improving the stability of the reverse austenite in these areas at room temperature. According to relevant research [26], these small-sized reverse austenites can exist as toughening phases, while improving the yield strength and tensile strength, ensuring that the material has good plastic toughness. This has better resistance to deformation during cavitation impact.

2.3 Microhardness analysis
As shown in Figure 7, the average microhardness of each group of samples. With the increase of aging temperature, the hardness of the cladding layer first increases and then shows a decreasing trend. The average hardness of the sample in the aging treatment at 530 ℃ reaches a maximum value of 423 HV0.3, which is about 23% higher than the deposited 346 HV0.3. However, when the aging temperature rises to 580 and 630 ℃, the average hardness of the cladding layer begins to decrease. This change is mainly related to the coarsening of the precipitate phase particles under high temperature conditions.

2.4 Cavitation resistance test and mechanism study
As shown in Figure 8a, the cumulative loss mass of each sample after 9 h cavitation test, the comparison shows that the maximum cumulative mass loss of the deposited sample is 145.4 mg, showing the worst cavitation resistance. In contrast, the cavitation resistance of the samples after aging treatment is greatly improved, and the cumulative loss mass shows a trend of first decreasing and then increasing with the increase of aging temperature: after aging treatment at 530 and 580 ℃, the cavitation mass loss of the samples is reduced to 60.6 and 69.8 mg respectively, which is more than twice the cavitation resistance of the deposited state. However, when the aging temperature is further increased to 630 ℃, the loss amount increases to 79.9 mg. From the perspective of cumulative mass loss, the cavitation resistance of the samples decreases in the following order: 530 ℃＞580 ℃＞630 ℃＞untreated deposited state. Figure 8b shows the mass loss rate curve of the samples. It can be seen that the mass loss rate of the samples after aging treatment is significantly reduced compared with the deposited state, and its average mass loss rate is only about half of that of the deposited sample, and the cavitation resistance is significantly enhanced.

Figure 9 shows the surface morphology of the deposited sample after cavitation for different lengths of time in 3.5wt.% NaCl solution. After 3 h of cavitation, as shown in Figure 9a, visible material peeling appeared on the surface of the sample, and the surface damage was serious. This peeling phenomenon was mainly due to the high residual stress introduced by laser cladding. The research of GAO et al. [27] showed that residual stress would reduce the cavitation resistance of the material, resulting in stress concentration in the “weak area” on the surface of the sample under cavitation collapse, which in turn caused the peeling of the surface material and the generation of cracks. As the cavitation was extended to 6 h, the surface damage of the sample became more serious due to the impact of the continuous microjet. This phenomenon was attributed to the influence of the lower austenite content in the deposited material. Since the lower austenite content only provided limited plastic toughness, it was difficult to withstand the impact force during the cavitation process, resulting in rapid surface deterioration and further brittle fracture, as shown in Figure 9b. As the cavitation progressed to 9 h, the surface fracture phenomenon became more serious, resulting in more obvious material mass loss. And under high magnification observation, the loss caused by the cavitation extends along the inside of the sample, forming deeper cavitation pits, which overall shows that the laser cladding deposition state has relatively poor cavitation resistance.

Figure 10 shows the surface morphology of the samples treated with direct aging after different cavitation times. As shown in Figure 10a, since the 530 ℃ aged sample failed to obtain a good homogenization treatment, a small area of ​​pits can still be observed on the surface after 3 h of testing. The samples aged at 580 and 630 ℃, thanks to the release of residual stress and the plasticity and toughness of austenite, effectively delayed the cavitation incubation period, thereby inhibiting the formation of pits and cracks in the early cavitation process. However, when the cavitation test reached 6 h, a pit group formed by the interconnection of many small pits was visible on the surface of the sample aged at 630 ℃, as shown in Figure 10h. This is because the hardness of the sample decreased at this aging temperature, causing its cavitation resistance to be weakened. In contrast, the surface of the samples treated at 530 and 580 °C did not change much, showing only slight deformation. As cavitation continued to 9 h, as shown in Figure 10i, the coarsening of the precipitated phase reduced the resistance to dislocation movement, aggravated cavitation damage, and formed large pits, while the weight loss rate of the samples at 530 and 580 °C was stable, the surface was relatively intact as a whole, and only small pits and surface micro-deformations appeared, indicating that the samples treated at this temperature had a high resistance to cavitation damage.

