1. Introduction
Equipment used in mining, oil and gas, metallurgy, and other industries operate in conditions that cause severe wear from friction, aggressive environments with a high content of abrasive particles, high temperatures, and cyclic and dynamic loads. Therefore, it seems necessary to use new, advanced, economically viable technologies that make it possible to achieve high reliability of modern equipment wear parts [
1,
2,
3,
4,
5,
6]. A number of requirements are placed on the technical condition of mechanical engineering products. The most important among them are the reliability and maintenance of the operational properties of the product throughout its life cycle [
7], since the occurrence of wear and corrosion emergencies, as well as the mechanical destruction of equipment parts, can lead to environmental risks [
8].
An extension of the service life of the equipment is achieved by reducing the wear and tear of the responsible parts through constructive, technological, and operational measures. When designing devices and machines, special attention is paid to the rational selection of construction materials [
9,
10,
11,
12], the study of wear processes [
13,
14,
15,
16,
17], corrosion [
18,
19] and tribocorrosion [
20,
21,
22], the investigation of temperature [
23,
24], and the stress state of components [
25,
26], including those with functional coatings [
27,
28]. The successful healing of cracks in solids is facilitated by welded [
29] and laminated [
30,
31,
32] layers. Also important is the problem of contact interaction of coatings with monolithic abrasives and loose abrasive particles [
33].
The specified accuracy and quality of the working surfaces of the parts is achieved using technological methods of mechanical processing [
34,
35,
36], taking into account the technological heredity [
37] and the rational selection of metalworking machines [
38], since the occurrence of errors in the processing of parts and the performance of assembly processes leads to growth stresses in components with coatings [
39].
The favorable combination of properties between the reinforced surface layer and the base metal can be achieved by using various surface engineering processes. Extension of the service life of critical parts is provided by the methods of hardening using surface deformation [
40,
41], electrochemical Cr plating [
42,
43], chemical-thermal treatment [
44,
45], electrospark doping [
46,
47], laser doping [
48,
49,
50], laser cladding [
51], plasma hardening [
52,
53], thermal spraying [
54,
55], supersonic spraying [
56,
57,
58], vacuum-arc deposition [
59,
60,
61], formation of oxide and nitride coatings by combined methods [
62,
63,
64,
65,
66,
67], manual arc hardfacing [
68,
69], and flux-cored arc welding [
70,
71,
72,
73,
74,
75,
76].
A comparison of different surface reinforcement techniques shows that cored wire arc welding has many advantages, the most important of which are high productivity and coating thickness, the possibility of depositing high-alloy alloys, and ease of use. Such a method can also be used to apply hardfacing to new wear parts as well as to restore worn ones. In addition to experimental methods for the development of materials for wear-resistant coatings, the results of thermodynamic modeling [
77,
78], the calculation of crystal lattice properties [
79,
80,
81], the determination of factors influencing corrosion resistance [
82], and a rational choice of deposition process parameters [
83,
84] are very important for achieving the maximum result of surface reinforcement. When developing technological processes for depositing coatings [
85] and modeling metal melting processes [
86], it is also necessary to take into account the influence of external factors, as well as to apply intelligent methods for comparing energy processes [
87].
As an alternative for the more expensive materials, such as W, Co, Nb, systems based on Fe-Cr-C, Fe-Mn-C-B, and Fe-Ti-C [
88,
89] are used more often, with the addition of metals from the IV-VI group of the periodic table of chemical elements, such as Nb [
67], Ti [
90,
91], Mo [
92], Ta [
93], V [
94], and W [
95]. Research is also being conducted into the use of boron (B) systems. Mostly, ferrous alloys are used as starting components for hardfacing materials [
64,
65,
68,
90,
92,
93]; however, there are studies on the microstructure and properties of materials manufactured using pure starting components, such as metal powders [
88,
96,
97] that have improved mechanical and tribological properties. For example, the results for systems with ferrous alloys (ferroalloys) as starting components [
92] indicate that the addition of Mo leads to intensive cracking, while in systems with pure metal powders as starting components [
97], cracking wasn′t observed with increasing amounts of Mo.
The analysis of recent scientific work on reinforcement technologies and the chemical composition of hardfacing electrodes shows that the development of new tungsten-free systems for flux-cored electrodes is a promising research area. As already mentioned, Fe-Cr-C and Fe-Ti-C, with the addition of some elements, are widely used systems for FCAW restoration and increasing the wear resistance of machine parts. However, there are also other systems that can be considered as prospects for the development of electrodes using pure metal powders instead of ferroalloys as starting components.
From this viewpoint Fe-rich alloys of the Fe-Mo-B-C alloying system is the promising candidates for hardfacing alloy development due to the presence of a hard and thermodynamically stable Fe(Mo,B)
2 phase, known as τ
2—phase [
98], which can be formed during welding, as shown in [
97,
99,
100]. Further improvement in the properties of such alloys can be achieved by refining the grain structure and increasing the properties of the steel matrix phase and expanding the alloy system. Therefore, the effects of the Ti and Mn additions on the microstructure, abrasion, and impact-abrasion wear resistance of the Fe-Mo-B-C hardfacing alloys were investigated.
