Surface hardening by in-situ grown composite layer on microalloyed steel employing TIG arcing process

Accepted Manuscript Surface hardening by in-situ grown composite layer on microalloyed steel employing TIG arcing process

Deepak Sharma, Prakriti Kumar Ghosh, Sudhir Kumar, Sourav Das, Ramkishor Anant, Nilesh Kumar PII: DOI: Reference:

S0257-8972(18)30814-4 doi:10.1016/j.surfcoat.2018.08.009 SCT 23667

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

4 May 2018 2 August 2018 3 August 2018

Please cite this article as: Deepak Sharma, Prakriti Kumar Ghosh, Sudhir Kumar, Sourav Das, Ramkishor Anant, Nilesh Kumar , Surface hardening by in-situ grown composite layer on microalloyed steel employing TIG arcing process. Sct (2018), doi:10.1016/ j.surfcoat.2018.08.009

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ACCEPTED MANUSCRIPT Surface Hardening by in-situ Grown Composite layer on Microalloyed Steel Employing TIG Arcing Process Deepak Sharmaa, Prakriti Kumar Ghosha*, Sudhir Kumar a, Sourav Dasa, Ramkishor Ananta, and Nilesh Kumara Department of Metallurgical and Materials Engineering, Indian Institute of Technology

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Roorkee, Roorkee, Uttarakhand-247667, India

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* Corresponding author: E-mail address: [email protected]

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Abstract:

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Surface of microalloyed steel, hereafter referred as steel, has been modified by developing an in-situ grown composite case on its surface for improved hardness. It is done

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through surface melting by employing Tungsten Inert Gas (TIG) arcing. The hard reinforcements were made to grow in the surface matrix of steel through chemical reactions

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of the inorganic powders present in the applied coating and the molten base. The distribution

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and incorporation of these reinforcements were taken care by addition of Al and TiO2 in the coating. Three different mixtures, comprising different proportions of Al and TiO2, were

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prepared to develop a hybrid composite primarily containing Al2O3 and a small fraction of TiC along with other oxides as reinforcements. The modified particulate composite surface

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was analyzed under Vickers’ micro-hardness tester confirming its significant improvement in hardness of the order of 1.88-2.24 times in comparison to that of the base metal, depending upon different chemistry of the powder mixture of the coating. Keywords: TIG arcing, Marangoni effect, surface hardening, in-situ grown composite, steel

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ACCEPTED MANUSCRIPT 1. Introduction: Steel is a commonly used economic structural material in various industries primarily due to its flexible mechanical properties that readily support the forming and fabrication practices. However, in case of relatively low and medium strength tough steels, their comparatively low hardness restricts them to be used in services involving friction and

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abrasion [1,2] Thus, by taking advantage of its good combination of strength and toughness

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with respect to the general needs of structural applications, a suitable surface modification

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can make such steels an economically competitive material for applications involving friction and abrasion by providing high hardness of faying surface supported by a relatively tough

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core [3]. It can be readily achieved by controlled hard phase transformation in the matrix up to a certain depth [4,5]. However, the extent of surface hardening of steel largely depends

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upon its chemistry, especially the carbon content [6]. Thus, to address this deficiency often it is considered to convert the steel surface to a suitable particle reinforced composite [7–10], or

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to introduce surface cladding [3,11–14] by suitable material having superior hardness in

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combination with other relevant properties. Out of all such options, the surface modification of substrate by making it a composite material has gathered wide attention due to its

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economically versatile nature of applicability on surface. The major advantage of this approach of surface modification is that it can be desirably designed and applied for

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improvement of any engineering properties including the strength, hardness and toughness as compared to those of the base metal [7–10]. Composite material is a continuous system of materials in which a relatively stiffer reinforcing material is well distributed in the matrix. The superiority in properties of a composite is primarily determined by the properties of the matrix and reinforcement as well as the type, amount, size, cluster free homogeneity and interfacial characteristics of the later in the matrix [15]. There are basically two approaches, such as ex-situ addition and in-situ

