Scouring erosion resistance of metallic materials used in slurry pump service

Low impact angle erosion resistance is a critical requirement of materials used in pumps, piping, valves, nozzles, cyclones and other components which transport and process most mineral slurries.The Coriolis method offers a suitable technique for assessing behavior under such scouring attack conditions. It is being used increasingly in support of the mining/mineral processing industry, to compare and discriminate between candidate materials and also assist in the development of new products and protection systems. The specific method used in the current study involves high velocity erosion with aqueous slurry containing 10 wt.% of AFS 50-70 silica test sand. This compares reasonably with the main, extremely abrasive solids constituent encountered in mining and processing oil sands deposits in northern Alberta, Canada. These are becoming a rapidly growing and critically important source of oil in North America.Abrasion resistant chrome white iron castings typically covered by the ASTM A532 standard, are used widely in slurry pump components particularly in oil sand operations. However, the development of proprietary cast hypereutectic chromium white irons with microstructures containing primary M₇C₃-type carbides, is providing the capability to significantly improve the wear performance of such parts. In certain applications where corrosion contributes significantly to overall attack, lower carbon and higher chromium-bearing variants are employed.A comparison of the Coriolis erosion behavior of a wide range of commercially available cast wear and corrosion resistant and high toughness alloys used in pump manufacture, has confirmed anticipated performance ranking and the superiority of the latest generation of hypereutectic chrome white irons. The ameliorative influence on scouring erosion behavior of high carbon content, hardness and carbide volume fraction and particularly of fine carbide size has been demonstrated. A correlation is drawn between test data and service performance.     

Surface morphologies and erosion rates of metallic building materials after sandblasting

Erosion behaviors of AISI 430 and 304 stainless steels, ARC-TEN weathering steel, brass, and 6063 aluminum alloy were experimentally investigated using a sandblasting type test rig and silicon carbide erodent particles. Surface morphologies of eroded specimens were examined using scanning electron microscopy. The volume loss was measured for the evaluation of erosion rates. This paper stresses the relationship between erosion rate and surface hardness (before and after erosion). It was found that the erosion rate is proportional to (Hv)⁻ⁿ, where Hv is the microhardness number after erosion testing, and the exponent n is 1.4 or 3, depending on conditions.     

Solid particle erosion behaviour of metallic materials at room and elevated temperatures

The behaviour of metallic materials subjected to solid particle erosion has been studied extensively over the last few decades. It is not the purpose of this paper to provide a comprehensive review of the above body of work especially since many such reviews already exist. Rather, the aim of this paper is to describe briefly the salient features characteristic of room temperature and elevated temperature erosion of metallic materials and follow it up with a review of some of the recent results, which in our opinion, have enhanced our current understanding in the area of solid particle erosion of metallic materials. As a natural consequence, the paper concludes with a critical review of the areas which require further study.     

The role of lubrication and corrosion in abrasion of materials in aqueous environments

A rubber wheel type test apparatus has been constructed which allows abrasion testing to be conducted in slurry or dry environments in otherwise identical conditions. Abrasion tests of a steel, a sintered tungsten carbide–cobalt hardmetal and an HVOF sprayed nickel chrome–chromium carbide cermet coating have been performed in dry and aqueous slurry conditions, the latter with both neutral and acidic carriers. It has been shown that the aqueous carrier acts as an effective lubricant and thus significantly reduces the abrasion rate over that observed in dry conditions. However, enhancement of corrosion by use of an acid slurry lead to an increase in the rate of material removal over that of the neutral aqueous conditions in all cases. Increases were small for the corrosion resistant cermet coating and moderate for the steel. Significant enhancement of wear was observed for the sintered WC–Co hardmetal where rapid removal of the cobalt binder by the acid resulted in a change in dominant mechanism of carbide removal from attritive wear to pullout.     

Particle size distribution effects on the solid particle erosion of brittle materials

The effects of distributions of projectile size and velocity on the erosion of brittle materials were assessed. Strong dependences of the steady state erosion rate on the width of the particle size distribution and on the velocity characteristics of the erosion equipment were predicted. The predictions were confirmed by experiment. The results indicate that careful design of controlled erosion experiments is essential in order to avoid misleading predictions of in-service erosion rates. The implications of the analysis for the interpretation of threshold effects and for experiments designed to verify erosion theories are also discussed.     

Upstream swirl-induction for reduction of erosion damage from slurries in pipeline bends

Industries which transport slurries and other particle-laden liquids in pipes expend the equivalent of millions of pounds every year to repair erosion damage caused by solid particle impingement. It is against this background that the perceived relationship between pipeline erosion and imposed swirling flow fields in pipe bends is important. Definitions of flow fields and particle dispersions which minimise erosive wear are sought to facilitate the development of new designs and geometries for slurry handling equipment. Such an approach is pertinent to industries handling valuable or hazardous material in the face of increasing safety, efficiency and economic requirements. Robust erosive wear models must be developed to explore the advantages of swirl flow and subsequent particle dispersion.

