Erosive wear is the phenomenon of removal of the surface of any component basing on different conditions due to high-speed impact of solid, liquid or gaseous particles. It is a common occurrence on most plant and machineries and manufacturing industries. The phenomenon of erosive wear has been studied for many years, and still research is going on the material-related aspects of particle impact erosion.
Different laboratory testing methods were used to determine the erosion resistance of all eroded materials. In order to predict the erosive wear rate during the erosion test, some major properties such as effect of erosion parameters, effect of abrasive particle characteristics were investigated.
The Aim of the project was to continue the work done by scholars and to investigate erosive characteristics of targets by using a small centrifugal accelerator erosion tester. The present project focuses on the erosion of ductile materials particularly mild steels as these are used in pneumatic conveyor plants where some of the most severe erosion problem occurs. An experimental process was conducted with olivine sand, mild steel as hitting and target specimens to calculate the erosion rate. During the experimental process a study was made to calculate the specific wear rate and analyse the wear properties under different testing conditions.
The report presented gives the detailed description of experimental process along with description of materials and tools used. Also report gives detailed analysis of relations of wear properties and rates of relations. Finally future works which could be possible are discussed in the report.
Chapter 1: Introduction
Wear is a progressive loss of substance resulting from mechanical interaction between two contacting surfaces, it is a common occurrence on most plant and machineries and manufacturing industries. In 1997, a survey of wear problems and costs in UK industry indicated that a typical average cost due to wear was about 0.25 percent of company turnover  . Wear can be abrasion, erosion, corrosion or other chemical reaction, appearance of failed surfaces (e.g. cracking, melting, phase transformation) etc… But processes involving erosive wears where there is an involvement of particle size, particle velocity, angle of incidence, and particle distribution are more effective than others.
1.2 Erosive wear of pneumatic conveying systems
Erosion involves the mass removed from the surface of any component at different conditions due to high-speed impact of solid particle or a liquid particle. Generally the erosive wear sizes between 5 and 500 microns by causing the particles. (W-1) . Today, many engineering industrial plants are facing the situation of the erosion wear.
Phenomenon of erosive wear has been studied for many years, and still research is going on the material-related aspects of particle impact erosion. There is a certain need for the analysis of wear process, before defining the exact problem. As stated earlier wear can be abrasion, erosion, corrosion or other chemical reaction, appearance of failed surfaces (e.g. cracking, melting, phase transformation etc.). The types of wear process will be able to tell what kind of testing is needed to find the wear problem. Sliding friction, erosion testing processes which involve surface fatigue can be modelled at a reduced scale. But processes involving abrasive wears, erosive wear where there is an involvement of particle size, particle velocity, angle of incidence, and particle distribution should be modelled at full scale. This can only happen with an appropriately designed test, and it will be carried out in specially designed erosion testers which involve impact of a stream of particles against a test sample. In order to predict the erosive wear rate during the erosion test, some following major aspects have been investigated (3 progress) .
Size and shape of particle
Material Properties of the target
Impact conditions (Impact angle, impact velocity)
Particle concentration on the target.
Now a day’s Different laboratory testing methods are used to determine the erosion resistance of all eroded materials. But the main challenge lies in the prediction of correct test conditions which should be adopted during the test. Current project study will involve calibration of test conditions such as particle velocities and impact angles to the targets. The work was mostly concentrated on erosive characteristics of ductile materials used in pneumatic conveyor plants where erosive problems occur most often. As mild steel is a commonly used ductile material in conveyor plants it was chosen as the target material and centrifugal accelerator type erosion tester was used to find the wear characteristics.
1.3 Project Aim and Objectives
The Aim of the project is to continue the work done by those scalars and to investigate erosive characteristics determination of mild steel targets by using a small centrifugal erosion tester. The present project focuses on the erosion of ductile materials particularly mild steels as these are used in pneumatic conveyor plant where some of the most severe erosion problem occurs. The experiment will be carried out with some of effective erosive wear factors.