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ACCEPTED MANUSCRIPT grown second phase, used for reinforcing a matrix. In the ex-situ method, occurrence of chemical incompatibility between the matrix and reinforcement may lead to poor interfacial bonding. Difference in the thermal properties of the composite phases (e.g., coefficient of thermal expansion, CTE) can increase the risk of cracking at the poorly bonded interface in case of ex-situ reinforcement. On the contrary, in-situ synthesis can remove the interface

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problem [16]. In the in-situ process one or more relatively harder phases are made to grow in

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the matrix under a favorable thermodynamic condition of heating, which may be also a TIG

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arcing process. Autogenous TIG arcing (without filler) process operating under inert shielding is found to be quite effective for surface modification of metal substrate by

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controlled melting up to a required depth for standard tribological applications at a relatively low cost [4,5]. Thermodynamically stable and homogeneously distributed in-situ grown

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reinforced phases with sufficient interfacial strength to transfer stresses in the matrix strengthens the matrix especially by reducing the probability of cracking and failure at the

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composite interface [16]. However, the distribution of in-situ grown phases in fused metal

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pool largely depends upon its co-efficient of surface tension as a function of temperature, presence of surface active elements like O, S and Se and the Marangoni effect of fluid flow

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[17–19]. The fluidity of molten weld pool and appropriate change in co-efficient of surface tension from negative to positive in presence of surface active element appreciably affects the

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kinetics of fluid flow under reverse Marangoni effect from outwards to inwards direction. It is also reported that presence of TiO2 also helps in reversing the Marangoni flow [20–22]. These behaviors may significantly influence the distribution of in-situ grown ceramic phases in the molten pool during arcing. The degree of uniformity of particle distribution throughout the matrix directly affects the homogeneity of the composite properties. It is therefore imperative to direct particle distribution during formation of composite melt by regulating the flow of weld pool with the

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ACCEPTED MANUSCRIPT help of addition of surface active elements. The presence of surface active elements, such as O, S and Se to the weld pool changes the ∂σ/∂T from negative to positive, where σ and T are the co-efficient of surface tension and temperature respectively. Thus, the Marangoni effect reverses its direction from outwards to inwards as shown in Fig. 1. Hence, TiO2 is added to the coating to help in reversing the flow [20–22]. Ali Emamian et. al. [16] reported that when

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Ti and C are added to the substrate through laser cladding, the in-situ growth of TiC takes

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place because the ∆G value for the formation of Titanium Carbide is much lower than that of

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formation of Iron Carbide [16]. The Ellingham diagram also demonstrates that the formation of Al2O3 in molten steel is favorable in comparison to the other oxides. However, in moderate

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condition of molten steel and in presence of acidic oxides arising out of binder a possibility of formation of Fe2O3 in minor amount cannot be ignored [23]. The presence of Al in excess in

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the molten pool of steel may also react with the Fe2O3 present in it and also give rise to the formation of Al2O3. These understandings have led to the usage of Al and TiO2 as the coating

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following reactions:

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constituents generating a hybrid surface composite containing TiC and Al2O3 based on the

TiO2

Ti + O2

(i)

4Al + 3O2

2Al2O3

(ii)

4Fe + 3O2

2Fe2O3

(iii)

2Al + Fe2O3 Ti + C

Al2O3 + 2Fe TiC

(iv) (v)

TiO2 added as a surface active element decomposes into Ti and [O] under arc heat [21], where Ti reacts with C that is already present in steel and forms TiC while oxygen reacts with the added aluminum to form Al2O3. Hence, developing a hybrid surface composite on steel substrate consisting of these ceramic hard reinforcements leads to significant improvement in surface hardness of steel substrate.

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Fig. 1. Marangoni convection mode by surface tension gradient in welding pool (a) ∂σ/∂T < 0 and (b) ∂σ/∂T > 0.

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In view of the above, an effort has been made to study the possibility and efficacy of

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development of an in-situ grown composite layer on steel surface during its melting by using autogenous TIG arcing process in order to improve its surface hardness. The approach

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involves introducing a combination of Al and TiO2 powders into molten pool to form in-situ grown TiC and Al2O3 reinforced hybrid composite layer on the steel substrate through

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appropriate chemical reactions. Based on the literature [21] of prior knowledge, three flux compositions (Mix1, Mix2 and Mix3) were prepared consisting of Al and TiO2 powders and applied as coating on steel surface allowing them to react in molten pool of steel to form desired reinforcements in a layer of composite material on the substrate with certain depth of penetration.

