Abrasion Resistance of Steels and Abrasion Resistant Steels
Abrasion Resistance of Steels and Abrasion Resistant Steels
Abrasion is a degradation process of the working surfaces of stressed parts, which gradually impairs the parameters of the machine parts and the equipment. This abrasive degradation process is caused by the interaction of hard, usually mineral particles with the working surface of the part. In marginal cases, intensive abrasive wear leads to the losses due to output stoppage caused by machine or equipment outage and further due to costs necessary for production of spare parts and/or their renovation and maintenance costs.
Abrasive wear of materials is a big challenge. It changes the surfaces and dimensions of the components, and can lead to failure creating hazards. Wear is also a significant economic issue since as per certain estimations, the costs of abrasive wear alone to be several percent of the national gross product in the industrialized world. Moreover, wear leads to indirect ecological consequences by raising the amount of replaced components. In this respect, steels with better endurance in harsh conditions offer an opportunity for decreasing the amount of material usage. In addition to lowering the number of replaced components, abrasion resistant steels enable the use of smaller material thickness, which makes the equipment lighter.
Abrasion is the process of scraping or wearing something away. Abrasion involves the removal of material from a solid object when loaded against hard particles which have equal or greater hardness. These particles can originate externally or from debris created by fracture of asperities. Examples of systems subjected to abrasive wear include chutes and hoppers, hydraulic systems with dirt, extruders, and rock crushers etc. For effective usage of different types of steels, it is indispensable to understand the phenomena of abrasion and the damage caused by hard particles. Considerable effort has been made to understand the response of various steels exposed to abrasion.
Abrasion resistant steels also known as wear resistant steels are the steels which are suitable for applications where resistance to wear is a critical demand. Examples of such applications can include (i) resistance to hard particles grinding under a surface sliding over the top of the steel surface, (ii) resistance to impact from rocks and other hard and heavy materials, (iii) resistance to high velocity abrasive dust and other particles. Boron, manganese, nickel, and chromium are the alloying elements which are normally used to make the steel abrasion resistant.
Wear caused by the impact and abrasion action of hard particles is a major problem in many industrial applications. Abrasive wear is characterized by separation and displacement of material particles during scratching and cutting caused by hard particles. These particles can either be free or in a certain cases bonded.
Another case of abrasive wear is known from experience in which hard particles are present between two working surfaces moving relatively one to another. This case is typical for crushing and grinding matter. However it can also be found in sliding pairs, where hard impurities penetrate into insufficiently sealed working surfaces. Hard particles can also be formed in the process of adhesive wear which can affect one or both surfaces abrasively.
Such a classification of types of abrasive wear has been generally accepted although it appears that the processes of interaction of abrasive particles and the material subject to wear are much more complicated. The thing which is required to be taken into account is the character and time response of forces acting between the abrasive particles and the surface subject to the wear. Ideally, the steels for resistance to abrasion are to have the hardness which is in excess of that of the mating surface or the abrasive particles. But in the most cases the hardness of abrasive particles is higher than the hardness of abraded steels.
Mechanism of abrasive wear is complex surface process in the context of the factors whose intensity of reaction depends on the operating environmental conditions under which the mechanical parts are applied, on operating parameters of equipments and material properties of contacting surfaces. Classifications such as two-body and three-body abrasive wear (Fig 2), low stress abrasion, high stress abrasion and gouging, and soft abrasion and hard abrasion have been proposed in order to describe the various types of abrasion processes. The physical interactions between the abrasive particles and the abraded surface have been studied in order to clarify the mechanisms of deformation and wear and can be divided into four types namely (i) micro-ploughing, (ii) micro-cutting, (iii) micro-fatigue and (iv) micro-cracking (Fig 1).
Fig 1 Schematic presentation of the micro-mechanisms of abrasive wear
The variety of the types of abrasive wear leads towards the use of steel materials, welding materials and coatings in order to ensure the highest possible wear resistance of the surface layers in working conditions.
