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Article

Tensile Strength and Microstructure of Rotary Friction-Welded Carbon Steel and Stainless Steel Joints

by
Hudiyo Firmanto
1,*,
Susila Candra
1,
Mochammad Arbi Hadiyat
2,
Yesa Priscilla Triastomo
1 and
Ivan Wirawan
1
1
Department of Mechanical and Manufacturing Engineering, University of Surabaya, Surabaya 60293, Indonesia
2
Department of Industrial Engineering, University of Surabaya, Surabaya 60293, Indonesia
*
Author to whom correspondence should be addressed.
J. Manuf. Mater. Process. 2023, 7(1), 7; https://doi.org/10.3390/jmmp7010007
Submission received: 15 November 2022 / Revised: 14 December 2022 / Accepted: 22 December 2022 / Published: 28 December 2022
Browse Figures
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Abstract

:
Due to the different properties of the materials, the fusion welding of dissimilar metals may be difficult. Structural irregularities may form as a result of various phase transformations during welding. Solid-state welding, as opposed to fusion welding, occurs below the melting temperature. As a result of the melting and solidification phenomena that happen in fusion welding, solid-state welding is expected to reduce the potential for phase transformation. This paper describes the use of a rotary friction welding technique to join carbon steel and 304 stainless steel. The purpose of this work is to investigate the characteristics of rotary friction welding (RFW) when joining 304 stainless steel to carbon steels with different carbon contents. Experiments were carried out on the RFW of low- and medium-carbon steels with 304 stainless steel. The investigation was carried out using the Taguchi method of experimental design. The joints’ tensile strengths and microstructures were evaluated. The parameters that had the greatest influence on the tensile strengths of the welding results were identified. The combination of parameters resulting in the greatest tensile strength is also suggested. A microstructural examination of the weldment revealed mechanical mixing and interlocking.

1. Introduction

The rotary friction welding (RFW) process has been used to join dissimilar metals on numerous occasions. This method can be used instead of fusion welding. When welding dissimilar metals, the latter process frequently encounters difficulties. Brittle intermetallic compound formation was found to be sensitive in nonferrous dissimilar gas metal arc welding [1]. Excessive heat-affected zones were also produced by fusion welding techniques such as electric arc welding [2]. Another significant issue in this process is the formation of residual stresses due to compositional differences in joining metals [3]. The differences in the physical properties of the joined metals influence the formation of the joint. As a result, the heat input must be properly controlled in order to improve the joint [4]. Methods involving the least amount of melting were suggested to avoid fusion problems when welding dissimilar metals [5].
RFW has also been used to join stainless steel to other materials. RFW was used to join this material to nonferrous metals such as copper [6,7,8,9,10], Inconel [11,12], titanium [13,14,15], and aluminum [16,17,18,19].
The joining of carbon steel to stainless steel is used in various industries. It has been applied in the petrochemical and power generation industries [20]. For medical applications, the materials have been combined to produce semi-biodegradable bone screws [21]. In the construction sector, a combination of carbon steel and stainless steel was used in the construction of concrete structures in a marine environment [22]. Other applications were piping system components [23] and aerospace industry components [24].
The work on the RFW of stainless steel with carbon steel or cast iron focused on the influence of RFW parameters on joint strength and microstructure. Forging pressure was said to improve the joint strength of 1045 steel and 316 L stainless steel [20]. The peak temperature of the joint increased as the friction increased. Nonetheless, increased friction pressure improved heat generation and the efficiency of converting mechanical work into heat [25]. While keeping the upset pressure constant, the most influential parameters of friction pressure and friction time were suggested in the RFW of AISI 1018–AISI 2205 steel [26]. In the RFW of low-carbon steel with nodular iron, however, the friction time is the most important determinant of tensile strength improvement, followed by the upset force and friction force [27]. The heating stage parameters (i.e., the friction time and friction pressure) were discovered to have a greater influence on the RFW of 304 stainless steel (SS) with carbon steel than the upset parameters (the forging pressure and forging time) [28]. A certain combination of heating pressure, upset pressure, and heating time was suggested to achieve the best tensile strength in RFW 304 SS with 316 stainless steel [29]. The parameter combinations of the rotating speed, friction pressure, forging pressure, and friction time for achieving the maximum tensile strength were found in the RFW of low-carbon steel and 202 stainless steel, which were used for semi-biodegradable bone screws [21].
Numerical and computational modeling were also used to study RFW process optimization. The method was used to simulate the thermomechanical phenomenon in relation to the process parameters. Using this technique, increasing the frictional pressure was expected to lower the peak temperature, while increasing the rotational speed would increase the RFW temperature of 1045 carbon steel and 304 SS [25]. The application of numerical analysis found the same effect of these parameters on heat generation in mild steel RFW [30]. Increasing any of three parameters (e.g., the workpiece radius, rotational speed, or frictional pressure) has been found to increase the maximum temperature range in the RFW of Cu [31]. For aluminum–steel RFW, this method showed that individually increasing the velocity, friction pressure, and friction time increased the maximum temperature [32].
The RFW of carbon steel and stainless steel has been reviewed and has produced a variety of results in terms of the factors that affected the tensile strength of the joint for different materials. This demonstrated that the welded materials’ combination also affects the mechanical properties of the RFW joint. However, not much research has been conducted to link the study to the type of material being welded. Many studies on the RFW of carbon steels and stainless steels have focused on the influences of process parameters on the mechanical properties of the joints. However, they did not link the research to the material type, such as the effect of the carbon content in carbon steel. In a thermomechanically controlled process, the mechanical properties and process stability during the process were influenced by the chemical composition, process parameter control, and optimization as well as post-forming cooling strategies and thermal treatments [33]. Therefore, the carbon content in carbon steel may influence the tensile properties of the RFW joint, as it involves the thermomechanical process. Hence, the different carbon contents in carbon steels can affect the strength of the connection to stainless steel. The current study investigated the RFW of 304 SS and carbon steels with varying carbon concentrations. The purpose of this work was to assess the characteristics of joining 304 SS and carbon steel using the RFW technique and to figure out how the carbon content of carbon steel affects the mechanical properties and microstructure of the joint. To achieve this goal, experiments on the RFW of 304 SS with low-carbon steel and medium-carbon steel were carried out. The studies were conducted to determine the RFW combination parameters that would provide the greatest tensile strength. The microstructure of the joint was evaluated in comparison with those of low-carbon steel and medium-carbon steel when they were welded with 304 stainless steel.

