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Surface hardening characteristics of microalloyed steel during ex-situ and in-situ Al2O3 reinforcement under TIG arcing
Journal Pre-proof Surface hardening characteristics of microalloyed steel during ex-situ and in-situ Al2O3 reinforcement under TIG arcing Deepak Sharma, Prakriti Kumar Ghosh, Nilesh Kumar, Ramkishor Anant PII:
S0257-8972(19)30991-0
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125002
Reference:
SCT 125002
To appear in:
Surface & Coatings Technology
Received Date: 23 July 2019 Revised Date:
31 August 2019
Accepted Date: 17 September 2019
Please cite this article as: D. Sharma, P.K. Ghosh, N. Kumar, R. Anant, Surface hardening characteristics of microalloyed steel during ex-situ and in-situ Al2O3 reinforcement under TIG arcing, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Surface hardening characteristics of microalloyed steel during ex-situ and in-situ Al2O3 reinforcement under TIG arcing Deepak Sharmaa,*, Prakriti Kumar Ghosha, Nilesh Kumar a, Ramkishor Ananta a
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand-247667, India * Corresponding author: E-mail address- [email protected]
Abstract: This work presents a comparative study of the influence of producing ex-situ and in-situ surface composite (by Tungsten Inert Gas (TIG) arcing process) on the hardenability of microalloyed steel, hereafter mentioned as steel. The ex-situ surface composite was prepared by adding hard Al2O3 particles into the fused surface matrix, while for in-situ surface composite, Al2O3 particles were made to grow in the fused surface matrix through the addition of Al. In the latter process, Al as deoxidizer reacts with the oxygen present in the steel to form the in-situ growth of Al2O3. The modified surface matrix exhibits the presence of martensite phase and the particle reinforcements when studied under optical microscopy, field emission scanning electron microscopy (FESEM) supported by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Vickers’ microhardness testing of the modified surface affirmed a noteworthy improvement in hardness (2.13 times) with respect to the base metal (BM) for both the cases (ex-situ and in-situ) of Al2O3 reinforcement. Although, the hardness improvement was found to be similar in both the cases of reinforcement, but the depth of peak hardening was observed to be greater in case of matrix reinforcement by in-situ grown Al2O3 (~1.25 mm) than the ex-situ added Al2O3 (~0.9 mm), which is more useful for tribological requirements of industries. Keywords: TIG arcing, Steel, Composite layer, Al2O3 dispersion, Surface hardening
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1. Introduction The steel is widely used in diverse industrial applications due to its ability to provide the best combination of mechanical properties such as strength, ductility, and toughness. Despite its versatility of choice for different usages, this material is generally not claimed to be a distinct option for applications involving high resistance to friction and abrasion because of its relatively low surface hardness [1,2]. This limitation can be largely resolved by surface modification through several techniques such as controlled hard phase transformation in the matrix up to a certain depth [3,4], conversion of the surface matrix to a particle reinforced composite [5–9], or surface cladding [10–14] contributing to the improvement of surface hardness. Out of all the above methods, surface modification by converting the surface matrix to a particle reinforced composite has gathered considerable attention in recent decades. This has happened primarily because of its economical and versatile nature in significantly improving the strength, hardness, and toughness of the matrix with respect to the base metal (BM) [5–9]. This process is especially interesting because of its independence of the chemical composition of the substrate. The composite material consists of well-dispersed hard reinforcements in a relatively softer matrix producing desirably improved properties under the complementary or supplementary influence of each component [15]. The approaches for the synthesis of composites are primarily classified into two categories i.e. ex-situ addition and in-situ growth of reinforcements in the matrix. In case of ex-situ addition, a hard-nonreactive reinforcing phase is incorporated into a matrix from outside while for in-situ production a hard phase is made to grow in the matrix by a chemical reaction. The chemical incompatibility of reinforcements and matrix can sometimes make the interface weak for ex-situ production while the in-situ produced reinforcements generally do not face such problems [16]. In the
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past, processes such as laser and electron beam surface melting have been successfully used to produce the surface composite up to a relatively thin depth of sub-millimeter range [17– 19]. These processes involved melting of the substrate coated with the desired reinforcements leading to the formation of surface composite. In recent years, successful use of highly versatile and economical TIG arcing process as a surface melting source has gathered significant interest in developing the ex-situ [9,20–22] and in-situ [23] grown surface composite. This is due to its capability to produce a surface reinforcement up to a depth of more than a couple of millimeters. The work [23] has shown that the thermal behavior of TIG arcing process appreciably supports the thermodynamics of chemical reactions for developing hard phase reinforcements and Marangoni flow in the molten matrix for the dispersion of reinforcements. However, the mechanism of surface reinforcement of steel by ex-situ addition or in-situ growth of particles during TIG arcing and their overall effect on surface hardening is not well understood. This is of large industrial importance especially with respect to the manipulation of depth of hardening that plays a critical role in improving the life of a component in tribological applications [24]. Considering the above facts, this work aims to understand the effect of producing exsitu and in-situ surface composite on the hardenability of steel. The reinforcement of steel surface was done using controlled surface melting of the substrate under TIG arcing process. The approaches involved the addition of Al2O3 particles as hard phase reinforcements into the matrix for ex-situ formation of surface composite and addition of Al for in-situ formation of a hard-nonmetallic compound of Al2O3 formed due to higher affinity of Al for O present in the steel. The primary aim of the current work is to study the mechanism of uniform hardening up to a maximum depth by following the process of ex-situ and in-situ reinforcement of matrix through fusion root by arcing.