Abrasion is a complex phenomenon present in both small and large scale in the industry. The ability of steel to resist abrasion is not an intrinsic mechanical property, as it depends on the tribo-system as a whole, including all the environmental and operational factors. Abrasive wear is becoming increasingly important in industrial applications, in particular due to its environmental impact through the reduction of the service life of the equipment.
Heavy wear, induced by harsh environments, leads to rapid material removal. Harsh environments expose the materials to both scratching and impacting contacts. Mining and construction industries are typical areas where high-stress wear is taking place. As per one estimate, one excavator bucket can require 6.35 tons of steel replacements during six months only. One other estimation shows that the material loss of crushers can be 24 kilograms per 1,000 tons of the processed ore. The components of equipment which are subjected to heavy wear are often made of abrasion resistant steels, which are harder and higher in strength than the normal structural steels and tolerate wear better. Abrasion resistant steels can be used in a variety of areas such as bins and hoppers handling abrasive materials, agriculture, earth moving, forestry, and mining.
In order to select the steels with better resistance to abrasive wear in a certain environment, it is important to know which factors are primarily influencing the wear rate and wear mechanisms in specific conditions. This way, it can be recognized, which properties need to be focused on and, on the other hand, what kind of conditions are beneficial or detrimental to the steels. Abrasive wear is a complex set of phenomena which are affected by many factors ranging from the environment and contact conditions to materials and their combinations. It is also affected by many parameters having inter-dependent effects.
Based on the type of contact with hard particles, the wear process can be categorized into two-body or three-body abrasion. In the former case, the hard particles remain rigid while in three-body abrasion they move during the wear process. Polishing a steel sample on paper impregnated with hard particles (sand paper) is an example of the two-body mechanism, while polishing the steel sample using a hard particle suspension on polishing cloth is an example of three-body abrasion.
It has been found that the wear rates are an order of magnitude less in three-body as opposed to two-body abrasion, since the abrasive particles spend around 90 % of the time rolling on the contact surface without causing much damage and only 10 % time in abrading the surface. Shear stresses at the surface for sliding and rolling contact differs (Fig 2). Two-body abrasion is similar to sliding contact, whereas three-body abrasion involves a certain amount of rolling contact as well. Hence, significantly different wear mechanisms apply in these two modes of wear.
Fig 2 Abrasion process causing wear of steel materials
Three-body abrasion is the combination of the micro-cutting wear mechanism and the plastic wear mechanism. It seems that abrasive particles at repeated passes create roll pits at the surface and lead to repeated plastic deformation of the material. In the three-body abrasion, particles are free to roll in the interface. The rolling of abrasive particles pressed in the interface leads to localized deformation on the surfaces, similar to a micro-indentation mark. The wear mechanisms correlated with the grooves are characterized by tips with plastic deformation, typical of micro-ploughing. Ridges are removed after repeated deformation due to successive passage of abrasive particles on wear surface.
Hence, it is important to consider the fact that certain phases and microstructures, although showing similar hardness, can be beneficial under some abrasive conditions. Hardness is to be considered the rough criterion for material selection on an element subjected to erosive and abrasive wear. Grain refinement can be expected as an effective way to improve the wear resistance. A number of studies showed that by decreasing the grain size the abrasion resistance increase continuously. Brittle properties, yield stress and tensile strength are considered mainly in terms of abrasion with slight impacts. However, in real situations during the steel use, dynamic load due to influence of slight impacts also has effect on wear.
The way of classifying abrasion into two- body abrasion and three-body abrasion defines the conditions through naming the active bodies participating in the process. However, this kind of over-simplification does not usually appreciate the complexity of real situations, where pure two- body abrasion or three-body abrasion is rather scarce, and often the two modes occur at the same time. They can also alternate in the same system, the conditions governing which of the modes is dominant. The division of abrasive processes into two- body and three-body situations is more of a description of the initial state than a precise observation of the ongoing process, which is greatly affected by the system in question.
Another classification for specifying the type of abrasive wear is the division to high- stress and low-stress abrasion. In high-stress abrasion, the load induced into the abrasive is so high that it breaks the abrasive, while in low-stress abrasion the abrasive remains intact. Also a division into mild and severe wear has been used, as it is frequently difficult to determine the exact conditions present in the interface.