The project has been divided in to several objectives. The main objectives of this project are listed below
Calibration of impact conditions on a centrifugal erosion tester
Controlling the impact characteristics for pre-set test condition
Determination of erosive wear for mild steel targets
Relationship between erosion rates for different impact conditions.
1.4 Project and its scope
The main purpose of the project was to determine the erosive wear which is a serious problem in the pneumatic conveyors. Here in this project an attempt was made to calculate erosive wear rate in the mild steel target material using the olivine sand as the eroded material. The results obtained from the experiment can be used for minimising the erosion rate in the mild steel made pneumatic conveyors. Also the work done can be taken as a guideline for several future works in the field of erosive wear.
CHAPTER 2 LITERATURE REVIEW
Ample of research has been carried out in the area of erosive wear impact on plant and machineries and manufacturing industries.
The theoretical concepts obtained from different references such as journals, websites, papers and books which are directly or indirectly related to the project are presented in this chapter.
2.2 History of wear concept
Erosive Wear is the concept which is not new to the manufacturing industry, it was first proposed in 17th century. Till 18th century until Osborne Reynolds paper on hydrodynamic lubrication it was not well known in manufacturing industry. In the paper author explained the causes of wear and need for lubrication for the first time. Gradual progress was made from 18th century and now days it has emerged as important factor in any manufacturing industry.
2.3 Basic concepts of erosive wear
Removal of material from the surface of the solid body is known as wear. Wear may combine effects of various physical and chemical processing preceding the friction between two contracting materials. obtained result was micro-cutting, micro-ploughing, Plastic deformation, cracking, fracture, welding, melting, chemical interaction(w-2). Before the start of project it is necessary to know the all basic concepts of wear.
Definition of wear coefficient K. (Lb-2) K=
Where V=volume worn away
L= total sliding distance
Erosive wear occur due to the impact of number of small particles on the surface of the any mechanical object. Generally erosive wear is caused by the particles of sizes between 5 and 500 microns. Surfaces are impacted by continuous stream of hard particles which is known as particle erosion (11) . One of such major place where erosion occurs is pneumatic conveyors, which may result in punctuation of conveyor bends or dust problem if the bends are penetrated. The main effects of the erosive wear are (2) .
Generated particles contribute more wear
Erosive wear resistance cannot be estimated only by hardness and characterizing resistance topenetration. The wear resistance of hard material with equal hardness level can be attributed to their resistance fracture (8) . There are two types of wear which are main reasons for erosion. The first one is due to the repeated deformation during the collision and the other one is due to the cutting action of the free moving particles. The rate of erosion depends on the particle velocity which results in the plastically formed concavity of similar radius of curvature to that of the particle (2).
Repeated deformation during the collision: This type of erosion occurs in the systems in which particles moves at high speeds for example pneumatic fluid bed systems, air mills etc. where the particles move free at high speed and impact on the solid which results in the wear (2).The formula for deformation wear is
Wd= Units volume loss
M= Total mass of impinging particles
V= Particle velocity
?= Impact angle
K= Maximum particle velocity at which the collision still is purely elastic and
?= the energy needed to remove a unit volume of material from the body by deformation wear.
Cutting action of the free moving particles: This type of wear is occurred due to the particle that impinges the component at an acute angle and due to this there is scratching of material from the surface of the component. This scratching of the material is highly influenced by the velocity and the impact angle of the particle (2).
2.4 Mechanisms of wear test
Wear is occurred by mechanical or chemical reactions generally accelerated by friction heating. There are six main wear phenomena which explain only removal of material from the solid surfaces (16) . In many cases, wear is initiated by one of these mechanisms and it may proceed by other wear mechanism, there by complicating failure analysis. Particle contamination can damage systems by causing a variety of types of wear (15) . The following table shows the primary cause of wear for each mechanism (1) .
