Open AccessArticle

Investigation of Surface Hardness and Microstructural Changes in S45C Carbon Steel Cylinders Through Arc Quenching

Faculty of Mechanical Engineering, HCMC University of Technology and Education, Ho Chi Minh City 71307, Vietnam

Author to whom correspondence should be addressed.

Metals 2024, 14(12), 1438; https://doi.org/10.3390/met14121438

Submission received: 8 November 2024 / Revised: 12 December 2024 / Accepted: 13 December 2024 / Published: 16 December 2024

(This article belongs to the Section Metal Casting, Forming and Heat Treatment)

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Figure 1
Schematic illustration of a double-ellipsoidal heat source. "> Figure 2
S45C steel tube sample and arc equipment: (a,b) arc quenching equipment, and (c) S45C steel tube after the quenching using the arc energy of the TIG gun corresponding to 25 cases designed by the Taguchi method. "> Figure 3
Influences of current intensity on the cylinder surface hardness of S45C steel tube after arc quenching. "> Figure 4
Influences of arc length on the cylinder surface hardness of S45C steel tube after arc quenching. "> Figure 5
Influences of shielding gas flow rate on the cylinder surface hardness of S45C steel tube after arc quenching. "> Figure 6
Influences of travel speed on the cylinder surface hardness of S45C steel tube after arc quenching. "> Figure 7
Influences of pulse time on the cylinder surface hardness of S45C steel tube after arc quenching. "> Figure 8
Influences of heating angle on the cylinder surface hardness of S45C steel tube after arc quenching. "> Figure 9
Influences of water cooling angle on the cylinder surface hardness of S45C steel tube after arc quenching. "> Figure 10
Main influences plot for SN ratios of the S45C steel tube hardness with cylinder surface (larger is better). "> Figure 11
Microstructure of hardened S45 steel tube of sample No. 6: (a) cross-section of the sample at 50× magnification, (b) boundary of the cross-section of the sample at 500× magnification, (c) base metal, (d) hardened zone, and (e) cross-section of the sample at 3000× magnification. "> Figure 12
Distribution of microhardness in the cross-section of S45C steel tubes of sample No.6. ">

Versions Notes

Abstract

Arc quenching has many advantages, including generating large amounts of heat in a short time, a self-quenching ability, and simple equipment. The electric arc energy from a TIG welding machine was used to modify the surface properties of S45C Carbon Steel Cylinders. The study focuses on the impact of arc length, current intensity, travel speed, gas flow rate, heating angle, and pulse on surface hardness after arc quenching an S45C steel tube with a cylinder surface. The study found that the hardness reduces from 45.1 HRC to 41.2 HRC as the current intensity increases from 125 A to 140 A. According to Taguchi’s results, the ranking of factors which have the greatest impact on surface hardness are pulse time, travel speed, intensity, gas flow rate, arc length, and heating angle. The pulse time has the highest impact because it directly influences the heating input, followed by the travel speed. Arc length and heating angle, on the other hand, have the least effect. The base metal, heat-affected area, and hardened area are the three distinct areas that make up the microstructure structure. After the arc quenching process, the case hardening depth is represented by the heat-affected zone at 1536 μm. A highly colored residual austenite and a needle-shaped martensite phase make up the hardened region. The hardened region is 1200 μm thick and has a hardness of more than 300 HV0.3. The study’s findings may improve the application and understanding of the arc quenching treatment procedure in the industrial sector.

Keywords:

surface hardness; cylinder shape; current intensity; microstructure; heat treatment

1. Introduction

Recently, many techniques have been applied to improve the surface hardness of steel products, such as quenching, carburizing, cold-working, and surface hardening [1,2,3]. Among them, to increase the hardness in a specific area, techniques such as laser hardening, flame hardening, electron-beam quenching, induction hardening, and arc hardening can be used [4,5,6]. These many forms of surface heat treatment each have advantages and disadvantages. For example, laser hardening could be focused accurately on a local zone, creating high-hardness steel or cast iron surfaces with minimal distortion [7]. Moreover, this quenching method could make a thin layer of high-hardness surface without having a cooling step, as the parts are self-cooling. Therefore, this technique is commonly used in hardening camshafts, gears, molds, and other wear-resistant parts [8,9,10].

