RESTORATION OF THE IMPACT CRUSHER ROTOR USING FCAW WITH HIGH-MANGANESE STEEL REINFORCED BY COMPLEX CARBIDES

: A new hardfacing alloy within the Fe-Ti-Nb-Mo-V-C alloying system was utilized to restore the working surfaces of cone crusher rotors using Flux-Cored Arc Welding (FCAW). TiC, NbC, Mo 2 C, VC, Mn, and ferromanganese powders were selected as the base materials for manufacturing the welding wire. The resulting hardfaced layer exhibits a composite structure, with manganese austenite as the matrix and complex solid solution reinforcements with a NaCl structure, closely resembling the formula (Ti 0.3 Nb 0.3 Mo 0.3 )C. The primary advantages of this hardfacing alloy include its capacity for intensive deformation hardening along with high abrasion resistance. The hardness of the hardfaced layer is approximately 47 HRC in the as-deposited state and increases to around 57 HRC after work hardening, surpassing typical hardfacing alloys derived from high manganese steel by about 10 HRC. The efficacy of the alloy was tested in restoring rotors made of Hadfield steel in a PULVOMATIC series crusher model 1145, during the milling of sand-gravel mixtures ranging from 25 to 150 mm into spalls measuring 5 to 20 mm. With an average productivity of approximately 60 tons per hour and a production volume of 300 tons, the utilization of this hardfacing alloy enabled multiple restorations of the rotor while maintaining productivity at a level of 15 thousand tons of spalls.


INTRODUCTION
Any construction regardless of its type -a new one or reconstruction of buildings, construction or repair of roads is always accompanied by the consumption of significant volumes of construction materials.At the same time, construction is one of the largest sources of waste in Ukraine and the world.These wastes include concrete, bricks, asphalt, soil, etc., and their processing allows for obtaining valuable secondary materials that can be reused in civil and industrial construction, road construction, and other industries.In particular, it is possible to obtain high-quality coarse aggregates for the production of concrete mixture from processed from the demolition or destruction of buildings and structures.The obtained secondary aggregates can be used as a partial or complete replacement of natural aggregates, contributing to the development of a circular economy and reducing the consumption of natural mineral resources.Therefore, managing the production of new construction materials and recycling waste from the construction and road industries is of primary importance to reduce the burden on the environment, as well as to obtain valuable secondary raw materials that can be reused in new construction projects.One of the urgent tasks, the solution of which will allow the implementation of the strategy of sustainable development, is the rational crushing of rocks to obtain construction materials and the processing of construction waste, and this requires the development of both new specialized equipment for effective crushing and the restoration of worn out ones.The article presents the results of strengthening and testing the rotor of the crusher, restored by electric arc welding of the coating from the developed tungstenfree alloy, directly at the place of operation of the crusher in the quarry.The work ends with conclusions relating exclusively to the results of rotor wear tests, as well as a several of modern literary references.

