Abstract
The present experimental work critically analyses the changes in microstructural orientation and mechanical properties of TIG (tungsten inert gas) welded AISI-304 (SS) plates which have undergone through various heat treatments. The characteristics of heat-treated samples have been compared with those of untreated samples which is denoted by the original sample in this study. Five SS-304 plates were welded by the TIG method. Out of five, four plates were heat treated and one plate was put in the original condition. Four heat treatment includes quenching, stepped heat treating (sand + water), tempering-1 (quick quench then sand cooling), and tempering-2 (quick quench then air cooling). In all the five plates, the tensile test and microhardness test have been performed. In addition, microscopic observation of cross-sectional surfaces has also been carried out. The base metal (BM) zone of all five plates mainly consists of equiaxed γ-grains, although twin can be observed only in the original sample. The heat-affected zone (HAZ) imparts both lathy and dendritic type δ-ferrite at the transition of the BM and welded zone (WZ). At the welded zone, the γ-matrix has become coarse with numerous spots and dendrites of δ-ferrite could be seen. Sample 3, tempered by means by water sprinkling for a very short time and then cooling in sand, has shown satisfactory results among all. The sample has achieved the 23% higher extension value and 10% higher hardness at WZ than these of the original sample, although a negligible loss (0.2%) of ultimate tensile strength (UTS) is also reported in sample 3.
1. Introduction
Metallography of AISI-304 steel, i.e., austenitic stainless steel (grade 304) has been taken under study by many researchers because of wide range of applications of this metal. This steel is alloyed with 8% Cr and 10% Ni which makes it highly corrosion resistant [1]. This grade of steel possesses lowers conductivity to heat and electricity than low-carbon steel. It is easy to mould it into different forms. SS-304 is widely utilized in the automobile, ship building, chemical plants, aircrafts, and construction areas [2]. SS-304 imparts a good weldability, and it can be conveniently welded by fusion welding techniques such as gas metal arc welding and gas tungsten arc welding. SS-304 has been widely explored with respect to its performance under different welding conditions which include similar and dissimilar metal joints [3], varying yield parameters [4], and postweld heat treatments [5]. Welding is an old but very important part of fabrication processes. In the present scenario, research on welding and its analysis is still happening. Mostly metals such as Al, Ti-alloys, and different grades of steels have been thoroughly analyzed by the researchers to enhance their application range in industries [6, 7]. The importance of heat treatment to metals has also been extensively discussed to modify the mechanical properties such as strength [8], hardness [9], bending strength [10], toughness [11], and ductility [12]. Some of the previous works on welding and heat treating of steel are discussed as follows.
In a work, 55Si7 grade, a medium carbon steel, was taken into consideration for welding followed by heat treatments. As a result, a considerable decrement in hardness was achieved and the heat affected zone was found as the lowest hard area in the plate [13]. Another grade, i.e., 120-Mn3Si2 steel was considered for analyzing the properties of the welded joint after heat treatment. Welded plates were quenched in water, and because of that, austenite was formed as layers in the welded joint. In addition, the residual austenite was reported in between the layers which further lead to formation of martensite. The result was obtained as a low value of tensile strength (i.e., 209.27 MPa) of the welded joint [14]. The effects of thermal aging at 400°C up to 10000 hours were investigated in welded joints of two metals, i.e., low alloy steel (SA 508) and nickel-based alloy steel (Alloy 52). SA508 is a ferritic type of steel, whereas Alloy 52 is an austenitic steel. Postweld treatment had not significantly affected the properties of the base metal and heat affected zone. The transition of two metals had been influenced by aging. It was seen that carbon had diffused from the welded zone to away from the weld centre line. Therefore, a reduction in hardness was reported in the welded zone [15]. The metal inert gas (MIG) welding technique was used to deposit the SS-304 layer on Q235 carbon steel for enhancing the physical properties of low-carbon steel, i.e., Q235. Both the welded joint and deposited surface were free from any type of defects. The surface was mainly composed of martensite along with traces of ferrite