GRADIENT LAYER STRUCTURE FORMATION DURING PLASMA TREATMENT OF WHEEL STEEL

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN (P): 2249–6890; ISSN (E): 2249–8001 Vol. 10, Issue 3, Jun 2020, 457–466 © TJPRC Pvt. Ltd.

GRADIENT LAYER STRUCTURE FORMATION DURING PLASMA TREATMENT OF WHEEL STEEL AMANGELDY KANAYEV1 & DUMAN ORYNBEKOV2 1 2

Professor, L.N. Gumilyov Eurasian National University, Nur-Sultan, Kazakhstan

Director of Science, L.N. Gumilyov Eurasian National University, Nur-Sultan, Kazakhstan

ABSTRACT Mechanism and kinetics of structure formation across the section of the hardened zone under surface plasma hardening of wheel steel are considered. It is shown that depending on the cooling rate and temperature conditions of austenite decomposition perlite and martensite, transformation takes place with formation of gradient layered structure leading to the surface steel layer structure upgrading at constant chemical composition, structure and properties of the central layers of the processed product. Diffusion-free martensite transformation develops in the surface zone, which leads to the formation of needle martensite. In the underlying layers, austenite decomposition flows by diffusion way and is followed by the formation of a plate ferrite-carbide mixture of different degrees of dispersion. The increase in cooling rate leads to a strong crushing of the structure characterized by increased dispersibility of ferrite-carbide mixture, which results in wheel steel in the surface layer makes it possible to eliminate the formation of a sharp transfer boundary from martensite structures to tropostite-martensite and mixed perlite structures. This increases the contact-fatigue strength of wheel steel and therefore its crack resistance.

KEYWORDS: Wheel Steel, Plasma Hardening, Transformation, Gradient-Layer Structure, Austenite, Martensite, Wear Resistance

Original Article

increased strength and reduced plasticity of the steel. The formation of a gradient-layer structure of plasma-strengthened

Received: Mar 07, 2020; Accepted: Mar 27, 2019; Published: Apr 17, 2020; Paper Id.: IJMPERDJUN202042

INTRODUCTION Wear resistance and contact-fatigue strength (cracking strength) of heavily loaded parts and units of railway transport, in particular, wheel pairs, are largely determined by the structure and properties of the surface steel layers. This is explained by the significant difference between the processes of wear and destruction of the surface layer and the processes of wear and volume destruction. At volume loading, the processes of plastic deformation are localized in a certain part of the material volume, where the structure defects occur, stress concentration and the beginning of the fracture zone take place. Under the surface loading, the processes of plastic deformation and destruction of surface layers are characterized, first of all by complex distribution of stresses over the whole contact zone. At any point of the surface layer, all layers of metal in the contact zone participate in the plastic deformation and fracture is likely in equal [1,2].

One more specific feature of the processes of wear and destruction of the surface layer is that during wearing there is a continuous overlay of plastic deformation and destruction cycles. Such dynamic character of processes leads to dynamic structural changes in the surface layer. The high concentration of internal stresses in the metal surface layer can be explained by the cyclical changes in the fine texture and structure of the surface layer. As a result, the fine texture and structure of the surface layer can be in the process of complete wear which would be www.tjprc.org

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different than the original structure and texture of the metal in the volume.

These features of wear and destruction of the surface layer led to the fact that in recent years, along with the traditional methods of strengthening thermal treatment, a new direction of unconventional methods of surface treatment of metallic materials based on the processing of details and components with highly concentrated energy flows (laser radiation and plasma energy flows, processing with ion and electronic beams, etc.) were developed and is rapidly progressing. During such treatment the structure and properties of the material are formed in its thin surface layer up to 1-3 mm thick with the constant chemical composition, structure and properties of the central (axial) zone. In this regard, it is important to note that among the above methods of treatment with highly concentrated energy flows, plasma treatment is one of the attractive and promising methods of fundamental change in the structure of the material, as a consequence, the surface properties of the processed components and products [3,4].

MATERIALS AND METHODS All-rolled wheels made of carbon steel grade 2, chemical composition (GOST10791-2011) of those are given in Table 1 and were processed by plasma treatment.

Table 1: Chemical Composition of Wheel Steel (% ) C Mn Si P S Ni Cr Cu 0,650 0,810 0,340 0,015 0,018 0,19 0,21 0,15 Mechanical properties of wheel rims, subjected to hardening heat treatment, according to the certificate of conformity, are presented in Table 2.

𝜎в, N/mm2 1105

Table 2:Mechanical Properties of Wheel Steel δ, Ψ, KCU. HB on Depth 30 mm HB Wheel Flange % % J/cm2 10,5 22,0 0,34 275 290

During plasma treatment (hardening) studies, the wheels were tested in the following mode: Electric arc current, A 275 Arc voltage, V 120.

