SLURRY AND CAVITATION EROSION OF THERMOMECHANICALLY TREATED TURBINE GRADE STEELS

Abstract

13Cr-4Ni and 16Cr-5Ni martensitic stainless steel hydroturbine materials are thermomechanically processed to identify the optimum hot working conditions and their performance is evaluated with regard to slurry and cavitation erosion. These materials are used in hydroelectric power plants because of their excellent mechanical properties and adequate corrosion resistance in severe aqueous environments. During running of hydro turbine, components like runner and blades are continuously impacted by hard and abrasive sand particles and affected by slurry erosion. Low pressure (below the saturation vapour pressure of liquid) inside the flow of liquid cause the cavitation, and implosion of bubbles leads to cavitation erosion. Erosion results in material removal which leads to breakdowns, less efficiency of power generation and expensive repairs. Slurry erosion and cavitation erosion are unresolved problems in hydraulic turbines. To increase the efficiency and life of the turbine blades, the design and selection of appropriate material plays an important role. Traditionally, 18Cr-8Ni was used as turbine material that was later replaced by 13Cr-1Ni steel. The commonly used materials for modern hydro turbines are 13Cr-4Ni or 16Cr-5Ni martensitic stainless steels. Some of the studies concerning improvement in the slurry and cavitation resistance involve heat treatments of turbine materials. Thermomechanical processing is a hot deformation process by which nucleation and growth of new grains is likely due to dynamic recrystallization, which replaces the initial microstructure with finer recrystallized form. The reduced prior austenite grain size and refined microstructure can lead to improved mechanical properties of turbine blade material. Some of the studies of thermomechanical processing like hot rolling and friction stir processing are found to be emerging techniques to refine the microstructure that leads to improved mechanical properties. Thermo-chemical surface modification process like plasma nitriding, laser beam, coating, cladding and laminated composites are some other techniques applied to improve the surface erosion resistance. In most of these studies 13Cr-4Ni and 16Cr-5Ni martensitic stainless steel have been considered as reference material for evaluating the performance of different modification techniques against slurry and cavitation erosion. The heat treated, thermomechanically processed, and surface modified materials are evaluated either by ASTM standard test rigs or by nonii standard test rigs. The slurry impingement test and slurry pot tester are widely acceptable methods to evaluate the solid particle erosion. Ultrasonic transducer is used in characterization cavitation erosion. These tests are performed to correlate the improved mechanical properties through thermomechanical processing with erosion behavior of the material. After the processing, microstructural modification cause variation in mechanical properties. Studies related to thermomechanical processing of 13Cr-4Ni and 16Cr-5Ni martensitic stainless steel and its slurry and cavitation erosion behaviour are limited. In view of the above information, objective is to obtain slurry and cavitation erosion resistant of martensitic stainless steels by carrying out the thermomechanical processing. With this objective, it is necessary to identify the hot deformation characteristics of these steels using the established constitutive equation and to find out the optimal thermomechanical processing condition from the processing map based on dynamic materials model. It is endeavored to obtain microstructural features which alter mechanical properties and thereby lead to improved resistance to slurry and cavitation erosion of martensitic stainless steels. The microstructural features and mechanical properties are correlated with erosion resistance of the materials. The most widely used erosion resistant materials 13Cr-4Ni and 16Cr-5Ni martensitic stainless steel have been investigated. These are investigated for their microstructures evolution, mechanical properties, and resistance against slurry and cavitation erosion. Slurry pot test setup is used for slurry erosion testing to physically simulate the service condition of a hydro-turbine. It is designed using a novel and simple test setup involving conventional slurry pot tester with baffle plate incorporated in it. Pot contained 10 parts of water and 1 part of erodent (silica sand) giving a 10 wt. percentage slurry concentration. Asreceived and thermomechanically processed 13Cr-4Ni and 16Cr-5Ni martensitic stainless steels are exposed to the slurry in the pot to evaluate their behavior under slurry erosion conditions. Similarly, the behaviour under cavitation erosion conditions is evaluated using an ultrasonic cavitation based setup. The First chapter describes the introduction to the topic and gives a brief idea about the failure of hydraulic turbines, developments of materials to reduce erosion. It also gives overview about the different techniques and methods employed in order to improve the iii erosion resistance. This chapter consists of insight into the present state of the art and steps taken to overcome the erosion loss. This also gives the importance about the work carried out in the present investigation. The Second chapter gives comprehensive literature review on problem of erosion in hydro turbine underwater parts. An extensive approach has been discussed about the causes for reduction in efficiency of hydraulic components. History of materials used in hydraulic turbines and steps taken to overcome the erosion loss are highlighted. Further, it outlines the requirement of stainless steels to show the better performance in erosion. It is focused on the need for thermomechanical processing, developments in various existing materials and methods, and performance of standard and non-standard erosion tests. This chapter highlights the research gap which is determined through literature review and defines the objectives of the present work. It details the scope of the work and the methodology adopted in the present investigation. Third chapter deals with the experimental procedures employed for the investigation of both 13Cr-4Ni and 16Cr-5Ni martensitic stainless steels. The Fourth chapter deals with the thermomechanical processing of 13Cr-4Ni and 16Cr-5Ni martensitic stainless steels. It explains the processing maps determined according to the dynamic materials model. Stable and unstable regimes are determined and these are discussed in light of microstructural evolution during thermomechanical processing under different processing conditions. It details about the different materials constants and Zener- Hollomon parameter. Fifth chapter details about the slurry and cavitation erosion of 13Cr-4Ni martensitic stainless steel. A section of the chapter explains about the slurry pot and ultrasonic cavitation tests to evaluate the materials at conditions similar to those in hydro turbines. Another section explains the thermomechanical processing conditions used to develop new recrystallized microstructures and improved mechanical properties that lead to erosion resistance. Results concerning microstructural characterization, mechanical properties and erosion behavior are explained. iv In Sixth chapter, the slurry and cavitation erosion of 16Cr-5Ni martensitic stainless steel is discussed. The TMP specimens are tested under both slurry pot and ultrasonic transducer. The details about the eroded surfaces are illustrated through the scanning electron microscopy. Results of slurry and cavitation erosion tests performed on variously thermomechanically processed 16Cr-5Ni martensitic stainless steel are presented and discussed. Erosion behaviour is correlated to the respective mechanical and metallographic results. The as-received and thermomechanically processed martensitic stainless steel are tested for their behavior for 24 h in slurry pot at room temperature. The variation in cumulative weight loss of as-received and thermomechanically processed martensitic stainless steels specimens confirm the effect of microstructural changes leads to mechanical properties. The thermomechanical processing conditions for best erosion resistance under actions of slurry erosion and cavitation erosion, were determined for both the MSS. The Seven and Eight chapter deal with the conclusions and scope for future work, respectively.