Relationship of mechanical property of steel with abrasion wear
Many studies as well as field tests, show that both abrasion wear rate and impact-abrasion wear-rate correlate linearly with hardness. Indeed, several steel grades have been developed assuming that wear resistance increases with bulk hardness. However, the increase in surface hardness increases abrasion wear resistance only if the steel material retains its toughness in the deformed layer.
However, this does not explain fully the behaviour of steel in their end product use. The wear of hard steel subjected to a complex wear environment which involves impact or gouging, correlates badly with the steel hardness. During testing of abrasion resistant steels under purely abrasion and impact-abrasion conditions, it has been seen that pure abrasion results found to have strong dependence on hardness, while wear loss in the latter case depended on hardness as well as toughness. Hence, it is obvious that the hardness alone cannot increase abrasion resistance of steels for high impact abrasion resistance applications which require high hardness components. Wear particles are removed by plastic deformation followed by fracture from the impact/abrading surface and hence other mechanical properties also play a vital role in determining wear resistance of abrasion resistant steels of high hardness. Therefore, other mechanical properties, micro-structure, and the steel composition also play an important role in determining abrasion resistance.
Abrasion-resistant steel comes in many grades. It is normally produced and used in the form of plate section. Alloys like carbon, manganese, nickel, chrome and boron are added in different proportions. The grades hence have different mechanical and chemical properties which results into different properties in the end product. Abrasion-resistant steels need maximum toughness yet with a low enough carbon content to maximize the steel weldability and maintain hardenability as the plate thickness increases. Carbon plays a key role in making the steel abrasion- resistant since it increases the hardness and toughness of the steel. But very high carbon content reduces the tensile strength of steel besides making it brittle and susceptible to cracking. Hence a balanced combination of alloy content, heat treatment, and chemical composition is essential for achieving the desired properties.
Effect of work hardening – The ability of steel to work-harden is important in enhancing the wear resistance, because it is the surface hardness which determines the interaction between the abrasive and the steel. It has been seen during a study that the steels with initial high hardness, harden less during surface deformation, explaining why the wear rate seems to become insensitive to hardness beyond a certain point. This is consistent with another study on high-stress abrasion, which has shown that the surface hardness of steels with an initial hardness of 500 HV to 700 HV increased to a much lesser degree than when soft, zone-refined steel has been deformed.
It is known that a strain-hardened layer increases the ability of the steel to resist further wear. It has also been reported that that the wear resistance correlates better with abraded surface hardness than with the bulk hardness, especially in quenched, and tempered steels. When the plastic strains involved in the cold working are much smaller than those associated with abrasion then in spite of the increase in hardness due to cold working, there is no improvement in abrasive wear resistance.
Fracture toughness – Fracture at various length scales is an integral part of most wear mechanisms, beginning with asperities to larger debris formation. It is obvious then that fracture toughness plays a role in some circumstances. In very brittle materials, fracture toughness is particularly prominent in determining the wear rate. In circumstances where steel is not too brittle, nor too tough, the wear rate varies inversely with the square of the fracture toughness.
In one of the study on tool steels, it has been seen that the wear resistance is low either at low or high toughness, with a maximum in-between. At first, it increases with fracture toughness in spite of decreasing hardness, presumably because detachment by fragmentation is reduced. Cutting or ploughing dominates at combinations where the toughness is high but the material is soft, again leading to poor wear-resistance. Increasing of the applied load also leads to more rapid abrasion.
One of the studies shows that there is dependence of abrasive wear resistance on toughness. There are three regimes. The first regime is the ductile range where wear takes place by plastic deformation or subcritical crack growth as in high fracture toughness steels in their annealed conditions. The second regime is the transition range in which wear rate starts to increase when the critical strain of the steel becomes smaller than the applied plastic deformation. The third regime is the brittle range where the plastic deformation is much larger than the critical strain. The wear volume per unit sliding distance varies with hardness in first and third regimes, but in second regime, toughness plays a crucial role.
