Kamaraj M

Hydropower generation from the Himalayan rivers in India face challenge in the form of silt-laden water. These sediments contain abrasive particles which can erode the turbine blades and reduce turbine life. This calls for the development of newer materials for turbine blade. To address this issue in the present investigation, 16Cr- 5Ni martensitic stainless steel has been selected and coated with polyurethane (PU) reinforced with boron carbide (B4C) nano particles to improve the wear resistance. With the increase of B4C content (0-2 wt %) in PU the mechanical properties and erosion wear resistance were investigated. The Shore hardness and pull off adhesion were found to increase with the increased content ofB4C nano particles and from contact angle measurement the coated surfaces are shown to be hydrophilic in nature. This condition reflects better wetting and may be good for cavitation wear resistance. Slurry erosive wear tests were done at various test conditions determined by Taguchi design of experiments of impact velocity, impingement angle, erodent size and slurry concentration. The erosion area of the PU coated samples were analyzed with scanning electron microscope (SEM) and the erosion wear mechanism is discussed Analysis of variance studies of erosion rate indicated that B4C content in PU material is the single most important parameter and interaction of impact velocity and impingement angle are proved to be significant Artificial Neural Network and Genetic Algorithm were employed to arrive at the worst possible scenario.

The cavitation erosion resistance ofl 6Cr5Ni martensitic stainless steel used in hydro turbine components has been studied in a vibratory cavitation test rig as perASTM G32 standard Three stages of rate of mass loss, surface roughness and the retained austenite content with cavitation time has been studied. Quantitative 3D surface roughness parameters such as average deviation (Ra), standard deviation ( Rq), mean roughness depth(Rz) and change in surface volume with cavitation time has been studied using confocai laser scanning microscope with a view to identify the damage mechanisms in these steels. The correlation between roughness profde parameters and the transition between the incubation and acceleration stage was established The present study indicates that the 3D surface parameters can become a useful tool to monitor the progress in cavitation damage.

The objective of the study was to carry out creep and creep crack growth studies on as received and aged steels with a view to develop a method for estimating remaining life of CrMoV steel components when a crack develops during service. In service, the material is subjected to elevated temperatures, which results in aging due to transformation of bainitic structure, nucleation and growth of carbides and changes in their morphology. This results in degradation in mechanical properties such as hardness, tensile strength and creep properties. Therefore, it was proposed to study the creep behaviour of as-received and aged steels and understand the role of microstructures in creep deformation and fracture of this steel and use this understanding to predict the remaining life of turbine components. The microstructural condition of the service exposed material was simulated by accelerated aging carried out at 873K (600°C) for 3648 hours which is equivalent to the material condition exposed at 813K (540°C) for 200,000h. Creep and creep crack growth (CCG) testing on as received (normalised and tempered) and aged 1Cr1Mo1/4V rotor forging steel has been carried out at 773, 813, 848 & 873K (500, 540, 575 and 600°C) at four different stress levels in each material condition in the range of 110 to 350 MPa. Results of 873K (600°C) only shall be discussed in this paper in detail. Single edge notch tension (SENT) type of specimens were used for creep crack growth testing.

Extensive creep testing was carried out on 1Cr1Mo1/4V low alloy forging and casing steels in as received (normalized and tempered) and aged condition. Both the steels exhibited considerable secondary and tertiary stages in the creep tests conducted in the temperature range between 813 and 873 K (540°C and 600°C). The primary stage, though present, was rather negligible in higher temperature ranges. Casting steel showed wedge type cavities which grew as cracks along the grain boundaries. In the case of forged steel, the voids were elliptical and flat which developed during the tertiary stage of creep deformation. Tertiary creep deformation and creep ductility of the two steels investigated have been analyzed based on the type of voids developed during the tertiary stage. Based on detailed microstructural investigations using scanning and transmission electron microscope, prevailing damage mechanisms such as structural transformation, particle coarsening, grain boundary thickening have been identified and applied for remaining life assessment of the two steels. Hardness and other mechanical properties were observed to decrease slightly on aging; both for rotor forging and casing casting steel. The softening occurred due to dissolution of M3C and Mo2C carbides and coagulation of others resulting in reduced creep strength. Gradual fall of creep strength at intermediate aging times was due to recovery in ferrite, gradual depletion of solid solution carbides from the ferrite matrix, metastability and transitional character of precipitated carbides. Creep crack growth studies have also been carried out on both the steels. Life assessment calculations have also been carried out using creep crack growth methodology. © 2013 The Authors.