LITERATURE REVIEW
Today, cone crushers of various sizes and modifications are widely used during the production of building materials by crushing rocks and processing waste from industrial, civil, and road construction [1,2,3].This technique is especially relevant for the implementation of strategies for effective management of waste generated in connection with the destruction of buildings and structures as a result of military operations, man-made accidents, natural disasters and natural phenomena, etc. [4,5] thereby ensuring a reduction of environmental pollution [6,7].One of the main problems during the operation of equipment for crushing rocks and construction waste is that their main nodes are subjected to large dynamic loads, which lead to intensive shock-abrasive wear [8,9], degradation [10] and fatigue damage of workers elements of parts with the formation of cracks [11].Papers [12,13] investigated the stress-strain state of twolayer coatings formed on a steel base under the action of an arbitrarily oriented local load.In the article [14] an assessment of the strength of the layered composition was carried out, taking into account the contact interaction of heterogeneous layers under axial load.The mechanical interaction of hard alloy coatings with fixed and unfixed abrasive was studied in [15].The effect of non-contrast surfacing on the effectiveness of healing surface cracks in solids was studied in papers [16,17,18].The authors of the articles [19,20,21] also note the positive role of composite layered and functional-gradient coatings in preserving the structural integrity of damaged thin-walled structures, which in particular include rotors.Engineers and technologists use several design, technological and operational measures to reduce the harmful effects of rotor wear and destruction on the production process.The first approach consists of optimizing the design parameters of the crusher, in particular the shape and profile of the working bodies and bearing assemblies [22], ensuring effective lubrication of the bearing supports of the rotors [23,24] and reliable connection of the rotors with their drives [25].The development of rational routes [26], the choice of processing modes of functional surfaces and equipment [27,28,29] to ensure the accuracy and quality of working surfaces [30,31,32] plays an important role during the manufacture of new parts for the restoration of worn parts of machines and equipment.Special attention is paid to careful static and dynamic balancing of rotors [33,34].Recently there was published a paper [35] on optimal control for rotor imbalance suppression.The authors presented a system of moving balancing masses and evaluated the optimal displacement of the balancing masses to minimize the imbalance on the rotor.The optimal displacement is given by open-loop control, which solves the optimal control problem posed in infinite time.The existence of an optimum is proved using the methods of calculus of variations and the corresponding optimality conditions are derived.The asymptotic behavior of the control system is rigorously investigated.Using Loyasevich's inequality, the optimums converge with time t → +∞ to a stable configuration.An explicit estimate of the convergence rate is given.This ensures that the optimal control stabilizes the system.In the case, when the imbalance is less than the calculated threshold, convergence is exponentially fast.Using the slice function and the directional derivative, as in [36,37], we can consider the optimal control problem for rotor imbalance in any direction, not just along the real axis.The phenomenon of severe vibration is common during crushing operation, and its vibration comes from many sources, primarily from the high-speed rotating impact of the cutting mechanism during stone shredding and rock drilling [38].Due to the structural shape and intermittent and uneven cutting, shredder vibration is a real problem that can only be optimized but cannot be completely avoided.Modal analysis is an effective method for studying the vibration characteristics of a structure and is the basis for dynamic analysis.The universal mechanical equation for the dynamics of the object is justified in [39]: [K] are the quality, damping, and stiffness matrices, respectively, {F(t)} is the force vector, {x}, {}, {} are the displacement, speed, and acceleration vectors, accordingly.It is the system of second-order linear non-homogeneous differential equations.Its exact solutions are given as entire curves [40,41] represented by a sum of exponential polynomials and a partial vector-valued solution generated by the non-homogeneous part, i.e. the force matrix.Moreover, the curves have interesting local and asymptotic properties as functions having bounded index in direction [36,37].Therefore, it is advisable to equip crushers with modern anti-vibration devices and dampers of various designs to minimize the harmful effects of dynamic loads on their parts and assemblies [42,43,44].No less important are also preventive measures, i.e. the introduction of modern innovations in the management of the mining industry [45,46], quality management [47] of intelligent computer control systems [48] of diagnostics and monitoring of the technical condition of crushers, which make it possible to detect signs of wear in advance [49] and plan timely repairs, including in production conditions [50].The application of technologies for the repair of equipment in industry [51] and the use of wear-resistant materials to strengthen their working surfaces during the manufacture and repair of parts that are subject to intensive wear [52,53,54] allows you to significantly save steel, which is relevant in view of the current state of their production [55].In this direction, the application of smoothing treatment even at the stage of obtaining workpieces of parts such as bodies of rotation by centrifugal casting with tungsten carbide reinforcement of the working zone provides a significant increase in wear resistance [56].However, the high cost of tungsten carbide holds back the widespread adoption of this technology in production.Typical materials used in alloys to ensure impact-abrasive wear resistance include tungsten carbide phases (WC and W2C).However, the cost of raw materials for obtaining tungsten carbide phases has significantly increased over the past decades and shows a further upward trend.WCbased materials, due to their versatility, are widely used in various industries such as material processing, oil and gas equipment, mining machinery, cermets, wear-resistant alloys, and coatings.In the latter case, the use of tungsten materials is not always justified, as several transition metal compounds demonstrate comparable or higher microhardness alongside relatively low cost according to data from the United States Geological Survey (USGS) [57] (see Fig. 1).

