Wear Resistance and Steels
Wear Resistance and Steels
Wear is described as ‘the phenomenon of metal surfaces which are moving relative to each other, getting worn out due to the surfaces scratching each other or due to metallic adhesion’. Wear resistance can be said to be the property in which such a phenomenon is difficult to occur. Wear caused by the impact and abrasion action of hard particles is a major problem in several industrial applications.
The wear resistant steels are characterized mainly by high resistance to wear friction, weldability, good ductility, and machinability. The disadvantage of wear resistant steels is low corrosion resistance which can limit their application in aggressive environments.
Wear and tear is familiar phenomena which people experience in their everyday lives. A study as early as 1803 dealt with wear of gold coins to understand whether the softness of the coins determines their susceptibility to friction during economic transactions. In 1833, a simple experiment established that hard metals have less friction and hence less wear. In the same study, it was stated that steel has a remarkable capacity to harden, and hence should ‘render it preferable to outperform every other substance yet discovered in reducing the friction of delicate instruments’. It was well-recognized even in those days that wear involves contact between at least two substances, so the behaviour of steel rubbing against ice would be different from when it abrades against brass. In other words, wear is not an intrinsic material property, and the environment in which it occurs also plays a role. Based on the damage mechanism, wear can be classified broadly into (i) adhesive wear, (ii) abrasive wear, and (iii) other forms which include erosion, fretting, fatigue, and corrosion.
Abrasion is the normal mode of wear which is experienced in several industries during the handling of materials. In the mining, lifting, and excavation processes, it contributes to around 60 % of the total wear losses. In a number of industries, steels are used extensively to resist wear because of their availability, ease of production, and phase transformations which can be exploited to control mechanical properties and microstructure. The properties of wear resistant steels enable them to resist wear, due to rubbing, impact, or compressive loads from external agents such as cement, sand, stones etc., and are intended for use in equipment construction and for replacement of wearing parts. Several structures, such as dump bodies, materials handling equipment, and crushing machines, for example, are exposed to continuous, abrasive, and impact wear, which is costly. As a solution, special steels have been developed which are highly resistant to wear and abrasion.
Given that the applications expose the steel to impact by abrading particles, the wear process is sometimes described as impact-abrasion. 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 efforts have been made to understand the response of various steels exposed to abrasion. Among the basic factors considered to select steels for abrasion wear conditions obviously belongs a criterion of material hardness. Microstructure of steels, size and type of carbide phase also participate in the resulting abrasion wear resistance.
Factors affecting wear resistance of steels
There are four main factors which have considerable effect on the wear resistance of steels. These are (i) heat treatment, (ii) alloying additions, (iii) influence of carbon content, and (iv) effects of carbides, both primary and secondary.
Factors affecting plastic deformation, such as grain size, recrystallization temperature, hardness, strain rate etc. also affect the wear of steels. Unlike single crystals which have free boundaries, the grains of a polycrystalline steel are influenced by their neighours during deformation, their constraining action on deformation is least when the average grain diameter is higher than the microscopic areas of contact. Thus contact over a large number of grains sharply reduces the wear rate. Hence, a large grain size is undesirable for a good wear resistance property of steel.
Classifications such as two-body and three-body abrasive wear, 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 inter-actions 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. The variety of the types of wear leads towards the use of steels, welding materials, and coatings in order to ensure the highest possible wear resistance of the surface layers in working conditions.
Mechanism of wear is complex surface process in the context of factors whose intensity of reaction depends on the operating environmental conditions under which the mechanical parts are applied, on operating parameters of machines, and material properties of contacting surfaces
Abrasion resistant steels in combination with a good formability and a desirable balance of strength and ductility are in high demand for industrial applications. Studies have shown that high strength low alloyed (HSLA) steels offer a good potential for use as wear resistant material. The correlation of microstructural features such as martensite, ferrite and pearlite, and ferrite and martensite with abrasion resistance for a HSLA steel are referred to offer a good wear behaviour. Some study results present the abrasion wear resistance of martensite and ferrite dual phase steel influenced by the microstructure and test conditions. As per the study, the wear resistance increases with increasing the volume fraction of martensite. Available information suggests that the abrasive wear resistance of materials depends on factors like microstructure (micro-constituents, their size, and quantity), and mechanical properties of materials. However, the effect of ferrite morphology on mechanical properties and wear resistance has rarely been taken into account.