and austenite. It was observed that the deposited layer surface and the welded joint were highly corrosion resistant as compared to carbon steel alone, although the hardness of the layer was found lower than the welded joint [16]. Dissimilar steels such as stainless and structural grades were welded and analyzed through microscopes and various mechanical tests to examine the reliability of joints. It was seen that 308LSI steel was found suitable as a filler rod for the welding process. The tensile test was performed to check the strength of the joint. It was noticed that the joints possess an average strength of 472 MPa. The area of the heat-affected zone was too high to cause fracture over there during the tensile test [17]. In an experimental work, it was noted that a significant reduction in weld penetration and a considerable improvement in the microstructure could have happened by rotating the filler wire during its downward feeding which causes an altered fluid flow in the fusion zone. Also, the strength of the joint was greatly improved after formation of the fine microstructure in the welded zone [18]. 2205 duplex stainless was welded by the TIG method, and the welded plate was critically analyzed at five different zones from the welded zone to the base metal zone to examine the grain structure, hardness, and corrosion behavior. It was observed that the second pass of welding had created a reheating phenomenon which had caused the formation of secondary austenite in the welded zone. Also, the coarse ferrite had come closer to the fusion line. Austenite was found tougher than ferrite from the weld metal to the fusion zone [19]. In a work, the AISI-304 metal plate was welded by MIG welding with Ar as inert gas and 308L as the electrode material. It was observed that δ-ferrite grains started to form as soon as heat input increased beyond 1.25 kJ/mm. As a result, the ultimate tensile strength and toughness of the material got decreased in the weld region. In addition, hardness was reported at its minimum value at the welded zone. The heat-affected zone was reported as highly hard due to formation of fine grains after rapid cooling [20]. SS-304 samples had undergone through pressure quenching at 1600°C temperature and 4 GPa pressure. Microstructures of these samples contained γ-Fe (FCC) and α-Fe (BCC), which were observed by X-ray profiles. Diffraction peaks of the pressure quenched sample were wider than other samples, possible causes could be grain refinement and microstrain during the solidification process. Pressure quenched samples were found to possess better hardness and yield strength, although there was no improvement in ultimate tensile strength, and also, the rate of elongation got decreased due to the presence of large density dislocations [21].
After going through the previous literature, it has come to the notice that various grades of steels had been analyzed at different experimental conditions. Sometimes, different results could also be observed for same working conditions. SS-304, being a commercial grade of stainless steel, is widely used all over the world, and therefore, a continuous assessment is required to increase its applicability.
In this direction, the present work provides a comparative analysis among four different heat treatment processes applied over the TIG-welded plates of AISI-304 (SS-304). The tensile strength, microhardness, and grain structure are the basis of this comparative analysis.
2. Experimental Work
Step 1: Collection of SS plates and welding ⟶ experimental work involves the collection of 10 nos. of AISI-304 (SS-304) plates with dimension 50 × 50 × 3 mm each. Five pairs of plates were welded in butt configuration by the tungsten inert gas (TIG) welding technique which had a tungsten electrode of 4 mm. The TIG 400 (Mosfet DC inverter) machine was utilized for welding purpose. The input parameters, i.e., voltage and current values of 25 V and 180 A, respectively, were selected for welding the plates. It was a random selection based on the suitable requirement for the 3 mm thick SS plate. The filler rod of SS-304 was selected in the form of a 2 mm thick wire. The “V” groove of 45° was created on the edges of two adjoining surfaces. Welding was carried out on both sides of the plate. During welding, arc shielding was carried out by providing argon gas (purity > 99%) with an identification number Ar 5.0. The flow of argon gas was maintained at 11.5 l/min. After welding, five pairs were named as sample 1, sample 2, sample 3, sample 4, and sample 5. Step 2: Heat treatment ⟶ postweld heat treatment methods were adopted after welding the plates. Sample 1 was left as welded and named as the original sample. Rest of the samples were heated up to 900°C for 1 h in