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Experimental: Steel plate of dimensions 100×40×17 mm having chemical composition as shown in

Table-I was used as base metal. The chemical composition was analyzed by using optical emission spectroscopy at spot diameter used in the range of 4-5 mm. Prior to processing for surface modification, the surface of the steel plate was cleaned by surface grinding followed

Table-I Composition of the steel base plate. Mn

0.135 1.62

S

P

Si

Al

0.01

0.03

0.35

.02

Cr

Nb

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Wt. %

C

.02

.05

Fe

rest

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Elements

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by polishing with 220 grade SiC grid paper and wiped by acetone.

2.1 Flux preparation

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Flux composition was estimated by performing reverse calculation leading to desired reactions as mentioned above in eq. (i-v). The reverse calculation was made in reference to

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the carbon content of the base metal (Table-I) acting as rate controlling factor of the reaction

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eq. (v) that determines the extent of availability of TiO2 in the flux coating. The actual amount of C present in the molten mass of steel, for reaction, was estimated by heat-

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temperature relationship as

(vi)

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Q = [m {Cp,solid *(Tm – T1) + L + Cp,liquid * (T2 - Tm)}

Where, Q is heat supplied due to TIG arcing, Tm is melting temperature of steel, T1 is room temperature (25oC), T2 is final temperature assumed as 2200oC [24], m is mass of steel melted, Cp,solid & Cp,liquid are the specific heat of steel in solid and liquid state respectively and L is latent heat of fusion for steel. The molten mass of steel is used to estimate the C available for reaction in reference to the base metal chemistry given in Table-I. The decomposition of TiO2 into Ti and [O] at high temperature of molten steel [21] and the possibility of formation of Fe2O3 [23] primarily indicates the amount of Al required for completing the reaction eq. 6

ACCEPTED MANUSCRIPT (ii) and (iv) with due consideration of the Al already present in base material. Al content of the flux with respect to TiO2 has been varied around its minimum requirement in different Mix. This led to designing of flux composition denoted as Mix1, which was applied in

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varying amount to coat the faying surface of the substrate, which gives sample number 1, 2 and 3. To study the effect of variation in composition of the coating, two other flux

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compositions denoted as Mix2 and Mix3 were also prepared which were applied on sample

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number 4 and 5 respectively. The powder mixtures were homogeneously dispersed in acetone by magnetic stirring followed by mixing with binder in a rotating tumbler. Sodium silicate

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(NaSiO3) as binder was mixed in drops of 0.18 gm each with all the fluxes (Mix1, Mix2 and

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Mix3). The binder was added in proportion of 4 drops for one gram of powder and the paste was applied to coat the faying surface of the substrate. Accordingly the Al and TiO2 content

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in the flux coating of the sample number 1-5 has been stated in Table-II.

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ACCEPTED MANUSCRIPT Table-II Showing Al and TiO2 content in the flux coating of different samples prepared by using different flux Mix. Composition of flux (in gm.)

Chemical composition of the paste (wt. %)

NaSiO3 ( no. of Al

TiO2

Al

3*0.18 =0 .54

0.18

0.45

15.38

8*0.18 = 1.44

0.58

1.42

16.86

20*0.18 = 1.51

1.44

3.56

8*0.18 = 1.44

0.4

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Samples drops * weight

TiO2

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of each drop)

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Mix1

38.46

46.15

41.27

41.86

22.11

54.68

23.19

1.6

11.62

46.51

41.86

1.28

20.93

37.20

41.86

Mix1

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(Sample 2) Mix1

Mix2

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(Sample 4)

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(Sample 1)

(Sample 3)

NaSiO3

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Mix3

8 * 0.18 = 1.44

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(Sample 5)

0.72

2.2 Surface modification procedure

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Mechanically cleaned surface of the steel substrate was coated by applying the paste on it with the help of a paint brush and then coating was dried overnight by keeping it in an oven at 800 C. Prior to application of coating, the surface was also made free from dirt and greases by wiping with acetone. Then TIG arcing (TIGA) was carried out on the coated part of the substrate by using an ESAB MEK 44 C power source with DCEN polarity. Arcing was carried out using tungsten electrode of diameter of 3.2 mm under argon gas (commercial) shielding at a flow rate of 10 liters per minute. The TIGA parameters were kept as the arcing

