Tribological properties and wear resistance of the metal deposited by chromium flux-cored wire with carbide-boride alloying

The wear resistance and the tribotechnical properties of chromium steel of the Fe-C-Cr-B system deposited by flux-cored wire with boron carbide alloyed were investigated. It is shown that this steel has high wear resistance. The average values of the mass wear of the deposited metal is 0.0028 g/m, and the linear values are 0.00723 mm/m. The average value of the friction moment was 21.82 N·m, and the coefficient of friction was 0.412. The hardness of such a metal reaches a maximum value of 58 HRC. The microhardness of structural objects for the matrix is 499-603 HV, for hardening phases is 859-924 HV. The metal structure is a martensitic matrix with a large number of reinforcing phases, formed on the basis of type carboboride (Cr, Fe)7(CB)3. It is shown that the mechanism of metal wear is associated with its high hardness due to the dispersed hardening by particles of compounds, which are effective obstacles to dislocation slip under conditions of plastic deformation of the surface during wear and a decrease in the role of the abrasive component of wear.


Introduction
One of the directions in modern engineering is the surface hardening of relatively cheap steels used for the manufacture of machine parts and mechanisms operating in difficult exploitative conditions. One of the methods of hardening, which is actively developing at the present time, is surfacing working surfaces by wear-resistant flux-cored wires.
A significant range of parts of various industries, is made of stainless chromium steel, combining a fairly high strength and corrosion resistance [1]. Such steels are used as surfacing materials to obtain wear-resistant coatings on parts for a wide range of purposes. Based on this, flux-cored wires of the Fe-Cr system have been developed [2,3]. At the same time, the wear resistance of the metal deposited by such wires is not very high. This is due to the small number of hardening phases in the structure of coatings deposited by such wires. On this basis, a promising way is the development of coatings of their chromium steel with reinforcing phases of high hardness. One of the ways to improve the properties of the deposited metal is dispersion hardening due to the introduction of boride compounds into it [4][5][6][7]. Alloying by boron carbide opens up special prospects [8][9][10][11]. At the same time, the tribotechnical properties and wear resistance during frictional interaction of friction pairs of such coatings has been insufficiently studied.

Objects and research methods
In connection with the above, structural transformations during wear and the main tribological properties of the metal deposited by flux-cored wire PPKh15 alloyed with 2% boron carbide were investigated.
The surfacing was carried out on plates made of St3 steel 200×50×10 mm in size by experienced flux-cored wires with a diameter of 2.4 mm in argon in three layers. weighed after every 300 revolutions of the disk, i.e. the time of one cycle was 3 minutes, and the friction path of the sample (finger) was 113.04 m. One of the compositions was tested until the wear value equal to the change of the sample length did not exceed 4-5 mm.
Metallographic studies of the deposited metal were performed on an AXIO Observer A1m (Carl Zeiss) optical microscope. The microstructure was detected by chemical etching in the reagent composition: CuSO 4 -4 g; HCl -20 ml; H 2 O -20 ml.
The hardness of the deposited metal was measured by the Rockwell method on a TK-2 instrument, and the microhardness of the structural components was determined by the Vickers method on a Shimadzu HMV-2 microhardness meter with loads of 10 g and 50 g.
Electron microscopic studies were performed on a JEOL JSM-6610-LV raster electron microscope with an Inca-350 attachment for energy dispersive analysis (EDA).

Experimental results and discussion
Metallographic studies have established that a composite structure is observed in metal obtained by surfacing with PPKh15+2.0% B 4 C wire (Fig. 1). It has a pronounced dendritic character. Along the boundaries of the dendritic cells is a eutectic. In the martensin matrix, a large number of precipitates of hardening phases are observed. The total hardness of such a metal after surfacing reaches 58 HRC. The results of the study of the microhardness of the structural components of such a metal are shown in Fig. 2 and are given in Table 1.  The results of a scanning electron microscopic analysis of such a metal with the location of the scanning areas and the distribution of the main alloying elements over the surface of the coating are shown in Fig. 3. It is noted that carbon and boron are located mostly in eutectic, while in the matrix are mainly iron and chromium. The results of the chemical composition of the scanned areas of the identified structure are given in Table 2. It has been established that all boron is in eutectic, which consists of borides and chromium and iron carboborides, while boron is absent in the matrix. From the research results, it follows that the structure of the metal under study is an ironchromium martensitic matrix with a eutectic component, formed mostly by chromium and iron borides of the (Fe, Cr) 2 B type and dispersed inclusions of carboboride of (Fe, Сr) 7 (С,В) 3 type. Due to their large number, boride eutectic is characterized by high microhardness. The test results on the adhesive wear of the metal obtained by surfacing PPKh15+2.0%B 4 C are given in Table 3-6 and Fig. 4.
The mass of the investigated samples with holders before testing and after testing are given in Table 3, and their mass wear and reduction of their length in Table 4 and 5. The main tribological properties of the deposited metal are given in Table 6. Studies show that the metal deposited by flux-cored wire PPKh15+2.0%B 4 C has a rather high wear resistance. Samples with such a metal withstood 1500 revolutions with a friction path of 565.2 m, and then were removed from tests. Mass wear for the friction path of 226.048 m is only 0.794 g. Mass wear at the friction path of 565.2 m has reached 1.5827 g (Table 4), and the shortening of the sample was 4.08 mm ( Table 5)  The fall of the friction coefficient of the coating during the tests is due to a decrease in the roughness of the friction surface and the area of actual contact and a decrease as a result of this contact pressure.
Additional information about the processes of destruction of the surface layer provides an analysis of its topography (Fig. 5). Microstructural studies of the metal end of the sample after frictional loading reveal the formation of a block structure that is quasi-evenly distributed over the volume of the surface layer (Fig. 6). In the surface layer, along with a high degree of hardening, a certain plasticity of the material is realized due to the high extent of the block boundaries and the boundary slippage. There is an increasing tendency to textured dislocation clusters in one direction and the development of microbands by localized plastic deformation.
The positive effect of boron carbide on the wear resistance of the metal is apparently connected not only with structural changes in the metal base of the matrix, but also directly with the nature of boron, which, due to its specific atomic-crystalline structure and physical and mechanical properties, serves as an internal lubricant that reduces the coefficient of friction in contact medium [12,13]. This is confirmed by the topography of the worn surface -the traces of abrasive interaction are very weakly expressed. This also indicates that boron contributes to the formation of durable passivating films, which do not break even with heavy forms of abrasive wear, thereby preventing the formation of hardening foci. As a result, such a metal is characterized by a smaller depth of traces of abrasive impact of the counterbody and, more importantly, a significantly smaller area of destruction of oxide films on a worn surface, which, as is well known, prevents the formation of setting points and slows down the rate of abrasion during abrasive wear [14,15].
Thus, the mechanism of metal wear, deposited by cored wire PPKh15+2.0%B 4 C, is associated primarily with its high hardness due to the dispersion hardening of particles of eutectic carboboride compounds, which are effective obstacles for dislocation slip [16] under conditions of plastic