The surface roughness of the four groups of samples after cavitation for 9 h was quantified by laser confocal microscopy, and the results are shown in Figure 11. It can be seen that a large area of ​​irregular peeling area appeared on the surface of the deposited sample, especially at the overlap of the cladding layer (the black dotted line in the figure), as shown in Figure 11a. This is because the overlap area undergoes melting and resolidification during the laser cladding process, and tends to form higher residual stress, which makes the area prone to stress concentration during the test, resulting in higher cavitation mass loss. As the aging temperature increases, these residual stresses are better released, significantly improving the surface damage morphology, especially for the sample aged at 580 °C. As shown in Figure 11c, its surface roughness decreases from 452 μm in the deposited state to 180 μm, showing good cavitation resistance. However, when the aging temperature is further increased to 630 °C, the surface roughness of the sample increases again to 257 μm. The cavitation resistance of the sample under this temperature condition shows a downward trend. This phenomenon has been analyzed in the previous article. It is believed that the reason is that the excessively high aging temperature causes the hardness of the sample to decrease, thereby weakening its cavitation resistance.

Figures 12a to 12c show the evolution of 17-4PH deposited state at different stages of cavitation. According to the above analysis, on the surface of the deposited sample, especially in the overlap area, due to the presence of high residual stress, when the cavitation collapses, a significant stress concentration will be formed in this area, which will trigger the peeling of the surface material and the formation of cracks. With the continuous cavitation impact, these cracks will further expand along the grain boundaries and deep in the organization, eventually leading to a large mass loss of the material and forming a significant cavitation pit.

After aging treatment, the residual stress inside the cladding layer is well released, which reduces the probability of forming a “weak mouth” during cavitation impact. At the same time, the aging treatment increases the austenite content inside the organization, thereby enhancing the resistance of the sample to impact deformation in the early stage of cavitation, resulting in only slight plastic deformation on the surface of the sample. In addition, the precipitation caused by the aging treatment forms an effective barrier inside the cladding layer, limiting the inward expansion of cavitation cracks, thereby improving the material’s cavitation resistance. However, it is worth noting that when the aging temperature is too high, it will cause the precipitate particles to coarsen, which will in turn weaken the cavitation resistance of the sample.

2.5 Physical repair of cavitation damaged blades
Finally, in order to verify the feasibility of this repair scheme, a turbine blade with local cavitation damage on the surface during actual operation was used as the object of the repair experiment. As shown in Figure 13a, the original state of the damaged blade is observed. It can be seen that many tiny pits are formed on the steam inlet side of the blade due to the impact of water droplets. If these damages are not dealt with in time, more serious material shedding problems will occur. To address this problem, this study first removed part of the material from the damaged surface of the blade and repaired it using laser cladding technology. As shown in Figure 13b, the initial morphology of the blade after laser cladding repair, a good metallurgical bond is formed between the cladding layer and the blade, which shows that the repair process can better meet the size recovery of the blade. Figure 13c shows the surface morphology of the blade after heat treatment and polishing in the furnace after repair. To further ensure the quality of the repair, the repair layer is subjected to X-ray nondestructive testing, and the test results are shown in Figure 13d. It can be seen from the X-ray flaw detection in the figure that there are no obvious crack defects in the repair area, which proves that this repair technology can better meet the needs of blade repair.

3 Conclusions
The effects of direct aging temperature on the microstructure, phase structure and cavitation damage of 17-4PH laser cladding layer were studied. The main conclusions are as follows:

(1) Direct aging treatment promotes the gradual decomposition of the columnar substructure inside the cladding layer with the increase of temperature, forming an obvious lath martensite structure and precipitating a large number of NbC precipitate particles, which can effectively improve the hardness of the material and hinder the generation and expansion of cracks during cavitation.

(2) Direct aging treatment promotes the homogenization of grain orientation and the release of residual stress. This reduces the stress concentration phenomenon in the cavitation process and the probability of the occurrence of cavitation “weak areas” on the surface of the sample. At the same time, the increase in the austenite content inside the cladding provides better impact deformation ability and effectively delays the impact damage of cavitation.

(3) After aging treatment at 530℃ and 580℃, the cavitation resistance of the 17-4PH cladding layer is significantly improved, and the cumulative mass loss is reduced to 60.6 mg and 69.8 mg, respectively, which is more than half of that of the deposited state. At the same time, it shows a lower mass loss rate. In addition, after aging treatment at 580℃, the surface roughness of the sample is reduced to the minimum, from 452 μm in the deposited state to 180 μm.

(4) Laser cladding plus subsequent direct aging heat treatment technology can be effectively applied to the physical repair process of the last-stage blade. There is a good metallurgical bonding between the cladding repair layer and the blade, and no obvious crack defects are found inside the repair layer.