Abrasion and impact-abrasion
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, hydraulic systems with dirt, extruders and rock crushers. 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 metallic sample on paper impregnated with hard particles (sand paper) is an example of the two-body mechanism, while polishing the metal 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 naturally differ, as shown in Fig 1. Two-body abrasion is similar to sliding contact, whereas the three-body abrasion involves a certain amount of rolling contact as well. Hence, considerably different wear mechanisms apply in these two modes of wear.
Fig 1 Schematic of the shear stress as a function of depth below the free surface
Impact by abrasives occurs in addition to abrasion in lifting and excavation activities common in the material handling processes, for example, during the loading / unloading buckets, conveyors, crushers, and dump truck liners. Wear and material loss because of the repetitive collision with abrasives are called impact-abrasion. Under repeated impact, macroscopic spalling and fine scale surface loss mechanisms occur and the damage can interact with abrasion wear. Impact energy, as small as 1 joule (J), can increase the wear loss in cast irons. A number of studies consisting of experiments, modelling, and field tests have been done on abrasion of steels. However, it has recently been identified that impact-abrasion is a real problem in equipment used in material handling, lifting, and excavation processes, and it needs to be addressed. The studies so far have been devoted mainly on developing and designing of test methods, testing of steels under such conditions, and ranking the steels based on their mechanical properties, mainly hardness. There are some studies on understanding the role of various metallurgical and micro-structural aspects influencing the impact abrasion wear of steel. Hence, it is important to compare and contrast the damage mechanism involved in abrasion, and impact-abrasion.
Material removal in abrasion is mainly through ploughing, cutting, and wedge formation and it depends on loading, and abrasive properties. Cutting leads to removal of material, while wedge formation and ploughing lead to displacement of material because of the plastic deformation. Wear in the later case occurs through a mechanism of delamination. In contrast, in impact abrasion, abrasive particles impact wear component at different angles from 0 degree to 90 degrees and also at varying velocities. Near 0 degree, the damage is abrasive, involving cutting, wedge formation, and ploughing. At other angles of impact, the material is displaced or removed from the site of the impact depending on the impact energy, and also impact craters are developed. When the impact occurs around normal to the surface, the displaced material from the crater is distributed as a lip around the crater, although some material can also be ejected from the sample depending on the energy of the impact. Hence, impact-abrasion includes chipping and fragmentation besides abrasion damage modes. Fig 2 shows major difference below the abrading surface of abrasion, and impact-abrasion.
In the Fig 2, steel A contains 0.03 % C (carbon) with 190 VHN (Vickers hardness number), steel B contains 0.17 % C with 320 VHN, and steel C contains 0.19 % C with 390 VHN. Cross-sectional microstructure of abrading surface under impact-abrasion produced a severely deformed subsurface. The surface of abrasion wear does not show mixing or craters as in case of impact-abrasion. Heavily deformed material with the presence of embedded abrasive particles can be noticed in case of impact abrasion.
Fig 2 Microstructure of worn sub-surface of the steel grades
In some applications, abrasion wear resistance can be improved by increasing the hardness but to address impact-abrasion, other mechanical properties need to be improved. For example, steel tested under only abrasion conditions normally has a strong correlation with hardness, i.e. decrease in wear loss with the increase in bulk hardness of the steel. However, with impact-abrasion wear the loss data show scatter (Fig 3) and it is possible that the loss increases with hardness.