Purpose and tasks of research.
The aim of this work was the design of the Fe-based hardfacing alloys within the Fe-Mo-Ti-B-C and Fe-Mo-Mn-B-C alloying systems for use as materials in FCAW hardfacing. To achieve this goal, the following tasks were set:
- (1)
Determination of the optimal composition of the hardfacing alloys of the Fe-Mo-Ti-B-C and Fe-Mo-Mn-B-C systems in a range that is suitable for the production of flux-cored wires from pure metal powders as starting components;
- (2)
Investigation of the effects of Ti and Mn additions on the microstructure and properties of Fe-Mo-B-C hardfacing alloys;
- (3)
Determination of wear resistance by dry sand and rubber wheel abrasion and impact-abrasion tests compared to the wear resistance of commercially available hardfacing alloys.
2. Materials and Methods
2.1. Thermodynamic Analysis
The determination of the optimal composition of the hardfacing alloys of the Fe-Mo-Ti-B-C and Fe-Mo-Mn-B-C systems were performed using Thermo-Calc version 2022a software [
101] with the Thermo-Calc Software TCFE12 Steels/Fe-alloys database [
102]. The starting relationship between components for the thermodynamic calculations was chosen according to the possible amounts of the alloy components that can be placed into flux-cored wire with the steel sheath while maintaining the equimolar ratio between Mo and B.
2.2. Preparing Starting Components
In order to obtain a high intensity of chemical reactions and dissolution precipitation processes as well as high degree of alloying of the hardfacing layer, the commercially available metal powders of Ti (PTS-1 TU 14-22-57-92 grade manufactured by JSC “Titanium Institute”), Mo (MPCh TU 48-19-69-80 grade, manufactured by CJSC «Promelectronica»), and Mn of MN95 grade with the fine particle size (in range of 1–5 μm) were used. The B-containing compound powders were used as a technologically favorable source of B and C, which ensure the formation of the refractory hard phases. To protect the arc from the atmosphere and improve arc stability during welding, fluorite and rutile were added to the powder mixture. To dry the components before weighing, they were dried in a SNOL-type drying oven at a temperature of 120 °C for 1.2 h. Mixing of the components was carried out in a laboratory gravity tumble mixer with an inclined axis of rotation for 8 h. An additional drying at 120 °C for 0.5 h was performed to prevent the influence of air humidity on the quality of the powder mixture.
2.3. Manufacture of FCAW Electrodes
Experimental hardfacing materials were fabricated as an FCAW wire (according to ISO 14343:2017) with overlap joint (
Figure 1) by drawing prepared flux powder into a sheath of 08kp low carbon steel (standard DSTU EN 10139:2018), with a size of 0.5 to 20 mm. Chemical composition and mechanical properties of 08kp steel are shown in
Table 1 and
Table 2. Such a construction of the FCAW wire contributes to more stable burning of the arc, as well as more uniform heat propagation through the flux charge. The experimental flux-cored wire was manufactured using the equipment of the Interdisciplinary Research and Production Center “Epsilon LTD”, Ivano-Frankivsk, Ukraine.
After the wire was made, it was cut into 420 mm long electrodes for convenient determination of electrode characteristics and application testing in manual arc mode. The determination of the chemical composition of the electrodes was based on the ratio (filling coefficient
) between powder filler and steel sheath
where
is the weight of the filled electrode and
is the weight of the empty electrode, which is equal to 28.5 g; the weighing accuracy was 0.01 g, and the calculation was performed by weighing three electrodes for each composition. The filling coefficient of the experimental electrodes was about 30%.
2.4. Hardfacing Process
A steel plate (material steel 40 standard DSTU 7809:2015) with a crosssection of 15 × 20 mm was chosen as the substrate. The chemical composition and mechanical properties of 08kp steel are shown in
Table 3 and
Table 4.
FCAW hardfacing mode was chosen to allow for a more intense transfer of the electrode material to the base material, with a direct current of 170 A with reversed polarity and an arc voltage of 30–32 V using a VDU-506 rectifier. Hardfacing coatings were manually deposited into three layers to minimize the effects of mixing the coating with the substrate material; thus, the coating thickness was about 5 mm. Air-cooling was carried out at 20 °C.
2.5. Microstructure Observation and Mechanical Properties Measurement
The microstructure of the hardfacing layer and the morphology of the worn surfaces was observed by scanning electron microscopy (SEM) using a ZEISS EVO 40XVP microscope at the Center for Collective Use of Scientific Instruments “Center for Electron Microscopy and X-ray Microanalysis” of the Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Lviv, Ukraine. The chemical composition of the phases was examined using energy-dispersive X-ray spectroscopy (EDS). Considering the difficulties in identifying boron and carbon using the EDS technique, the composition of boride phases was determined based on the ratio of metal components, and the carbon content was determined only for carbide phases. The microhardness distribution at the interface between the hardfacing layer and the base metal was measured with the Vickers pyramid at a load of 0.1 N, using a PMT-3 hardness tester. Macro-hardness was measured by means of the average measurements taken from the top surface of the experimental hardfacing coatings and using the Rockwell method, scale “C”, with a modernized TK-2 hardness tester.