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ACCEPTED MANUSCRIPT current (I) = 180±5A, arcing voltage (V) = 12±1V and arc travel speed (S) = 10cm/min giving rise to a heat input about 10.12 kJ/cm. 2.3 Sample preparation and characterization Samples were cut from the surface modified part of base metal for characterization

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and then mechanically prepared by sequential polishing with SiC emery papers of grid size 100-2000μm. The polished surface was etched with nital solution (2 vol. % nitric acid in

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ethanol). The sample collected from the untreated base metal was also prepared by the same

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procedure to use as a reference material for all kinds of characterization. All the specimens having surface preparation of metallographic quality were studied for characterization using

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various instruments and techniques, such as optical microscopy, field emission scanning

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electron microscopy (FESEM) with energy dispersive spectroscopy (EDS) facility, and XRay diffraction (XRD) analysis. The FESEM studies were carried out at 20kV. The XRD

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studies were carried out using CoK radiation and Cuβ filter at 2 from 35o to 105o. The scan

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rate for the XRD analysis was kept as 1 degree per minute. Further, the hardness of the modified surface was measured by micro-hardness indentation using a load of 300 gm with a dwell time of 30 sec. Under a given condition the hardness is reported as an average of at

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least five measurements.

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3. Results and Discussion:

3.1 Characteristics of Powders All the powders and their mixtures were characterized under FESEM using EDS confirming their size and purity. FESEM images of morphology of the Al and TiO2 powders and their mixtures as Mix1, Mix2 and Mix3 are shown in the Fig. 2. Average size of Al and TiO2 particles was found to be 65 µm and 0.5 µm respectively as calculated using the Image J software. The characteristic of Al and TiO2 powder, as confirmed by EDS studies has been 9

ACCEPTED MANUSCRIPT shown in Fig. 2(a2) and (b2) respectively. Similarly the characteristic of the mixture of powders, Mix1, Mix2 and Mix3, as confirmed by EDS studies has been shown in Fig. 2(c2), (d2) and (e2) respectively. The EDS analysis of the Al and TiO2 powders presented in Fig. 2(a2) and (b2) shows the purity of them without any contamination of other elements. The presence of Ti in oxide form is confirmed by the presence of [O] in Fig. 2(b2) which is not

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there in Fig. 2(a2) for Al powder. However, the EDS analysis of the powder mixtures as

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Mix1, Mix2 and Mix3 shown in Fig. 2(c2), (d2) and (e2) respectively confirms the presence

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of both the Al and Ti (possibly as oxide) in the Mix. The variation of [O] content is analogous to the change in Ti content of the matrix signifying its signature appearance from

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the TiO2 powder present in the Mix as mentioned in Table-II. The homogeneity in dispersion of Al and TiO2 in the powder mix of Mix 1, Mix2 and Mix 3 and the fraction of each element

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present in the mixture, as observed under X-ray mapping have been shown in Fig. 3 (a), (b)

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and (c) respectively.

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ACCEPTED MANUSCRIPT Fig. 2. FESEM images and EDS analysis, where ‘1’ represents FESEM image and ‘2’ represents EDS analysis, for (a) Al and (b) TiO2 powder and flux mix (c) Mix1, (d) Mix2,

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and (e) Mix3.

Fig. 3. Typical mapping observed for different mixture of powders: (a) Mix 1, (b) Mix2 and

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(c) Mix3.