2. Materials and Methods

2.1. Materials

The materials used in the experiments were 16 mm diameter rods of low-carbon steel, medium-carbon steel, and 304 SS. Table 1, Table 2 and Table 3 show the chemical compositions of the materials, respectively. The materials were cut into 20 mm lengths prior to the RFW experiment. To perform an RFW process, a flat surface is necessary to create a perfect mating between the surfaces of the welded materials. To obtain the flat surface, a facing process using a lathe machine was carried out on the surfaces of the two materials. The end of the bar was machined perpendicular to the axis of the bar. Shallow depth-of-cut machining was performed, creating a flat, smooth surface at the rod’s end.

2.2. Experiment Setting and Procedure

The friction welding was performed with a lathe machine with a hydraulic power pack and a rod pusher. Figure 1 shows a representation of the machine. The hydraulic pack and the timer were to control the pressure and timing of its application. The hydraulic mechanism could withstand axial pressures of up to 200 bar.
A fixture was installed on a lathe for the welding process. It was located on the machine guideways and could be moved toward the spindle. This equipment was designed so the center of the specimen holder was aligned with the center of the spindle to maintain the axial alignment of the two specimens. The fixed sample holder was installed in a sleeve cylinder so that it could slide and move axially. The position of the device was fixed using a bolted clamp when the required position was reached so that it could resist the force during welding. Additionally, the length of the free end of the specimen was adjusted to avoid excessive vibration and maintain the axiality of the force. Prior to welding, the positions of both specimens were adjusted so that their faces were at around a 1–3 mm distance. Then, the axial force was supplied by the hydraulic piston rod of the power pack unit to provide interfacial contact between the specimens. The 304 SS was used as a fixed specimen in the RFW experiments, while the carbon-steel sample was placed on the spindle of the machine and rotated continuously. Based on the implemented experimental design, two welding processes were performed for each experimental condition.
The study used a Taguchi L8 orthogonal array design with two replications (Table 4), which accommodated the main effect and potential interactions among factors based on its linear graph [34]. In addition to providing a useful interpretation, this Taguchi design supported an ANOVA analysis to statistically evaluate factor influences [35]. Based on the preliminary experimental results, the RFW parameters for low-carbon steel–stainless steel and medium-carbon steel–stainless steel are presented in Table 5 and Table 6, respectively.
Three variables, namely the friction pressure (Pf), friction time (Tf), and upset pressure (Pu), were the focus of the experimental procedure. The two previous parameters represent the heating parameters, while the most recent parameter represents the joining stage. They were chosen because they could potentially influence the tensile strength as an experimental response. To determine the high and low settings for each factor, an initial one-factor-at-a-time experiment was carried out. Other parameters, such as the spindle speed and upset time, remained constant. The spindle speeds for medium-carbon steel–304 SS and low-carbon steel–304 SS were set at 2000 RPM and 1330 RPM, respectively, while the upset time for both joints was set at three seconds.
The primary goal of the Taguchi analysis was to identify the optimal factor level combinations that maximized the experimental response, i.e., the tensile strength. Taguchi provided a transformation formula for the experimental data based on the signal-to-noise ratio (see Equation (1)). The transformed value was then analyzed using a standard ANOVA and a response table to evaluate significant factors, including their interactions, and optimal factor levels were obtained. The signal-to-noise ratio based optimization process yielded an optimal solution while minimizing the variation within factors [35].
S i g n a l - t o - n o i s e   r a t i o   = − 10 l o g ( 1 n ∑ i = 1 n 1 y i 2 )

2.3. Microstructure Analysis

To prepare metallographic samples, the welded specimen was cut, leaving the welded joint. The joint was then cross-sectioned with an abrasive cutting machine and mounted using epoxy, exposing the surface for polishing. The sample surface was mechanically ground and polished using different grits of emery paper and alumina powder on a rotating disk of a polishing machine. Subsequently, the specimen’s surface was etched using nital and aqua regia etching reagents for the carbon steel and stainless steel, respectively. The nital reagent was a mixture of 90% ethanol (100 mL) and HNO3 (1–10 mL), while the aqua regia reagent consisted of 15 mL of HCl and 5 mL of HNO3. The characterization of the microstructure was then carried out on the sample using Amscope ME300TC-14M3 optical microscopy. Finally, the sample was used for elemental analysis using FEI Inspect S50 scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS).