Fig. 1 Comparative characteristics of the cost of refractory metals used in hardfacing alloys and the hardness of their carbide phases
Furthermore, tungsten carbides interact intensively with the melt of high-manganese steel, leading to the formation of complex carbide phases such as (W, Fe, Mn)6C.High-chromium alloys, as an alternative to tungsten materials, are not rational for use under impact-abrasive wear conditions due to their low toughness resistance.Another effective improvement in repair technology is, in particular, the use of new tungsten-free alloys for the restoration of worn surfaces of parts by surfacing.Theoretical approaches to the development of such new compositions of wear-resistant alloys are considered in papers [58,59,60,61].The authors [62,63] investigated the influence of the filling factor of powder electrodes and exothermic applications on stable arc combustion during electric arc welding of coatings with powder electrodes.In the paper [64] it is proposed to use oxygen-acetylene powder surfacing of self-fluxing NiCrSiB powders to prevent wear of cast iron.It was established that the amount of weight wear of the NiCrSiB alloy is smaller than the reference samples from X5CrNi18-10 and EN-GJL-200.To increase the wear resistance of low-carbon steel and gray cast iron, researchers [65] proposed forming composite layers of the TiC/Fe type using self-propagating high-temperature synthesis, which are characterized by high resistance to various types of wear.The proposed measures make it possible to increase the abrasive wear resistance [63,66] and impact-abrasive wear resistance of steels [64, 65 67, 68], which ensures the extension of the service life of various steel parts.The results of research [66,67,69,70] indicate the prospects of using tungsten-free alloys for electric arc surfacing of working elements of rotors of expensive equipment for crushing rocks and destroyed building elements.The purpose of the research is to develop a technology for restoring worn surfaces of cone crusher rotors with a tungsten-free alloy.To achieve the goal, the following tasks must be solved: − examin of the worn rotors of the cone crusher; − to develop an alloy of Fe-Ti-Nb-Mo-V-C alloying system and rotor restoration technology; − conduct industrial tests of restored cone crusher rotors.
As observed in the figure, the initial NbC particles are characterized by a shape close to spherical and an average size of approximately 1 µm (Fig. 2a).The pseudo-alloyed TiC (Fig. 2b) and Mo2C carbides used were characterized by angular shapes and sizes ranging from about 50 to 100 µm, while VC powders comprised both dispersed particles and conglomerates up to 200 µm in size.Carbide phases with this particle size distribution allow for a high level of manufacturability in the production of powder electrode materials.Considering that the use of exclusively fine-dispersed powders does not provide the necessary level of flowability to achieve a high level of powder strip filling coefficient, the chosen carbide sizes are advantageous.To ensure the required manganese content, a combination of Mn powders (grade MN997 (GOST 6008-90)) and ferrosilicomanganese grade MNS17 (GOST 4756-91, ISO 5447-80), produced by PJSC "Nikopol Ferroalloy Plant", Ukraine, was used for alloying.The technological process of producing powder strips was conducted at MNVC "Epsilon LTD", Ukraine, using a stand for manufacturing electrode materials (Fig. 3a) in the form of single-lock powder strips (Fig. 3b).

Fig. 2 Morphology of carbide phase powders used for the production of experimental hardfacing alloys: a -NbC, b -TiC, c -VC, d -Mo2C
The stand for manufacturing electrode materials consists of a dispenser for controlled delivery of the powder mixture (item 1, Fig. 3a), forming rollers (item 2, Fig. 3a), rollers for feeding strips of low-carbon steel 08kp (DSTU EN 10139:2018) with a width of 20 mm and a thickness of 0.5 mm (item 3, Fig. 3a), a mechanism for feeding the core from the powder strip (if necessary) (item 4, Fig. 3a), exhaust ventilation (item 5, Fig. 3a), control unit (item 6, Fig. 3a), and cutters for cutting the strip into electrodes with a length of 400 mm (if necessary).

a) b) Fig. 3 Stand for manufacturing powder strips (as labelled in the text) (a) and its construction (b)
Considering the constraints imposed by the design of powder electrode materials (the powder content of carbide phases), the optimal elemental composition of the hardfacing alloy for the "high-manganese steel -multi-component carbide" system will correspond to that outlined in Table 1 and marked as 360G15M6B6T3C3F.

Table 1 Composition of the alloy for hardfacing in the "high-manganese steel -multi-component carbide" system
The content of components in the hardfacing alloy 360G15M6B6T3C3F, wt.% Based on macrohardness measurements using Leeb's dynamic hardness testing method and Rockwell surface hardness indentation, the hardness of the deposited layer with this alloy is approximately 47 HRC after deposition and approximately 57 HRC after deformation strengthening.A Fe-Ti-Nb-Mo-V-C alloy was developed to restore the worn surfaces of the cone crusher rotors.FCAW surfacing mode: direct current 170 A with reverse polarity and arc voltage 30-32 V. Power source -rectifier VDU-506.The deposited coatings were manually applied to the rotor blade in industrial conditions in several layers.Cooling of the welded coating was carried out in air at an ambient temperature of 20°C.