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2. Experimental details The steel plate of size 100 mm × 40 mm × 17 mm was used in this work. The chemical composition of the BM is demonstrated in Table I. Optical emission spectroscopy with a spot diameter of the order of 4–5 mm was used to examine the chemical composition. Before modifying the surface, the steel surface was ground using 220 grit size SiC abrasive paper. The prepared samples were further cleaned with acetone to remove the rust and grease. Table I. Chemical composition of the steel base plate. Elements Wt. %
C 0.13
P 0.03
S 0.01
Al 0.02
Si 0.35
Mn 1.62
Nb 0.05
Cr 0.02
Fe rest
2.1 Flux preparation Two fluxes in paste form containing Al2O3 and Al powders were prepared by adding sodium silicate (NaSiO3) as a binder to them. The binder was mixed in drops of 0.18 g to both the fluxes containing Al2O3 or Al powders. For one gram of powder four drops of the binder were added. The prepared paste was coated on the surface of the substrate. The composition of each paste is stated in Table II. Table II. Chemical composition of flux pastes containing Al2O3 and Al powders. Chemical composition of each paste (wt. %)
Composition of flux (in g) Samples P1 P2
Containing Al2O3 Containing Al
NaSiO3 (no. of drops × weight of each drop) 8 × 0.18 = 1.44 8 × 0.18 = 1.44
Al
Al2O3
Al
2 2
0.58
Al2O3
NaSiO3
0.58
0.42 0.42
2.2 Surface modification procedure A paintbrush was used to coat the cleaned surface of the steel substrate. The coating was applied to an area of 1 × 40 mm2. The weight density of the coating was also calculated as: (Weight of powder applied/applied area) ×η and is shown in Table III. The efficiency (η) of applying the flux with the brush is estimated to be about 75 %, where η = [[1− (Prior 4
weight of brush with the flux of different compositions − weight of brush after application of flux in the coating) / (weight of powder present in flux)] × 100]. The estimation of volume fraction (Vol%) of the inclusions having no depth of pores on the matrix was made by using standard line intercept method and the results are shown in Table III. Table III. Weight density of different flux coatings, and the volume fraction of developed inclusions in the modified matrix. Sample modified by using different flux coatings P1 Coating, Al2O3 P2 Coating, Al
Inclusions content (Vol. %) 1.6 3.5
Weight density of coating (g/cm2) 1.875 1.875
The coated part of the substrate was melted under TIG arcing (TIGA). An ESAB MEK 44C power source with DCEN polarity was used for this purpose. Arcing was carried under argon gas (commercial) shielding at a flow rate of 10 l/min. A tungsten electrode of diameter 3.2 mm was used to perform TIGA. The TIGA parameters were kept as- arcing current (I) = 180 ± 5 A, arcing voltage (V) = 12 ± 1 V and arc travel speed (S) = 10 cm/min, giving rise to a heat input of about 10.12 kJ/cm. TIG arcing instantly melts the substrate up to a certain depth and makes it superheated due to well-known high arc temperature ≥ 3000 °C. The superheated low viscous fusion zone (FZ) allows the flux to get mixed with it and starts the process of reinforcing the matrix differently, depending on the flux composition as discussed later. 2.3 Sample preparation and characterization Transverse section of the samples machined out from the modified BM were prepared for further characterization using standard metallographic procedure. The samples were first ground with SiC abrasive papers of grit size 100-2000 followed by cloth polishing using alumina of size 0.024 µm. Nital solution (2 vol. % nitric acid in ethanol) was used to etch the polished surface. An untreated BM was also prepared by the same procedure to use as 5
reference material for all kinds of characterization. All the metallographic specimens were studied under optical microscopy, FESEM with EDS facility, and XRD analysis. An accelerating voltage of 20 kV was used to perform FESEM analysis. The XRD studies were performed using CoKα X-ray radiation and Cuβ filter for a 2θ range of 35o to105o. A slow scan rate (1°/min) was used for XRD measurements. The powders used in this work were characterized using FESEM and EDS, where EDS confirmed the purity of Al2O3 and Al powders as stated by the supplier. The average size of the powders was also studied under the FESEM. The hardness of samples was measured using Vickers’ microhardness indentation across the depth of the treated surface, starting from the top of fusion zone (FZ) and subsequently moving to the heat-affected zone (HAZ) and BM as appeared in transverse section of the samples. The hardness was measured using an indentation load of 300 g with a dwell time of 30 s. At a given processing condition, an average of five hardness measurements is reported. 3.
Results and discussion
3.1 Characteristics of powders FESEM images showing the morphology of Al2O3 and Al powders are presented in Fig. 1(a1) and (b1) respectively. The images presented in Fig. 1(a1) and (b1) show that average size of the Al2O3 and Al particles is well below 1 and 25 µm respectively. The characteristics of Al2O3 and Al powders as confirmed by EDS studies are shown in Fig. 1(a2) and (b2) respectively, which show that the powders are practically free from any contamination.
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Fig. 1. FESEM images (1) and EDS analysis (2) indicating the morphology and composition respectively of different powders: (a) Al2O3 and (b) Al. 3.2 Matrix reinforcing process The mixing and distribution of constituents, arriving from the flux, take place simultaneously in the molten surface of the substrate with the help of Marangoni flow of liquid metal during arcing. In case of the flux containing Al2O3 (P1), the fine oxide particles directly go into the fused matrix and get distributed in it under the assistance of Marangoni flow of molten pool. This incorporation of particles from the P1 coating to the TIG arc modified FZ can be considered as the reinforcement by ex-situ addition of Al2O3 particles that are practically nonreactive for further transformation under the existing state of the molten pool. However, during arcing on the flux containing Al (P2), the metallic particles quickly react with the available oxygen in the FZ and form fine Al2O3 particles in the matrix. Oxidation of Al starts relatively slow at about 650 °C with the growth of γ-alumina
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accompanied by a further phase transformation such as δ-alumina and θ-alumina leading to the formation of stable α-alumina at about 1050 °C [25–27]. Thus, for the in-situ process of surface modification by TIG arcing, the formation of stable α-alumina in the superheated molten steel substrate is very much expected. Along with their formation, these alumina particles also simultaneously get distributed in the FZ under the Marangoni flow of liquid metal. The Ellingham diagram states that in the presence of Al, the formation of Al2O3 in molten steel (eq. i) is favorable when compared to other oxides. This can be correlated with the current case of surface modification by in-situ grown Al2O3 particles in the matrix during TIG arcing on P2 coating. However, at the moderate condition of molten steel due to presence of acidic oxides arising out of the binder, a possible generation (eq. ii) of a minor amount of Fe2O3 in the matrix cannot be ignored [28]. The available excess Al from P2 coating in the molten pool may also react (eq. iii) with part of the Fe2O3 present in it giving rise to further generation of Al2O3. Such reactions are justified in a similar work reported earlier [23]. Thus, for surface modification by TIG arc melting with P2 coating, the volume fraction of inclusions observed in the matrix may contain the predominant presence of in-situ formed fine Al2O3 along with some amount of Fe2O3. 4Al + 3O2
2Al2O3
(i)
4Fe + 3O2
2Fe2O3
(ii)
Al2O3 + 2Fe
(iii)
2Al + Fe2O3
This may be the possible mechanism of how a particle reinforced surface composite of the substrate is prepared under TIG arcing process. The amount of nonmetallic reinforcements present in the fusion modified matrix was studied by estimating the volume fraction of inclusions (dark spots) appeared in it under the optical microscope.