an induction furnace. In other words, four samples had soaked heat for 1 hour. The purpose of heating is to make the microstructure equiaxed and stress-free. Three samples, i.e., sample 2, sample 3, and sample 4 were quenched by dipping them into water. Heat treating of sample 2 became over as soon as it was quenched. It was named as water quenched. Sample 5 was cooled in two steps: first 1 min in sand (fully buried inside it) and then in water till room temperature. Hence, sample 5 is termed as the sand + water-cooled sample. Step 3: Tempering of sample 3 and sample 4 ⟶ after quenching, both, sample 3 and sample 4 were again heated in the furnace for a holding time of 1 h. Now, the cooling methods were varied for both the samples. Sample 3 was cooled in two steps: (i) a quick water sprinkle for 1 s for possible rapid cooling of surface and (ii) then, cooling inside sand up to room temperature. Hence, sample 3 was named as the water + sand-cooled sample. Sample 4 was also cooled in two steps: (i) a quick water sprinkle for 1 s and (ii) cooling in atmospheric air. Therefore, sample 4 was indicated by the water + air-cooled sample. To properly understand the processes of experiments, an outline is provided in Figure 1. The actual welded plates taken under the study are shown in Figure 2(a). The heat-treated samples are shown in Figure 2(b). Step 4: Preparation of tensile test specimens ⟶ from all the five welded plates, the specimens for the tensile test were prepared by using wire-electric discharge machining. These samples were prepared as per the ASTM-E8 standard. The specimens are shown in Figure 2(c) (i). Step 5: Preparation of samples for hardness and the microstructural test ⟶ all the plates were cut across the welded joint to make other plates of 100 × 10 × 3 mm dimension. These cut sections were used for the hardness test (on one cross section) and microstructural observation (on another cross-section). The specimens are shown in Figure 2(c) (ii). The cross-sectional surfaces were properly finished for the microhardness test. For microscopic analysis, the other side of the cross-sectional surface was highly polished by using the rotating disc polisher. Five different grit sizes, i.e., 1000, 1400, 1800, 2200, and 2500 of sand papers were used for superfinishing the work. Adler’s etchant (ferric chloride + copper ammonium chloride + hydrochloric acid + distilled water) was applied 1 min before the microscopic observation. All the polished samples are shown in Figure 3. Three zones, i.e., the base metal zone (which is affected by heat), welded zone (gets converted in molten state), and heat-affected zone (not melted by highly heated) were focused for any microstructural changes. Step 6: Tensile testing ⟶ the tensile test was conducted on a universal testing machine which was controlled by the hydraulic system. There was a data acquisition system attached to it so that the load-displacement curve could be recorded during the test. The strain rate of 0.001 s −1 was selected for the test. All the specimens got fractured through the welded joint during the tensile test. The broken tensile test specimens are given in Figure 4. Step 7: Fractographic testing of broken specimens ⟶ the fractured parts of the used tensile test specimens were cut by the EDM method in such a way that the height of each sample should not exceed 5 mm. These samples were, then, observed through the field emission scanning electron microscope (FESEM). The central part of the fractured area was focused for capturing the images. Step 8: Hardness testing ⟶ the Vickers microhardness test was conducted on each welded plate to know the variation of hardness among three zones, i.e., the base metal zone, welded zone, and heat-affected zone. A load of 300 gm was considered for the test.3. Result Analysis and Discussion
The outcomes of the tests conducted are discussed in this section.
3.1. Microstructural Test Results
The SS-304 grade is known as austenitic steel in which main alloying elements are Cr and Ni apart from Fe. It contains mainly austenite (FCC) and ferrite (BCC) based on the circumstances of solidification. If solidification happens in the equilibrium condition; then, the final microstructure will be mostly γ-austenite. If solidification is nonequilibrium, there is variation in the cooling rate or in case of rapid cooling, and then, the transformation from δ-ferrite to γ-austenite does not take place fully. In this case, a metastable δ-ferrite may remain with the γ-matrix [22]. In general, the austenitic steel like SS-304 may contain 2–10% of δ-ferrite, especially in the case of the weld metal. Due to the presence of the δ-phase, the occurrence of embrittlement and hot cracking is avoided during solidification of weldment [23].