Fe
The microstructure and element distribution of the hardfaced layer were determined by scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (EDS) using scanning electron microscope ZEISS EVO 40XVP (ZEISS Group, Jena, Germany) with a micro Xray spectral analysis system and an INCA ENERGY 350 (Oxford Instruments, Abingdon, UK) energy dispersive X-ray spectrometer.

RESULTS AND DISCUSSION
The necessity to intensify infrastructure reconstruction efforts in Ukraine during both wartime and post-war periods requires the application of a wide range of equipment for the production of construction materials, including foreign-made equipment designed for operation in challenging conditions.Among such equipment are impact crushers from the PULVOMATIC series crusher model 1145 (Spain) (Fig. 4a), designed for crushing a wide range of materials such as gravel, crushed stone, concrete, etc.
The main working element of these crushers is a multisection rotor made of Hadfield steel using casting methods (Fig. 4b).

Fig. 5 General view of PULVOMATIC series crusher model 1145 (a), raw material (b), and finished product (c)
The analysis of the operation of the PULVOMATIC series crusher model 1145 (Fig. 5a) at a rotor rotation frequency of 1500 rpm during the crushing of a sand-gravel mixture with an initial particle size distribution of 25-150 mm (Fig. 5b) to produce gravel with a primary final particle size distribution of 5-20 mm (Fig. 5c) revealed an average processing capacity of approximately 60 t/hour, with an operating period exceeding 300 hours.
The results indicate significant wear of the rotor's working surfaces under these conditions, leading to the catastrophic impact-abrasive wear (Fig. 6), which is the cause of fatigue failure of the rotor at weakened areas and, consequently, the occurrence of potentially hazardous situations.

Fig. 6 Character of catastrophic wear of the rotor of the PUL-VOMATIC series crusher model 1145
Considering that the cost of a new rotor is approximately €6000, and taking into account additional expenses related to logistics, ensuring the profitability of operating this crushing equipment in the conditions of Ukraine requires a more optimal approach.Moreover, material costs associated with equipment downtime due to complicated logistics, especially during wartime, are an additional factor negatively impacting production profitability.Therefore, it was proposed to reduce the volume of processed raw materials from 20 to 15 thousand tons, i.e., to conditions where catastrophic wear does not occur.Under these conditions, the wear of the working surfaces (Fig. 7a) allows for multiple restoration of the working surfaces using the hardfacing method (Fig. 7b) by using the developed alloy Fe-Ti-Nb-Mo-V-C to restore the worn surfaces of the rotors of cone crushers.It should be noted that after the restoration of worn rotors, it is necessary to balance the rotors, including in production conditions.The deposited layer (Fig. 8) consists of an austenite matrix reinforced with a complex carbide solid solution corresponding to the formula (Ti0.3Nb0.3Mo0.3)C,which is uniformly distributed within the steel matrix in the form of grains with cuboidal morphology.The average size of the carbide grains is ∽3 µm, and their microhardness is higher than 20 GPa.The EDS analysis of the fusion zone shows a smooth transition from the base material to the hardfaced layer.Defects like pores, cracks, and delamination were not observed, indicating strong metallurgical bonding.In the regions corresponding to the carbide inclusions, simultaneous peaks of the carbideforming elements were detected, indicating the presence of a solid solution.The obtained composite structure of the hardfaced layer creates prerequisites for the combined deformation and dispersion strengthening of the surface layer during impact loads in abrasive environments.

Fig. 8 The microstructure and results of the EDS analysis of the fusion zone of the hardfaced layer with 360G15M6B6T3C3F alloy
In further studies, it is planned to optimize the technological regimes of flux-cored arc welding rotors.

CONCLUSIONS
As a result of the analysis of the working conditions and examination of the rotors of the cone crusher, it was established that the main reasons for their failure are shockabrasive wear and fatigue failure.
The results of industrial tests showed high wear resistance of the deposited coating from the developed Fe-Ti-Nb-Mo-V-C alloy system and its industrial suitability for restored cone crusher rotors.
Our developed technology allows us to perform restoration work directly at the crushers' operation site, reducing non-productive downtime.This is particularly important due to the increasing demand for raw materials needed for producing construction products essential for the reconstruction of Ukraine's industrial and civil infrastructure.

Fig. 7
Fig. 7 Worn and restored sections of the rotor of the PULVOMATIC series crusher model 1145 (a) and the overall appearance of the rotor after restoring the geometry of worn surfaces (b)