3.2 Microstructural Observations

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Microstructure of the modified surfaces, carried out at the heat input about 10.12 kJ/cm, was first examined across its cross-section using optical microscopy. Fig. 4 typically demonstrates the overall nature of distribution of different microstructural characteristics primarily defined as the fusion zone (FZ) and heat affected zone (HAZ) of the base metal (BM) in cross-section of the modified sample. Microstructure of the base metal shown in Fig. 5(a) reveals the presence of pro-eutectoid ferrite and pearlite in the matrix [25]. Microstructure of HAZ of the without Mix TIG arcing modified matrix is shown in Fig.5(b),

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ACCEPTED MANUSCRIPT which demonstrates the presence of relatively finer grains when it becomes close to the base metal [25], which has happened due to rise in temperature to about 1000-1200K in this location. Microstructure of the FZ modified by TIG arcing without or with Mix always found to be consist of lathe type of martensite and some amount of ferrite and bainite in the matrix as shown in the Fig. 5 (c-h). The presence of martensitic phase in the fused zone has been

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confirmed by XRD studies as discussed latter. This is in agreement to the observations

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reported by Tewary et al. (2014) for martensite formation in the low carbon steel [26].

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However, at given TIG arcing parameters the observed variation in morphology of FZ microstructure (Fig. 5(d-h)) may be attributed to the change in thermal behavior of chemical

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reactions occurred during fusion in presence of Mix of different amount (Mix1 of sample 1-3) and composition (Mix2 and 3). The thermal behavior of chemical reaction in this process of

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TIG arcing is possibly also partly predominated by the exothermic reaction in presence of reducing Al in excess in the molten steel that may have certain amount of Fe2O3. It is clearly

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justified by more coarsening of matrix morphology revealed in microstructure of FZ

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developed in presence of larger amount of Al in the flux coating. The observed maximum coarsening of microstructure of FZ developed during TIG arcing in presence of coating of

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Mix1 (sample 3) containing maximum amount of Al (Table-II) has occurred in confirmation to this phenomenon. The observation on morphology of FZ prepared in presence of other Mix

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of different sample containing different amount of Al is also broadly found in agreement to this understanding.

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Fig. 4. Typical overall microstructure of cross section.

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ACCEPTED MANUSCRIPT Fig. 5. Typical microstructure of (a) base material, (b) HAZ and (c) FZ of without Mix and of FZ with different Mix as (d) Mix1(sample 1), (e) Mix1(sample 2), (f) Mix1(sample 3), (g) Mix2(sample 4), and (h) Mix3(sample 5) in TIG arcing modified matrix. 3.3 Hardness

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Hardness of the base material is measured as 205 ± 10 HV. Fig. 6 shows the variation in hardness from the top surface to the base metal along the FZ and HAZ measured

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on cross section of the modified substrate. It is found that maximum hardness belongs to the

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matrix close ( 0.3 mm) to the surface followed by its reduction to reach the stable base metal hardness at certain depth (Table-III). In case of surface hardening due to martensite

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transformation in the matrix during normal TIG arcing the maximum hardness around surface

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has been found as about 386 HV. However, the further increase in surface hardness beyond this during TIG arcing on coated substrate is largely attributed to the formation of

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reinforcement particles in the FZ zone over the martensite transformation in the matrix. The

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maximum increase in surface hardness has been found to be around 460 HV when the flux coating of Mix1 (sample 3) is used. It is further noted (Fig. 6) that the depth of hardening before reaching the base metal hardness is appreciably more (6.0 mm) in case of using the

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flux coating than that (4.25 mm) of arcing without flux. The depth of hardening during arcing

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on flux coating is found to vary also with the chemistry of the applied flux Mix justifying the role of Marangoni fluid flow. It may have primarily occurred due to occurrence of effective Marangoni fluid flow as a function of the chemistry of flux coating [21]. The hardness value also depends upon volume fraction and distribution of the reinforcement particles over the martensite transformation in the matrix. The variation in presence of different volume fraction of reinforcement in the matrix may have largely resulted from different amount of Al and TiO2 present (Table-II) in the flux. The average hardness values, in the matrix at about 0.3 mm below the differently modified surfaces are given in the Table-III. Out of these 16

ACCEPTED MANUSCRIPT observations it may be noted that in order to enhance the depth of hardening, strengthening of the Marangoni effect of fluid flow should be studied further by changing the favorable chemistry of the flux as well as TIG arcing parameters affecting fluidity of the fused matrix. The table shows that the hardness of every modified surface is significantly higher than the base material. However, the maximum increase in hardness was found in case of surface

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modification using Mix1 (sample 3), which is about 2.24 times more than the base meal. The

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efficiency of increasing the surface hardness by this reinforcement technique is found to be

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appreciably higher than that observed in earlier works [4,7] of TIG arc surfacing of steel. At this point as a merit of this surface hardening process by introducing in-situ

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reinforcing composite matrix on steel substrate, this process has ability to produce any level of hardness by controlling the extent and characteristics of reinforcement on large surface of

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substrate using TIG arcing. Whereas during multi-pass surfacing of large surface by TIG arcing, the reduction of hardness by tempering of hard phase restricts the extent of surface

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hardening of steel substrate [27].