Compared to laser hardening, flame hardening often requires rapid cooling after heating the surface. This process applies the oxy-gas flame to heat the surface to a critical temperature [11]. Flame hardening also produces minimal distortion, which is suitable for large and complex products, requiring only conventional equipment [12,13,14]. On the contrary, electron-beam hardening uses high-end equipment with a high-energy electron gun to heat the surface of a metal to a critical point [15]. The heated surface is also rapidly cooled down by transferring the heat input to its core, creating a thin layer with a harder structure, for example, martensite. Induction hardening uses electromagnetic induction to heat the surface of magnetic materials [16,17,18]. The coils surrounding the metal parts generate an induced eddy current flowing on the surface; therefore, it is heated rapidly. Similar to traditional flame quenching, induction quenching also requires a quenching medium such as water, or oil. After cooling, the surface structure transforms into martensite, which is much harder than the original structures [19].

Arc hardening conducts an electrical arc to heat the metal surface rapidly. The surface is self-cooling without requiring a further cooling step, which is similar to laser and electron-beam hardening methods. This technique requires only conventional equipment to generate electrical arcs. Therefore, this hardening method has the combination of self-cooling as a high-end method and simple equipment as a traditional method. Some authors have tried to investigate arc hardening techniques. For example, Safonov et al. [20] investigated the arc-quenching process of iron–carbon alloys. The results indicated that the maximum hardness that could be achieved is 50–60 HRC, with the highest hardening depth of 1.5–2 mm, depending on the carbon content of the sample. Notably, there is compressive stress on the hardened surface, creating a high-quality layer. There is more than 60% retained austenite in the 2.0 mm depth layer, which is relatively high. The wear resistance also improves about 1.5–4 times after the arc hardening process. Mikheev et al. [21] compared the arc quenching process of low-carbon, medium-carbon, and high-carbon steels. The microhardness could improve from 4.0 GPa to 7.5 GPa when the carbon content increases from 0.2% to 0.4%. With 0.8% carbon, the microhardness could reach 9.5 GPa. Moreover, the optimal treatment speed is 0.06–0.09 m/s, creating maximum surface hardness. Increasing the current intensity results in a deeper hardening layer, but with a lower surface hardness. The wear resistance increases about four times compared to the untreated one.

These above hardening processes could be optimized by using optimization tools. For instance, Wagh et al. [22] applied the Taguchi method to optimize the laser hardening process of Ck45 steel. Three parameters, including standoff distance, laser scan speed, and laser beam power are examined. Laser scan speed is the most important factor that impacts the hardness depth of Ck45 steel, followed by laser beam power and standoff distance. The maximum hardness depth is about 340 µm, achieved by the 300 mm standoff-distance parameter set, the 1.0 mm/s laser scan speed, and the 330-watt laser beam power. Marichamy et al. [23] investigated the hardness of Eglin steel by using flame hardening and Taguchi optimization. They examined the impacts of surface temperature, cooling time, and standoff distance. The Taguchi technique reveals that surface temperature plays a critical role in hardness. Moreover, the highest hardness is achieved by the parameter set of surface temperature 1000 °C, a quenching time of 40 s, and a standoff distance of 40 mm. Jean et al. [24] surveyed the high-energy electron-beam hardening for cast iron using the Taguchi method. The study investigates the influences of travel speed, substrate material matrix, voltage, current, beam oscillation, melted width, and heat treatment after quenching. The studies reveal that the most critical parameters are travel speed, voltage, and post-heat treatment. The maximum hardness of the optimal parameters is about 941 HV. Moreover, the microstructure of the melted layer is martensite and primary dendrites. Parvinzadeh et al. [25] applied the Taguchi method to examine the induction hardening process of 4340 steel discs. The study tried to reduce the edge influence via magnetic flux concentrations by controlling the machine power, gaps, and heating time. The test findings indicate that the maximum axial gap, maximum radial gap, medium machine power, and maximum heating time reduce the edge influence.

With many of the advantages of generating large amounts of heat in a short time, the merits of self-quenching ability, and simple equipment, the method of quenching steel by electric arc energy was applied in this study. Despite this, the arc quenching method is not popular, due to the difficulty of applying it on complex surfaces such as curves and cylinder shapes. Therefore, this study focuses on the cylinder surface, which is a popular shape for axes and tubes in mechanical applications. In addition, optimizing the arc quenching parameters is also rarely considered. Therefore, in this study, the tungsten inner gas (TIG) arc source is controlled by a computer numerical control (CNC) machine to harden a carbon steel tube. The study’s findings might enhance the use of the arc quenching treatment method in the industrial sector and offer additional details about it.

