SURFACE MODIFICATION OF MARTENSITIC STAINLESS STEELS FOR HYDROTURBINE BLADES

Abstract

Hydropower stations are renewable, green and inexpensive source of power. The turbine is the main component in a hydropower station. The turbines are underwater components and operate in severe erosive condition. The blades of turbines are mainly fabricated from 13Cr-4Ni (13/4MSS) and 16Cr-5Ni (16/5MSS) martensitic stainless steels. During operation the blades are exposed to slurry erosion and cavitation which severely damages it inspite of 13/4MSS and 16/5MSS possessing high toughness, high strength and excellent corrosion resistance properties. Repair welding is done to restore the blades which requires shutdown of the plant. This leads to reduction in the efficiency and productivity of the hydropower plant. Slurry erosion is mainly caused by silt particles in the Himalayan rivers in India. The silt is mainly comprised of silica (SiO2) which has a hardness ranging from 800-1100 VHN. The hardness of 13/4MSS is in the range of 300-350 VHN, while that of 16/5MSS is in the range of 350-400 VHN. The hard silt particles easily erode the soft 13/4MSS and 16/5MSS leading to material removal and this type of erosion is termed as slurry erosion. Cavitation is due to generation of vapour/gas bubbles in the low pressure zone and they implode in the high pressure zone. The implosion releases high velocity microjets of water which impact the blades of the turbine and generate stress. The high level of stress leads to material removal and this type of erosion is commonly known as cavitation. Researchers have attempted to improve the slurry erosion resistance and cavitation resistance of 13/4MSS by surface modification techniques, proper heat treatment, thermomechanical processing and friction stir processing. Surface modification of 13/4MSS includes depositing hard coating by thermal spray process, laser surface treatment and nitriding treatment. Voith and Aldritz who are world leaders apply patented coating very similar to WC-10Co-4Cr on the blades of turbine by high velocity oxy fuel (HVOF) process to enhance the slurry 6 erosion resistance and cavitation resistance. Researchers have carried out erosion studies and cavitation studies of WC-10Co-4Cr coated 13/4MSS. Plasma nitriding of 13/4MSS and its cavitation studies has been reported. Researchers have studied the slurry erosion behaviour and cavitation of 16/5MSS by proper heat treatment, surface modification technique and thermomechanical processing. It is observed that the erosion and cavitation studies of 16/5MSS are scarce. Low temperature salt bath nitriding of many types of stainless steel has been reported. However, it hasn’t been reported for 13/4MSS and 16/5MSS. It is a thermochemical process by which N atoms are diffused in the metal substrate which leads to increase in the hardness and corrosion resistance of the metal and as a result increases the life-span of the components. Slurry erosion test is conducted in laboratory by slurry pot tester or jet type test rig. Slurry pot tester produces results with good repeatability and has been used by many researchers to study slurry erosion behavior of materials. Cavitation test is carried out in laboratory as per ASTM G-32 standard by probe sonicator. It is very reliable equipment to conduct cavitation studies in laboratory and has been used by many researchers for cavitation studies. In view of the above mentioned facts, it was found that low temperature salt bath nitriding of 13/4MSS and 16/5MSS followed by slurry erosion and cavitation studies have not been conducted. In the present work low temperature salt bath nitriding of heat treated 13/4MSS (13/4HTT) has been carried out at 450ºC (N450) and 500ºC (N500) for 10h each. In separate batch process, low temperature salt bath nitriding of heat treated 16/5MSS (16/5HTT) was done at 450ºC (16/5M450) and 500ºC (16/5M500) for 10h each. The characterization of nitrided 13/4HTT and 16/5HTT was done by optical microscopy, x-ray diffractometry, energy dispersive spectroscopy, and field emission scanning electron microscopy (FESEM). 7 It is also observed that slurry erosion of nano sized TiC and micron sized TiC mixed WC-10Co-4Cr applied by high velocity oxy fuel process on 13/4MSS has not been studied. Slurry erosion test was done by slurry pot tester. Silica was used as erodent. The concentration of erodent in the slurry was 10%. The relative speed between the erodent particle and the specimen was 4.55m/s. The test was conducted for 48h and weight loss was measured after every 6h. The worn surface was examined under FESEM and surface roughness was measured by surface profilometer. The erosion mechanism is correlated with the microstructure and mechanical properties. It was observed that low temperature (450° C) salt bath nitriding enhances the slurry erosion resistance of 13/4MSS. The weight loss of N450 after 48h slurry erosion test was found to be 83% less than that of 13/4HTT. However when the salt bath nitriding was done at 500° C, then the weight loss after 48h of slurry erosion test was found to be 92% more than that of 13/4HTT. Low temperature (450° C) salt