Plasma coating of high-speed molybdenum steel after tempering and electron-beam processing: Structural-phase states and properties

Introduction

Over the past century since the development of the classical high-speed steel R18, dozens of new steels have been created for specific applications in the machine-building, metallurgical, and mining industries [1], and these have been included in handbooks and catalogs of leading foreign countries. Recently, in high-speed steels, relatively expensive and scarce tungsten has been replaced by molybdenum, which exerts a similar effect on the formation of structure and properties [2]. High-speed molybdenum steels exhibit higher performance properties than tungsten or tungsten–molybdenum steels [3 – 5].

An important challenge in improving the reliability and service life of machine and mechanism components is the purposeful and controlled modification of the properties of their working surfaces. The creation of coatings subjected to mechanical loading, wear, corrosion, and aggressive environments, while possessing high functional properties, is a fundamental task of great practical importance [6 – 8]. One approach to addressing this challenge is plasma surfacing using nitrogen as an alloying element [9 – 13].

Improvement of surface quality and its tribological and mechanical properties can be achieved by subsequent electron-beam processing. Electron-beam processing using low-energy high-current electron beams is carried out in a pulse-periodic mode, which, on the one hand, makes it possible to increase the time during which the surface layer remains molten and to homogenize its elemental composition and, on the other hand, to preserve the hardening effects leading to the formation of a submicro- and nanosized structure [14 – 17].

A fundamentally important feature of surface-layer modification by low-energy high-intensity electron beams is the absence of a pronounced interface between the modified layer and the bulk of the material. This determines the good damping properties of the material under external mechanical and thermal loading and prevents the premature initiation and propagation of brittle microcracks from the surface into the bulk of the material, which lead to fracture [18; 19].

The lack of studies clarifying the physical nature and mechanisms of structural-phase transformations in plasma-surfaced high-speed molybdenum steel coatings during deposition and subsequent treatment considerably restricts their practical application.

Materials and methods

Samples were prepared by plasma surfacing onto 30KhGSA steel in a nitrogen atmosphere using MoCrCoC flux-cored wire. The deposited layer was formed under the following conditions: welding current, 145 – 150 A; arc voltage, 50 – 55 V; surfacing speed, 18 m/h; wire feed rate, 60 m/h; and arc length, 20 mm. Surfacing was performed in four layers to a total thickness of 9 – 10 mm.

Irradiation of the deposited layer was carried out using the SOLO facility (Institute of High Current Electronics, Siberian Branch of the Russian Academy of Sciences) under the following conditions: electron energy, 18 keV; electron beam energy density, 30 J/cm²; pulse duration, 50 μs; number of pulses, 10; pulse repetition rate, 0.3 s^–1; and residual gas pressure (argon) in the chamber, 0.02 Pa.

The phase composition of the deposited layer was analyzed by X-ray diffraction using a DRON-8N diffractometer. Phase identification and qualitative and quantitative phase analysis, as well as refinement of structural parameters, were performed using the KDA – Crystallography and Diffraction Analysis software package with an integrated powder diffraction database (Burevestnik JSC, version 2023-01-24-144022.8dec10c0f).

The structure and elemental composition of the deposited layer were examined by scanning electron microscopy using a KYKY-EM6900 microscope equipped with a thermionic tungsten cathode and an X-ray microanalysis attachment.

The defect substructure and the elemental and phase compositions of the deposited layer were investigated by transmission electron microscopy of thin foils in both transmission and scanning modes using a JEOL JEM-2100 microscope (Japan) [20 – 22]. Microdiffraction combined with dark-field analysis was used to determine the phase composition of the material.

The hardness and Young’s modulus of the deposited layer were measured using a NanoScan Scanning Nano-Hardness Tester at an indenter load of 0.1 N. The tribological properties of the deposited layer were evaluated in a pin-on-disc configuration under circular sliding using an Oscillating TRIBOtester tribometer (ASTM G99). Wear track profiles were recorded with a contact profilometer attached to the Oscillating TRIBOtester tribometer.

Results and discussion

The structure of the deposited metal consists of eutectic grains and grains of a solid solution based on iron (Fig. 1, a). Double tempering of the deposited layer did not alter the polycrystalline state of the material; as in the initial state, the deposit retained a structure consisting of eutectic grains and grains of a solid solution based on iron (Fig. 1, b). Subsequent irradiation of the deposited layer with a pulsed electron beam resulted in a marked change in the morphology of the surface layer (Fig. 1, c, d). First, numerous craters formed on the deposit surface (Fig. 1, c). Second, a fine-grained structure with grain sizes on the order of several micrometers developed (Fig. 1, d). Third, the eutectic structure was no longer observed. These changes should be accompanied by modifications in the phase composition of the surface layer.

This assumption is supported by the results of X-ray diffraction analysis, which revealed a significant change in the phase composition of the deposited material after irradiation with pulsed electron beams (Fig. 2; Table).

Comparison of the data presented in the Table shows that both tempering of the deposited material and subsequent irradiation of the tempered material with pulsed electron beams led to a substantial decrease in the carbide-phase content, from 23 wt. % in the initial deposit to 3 wt. % after irradiation. Tempering of the deposited layer was accompanied by additional transformation of the residual austenite, whereas subsequent irradiation again led to the retention of residual austenite in the surface layer. It may be assumed that the decrease in the relative carbide-phase content after tempering, and especially after additional irradiation, was associated with the formation of a supersaturated solid solution based on α- and γ-iron. However, the crystal lattice parameter of α-iron was highest in the initial state and lowest after additional irradiation (see Table). This finding indicates, among other things, the release of alloying elements from the crystal lattice of α-iron, which in turn suggests the formation of carbide-phase particles. These apparently contradictory results may indicate that the second-phase particles formed during additional treatment of the deposited metal are nanosized.