2.4. Tensile Test

The welded rods were machined to make tensile test specimens on a Leadwell LTC-20B CNC lathe. During the machining, one end of a rod was fixed to the spindle of the machine, while the other end was supported by the tail stock. This was performed to prevent any defects in the samples during the machining process. The tensile samples were prepared according to ASTM EM8 [36], as shown in Figure 2. The welded joint was positioned in the middle of the gauge length. To confirm that the machining process did not affect the joint, a non-destructive inspection using a liquid penetrant was carried out on the machined surfaces of several samples. The inspection revealed that there were no defects or cracks found on the machined surfaces. The tensile test was then performed on a Tarno Grocki UPH 100KN universal testing machine, which was operated in displacement control and had a crosshead speed of 5 mm/min.

3. Results and Discussion

3.1. Visual Observation

Figure 3a,b show several samples of low-carbon steel and medium-carbon steel welded with 304 stainless steel. The illustrations exhibit sound joints for the two materials. Both the carbon-steel and stainless-steel sides of both pairs produced flash. The samples depict a variety of flash conditions at the joint. The higher the values of the experiment parameters, the more flash that was produced. Higher and longer friction pressures (Pf and Tf) and upset pressure (Pu) caused more heat and deformation. This caused more flash at the joint. Figure 3a shows that the setting parameters of Pf = 16, Tf = 4, and Pu = 40 (sample 4) produced the highest flash at the joint of low-carbon steel–304 SS. Conversely, the parameters of Pf = 16, Tf = 4, and Pu = 40 led to the smallest flash at the joint (sample 1). Similarly, Figure 3b illustrates that sample number 4, which had high values for the RFW parameters (i.e., Pf = 90, Tf = 11, and Pu = 120) yielded the highest flash compared to sample 1 (Pf = 10, Tf = 7, and Pu = 95), which gave the smallest flash at the joint of medium-carbon steel–304 SS.
A higher flash was produced at the carbon steel part in both the low-carbon steel–stainless steel and medium-carbon steel–stainless steel joints. The higher heat conductivity of the carbon steel compared to the stainless steel allowed the material to soften and deform more severely. Because of its lower deformation resistance, more materials on the side of the carbon steel workpiece were forced radially outward from the flash [25]. As a result, more flash was formed on the carbon steel parts.