Rated arc power, kW 35 Nozzle diameter, mm 3.5 Distance from cut to hardened surface, mm 10- 15 Shielding gas flow rate, l/min 3-6 Wheelset speed, rpm 0.25-0.14 (5.0-7.0 revolutions per minute) The microstructure of the samples, depth of the hardened layer, parameters of mechanical properties and microhardness distribution were studied by the PMT-3 device. The microscopic researches were carried out on optical microscope "Neophot" at magnification x 200 on the microsections cut in a cross direction from a segment with the condition of preservation of the hardened layer. Electron-microscopic examinations were carried out on the Jeol JEM 2100 Impact Factor (JCC): 8.8746

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transmission electron microscope. The strength and plastic characteristics were determined at the Instron testing machine, the samples were preliminary prepared in accordance with GOST 1497-94. Samples were cut out from wheel rims for testing in accordance with paragraph 3.4 of GOST 10791-2011.

RESULTS AND DISCUSSIONS The processes of structure formation under the exposure to highly concentrated energy flows, in particular, plasma treatment, are under general laws of structure formation described by the iron-carbon (Fe-C) phase equilibrium diagram.

The depth of strengthened structural zones at plasma hardening depends on heating and cooling parameters, is determined by the mechanism and kinetics of phase (γ→α) transformation in non-equilibrium states [5].

Let's consider the kinetics and formation trends of gradient-layer structure on the basis of a typical thermo kinetic diagram of pre-eutectoid carbon steel (0.59-0.63%C), which is an analogue of wheel steel according to GOST 10791-2011.

Figure 1: Decomposition Diagram of Subcooled Austenite of Pre-Eutectoid (0,60-0,65% C) Structural Steels [6]. (Hatch Lines Represent the Isothermal Transformation of Austenite, Solid Lines Represent the Transformation of Austenite during Continuous Cooling (Thermokinetic Diagram).

It can be seen that the thermokinetic diagram at temperatures above the martensite point Mn (~260 °C) is characterized by only one kinetic maximum, which means that there is no intermediate mechanism for the decay of austenite (shaded part of the diagram). At the temperatures above this maximum, austenite decomposition (γ→α) flows in a diffusion path and is followed by the formation of a ferrite-carbide mixture of varying levels of dispersion, forming the socalled perlite structures (perlite, sorbitol, tropostite). At the temperatures below this maximum, the transformation (γ→α) occurs by diffusion-free (martensite) mechanism with the formation of martensite [7, 8].

From figure 1, it follows that at cooling of the austenite with speed V3 (critical speed of hardening) and above the lamellar martensite is formed. At lower cooling rate V2 the overcooled austenite transforms into troostite-martensite, i.e. partially by the perlite (diffusion), partially by the diffusion-free mechanism. At even lower cooling rates of V1, the transformation is developed by the diffusion mechanism with the formation of tropostitis and sorbite.

At the same time, the thermokinetic diagram clearly shows that the intermediate mechanism of transformation of austenite with the formation of bainite structures cannot be realized, because the transformation in this case is developing www.tjprc.org

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either by the perlite mechanism (cooling rates V1 and below), or a mixed perlite-martensite (cooling rates between V1 and V2), or martensite mechanism (cooling rates V3 and above).

Note that both perlite and martensite transformations are based on the polymorphic transition (γ→α) of austenite hexagonal lattice into a volume-centered lattice of equilibrium or supersaturated ferrite.

The comparison of the kinetics of austenite transformation under isothermal conditions and during continuous cooling shows that the corresponding lines on the thermo kinetic diagrams are located to the right and below similar lines of the isothermal diagram. This indicates that the stability of subcooled austenite under continuous cooling is slightly higher and that the transformation flows at lower temperatures than in the isothermal decomposition of subcooled austenite.

In practice, this is shown in the value of the interplate distance, which is the most important structural characteristic of structural steels. It is known that the plate spacing is an average of the thicknesses of two adjacent ferrite and cementite plates in perlite structures. The higher the cooling rate, the smaller the interplate spacing and the more disperse the ferrite-carbide mixture, the higher the microhardness (hardness) of the steel [9, 10].

It is necessary to note that in the analysis of structural transformations, the separation of ferrite and cementite structures into perlite, sorbitol, or tropostite is conditional, since there is no clear boundary between these structures. This is because practically it is not possible to differentiate the processes of formation of pure tropostitis, sorbitol or perlite, since the temperature change rate over the cross section of the cooled product in the process of continuous cooling does not remain constant, as noted above. It is variable and varies according to a certain defined law, depending on the thermal physical properties of the steel.

In addition, compared to perlite, sorbitol and troostite are not equilibrium structures. Under real production conditions, the cooling is generally uneven, and this leads to a certain carbon enrichment of sorbitol and cane ferrite, which naturally affects the mechanical properties of the material. In particular, the mechanical properties of steel with perlite, sorbite- and troostite structures are directly proportional to the interface area between ferrite and cementite. Therefore, with a decrease in the temperature of decomposition of austenite and the appropriate grinding of the structure (increase in the degree of dispersion), ferrite plates are slightly oversaturated with carbon, the strength characteristics (strength - σв hardness - HB) are increased, and the plastic characteristics (relative elongation - δ and contraction -ψ) are reduced [11, 12].

The two-phase structure of sorbite and troostite is observed only under the electron microscope, because ∆ - the interplate distance of these structures is within the resolution limit of the optical microscope (~0.2 µm). Figure 2 shows pictures of sorbite and tropostite at x5000 magnification, captured by the electron microscope. It is clearly seen that both these structural components consist of alternating ferrite and cementite plates.