bath nitriding also enhances the slurry erosion resistance of 16/5MSS. The weight loss of 16/5M450 after 48h slurry erosion test was found to be 73% less than that of 16/5HTT. However when the salt bath nitriding was done at 500° C, then the weight loss after 48h of slurry erosion test was found to be 6.7% more than that of 16/5HTT. Cavitation test was done as per ASTM G-32 standard. The frequency for the test was set at 20 kHz with peak to peak displacement amplitude of 50 μm. The temperature of the water was set at 25±2ºC and test was conducted for 12h and weight loss was periodically measured after 1h. The worn surface was investigated under FESEM and surface roughness was measured by surface profilometer. The cavitation mechanism is correlated to microstructure and mechanical properties of the specimen. It is observed that low temperature salt bath nitriding treatment enhances the cavitation resistance of 13/4MSS. The weight loss of N450 after 12h of cavitation erosion test was found to be 8 58% less than that of 13/4HTT. However when the salt bath nitriding was done at 500° C, then the weight loss after 12h of slurry erosion test was found to be 48% more than that of 13/4HTT. Low temperature (450° C) salt bath nitriding also enhances the cavitation resistance of 16/5MSS. The weight loss of 16/5M450 after 12h of cavitation test was found to be 31% less than that of 16/5HTT. However when the salt bath nitriding was done at 500° C, then the weight loss after 12h of cavitation test was found to be 7% more than that of 16/5HTT. An attempt has also been done to enhance the erosion resistance of 13/4 and 16/5MSS by applying nano sized TiC and micron sized TiC mixed WC-10Co-4Cr coatings on 13/4 MSS. The slurry erosion behaviour of nano sized TiC mixed WC-10Co-4Cr and micron sized TiC mixed WC-10Co-4Cr coated steel is compared with bare 13/4MSS and WC-10Co-4Cr coated steels. The erosion behaviour of the modified coating was analysed by structure property correlation. The nano sized TiC mixed WC-10Co-4Cr coating and micron sized mixed WC-10Co-4Cr coating has higher slurry erosion resistance in comparison to bare 13/4MSS, however the two modified coatings have less slurry erosion resistance in comparison to conventional WC-10Co-4Cr coating. The thesis is organized into 7 chapters. Chapter 1 presents the introduction of the thesis and briefly gives the insight of current scenario of total hydropower generation in the world and India, the problem faced by the erosive wear of the blades of the hydroturbines and methods to combat slurry erosion and cavitation. It also presents the work done by various researchers to enhance the erosion resistance of 13/4MSS and 16/5MSS. In the end work done in the present study is discussed briefly. Chapter 2 presents extensive literature survey related to the erosive wear of 13/4MSS and 16/5MSS and measures to reduce it. It gives brief insight of the hydropower plant, types of turbines, material used for fabricating blades of turbines and their evolution, types of erosion 9 encountered by the blades of turbines, erosion mechanism, salt bath nitriding process and high velocity oxy fuel process. The chapter also presents the various techniques employed by various researchers to enhance the slurry erosion resistance and cavitation resistance of 13/4MSS and 16/5MSS. From the extensive literature survey, the gap has been identified, problem formulation has been done and the objective of the present work is finalized. Chapter 3 illustrates the detailed experimental procedure followed for i) Low temperature salt bath nitrided 13/4HTT and 16/5HTT. ii) Slurry erosion and cavitation test of low temperature salt bath nitrided 13/4HTT and 16/5HTT. iii) Development of a) nano sized TiC mixed WC-10Co-4Cr coating applied on 13/4MSS by HVOF process and b) micron sized TiC mixed WC-10Co-4Cr coating applied on 13/4MSS by HVOF process iv) Slurry erosion of nano sized TiC mixed WC-10Co-4Cr coating, micron sized TiC mixed WC-10Co-4Cr coatings, WC-10Co-4Cr and bare 13/4MSS. Chapter 4 outlines the results and discussion of i) Slurry erosion test of low temperature salt bath nitrided 13/4HTT ii) Cavitation test of low temperature salt bath nitrided 13/4HTT The characterization details, structure property correlation with slurry erosion and cavitation mechanism of nitrided 13/4HTT are discussed in details Chapter 5 presents the results and discussion of i) Slurry erosion test of low temperature salt bath nitrided heat treated 16/5MSS (16/5HTT) ii) Cavitation test of low temperature salt bath nitrided heat treated 16/5HTT 10 The characterization details, structure property correlation with slurry erosion and cavitation mechanism of nitrided 16/5HTT are discussed in details. Chapter 6 presents the results and discussion of nano sized TiC mixed WC-10Co-4Cr and micron sized TiC mixed WC-10Co-4Cr coating deposited on 13/4MSS by HVOF process and slurry erosion test. Chapter 7 presents the overall conclusions of the present work. It also briefly highlights the future scope of work which can be performed by potential researchers to enhance the erosion and cavitation resistance of 13/4MSS and 16/5MSS.