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3.3 Microstructural observations The typical appearance of the microstructurally different zones (Fig. 2) in the modified surface, prepared at a given heat input of about 10.12 kJ/cm, was examined across its cross-section under the optical microscope. Fig. 2 shows that the FZ has a co-axial dendritic growth from the fusion line followed by HAZ extended up to about 700 µm in the BM. The typical geometry of the fusion modified surface of the substrate during TIG arcing at different processing conditions (with and without the coating of different fluxes) are shown in Table IV. The geometry of the fusion modified surface of the substrate exhibits the presence of different microstructural zone with respect to the BM. Table IV also shows that the depth (D), width (W) and the ratio D/W of the FZ are different under different conditions of processing i.e. with and without flux coating of different compositions where the heat input always remained constant as about 10.12 kJ/cm. It shows that the use of flux coating significantly reduces the depth and width of the FZ as compared to the FZ obtained for the without flux modified surface. This may have possibly happened because heat consumed by melting of the flux relatively reduces superheating of the FZ and thus, restricts its depth of penetration in comparison to that occurring in case of only TIG arcing modified surface i.e. without application of flux coating on the substrate. However, during TIG arcing on the flux coated surface, the thermal behavior is largely predominated by the thermodynamics of chemical reactions taking place in the molten pool. Thus, in case of the flux composition that gives rise to chemical reactions with the favorable constituents of the molten pool, the thermal behavior is governed by the arc heating (+), heat consumed by mass melting (-) and heat generated (+) by the chemical reactions taking place during the arcing process. It finally controls the depth of fusion and matrix cooling rate and consequently affects the matrix
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hardening by influencing both the basic criterion such as the extent of martensitic/bainitic transformation and formation of reinforcing particles in the matrix. In this regard, it may be noted that the thermal behavior of chemical reactions during TIG arcing on P2 flux coating may have been partly dominated by the exothermic reaction. This is due to the excess presence of reducing Al in the molten steel along with a certain amount of Fe2O3. This is also circumstantially justified with evidence in the earlier reported work [23] of similar nature. The exothermic reaction enhances superheating of the molten pool that increases its fluidity and consequently strengthens the kinetics of Marangoni fluid flow which extends the depth of fusion. Marangoni convection of fluid flow in the weld pool largely depends upon the temperature coefficient of surface tension (dγ/dT) in the weld pool, which is strongly controlled by distribution of temperature and composition of the FZ. Here γ and T are the surface tension and temperature of the FZ. The dγ/dT is, however, affected by the enthalpy of the reaction [29,30]. Thus, it is interesting to note that the use of flux composition (P2) containing Al gives appreciably higher D and W of the FZ (Table IV) in comparison to the one found due to the use of Al2O3 containing flux (P1).
Fig. 2. The optical micrograph showing the typical macrostructure of a cross-section of TIGA modified steel.
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Table IV. The geometry of the fusion modified surface under TIG arcing, without and with flux coating of different compositions, at a given heat input of 10.12 kJ/cm. Flux
FZ Depth, D (mm)
FZ Width, W (mm)
D/W
TIGA
3.5
7.5
0.47
Al2O3 (P1)
1.5
4.5
0.34
Al (P2)
2.5
6
0.42
Bead Picture
The representative microstructure of BM, as shown in Fig. 3(a), exhibits the presence of pro-eutectoid ferrite and pearlite in the matrix as also reported earlier [31]. The microstructure of HAZ developed by the influence of plain (without flux) TIG arc melting exhibits the presence of relatively finer grains (Fig. 3 (b)) when observed close to the BM [31]. This has happened because of the temperature rise in this region to about 1000-1200 K. The microstructure of FZ has always been studied adjacent (0.6 mm) to its surface. The microstructure of the FZ of the plain TIG arcing is shown in Fig. 3 (c). It consists of lathe type coarse martensite along with some amount of bainite and ferrite. The XRD studies have confirmed the presence of martensite in the fused zone and are discussed later. This is also in agreement with the findings of similar TIG arc processing reported earlier for the formation of martensite in low carbon steel [32]. Fig. 3 (d) and (e) show the presence of nonmetallic reinforcements appearing as black spots in the matrix of TIG arcing modified FZ produced by using P1 and P2 flux coating containing Al2O3 and Al respectively. Fig. 3(e) shows the presence of relatively well-distributed fine in-situ grown particle reinforcements in the
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matrix, whereas the Fig. 3(d) exhibits the presence of coarse clusters of ex-situ added reinforced Al2O3 particles in the matrix.
Fig. 3. The optical micrographs showing the typical microstructure of (a) BM, (b) HAZ and (c) FZ of plain TIG arcing. The optical micrograph in (d) shows the FZ of TIG arcing with P1 flux containing Al2O3 while (e) shows the FZ with P2 flux containing Al. 3.4 XRD Analysis XRD analysis of BM and the fusion modified surface was carried out to confirm the presence of different phases in the matrix of the steel. The XRD analysis of the BM is shown 12
in Fig. 4(a), which depicts the presence of ferrite in the matrix. However, the XRD of the fusion modified surface clearly shows the presence of martensite (Fig. 4(b-d)), while no martensite is seen in the BM (Fig. 4(a)). The peaks confirming the presence of martensite is observed at the same value of 2θ as that of ferrite. It is because, at the low amount of C in the steel, the c-axis of the body-centered tetragonal martensite does not shift leading to the same 2θ position of the peak for ferrite and martensite [33–35]. The variation in the intensity of the peaks for the presence of martensite in the matrix primarily indicates the different extent of its formation in the matrix along with the possible presence of reinforcement particles formed in the FZ. The observed presence of a relatively low intensity peak of martensite (Fig. 4 (d)) in the FZ indicates the formation of relatively less amount of martensite in the matrix modified under the P2 flux coating. This possibly happened due to its comparatively slower cooling than that of the P1 flux modified matrix (Fig. 4 (c)). It is further discussed in section 3.6 in the context of the martensite formation as a function of the cooling rate of the matrix affecting its hardness. However, in the XRD plot (Fig. 4 (c)) of the matrix modified by using the flux (P1) containing Al2O3, the presence of reinforced particles is not detected. Whereas, in the XRD plot (Fig. 4 (d)) of the surface modified with the flux (P2) containing Al, a weak presence of the peak from Al2O3 could be marked due to its relatively larger presence (Table III) in the matrix. This is considered as the fulfillment of the primary goal of reinforcement in the fusion modified matrix as mentioned above. The absence or weak presence of peaks of Al2O3 reinforced particles in XRD plots may have primarily happened due to their presence in very low volume fraction in the FZ matrix. An appreciable response from the presence of the Al2O3 reinforced particles in the matrix could be detected by EDS analysis under FESEM of both ex-situ and in-situ grown composites as discussed later.
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Fig. 4. The XRD patterns for (a) BM and different TIGA modified surfaces: (b) without flux, (c) with flux containing Al2O3 and (d) with flux containing Al. 3.5 FESEM and EDS Analysis FESEM analysis performed to study the amount and distribution of the reinforced particles in the FZ (adjacent to its surface) show (Fig. 5) the presence of particles and their discrete occurrence in the matrix. Fig. 5 (a) and (b) show the matrix appearance of the surface modified by using Al2O3 and Al-containing fluxes respectively. Both figures reveal the appreciable presence of reinforced particles in the matrix of the FZ. However, Fig. 5(b) shows the existence of relatively more amount of in-situ grown particles in the matrix when compared to the amount of ex-situ added Al2O3 particles (Fig. 5(a)). It justifies the reason stated above for non-detection of reinforcing particle during XRD analysis of the modified FZ produced by using the P1 flux containing Al2O3.