Type of mechanismPrimary cause
Abrasive wearParticle between adjacent moving surface
Adhesive wearParticle and high fluid velocity
Fatigue wearSurface to surface contact (loss of oil film)
Corrosive wearParticle damaged surface subjected to repeated stress
Erosive wearWater or chemical
Table 2.1: Different types of wear mechanisms. (1)
Finnie in his work Observations of the erosion of ductile metals has made an attempt to clearly discuss the factors affecting the erosion in ductile metals. Also Gwidon W. Stachowiak, Andrew W. Batchelor, GrazynaStachowiak (2004) in their journal Experimental methods in Tribology discussed the mechanism of erosion of ductile metals analytically and experimentally. The analysis was carried out by writing the equation of motion of a single particle impaction on the surfaces, volume for the same was then calculated using the trajectory of particles . Such type of approaches was obviously unsuitable for brittle materials. In the experiment the particle velocity on the surface of the ductile metals were obtained by using high speed double flash light source. The authors mainly concentrated on the calculation of single particle of different sizes and at different angles with which he then calculated for whole of the volume of particles. The authors also explained about the drawbacks that they came across during the experiment and mentioned about the most unsatisfactory feature of his analysis. It underestimates the weight loss at large angles and, in particular, predicts no erosion at 90°, and explained the three effects which cause the relative erosion at high angles to be greater than that predicted by the sample theory (9).
2.5 Factors effecting the erosive wear
The numbers of factors which significantly affect the rate of erosive wear were discussed by Dr.Dmitri Kopeliovich in his work Mechanisms of wear in 2009.According to him the factors are (12)
Impingement angle: the angle at which a particle hits the surface is very important to erosive wear rate. By the experience it was found that the wear rate was high at an angle of 30° for ductile material and 90° angle for brittle materials.
Impact speed: high speed particles impacts cause more damage than low speed collisions.
Particle properties: particle properties were more important in erosive wear phenomenon, sharp and hard particles causes more wear rate.
Temperature: High temperatures cause high erosive wear because increase in softness of material in high temperatures
Particle flux rate: Particle flux rate is the mass of particles hitting an area of surface per unit of time, which can vary greatly from 100 kg/m^2/s to 10,000 kg/m^2/s.
2.6 Test facilities and conditions
2.6.1 Wear Test facilities
The national laboratory in the UK identified over 400 wear testing standards all over the world and American society of lubrication engineers indentified 300 tests that were used in various test laboratories in year of 1973. From these tests some type of tests have slight variation on others, less than 100number of tests are useful to find out the wear problems (15). As early as Mr. Neale was carried out a survey on wear in 1997 as the result in his survey seven types of wear mechanism were identified, most significant wears were abrasive, erosive and adhesive wear. More than one test method is justified for a particular wear type and more than ten test methods are justified to meet industrial need for seven types of methods (14).There are three major types of erosion testing devices that are used throughout the world for quantifying particle impact erosion against a solid surface (10).
2.6.2 Test condition for erosive wear impact
One of the important tasks of the project was to identify the ideal testing conditions. A journal by B.G Mellor in Surface coatings for protection against wear gives the brief idea of various testing standards. According to Mellor the main equipment that is required to find the erosive wear of the pneumatic bed is the centrifugal accelerator erosion tester. Here in the experiment the different test conditions like using different velocities at different impinging angles are used to find the erosive wear of the component. The investigation is mainly focused on the effect of the particle concentration, effect of progression of wear and the effect of bend radius (3) .
The pneumatic conveyor test rig should also be inspected prior to the experiment. The equipment should be set to the conditions that would exactly suit the experiment. Hence, the rotational speed of the screw feeder, the blow tank pressure and the mass flow rate of the air supplied to the conveying pipe all should be adjusted to give a positive experimental result (4). As mentioned the test rig material is the mild steel on which the experiment is to be performed.
The material that is to be used in the experiment which is to be impinged on the mild steel component is the olivine sand abrasives. Olivine sand consists of 98.5% (Mg,Fe)2SiO4, with the balance being made up of traces of metallic oxides. The particles density of the abrasive material should be found out prior to the experiment. The grain size should also be measured and found before the experiment (3).
2.6.3 Centrifugal accelerator erosive tester
The test equipment that is to be used for the determination of the erosive wear for mild steel impacted by olivine sand is the centrifugal accelerator erosion tester. The schematic figure of the centrifugal accelerator erosion tester is shown below (4).