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(а) (b) Figure 2: Structure of Austenite Decomposition Products during Continuous Cooling (a - Sorbite, x5000); (b - Troostite).

The microstructures shown in Figure 3 illustrate the overlay of martensite transformation on perlite. So, at cooling rate V2 in the temperature range ~ 550 °С - 460 °С the part of austenite by the diffusion mechanism turns to troostitis, the remaining part below the point Mn (~260 °С) without diffusion turns to martensite. Here, it is necessary to note, that experimental research of processes of structure formation at cooling with variable speed is a difficult task, because depending on a number of factors and, first of all, on cooling speed, the kinetics and temperature conditions of this or that transformation process development can change within certain limits.

Therefore, it is not always possible to show the temperature ranges in which the transformation proceeds by only one mechanism (perlite-diffusion, intermediate or martensite-non-diffusion) and clearly distinguish the structural zones of formation of troostitis, sorbite or perlite. In fact, the transformation processes of supercooled austenite can overlay one another in temperature and time of its development, leading to the formation of mixed plate type structures.

Figure 3: Gradient-layer Structure of the Wheel Ridge in a Hardened Area, Captured by an Optical Microscope at x200 Magnification.

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Figure 4: Microhardness Change Curve across the Hardened Zone Cross Section.

Changes in microstructure and microhardness of steel exposed to surface plasma hardening (Figures 3 and 4) show that the formation of several structural zones of different microhardness (~860-250HV) is clearly observed by the depth of hardening. The surface has a zone with chemical composition equal to that of steel with carbon content of 0.63 %.

At shock cooling, it transforms into needle martensite with dispersion of 5-15 microns. Next comes the zone of transformed austenite into troosto-martensite. In the microstructure of these layers, there is a small amount of residual austenite, the amount of which varies and depends on the depth of the hardened layer.

Next comes the troostitis layer, where the microhardness decreases and depends on the volume content of the phases present. Then sorbite appears in the structure. The location area of the appeared sorbite is determined by the central parts of former austenitic grains, characterized by lower dispersion of ferrite and cement components in them. The microhardness in this area also depends on the volume number of the existing phases. Then, with deepening inside the sample, ferrite appears at the interface of the former austenitic grains and its quantity gradually increases. The structure remains ferrite-sorbit and then changes quietly into ferrite-perlite. The total microhardness is reduced to the initial one. The initial structure is a mixture of ferrite and perlite grains with the volume proportion of each phase of 40-60 % respectively [13].

The formation of a gradient-layer structure of plasma-strengthened wheel steel in the surface layer makes it possible to eliminate the formation of a sharp transfer boundary from martensite structures to tropostite-martensite and mixed lamellar structures (tropostite, sorbite). This is one of the main factors that increase the contact-fatigue strength of wheel steel and contribute to its crack resistance [14, 15].

CONCLUSIONS 

Mechanism and kinetics of structure formation along the cross section of the plasma-thermal zone of wheeled steel changes is dependent on the cooling rate and temperature conditions of the transformation process. With the increase of cooling rate, the transformation of austenite, which is based on shear phase transition γ→α, moves down the temperature scale.



The analysis of the microstructure on the basis of thermokinetic diagram and distribution of microhardness over

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the section of the hardened zone shows that at surface plasma hardening in wheel steel is dependent on cooling rate perlite and martensite transformation with formation of gradient-layer structure occurs. Diffusion-free martensite transformation develops in the surface zone, which leads to the formation of needle martensite. In the underlying layers, austenite decomposition flows by diffusion way and is followed by the formation of a plate ferrite-carbide mixture of different degrees of dispersion. 

3 The higher the cooling rate, disperses the resulting ferrite-carbide mixture, the smaller the interplate distance.

Therefore, as the degree of dispersion increases, the strength characteristics (strength - σв hardness - HB) increase, and the plastic characteristics (relative elongation - δ and contraction - ψ) decrease.



The formation of a gradient-layer structure of plasma-strengthened wheel steel in the surface layer makes it possible to eliminate the formation of a sharp transfer boundary from martensite structures to tropostite-martensite and mixed lamellar structures (tropostite, sorbite). This is one of the main factors that increase the contact-fatigue strength of wheel steel and contribute to its crack resistance.

ACKNOWLEDGEMENT The work was carried out as part of the grant financing project AR05131581 "Development of Railway Engineering Cluster in Kazakhstan by implementing Innovative Plasma Technology for Wheelset Hardening" under Contract No 132 of March 12, 2018.

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AUTHOR PROFILE

Prof. Amangeldy Kanayev is from Department of Standardization, Certification and Metrology of L.N. Gumilyov Eurasian National University, Nur-Sultan, Kazakhstan and his Scientific directions are Materials Science and Engineering.

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Dr. Duman Orynbekov is the Director of Science and Innovation Department of L.N. Gumilyov Eurasian National University, Nur-Sultan, Kazakhstan and his Scientific directions are Mechanical Engineering, Materials Science, Food Engineering

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