Fig 3 Wear rate in steels as a function of Brinell hardness
Mechanical property-wear relationships
The mechanical properties of steels have relationship with their wear resistance. These relationships are described below.
Hardness – A big factor affecting wear resistance is ‘hardness’. In general, the wear resistance increases as the material becomes harder. There is a direct relationship between hardness and wear resistance. The resistance of a steel surface against wear is primarily a function of the ‘effective hardness’ resulting from the destructive action of the abrasive particles and depend on the strain hardening rate of the steel under the applied conditions.
Considerable studies as well as field tests indicate that both abrasion and impact-abrasion wear-rates correlate linearly with hardness. Indeed, commercial steels mostly are developed assuming that wear resistance increases with bulk hardness (Fig 4). However, this cannot be the full explanation and it is interesting to examine the roles of other mechanical properties, microstructure, and the steel composition in determining wear resistance. These relationships can lead to a better insight into the mechanisms involved, and hence the possibility of better steel design.
Fig 4 Abrasive wear resistance of materials as a function of their bulk hardness
Field test data for wear resistance steel tools used in ceramic industry show huge scatter when plotted against hardness (Fig 5a). This is an exaggerated example which shows the possible role of factors other than hardness. Similarly, during wet abrasion (Fig 5b), the dependence of wear on hardness is definitely not linear.
Fig 5 Dependence of wear on steel hardness
Further, based on laboratory abrasion experiments, Moore proposed that the wear resistance and hardness change with the square root of the carbon content in martensitic steels. However, Rosenburg’s results (Fig 6a) of wear loss of different martensitic steels in the sand blast test do not confirm to the equation proposed by Moore. The rate of decrease in wear loss up to 0.4 % of carbon is very prominent. Further increase in carbon does not show considerable decrease in wear loss. The wear loss data also do not show a linear relationship. It can be because of possible impact loads in the application of the tools. Further, wear rate changes considerably with change in surface hardness. An increase in wear resistance takes place only if there is a adequate depth of hardening to resist cracking. The ratio of surface hardness (Hs) of the wear material to the hardness of the abrasive (Ha) is a rate controlling parameter in abrasive wear. As per Tabor, surface is scratched by an abrasive only when Ha is higher than / equal to 1.2 Hs.
The change in wear rate because of the ratio results from a change in the nature of the contact mechanics. At Hs / Ha ratios between 0.6 and 0.8, the fracture mode is dominated by micro-ploughing, or cutting because of the plastic deformation, while at higher Hs / Ha ratios, the material is removed by fragmentation. However, the increase in surface hardness increases wear resistance only if the material retains its toughness in the deformed layer.
The wear of hard steel is subjected to a complex wear environment which involves impact or gouging, correlates badly with hardness. Rendon and co-workers tested commercially available steels under purely abrasion and impact-abrasion conditions. Pure abrasion results were found by them to have strong dependence on hardness, while wear loss in the latter case depended on the hardness as well as the toughness. For example, Miyoshi and co-workers study on-wear loss with hardness in sand abrasion test has shown that the wear loss decreases with increase in hardness but at decreasing rate (Fig 6a). The decrease is small once the hardness exceeds 500 VHN which corresponds to around 0.3 % C. Hence, it is obvious that the hardness alone cannot increase wear resistance of steels for high impact abrasion resistance applications which need high hardness components. Wear particles are removed by plastic deformation followed by fracture from the impact /abrading surface and hence other mechanical properties play an important role in determining wear resistance of steels of high hardness. It is obvious from the totality of results that harder steels normally wear less, but there are diminishing returns once the hardness exceeds about 500 VHN.