This study assumes that crack growth determines the wear behaviour in transition range except in second regime where fracture toughness plays a key role. A sharp contact between an abrasive particle and the substrate results in an elastic-plastic indentation. Fracture then does not occur until the indentation reaches a critical length. Micro-cracking occurs above the critical length which increases with fracture toughness. In normal steels, toughness decreases as hardness increases. It is obvious in impact wear that the wear resistance of the pure metals increases with material hardness but it does not apply in the case of hardened steel and increase in hardness beyond certain value decreases wear resistance.
The study has explained the observations qualitatively. However, all mechanical properties of different steels and wear data of the steels are required to evaluate quantitatively. Further, the study has been developed based on Archard’s equation which was based on asperity contacts / junctions and hence for its validity beyond asperity length scale (order of micro-metres), and also under impact loads needs further studies. Hence, under the circumstances, where steel is not too brittle, nor too tough, the wear rate varies inversely with the square of the fracture toughness.
Ductility – It has been shown theoretically that plastic deformation accounts for the major part of the energy absorbed in the abrasive wear of a ductile material. It is quite reasonable that the work of creating new surfaces during debris creation is very small and around 0.0001 times the plastic work contribution. The definition of ‘ductile’ in this context is therefore mean that the steel is well above its ductile-brittle transition temperature. Another calculation based on conservation of energy reaches a similar conclusion, that some 95 % of the energy during abrasive wear is consumed in structural changes and deformation at the surface. Structural changes include phase transformation, for example that of retained austenite. It is also argued that plastic deformation consumes major amount of input energy.
Indeed, the correlation of wear resistance with hardness can, for a ductile material, be interpreted in terms of ductility alone. It has also been found that the wear resistance of the normal steels tested in abrasion is related to both hardness and strain to fracture. However, it is difficult to identify the independent effects of hardness and ductility.
Effect of steel micro-structure constituents on abrasion resistance
Conventional wear resistance steels are mainly medium carbon (around 0.2 % to 0.4 %) martensitic steels in either quenched and tempered or auto-tempered condition. Microstructure is one of the key factors in abrasion, and impact wear resistance of alloys as it affects how load influences the wear rate, and changes in sub-surface micro-structure influences wear behaviour, but it is difficult to assign an effect of structure which is independent of the mechanical properties. For example, role of retained austenite on wear resistance is inconclusive as there are some claims of improved wear resistance due to work hardening, while there are other claims to show harmful or no effect of retained austenite on wear resistance depending on loading conditions. The role of retained austenite is important since the normal steels can contain around 10 % to 15 % of retained austenite.
Further, high austenite containing Hadfield steel crusher liners show short service life when exposed to impact wear in the field of ore crushing. The improvement in abrasion wear resistance is related to both the hardening effect of the retained austenite and/or the strain induced transformation of austenite into martensite. Such transformation also leads to compressive stresses at the surface. This improves the local ductility, and hence, permits the wear surface to achieve higher hardness.
In shot peening study on Hadfield steels, it has been shown that surface hardness increases greatly due to formation of refined micro-structure at sub-surface. In this study, it has been found that three-body wear resistance of the steel after shot peening is increased when subjected to soft abrasives, but failed to show any improvement when exposed to hard abrasives in two-abrasion wear due to severe plastic deformation caused during the test. It has also been reported that in impact wear, material loss increases under heavy impact energy where wear is caused mainly by plastic deformation as the local ductility improvement because of the small transformation.
Increase in hardness not only depends on amount of austenite transformed but also work-hardening mechanism. As an example, when tested under impact wear, medium manganese steel shows different hardness values, (465 HV and 580 HV), despite similar amount of martensite produced by two impact loads 1.5 joules and 3.5 joules respectively. Lower impact energy causes formation of dislocations cells and fine twins, while at higher energy, the density of dislocations increases steeply causing formation of islands and wider twins. The high dislocation density increases resistance to plastic deformation, while twin structure cuts the matrix and increases the strength. Hence, the role of retained austenite in impact-abrasion can be very complex depending on wear-component and loading conditions. However, retained austenite films are special in this context since they are known to have complex interactions with abrasives, by enhancing toughness during deformation, and by absorbing load prior to any transformation into martensite.