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Fig. 5. FESEM images of TIGA modified surfaces using different fluxes containing (a) Al2O3 and (b) Al. The EDS analysis of the matrix and reinforced particles present in the fusion modified surface modified using two fluxes containing Al2O3 and Al is shown in Fig. 6 and 7 respectively. The figures show the presence of Al and O in the particles that confirms the desired presence of Al2O3 after ex-situ and in-situ surface modification of the substrate as discussed above. In this regard, it is also interesting to note that the EDS analysis of the P1 flux assisted modified matrix shows feeble (Fig. 6) presence of Al and O, but their good presence is visible (Fig. 7) in the analysis of the modified matrix prepared by application of the P2 flux. It indicates the possible in-situ growth of fine Al2O3 particles by chemical reactions during surface modification of the steel under TIG arcing on P2 flux coated substrate. It is also noted that both the ex-situ added and in-situ grown Al2O3 particles do not show any discontinuity at their interface with the matrix (Fig. 6 and 7 respectively) which indicates that they are well bonded with the matrix and positively contributes to its enhancement of hardness. However, for advanced understanding the morphology of the particle and its bonding with the matrix should be studied further under TEM.
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Fig. 6. FESEM image and EDS analysis showing the presence of reinforced particles in FZ treated with a flux containing Al2O3.
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Fig. 7. FESEM image and EDS analysis showing the presence of reinforced particles in FZ treated with a flux containing Al. 3.6 Hardness The hardness of the steel BM is measured as 205 ± 10 HV, which is not appreciable for its application in friction or abrasion until surface modification is done. Surface modification of the BM by ex-situ added and in-situ grown Al2O3 particles in the matrix by TIG arcing is largely characterized by the hardness characteristics in the matrix. The variation in hardness from the top of the modified surface to the BM across the FZ and HAZ, as measured on the cross-section of the substrate, is shown in Fig. 8. The hardening of TIGA modified substrate up to a certain depth from the surface produced by using no flux coating is primarily happened due to formation of hard martensitic or bainitic phase at the lower part of the FZ in the matrix as typically shown in Fig. 9. However, with respect to this, a relatively higher hardness up to a certain depth from the surface, observed in different flux (P1 and P2) 17
coated TIGA modified substrate, may have happened due to the presence of martensite and reinforced particles in the fused matrix. The hardness of the reinforced matrix is also dependent on the volume fraction, size and distribution of the reinforced particles in it. The average hardness values observed in the matrix at 0.6 mm below the surface of differently modified steel substrate are given in Table V. The table shows that the hardness of all the modified surfaces produced by TIG arcing (with or without application of flux coating) is significantly higher than the BM. But, the improvement of hardness in case of the modified surface developed with both the flux coatings is much higher than the without flux modified surface.
Fig. 8. The hardness characteristics of TIGA modified surfaces (with or without flux) measured along the depth starting from the top surface, across the FZ and HAZ, to BM.
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Fig. 9. The FESEM micrographs showing the typical presence of bainite towards the bottom of the FZ. It indicates that the reinforcement of particle does have a significant influence on the matrix hardness. However, it is noted that the depth of peak hardness region is considerably lower for flux coated modification with respect to the without flux one. Such a difference in depth of hardening with respect to the use of Al2O3 and Al bearing fluxes may be understood from thermal characteristics of the fused region as stated above. In the line of earlier discussions on the effect of exothermic reaction in case of arcing on P2 flux coating, it enhances the fluidity of molten pool and consequently strengthens the kinetics of Marangoni convective fluid flow that extends the depth of fusion and plays a primary role in improving the dispersion of in-situ grown reinforcing particles up to a relatively larger depth of FZ. The appearance of the transition of the depth of effective dispersion of particles in FZ due to Marangoni fluid flow is typically shown in Fig.10, which has to be studied further in detail in the future. The relative improvement in uniform distribution of the reinforcing particles is apparently marked in the matrix modified with the P2 flux (Fig. 3(e)) with respect to that observed in the P1 flux assisted one (Fig. 3(d)). Both the conditions clearly support the occurrence of the relatively more hardened depth of the fused region for P2 flux (3.87 mm) assisted modification than that of the P1 flux (2.7 mm) assisted one as observed in Fig. 8.
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Fig. 10. FESEM micrograph showing the typical appearance of transition in the amount of distribution of ex-situ reinforcing particles from top to bottom in FZ. At this juncture, it may be clearly understood that hardness enhancement in the FZ is basically a function of hard phase transformation and nature of particle reinforcements in the matrix, where the latter one plays an additional role to offer the peak hardness. It is significantly noted that the depth of peak hardening region in the surface matrix due to particle reinforcements is appreciably lower in case of using the Al2O3 bearing flux (P1) than the Al-bearing (P2) one, found as ~0.9 mm and ~1.25 mm respectively. The relatively higher depth of Al2O3 reinforcement for P2 flux containing Al may be primarily attributed to more effective Marangoni flow and comparatively more fluidity (low viscosity) of FZ due to higher superheating as discussed above. Thus, it infers that the use of Al containing flux is more useful than direct use of Al2O3 for reinforcing the steel during surface modification by TIG arcing. Fig. 8 also shows that the spread of HAZ, characterized by a gradual fall of matrix hardness starting from the fusion line up to a point of its sharp fall near the BM, is comparatively larger in case of the modification assisted by the P2 flux than P1 flux. This is attributed to relatively more superheating of the FZ due to exothermic reaction occurring due to the use of the P2 flux coating under TIG arcing. The extension of HAZ reduces its cooling
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rate by pushing away the effective heat sink to the base metal and thus it encourages more bainite transformation in the matrix than martensite as typically shown in Fig. 11. It lowers the hardness of this region, as clearly observed in Fig. 8, during surface modification by TIG arcing with a P2 flux coating. Table V. The average hardness of the modified surface taken at 0.6 mm below the top surface. Surface modified by TIGA
Average hardness (Hv)
Without flux coating P1 Coating, Al2O3 P2 Coating, Al
392 ± 7 437 ± 17 436 ± 17
Fraction of increase w.r.t. BM hardness (%) 91 113 113
Fig. 11. A typical microstructure of HAZ observed during surface modification by TIG arcing with P2 coating.
4. Conclusions The surface hardness of steel can be significantly improved (2.13 times) by using a flux activated particle reinforcing TIG arcing process. The surface hardness has been improved up to a considerable depth due to the formation of a composite layer on it. However, the hardenability is dependent on the flux chemistry, where the use of Alcontaining flux forming in-situ Al2O3 reinforcement in the matrix is found to be more
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attractive than using ex-situ added Al2O3-containing flux. The considerable improvement in surface hardness of steel by using flux activated TIG arcing process is dependent on the martensite transformation as well as particle reinforcement in the matrix. The mechanism of improving the depth of hardening with an appropriate dispersion of reinforcements by the control of Marangoni flow is understood up to a reasonable extent. This may be concluded as quite encouraging to open an avenue for further research to establish its multipurpose usage in wear-resistant applications.