All the microstructural images in this study were captured at a scale of 100 µm and a magnification of 100X. The microstructures of sample 1 are shown in Figures 5(a)–5(c). Figure 5(a) depicts the grains in the base metal part in which a fine the γ-matrix could be seen surrounded by δ-ferrite in the boundaries of the γ phase. Some of the δ-phases are linear and a very small number of δ-ferrite globules could be noted. It is the only sample where twins could be properly seen in the base metal zone. The γ-matrix, δ-ferrite, and twins are indicated in Figure 5(a). Figure 5(b) shows the HAZ in which the boundary between BM and WZ is clearly indicated by black colored lines. It is the area where the microstructure is completely different from BM and WZ. Despite having the coarse, fine, and equiaxed structure, this zone contains the lathy (δ + γ) structure at one part and some dendritic structures at another part. Figure 5(c) shows WZ, where γ-austenite is significantly coarser than that of BM. The distribution of δ-ferrite is mainly in the form of globules and spots. Some lathy-type δ-ferrite appearance can also be noticed in the image.
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The microstructure of sample 2 is explained in Figure 6. Figure 6(a) is an image of BM which is completely different from the same of sample 1. In this, the γ-matrix is equiaxed and coarser than that of sample 1. Twin is not visible anywhere. The appearance of δ-ferrite is somewhat like sample 1. Figure 6(b) is the HAZ of sample 2. The boundary between WZ and BM shows that the area nearer to BM is mainly containing lathy ferrite, whereas it is dendritic towards the WZ. As compared to sample 1, the ferrite dispersion in the transition zone (between WZ and BM) in sample 2 is lower and widely spaced. Figure 6(c) is the image of WZ which imparts an equiaxed γ-matrix but a little finer than that of sample 1. The γ-matrix is fully dispersed with δ-dendrites (black colored).
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The grain orientation of sample 3 is shown in Figure 7. The BM is showing the coarser γ-matrix than the same of sample 2. Rest of the things is almost similar. No twin is formed in this sample also. By the virtue of slow cooling inside the sand, the WZ possesses a very coarse γ-matrix with numerous δ-ferrite as dots or tiny globules. This is the condition which improves the ductile property of austenitic steel. The HAZ (Figure 7(c)) possesses a narrow transition zone between WZ and BM. It imparts a widely spaced lamella of the γ and δ phase. Some lathy structure can also be seen in this zone.
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The microstructure of sample 4 can be seen in Figure 8. The images of all the three zones are very similar to those of sample 3. Both samples 3 and 4 are tempered after quenching. Sample 4 is cooled at a relatively faster rate (medium was air). Hence, the only change was observed in the γ-matrix of the welded zone. It is comparatively finer than sample 3. The HAZ imparts lathy ferrite which is widely spread.
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The grains structure of sample 5 is shown in Figure 9. This sample has more uniform γ-matrix as compared to others. The clearly defined boundaries of the γ-phase are like spherical in shape. Twin formation is completely restricted. The HAZ (Figure 9(b)) is containing both lathy and dendritic ferrite across the transition zone. The WZ is mainly containing coarse γ-matrix with dendritic appearance of δ-ferrite. All the microscopic results of this work are comparable with the same of Kellai et al. [24].
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3.2. Tensile Test Results
The load-displacement graphs of all the samples are shown in Figure 10. The profile of all the graphs is similar. Sample 1, i.e., original sample exhibited UTS of 470 MPa at an elongation of 6.6 mm. These values are considered as the standard for comparison basis. Sample 2 had shown UTS of 434 MPa with 6.5 mm of elongation. Both the values are 7.6% and 1.5%, respectively, lower than the UTS and elongation of sample 1. Hence, only water quenching has reduced the strength and ductility of the sample. Sample 3, which was initially quenched and then tempered, had shown a UTS value of 469 MPa, which is almost same as the original sample. The elongation shown by this sample, i.e., 8.1 mm, is 23% higher than that of the original sample. Hence, the sand cooling method has improved the ductile behavior of the sample. This has happened by the coarse structure of the γ-matrix at the WZ. Unlike sample 3, sample 4 had shown a considerable reduction in elongation which is 18% less than that of the original sample. Also, it showed a UTS value of 441 MPa which is smaller that of sample 1. Sample 5 had exhibited the UTS of 437 MPa and elongation of 5.2 mm which are almost 7% and 21% lower as compared with sample 1. Hence, quick cooling through sand followed by rapid quenching has negative effects on ductility. A comparative bar chart among the UTS and elongation of five different samples is shown in Figure 10(f).
