EROSION STUDIES ON NITRONIC STEELS FOR UNDERWATER PART APPLICATIONS

Abstract

Energy is the primary requirement of economic development. Every sector needs input of energy. Power generation is mostly dependent on the nonrenewable fossils energy sources, these sources are fast depleting. The natural renewable power generation sectors like solar, wind and hydro sources needs to be developed for sustainable energy production. India is blessed with huge amount of hydroelectric potential. In the hydroelectric power plants, 13wt%Cr-4wt%Ni (all compositions in wt %) martensitic stainless steel (termed as 13/4 steel) is widely used as turbine blade material. Hydroelectric turbine blades are exposed to erosion by cavitation and by silt laden water. These environments lead to damage and failure of components resulting in decrease of turbine efficiency. It is difficult to sustain the long duration performance because of several repairs and maintenance related problems associated with 13/4 steel as the steel is difficult to weld. There is need to develop appropriate materials, which increase the efficiency and the life of hydroturbine underwater parts. The worn out/broken component of hydroturbine cannot be replaced so easily. Repair welding is the only solution to reduce the burden of replacing whole components. Nitrogen containing steels exhibit excellent weldability and have promising wear resistance. In this research work attempts are made to develop an erosion resistant nitrogen alloyed austenitic stainless steels also called nitronic steels which have potential to replace the 13/4 steel. So far no investigation has been studied erosion behaviour comparative study of 23Cr-8Ni-N (23/8N) and 21Cr-12Ni-N (21/12N) steels. Nitrogen stabilizes the austenitic phase at room temperature. It enhances the strain hardening ability which results improved mechanical deformation. In the present work the weldability of 23/8N steel is studied and compared with suitable grade of austenitic stainless steel (309L). The solid particle erosion behavior as well as cavitation erosion of weld coating applied on the surface of 23/8N steel is also studied. The nitrogen containing steels were received from M/S star Wire (India) Ltd. Ballabhgarh, Haryana, India in form of bar with 100x100 mm cross section. The heat treatment of 23/8N and 21/12N steels was carried out at various temperatures (1000oC, 1050oC, 1100oC and 1150oC) for 3 hrs followed by water quenching. 23/8N steel samples quenched from 1100oC were aged at 700oC for 20 hrs. Weld joining and weld overlay of 23/8N steel by 309L ii austenitic stainless steel has also been studied. For comparison 13/4 steel was also investigated. The microstructure of 13/4 steel consists of fine martensitic laths also exhibited with δ- ferrite. The microstructure of 23/8N and 21/12N steels made up of austenitic matrix with chromium carbides along the grain boundaries. High C and Cr in 23/8N (~ 6-9 %) steel compare to 21/12N (~ 2-4 %) steel possesses higher volume fraction of carbides. Different solution annealing heat treatment temperatures are used to dissolve most of the carbides in the austenitic matrix. Reprecipitation of carbides is observed in the microstructure after the aging heat treatment. 23/8N and 21/12N steels possess higher values of tensile toughness, ductility, strain hardening exponent and impact energy than 13/4 steel. The solution annealing heat treatment of 23/8N steel enhanced yield strength, ultimate tensile strength, ductility and the tensile toughness while decreased on aging heat treatment. Cumulative weight loss (CWL) and mean depth of erosion (MDE) of as received, heat treated and weld coated steels were determined by means of cavitation using ultrasonic processor. A piezoelectric ultrasonic transducer was used to produce oscillations at a frequency of 20±0.5 kHz and peak to peak amplitude 50 μm in distilled water for the duration of 24 hrs. Water temperature was maintained in the range of 25±2 oC. Solid particle erosion test was performed using Air jet erosion tester. The abrasive alumina (Al2O3) particle of size 53-75 μm with velocity 32m/s and feed rate 3±0.3g/min has been employed. The tests were performed at various 30o, 45o, 60o and 90o impact angles at room temperature. The respective volume loss was calculated after every 3 min of testing. The ratio of volume loss to the weight of eroded particles (i.e. particle feed rate x testing time) causing the loss was then calculated as erosion rate in mm3/g. The erosion test was repeated with each subsequent test of 3 min duration until a steady state in erosion rate was obtained. Scanning electron microscope was used to study the mechanism of erosion. The best results regarding the repair techniques applied to the cavitation affected areas of runner blades were obtained by overlay welding of work hardening austenitic steels. Weld coatings and joining was performed using filler metal austenitic AWS E309 stainless steel (SS 309L). Dye penetration and X-ray radiography tests were conducted to determine the weld defect. For the filler metal, the values for current and voltage during welding were 80A and 27.5V respectively. The result shows that weld coating by 309L steel provides good weldability as well as good erosion resistance. iii The entire research work has been presented in six chapters in proposed thesis. Chapter 1 presents a critical review of the available literature on nitrogen alloyed austenitic stainless steel, cavitation and solid particle erosion, erosion resistance materials and erosive wear of welded surface coating. Chapter 2 consists of formulation of problem, objectives of present work based on literature review and planning of experimental works. Chapter 3 deals with the experimental procedures employed for present work. Details of ultrasonic processor used for cavitation erosion, air jet erosion test rig has been described. Various heat treatments given to steels are described in details in this study. The techniques employed in the mechanical testing and metallographic study and instruments, machines used for welding are described in details. Chapter 4 deals with characterization of microstructural and mechanical properties as well as the cavitation and solid particle erosion behaviour of 13/4, 23/8N and 21/12N steels have been analyzed as a function of microstructure, mechanical properties and alloying elements. Generally in power plant gray cast iron (GCI) and bronze were used as turbine materials for available impulse and reaction turbines. So, for cavitation erosion the grey cast iron (used in underwater parts as a pivot ring in the Mohhmadpur power plant, Uttarakhand, INDIA) is studied for the comparison purpose. Chapter 5 is described the effect of heat treatment on cavitation and solid particle erosion behaviour of 23/8N and 21/12N. Chapter 6 is deals with the weldability of 23/8N in terms of microstructural and mechanical properties. Also the erosion behaviour of the weld surface coating applied by 309L austenitic stainless steel has been studied. Chapter 7 described the future directions in which these studies can be extended have been suggested at the end.