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Highlights: •
Ex-situ and In-situ development of surface composite on microalloyed steel surface to improve the surface hardness
•
Employing TIG arcing to develop surface composite
•
Influence of ex-situ and in-situ reinforcement on hardenability of microalloyed steel surface
•
Influence of Marangoni effect during ex-situ and in-situ reinforcement on hardenability of microalloyed steel surface
S0257-8972(19)30991-0
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125002
Reference:
SCT 125002
To appear in:
Surface & Coatings Technology
Received Date: 23 July 2019 Revised Date:
31 August 2019
Accepted Date: 17 September 2019
Please cite this article as: D. Sharma, P.K. Ghosh, N. Kumar, R. Anant, Surface hardening characteristics of microalloyed steel during ex-situ and in-situ Al2O3 reinforcement under TIG arcing, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Surface hardening characteristics of microalloyed steel during ex-situ and in-situ Al2O3 reinforcement under TIG arcing Deepak Sharmaa,*, Prakriti Kumar Ghosha, Nilesh Kumar a, Ramkishor Ananta a
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand-247667, India * Corresponding author: E-mail address- [email protected]
Abstract: This work presents a comparative study of the influence of producing ex-situ and in-situ surface composite (by Tungsten Inert Gas (TIG) arcing process) on the hardenability of microalloyed steel, hereafter mentioned as steel. The ex-situ surface composite was prepared by adding hard Al2O3 particles into the fused surface matrix, while for in-situ surface composite, Al2O3 particles were made to grow in the fused surface matrix through the addition of Al. In the latter process, Al as deoxidizer reacts with the oxygen present in the steel to form the in-situ growth of Al2O3. The modified surface matrix exhibits the presence of martensite phase and the particle reinforcements when studied under optical microscopy, field emission scanning electron microscopy (FESEM) supported by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Vickers’ microhardness testing of the modified surface affirmed a noteworthy improvement in hardness (2.13 times) with respect to the base metal (BM) for both the cases (ex-situ and in-situ) of Al2O3 reinforcement. Although, the hardness improvement was found to be similar in both the cases of reinforcement, but the depth of peak hardening was observed to be greater in case of matrix reinforcement by in-situ grown Al2O3 (~1.25 mm) than the ex-situ added Al2O3 (~0.9 mm), which is more useful for tribological requirements of industries. Keywords: TIG arcing, Steel, Composite layer, Al2O3 dispersion, Surface hardening
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1. Introduction The steel is widely used in diverse industrial applications due to its ability to provide the best combination of mechanical properties such as strength, ductility, and toughness. Despite its versatility of choice for different usages, this material is generally not claimed to be a distinct option for applications involving high resistance to friction and abrasion because of its relatively low surface hardness [1,2]. This limitation can be largely resolved by surface modification through several techniques such as controlled hard phase transformation in the matrix up to a certain depth [3,4], conversion of the surface matrix to a particle reinforced composite [5–9], or surface cladding [10–14] contributing to the improvement of surface hardness. Out of all the above methods, surface modification by converting the surface matrix to a particle reinforced composite has gathered considerable attention in recent decades. This has happened primarily because of its economical and versatile nature in significantly improving the strength, hardness, and toughness of the matrix with respect to the base metal (BM) [5–9]. This process is especially interesting because of its independence of the chemical composition of the substrate. The composite material consists of well-dispersed hard reinforcements in a relatively softer matrix producing desirably improved properties under the complementary or supplementary influence of each component [15]. The approaches for the synthesis of composites are primarily classified into two categories i.e. ex-situ addition and in-situ growth of reinforcements in the matrix. In case of ex-situ addition, a hard-nonreactive reinforcing phase is incorporated into a matrix from outside while for in-situ production a hard phase is made to grow in the matrix by a chemical reaction. The chemical incompatibility of reinforcements and matrix can sometimes make the interface weak for ex-situ production while the in-situ produced reinforcements generally do not face such problems [16]. In the
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past, processes such as laser and electron beam surface melting have been successfully used to produce the surface composite up to a relatively thin depth of sub-millimeter range [17– 19]. These processes involved melting of the substrate coated with the desired reinforcements leading to the formation of surface composite. In recent years, successful use of highly versatile and economical TIG arcing process as a surface melting source has gathered significant interest in developing the ex-situ [9,20–22] and in-situ [23] grown surface composite. This is due to its capability to produce a surface reinforcement up to a depth of more than a couple of millimeters. The work [23] has shown that the thermal behavior of TIG arcing process appreciably supports the thermodynamics of chemical reactions for developing hard phase reinforcements and Marangoni flow in the molten matrix for the dispersion of reinforcements. However, the mechanism of surface reinforcement of steel by ex-situ addition or in-situ growth of particles during TIG arcing and their overall effect on surface hardening is not well understood. This is of large industrial importance especially with respect to the manipulation of depth of hardening that plays a critical role in improving the life of a component in tribological applications [24]. Considering the above facts, this work aims to understand the effect of producing exsitu and in-situ surface composite on the hardenability of steel. The reinforcement of steel surface was done using controlled surface melting of the substrate under TIG arcing process. The approaches involved the addition of Al2O3 particles as hard phase reinforcements into the matrix for ex-situ formation of surface composite and addition of Al for in-situ formation of a hard-nonmetallic compound of Al2O3 formed due to higher affinity of Al for O present in the steel. The primary aim of the current work is to study the mechanism of uniform hardening up to a maximum depth by following the process of ex-situ and in-situ reinforcement of matrix through fusion root by arcing.