Fig 6 Wear resistance and steel
Work hardening – In a number of interesting experiments, Richardson deformed the surfaces of a variety of materials by shot peening, by wear in stony soil, and by burnishing with a tool. Fig 6b shows his data analyzed by plotting the increment in surface hardness due to the variety of deformations, against the initial hardness. It is obvious in hindsight that the steels which have high initial hardness, harden less during surface deformation, explaining the reason for the wear rate seems to become insensitive to hardness beyond a certain point. This is consistent with independent studies, for example a recent study on high-stress abrasion has shown that the surface hardness of steels with an initial hardness of 500 VHN to 700 VHN has 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 seen that that wear resistance correlates better with abraded surface hardness than with the bulk hardness, especially in quenched and tempered steels (Fig 9b). The ability of a steel to work-harden is important in increasing the wear resistance, since it is the surface hardness which determines the interaction between the abrasive and the steel.
Further, it seems that in spite of the increase in hardness due to cold working, there is no improvement in abrasive wear resistance. However, this is since the plastic strains involved in the cold working are much smaller than those associated with abrasion. It is also, seen that retained austenite does play a role in the work hardening rate.
Fracture toughness – Fracture at different length scales is an integral part of the majority of wear mechanisms, beginning with asperities to larger debris formation. It is obvious then that fracture toughness plays a role in some conditions. In case of very brittle materials such as ceramics, fracture toughness is particularly prominent in determining the wear rate. Based on experiments on ceramics and tool steels, a generalized relation between wear resistance, hardness, and fracture toughness is given in Fig 7a, although it is assumed that the fracture toughness increases monotonically as the hardness decreases. 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 since 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 the applied load, of course, leads to more rapid abrasion. These trends are consistent with actual data, as shown in Fig 7b and 7c.
Fig 7 Relationship of wear resistance of steel with other parameters
Hornbogen modified Archard’s model to explain the dependence of abrasive wear resistance on toughness. His modified model postulates three regimes namely (i) ductile range where wear takes place by plastic deformation or subcritical crack growth as in high fracture toughness steels in their annealed conditions, (ii) transition range in which wear rate starts to increase when the critical strain, lc, of material becomes smaller than the applied plastic deformation ld, and (iii) brittle range where the ld is much larger than lc. The wear volume per unit sliding distance (V) varies with the hardness in the cases (i) and (iii), but in case (ii) toughness plays a key role. V is proportion to [(W3/2).n2.E.Ys] / [(H3/2).(Kic2)] where W is the applied load, n is the exponent of work hardening, E is the Young’s modulus, Ys is yield strength, H is the hardness of the abrading material , and Kic is the plane strain fracture toughness.
The modified model assumes that crack growth determines the wear behaviour in transition range (ii) 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 conventional steels, toughness decreases as hardness increases. It is obvious from Fig 7b that in impact wear, the wear resistance of the pure metals increases with material hardness but it does not apply in the case of hardened steel and in Fig 7c the increase in hardness beyond certain value decreases wear resistance.
The modified model can explain the observations qualitatively in Fig 7b and 7c. However, all mechanical properties of different materials and wear data of the materials are needed to evaluate the model quantitatively. Further, the modified model was developed based on Archard’s equation which was based on asperity contacts / junctions and hence further investigation is needed to study if the model is valid beyond asperity length scale (order of micrometers), and also under impact loads. In conditions where steel is not too brittle, nor too tough, the wear rate varies inversely with the square of the fracture toughness.
Ductility – Moore and co-workers have shown theoretically that plastic deformation accounts for the major part of the energy absorbed in the abrasive wear of a ductile material. They argued reasonably that the work of creating new surfaces during debris creation is very small and around (10)−4 times the plastic work contribution. The definition of ‘ductile’ in this context hence means that the steel is well above its ductile-brittle transition temperature. Another calculation based on conservation of energy reaches a similar conclusion, that around 95 % of the energy during abrasive wear is consumed in structural changes and deformation at the surface. Structural changes include phase transformation, for instance that of retained austenite. Uetz and co-workers 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, as shown in Fig 8a and Fig 8b. Rendon and co-workers also found that the wear resistance of commercial steels tested in abrasion is related to both hardness and strain to fracture.