2. Analytical Estimation of Thermal Characteristics of TIG

Nguyen et al. [26] proposed a solution for the temperature field in general arc TIG using a moving heat source with a double ellipsoidal power density (Figure 1), which was shown to be reasonably similar to experimental results.

The proposed solution for the transient temperature area (T_d) at every location (x, y, z) of a semi-infinite thick plate from time t′ = 0 to t′ = t is given below.

T_{d} = \frac{3 \sqrt[3]{3} Q_{e}}{2 ρ . c . π \sqrt{π}} \int_{0}^{t} [\frac{{d_{t}}^{'}}{\sqrt{12 a (t - t^{'}) + b_{h}^{2}} . \sqrt{12 a (t - t^{'}) + a_{h}^{2}}} . (\frac{K^{'}}{\sqrt{12 a (t - t^{'}) + c_{h f}^{2}}} + \frac{H^{'}}{\sqrt{12 a (t - t^{'}) + c_{h b}^{2}}})] + T_{0}

(1)

The a,

ρ

, and c are thermal diffusivity of the workpiece, mass density, and specific heat.

The ellipsoidal heat source parameters a_h, b_h, c_hf, and c_hb are defined as a place with a minimum power density of 5% of that of the ellipsoid’s surface center.

Q_e is the arc heat transmitted to the arc TIG pool, and is calculated using the following Equation (2).

Q_e = η.V.I_e (Joule)

(2)

η and V are arc efficiency and arc voltage.

I_{e} = \sqrt{j_{p} . I_{p}^{2} + (1 - j_{p}) I_{b}^{2}}

(3)

While I_b and I_p are the base/background current and peak current (units are Ampe).

j_p is the pulse duty cycle, defined as

j_{p} = \frac{t_{p}}{t_{p u l}}

(4)

t_p: peak current duration (s)

t_pul: total pulse time (s)

Equations (2)–(4) show that the pulse conditions greatly influence the heat generation process on the TIG head.

K^{'} = r_{f} . e x p (- \frac{3 {(x - v . t^{'})}^{2}}{12 a (t - t^{'}) + c_{h f}^{2}} - \frac{3 y^{2}}{12 a (t - t^{'}) + b_{h}^{2}} - \frac{3 z^{2}}{12 a (t - t^{'}) + a_{h}^{2}})

(5)

H^{'} = r_{b} . e x p (- \frac{3 {(x - v . t^{'})}^{2}}{12 a (t - t^{'}) + c_{h b}^{2}} - \frac{3 y^{2}}{12 a (t - t^{'}) + b_{h}^{2}} - \frac{3 z^{2}}{12 a (t - t^{'}) + a_{h}^{2}})

(6)

where the fraction coefficients in front of and behind the heat source are denoted by r_f and r_b, which are calculated as

r_{f} = \frac{2 . c_{h f}}{c_{h f} + c_{h f}}

(7)

r_{b} = \frac{2 . c_{h b}}{c_{h f} + c_{h f}}

(8)

Arc TIG geometry measurements are used to determine the proper ellipsoidal axes parameters for the TIG arcing process, which has a 75% arc efficiency. c_hf = b_h and c_hb = 2 c_hf are the values for the c_hf (in front of the heat source) and c_hb (behind the heat source).

From Equations (1) and (5)–(8), the importance of the dimensions a_h, b_h, c_hf, and c_hb is shown. These dimensions depend on the distance from the TIG to the base plate (called the arc length).

The research of Ravindra Kumar et al. [27] shows that the input heat of TIG technology played an extremely important role in the arc quenching process. The estimation of the heat input had been studied, as in Equation (9).

Heat input = \frac{η . I . V}{Travel speed (\frac{m m}{s})} (J / m m)

(9)

η = 0.75: the arc efficiency for the TIG arcing process

I(A): intensity

V(V): voltage

Equation (9) shows that the travel speed greatly influences the heat input of the quenching process.

3. Experimental Methods

In this study, a medium-carbon steel-grade tube is used for the arc quenching process, having a dimension of 76 mm diameter, 4.8 mm thickness, and 500 mm length. The nominal chemical composition of the S45C steel tube is shown in Table 1.