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Fig. 6. Hardness distribution along the depth from top surface across the FZ and HAZ to base

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material in TIGA modified substrate without Mix and with samples of different Mix. Table-III Average hardness at depth of 0.3 mm below the modified surface. Fraction of increase w.r.t.

(HV)

base metal hardness (%)

Without flux coating

386

88

Coating, Mix1(sample 1)

402

96

Coating, Mix1(sample 2)

412

100

Coating, Mix1(sample 3)

460

124

Coating, Mix2(sample 4)

402

96

Coating, Mix3(sample 5)

402

96

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Average hardness

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Surface modified by TIGA

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(Table-II)

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Mix3(sample 5) respectively. The XRD analysis of the base metal has been shown in Fig. 8,

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which depicts no presence of martensite in the matrix. Due to less amount of carbon in steel, the shift in the c-axis for body centered tetragonal martensite is negligible, hence the peaks

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for ferrite and martensite coincide [28–30]. However, microstructures of the modified surfaces clearly shows the presence of martensite , as shown in Fig 5(c-h), while no

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martensite is observed for base metal in Fig. 5 (a). Moreover, the variation in intensity of the peaks from the presence of martensite in the matrix indicates the possible presence of other

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phases in the matrix, which may be the reinforcement particles formed in the FZ. But, in the

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XRD plots of the modified surface prepared by using the coating of any Mix the presence of TiC or Al2O3 reinforced particles is not detected. This may have primarily happened due to

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their presence in low volume fraction in the matrix, which is unable to produce easily detectable peak intensity in general technique of XRD analysis. However, in the surface

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modified with the flux coating of Mix1(sample 3) as shown in Fig. 7(d), a feeble indication of characteristic peak of TiC could be marked at high magnification of the plot, which is the primary objective of reinforcement in fusion modified matrix as stated above. However, the indication of in-situ developed reinforced particles in the matrix could be significantly detected by EDS analysis of the matrix under FESEM as discussed latter.

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Fig 7. XRD plots of TIGA modified surfaces for (a) without Mix and with (b) Mix1 (sample 1), (c) Mix1 (sample 2), (d) Mix1 (sample 3), (e) Mix2 (sample 4) and (f) Mix3 (sample 5).

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Fig. 8. XRD analysis of base metal. 3.5 FESEM and EDS Analysis of Composite FZ

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FESEM analyses carried out to check the amount and distribution of the black colored reinforced particles in the FZ is shown in Fig. 9. Fig. 9(d) clearly shows the existence of

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relatively more number of uniformly distributed relatively finer particles in the matrix in comparison to that observed in case of other fusion modified zones developed with other flux mixtures. Possibly this is the reason why the XRD analysis could not detect the particles at all in FZ for Mix1 (sample 1 and 2), Mix2 (sample 4) and Mix3 (sample 5). The EDS analyses of the particles present in the fusion modified surfaces developed by using Mix1 (sample 1), Mix1 (sample 2), Mix1 (sample 3), Mix2 (sample 4) and Mix3 (sample 5) have been shown in Figs. 10-14 respectively. The figures show the significant presence of Al, C and O almost

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formation of SiO2 or aluminum silicate (Fig. 13), or the formation of oxides other than that of

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Al and Si like Fe2O3 (eq. (iii)) cannot be ignored (Figs. 10, 11 and 14), whereas in every

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cases a minor possibility of formation of TiC particles (eq. (v)) is marked in presence of Ti and C. However, the desired formation of Al2O3 and TiC could not be detected in XRD

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analysis as stated above due to their small volume fraction in the matrix as presented later.

