Registration of melting temperature at phase boundaries in melting powder mixtures

An integral constituent of the induction surfacing process for parts hardening pertains to the thermal treatment of rigid alloy particulates and the flux contained within the surfacing amalgam. This scholarly exposition delineates the outcomes of a comprehensive investigation, oriented towards the quantification and simulation of thermal gradients at interphase boundaries within intricate amalgamations of melting and thermosetting powdery substrates. In order to monitor the thermal dynamics during induction surfacing, the application of the CA-microthermocouple methodology and the thermal indication technique utilizing self-propagating SHS-process compositions is posited. The study encompasses both computational simulations and the resolution of the unsteady-state heat conduction problem within the chosen model of composite contacting materials. The devised methodologies for the intricate tracking of thermal profiles during induction surfacing offer the capability to ascertain the temperatures at which the surfacing amalgam and its individual constituents undergo liquefaction, in conjunction with the temperature gradient exhibited by the surface of the targeted component. This facet holds marked significance in the context of optimizing the hardening regimen and ensuring the requisite attributes of the amalgamated materials post-fusion. The outcomes of this research bear practical relevance in industrial applications, wherein enhancements to the caliber and dependability of hardened components are sought, simultaneously facilitating the curtailment of production overheads.


Introduction
Contemporary investigations into multicomponent powder amalgamations necessitate a profound comprehension of the temperature dynamics and its spatial dissemination at interphase demarcations.This comprehension serves as a prerequisite for effectively regulating intricate phenomena including but not limited to the amalgamation's phase transitions, alloy and coating formation, chemical reactions, and the emergence of novel compounds subsequent to thermal excitation.This imperative knowledge finds applicability in overseeing processes encompassing induction surfacing (IS), welding operations, self-propagating high-temperature synthesis (SHS), alongside numerous other technological domains.
Amidst a plethora of surface treatment and coating methodologies elucidated in the literature [1], approaches predicated upon chemical vapor deposition (CVD) have garnered substantial attention.Particularly noteworthy among these is the borating process, which has shown tremendous promise due to its ability to bestow protection upon diverse surfaces of components composed of various ferrous alloys [2].Within the ambit of surface modification, composite matrices founded upon the Fe-B-FenB ternary system have emerged as a propitious candidate for the development of fortifying protective coatings.
Pertinently, steel components subject to the aforementioned treatment exhibit commendable tribological attributes, inclusive of heightened resistance against abrasive, adhesive, fatigue-induced, and corrosion-induced wear mechanisms [3].It is worth underscoring that one of the cardinal merits intrinsic to this modality of fortification pertains to the remarkable augmentation in hardness exhibited by boride-reinforced steels, showcasing hardness values that span the range of 1600 to 2000 HV [4,5].
Boriding, in its conventional interpretation, constitutes a chemical-thermal treatment process characterized by the diffusion-driven impregnation of the surfaces of metallic substrates with boron.This impregnation is accomplished through elevated temperature exposure within a chemically reactive milieu.This technique finds wide-ranging application in the treatment of steel and its derivatives.In numerous instances, the treated surface assumes a biphase composition: a region featuring a confluence of FeB and Fe2B borides, alongside an interstitial transition region wherein a solid solution of boron integrates with other alloying elements present in the α-Fe matrix of steel.
Upon meticulous examination, the microstructure of these surface coatings unveils an arrangement of boride needles that are fused at their basal junctions.However, this microstructural arrangement engenders notable internal stresses within the coatings, thereby diminishing their mechanical robustness.Consequently, even relatively modest mechanical loads, such as impact, bending, or compression, can precipitate the delamination or detachment of these coatings.The presence of alternating stress conditions and vibrational forces exacerbates the propensity for coating degradation.Given these considerations, the conventional practice of isothermally depositing hardening compositions presents significant limitations when contemplated for widespread integration into industrial manufacturing and engineering endeavors.
Conversely, the utilization of induction heating for boriding purposes offers a multitude of merits that warrant acknowledgment.Notably, the expedited heating rate associated with induction heating accelerates the process of coating formation manifold.These confluences of circumstances position the induction-based boriding technique as a propitious avenue for generating fortified coatings within continuous production lines, particularly within mass manufacturing settings or when confronted with substantial part volumes per operational shift [6][7][8].
A pivotal facet within the domain of parts hardening through induction surfacing encompasses the thermal treatment of rigid alloy particulates in conjunction with the flux constituent present within the surfacing composition, as outlined in reference [8].Simultaneously, the act of attaining temperature measurements of acceptable precision during the course of induction surfacing presents inherent technical and methodological intricacies.Foremost among these challenges pertains to the variability of coating thickness achieved through the surfacing induction (SI) technique, which fluctuates within the range of 1 to 3 mm.Consequently, the utilization of conventional industrial and research-oriented thermocouples is rendered unfeasible in this context.Secondly, during the progression of induction heating, the underlying base metal surface releases a greater quantum of energy-both in the form of thermal radiation and heat-compared to the coating that is concurrently being formed.Consequently, the utilization of pyrometric methods for temperature measurement proves inefficacious, capturing merely the thermal status of the base metal surface.Thirdly, a composite heterogeneous amalgamemploying any of the well-established contact thermometric methodologies results in conspicuously irreproducible outcomes within the SI process.
An even more intricate endeavor lies in the empirical investigation of the non-steady-state thermal profile within the realm of composite contact material (CCM) fabrication employing the SHS methodology.This challenge is compounded by the fact that the dimensions of the subject of inquiry are significantly smaller than those accommodated by conventional temperature sensors.
Resolution of the aforementioned challenges can be pursued through a multifaceted approach, encompassing: 1. Diminution of the primary sensor, temperature-responsive coating, metallic substrate, surfacing charge, and its individual constituents; 2. Exploitation of temperature-dependent material processes, whether inherent within the system or introduced externally, to enable unequivocal, reliable, and replicable temperature recording within the system; 3. Deployment of numerical simulation techniques to elucidate potential mechanisms of SHSinitiated reactions occurring at interphase interfaces.
The prime aim of this investigation was to formulate sophisticated methodologies for the comprehensive recording and simulation of temperature gradients at phase boundaries within heated amalgamations of metal powders.This pertains to scenarios encompassing both the materials' melting progression and the chemical interactions among their constituents.