The quenching equipment (TIG200 W223, JASIC Company, Ha Noi Capital, Vietnam) and the S45C tube after quenching are presented in Figure 2. The arc is generated by a TIG gun (Nam Son JSC, Ha Noi Capital, Vietnam), while the gun is controlled by a CNC machine (The main equipment of the cnc machine is assembled from different places such as: three-jaw self-centring chucks of SAN OU Company, Zhejiang, China; 23KM Series Stepper Motors of MinebeaMitsumi, Nagano, Japan; PLC FX3U-32MT-6AD-2DA of Mitsubishi, HIMEJI, JAPAN; the remaining auxiliary equipment is manufactured in Ho Chi Minh City, Vietnam). The environment during the experiment was about 30 °C, and the cooling water was also at this temperature. The TIG needle had a diameter of 2.5 mm. The experimental voltage was fixed constant at 80 V. Before implementing the Taguchi design, preliminary tests were performed to ensure the arc and specimen surface stability and to prevent overheating. This test sought to stabilize and optimize the surface hardening parameters, such as current intensity, travel speed, arc length, gas flow, cooling angle, and pulse conditions. To evaluate the influence of each parameters, when designing the test, the tested parameters were varied, with 5 different levels and the remaining parameters remained unchanged, as shown in Table 2. Column 4 of this table estimates the heat input value, which was calculated using Equation (9).

Rockwell hardness testing is conducted by using the HR-150A Rockwell hardness tester, Yisite, Shen Zhen, China. The surface hardness of the initial sample is 90 HRB, which is about 183 HV (or about 11 HRC). After hardening, samples are also cut by a wire electric discharge machine (EDM) (JSEDM, Taichung City, Taiwan) to measure the microstructure and microhardness. The microstructure is examined by the Vickers hardness tester (HV_0.3) HM101 Mitutoyo, Tokyo, Japan. This measuring method employs a square-based pyramid indenter, whose opposite sides meet at the apex at an angle of 136°. Vickers microhardness was calculated by Equation (10).

H V = \frac{2 F s i n \frac{136^{o}}{2}}{d^{2}} 0.1 \approx 0.1854 \frac{F}{d^{2}} (N / {m m}^{2})

(10)

F(N): the force applied to the diamond

d(mm): arithmetic mean of the two diagonals of indentations

The microstructure is observed via a microscope named Oxion OX.2153-PLM EUROMEX, Duiven, The Netherlands. The scanning electron microscope (SEM) named JEOL 5410 LV, Tokyo, Japan is also used to observe the microstructure at a higher magnification.

4. Results and Discussion

4.1. Effects of Individual Parameters

This study first examines how each parameter impacts the sample’s hardness with the cylinder shape. Before quenching, the average surface hardness was 183 HV (about 11 HRC). After arc quenching, the surface hardness increases to an average of 41.6 HRC, which is 2.1 times higher than the original state, demonstrating a significant improvement in hardness quality. The following figures will explore the influence of each parameter on cylinder surface hardness.

Figure 3 shows the influences of current intensity on the cylinder surface hardness of the S45C steel tube after arc quenching. The surface hardness range is 42.2–46.2 HRC. Initially, the hardness increases slightly from 45.6 HRC to the highest value of 46.2 HRC when the current intensity rises from 120 A to 125 A. Compared to the original hardness of 193 HV, the highest hardness value has a 135% improvement. When the current intensity increases further, from 125 A to 140 A, the hardness gradually declines, from 46.2 HRC to 42.2 HRC. The reduction of the hardness could be caused by the overheating phenomenon. When the current intensity increases, the heat input energy increases proportionally. When researching the TIG technique, Uyen et al. [28] also indicated that the heat input values are proportional to the current intensity and current voltage. Therefore, a high current intensity leads to the overheating phenomenon of the steel surface, resulting in the reduction of the surface hardness. Overall, conducting arc quenching at 125 A leads to the highest surface hardness of the S45C tube.

Figure 4 shows the influences of arc length on the cylinder surface hardness of the S45C steel tube after arc quenching. The surface hardness value ranges from 40.1 HRC to 46.2 HRC, and it mostly varies around 42 HRC when the arc length increases from 1.0 mm to 2.0 mm. The surveyed range of the arc length is controlled from 1.0 mm to 2.0 to create a suitable arc, because if the arc length is less than one millimeter, the electrode will adhere to the steel surface. The arc length directly impacts the heat flux distribution during the heating process [29]. Therefore, an arc length that is too short could cause overheating of the steel surface. In reverse, if the arc length exceeds 2.0 mm, it cannot be properly created. The impact of the arc length will be discussed more in the Taguchi analyses.