Figure 2.1: Schematic of the centrifugal accelerator type erosion tester (4).
The centrifugal accelerator erosion tester is the testing equipment that is to be used in the experiment in the laboratory. The main parts of the centrifugal erosion tester are feeder, an accelerating rotating disc and a target holding system. The abrasive olivine sand is fed into the central hole of the rotating disc and is accelerated through the several radial ceramic tubes by the centrifugal force and ejected from the end of the tubes. The specimen targets are set for the reorientation relative to the direction of parallel flow by the disc on holders. The edges and back face of the specimens are protected from particle impact to prevent unwanted erosion. The target holder is the main equipment that is used for the change of angle for the particles that to be feed into the system.
There are many experiments performed using the same equipment by many researchers which would help the present work with their papers. They clearly explained about the working procedure of the experiment and the methodology and also the result which would be a great reference for the present piece of work (5).
2.6.4 Glass-blast erosive wear test
This test used for determined the wear factors .the glass blast test is consist of carbide nozzle 90mm in length with inter diameter 3mm and it having air injection for particle acceleration. A screw feeder mounted to form a loss in weight, this feeder fed the abrasive into the injector housing. And the object was placed on the adjustable table to control impact angles on the object. The author T.Deng was compared the wear factors using the two wear test and compared the results (6) . The figure 2.2 show below was glass-blast erosive wear test.
Figure 2.2: Glass blast erosive wear test (6)
2.6.5 Measurement of wear
The measurement of wear is partly determined by the change of mass occurred in the object. A journal by Gwidon W. Stachowiak, Andrew W. Batchelor, GrazynaStachowiak in Experimental methods gave the brief idea about measurement of wear. Another problem in the experiment of wear is loss of data when wear is measured over a series of intervals as opposed to continuous monitoring and this monitor shows all worn loss on the object figure 2.3.
Figure2.3: Loss of information when continues wear recording is substituted by periodic measurement (17).
At present there are methods of measuring wear:
Detection of change in mass.
Measurement of reduction in dimension of a worn specimen.
Profilometry of the worn specimen.
2.6.6 Measurement of solid particle velocity in erosive wear
Authors A. W. Ruff and L. K. Ives in their paper explained about measurement of solid particle velocity in erosive wear that is being performed in their experiment in the laboratory. Measuring of solid particle velocity is mostly depends on the best erosive testing equipments. Here they have performed the experiment using three different erosion testing apparatus, and explained the working procedure of each of the three different apparatus used. The motion of velocity of particles was determined by high speed photo graphics methods. Also they have shown methods of increasing and decreasing the velocity of particles through motors and transmitted them on transducer output which can be viewed directly on an oscilloscope. As the project is based on experiment the paper would be of great use because the experimental procedure can be useful in many ways (7).
The chapter gave detailed literature review of the concept of erosion and its developments over the years. Literature review was very much useful for the proper understanding of the topic and gaps in research. Based on these gaps in research the author proceeded further in project work and the next chapter gives detailed methodologies, equipments and tools used during project work.
CHAPTER 3 METHODOLOGY
This chapter explains about the different testing methods and tools used during the experimental process. Also a brief description of materials and material handling equipment is given.
3.2 Experimental requirements
olivine sand was prepared to required sizes:
Due to vast availability of olivine sand in the nature it is been preferred in the present project. With the use of sieving machine olivine sand was prepared to required sizes ranging from 45-425µm.
impact velocity and angle were calibrated by double disc velocity meter:
The double disc velocity meter was used for calibration of impact angles and impact velocities. The calibration results were used for calculating the erosion rate of the mild steel target at proposed conditions.
Setting up of right test conditions:
To satisfy the proposed test conditions the correct setup of the experiment was required. The setup process would include setting up the position of the target angle, rotating speed of the accelerating rotating disc and feeding rate of the olivine sand.
