B. S. Mann

Abrasive and erosive wear characteristics of plasma nitriding and HVOF coatings: their application in hydro turbines

Abstract This paper describes the abrasion and silt erosion characteristics of plasma nitriding and HVOF coatings along with commonly used steels in hydro turbines. For silt erosion characterisation, hydrofoils scaled down to 1/10 of the actual hydro turbine blade were selected. Angles of incidence, velocity and Reynolds numbers were maintained similar to those that commonly occur in hydro turbines, simulating low as well as high-energy impingement wear. The abrasive wear characterisation was carried out as per ASTM G-65. HVOF coated steel performed much better than plasma nitrided 12Cr and 13Cr–4Ni steels. Plasma nitrided 12Cr steel performed better than plasma nitrided 13Cr–4Ni steel. This is due to its higher microhardness and its ability to absorb more nitrogen under identical plasma nitriding experimental conditions. Based on this experimental study, HVOF and plasma nitrided 12Cr steel are being field-tried on a hydro turbine component, which is severely affected due to abrasion and silt erosion.

An experimental study to corelate water jet impingement erosion resistance and properties of metallic materials and coatings

Abstract This paper describes the water jet impingement erosion characteristics of titanium alloy (Ti6Al4V), Hadfield steel, laser hardened, plasma nitrided and pack borided 12Cr steel along with most commonly used steels in hydro turbines. Round samples as per ASTM G73-98 were tested for water jet impingement erosion study. While testing, in the incubation period, plasma nitrided and pack borided 12Cr steel performed much better than 12Cr and 13Cr4Ni steels. Plasma nitrided 12Cr steel performed much better than pack borided 12Cr steel. This is due to the integrity of plasma nitrided layers and their ability to absorb shocks due to jet impingement. During incubation as well as in the long run, Hadfield steel and laser hardened 12Cr steel performed exceptionally well followed by 17Cr4Ni ‘PH’ steel. Based on this experimental study, a suitable criterion based on ultimate resilience (UR) for metallic materials and a composite modified resilience (CMR) for hard metallic coatings has been discussed. Water jet impingement erosion test results along with the mechanical properties of materials and coatings, and their scanning electron microstructural details are reported in this paper.

High-energy particle impact wear resistance of hard coatings and their application in hydroturbines

Abstract The problem of erosion due to silt in a number of Indian hydro power stations is found to be quite serious, especially in those located in the Himalayan region. Erosion is a function of different parameters such as silt size, hardness, concentration, quantity, shape, velocity and base material properties. In most cases, this can be minimised by controlling the above-mentioned parameters. During monsoon season, it becomes impossible to control these parameters which cause erosion. So, it is essential to know their effects. Wear, which occurs due to low and high-energy particle impact, can be controlled by velocity or by controlling silt size, shape and concentration. The low energy impact wear can also be controlled by providing suitable hard coatings. However, this becomes critical for high particle impact wear. An experimental study was undertaken for understanding its nature. In this study, hard coatings such as hard chrome plating, plasma nitriding, D-gun spraying and boronising were studied for high-energy impact wear resistance. Boronising appears to be excellent for this, followed by D gun sprayed WC+12Co coating. Based on this experimental study, boronising is being field-tried on a component which is prone to erosion due to high-energy particle impacts.

HVOF coating and surface treatment for enhancing droplet erosion resistance of steam turbine blades

Abstract This paper describes the water droplet erosion characteristics of high velocity oxygen fuel sprayed (HVOF) coated and laser hardened 12Cr steels along with steels and titanium alloys used in steam turbine blades at two different energy fluxes. For droplet erosion study, round samples as per ASTM G73-98 were used. At low energy flux, the HVOF coated 12Cr steel performed much better than 12Cr and 13Cr–4Ni steels. This is due to integrity of hard carbide particles in cobalt chrome matrix and its ability to absorb shocks due to high hardness of the carbide particles. During incubation as well as in the long run, laser hardened 12Cr steel performed exceptionally well followed by 17Cr–4Ni ‘PH’ and heat-treated 12Cr steel. From the experimental study, it appears that ultimate and modified resilience of materials play significant role to combat droplet erosion. Droplet erosion test results of all these materials and HVOF coating along with their properties and scanning electron micrographs are reported and discussed in this paper.

Boronizing of cast martensitic chromium nickel stainless steel and its abrasion and cavitation-erosion behaviour

Abstract Boronizing of steels has been very effectively used in overcoming adhesive, sliding and abrasive wear. However, little information is available on boronized cast chromium–nickel steel (13Cr–4Ni) with regard to cavitation-erosion and abrasion resistance as well as their effect on mechanical properties. In this paper, the cavitation-erosion characteristics along with abrasive wear of boronized 13Cr–4Ni steel are studied in detail. Cavitation-erosion resistance was evaluated by using a rotating disc apparatus, whereas abrasive wear was studied as per ASTM G-65. These phenomena are being correlated with mechanical properties such as resilience and strain-energy. Drastic reduction in elongation and strain-energy of borided steel has resulted in poor cavitation-erosion resistance whereas improvement of the order of 300% was observed in abrasive wear resistance. All these results are being reported in this paper.