Collaboration between the universities of Nottingham and Southampton is aimed at the reduction of wear at critical locations in slurry handling pipelines by applying swirl-inducing pipes upstream of pipe bends. This paper details the improved particle distributions, particle impingement conditions and lower flowrates resulting from such swirl flow. These factors are discussed in terms of current erosion models and the predicted reduction in wear rates. Parallel visualisation studies using simulant particle-laden liquids augment computational modelling of the flow patterns.     

Vibratory cavitation erosion in aqueous solutions

Vibratory cavitation erosion tests on AISI-SAE 1018 carbon steel in tap water and in mild (0.1 M) aqueous solutions of CaCO₃, CaO, NaHCO₃ and NaOH were conducted at a temperature of 80 °F (27 °C), a double amplitude of 1.38 × 10^?3 in (35.1 μm) and a pressure of 1 atm. For the maximum (150 min) test duration the weight loss in tap water (no additive) is the smallest. However, this is not the case for shorter test times. The biggest difference between weight losses among the various solutions is about 10% – 30%, which is somewhat beyond natural data scatter for such vibratory tests. Released gases and also particles may play an important role in the results.There are three easily distinguishable damage regions for all cavitated surfaces, i.e. generally undamaged rim, central heavily damaged region and transition region, as for most vibratory tests. The relative areas of the three regions are about 53.5%, 0.13% and 46.4% respectively for the present tests.The erosion rate and extent of the damaged regions do not depend substantially on the solute tested. The very small area of the heavily damaged central region is presumably due to the relatively low horn amplitude used in these tests. The increase in damage rate with respect to tap water is about 50% for the maximum test duration.Surface photographs and scanning electron microscopy photomicrographs (for a test duration of 150 min) are presented. Cracks, intercrystalline fractures and single-blow craters are most concentrated in the central region, as would be expected.     

A CFD model of particle concentration effects on erosion–corrosion of Fe in aqueous conditions

A CFD (computational fluid dynamics) model has been developed to evaluate the effects of particle concentration on the erosion–corrosion of the inner surfaces of a circular pipe of 90° bend at room temperatures. The relative intensity of erosion and corrosion around the pipe geometry results in transitions between various erosion–corrosion regimes, for a given inlet particle concentration. The results indicate that the corrosion-dominated regime at the pipe bend is reduced with an increase in particle concentration. Typical results from the model are shown illustrating how this 3D mapping method can be used to model parameters such as particle concentration on the erosion–corrosion regimes over the surface.     

Material and velocity effects on cavitation erosion pitting

Cavitation erosion during the incubation period was investigated via pitting tests conducted on three different materials: an Aluminum alloy, a Nickel Aluminum Bronze alloy and a Duplex Stainless Steel. Pitting tests were conducted in a cavitation tunnel in the velocity range 45–90 m/s at a constant cavitation number. The test section was made of a straight nozzle 16 mm in diameter discharged into the radial 2.5 mm space between two flat walls. Cavitation appears in the form of a toroidal cavity attached to the nozzle exit and damage on the samples facing the nozzle is concentrated in a circular ring centered in the cavity closure region. The exposure time was adjusted to avoid pit overlapping. The material surface was examined using a conventional contact profilometer which allowed us to identify the pits, count them, and measure their main characteristics such as depth, surface area, and volume. From these the pitting rate, the coverage rate, and the depth of deformation rate were defined. Pits were classified according to their diameter. For all materials and operating conditions, pitting rate appears to follow an exponential law in relation to the pit diameter. This law depends upon two parameters only, which were identified as the coverage time τ (i.e. the time required for the surface to be covered by erosion pits) and a characteristic pit diameter δ, which corresponds to the pits whose contribution to the coverage process is the highest. Scaling laws for pitting were derived accounting for both material properties and flow velocity, and a procedure to make pitting test results non-dimensional is proposed. The influence of the material on pitting test results was analyzed. It is shown that the damage is not correlated in simple terms with the elastic limit determined from conventional tensile tests and it is conjectured that other parameters such as the strain rate might play a significant role and should be included in the analysis. The effect of flow velocity on both parameters τ and δ was analyzed and a classical power law was found for the influence of the flow velocity on pitting rate for all three materials. Finally, some analysis and discussion is given concerning distributions of pit volume and pit depth.     