STEM analysis showed that the structure of the surface layer after irradiation with a pulsed electron beam consisted of high-speed crystallization cells ranging in size from 0.15 to 0.25 μm (Fig. 3).

Two isolated areas containing different types of high-speed crystallization cells were identified. In the first type of cells, the boundaries were nearly free of second-phase precipitates, whereas in the second type, extended interlayers of the second phase were located along the boundaries. The sizes of these inclusions ranged from 25 to 43 nm. It may be assumed that the first-type crystallization cells formed as a result of high-speed crystallization in molten regions corresponding to grains of a solid solution based on α-iron, whereas the second-type crystallization cells formed during crystallization of molten eutectic grains. This explains the marked difference in the content of second-phase particles between the two cell types.

X-ray microanalysis showed that the interiors of crystallization cells of both types were enriched predominantly in iron atoms (Fig. 4, b), whereas extended interlayers enriched predominantly in molybdenum (Fig. 4, c), chromium (Fig. 4, d), and carbon (Fig. 4, e) were detected along the cell boundaries.

Dark-field analysis and indexing of TEM electron diffraction patterns were used to determine the phase composition of the high-speed crystallization cells. In the first-type crystallization cells, the cell interior was found to consist of a solid solution based on α-iron. In some cases, nanosized grains of residual austenite as well as nanosized particles of carbides of the Me₂₃C₆ and MoC types were detected (Fig. 5). A dislocation substructure in the form of randomly distributed dislocations was observed within the crystallization cells.

Analysis of the phase composition of surface-layer regions containing second-type crystallization cells showed that the cell interior also consisted of a solid solution based on α-iron. In some cases, reflections corresponding to residual austenite were detected in the electron diffraction patterns. Numerous second-phase inclusions were located along the crystallization cell boundaries, and analysis of the corresponding electron diffraction patterns indicated that they were carbides of the Me₂₃C₆, Cr₃C₂ and Cr₇C₃ compositions.

Following this combined treatment of the surface layer of the high-speed molybdenum steel, both the mechanical and tribological properties changed. Additional irradiation of the deposited layer with a pulsed electron beam reduced the wear parameter and the coefficient of friction from k = 2.0·10^–6 mm³/(N·m) and μ = 0.75 for the deposit in the initial state to k = 1.25·10^–6 mm³/(N·m) and μ = 0.60 for the deposit after tempering and additional pulsed-electron-beam irradiation. For the substrate (30KhGSA steel), k = 10.0·10^–6 mm³/(N·m) and μ = 0.44. Thus, irradiation of the deposited layer with pulsed electron beams resulted in a slight decrease in the wear parameter by a factor of 1.6 and in the coefficient of friction by a factor of 1.23. Compared with 30KhGSA steel, the wear parameter decreased eightfold, whereas the coefficient of friction increased by a factor of 1.36.

The nanohardness of the surface layer of the deposited high-speed molybdenum steel after irradiation with pulsed electron beams ranged from 9.5 to 14.7 GPa and averaged 10.7 GPa within a surface layer 100 μm thick. The mean nanohardness of the deposited layer in the initial state was 12.4 GPa. Thus, irradiation of the deposited high-speed molybdenum steel with pulsed electron beams resulted in a 1.16-fold decrease in nanohardness. The nanohardness of the substrate (30KhGSA steel) was 3.2 GPa and was substantially lower, by a factor of 3.3, than the mean hardness of the deposited metal after irradiation with a pulsed electron beam.

The Young’s modulus of the surface layer of the high-speed molybdenum steel irradiated with pulsed electron beams, averaged over 10 indents, was 215 GPa. Given that the mean Young’s modulus of the deposited metal in the initial state was 234.9 GPa, irradiation of M9 high-speed molybdenum steel resulted in a slight decrease in this parameter by a factor of 1.09.

Conclusions

A 9 – 10 mm thick deposited layer of high-speed molybdenum steel was formed on medium-carbon 30KhGSA steel by plasma surfacing in a nitrogen alloying atmosphere. The deposited layer was then subjected to double tempering followed by irradiation with pulsed electron beams. Mechanical and tribological tests were carried out for the deposited layer in the initial state, after tempering, and after electron-beam processing. After irradiation, the nanohardness of the deposited layer was found to be 3.3 times higher than that of 30KhGSA steel. The wear parameter of the irradiated deposited layer was eight times lower than that of 30KhGSA steel, whereas its coefficient of friction was 1.36 times higher. After electron-beam processing, the high-speed crystallization cells measured 0.15 – 0.25 μm in size. Two isolated areas containing different cell types were identified. In the first-type cells, the boundaries were almost free of second-phase precipitates. In the second-type cells, extended carbide-phase interlayers were present along the boundaries. These interlayers consisted of complex carbides of the Me₂₃C₆ type, chromium carbides Cr₇C₃ and Cr₃C₂, and molybdenum carbides MoC, with particle sizes ranging from 25 to 43 nm. The pronounced difference in the content of second-phase particles between the two cell types is attributed to their different origins. The first-type crystallization cells formed during high-speed crystallization of molten regions corresponding to grains of a solid solution based on α-iron, whereas the second-type cells formed during crystallization of molten eutectic grains.