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2. Experimental details The steel plate of size 100 mm × 40 mm × 17 mm was used in this work. The chemical composition of the BM is demonstrated in Table I. Optical emission spectroscopy with a spot diameter of the order of 4–5 mm was used to examine the chemical composition. Before modifying the surface, the steel surface was ground using 220 grit size SiC abrasive paper. The prepared samples were further cleaned with acetone to remove the rust and grease. Table I. Chemical composition of the steel base plate. Elements Wt. %
C 0.13
P 0.03
S 0.01
Al 0.02
Si 0.35
Mn 1.62
Nb 0.05
Cr 0.02
Fe rest
2.1 Flux preparation Two fluxes in paste form containing Al2O3 and Al powders were prepared by adding sodium silicate (NaSiO3) as a binder to them. The binder was mixed in drops of 0.18 g to both the fluxes containing Al2O3 or Al powders. For one gram of powder four drops of the binder were added. The prepared paste was coated on the surface of the substrate. The composition of each paste is stated in Table II. Table II. Chemical composition of flux pastes containing Al2O3 and Al powders. Chemical composition of each paste (wt. %)
Composition of flux (in g) Samples P1 P2
Containing Al2O3 Containing Al
NaSiO3 (no. of drops × weight of each drop) 8 × 0.18 = 1.44 8 × 0.18 = 1.44
Al
Al2O3
Al
2 2
0.58
Al2O3
NaSiO3
0.58
0.42 0.42
2.2 Surface modification procedure A paintbrush was used to coat the cleaned surface of the steel substrate. The coating was applied to an area of 1 × 40 mm2. The weight density of the coating was also calculated as: (Weight of powder applied/applied area) ×η and is shown in Table III. The efficiency (η) of applying the flux with the brush is estimated to be about 75 %, where η = [[1− (Prior 4
weight of brush with the flux of different compositions − weight of brush after application of flux in the coating) / (weight of powder present in flux)] × 100]. The estimation of volume fraction (Vol%) of the inclusions having no depth of pores on the matrix was made by using standard line intercept method and the results are shown in Table III. Table III. Weight density of different flux coatings, and the volume fraction of developed inclusions in the modified matrix. Sample modified by using different flux coatings P1 Coating, Al2O3 P2 Coating, Al
Inclusions content (Vol. %) 1.6 3.5
Weight density of coating (g/cm2) 1.875 1.875
The coated part of the substrate was melted under TIG arcing (TIGA). An ESAB MEK 44C power source with DCEN polarity was used for this purpose. Arcing was carried under argon gas (commercial) shielding at a flow rate of 10 l/min. A tungsten electrode of diameter 3.2 mm was used to perform TIGA. The TIGA parameters were kept as- arcing current (I) = 180 ± 5 A, arcing voltage (V) = 12 ± 1 V and arc travel speed (S) = 10 cm/min, giving rise to a heat input of about 10.12 kJ/cm. TIG arcing instantly melts the substrate up to a certain depth and makes it superheated due to well-known high arc temperature ≥ 3000 °C. The superheated low viscous fusion zone (FZ) allows the flux to get mixed with it and starts the process of reinforcing the matrix differently, depending on the flux composition as discussed later. 2.3 Sample preparation and characterization Transverse section of the samples machined out from the modified BM were prepared for further characterization using standard metallographic procedure. The samples were first ground with SiC abrasive papers of grit size 100-2000 followed by cloth polishing using alumina of size 0.024 µm. Nital solution (2 vol. % nitric acid in ethanol) was used to etch the polished surface. An untreated BM was also prepared by the same procedure to use as 5
reference material for all kinds of characterization. All the metallographic specimens were studied under optical microscopy, FESEM with EDS facility, and XRD analysis. An accelerating voltage of 20 kV was used to perform FESEM analysis. The XRD studies were performed using CoKα X-ray radiation and Cuβ filter for a 2θ range of 35o to105o. A slow scan rate (1°/min) was used for XRD measurements. The powders used in this work were characterized using FESEM and EDS, where EDS confirmed the purity of Al2O3 and Al powders as stated by the supplier. The average size of the powders was also studied under the FESEM. The hardness of samples was measured using Vickers’ microhardness indentation across the depth of the treated surface, starting from the top of fusion zone (FZ) and subsequently moving to the heat-affected zone (HAZ) and BM as appeared in transverse section of the samples. The hardness was measured using an indentation load of 300 g with a dwell time of 30 s. At a given processing condition, an average of five hardness measurements is reported. 3.
Results and discussion
3.1 Characteristics of powders FESEM images showing the morphology of Al2O3 and Al powders are presented in Fig. 1(a1) and (b1) respectively. The images presented in Fig. 1(a1) and (b1) show that average size of the Al2O3 and Al particles is well below 1 and 25 µm respectively. The characteristics of Al2O3 and Al powders as confirmed by EDS studies are shown in Fig. 1(a2) and (b2) respectively, which show that the powders are practically free from any contamination.
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Fig. 1. FESEM images (1) and EDS analysis (2) indicating the morphology and composition respectively of different powders: (a) Al2O3 and (b) Al. 3.2 Matrix reinforcing process The mixing and distribution of constituents, arriving from the flux, take place simultaneously in the molten surface of the substrate with the help of Marangoni flow of liquid metal during arcing. In case of the flux containing Al2O3 (P1), the fine oxide particles directly go into the fused matrix and get distributed in it under the assistance of Marangoni flow of molten pool. This incorporation of particles from the P1 coating to the TIG arc modified FZ can be considered as the reinforcement by ex-situ addition of Al2O3 particles that are practically nonreactive for further transformation under the existing state of the molten pool. However, during arcing on the flux containing Al (P2), the metallic particles quickly react with the available oxygen in the FZ and form fine Al2O3 particles in the matrix. Oxidation of Al starts relatively slow at about 650 °C with the growth of γ-alumina
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accompanied by a further phase transformation such as δ-alumina and θ-alumina leading to the formation of stable α-alumina at about 1050 °C [25–27]. Thus, for the in-situ process of surface modification by TIG arcing, the formation of stable α-alumina in the superheated molten steel substrate is very much expected. Along with their formation, these alumina particles also simultaneously get distributed in the FZ under the Marangoni flow of liquid metal. The Ellingham diagram states that in the presence of Al, the formation of Al2O3 in molten steel (eq. i) is favorable when compared to other oxides. This can be correlated with the current case of surface modification by in-situ grown Al2O3 particles in the matrix during TIG arcing on P2 coating. However, at the moderate condition of molten steel due to presence of acidic oxides arising out of the binder, a possible generation (eq. ii) of a minor amount of Fe2O3 in the matrix cannot be ignored [28]. The available excess Al from P2 coating in the molten pool may also react (eq. iii) with part of the Fe2O3 present in it giving rise to further generation of Al2O3. Such reactions are justified in a similar work reported earlier [23]. Thus, for surface modification by TIG arc melting with P2 coating, the volume fraction of inclusions observed in the matrix may contain the predominant presence of in-situ formed fine Al2O3 along with some amount of Fe2O3. 4Al + 3O2
2Al2O3
(i)
4Fe + 3O2
2Fe2O3
(ii)
Al2O3 + 2Fe
(iii)
2Al + Fe2O3
This may be the possible mechanism of how a particle reinforced surface composite of the substrate is prepared under TIG arcing process. The amount of nonmetallic reinforcements present in the fusion modified matrix was studied by estimating the volume fraction of inclusions (dark spots) appeared in it under the optical microscope.