High temperature friction and wear characteristics of various coating materials for steam valve spindle application

Abstract Various coatings such as chromium carbide (deposited by plasma spraying and detonation gun techniques), chromium oxide, chromium oxide+titania+silica, NiCrAlY, and Al2O3+NiAl, all deposited by plasma spraying; stelliting, and surface nitriding have been applied on X20CrMo V121 steel. This steel is used for high temperature applications such as steam turbine valve spindle. Friction and wear behavior of the surface coated and treated materials have been studied at an elevated temperature of 550°C while rubbing against graphite-filled stellited steel. These studies have been carried out on SRV Optimol reciprocating tribometer. Test parameters for tribological studies have been selected with a view to simulate operating conditions encountered in operation. Additionally, the structure, porosity, hardness, bond strength, and thermal cycling behaviour of these surface coated/treated materials have been characterised. Based on these laboratory investigations, chromium carbide coating deposited by plasma spraying technique has been identified as the most suitable coating for steam turbine valve spindle application. Process parameters have been established for deposition of chromium carbide coating by plasma spraying technique on actual valve spindles. The field results obtained are found to be commensurate with the laboratory findings.

Solid-particle erosion and protective layers for steam turbine blading

Solid particle erosion of steam path surfaces is a major concern in steam turbines. This is mainly due to the formation of magnetite on the inside of boiler tubes. This builds up, spalls and travels along with the steam causing erosion in areas where steam velocities are the highest. The steam velocities are the highest in the high pressure sections of steam turbines. The erosion depends upon the base material and steam flow conditions. The base material is generally protected by plasma/detonation sprayed and thermo-chemically formed diffusion coatings. In this paper prevention and visualisation of erosive wear of two dimensional steam turbine 12Cr steel blading under similar Reynolds number to that of steam turbines is described. For erosion visualisation, a rotating disc apparatus has been used. From the study, it was observed that erosion of complicated steam turbine blading depends upon the flow conditions especially flow separation, reattachment and boundary layer growth. The flow visualisation was obtained from the replicas which were etched on the aluminium disc due to erosive wear. Further, to control the erosive wear of steam turbine blading, these were given a hard diffusion layers based upon boronising. The performance of these hard diffused layers along with erosion visualisation of 12Cr steam turbine blading are reported in this paper.

Erosion visualisation and characteristics of a two dimensional diffusion treated martensitic stainless steel hydrofoil

Abstract Erosive wear of complicated water pump and hydro turbine blades is a complex problem. This is due mainly to the many variables involved in the erosive wear. These depend upon type of erodant, base material and flow conditions. In this paper, visualisation and prevention of erosive wear on two dimensional forged 12Cr and 13Cr4Ni cast steel hydrofoils under flow conditions similar to that of hydroturbines and water pumps is described. An experimental study was taken using a rotating disc apparatus. From the study, it was observed that the erosion of complicated hydrofoils depends on the flow conditions, especially flow separation, reattachment and boundary layer growth. The visualisation of wear on the hydrofoils was obtained from the wear replicas which were etched on the aluminium rotating disc. Further, to control the wear of these hydrofoils, these were given a hard diffused layer based on boronizing. The performance of these hard diffused layers along with wear prediction on 12Cr and 13Cr4Ni steel hydrofoils are reported in this paper.

Advanced high-velocity oxygen-fuel coating and candidate materials for protecting LP steam turbine blades against droplet erosion

This paper describes the water droplet erosion characteristics of advanced high-velocity oxygen-fuel (HVOF) coating and the materials used for steam turbine blading. For droplet erosion study, round samples as per ASTM G73-98 (“Standard Practice for Liquid Impingement Erosion Testing,” Vol 03.02, Annual Book of ASTM Standards, ASTM, 2004) were selected. The materials commonly used for steam turbine blading are X20Cr13, X10CrNiMoV122, and Ti6Al4V. During incubation as well as in the long run, advanced HVOF coating has performed much better than all these materials. This is due to enhanced particle kinetic energy caused by optimum flow of oxygen and fuel injection by modifying the fuel injector. Droplet erosion test results of these materials and advanced HVOF coating along with their properties and damage mechanism are reported and discussed in this paper.

Cavitation Erosion Behavior of HPDL-Treated TWAS-Coated Ti6Al4V Alloy and Its Similarity with Water Droplet Erosion

Twin wire arc-sprayed (TWAS) coating of commercially available SHS 7170-cored wire was obtained on Ti6AL4V alloy, and to improve its properties, it was further surface treated with high-power diode laser (HPDL). The cavitation erosion (CE) resistance of TWAS-coated samples was evaluated as per ASTM G-32-2003 and it was compared with laser-treated and untreated Ti6Al4V alloys. The CE resistance of TWAS-coated SHS 7170 samples after HPDL treatment has improved significantly. The main reasons for its improvement are elimination of pores, increased fracture toughness, reduced hardness, and brittleness. The CE resistance of HPDL-treated TWAS coating is compared with water droplet erosion resistance. It is observed that there is a similarity in the both the phenomenon.