Figure 5 shows the influences of shielding-gas flow rate on the cylinder surface hardness of the S45C steel tube after arc quenching. This shielding gas will be used before performing the arc quenching process, to protect the surface of the cylinder tube. Similar to the arc length impact, the surface hardness also oscillates about an average value of 44.5 HRC, ranging from 42.2 to 46.2 HRC when the gas flow rate increases from 8 to 16 L/min. The hardened surface achieves an improvement of 120%, or 2.2 times higher than the original S45C tube. Compared to the average hardness from the arc length examination, this value in the examination of the gas flow rate is slightly higher. The gas flow rate in this study is controlled in a well-designed range; therefore, it does not strongly impact the surface hardness, because without a suitable gas flow rate, the arc can not be stable [30]. The comparison between these two factors will be surveyed in the following Taguchi section.

Figure 6 shows the influences of travel speed on the cylinder surface hardness of the S45C steel tube after arc quenching. The surface hardness range after hardening is 43.3–46.2 HRC. Interestingly, the average hardness value of the travel speed surveyed is 44.6 HRC, which is similar to the result when evaluating the gas flow rate parameter. The highest hardness of 46.2 HRC is attained at 480 mm/min. Different from the arc length and gas flow rate effects, with the oscillation values of the surface hardness, there is mostly a trend for surface hardness when the travel speed increases. The surface hardness tends to increases when the travel speed increases. This phenomenon could be caused by the faster cooling rate when the travel speed increases or by the travel speed impacting the thermal distribution of the arc [27]. This factor needs more investigation, and will be noted in the following section.

The pulse time of the arc directly impacts the heat input rate of this source. Figure 7 shows the influences of pulse time on the cylinder surface hardness of the S45C steel tube after arc quenching. If there is no pulse time, the surface hardness is 35.3 HRC. The surface hardness of the samples is 44.7 HRC, 41.7 HRC, 46.2 HRC, 41.1 HRC, and 42.5 HRC, corresponding to the Ton values of 0.6 s, 0.7 s, 0.8 s, 0.9 s, and 1.0 s. The average surface hardness of 43.5 HRC is also 20% higher than in the case without pulse time. Moreover, applying a pulse at 0.8 s could lead to a high surface hardness of 46.2 HRC, which is much higher than the original one. In general, applying a pulse leads to better surface hardness of the S45C steel tube, because applying a longer pulse time leads to a higher rate of thermal input and higher surface temperature [31]. The pulse time that is higher than 1.0 s may be further investigated in future research.

Changing the heating angle of the TIG source influences the heat input; therefore, the surface hardness could be modified. Figure 8 shows the diagram that presents the influences of the heating angle on the cylinder surface hardness of the S45C steel tube after arc quenching. The surface hardness of the samples is 36.3 HRC, 39.1 HRC, 46.2 HRC, 44.5 HRC, and 38.2 HRC, corresponding to the heating angle of 0°, 5°, 10°, 15°, and 20°. This effect has two stages. Firstly, increasing the heating angle from 0° to 10° increases the surface hardness from 36.3 HRC to 46.2 HRC. Secondly, the surface hardness reduces from 46.2 HRC to 38.2 HRC when the heating angle increases from 10° to 20°. The reason for this trend is that changing the heating angle results in different heat inputs of the arc and cooling rate of the steel substrate [32,33]. This result will be further investigated in the following Taguchi section.

The cooling angle of the water also can change the cooling rate of the heated steel; therefore, the surface hardness is impacted. The effects of the water cooling angle on the cylinder surface hardness of the S45C steel tube after arc quenching are shown in Figure 9. The surface hardness of the samples is 41.3 HRC, 44.4 HRC, 46.2 HRC, 42.8 HRC, and 42.3 HRC, corresponding to the heating angle of 60°, 70°, 90°, 120°, and 150°. It is noteworthy that these effects can also be divided into two stages, which is similar to the heating angle effects. In the first stage, increasing the cooling angle from 60° to 90° leads to an increase in the surface hardness, from 41.3 HRC to the highest value of 46.2 HRC. Thereafter, in the second stage, the surface hardness reduces from 46.2 HRC to 42.3 HRC when the cooling angle further increases from 90° to 150°. Overall, the best cooling angle is 90°, generating the highest surface hardness.

4.2. Taguchi Analysis and Microstructure

In this part, the arc hardening parameters are designed via Taguchi methods, with six parameters and five levels for a parameter. The cooling angle is fixed at the optimal value of 90°. The L25 orthogonal array was designed by Minitab software, and is shown in Table 3.