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3.3 Microstructural observations The typical appearance of the microstructurally different zones (Fig. 2) in the modified surface, prepared at a given heat input of about 10.12 kJ/cm, was examined across its cross-section under the optical microscope. Fig. 2 shows that the FZ has a co-axial dendritic growth from the fusion line followed by HAZ extended up to about 700 µm in the BM. The typical geometry of the fusion modified surface of the substrate during TIG arcing at different processing conditions (with and without the coating of different fluxes) are shown in Table IV. The geometry of the fusion modified surface of the substrate exhibits the presence of different microstructural zone with respect to the BM. Table IV also shows that the depth (D), width (W) and the ratio D/W of the FZ are different under different conditions of processing i.e. with and without flux coating of different compositions where the heat input always remained constant as about 10.12 kJ/cm. It shows that the use of flux coating significantly reduces the depth and width of the FZ as compared to the FZ obtained for the without flux modified surface. This may have possibly happened because heat consumed by melting of the flux relatively reduces superheating of the FZ and thus, restricts its depth of penetration in comparison to that occurring in case of only TIG arcing modified surface i.e. without application of flux coating on the substrate. However, during TIG arcing on the flux coated surface, the thermal behavior is largely predominated by the thermodynamics of chemical reactions taking place in the molten pool. Thus, in case of the flux composition that gives rise to chemical reactions with the favorable constituents of the molten pool, the thermal behavior is governed by the arc heating (+), heat consumed by mass melting (-) and heat generated (+) by the chemical reactions taking place during the arcing process. It finally controls the depth of fusion and matrix cooling rate and consequently affects the matrix
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hardening by influencing both the basic criterion such as the extent of martensitic/bainitic transformation and formation of reinforcing particles in the matrix. In this regard, it may be noted that the thermal behavior of chemical reactions during TIG arcing on P2 flux coating may have been partly dominated by the exothermic reaction. This is due to the excess presence of reducing Al in the molten steel along with a certain amount of Fe2O3. This is also circumstantially justified with evidence in the earlier reported work [23] of similar nature. The exothermic reaction enhances superheating of the molten pool that increases its fluidity and consequently strengthens the kinetics of Marangoni fluid flow which extends the depth of fusion. Marangoni convection of fluid flow in the weld pool largely depends upon the temperature coefficient of surface tension (dγ/dT) in the weld pool, which is strongly controlled by distribution of temperature and composition of the FZ. Here γ and T are the surface tension and temperature of the FZ. The dγ/dT is, however, affected by the enthalpy of the reaction [29,30]. Thus, it is interesting to note that the use of flux composition (P2) containing Al gives appreciably higher D and W of the FZ (Table IV) in comparison to the one found due to the use of Al2O3 containing flux (P1).
Fig. 2. The optical micrograph showing the typical macrostructure of a cross-section of TIGA modified steel.
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Table IV. The geometry of the fusion modified surface under TIG arcing, without and with flux coating of different compositions, at a given heat input of 10.12 kJ/cm. Flux
FZ Depth, D (mm)
FZ Width, W (mm)
D/W
TIGA
3.5
7.5
0.47
Al2O3 (P1)
1.5
4.5
0.34
Al (P2)
2.5
6
0.42
Bead Picture
The representative microstructure of BM, as shown in Fig. 3(a), exhibits the presence of pro-eutectoid ferrite and pearlite in the matrix as also reported earlier [31]. The microstructure of HAZ developed by the influence of plain (without flux) TIG arc melting exhibits the presence of relatively finer grains (Fig. 3 (b)) when observed close to the BM [31]. This has happened because of the temperature rise in this region to about 1000-1200 K. The microstructure of FZ has always been studied adjacent (0.6 mm) to its surface. The microstructure of the FZ of the plain TIG arcing is shown in Fig. 3 (c). It consists of lathe type coarse martensite along with some amount of bainite and ferrite. The XRD studies have confirmed the presence of martensite in the fused zone and are discussed later. This is also in agreement with the findings of similar TIG arc processing reported earlier for the formation of martensite in low carbon steel [32]. Fig. 3 (d) and (e) show the presence of nonmetallic reinforcements appearing as black spots in the matrix of TIG arcing modified FZ produced by using P1 and P2 flux coating containing Al2O3 and Al respectively. Fig. 3(e) shows the presence of relatively well-distributed fine in-situ grown particle reinforcements in the
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matrix, whereas the Fig. 3(d) exhibits the presence of coarse clusters of ex-situ added reinforced Al2O3 particles in the matrix.
Fig. 3. The optical micrographs showing the typical microstructure of (a) BM, (b) HAZ and (c) FZ of plain TIG arcing. The optical micrograph in (d) shows the FZ of TIG arcing with P1 flux containing Al2O3 while (e) shows the FZ with P2 flux containing Al. 3.4 XRD Analysis XRD analysis of BM and the fusion modified surface was carried out to confirm the presence of different phases in the matrix of the steel. The XRD analysis of the BM is shown 12
in Fig. 4(a), which depicts the presence of ferrite in the matrix. However, the XRD of the fusion modified surface clearly shows the presence of martensite (Fig. 4(b-d)), while no martensite is seen in the BM (Fig. 4(a)). The peaks confirming the presence of martensite is observed at the same value of 2θ as that of ferrite. It is because, at the low amount of C in the steel, the c-axis of the body-centered tetragonal martensite does not shift leading to the same 2θ position of the peak for ferrite and martensite [33–35]. The variation in the intensity of the peaks for the presence of martensite in the matrix primarily indicates the different extent of its formation in the matrix along with the possible presence of reinforcement particles formed in the FZ. The observed presence of a relatively low intensity peak of martensite (Fig. 4 (d)) in the FZ indicates the formation of relatively less amount of martensite in the matrix modified under the P2 flux coating. This possibly happened due to its comparatively slower cooling than that of the P1 flux modified matrix (Fig. 4 (c)). It is further discussed in section 3.6 in the context of the martensite formation as a function of the cooling rate of the matrix affecting its hardness. However, in the XRD plot (Fig. 4 (c)) of the matrix modified by using the flux (P1) containing Al2O3, the presence of reinforced particles is not detected. Whereas, in the XRD plot (Fig. 4 (d)) of the surface modified with the flux (P2) containing Al, a weak presence of the peak from Al2O3 could be marked due to its relatively larger presence (Table III) in the matrix. This is considered as the fulfillment of the primary goal of reinforcement in the fusion modified matrix as mentioned above. The absence or weak presence of peaks of Al2O3 reinforced particles in XRD plots may have primarily happened due to their presence in very low volume fraction in the FZ matrix. An appreciable response from the presence of the Al2O3 reinforced particles in the matrix could be detected by EDS analysis under FESEM of both ex-situ and in-situ grown composites as discussed later.
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Fig. 4. The XRD patterns for (a) BM and different TIGA modified surfaces: (b) without flux, (c) with flux containing Al2O3 and (d) with flux containing Al. 3.5 FESEM and EDS Analysis FESEM analysis performed to study the amount and distribution of the reinforced particles in the FZ (adjacent to its surface) show (Fig. 5) the presence of particles and their discrete occurrence in the matrix. Fig. 5 (a) and (b) show the matrix appearance of the surface modified by using Al2O3 and Al-containing fluxes respectively. Both figures reveal the appreciable presence of reinforced particles in the matrix of the FZ. However, Fig. 5(b) shows the existence of relatively more amount of in-situ grown particles in the matrix when compared to the amount of ex-situ added Al2O3 particles (Fig. 5(a)). It justifies the reason stated above for non-detection of reinforcing particle during XRD analysis of the modified FZ produced by using the P1 flux containing Al2O3.