In the carbide-free bainitic and high toughness martensitic steels, the abrasive wear resistance is very high in carbide-free bainitic steels when compared to normal quench and tempered steels, mainly because of the relatively stable retained austenite and the absence of any carbides. The wear loss is controlled by micro-cutting and ploughing in these steels. While in normal steels containing carbides, it has been observed that the carbides can increase hardness but enhance wear rate by causing disruption of plastic flow during particle impact. The inhomogeneous nature of the plastic flow results in very high strain gradients which can lead to void formation near to and cracking of the carbides. Further, large carbides can also act as abrasive and increase the wear rate during abrasion. Hence, it is possible to increase the wear resistance of the normal steels by refining its micro-structure and increasing the austenite in the film form.
Precipitation – Commercial wear resistance steels are produced by quenching followed by tempering. Tempering results in the formation of iron carbides and/or other metallic carbides. This depends upon the tempering temperature and alloy composition. Normally these steels are tempered below 300 deg C to avoid temper embrittlement. Role of precipitation of iron carbides in steels on wear resistance depends on the particle size, morphology, and the hardness. Hard and randomly distributed fine carbides resist micro-cutting more efficiently than the large and low hardness precipitates.
For example, precipitation strengthened alloys show no increase in wear resistance with hardness. Abrasion resistance increases if there is an increase in strength at high strains. It is possible in fine precipitation in steel on tempering at low temperature but this is not the case in at high temperature tempering. Abrasive wear resistance of steels with carbon ranging from 0.04 % to 1.23 % in quenched and tempered (between 300 deg C to 600 deg C) does not increase substantially. However, the wear resistance is increased greatly when tempered between 20 deg C to 200 deg C.
In a study, in which 0.32 % C steel is quenched, and then tempered at different temperatures, it has been shown that the wear resistance increases if the drop in hardness is compensated by improved toughness properties at low temperature tempering below 190 deg C. However, wear resistance dropped when both hardness and impact energy are decreased. It has been observed that the carbides in steel can increase hardness but enhance wear rate by causing disruption of plastic flow during particle impact. The inhomogeneous nature of the plastic flow results in very high strain gradients which can lead to void formation near to and cracking of the carbides. It has been shown that large carbides can also act as abrasive and increase wear rate during abrasion. Precipitation has limited role in increasing wear resistance of normal steels containing 0.25 % to 0.4 % carbon.
Fine grain size of steels increase the hardness at low strains but after sufficient strain, the mechanical properties and energy stored during plastic deformation become similar to that of large grain material. The strain levels reached at abrading surfaces are extremely high compared with those reached under conventional cold working processes. Hence, change in grain size may not improve wear resistance. In a study, it has been shown that there is no increase in wear resistance with change in grain size. Also, non-strengthened boundaries, and dislocations walls, as in cold worked steels, with a higher degree disorientation are not effective against abrasive wear. Hence, there is no strong evidence to show that grain size effects abrasion or impact-abrasion wear resistance.
However, grain size of prior austenite in steel has indirect effect on wear resistance. Change of prior austenite grain size from 50 micro-meters to 200 micro-meters changes hardness of quenched martensite from 390 HV to 280 HV in medium carbon steel. Deformed hot-rolled structure shows severely pan-caked un-recrystallized austenite grains, which contain deformation bands with increased number of defects such as sub-grain boundaries, and dislocations cells. These defects ensure a fine martensite structure, consisting of packets, blocks and laths, which are conducive to good toughness since the tendency to crack under load decreases with lath size. It has been experimentally proved that decrease in prior austenite grain size decreases the packet size and the block length of transformed and hence strength-ductility combination and toughness are significantly increased by refining packet/block size. Fracture toughness of normal abrasion resistance steels can be improved by severe thermo-mechanical treatment to refine prior austenite grain size and hence increasing their performance under impact-abrasion damage.