Fig 8 Wear resistance and steel properties
Microstructural constituents – The surface texture of the wear track is largely influenced by the microstructure of the material. A discontinuous structure is an advantage in oder to inhibit severe grain growth . Hence, carbon steels are less prone to wear than the homogeneous stainless steel. Due to ferrite- pearlite structure in C steels, the wear is limited to the ferrite constituent only and hence, by increasing the carbon, ferrite content can be reduced and the hardness can be increased resulting into increase in the wear reistance. Tempering becomes easy as the quantity of carbon increases. If the quantity of carbon exceeds 0.6 %, the tempered hardness becomes almost constant. Although when the hardness becomes constant, the wear resistance does not become stable at that point, but the wear resistance increases further as the carbon content increases.
When a steel is tempered, iron and carbon bond together and the material changes into a martensite. This martensite is effective for wear resistance. However, in high carbon steel or high alloy steel, not all the material is converted into martensite by tempering and annealing, and about 20 % to 30 % of the material remains as austenite. This residual austenite is not good for wear resistance.
Conventional wear resistant steels are mainly medium carbon (around 0.2 % C to 0.4 % C) martensitic in either quenched and tempered or auto-tempered condition. Microstructure is one of the key factors in abrasion, and impact wear resistance of alloy steels as it affects how load influences the wear rate, and changes in subsurface microstructure influences wear behaviour, but it is difficult to assign an effect of structure which is independent of mechanical properties. For example, role of retained austenite on wear resistance is inconclusive as some studies claim improved wear resistance is because of the work hardening, while others have shown harmful or no effect of retained austenite on wear resistance depending on loading conditions.
The role of microstructure is important to study since the conventional steels can contain around 10 % to 15 % 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 which improves the local ductility and hence permits the wear surface to achieve higher hardness.
In a shot peening study on Hadfield manganese steels, it has been shown that surface hardness has increased considerably due to the formation of refined microstructure at subsurface. In the same study, it has been found that three-body wear resistance of the steel after shot peening increased when subjected to soft abrasives, but failed to show any improvement when exposed to hard abrasives in two abrasion wear because of the 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 transformation is small.
Increase in hardness not only depends on the quantity of austenite transformed but also work-hardening mechanism. For example, when tested under impact wear a medium manganese steel has shown different hardness values, 467 VHN and 579 VHN, despite similar quantity of martensite produced by two impact loads 1.5 J and 3.5 J, respectively. Lower impact energy causes formation of dislocations cells and fine twins, while at higher energy the density of dislocations increases steeply causing to form islands and wider twins as shown in Fig 8c. 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 the wear component and loading conditions. However, retained austenite films are special in this context and they are known to have complex interactions with abrasives, by improving the toughness during deformation, and by absorbing load prior to any transformation into martensite.
Carbide-free bainite and high toughness martensitic steels have relatively recently become prominent in wear studies. The abrasive wear resistance is very high in carbide-free bainitic steels when compared to conventional quenched and tempered steels, mainly because of the relatively stable retained austenite and the absence of carbides.
The wear loss is controlled by micro-cutting and ploughing in these steels. In conventional steels containing carbides, it has been observed that the carbides can increase hardness but improve the 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 also been shown that large carbides can also act as abrasive and increase wear rate during abrasion. Hence, it is possible to improve the wear resistance of the commercially available steels by refining its microstructure and increasing the austenite in the film form.
Results of recent studies on wear resistance of dual phase and multiphase steels seems promising, mainly at laboratory stage. It has been shown that wear resistance of the steel increases with its ductility in these steels. A study has reported development of medium carbon two-phase microstructure steel (2 % manganese, and 4 % chromium) for truck liner which exposes to both abrasion and impact damage. These steels can be mass produced in conventional way in hot rolled mills. However, field testing of the steels is yet to be evaluated. Carbide-free bainite steels can be a wear resistant steel. However, the challenge is the commercial viability of mass production of this steel.