3.2. Microstructure of the Joint

The macrostructures of the low-carbon steel–304 SS and medium-carbon steel–304 SS joints are presented in Figure 4a,b, respectively. Both figures identify three parts of the structures at the joints. They are the interface (IF) layer, the thermomechanically affected (TMA) zone, and the heat-affected zone (HAZ). As shown in Figure 2b, the TMA and HAZ at the joint of medium-carbon steel and 304 SS are thicker than those of low-carbon steel and medium-carbon steel. The higher hardness of medium-carbon steel generated more heat around the joint when it was in friction with the 304 SS; thus, it resulted in thicker TMA and HAZ at the medium-carbon steel–304 SS joint.
Heat was generated during the application of friction pressure due to the friction of the mating surfaces. The applied pressure caused heavy deformation in both metals at the same time. The microstructural change was caused by a temperature increase and heavy deformation. Deformation, dynamic recovery, and recrystallization occurred, stimulating microstructural change around the joint. Aside from structural changes, the different element concentrations in the metals may cause element diffusion. As a result, elemental irregularities and the formation of new phases around the joint were also possible.
Figure 5 presents the microstructure of the low-carbon steel–stainless steel joint (a). It illustrates three parts, namely the base metal of carbon steel, the interface zone, and the base metal of stainless steel. However, in low-carbon steel and 304 SS, two parts of the microstructure zone were indicated in the macrostructure of the joint, as shown in Figure 4, i.e., the TMA and HAZ. The structure of the carbon steel consisted of ferrite and pearlite (Figure 5a). At the 304 SS part, austenite grain was observed as a typical structure of the austenitic stainless steel. Near the interface zone, some areas with different microstructures were observed on both the carbon-steel and stainless-steel sides. The figure depicts finer ferrite and pearlite grains on the carbon-steel side closest to the interface. This part was the structure of the TMA in low-carbon steel. An enlarged view of the 304 SS is given in Figure 5c. The figure shows that the TMA structure, which was closed to the interface, contained a very fine deformed grain. The coarser elongated grain appeared beyond this area. The structure at the TMA indicated the process of deformation, which was followed by recovery and recrystallization. The product of this process was a very fine grain. As the process continued, the grain grew into the larger size that appeared at the HAZ.
The interface of the low-carbon steel–304 SS joint illustrated a mixing zone of the low-carbon steel and stainless steel (see Figure 6). The severe deformation of both materials had built this mixing area, which produced an interlock between the materials. A darker zone at the joint was part of the carbon steel, which moved and mixed with the 304 SS. This was observed in a previous work in the RFW of mild steel with 304 SS [37]. This could happen due to a combination of heating and deformation. The axial force exerted by the upset pressure caused excessive deformation, which pushed the materials. Previous research suggested that angular velocity during the friction [38] led the softened metals to move away from the center. However, some materials remained at the center and mixed with the other metal. This resulted in an intermixing zone at the joint, as also seen in the current work. It seems that this mixing phenomenon occurs especially in the joining of carbon steel with 304 SS. Previous studies in the RFW of mild steel [37] and 1045 carbon steel with 304 SS [38] discovered this. As a result, they proposed that it was also dependent on the type of welded metal.
The EDS mapping of the elements at the joint, as given in Figure 7, shows the distribution of Cr (dominant elements of SS) and Fe elements. The figure exhibits less Cr at the island on the SS side, indicating that the island was a part of the carbon steel. This phenomenon was observed previously in the RFW of 304 SS with mild steel [37]. A higher portion of carbon steel that moved and mixed with stainless steel was found at the periphery of the joint. This part also increased with an increasing friction time [38]. The two conditions indicated that the mixing of carbon steel and stainless steel at the joint was caused by the deformation of both metals. In the present work, the mixing zone at the joint of low-carbon steel and stainless steel was higher than at the joint of medium-carbon steel and stainless steel. Since low-carbon steel is more ductile than medium-carbon steel, the low-carbon steel experienced more deformation at the joint and produced a larger mixing zone.
The general microstructure of medium-carbon steel–stainless steel is depicted in Figure 5b. The figure shows a dark, thin layer closed to the interface on the stainless-steel side. This is the carbon steel that was pushed to the stainless-steel side and created a mixing zone. This intermixing was possible due to hot deformation during the RFW process.
The formation of thicker TMA and HAZ at the medium-carbon steel–stainless steel joint can be seen in the figure. This suggested that friction generated more heat during the RFW process and allowed for more deformation. The higher temperature at the medium-carbon steel–stainless steel joint was also confirmed by temperature measurements taken during the RFW process experiment. Furthermore, the higher thermal conductivity of medium-carbon steel enabled longer heat transmission, resulting in a larger TMA and HAZ.
A fine grain of ferrite and pearlite could be seen at the interface between low-carbon steel–304 SS and medium-carbon steel–304 SS. Figure 6 illustrates this. A ferrite structure appeared at the interface and border of the medium-carbon steel. This was identified as proeutectoid ferrite [39], which was formed earlier during the hypoeutectoid steel phase transformation of austenite to pearlite. As claimed in the friction welding of 1045 steel, martensite could also form in this area [40,41]. In the current work, a needle-shaped structure was observed at the TMA of medium-carbon steel. This can be seen in Figure 6b. The needle-plate structure resembles acicular ferrite, which is commonly formed in weld metal/HAZ [42] and thermomechanically treated steel [43]. It was discovered in medium-carbon steel–316L stainless steel RFW [20]. The authors hypothesized that it was formed as a result of the TMA zone overheating. The isothermal or continuous cooling transformation of medium-carbon microalloyed forging steel produced this [44]. The acicular ferrite was most efficiently produced when the steel had a medium carbon content [45]. It was identified as a microstructure that could potentially improve a combination of weld metal and HAZ strength, toughness, and fatigue properties in high-strength low-alloy (HSLA) steels [44]. The welding efficiency of medium-carbon steel and stainless steel joined by RFW in this work was 109.5% compared to stainless steel, the weaker material, and 98.7% compared to medium-carbon steel. This was much better than the strength of a low-carbon steel–304 SS joint. The possible reason for this strength improvement is discussed in the next section.
The interface of the medium-carbon steel–304 SS joint was like that of the previously discussed low-carbon steel–304 SS joint. At the joint, a medium-carbon steel–304 SS mixing zone was observed. Figure 5b and Figure 6b demonstrate this. The medium-carbon steel is the thin, dark island in the stainless steel that transferred and mixed with the stainless steel because of heating and deformation. The EDS mapping of the elements at the joint, as shown in Figure 8, confirmed this. A lower Cr concentration in the thin layer at the stainless steel revealed that it was medium-carbon steel that mixed with the stainless steel. The intermixing zone in medium-carbon steel–stainless steel was thinner than the mixing zone in low-carbon steel–stainless steel. This could be due to the lower yield strength and greater ductility of low-carbon steel, which allow for greater deformation. Figure 8 depicts the distribution of elements at the joint but does not depict the colony of a compound or a specific phase. As a result, no intermetallic compound was detected at the joint in this study.
A flow of transversal deformation appeared near the medium-carbon steel–304 SS joint. This part was the TMA zone, which indicated the deformation, recovery, and recrystallization areas during the RFW process of medium-carbon steel–304 SS. The clearer, elongated grain, indicating the deformed structure and its boundary with the coarse structure of the HAZ, can be seen in its enlarged picture in Figure 5d. Beyond this area, the microstructure of the stainless steel consisted of larger equiaxed-grain austenite. Comparing the joints of low-carbon steel–304 SS with those of medium-carbon steel–304 SS, Figure 5a,b) showed that a thicker TMAZ and HAZ were indicated at the joint of medium-carbon steel–304 SS. This could indicate that more heat was generated and transmitted further in medium-carbon steel–304 SS than in low-carbon steel–304 SS. The deformation zone terminated at the HAZ, where the microstructure was a small, equiaxed, austenite grain (Figure 5b). Beyond the HAZ, the structure was austenite with a larger grain because it was owned by the base metal structure of the stainless steel.
The preceding discussion described similar microstructural joint conditions in the RFW of low-carbon steel–304 SS and medium-carbon steel–304 SS. The TMA zone was observed in the carbon steel and 304 SS as a deformation, dynamic recovery, and recrystallization zone. The microstructure of this part was composed of fine ferrite and pearlite at the low-carbon steel–304 SS joint. At the HAZ, a larger regular structure was formed until it reached the base metal, which had the typical low-carbon-steel structure of ferrite and pearlite. A fine structure was also observed at the medium-carbon-steel portion of the medium-carbon steel–304 SS joint. However, a needle-plate structure was discovered near the joint interface. This structure may have been acicular ferrite formed during the deformation and thermomechanical treatment [43,45]. The higher carbon content in medium-carbon steel may promote the formation of acicular ferrite during the thermomechanical treatment’s continuous cooling transformation [45]. As previously stated, the structure was acknowledged to improve the material’s strength.
The metallographic analysis of the joint revealed a zone of carbon steel and 304 SS intermixing at both the low-carbon steel–304 SS and medium-carbon steel–304 SS joints. It was observed that carbon steel was transferred to stainless steel and vice versa. The element distribution obtained from the EDS analysis confirmed the presence of carbon steel both on the 304 SS side and the carbon-steel side. The mechanical interlocking of the materials was caused by heating and severe deformation during the RFW process. It was discovered that friction time influenced the width of the intermixing zone [38]. However, the current study found that the low-carbon steel–304 SS joint experienced more material mixing than the medium-carbon steel–304 SS joint. This was due to the low-carbon steel having a lower yield strength and greater ductility than the medium-carbon steel. This allowed for more deformation and material mixing at the joint. As a result, in addition to the friction time, the types of materials influenced the width of the intermixing zone.
According to the EDS analysis, elements transferred at the joint. The diffusion of these elements has been widely recognized in the RFW of dissimilar materials [26,37]. However, in the current work, the element distribution did not preclude the occurrence of intermetallic compounds.