Effect of alloying elements – Normal abrasion resistance steels are marketed based on their bulk hardness and carbon equivalent (an indication of weldability). These steels are either in the quenched or quenched and tempered martensitic condition with hardness in the range 300 BHN to 550 BHN and carbon from 0.15 % to 0.4 %. These steels are alloyed with manganese, molybdenum, and chromium for hardenability, silicon for solid solution strengthening, and micro-alloying elements like niobium, vanadium, and titanium added for austenite grain refinement during hot rolling. Their impact energy is around 20 joules to 40 joules at -40 deg C and this is relevant for low-temperature applications.
Medium carbon steels, containing around 0.3 % to 0.4 % of carbon are most commonly used for wear resistance applications possibly due to its weldability and ease of processing in steel plants. It is important that steels produced by thermo-mechanical processes without any complementary heat treatment make them more cost effective compared with quenched and tempered steels or high carbon carbide-free bainitic steels which need long heat treatment schedules. However, there are no steels specifically designed commercial steels for impact-abrasion wear applications. The narrow carbon range not only helps to have martensite start temperature about 300 deg C to develop a heavily dislocated lath martensite matrix with retained austenite interlath films as the second phase, but also possible to produce in the hot rolling mill. It has also been suggested that micro-structure with martensite and finer precipitates enhances wear resistance in steels due to quenched martensite and fine precipitates.
In steels, carbon is most effective in increasing hardness and hence abrasion resistance. Not surprisingly, the wear resistance of pearlitic steels increases with its carbon content. The rate of increase of wear resistance is low in hyper-eutectoid steels where networks of carbides can cause brittleness in the steels. Similarly, the wear resistance of quenched-martensitic, and quenched and tempered steels also increases with increase in carbon content. The hardness of bainitic steels increases linearly with carbon by around 190 VHN per percent. Other alloying elements, like manganese, chromium, molybdenum, and boron etc., are added to steel to improve hardenability so that a full martensitic structure can be obtained on quenching from the austenite phase field to room temperature. In general the abrasion resistance steels are produced in thick sections and hence the addition of alloying elements is needed to increase the hardenability. Though silicon is a strong solid solution strengthening element, its addition is restricted to 0.5 % to avoid red scale formation during hot rolling. Micro-alloying elements, niobium, vanadium and titanium are added to control the austenite grain size during hot rolling.
A study has been carried out for understanding the role of chemical composition on wear properties with 15 commercially available abrasion steels having hardness of 400 BHN. The steels have been in the quenched condition with similar amount of carbon, carbon equivalent and alloying additions. It has been found that steel containing high amount of boron and combined nickel and molybdenum contents have performed better than other steels. The wear loss difference is minimum 20 % to the next best steel. However, boron is added in very small concentrations which are difficult to control during the steelmaking process. Nickel increases ductility and toughness while molybdenum promotes secondary hardening during tempering.
Besides micro-alloying elements, rare earth metals addition can be added to refine the austenite grain size. It has been reported that addition of rare earth metals improves impact energy and also the steel performance against wear for a particular application. It has also been found that the elements acted as deoxidizers and desulphurizers which results in clean steels. However, rare earth metals are expensive to use on a large scale. In a study on three-body abrasion resistance of steels containing different amounts of carbon, boron, and chromium for agricultural tools, it has been found that steel containing 0.3 carbon with either 0.4 % chromium or 25 parts per million (ppm) boron in quenched condition performed better than other combinations due to the combination of martensite and fine carbides in the steels.
Effect of carbon, chromium, nickel, and manganese on change in abrasion wear of line pipes by sand is shown in Fig 3. It seems that chromium is most effective element to increase wear resistance after carbon. Addition of at least 2 % to 5 % of chromium enhances wear resistance. Chromium increases hardenability, along with carbon form a variety of carbides and it can replace part of iron to form composite cementite (complex carbides) which plays a significant role in increasing wear resistance of steel.
Fig 3 Effect of alloying elements on the wear rate of carbon steel
A study for developing very high wear resistance steel has suggested that high strength medium carbon steels (0.3 % to 0.4 % carbon) which are alloyed with upto 2 % manganese, 2 % to 4 % chromium and 0.5 % molybdenum in quenched and tempered condition have high wear resistance in high stress abrasion. The steel also has high fracture toughness compared to normal steels and hence this steel performs better when exposed to impact damage besides abrasion.