The erosion characteristics of mild steel were determined at certain conditions:
Most of the pneumatic conveyers are designed by specific characteristics conditions , based on this characteristics condition have been choose some specific conditions such as an impact angles 20°,30°,45° and 90°and velocity 20,30 and 40 m/sec during this experiment . The main intention was to finding impact angles, impact velocities and size of the material were to determine the erosion rate.
Determination of specific erosion rate on mild steel:
The specific erosion rates of mild steel target were calculated at different conditions by changing the impact angles and impact velocities, the results were analysed. The highest erosion rate and the least erosion rate of the target were calculated. These results were useful for the predicted models.
3.3 Test facilities
There were number of tools used for analysing the test during the experiment which was described below.
Erosion wear test apparatus built by T.deng at the wolfson centre was available for this wear test. Centrifugal accelerator tester which is illustrated in figure 3.1 below consists of a feeder, an accelerator rotation disc and target holding systems. There were two types of feeders to feed the materials into the rotating disc which were vibratory feeder and conical feeder. Conical feeder was used for this present project. The target particles continuously fed to the centre hole of the accelerator rotating disc through special type of funnel, and then from the feeder they travel outwards along the radial tubes in the rotating disk. Material leaves disc at the periphery with tangential velocity equal to disc velocity. Targets are mounted at the points surrounding the periphery of the disc and the specimens are exposed to a series of eroded streams particles from the radial tubes of the rotating disc.
Figure 3.2 schematical diagram of centrifugal accelerator tester:
Figure 3.1: Centrifugal accelerator type erosion tester
This method is more advantageous over the gas jet method and well suited to perform the screening tests on many different materials. The specimens were mounted on the holding system which rotates slowly around the accelerator rotating disc but it rotates only in one direction, to eliminate any possible bias associated with specific specimen locations. The maximum numbers of specimens per test have ranged to 30 in different design of rigs. 
Accelerator rotating disc is one of the most important parts of the centrifugal accelerator tester. Usually the rotating disc diameter varies between 200 and 600 mm. The apparatus shown in figure 3.1 has a disc of 240 mm in diameter and it contains six radial alumina ceramic tubes with an internal bore diameter of 2.6 mm. The erosion centre was placed on rotating disc of 0.24m diameter . The disc was attached to an infinitely variable inverter controlled motor that was driven by V- shape belt drive. The motor requires 220-240 voltage and which has maximum of 2850 revolution per minute.
3.3.2 Description of tabular feeder system
In order to obtain a constant mass flow rate of the abrasive particle, a tabular feeder is used to feed the abrasive material into conveyers or into the erosion test rig and serves as an auxiliary machine in coal washing, electric power, sintering plant and building industries . Tabular feeder consist two main parts.
2) Rotating control disc
Hopper acts as storage for olivine sand before feeding into the accelerator disc, rotating control disc controls flow rate of material is required. The speed of the rotating disc can be altered using this speed control unit as a result the material flow will change according to the speed of the rotating disc. Figure 3.2 shows the laboratory tabular feeder used for the project work. Calibration of the tabular feeder was achieved by measuring the flow rate of mass of materials.
Figure 3.2: Tabular feeder [U1]
3.3.3 Description of Double Disc Velocity meter
There are two types of instruments which were used to measure the particle velocity vector in an erosion tester, they are optoelectric velocity meter and laser Doppler anemometer (k89) . The accuracy of the particle velocity measure was low while using optoelectric velocity meter in centrifugal erosion tester. Therefore, a static form of the double–disc velocity meter and displaced wear scar method (R75) were used for calibration test of the particle velocity vector. Static form of double disc velocity meter is used to measure the particle velocity vector and direction of particle travel. The slot on the double disc velocity meter from which continuous flow of eroded particle travels and hits the surface of the target forms a scar on target material. The exit angle of the actual particle jet can be calculated by the following equation. A schematic diagram of double disc velocity meter is shown in figure 3.3.
Figure 3.3: Double disc velocity meter
Tachometer is a mechanical instrument used to measure the speed of rotating devices. Here in the present experiment it is used for measuring the speed of the accelerating rotating disc. It has the digital display which when placed against the rotating device shows the speed in rpm. The figure of tachometer is shown below.