3.3. Tensile Strength

The tensile strength data from the RFW experiments on low-carbon steel–304 SS and medium-carbon steel–304 SS are shown in Table 7 and Table 8, respectively. During the tensile test of low-carbon steel–304 SS, most fractures occurred at the joints; however, fractures at the carbon-steel base metal were also found in some conditions. Meanwhile, some medium-carbon steel–304 SS specimens showed fractures in the 304 SS material, indicating that the strength of joined parts was greater than those of the weaker base materials (Figure 9). The tensile strengths of the low-carbon steel and 304 SS were found to be in the range of 296.01–492.40 MPa, while the tensile strengths of the medium-carbon steel and 304 SS were found to be in the range of 246.27–745.49 MPa.
The welding efficiency is the percentage of the joint strength relative to the strength of the base metal. Thus, it was calculated as (tensile strength of the welding joint/tensile strength of the base metal) × 100%. The maximum RFW efficiencies of low-carbon steel and 304 SS were 103.3% and 72.4%, respectively, when compared to the tensile strengths of the two materials. When compared to 304 SS, as the weaker material, the RFW of medium-carbon steel–304 SS had the highest welding efficiency of 109.5%. The RFW efficiency for these materials was 98.7% when compared to medium-carbon steel. As a result, the joint strength of medium-carbon steel and SS 304 was nearly equal to that of the stronger material. Assuming that the welding process remained highly efficient, all of the experiment data (tensile strength) could be statistically analyzed.
The joint tensile strength data revealed that welding 304 SS to medium-carbon steel produced a higher welding efficiency than welding 304 SS to low-carbon steel. This could relate to the microstructure of the joint. The microstructure of the joint consisted of three parts: the interface layers, TMA, and HAZ. The deformed microstructure at the TMA was indicated by the elongated grain. This can be seen in Figure 5. The TMA zone ended with fine grain as a result of recovery and recrystallization. This was found at the joint of low-carbon steel and 304 SS as well as in medium-carbon steel and 304 SS joints. Nevertheless, the TMA zone at the joint of medium-carbon steel–304 SS was thicker than at the joint of low-carbon steel–304 SS. Several studies have claimed the existence of intermetallic compounds or carbide precipitations in the carbon steel–stainless steel joint [20,39,46]. The intermetallic compound is normally brittle, which decreases the joint’s strength. In this work, however, intermetallic compounds or precipitations were not observed. Although the distribution of elements was detected in both welded materials, no precipitate could be detected.
The better strength of the medium-carbon steel–304 SS joint compared to the low-carbon steel–304 SS joint could be due to the thicker TMA produced at the joint. It consisted of a deformed and fine-grained structure. It was well known in the literature that the deformed structure was related to the dislocation density and improved the material’s strength. The deformation due to the compressive force created an accumulation of dislocation density in the microstructure of the high-carbon steel. The dislocation increased as the deformation lengthened. The dislocation motion became constrained as the dislocation density increased. This increased the hardness of the structure [47]. In low-alloy carbon steel, both deformation and phase transformation resulted in a high dislocation density [48]. Hot forming combined with quenching, partitioning, and tempering in medium-carbon steel increased the dislocation density and improved the tensile strength [49]. Based on the relations between deformation, dislocation, and strength improvement suggested by the reviewed studies, it could be expected that the presence of the deformed and fine-grained structure in the joint of medium-carbon steel–304 SS contributed to the strength of the joint. Furthermore, a thicker TMA in the joint may also result in a higher joint strength than in a low-carbon steel–304 SS RFW joint.
Besides the deformed structure, the influence of fine grain on the material’s strength has also been revealed. Increasing the grain size decreased the yield and the ultimate strength of manganese austenitic steel [50]. Fine grain also improved the mechanical properties of high-carbon steel [51]. A reduction in grain size significantly increased the strength of low-carbon, high-Mn austenitic steel [52]. Thus, the small grain size at the joint of the RFW seems to improve joint strength.
As a summary, a microstructural change in the joint of carbon steel and 304 SS took place during the RFW process. This was caused by the thermomechanical processing. Intermetallic compounds may be formed as the result of the reaction of elements from the welded metals. In this work, however, we were unable to observe any precipitations at the joint. Mechanical interlocking caused by mechanical mixing due to excessive deformation occurred at the joint. Near the interface, the thermomechanical processing produced a deformed structure and fine grain, which resulted from the recovery and recrystallization processes. Beyond the TMA, coarser-equiaxed grain was formed in the HAZ. This zone ended with an unaffected base metal that retained the metal’s original coarse structure. The structure of the TMA, which consisted of a deformed and fine-grained structure, may have enhanced the strength of the welded joint.