Precipitation – Commercial wear resistant steels are produced by quenching followed by tempering. This quenching and tempering treatments result into the formation of iron carbides and / or other metallic carbides depending upon the tempering temperature and alloy composition. Normally, the 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 their hardness. Hard and randomly distributed fine carbides resist micro-cutting more efficiently than the large and low hardness precipitates.
For instance, precipitation strengthened alloy steels show no increase in wear resistance with hardness (Fig 4). 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 and 600 deg C) condition do not increase substantially. However, the wear resistance increases substantially when the steel is tempered between 20 deg C to 200 deg C. In a study on 0.32 % C steel in quenched, and then tempered at different temperatures, it has been shown that 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 has dropped when both hardness and impact energy are decreased.
It has been observed that the carbides in steel can increase hardness but improve the 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 commercial steels containing 0.25 % C to 0.4 % C.
Grain size – Fine grain size of metals increase 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. A study on brass has shown that the strain levels reached at abrading surfaces are extremely high compared with those reached under conventional cold working processes. Hence, it is not necessary that the change in grain size improves the wear resistance.
Experimental results of a study have shown no increase in wear resistance with change in grain size. The study proposed that non-strengthened boundaries, and dislocations walls, as in cold worked metals, with a higher degree disorientation are not effective against abrasive wear. It has been claimed in another study that with the decrease in grain size there is increase in the wear resistance. However, when data has been looked carefully, it seems it can be coincidence as the steels compared have been of different composition and also change in grain size has not correlated to change in their hardness. Hence, there is no strong evidence to show that grain size affects 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 micrometers to 200 micrometers changes hardness of quenched martensite from 390 VHN to 280 VHN 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 increases considerably by refining packet / block size.
Fracture toughness of commercial wear resistant 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
The presence of alloy carbides improves the wear resistance of steels. Hence alloying elements such as chromium, vanadium, tungsten, and molybdenum contribute to wear resistance in steels. The carbides being the hardest component in the microstructure has a decisive influence on the wear resistance. Further the smaller is the size of the carbides in the steel, higher is its wear resistance.
Commercially available steels for wear resistance are normally 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 Brinell hardness in the range 300 BHN to 550 BHN and carbon content in the range of 0.15 % C to 0.4 % C. 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 J to 40 J at -40 deg C and this is relevant for low-temperature applications.
Medium carbon steels, containing around 0.3 % to 0.4 % C are normally used for wear resistance applications possibly because of their weldability and ease of processing in the 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 commercial steels which are specifically designed for impact-abrasion wear applications.
The narrow carbon range not only helps to have martensite start temperature of around 300 deg C to develop a heavily dislocated lath martensite matrix with retained austenite inter lath films as the second phase, but also possible to produce in conventional hot rolling mills. It has also been suggested that microstructure with martensite and finer precipitates improves wear resistance in steels because of quenched martensite and fine precipitates.
In steels, carbon is the most effective element in increasing the 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 hypereutectoid steels where networks of carbides can cause embrittlement in the steels. Similarly, the wear resistance of quenched martensitic, and quenched and tempered steels also increases with increase the 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 wear 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.
In a study, 15 commercially available abrasion resistant steels with 400 BHN have been studied to understand the role of chemical composition on wear properties. The steels have been in the quenched condition with similar amount of carbon, carbon equivalent, and alloying additions. Samples have been tested using a crushing pin-on-disk wear test. It has been found that steel containing high amount of boron and combined nickel and molybdenum contents performed better than other samples. The wear loss difference is minimum 20 % to the next best sample. However, boron is added in very small concentrations which are difficult to control during steelmaking. Nickel increases ductility and toughness while molybdenum promotes secondary hardening during tempering.
