Composition and treatment of wear resistant steel for application in mining industry

The service life of machines in the mining industry is in the most cases determined by wear rate of parts in contact with the rock and soil. This work demonstrates the results of investigation on wear resistance of high-carbon low-alloy steel under abrasive wear. Steel samples containing 1.2 wt.% of carbon, 3 wt.% of manganese and 2 wt.% of silicon were hardened from 900 °C and 1000 °C and subjected to two-body and three-body abrasion tests. It has been established that after heat treatment this steel has increased abrasive wear resistance due to the phase transformation of unstable austenite into deformation-induced martensite. The results of study of near-surface region microstructure of samples after wear are presented. X-ray diffraction analysis and measurement of the microhardness of the worn surface were also carried out. It has been established that during abrasive wear a continuous layer of deformation martensite with a microhardness of 1300-1400 HV 0.05 is formed at a depth of up to 10 μm. Such a microhardness significantly exceeds the microhardness of quenching martensite. The results of the work can be used to increase the service life of wear parts of mining equipment.


Introduction
Wear of machine parts is one of the main factors limiting the lifetime of mining equipment [1].Surfaces in direct contact with the rock or soil are subjected to the abrasive wear, which is the most aggressive one among the all wear modes.Total global energy losses from abrasive wear are equal to losses from all other types of wear [2][3].The rate of abrasive wear exceeds the rate of other types of wear by several orders of magnitude.
The mechanism of material destruction during wear (surface destruction) is identical to the mechanism of its volumetric destruction under cyclic loading.The difference is only in the size of the volume in which the processes accompanying cyclic loading are going on.During abrasive wear, a low-cycle failure mode occurs when contact stresses exceed the yield strength of the material.
Steels and cast irons are the main materials for wear parts of mining machines.The problem of increasing their wear resistance during abrasive wear still attracts the attention of researchers [4][5][6][7][8].Carbide alloys and high-chromium cast irons have the highest possible wear resistance during abrasive wear due to the large amount of carbides with high hardness [9][10][11][12].However, their use is not always possible or advisable due to high cost and low ductility.
In [7,8,[13][14][15], it was proposed to use the effect of strain-induced martensitic transformation of unstable austenite to increase the wear resistance of steels during abrasive wear.It has been shown that to achieve maximum effect, the carbon content in steel should be at the level of 1.2 wt%.
The purpose of research was to determine the wear resistance, microstructure, microhardness and peculiarities of X-ray diffraction patterns of high-carbon low-alloy steel containing 1.2 wt% of carbon after hardening at different temperatures with different content of retained austenite.

Methods
The object of study is high-carbon steel containing 1.2 %C (by mass), 2.9 %Mn, 1.5 %Si and the rest are impurities in acceptable quantities.In accordance with the chemical composition, the steel is designated as 120Mn3Si2.
Abrasive wear tests were carried out according to the two-body abrasion and three-body abrasion schemes.In the first case, the samples were round or square cylinders with a diameter of 2 mm or a side of 2 mm, respectively, and a length of 10-15 mm.In the second case, the samples were tiles with dimensions 30 x 90 x (5-10) mm.Test methods are described in works [13,15].The wear of the samples was determined by weight loss using an analytical balance with a scale resolution of 0.1 mg.
Cross-sections for studying the microstructure were prepared by grinding and polishing with abrasives of various grain sizes, followed by etching in a 3% nital.The microstructure of the samples and the friction surface were studied using scanning electron microscopes JSM-7000F (JEOL, Japan) and Vega3 (TESCAN, Czech Republic).Microhardness was determined using a Wilson® Hardness Tester (BUEHLER, Germany) with a load of 0.05 kg.Macrohardness was determined using a Vickers hardness tester TVP-5012 using 98.1 N load.
Heat treatment of the samples was carried out in electric resistance furnaces in an air atmosphere.Before testing for abrasive wear, the decarburized layer was removed from the surface of the samples.