Table 4 shows the response table for means of the S45C steel tube hardness with cylinder surface, with “larger is better” criteria. The impact rankings on the surface hardness are pulse, travel speed, intensity, gas flow rate, arc length, and, finally, heating angle. The pulse time has the highest impact level, due to its direct influence on the heating input, followed by the travel speed. The intensity and gas flow rate has the appropriate impact on the surface hardness. On the contrary, the arc length and heating angle have the lowest impact level. Notably, the reason for the low impact of arc length and heating angle could be the good survey range, as these parameters were initially surveyed in the previous section, to choose the good range.

Figure 10 shows the plot of the main influences on the SN ratios of the S45C steel tube hardness with cylinder surface, with “larger is better” criteria. These results show that the optimal parameters are the current intensity of 125 A, arc length of 1.8 mm, travel speed of 460 mm/min, gas flow rate of 10 L/min, heating angle of 90°, and pulse of 1.0 s. In addition, the regression equation calculated by the Taguchi method is

HRC = 44.69 + 0.0017 Intensity + 0.921 Arc length − 0.0060 travel speed − 0.141 Gas flow rate + 0.00584 Heating angle + 4.06 Pulse

(11)

Figure 11 shows the microstructure of the S45C steel tube sample with cylinder surface after the arc quenching process. This structure can be divided into three areas: base metal, heat-affected area, and hardened area, as shown in Figure 11a. Also, according to this Figure 11a, the hard area has many longitudinal marks, which may be due to the surface being partially melted when heated with a large amount of heat. The base metal consists of ferrite, with a bright color, and pearlite, with a darker color, indicating the typical medium-carbon steel of S45C. The heat-affected zone is located at 1536 μm, which lies between the hardened area and base metal, where the bainite phase may be present [29]. The hardened area, as shown in Figure 11d,e, consists of a martensite phase in a needle shape and residual austenite in a bright color. The presence of bainite and martensite phases leads to an improvement in the surface hardness of the steel substrate [8].

The microstructure could reveal the case hardening depth of the samples. However, measuring microhardness directly provides more precise information. Figure 12 displays the distribution of microhardness in the cross-section of S45C steel tube sample No. 6. The hardened area, which has a hardness above 300 HV_0.3, has a thickness of 1200 μm. The heat-affected area spreads from 1200 μm to 1600 μm depth, having a thickness of 400 μm. The hardness value of this area suffers a rapid drop, due to the transition from the hardened area to the base area. Finally, the base metal has the lowest hardness of above 180 HV_0.3, starting from a depth of 1600 μm. The microhardness of the sample reduces gradually as the microstructure changes from martensite and bainite to the pearlite and ferrite of the substrate. Overall, the arc quenching process on the S45C steel tube could create a case hardening depth of 1200 μm.

5. Conclusions

Arc quenching has many of the advantages of generating large amounts of heat in a short time, the merits of a self-quenching ability, and simple equipment. This method is very suitable for the use of the heat treatment method in the industrial sector. This study investigates the effects of arc length, current intensity, travel speed, gas flow rate, heating angle and pulse on surface hardness after arc quenching an S45C steel tube with cylinder surface. The following noteworthy findings may be discovered:

The hardness progressively decreases from 45.6 HRC to 42.2 HRC as the current intensity rises from 125 A to 140 A. Consequently, an excessive current intensity causes the steel surface to overheat, which lowers the surface hardness. Moreover, 90° is the ideal cooling angle, since it produces the highest level of surface hardness.
The Taguchi results show that the sequence of impact rankings on surface hardness is as follows: pulse, travel speed, intensity, gas flow rate, arc length, and heating angle. The pulse time has the greatest impact because it directly affects the heating input, followed by the travel speed. The intensity and gas flow rate have a suitable impact on surface hardness. On the contrary, arc length and heating angle have the least impact.
The highest surface hardness achieved by the Taguchi analysis was when the current was 125 A, the travel speed was 560 mm/min, the arc length was 1.8 mm, the gas flow rate was 10 L/min, the heating angle was 90°, and the pulse was 1 s.
The microstructure structure can be separated into three sections: base metal, heat-affected area, and hardened area. The base metal is composed of ferrite, which is bright in color, and pearlite, which is darker in color. The heat-affected zone at 1536 μm represents the case hardening depth from the arc quenching procedure. The hardened area consists of a needle-shaped martensite phase and a brightly colored residual austenite.
The hardened area, with a hardness above 300 HV_0.3, is 1200 μm thick. The heat-affected area is 400 μm thick and ranges from 1200 μm to 1600 μm in depth. The hardness value of this area drops rapidly as it transitions from the hardened to the base area. The base metal has the lowest hardness (over 183 HV_0.3) at a depth of 1600 μm. Arc quenching of the S45C steel tube resulted in a case hardening depth of 1200 μm.