Materials and methods
To enhance the operational efficiency of production methods pertinent to the diffusion saturation processes of metal and alloy surfaces with boron, a comprehensive array of strategies has been proposed, drawing from an in-depth examination of extant literature.These strategies encompass the utilization of high-frequency currents (HFC) to heat the steel undergoing boriding, in conjunction with the saturating medium.Additionally, a coherent amalgamation of procedural stages involves the diffusion-based borating of both liquid and solid media, alongside the transition of the borating diffusion process towards a chemical interaction between the constituent elements of Fe and B [1,2].This intricate procedural framework is executed upon the surfaces of the materials under investigation.
The foundational premise underpinning this line of inquiry in exploring a novel approach to the diffusion-based saturation of metal and alloy surfaces with boron, centered on the orchestration of a meticulously controlled topochemical reaction Fe + B, is rooted in a pivotal study denoted as reference [17].This study introduces a methodology termed differential thermal analysis (DTA).Herein, the authors of this study elucidate the chemical interplay transpiring within an inert, rarified atmosphere between iron, alongside its oxide species prevalent upon the steel surface, and boron.The culmination of this interaction engenders the formation of iron borides, a sequence commencing at temperatures in the approximate range of 500-700 °C, ultimately culminating in the generation of d-metal borides.
The subject of investigation encompassed specimens comprising the feedstock for the exothermic initiation process, with the subsequent composition denoted in weight percentages (%wt.):85% hard alloy and 15% fused flux.The hard alloy component consisted of PG-US27 and PS-14-60 (particle size: 0.5-1.5 mm) powders, while the flux constituents comprised industrially formulated compositions designated under ASM classifications (Rubtsovsk), CSM (Astana), and ZOR (Odessa).
For the fabrication of microthermocouples, wires of chromel and alumel with a diameter of 0.2 mm (conforming to GOST 1790-77) were encased within fiberglass insulation.The establishment of junctions was realized through the fusion of wires utilizing capacitor welding under direct current employing a carbon electrode.
In the context of crafting thermal indicator compositions, nickel powders of the PNE classification, along with aluminum of ASD-1 grade, and titanium classified as PTK, were employed.
The emulation of thermal distributions within the Self-Propagating High-Temperature Synthesis (SHS) reactive Continuous Casting Mold (CCM) system was executed via the solution of the transient heat equation through numerical methodologies facilitated within the Mathcad 2011 computational environment.