The nano tungsten carbide particles were produced by wire explosion process (WEP) in different carburizing mediums. Certain physic-chemical diagnostic studies (including Wide angle X-ray diffraction studies, Fourier transform infra red spectroscopy, UV analysis) were carried out to understand the characteristics of nano tungsten carbide powder produced by wire explosion process. TEM analysis indicates that tungsten carbide particle are spherical in shape and the size distribution follows log-normal distribution. The size of the particles could be controlled by varying the operating pressure and by varying the deposited energy. The high speed photograph obtained during the explosion process indicates that the activity of nano particle formation is larger in methane-paraffin ambience, while compared to the process of nano particle formation in pure methane ambience. © 2010 IEEE.

Hydropower generation faces challenge in withdrawal of clean water from sand-laden rivers. This may lead to silt erosion and which, when compounded with cavitation-induced erosion of turbine blades, may lead to unexpected shutdowns. To address this issue, we have developed novel polyurethane (PU) coatings reinforced with boron carbide (B4C) or silicon carbide (SiC) nanoparticles on 16Cr-5Ni martensitic stainless steel substrate to improve resistance to silt and cavitation erosion. To the best of our understanding, the synergy between silt and cavitation erosion behaviors cannot be simulated with the available ASTM or non-standard test rigs. To overcome such shortcoming we developed a new test facility, where the submerged slurry jet impinges upon a stationary specimen, and focused ultrasonic transducer is used for cavitation erosion studies and this facility explores the possibility of synergetic effect of silt-cavitation erosion mechanisms. An interesting finding from the results is that there is an optimum amount of nanoparticles (20 wt.% B4C and 10 wt.% SiC) with respect to silt erosion and (10 wt.% B4C and 2 wt.% SiC) with respect to cavitation erosion, where mass removal is the minimum. Comparison of pure silt, pure cavitation and combine silt-cavitation erosion characteristics obtained using this experimental test facility, demonstrate enhanced erosion in case of combined silt-cavitation studies. This has been explained in terms of surface characteristics of coatings using scanning electron microscopy (SEM) images.

The wear behaviour of a sintered Fe-based alloy reinforced with mechanically alloyed intermetallic has been studied using pin-on-disk wear testing machine in dry condition. A sintered iron alloy made by powder metallurgy was used as matrix. The intermetallics Fe3Al particles prepared by mechanically alloying (15 hours) have been used as reinforcement (5-10%). The processing of the composites includes mixing and cold compacting followed by sintering at 1120°C in Nitrogen atmosphere. The presence and distribution of intermetallic Fe3́Al phase on the surfaces were identified with optical microscopy and further confirmed by XRD. The influence of mechanically alloyed Fe3Al intermetallics additions on the wear behaviour was studied with different sliding conditions. The experimental results indicate that improvement in wear properties in MMCs compared to plain sintered steel as standard material. Wear tracks were characterised by SEM techniques. Studies reveal that unreinforced sintered steel shows lower wear resistance, while composites with mechanically alloyed Fe3Al are the most wear resistant. Copyright © 2004 by Society for Advancement of Heat Treatment & Surface Engineering (SAHTSE).

Aluminum alloy composites are extensively used for tribological applications due to excellent wear resistance especially during sliding under different operating conditions such as dry, wet and high temperature environments. In this present study, dry sliding wear behavior of aluminum silicon alloy reinforced with graphite (3 wt.%) was investigated by using Pin on Disc wear tester with different temperatures up to 350°C in air. The wear rate of alloy and composite was decreased with increasing temperature due to oxide and glazing layers formation. However, the wear behaviour of composites were superior than that of alloy for all temperature conditions due to formation of rich tribo film between sliding surfaces by smeared graphite particulates. The alloy could sustain only up to 300°C during sliding, but the composite could prolong till 350°C. The worn surfaces of alloy and composites were characterized by scanning electron microscope to understand the wear mechanism.