Relationship of hard micro-structural components with abrasion failure of the steels – The basic concept of the effect of micro-structural components on abrasion resistance is based on assumption that the mechanism of wear is identical for all micro-structural components. The abrasion resistance of heterogeneous materials is given by the sum of products of the volume shares of individual micro-structural phases and their relative abrasion resistance (additive law). In the assumption, the thermal affection of the process by wear and by micro-structural modifications is not considered. In another concept, the behaviour of heterogeneous materials is not controlled by a single phase but that the contribution of each phase is linearly proportional to its volume share.
Abrasive wear decreases linearly with the growing volume of the hard phase. Both the concepts assume that all the components of the steel micro-structure are subject to wear in the same way and that the contribution of each component depends only on its volume share and proportional wear. The effect of further important factors such as the properties of inter-phase boundaries, relative size, and fracture toughness of the phases are not considered by these concepts although is obvious that they have a significant effect on the abrasive wear of a steel having heterogeneous micro-structure.
Abrasion resistance of steel materials depends on the hardness, shape, size and amount of hard structural components and their distribution in the basic metallic matrix. The growing hardness of these phases and their amount in the micro-structure increase the abrasion resistance of the steel. The share of carbides, however, cannot be considered separately from the basic material. For example, in pearlite-carbide micro-structures the hardness and abrasion resistance grow with the growth of the carbide volume upto 35 %. However in ferrite-carbide micro-structures abrasion resistance grows with the share of carbides upto 5 % to 6 % and further growth does not affect abrasion resistance any more.
Besides the share of carbides important factors are the type of the carbide phase and its size. In an study regarding abrasion resistance in steels with a constant content of chromium, tungsten, and vanadium with a ferritic matrix and uniformly distributed carbides, it has been seen that the abrasion resistance of steels grows with the change of a type of carbide to a type richer in the alloy element. For example, when changing from carbide of the M3C type to a more complex carbide type M7C3, abrasion resistance grows. Special carbides of alloy elements increased abrasion resistance compared with steels containing complex carbides. A typical example is the addition of vanadium in ledeburitic chromium steels which results in an increase of abrasion resistance due to formation of very hard vanadium carbide (2100 HV to 2800 HV).
For this range of hardness the share of retained austenite in high-alloy steels is low and affects overall abrasion resistance only slightly. The abrasion resistance is found to be low in steels with Fe3C and/or M3C carbides. Higher abrasion resistance is normally found in steels with M7C3 carbides and maximum abrasion resistance in steels containing special MC carbides.
High dispersity of carbides is most favourable for the achievement of high wear resistance. This is why, abrasion resistance grows in tempered steels with the growing dispersity of cementite particles.
Effect of matrix on the abrasion process in the steel – Steels with a ferritic matrix show the lowest abrasion resistance. Substitutional hardening of ferrite by carbide-forming elements (chromium, tungsten, and vanadium) does not result in an enhancement of abrasion resistance. The growing share of pearlite in ferritic-pearlitic steels results in enhancement of resistance to wear. Steels with a martensitic matrix have a higher abrasion resistance than pearlitic steels with a similar chemical composition.
The higher content of carbon and alloy elements leads to a marked enhancement of abrasion resistance. The presence of carbides in the martensitic matrix results in a further increase of resistance to abrasive wear by particles. The hardness and quantity of these carbides makes a significant contribution to the abrasive resistance of the steel. The matrix and carbides resist penetration of abrasive particles which start a cutting action. During heat treatment of steels with a higher content of carbon and alloy elements, which shift Ms and namely Mf temperatures towards lower values, retained austenite is formed in the micro-structure. It has its influence on the abrasion resistance. It has been seen that in various grades of steels, the function of retained austenite during sliding abrasion is positive. An explanation for this behaviour is that the strain-induced martensitic transformation can contribute to the enhancement of wear resistance at low impact energies.