Experimental results
Temperature assessment for discrete components within the surfacing amalgam, the substrate's surface, and the air-melt interface adjoining the hard alloy is feasible with a tolerable accuracy of up to 3-5%.This is achievable through the application of microthermocouples, provided their junction dimensions range from 0.1 to 0.3 times the characteristic size of the investigated structural element.In this context, microthermocouples based on the chromel-alumel composition (termed as MTPs) were meticulously devised and subjected to calibration procedures.
The initial phase of the research entailed temperature measurement concerning powder materials, encompassing the hard alloy employed in the surfacing induction (SI) process.For this purpose, the operational junction of the thermocouple was precisely welded to a distinct powder particle, under the magnification of a microscope (MBS-10), thereby facilitating precise temperature measurement during the heating phase.Illustrated in figure 1 are images depicting hard alloy particles coupled with thermoelectrodes affixed to them.The attachments are welded onto both singular and dual planes of the particle.
The introduced methodology satisfying the fundamental prerequisites for contact-based temperature sensing protocols [2].The heating temperatures pertaining to the hard alloy were ascertained at two distinct interfaces, specifically the charge-base metal and charge-air interfaces.The resultant findings exhibit a commendable degree of reliability, primarily attributed to the congruence between the shape and chemical composition of the operative layer and those of the individual particles comprising the heated powder material [3].Illustrated in figure 2, and captured by an autonomous recording apparatus, are the distinctive heating profiles delineating the surface ( 1) and ( 2).The conducted experiments have unveiled a discernible relationship: an augmentation in the dimensions of hard alloy particles translates to a commensurate reduction in the extent of base metal infiltration, while the surfacing parameters remain unchanged.For instance, during the application of a surfacing charge featuring hard alloy particles of sizes ranging from 0.5 to 1.2 mm, the resultant base metal penetration depth oscillated between 0.40 and 0.50 mm.Contrastingly, employing larger particles of sizes between 2.0 and 3.0 mm yielded a diminished base metal penetration of 0.30 to 0.35 mm.This phenomenon can be attributed to the induction of electrical currents within larger particles, inducing their heating.Consequently, especially when augmenting the thickness of the deposited stratum, a propensity to amplify the dimensions of the hard alloy grains is recommended.
Further investigations have also revealed that the heating potency within the surfacing charge experiences attenuation as the proportion of its composition diminishes.The employment of microthermocouples (MTPs) facilitates the measurement of the surfacing charge's heating temperature through height measurements of its fill level, with precision achievable at virtually any point along its composition, encompassing both the charge-air interface and its adjacency to the part's surface.
Nevertheless, practical implementation mandates the monitoring of the surface heating temperature within the context of the surfacing induction (SI) process.Traditional techniques for temperature assessment, such as thermocouples or pyrometers, have not been embraced within production environments, primarily due to the intricacy of the instrumentation in the former case, and substantial measurement errors inherent to the latter.
For the quantification of the part surface's elevated temperature induced by high-frequency currents, two distinct thermal indicator (TI) methodologies were employed.The first involves thermite mixtures, while the second entails amalgamations of titanium, nickel, and aluminum powders, engendering an exothermic self-propagating high-temperature synthesis (SHS) reaction upon mutual interaction [8].The SHS process studied by us is visible visually (figure 3).In the process of calibrating the examined nickel-aluminum (Ni-Al) and nickel-titanium (Ni-Ti) thermal indicators (TIs), a tungsten-rhenium microthermocouple (MTP) with a diameter of 0.2 mm was employed.This MTP was affixed to a metallic plate via capacitor welding.Adjacently positioned to the MTP, a thermal indicator pellet was situated, and the assembled sample underwent heating through induction.The temperature profile of the thermal indicator during the heating process conspicuously depicts a characteristic temperature surge upon ignition of the powder mixture.This surge corresponds to the incitement of SHS-process within the mixture, unfolding in a thermal explosion mode.Through the utilization of a software and hardware complex, the aggregate temperature measurement error inherent to this methodological configuration did not surpass 3.5-4.5%.
Consequently, the elaborated methodologies facilitating the comprehensive recording of temperature dynamics throughout the course of induction surfacing proffer the capability to discern the melting temperatures characterizing the surfacing charge and its discrete constituents, the thermal state of the part's surface, and the investigation of thermal processes inherent to surfacing induction.Moreover, these methods enable the determination of temperature gradients across the treated surface as the part is traversed through the radiofrequency (RF) inductor.Such insights permit timely adjustments to the operational parameters of the RF generator.
In the ensuing segment of the study, encompassing numerical simulation and the resolution of the non-steady-state heat conduction problem within the chosen composite contact material (CCM) model, several assumptions will be posited: The developmental trajectory of processes during sintering within each cell can be presumed to exhibit similarity.
The volumetric electrical power remains consistent throughout the synthesis of the composite contact material.
Temperature variations are constrained within the range of 20 to 1500 °C.Lateral heat losses are realized via convective exchange mechanisms.Taking into account these assumptions, the formulation of the problem is reduced to solving the wellknown non-stationary heat quation: Figure 4 illustrates the temperature distributions along the longitudinal axis of the Cellulose Microcrystalline (CMC) unit.Evidently discernible from the plot is the presence of a paramount temperature peak occurring precisely at the interstice of the copper fiber and powder amalgamation.Subsequently, a plateau-like temperature profile is evident within the powder blend domain, persisting until the temporal point t = 6.0 seconds.Subsequent to this interval, a secondary temperature apex emerges, positioned centrally within the powder compact, and is sustained approximately until the 8.0second juncture of the heating process.

Conclusion
Advanced techniques for temperature monitoring during the induction surfacing process have been formulated, utilizing a combination of microthermocouples and thermal indicators based on SHSprocess.Through the utilization of microthermocouples for temperature registration, it becomes viable to accurately record the melting temperatures associated with individual components within the surfacing charge, alongside capturing temperature profiles at interphase boundaries.
Incorporating thermal indicators into the temperature registration process yields errors within a range of no more than 4.5%, demonstrating the method's reliable accuracy.
Furthermore, computational investigations have been undertaken to model the temperature distribution throughout the pre-reaction phase of electrosynthesis in composite contact materials (CMC), with particular attention directed towards the inclusion of pertinent contact phenomena.

Figure 2 .
Figure 2. Generalized graphs of heating of the surfacing charge at the boundaries: the main metal charge (1) and charge-air (2).

Figure 3 .
Figure 3. Ignition of the Ni-Al thermal indicator.

Figure 4 .
Figure 4. Temperature profile along the axis of the CMC cell (after 1 second SHS).