Two-body abrasion test
To obtain comparative data on the wear resistance of120Mn3Si2 steel the following traditionally wear-resistant materials were also tested: U8 steel (1.1625 tool steel according to DIN) after quenching to maximum hardness, as well as after quenching and tempering at different temperatures; VK8 hardmetal (HG30 hardmetal according to DIN); high carbon white cast iron (3.3 %С) quenched to maximal hardness and also in annealed state.
All samples except sample VK8 were tested on an electrocorundum abrasive cloth.The hardness of electrocorundum is about 2200 HV or 22 GPa, which is sufficient to provide the abrasive wear mode of all samples except VK8 one.Since the hardness of VK8 is 15 GPa (and tungsten carbides is about 22 GPa), the hardness of electrocorundum is not enough to micro-scratch the friction surface of VK8 sample.To test VK8, a silicon carbide abrasive cloth was used (silicon carbide hardness is about 35 GPa).
The results of two-body abrasive wear tests with loose abrasive are presented in table 1. Relative wear resistance ε of samples was calculated in comparison with pure iron (εFe = 1).
The test results demonstrate higher wear resistance of 120Mn3Si2 steel compared to all tested materials except the VK8 hard alloy.However, the use of hard alloy to protect working surfaces is complicated by a number of circumstances: the high cost of the material, the inconvenience of protecting large working surfaces (the need to solder individual hard alloy plates), low impact resistance.The high wear resistance of 120Mn3Si2 steel is explained by hardening of the friction surface in course of abrasive wear during plastic deformation of the material by abrasive grains.The hardening mechanism is the transformation of unstable austenite into deformation-induced martensite with a hardness that is significantly higher than that of quenching martensite.The results of measuring the friction surface hardness of 120Mn3Si2 steel are discussed below.

Three-body abrasion test
Three-body abrasion tests with belong to the category of bench tests, which are created for the purpose of laboratory reproduction of the wear of materials under certain operating conditions of real machine parts.
120Mn3Si2 steel was tested in comparison with the same materials except VK8.Abrasive was a silicon carbide with a grain size of the main fraction of 0.8 mm.The test results are presented in table 2. Hardened steel U8 (εU8 = 1) was taken as the standard.It can be seen that also in the case of three-body abrasive wear, high wear resistance of 120Mn3Si2 steel is achieved after hardening from 900 °C and 1000 °C.When testing materials with low hardness (annealed steel, iron, etc.) using this method, abrasive grains are pressed and embedded into the friction surface.Subsequently, friction occurs not between the abrasive and the material being tested, but between the free abrasive and the abrasive particles that are fixed in the surface.In this situation there is no wear of the sample, therefore softer materials, for example iron or annealed steels, may appear to be "more wear resistant".In this regard, only materials with relatively high hardness (initial or acquired during wear) are subject to testing; in this case, the friction surface is free from embedded abrasives and the test results obtained reflect the actual wear resistance of the materials.

Micro-hardness of worn surface
Measurements of the microhardness of the worn surface were carried out on samples after three-body abrasive wear testing due to their rather large sizes.Indentation was carried out directly on the worn surface without preliminary polishing.This made it possible to preserve the hardened surface layer, the thickness of which is about 10 microns, and to measure its microhardness.
Indentation of a roughly rough surface after abrasive wear is accompanied with some inconveniences associated with finding a place for an indentation.However, experience has shown that if the load is 50 g, than the imprint size is small enough, and it is possible to find surface areas suitable for indentation.figure 1 illustrates several imprints on a worn surface after measuring microhardness.
It was established that the microhardness of the friction surface of 120Mn3Si2 steel sample after hardening from 1000 °C and abrasive wear varied in the range of 950-1800 HV0.05.The maximum probability density corresponded to a microhardness of 1400 HV0.05.
The microhardness of the friction surface of 120Mn3Si2 steel sample after quenching from 900 °C and abrasive wear varied in the range of 800-1700 HV0.05.The maximum probability density corresponded to a microhardness of 1300 HV0.05.
The recorded microhardness values significantly exceed those for high-carbon steels in the hardened state (~860 HV) and cementite-type carbides (~1000 HV).This indicates a significant difference in the structure of the martensite of the strengthened layer and the martensite that as a result of quenching.