Besides micro-alloying elements, rare earth elements can be added to refine the austenite grain size. It has been reported that addition of rare earth elements improves impact energy and also the material performance against wear for a particular application. It has also been found that these elements also act as deoxidizers and desulphurizers which results in clean steels. However, rare earth elements are expensive to use on a large scale and are sparsely distributed in the world.
Three-body abrasion resistance of steels containing different quantities of carbon, boron, and chromium have been studied for agricultural tools and it has been found that steel containing 0.3 % carbon with either 0.4 % chromium or 25 ppm (parts per million) boron in quenched condition performed better than other combinations due to 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 9a. It seems that chromium is the most effective element to increase wear resistance after carbon. Further, it is seen that addition of at least 2 % chromium to 5 % chromium improves wear resistance. Chromium increases hardenability, along with carbon, form a variety of carbides, and it can replace part of Fe (iron) to form composite cementite and form complex carbides which play a considerable role in increasing wear resistance of steel.
Fig 9 steel parameters and wear rate
While developing very high wear resistance steel, it has been suggested that high strength medium carbon steels (0.3 % C to 0.4 % C) which are alloyed with up to 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 commercially available steels and hence it is expected that this material perform better when exposed to impact damage besides abrasion.
Other issues related to the wear resistance of steels
The oxide film produced in air due to mechanical oxidation prevents metal / metal contact and reduces the wear rate as long as the oxide layer remains bonded to the surface. Steels which resist the oxidation effect are more likely to wear by a severe mechanism of adhesion and metal transfer, particularly if they also possess low hardenability.
There are three principal ways of strengthening the structure of steels by (i) alloying, (ii) heat treating (ii) work hardening. The effect of work hardening on carbon steels on wear resistance is shown in Fig 9b.
Examples of wear resistant steels
Wear-resistant special structural steels are, as a rule, quenched or quenched and tempered, and have martensitic or martensitic-bainitic microstructure. Quenched and tempered steels are tailored for different applications with sufficient hardness and toughness achieved either by heat treatment process or by thermo mechanical rolling process. The hardness of these steels is tailored to have the required wear resistance along with the needed toughness in an economical way. These steels are produced in thicknesses up to 120 mm. They are produced under the trade names XAR, BRINAR, DILLIDUR, ABREX, EVERHARD and HARDOX etc.
Normalized special structural steel with hardness of 300 BHN is now available for structures exposed to low or moderate levels of wear, such as scrap grabs, while the 600 HB grade meets extreme wear resistance requirements. Covering a hardness spectrum from 300 BHN to 600 BHN, a suitable material is hence available for every type of wear-exposed application. The grade most in use at present is the steel with hardness of 400 BHN, which is around five times as durable as conventional structural steel. The steels with 450 BHN, a further modified grade, display even higher hardness and, at the same time, good toughness. It enables the realization of more stable and lighter structures which are also highly resistant to impact wear. The main fields of use for the 450 BHN steel include the manufacture of dump bodies and cutting edges.
All the wear resisting steels contain chromium as an alloying element, which has proven very effective especially in low-acid media. The high strength ensures good shape stability and hence little deformation. Thin plate structures allowing a higher net load are also possible. The steels have a level of toughness which guarantees a high impact resistance even under the most difficult conditions, such as sub zero temperatures, for example. Wear resisting steels present no problems when subjected to flame, plasma and laser cutting. They display good weldability and low susceptibility to cold cracking.
Austenitic manganese steel is a very tough and ductile material having high impact toughness. Manganese steel is a soft material having an initial hardness of around 220 BHN to 240 BHN. The wear resistant of manganese steel is based on the work hardening phenomenon. When the surface of manganese steel is under heavy impact load or a compressive load, it hardens from the surface while the base material remains tough. The depth and hardness of the work hardened surface vary depending on application and manganese steel grade. The work hardened layer can be 10 mm to 15 mm deep and hardness can be up to 560 BHN in primary applications. The manganese / carbon ratio and the amount of chromium are also relevant for the desired wear resistance of these steels.