Recommendations for further investigations are as follows: the two most influential factors are pulse and current intensity. Therefore, focus should be on investigating pulse and intensity factors when applicable to cases requiring hardening. The hardening layer is 1200 μm thick, which is suitable for the surface hardening technique.

Author Contributions

P.S.M., V.-T.N., T.T.N. and N.H.: conceptualization, funding acquisition; V.-T.N., T.T.N. and N.H.: writing original draft, investigation; T.T.N., V.-T.N. and N.H.: analyzing, visualization; T.T.N., P.S.M., N.H. and V.-T.N.: project administration; N.H., P.S.M. and V.-T.N.: investigation; N.H., P.S.M., T.T.N. and V.-T.N.: writing, review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the funding from HCMC University of Technology and Education under grant No. T2024-35 for this study.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

Acknowledgments

The authors acknowledge the support of HCMC University of Technology and Education for this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of a double-ellipsoidal heat source.

Figure 2. S45C steel tube sample and arc equipment: (a,b) arc quenching equipment, and (c) S45C steel tube after the quenching using the arc energy of the TIG gun corresponding to 25 cases designed by the Taguchi method.

Figure 3. Influences of current intensity on the cylinder surface hardness of S45C steel tube after arc quenching.

Figure 4. Influences of arc length on the cylinder surface hardness of S45C steel tube after arc quenching.

Figure 5. Influences of shielding gas flow rate on the cylinder surface hardness of S45C steel tube after arc quenching.

Figure 6. Influences of travel speed on the cylinder surface hardness of S45C steel tube after arc quenching.

Figure 7. Influences of pulse time on the cylinder surface hardness of S45C steel tube after arc quenching.

Figure 8. Influences of heating angle on the cylinder surface hardness of S45C steel tube after arc quenching.

Figure 9. Influences of water cooling angle on the cylinder surface hardness of S45C steel tube after arc quenching.

Figure 10. Main influences plot for SN ratios of the S45C steel tube hardness with cylinder surface (larger is better).

Figure 11. Microstructure of hardened S45 steel tube of sample No. 6: (a) cross-section of the sample at 50× magnification, (b) boundary of the cross-section of the sample at 500× magnification, (c) base metal, (d) hardened zone, and (e) cross-section of the sample at 3000× magnification.

Figure 12. Distribution of microhardness in the cross-section of S45C steel tubes of sample No.6.

Table 1. Chemical composition of S45C steel samples.

Weight%	C	Si	Mn	P	S	Ni	Cr	Fe
S45C	0.42–0.50	0.17–0.37	0.5–0.8	0.035 max	0.035 max	0.25 max	0.25 max	Remaining

Table 2. Experiment parameters for arc quenching with cylinder surface samples.

No.	Intensity (A)	Travel Speed (mm/min)	Heat Input (J/mm)	Arc Length (mm)	Gas Flow Rate (L/min)	Heating Angle (°)	Cooling Angle (°)	Pulse (s)	HRC
1	130	480	975.0	1.5	12	10	90	0.8	45.9
2	125	480	937.5	1.5	12	10	90	0.8	46.2
3	120	480	900.0	1.5	12	10	90	0.8	45.6
4	135	480	1012.5	1.5	12	10	90	0.8	44.3
5	140	480	1050.0	1.5	12	10	90	0.8	42.2
6	125	480	937.5	1.5	10	10	90	0.8	45.6
7	125	480	937.5	1.5	8	10	90	0.8	44.6
8	125	480	937.5	1.5	14	10	90	0.8	42.2
9	125	480	937.5	1.5	16	10	90	0.8	43.6
10	125	480	937.5	1.5	12	10	60	0.8	41.3
11	130	480	975.0	1.5	12	10	70	0.8	44.4
12	125	480	937.5	1.5	12	10	150	0.8	42.3
13	130	480	975.0	1.5	12	10	120	0.8	42.8
14	125	500	900.0	1.5	12	10	90	0.8	45.3
15	125	520	865.4	1.5	12	10	90	0.8	44.2
16	125	460	978.3	1.5	12	10	90	0.8	44.1
17	125	440	1022.7	1.5	12	10	90	0.8	43.3
18	125	480	937.5	1	12	10	90	0.8	42.4
19	125	480	937.5	1.2	12	10	90	0.8	40.1
20	125	480	937.5	1.8	12	10	90	0.8	41.1
21	125	480	937.5	2	12	10	90	0.8	42.1
22	125	480	937.5	1.5	12	10	90	0.6	44.7
23	125	480	937.5	1.5	12	10	90	0.7	41.7
24	125	480	937.5	1.5	12	10	90	0.9	41.1
25	125	480	937.5	1.5	12	10	90	1	42.5
26	125	480	937.5	1.5	12	10	90	0	36.4
27	125	480	937.5	1.5	12	15	90	0.8	44.5
28	125	480	937.5	1.5	12	20	90	0.8	38.2
29	125	480	937.5	1.5	12	5	90	0.8	39.1
30	125	480	937.5	1.5	12	0	90	0.8	36.3