Analysis of the microstructure and friction surface of samples
The electronic images of cross-sections of the near-surface region of 120Mn3Si2 steel samples after quenching from 900 °C with subsequent wear (figure 2, a) and after quenching from 900 °C with subsequent wear (figure 2, b) are presented on figure 2.
At 900 °C, the structure of the metal base of the steel is austenitic with inclusions of undissolved cementite.After hardening, cementite remains in the structure in the form of relatively large carbide inclusions (3 in figure 2, a).At a temperature of 1000 °C, the material passes into the single-phase region, and the carbides are almost completely dissolved.After quenching, austenite with single small carbide inclusions (3 in figure 2, b) is fixed.
The specific surface profile is formed by multiple contacts with abrasive grains.Slip lines decorated with etching pits at their intersections indicate intense plastic deformation of the subsurface layer of the material.Such places are clearly visible in areas 1 in figure 2, a and 2, b (15-20 μm from the surface).Austenite that is susceptible to plastic deformation is still preserved here.
At a shallower depth from the surface (down to 10 µm), both images show a continuous layer of material without slip lines, but with a specific relief pattern.Judging by the high microhardness, significantly exceeding 1000 HV0.05, this layer is formed by martensite, obtained as a result of the transformation of unstable austenite during plastic deformation by an abrasive.However, both the hardness and wear resistance of deformation martensite significantly exceed the hardness and wear resistance of quenched martensite.Possible reasons are discussed below.
Figure 3 shows electronic images of the friction surface of 120Mn3Si2 steel samples after quenching from 1000 °C and wear in two-body abrasive wear mode (figure 3, a) and three-body abrasive wear mode (figure 3, b).When worn in two-body abrasive wear mode, the destruction of the surface occurs as a micro-scratching or removal of material by scratching or shearing during a single interaction with the abrasive.There are virtually no signs of fatigue failure.The friction surface of the sample after three-body abrasive wear is significantly different (figure 3, b).Multiple traces of fatigue spalling are observed, which indicates repeated interaction of the same surface area with the abrasive.

XRD analysis of worn surface
Figure 4 shows the diffraction maxima (211) of martensite for steel 120Mn3Si2 after quenching from 900 °C and after quenching from 900 °C with subsequent three-body abrasive wear.Analysis of profiles obtained before and after wear indicates significant changes in the material.
The broadening of the (211) line indicates an increase in the level of internal stress in the crystal lattice.This is understandable, because martensite, which is formed during abrasive wear, inherits the dislocation structure of austenite, which is significantly strengthened by plastic deformation before transformation.Calculation has shown that the dislocation density in the hardened material of the friction surface is estimated at 10 14 cm -2 and corresponds to the maximum possible dislocation density, which, during fracture, occurs in the immediate vicinity of the incipient micro-crack.It should also be noted that the diffraction maximum (211) of martensite shifts towards smaller 2Θ angles.This indicates an increase in the α-lattice parameter and can be explained, for example, by an increase in the carbon content in the solid solution during plastic deformation due to the dissolution of cementite [16].However, an accurate determination of the reasons for this phenomenon and a quantitative assessment of the increase in the content of carbon or other elements in the solid solution after abrasive wear requires further research.

Conclusions
In the course of abrasive wear of 120Mn3Si2 steel, that is hardened at temperatures in the range of 900-1000 °C, a deformation-induced martensitic transformation of unstable retained austenite occurs.As a result, high wear resistance is achieved during both two-body and three-body abrasive wear.
A continuous layer with a microhardness of 1300-1400 HV 0.05 is formed on the friction surface, which significantly exceeds the hardness of most soils with which the working parts of mining machines come into contact during operation.Such hardening can be used to increase the wear resistance of wear parts operating under abrasive wear conditions and to increase the overhaul life of equipment.

Figure 4 .
Figure 4. Diffraction maxima (211) of martensite for steel 120Mn3Si2 after quenching from 900 °C (black line) and quenching from 900 °C with subsequent three-body abrasive wear (red line).

Table 1 .
Results of two-body abrasive wear tests.

Table 2 .
Results of three-body abrasive wear tests.