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Fig. 5. FESEM images of TIGA modified surfaces using different fluxes containing (a) Al2O3 and (b) Al. The EDS analysis of the matrix and reinforced particles present in the fusion modified surface modified using two fluxes containing Al2O3 and Al is shown in Fig. 6 and 7 respectively. The figures show the presence of Al and O in the particles that confirms the desired presence of Al2O3 after ex-situ and in-situ surface modification of the substrate as discussed above. In this regard, it is also interesting to note that the EDS analysis of the P1 flux assisted modified matrix shows feeble (Fig. 6) presence of Al and O, but their good presence is visible (Fig. 7) in the analysis of the modified matrix prepared by application of the P2 flux. It indicates the possible in-situ growth of fine Al2O3 particles by chemical reactions during surface modification of the steel under TIG arcing on P2 flux coated substrate. It is also noted that both the ex-situ added and in-situ grown Al2O3 particles do not show any discontinuity at their interface with the matrix (Fig. 6 and 7 respectively) which indicates that they are well bonded with the matrix and positively contributes to its enhancement of hardness. However, for advanced understanding the morphology of the particle and its bonding with the matrix should be studied further under TEM.
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Fig. 6. FESEM image and EDS analysis showing the presence of reinforced particles in FZ treated with a flux containing Al2O3.
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Fig. 7. FESEM image and EDS analysis showing the presence of reinforced particles in FZ treated with a flux containing Al. 3.6 Hardness The hardness of the steel BM is measured as 205 ± 10 HV, which is not appreciable for its application in friction or abrasion until surface modification is done. Surface modification of the BM by ex-situ added and in-situ grown Al2O3 particles in the matrix by TIG arcing is largely characterized by the hardness characteristics in the matrix. The variation in hardness from the top of the modified surface to the BM across the FZ and HAZ, as measured on the cross-section of the substrate, is shown in Fig. 8. The hardening of TIGA modified substrate up to a certain depth from the surface produced by using no flux coating is primarily happened due to formation of hard martensitic or bainitic phase at the lower part of the FZ in the matrix as typically shown in Fig. 9. However, with respect to this, a relatively higher hardness up to a certain depth from the surface, observed in different flux (P1 and P2) 17
coated TIGA modified substrate, may have happened due to the presence of martensite and reinforced particles in the fused matrix. The hardness of the reinforced matrix is also dependent on the volume fraction, size and distribution of the reinforced particles in it. The average hardness values observed in the matrix at 0.6 mm below the surface of differently modified steel substrate are given in Table V. The table shows that the hardness of all the modified surfaces produced by TIG arcing (with or without application of flux coating) is significantly higher than the BM. But, the improvement of hardness in case of the modified surface developed with both the flux coatings is much higher than the without flux modified surface.
Fig. 8. The hardness characteristics of TIGA modified surfaces (with or without flux) measured along the depth starting from the top surface, across the FZ and HAZ, to BM.
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Fig. 9. The FESEM micrographs showing the typical presence of bainite towards the bottom of the FZ. It indicates that the reinforcement of particle does have a significant influence on the matrix hardness. However, it is noted that the depth of peak hardness region is considerably lower for flux coated modification with respect to the without flux one. Such a difference in depth of hardening with respect to the use of Al2O3 and Al bearing fluxes may be understood from thermal characteristics of the fused region as stated above. In the line of earlier discussions on the effect of exothermic reaction in case of arcing on P2 flux coating, it enhances the fluidity of molten pool and consequently strengthens the kinetics of Marangoni convective fluid flow that extends the depth of fusion and plays a primary role in improving the dispersion of in-situ grown reinforcing particles up to a relatively larger depth of FZ. The appearance of the transition of the depth of effective dispersion of particles in FZ due to Marangoni fluid flow is typically shown in Fig.10, which has to be studied further in detail in the future. The relative improvement in uniform distribution of the reinforcing particles is apparently marked in the matrix modified with the P2 flux (Fig. 3(e)) with respect to that observed in the P1 flux assisted one (Fig. 3(d)). Both the conditions clearly support the occurrence of the relatively more hardened depth of the fused region for P2 flux (3.87 mm) assisted modification than that of the P1 flux (2.7 mm) assisted one as observed in Fig. 8.
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Fig. 10. FESEM micrograph showing the typical appearance of transition in the amount of distribution of ex-situ reinforcing particles from top to bottom in FZ. At this juncture, it may be clearly understood that hardness enhancement in the FZ is basically a function of hard phase transformation and nature of particle reinforcements in the matrix, where the latter one plays an additional role to offer the peak hardness. It is significantly noted that the depth of peak hardening region in the surface matrix due to particle reinforcements is appreciably lower in case of using the Al2O3 bearing flux (P1) than the Al-bearing (P2) one, found as ~0.9 mm and ~1.25 mm respectively. The relatively higher depth of Al2O3 reinforcement for P2 flux containing Al may be primarily attributed to more effective Marangoni flow and comparatively more fluidity (low viscosity) of FZ due to higher superheating as discussed above. Thus, it infers that the use of Al containing flux is more useful than direct use of Al2O3 for reinforcing the steel during surface modification by TIG arcing. Fig. 8 also shows that the spread of HAZ, characterized by a gradual fall of matrix hardness starting from the fusion line up to a point of its sharp fall near the BM, is comparatively larger in case of the modification assisted by the P2 flux than P1 flux. This is attributed to relatively more superheating of the FZ due to exothermic reaction occurring due to the use of the P2 flux coating under TIG arcing. The extension of HAZ reduces its cooling
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rate by pushing away the effective heat sink to the base metal and thus it encourages more bainite transformation in the matrix than martensite as typically shown in Fig. 11. It lowers the hardness of this region, as clearly observed in Fig. 8, during surface modification by TIG arcing with a P2 flux coating. Table V. The average hardness of the modified surface taken at 0.6 mm below the top surface. Surface modified by TIGA
Average hardness (Hv)
Without flux coating P1 Coating, Al2O3 P2 Coating, Al
392 ± 7 437 ± 17 436 ± 17
Fraction of increase w.r.t. BM hardness (%) 91 113 113
Fig. 11. A typical microstructure of HAZ observed during surface modification by TIG arcing with P2 coating.
4. Conclusions The surface hardness of steel can be significantly improved (2.13 times) by using a flux activated particle reinforcing TIG arcing process. The surface hardness has been improved up to a considerable depth due to the formation of a composite layer on it. However, the hardenability is dependent on the flux chemistry, where the use of Alcontaining flux forming in-situ Al2O3 reinforcement in the matrix is found to be more
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attractive than using ex-situ added Al2O3-containing flux. The considerable improvement in surface hardness of steel by using flux activated TIG arcing process is dependent on the martensite transformation as well as particle reinforcement in the matrix. The mechanism of improving the depth of hardening with an appropriate dispersion of reinforcements by the control of Marangoni flow is understood up to a reasonable extent. This may be concluded as quite encouraging to open an avenue for further research to establish its multipurpose usage in wear-resistant applications.
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Highlights: •
Ex-situ and In-situ development of surface composite on microalloyed steel surface to improve the surface hardness
•
Employing TIG arcing to develop surface composite
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Influence of ex-situ and in-situ reinforcement on hardenability of microalloyed steel surface
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Influence of Marangoni effect during ex-situ and in-situ reinforcement on hardenability of microalloyed steel surface