2.6. Abrasive Wear Test in Loose Abrasive Condition
Tribological tests of hardfacing alloys were performed in the loose abrasive state, since such a wear mechanism is most common for machine parts in the metallurgical, agricultural, woodworking, and mining industries. Such a wear mechanism is also one of the main causes of wear during the operation of machine parts under conditions of intense abrasive action. The abrasive wear tests were performed on the developed testing machine of material in a loose abrasive condition (
Figure 2), which was designed with regard to the recommendations of the standard (ASTM G65-16 (2021) Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus). The test process is as follows:
The fixing lever (1), installed on a hinged support, presses the hardfacing flat sample (10) to the rubber wheel (5). The required value of pressing force is provided with the appropriate selection of the value of Li, installation of the removable weight (2), and selection of its mass. The supply of abrasive particles (9) flows through the bowl (4) into the friction zone formed by contact of the flat sample (10) with the rubber wheel’s cylindrical surface (5). The linear velocity vector of the rubber wheel (5) coincides with the direction of flow of the abrasive particles (9) into the friction zone. Due to the friction between the rubber wheel’s cylindrical surface (5) and the surface of the flat sample (10), abrasive particles (9) are thrown into the friction zone, partially deepened into the rubber, and then interact with the hardfacing, causing wear due to scratching and cyclic deformation.
The properties of the rubber wheel are as follows: diameter, D = 50 mm; width, b = (15 ± 0.1) mm; hardness of the rubber, 78–85 Shore units; relative residual elongation of the roller material at break, 15–20%. The harness of the rubber surfaces was measured according to the ASTM 2240 Standard using a tire durometer of type “D”. The proposed installation of the fixing lever at an angle to the horizontal creates improved conditions for the supply of abrasive particles into the friction zone. The non-parallelism of the horizontal axis of the rubber wheel to the working surface of the flat sample was not more than 0.1 mm.
The testing machine (
Figure 3) consists of an electric motor (6) equipped with a device for regulating the speed of the motor (7), causing the rubber wheel (5) to rotate about a horizontal axis.
The wear tests were performed using a silica “Alfa-Quartz” (SiO
2) sand abrasive manufactured by Ekkom Plus, Kyiv, Ukraine with a grain size in range of 0.2–0.4 mm. The content of impurities in abrasive mass was not more than 2% (Standard DSTU B V.2.7-131:2007). As can be seen (
Figure 4), particles are characterized by a non-equiaxed grain shape with sharp edges at the vertices, indicating their high abrasive ability.
Samples with the experimental hardfacing layer and standard materials were made in the form of samples measuring 15 mm wide, 20 mm long, and 25 mm high, with dimensional tolerances according to the 14th quality of accuracy and roughness and a work surface not lower than 1.25 μm.
The friction mode for wear resistance tests was chosen as follows: load, p = 2.4 kN; roller speed converted to linear speed, 30 m/min; test duration, 220 min. The duration of the tribological tests was measured with a stopwatch.
The amount of weight loss due to wear was determined by gravimetric method: weighing the test specimens on the analytical axis of VLA-200 g-M (weight accuracy of 0.1 mg) before and after the test. The weight loss of the sample due to wear during the tests was at least 5 mg. Before each weighing, the samples were thoroughly wiped with alcohol and dried. The arithmetic mean value from three tests for wear of the hardfacing layer was chosen as the resulting value for determining the wear resistance.
The impact-abrasion wear tests were conducted using a laboratory machine that provided the mode in which a cemented carbide striker impacts the sample with an abrasive environment present in the contact zone. The impact-abrasion wear tests were conducted using a laboratory machine described in [
103] providing the mode in which a cemented W-carbide striker impacts the sample with an abrasive environment present in the contact zone. The impact energy was set to 2.4 J/cm
2 and the chilled white cast iron particles with an average size of 1 mm were chosen as the abrasive environment.
4. Conclusions
The optimal composition of the Fe-rich alloys of Fe-Mo-Ti-B-C and Fe-Mo-Mn-B-C was determined using the CALPHAD approach as implemented in the Thermo-Calc software. Comparing the modeling results with experiments agrees well with the compositional ranges that are promising for the development of hardfacing alloys.
It has been observed that the addition of Ti to the Fe-Mo-B-C hardfacing alloy results in significant grain refinement due to the formation of TiC, which acts as a structural modifier, while the Mn addition results in the formation of Mn-austenite in the shape of dendritic grains and lamellar eutectic Mn-austenite and Fe(Mo,B)2.
The abrasion and impact-abrasion wear tests show that alloying the Fe-Mo-B-C system hardfacing alloys with Ti results in an increase of up to 1.2 times in their abrasion wear resistance, and the addition of Mn results in an increase in impact-abrasion resistance up to 1.3 times. That creates prerequisites for the industrial use of developed alloys in the form of flux-cored wires for FCAW hardfacing.
Further investigations need to be carried out on the effects of Mn dissolution on the mechanical properties and thermodynamic stability of Fe(Mo,B)2.