Influence of cavitation erosion on corrosion fatigue and the effect of surface coatings on resistance

The fracture of S35C test pieces under cavitation erosion in 3% salt water occurs in the area of corrosion as in the case of corrosion fatigue without cavitation erosion. However, erosion fatigue strength decreases more than corrosion fatigue strength owing to the formation of a macrogalvanic cell between the erosion and corrosion areas. When the surface of the test piece is coated with either a less noble or a more noble metal than that of the matrix, its fatigue strength is recovered. The effects of different materials, test liquids and distances between the disc and test piece are also considered.     

金属材料的喷丸强化及其应用

介绍了传统喷丸强化的原理、工艺、作用、研究现状及发展趋势,概述了包括预应力喷丸、热喷丸、超声喷丸及高压水喷丸等新技术,并介绍了计算机仿真技术在喷丸强化中的应用.     

Scale-dependent subsurface deformation of metallic materials in sliding

Using laser speckle decorrelation, TEM, optical microscopy and AFM we study deformation structures generated in subsurface layers of metals and alloys by sliding wear. Strain localization process in sliding wear as well as its effect on sliding-induced structures is considered. As shown, strain localization leads to intense fragmentation in subsurface layers and generation of shear bands in previously fragmented materials. These shear bands first exist at the microscale level under mild wear but may reveal at the mesoscale deformation level when there occurs mild-to-catastrophic wear mechanism transition. The result of such a transition is a thick (tens and hundreds of micrometers) nanocrystalline layer at the surface of metals. Hadfield steel shows another type of tribological behavior when only thin nano crystalline layer is formed. We relate such a behavior to the specificity of fragmentation in this steel. High wear resistance of high manganese steel is analyzed too.     

Aerodynamic effects in the erosion process

Experimental investigations of velocity effects on the erosion of a ductile material by aerodynamically entrained solid particles indicate that erosion varies with the fourth power of velocity in normal or 90° impacts. For smaller angles of attack the exponent is less than 4 but greater than 2. Previous quantitative erosion models do not predict these high exponent values. In this study the two-phase fluid mechanical system is analyzed and an analytical expression is presented that predicts particle impact speeds varying with the square of the fluid free stream speed in normal impacts. It is shown that the high values found experimentally are the result of aerodynamic effects alone.     

Particle size effects in bend erosion

From work on the erosion of flat plates in test rigs of the sand blast type published in recent years it has generally been concluded that there is a critical particle size above which erosion is not influenced by size. Recent work on the erosion of pipe bends by sand in a pneumatic conveying line tends to confirm this theory. For pipe bends, however, it was observed that particle size has two additional effects of major significance: the appearances of the eroded surfaces are totally different and the depth of penetration by erosion is much greater for smaller particles; this means that pipe bends will fail much earlier if smaller particles are conveyed.     

The reference model of the impact of cavitation erosion on materials

This investigation is devoted to the problems concerning the reliability of units subjected to an erosion attack of heterogeneous liquid mediums containing gas vapor and denser fine-dispersed substances. The results of the simulation of an ambient dynamic action on solid barrier surfaces, and the response of the materials with heterogeneous structures and protecting covers to repeated external pulse attacks with different intensities are presented. The structural-energetic model of the materials’ cavitation and erosion wear is given. The physical-mechanical criteria and time wearing parameters are also presented.     

Erosion–corrosion performance of high-Cr cast iron alloys in flowing liquid–solid slurries

In many slurry transportation systems, such as in FGD (Flue Gas Desulphurization) and chemical processing applications, corrosion and erosion are the two main mechanisms of material degradation of the pump wet-end components including pump casing, impeller and liners. The performance of a selected material is mostly dependent upon its relative corrosion and erosion resistance to the service environment. In these cases erosion, corrosion and the related synergistic effects can be very complicated since they are affected by numerous factors including solid and slurry properties, chemical contents, hydraulic conditions and temperatures. In this experimental study, sliding Coriolis erosion testing has been performed with various corrosion factors such as pH value, chlorides content and temperature to evaluate the erosion–corrosion resistance of some high-alloyed white cast irons containing different levels of chromium and other elements. Optical microscope and SEM-EDS have also been used to examine microstructure and surface conditions of tested materials. Results indicated that material loss due to corrosion factors increased as acidity-chlorides and temperature increased. At relatively high corrosion intensity, the white cast irons with higher alloy content (especially chromium) clearly showed improved corrosion resistance and combined erosion–corrosion resistance over those with lower alloy content. Under certain corrosion and hydraulic conditions, particle size is perhaps the single most influential factor on erosion–corrosion rate of the high-Cr cast iron alloys. Relatively large particles are much more effective than small ones at removing both the corroded surface layer and the fresh material, causing substantially higher rate of material loss. Some other related factors have also been addressed.