Table 3. Experiment parameters designed by Taguchi method for arc quenching with cylinder surface samples.

No.	Intensity (A)	Travel Speed (mm/min)	Heat Input (J/mm)	Arc Length (mm)	Gas Flow Rate (L/min)	Heating Angle (°)	Pulse (s)	HRC
1	120	440	981.8	1	8	60	0.6	43.4
2	120	460	939.1	1.2	10	70	0.7	45.4
3	120	480	900.0	1.5	12	90	0.8	45.8
4	120	500	864.0	1.8	14	120	0.9	46.8
5	120	520	830.8	2	16	150	1.0	44.7
6	125	460	978.3	1	12	120	1.0	47.7
7	125	480	937.5	1.2	14	150	0.6	45.8
8	125	500	900.0	1.5	16	60	0.7	45.1
9	125	520	865.4	1.8	8	70	0.8	47.3
10	125	440	1022.7	2	10	90	0.9	46.4
11	130	480	975.0	1	16	70	0.9	42.4
12	130	500	936.0	1.2	8	90	1.0	47.5
13	130	520	900.0	1.5	10	120	0.6	43.8
14	130	440	1063.6	1.8	12	150	0.7	44.0
15	130	460	1017.4	2	14	60	0.8	46.6
16	135	500	972.0	1	10	150	0.8	47.4
17	135	520	934.6	1.2	12	60	0.9	42.4
18	135	440	1104.5	1.5	14	70	1.0	45.9
19	135	460	1056.5	1.8	16	90	0.6	46.2
20	135	480	1012.5	2	8	120	0.7	44.9
21	140	520	969.2	1	14	90	0.7	44.0
22	140	440	1145.5	1.2	16	120	0.8	45.3
23	140	460	1095.7	1.5	8	150	0.9	46.5
24	140	480	1050.0	1.8	10	60	1.0	48.1
25	140	500	1008.0	2	12	70	0.6	45.1

Table 4. Taguchi results for response table for means of the S45C steel tube hardness with cylinder surface (larger is better).

Level	Intensity	Arc Length	Travel Speed	Gas Flow Rate	Heating Angle	Pulse
1	45.21	45.00	45.01	45.93	45.13	44.87
2	46.45	45.27	46.48	46.24	45.21	44.68
3	44.89	45.42	45.40	45.00	45.99	46.48
4	45.36	46.50	46.38	45.83	45.70	44.91
5	45.81	45.52	44.46	44.72	45.68	46.78
Delta	1.57	1.50	2.02	1.51	0.86	2.10
Rank	3	5	2	4	6	1

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Son Minh, P.; Nguyen, V.-T.; Nguyen, T.T.; Ho, N. Investigation of Surface Hardness and Microstructural Changes in S45C Carbon Steel Cylinders Through Arc Quenching. Metals 2024, 14, 1438. https://doi.org/10.3390/met14121438

AMA Style

Son Minh P, Nguyen V-T, Nguyen TT, Ho N. Investigation of Surface Hardness and Microstructural Changes in S45C Carbon Steel Cylinders Through Arc Quenching. Metals. 2024; 14(12):1438. https://doi.org/10.3390/met14121438

Chicago/Turabian Style

Son Minh, Pham, Van-Thuc Nguyen, Thanh Tan Nguyen, and Nguyen Ho. 2024. "Investigation of Surface Hardness and Microstructural Changes in S45C Carbon Steel Cylinders Through Arc Quenching" Metals 14, no. 12: 1438. https://doi.org/10.3390/met14121438

APA Style

Son Minh, P., Nguyen, V. -T., Nguyen, T. T., & Ho, N. (2024). Investigation of Surface Hardness and Microstructural Changes in S45C Carbon Steel Cylinders Through Arc Quenching. Metals, 14(12), 1438. https://doi.org/10.3390/met14121438

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