3.4. Taguchi Analysis and Optimization

The Taguchi design and analysis have the advantage of identifying the best factor combinations to maximize the tensile strength while minimizing variance. The measured value was converted into a signal-to-noise ratio using the larger-is-better formula (see [27] and [28]). As a result, the signal-to-noise ratio as well as the ANOVA and effect plots were included in the statistical analysis in this paper, which followed the standard Taguchi procedure.
Table 9 and Table 10 show the ANOVA for low-carbon steel–304 SS and medium-carbon steel–304 SS welding based on the signal-to-noise ratio. All of the individual main effects were significant when compared to the p-value using a significance level of 5%, indicating that this experiment successfully captured the influencing factors in RFW. Furthermore, except for one interaction in low-carbon steel–304 SS (see Table 6), almost all interactions were significant. These results indicated that these predetermined factors cannot be separated; all factors influence tensile strength simultaneously. The ANOVA and effect plots, on the other hand, revealed some differences between low-carbon steel–304 SS and medium-carbon steel–304 SS welding (Figure 10 and Figure 11).
Although all the individual effects were significant, the contribution ratio varied, depending on the welded material. In medium-carbon steel–304 SS, the friction pressure had a high contribution ratio (see Table 10 and Figure 11); in low-carbon steel–304 SS, the friction time had a high contribution ratio (see Table 9 and Figure 10). As a result of these discoveries, special characteristics for welding carbon steel with SS 304 were discovered, namely that the hardest factor influencing joint strength is the material’s hardness. When welding a harder material (i.e., medium-carbon steel) with 304 SS, careful control of the friction pressure was required, whereas welding low-carbon steel, as the softer material, required more consideration of the friction time variation. This phenomenon could be explained by the suggestion that the friction time was required to bring the material to its peak temperature [21]. As a result, a sufficient friction time was required to heat up the softer materials. In other words, to obtain a stronger joint, a longer friction time was needed. Hence, increasing the friction time in the RFW of low-carbon steel–304 SS improved the tensile strength more than the other parameters.
For the case of a harder material, e.g., medium-carbon steel as compared to low-carbon steel, sufficient friction pressure was also required to promote deformation. Thus, a higher friction pressure was required when joining the harder material. The increased friction pressure improved the heat generation and enhanced the efficiency of converting mechanical work into heat. A higher pressure was needed to spur the deformation required for the joining of the harder material, such as medium-carbon steel, when compared to low-carbon steel [25]. As a result, a higher friction pressure may result in a higher tensile strength in a medium-carbon steel with a 304 SS joint. Therefore, in the RFW of medium-carbon steel with 304 SS, the friction pressure was a more influential factor in improving the joint strength than the other parameters.
Finally, the goal of the Taguchi analysis was to find the best factor level combination that maximized the tensile strength based on the signal-to-noise ratio. The Taguchi method recommended selecting 50% of the significant factors as the basis for selecting the optimal level [28] and remaining involved in existing interactions. Nevertheless, in this case, with only three factors included in the experiment, all these factors must be involved to find an optimal level. The selection of optimal factor levels referred to the highest signal-to-noise ratio value, as shown in Table 11. Therefore, besides the dominant factor discussed previously, the prediction of these selected optimal levels gave a high tensile strength compared to those in Table 7 and Table 8. This optimal condition suggested the combination of RFW parameters (i.e., the friction force, friction time, and upset pressure) that would provide the highest tensile strength.

4. Conclusions

Medium-carbon steel and low-carbon steel were successfully welded with 304 SS using the RFW method. The RFW of medium-carbon steel–304 SS produced a stronger joint and better efficiency than the RFW of low-carbon steel–304 SS. The welding efficiency for medium-carbon steel–304 SS was 109.5% compared to SS 304, as the weaker material, while the welding efficiency for low-carbon steel–304 SS was 103.3% compared with the low-carbon steel.
Three factors, i.e., Pf, Tf, and Pu, had a significant influence on the tensile strength, including their interaction. Among those parameters, Pf was more influential when joining medium-carbon steel (the harder material) with 304 SS, whereas the friction time had a greater effect when joining low-carbon steel with 304 SS. Optimization processes using a Taguchi analysis revealed that the highest tensile strength of low-carbon steel–304 SS, i.e., 476.56 MPa, was produced by the parameter combination of Pf = 35 bars, Tf = 4 s, and Pu = 70 bars. For the medium-carbon steel–304 SS, the highest tensile strength was 739.39 MPa, which resulted from the parameter combination of Pf = 90 bars, Tf = 7 s, and Pu = 95 bars.
At the interface of carbon steel and 304 SS, a metal mixing zone was formed. Because low-carbon steel has a lower yield strength and better ductility than medium-carbon steel, a thicker mixing zone tends to form at the joint of low-carbon steel and 304 SS. Medium-carbon steel produced more heat and transmitted it further during friction due to its higher carbon content and heat conductivity. It resulted in a thicker deformation zone and a larger HAZ. These factors also influenced the formation of an acicular ferrite phase, which increased the strength of the joint.