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ACCEPTED MANUSCRIPT Fig. 9. FESEM images of TIGA modified surface in different samples (a) overall appearance of cross-section and of FZ developed using (b) Mix1(sample 1), (c) Mix1(sample 2), (d)

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Mix1(sample 3), (e) Mix2(sample 4) and (f) Mix3(sample 5).

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ACCEPTED MANUSCRIPT Fig. 10. FESEM image and EDS analysis showing presence of reinforced particles in FZ with

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Mix1 (sample 1).

Fig. 11. FESEM image and EDS analysis showing presence of reinforced particles in FZ with Mix1 (sample 2).

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Fig. 12. FESEM image and EDS analysis showing presence of reinforced particles in FZ with

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Mix1 (sample 3).

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Fig. 13. FESEM image and EDS analysis showing presence of reinforced particles in FZ with

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Mix2 (sample 4).

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Fig. 14. FESEM image and EDS analysis showing presence of reinforced particles in FZ with Mix3 (sample 5).

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ACCEPTED MANUSCRIPT 3.6 Volume Fraction The amount of particles present in the fusion modified matrix was checked under microscope by estimation of their volume fraction by using the line interception method on the inclusions marked in the matrix, as shown in Table-IV. In this context as a reference the weight density of the coating was also calculated ((Weight of powder applied/ applied area)

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×), as shown in Table-IV. The coating was applied on the area (1 × 40 mm2). The efficiency

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of applying the flux with brush () is estimated about 75 % using the following expression.

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 = [[1 (Prior weight of brush with flux of different Mix  weight of brush after application of flux in coating) / (weight of powder present in flux Mix)] × 100]

(v)

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The presence of total volume fraction of all the particles maximum within 3.6 % has deprived

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any kind of particles to be detected in the XRD analysis as stated above. Table-IV Volume fraction and concentration of inclusions in the modified matrix prepared

Sample Modified by

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with different coatings.

Weight Density of Coating

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Volume Fraction (%) (g/cm2)

Different Coatings

0.9

1.185

Mix 1 (sample 2)

1.4

3.75

Mix 1 (sample 3)

3.6

9.375

Mix 2 (sample 4)

1.6

3.75

Mix 3 (sample 5)

2.0

3.75

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Mix 1 (sample 1)

4. Conclusions Surface modification of steel through fusion by cost effective and versatile TIG arcing on activated flux coating significantly improves the surface hardness to 460 Hv by using the Mix1(sample 3) having coating composition as 22.11, 54.68, and 23.19 wt% for Al, TiO2, 29

ACCEPTED MANUSCRIPT and NaSiO3, respectively. The hardness was found to be improved up to an appreciable depth by making it an in situ developed composite. However, the improvement of hardness of base metal from 205 ± 10 HV to 460 HV is a function of the chemistry of flux coating that governs the fluid flow in weld pool and also facilitates the chemical reactions producing the reinforcement particles in the matrix. The optical microscopy, EDS, XRD and FESEM

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studies confirm that the improvement in hardness of modified surface is primarily attributed

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to martensite transformation along with possible in-situ formation of Al2O3 reinforcement

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particles in the matrix. In some cases the indication of the presence of other oxides of Si and Fe is also noted in EDS analysis of the matrix. However, in view of EDS analysis the

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possibility of minor presence of TiC in modified matrix cannot be ignored. The presence of significantly low fraction of reinforcing particle in the modified matrix did not allow them to

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be detected in the XRD analysis. Thus, it may be concluded that TIG arcing using activated flux coating can be successfully used for surface hardening of steel by developing a particle

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reinforced martensitic matrix up to an appreciable depth, which can facilitate its multipurpose

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use in precise applications. However, studies should be carried out further on chemical composition of flux mixture governing the chemical reaction and its kinetics producing more

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parameters.

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reinforcing particles in fusion modified surface of steel produced by TIG arcing at varied

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ACCEPTED MANUSCRIPT Highlights 1. In-situ grown surface composite to improve surface hardness 2. Employing TIG arcing to develop surface composite 3. Reversing Marangoni effect to control the incorporation and distribution of reinforcement

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4. Reaction between inorganic powders in the coating to form hybrid composite

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