Author Contributions

Conceptualization, H.F. and S.C.; methodology, M.A.H.; software, M.A.H.; investigation, Y.P.T. and I.W.; writing—original draft preparation, H.F.; writing—review and editing, M.A.H.; All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Directorate of Research, Technology, and Community Services, Ministry of Education, Culture, Research, and Technology, Republic of Indonesia, grant number 073/E5/P6.02.00.PT/2022, dated 16 March 2022, contract number 004/SP2H/PT-L/LL7/2022.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Lathe machine equipped with hydraulic pack for RFW process.
Figure 1. Lathe machine equipped with hydraulic pack for RFW process.
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Figure 2. Dimensions of tensile test specimen based on ASTM E8M.
Figure 2. Dimensions of tensile test specimen based on ASTM E8M.
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Figure 3. Samples of welded specimens of (a) low-carbon steel–304 SS: 1. Pf = 16, Tf = 4, and Pu = 40; 2. Pf = 16, Tf = 4, and Pu = 70; 3. Pf = 35, Tf = 4, and Pu = 40; and 4. Pf = 35, Tf = 9, and Pu = 70 and (b) medium-carbon steel–304 SS: 1. Pf = 10, Tf = 7, and Pu = 95; 2. Pf = 10, Tf = 11, and Pu = 95; 3. Pf = 90, Tf = 7, and Pu = 95; and 4. Pf = 90, Tf = 11, and Pu = 120 (units: P = bar, T = second).
Figure 3. Samples of welded specimens of (a) low-carbon steel–304 SS: 1. Pf = 16, Tf = 4, and Pu = 40; 2. Pf = 16, Tf = 4, and Pu = 70; 3. Pf = 35, Tf = 4, and Pu = 40; and 4. Pf = 35, Tf = 9, and Pu = 70 and (b) medium-carbon steel–304 SS: 1. Pf = 10, Tf = 7, and Pu = 95; 2. Pf = 10, Tf = 11, and Pu = 95; 3. Pf = 90, Tf = 7, and Pu = 95; and 4. Pf = 90, Tf = 11, and Pu = 120 (units: P = bar, T = second).
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Figure 4. Macrostructures at the joints of (a) low-carbon steel–304 SS (Pf = 16, Tf = 4, and Pu = 40) and (b) medium-carbon steel–304 SS (Pf = 90, Tf = 7, and Pu = 95).
Figure 4. Macrostructures at the joints of (a) low-carbon steel–304 SS (Pf = 16, Tf = 4, and Pu = 40) and (b) medium-carbon steel–304 SS (Pf = 90, Tf = 7, and Pu = 95).
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Figure 5. Microstructure of the interface at the joint of carbon steel–304 SS: (a) low-carbon steel–304 SS joint; (b) medium-carbon steel–304 SS joint; (c) enlarged view of deformed structure at TMA and coarse austenite in HAZ in low-carbon steel–304 SS joint; (d) enlarged view of deformed structure and coarse grain of austenite in HAZ in medium-carbon steel–304 SS joint.
Figure 5. Microstructure of the interface at the joint of carbon steel–304 SS: (a) low-carbon steel–304 SS joint; (b) medium-carbon steel–304 SS joint; (c) enlarged view of deformed structure at TMA and coarse austenite in HAZ in low-carbon steel–304 SS joint; (d) enlarged view of deformed structure and coarse grain of austenite in HAZ in medium-carbon steel–304 SS joint.
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Figure 6. Microstructure of the interface at the joint of low-carbon steel–304 SS (a) and medium-carbon steel–304 SS (b).
Figure 6. Microstructure of the interface at the joint of low-carbon steel–304 SS (a) and medium-carbon steel–304 SS (b).
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Figure 7. EDS element (Fe and Cr) mapping at the joint of low-carbon steel–304 SS.
Figure 7. EDS element (Fe and Cr) mapping at the joint of low-carbon steel–304 SS.
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Figure 8. EDS element (Fe and Cr) mapping at the joint of medium-carbon steel–304 SS.
Figure 8. EDS element (Fe and Cr) mapping at the joint of medium-carbon steel–304 SS.
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Figure 9. Broken specimens of low-carbon steel–304 SS (a) and medium-carbon steel–304 SS (b) after tensile testing.
Figure 9. Broken specimens of low-carbon steel–304 SS (a) and medium-carbon steel–304 SS (b) after tensile testing.
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Figure 10. Low-carbon steel–304 SS main effect (a) and interaction (b) plots for SNR.
Figure 10. Low-carbon steel–304 SS main effect (a) and interaction (b) plots for SNR.
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Figure 11. Medium-carbon steel–304 SS main effect (a) and interaction (b) plots for signal-to-noise ratio.
Figure 11. Medium-carbon steel–304 SS main effect (a) and interaction (b) plots for signal-to-noise ratio.
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Table
Table 1. Chemical composition of low-carbon steel.
Table 1. Chemical composition of low-carbon steel.
CSiMnPSFe
0.090.080.550.0090.010Balance
Table
Table 2. Chemical composition of medium-carbon steel.
Table 2. Chemical composition of medium-carbon steel.
CSiMnPSNiCrCuFe
0.450.250.700.0140.0030.010.360.01Balance
Table
Table 3. Chemical composition of SS 304.
Table 3. Chemical composition of SS 304.
CSiMnPSNiCrCuFe
0.0190.281.600.0380.0248.0718.220.01Balance
Table
Table 4. Taguchi L8 orthogonal array.
Table 4. Taguchi L8 orthogonal array.
Experiment Run
Number		PfTfPu
1111
2112
3121
4122
5211
6212
7221
8222
Lower and higher factor levels are represented by 1 and 2, respectively. Pf: pressure during friction. Tf: time spent during friction. Pu: forging pressure after the friction stops (upset).
Table
Table 5. Input parameters for each level in RFW of low-carbon steel–304 SS.
Table 5. Input parameters for each level in RFW of low-carbon steel–304 SS.
No.ParameterLowHigh
1Pf (bar)1635
2Tf (second)49
3Pu (bar)4070
Table
Table 6. Input parameters for each level in RFW of medium-carbon steel–304 SS.
Table 6. Input parameters for each level in RFW of medium-carbon steel–304 SS.
No.ParameterLowHigh
1Pf (bar)1090
2Tf (second)711
3Pu (bar)95120
Table
Table 7. Tensile strength of RFW low-carbon steel–304 SS.
Table 7. Tensile strength of RFW low-carbon steel–304 SS.
ParametersTensile Strength (MPa)
Pf (sec)Tf (sec)Pu (bar)Tu (sec)R Speed
(RPM)		Replication 1Replication 2
1644031330443.82492.40
16470437.47420.33
35940319.65302.89
35970334.23353.87
16940451.95484.10
16970487.03473.96
35440356.58296.01
35470356.21403.59
Table
Table 8. Tensile strength of RFW medium-carbon steel–304 SS.
Table 8. Tensile strength of RFW medium-carbon steel–304 SS.
ParametersTensile Strength (MPa)
Pf (sec)Tf (sec)Pu (bar)Tu (sec)R Speed
(RPM)		Replication 1Replication 2
1079532000333.06314.67
107120444.15246.26
901195692.02751.23
9011120704.82714.66
101195417.05466.64
1011120386.72462.23
90795745.49744.44
907120701.40752.93
Table
Table 9. Low-carbon steel–304 SS ANOVA analysis for the signal-to-noise ratio (the larger the better).
Table 9. Low-carbon steel–304 SS ANOVA analysis for the signal-to-noise ratio (the larger the better).
SourceDFSeq SSAdj SSAdj MSFp% Contrib.
Pf10.56990.56990.569940.870.0243.42%
Tf114.465414.465414.46541037.270.00186.80%
Pu10.38850.38850.388527.860.0342.33%
Pf*Tf
Pf*Pu10.27590.27590.275919.780.0471.66%
Tf*Pu10.93850.93850.938567.30.0155.63%
Residual 20.02790.02790.0139
Total716.666
Table
Table 10. Medium-carbon steel–304 SS ANOVA analysis for the signal-to-noise ratio (the larger the better).
Table 10. Medium-carbon steel–304 SS ANOVA analysis for the signal-to-noise ratio (the larger the better).
SourceDFSeq SSAdj SSAdj MSFp% Contrib.
Pf169.830769.830769.830772982820.00090.03%
Tf13.06783.06783.06783206260.0013.96%
Pu10.20560.20560.205621491.80.0040.27%
Pf*Tf14.41154.41154.4115461065.60.0015.69%
Pf*Pu10.04260.04260.04264454.280.0100.05%
Tf*Pu10.00580.00580.0058607.830.0260.01%
Residual 1000
Total777.564
Table
Table 11. Optimal factor levels based on Taguchi analysis.
Table 11. Optimal factor levels based on Taguchi analysis.
Welded MaterialSelected Factors for OptimizationInteraction
between Factors		Optimal
Selected Factor Level		Tensile Strength Prediction (MPa)
MeanSNR
Low-carbon steel–304 SSPf, Tf, PuNot significant (optimal factors ignore the interaction)Pf = 35 bar
Tf = 4 s
Pu = 70 bar
(based on Figure 10a)		476.56 53.59
Medium-carbon steel–304 SSPf, Tf, PuSignificantPf = 90 bar
Tf = 7 s
Pu = 95 bar
(based on Figure 11a)		739.39 57.44
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Firmanto, H.; Candra, S.; Hadiyat, M.A.; Triastomo, Y.P.; Wirawan, I. Tensile Strength and Microstructure of Rotary Friction-Welded Carbon Steel and Stainless Steel Joints. J. Manuf. Mater. Process. 2023, 7, 7. https://doi.org/10.3390/jmmp7010007

AMA Style

Firmanto H, Candra S, Hadiyat MA, Triastomo YP, Wirawan I. Tensile Strength and Microstructure of Rotary Friction-Welded Carbon Steel and Stainless Steel Joints. Journal of Manufacturing and Materials Processing. 2023; 7(1):7. https://doi.org/10.3390/jmmp7010007

Chicago/Turabian Style

Firmanto, Hudiyo, Susila Candra, Mochammad Arbi Hadiyat, Yesa Priscilla Triastomo, and Ivan Wirawan. 2023. "Tensile Strength and Microstructure of Rotary Friction-Welded Carbon Steel and Stainless Steel Joints" Journal of Manufacturing and Materials Processing 7, no. 1: 7. https://doi.org/10.3390/jmmp7010007

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