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3.3. Fractography Analysis
The fracture behavior of the tensile test specimens was analyzed by FESEM images. All the images were captured at a same magnification and at same scales, i.e., 500X and 10 µm, respectively. Figure 11(a) shows the magnified fractured surface of sample 1 which was “as welded.” The plastically deformed location with whirl-like appearance could be observed in the same. Some pores and dimples along with microdimples were also reported in the image. All these symptoms are of ductile failure. Sample 2 (water quenched) showed dimples and very less pores in the image. No microdimples and plastic flow could be noted (Figure 11(b)). Hence, it is comparatively less ductile than the original sample. Figure 11(c) is the fracture part of sample 3 which was mainly sand-cooled. A numerous plastically deformed zones were reported which seems like whirls. In addition, numerous microdimples, dimples, and pores were also identified. Its appearance is very similar to that of the original sample. Therefore, by the virtue of high ductility, a good elongation was achieved in sample 3 during the tensile test. Figure 11(d) is the fractography image of sample 4 which was primarily cooled in air which provided the faster cooling rate than that of sand. As a result, the plastically deformed zones were almost negligible in the image. Only dimples could be observed. These symptoms proved the less ductile behavior of sample 4. Figure 11(e) is an image of sample 5. This is the sample which was first cooled in sand then in water. Due to slow cooling at the initial stage, some plastically deformed zones were reported along with an area which was full of pores and dimples over there. Although these are the indications of good ductility, the extension provided by sample 5 during the tensile test is very less.
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3.4. Hardness Test Results
As already mentioned above, the Vickers microhardness test was carried out on the samples. According to the microscopic images, three zones (BM, HAZ, and WZ) were investigated for hardness variation. In each zone, three different indentations were made, and their average values were calculated. The hardness values of sample 1, sample 2, sample 3, sample 4, and sample 5 are shown in Tables 1–5, respectively.
The original sample, water-quenched sample, and stepped-cooled sample showed the highest hardness at the welded zone, whereas both the tempering processes have made the heat-affected zone harder than other parts. If only the welded zone is focused, then all the heat-treated samples had shown an increment in hardness in comparison to the original sample. The percentage improvement in hardness at the welded zone of water-quenched, water + sand-cooled, water + air-cooled, and sand + water-cooled is 9%, 10%, 5%, and 1%, respectively. A comparative bar chart of hardness, with respect to microstructure, in various zones is shown in Figure 12.
4. Discussion
The present work provides a comparative assessment among quenching, tempering (two types), and stepped heat treatment on the basis of tensile strength and hardness results along with a detailed microstructural appearance. Based on the present experimental work, a comparative analysis of mechanical properties is discussed in Table 6. The fundamental behind this work was to determine the difference between tempering and stepped heat treating. The problems generated by quenching are removed by tempering methods, but the tempering involves a repetitive work, heating, holding, and cooling again. Besides a time consuming work, this may cause high energy loss also. Hence, to find an alternative of tempering, step heat treatment has been adopted in this work. Although the results of stepped heat treatment have not been achieved satisfactorily, an idea of further improvement is developed. In future, the approach of tempering and stepped heat treatment will be significantly varied by increasing the number of samples.
Plain TIG welding may sometimes lead to formation of poor microstructures and distorted weld properties. The idea of using a filler metal in keyhole-TIG has overcome the problem of poor properties. As suggested by Fei et al. [25] and Su et al. [26], the use of the filler material has remarkably enhanced the capability of the keyhole-TIG process.
5. Conclusion
An attempt has been made to critically analyze the microstructure, tensile strength, and hardness of the TIG-welded joints in SS-304 plates with and without heat-treated conditions. Also, a comparative measure has been provided to find out the suitable heat treatment process. The following conclusion can be drawn:(i)The tensile strength of SS-304-welded joint gets reduced after heat treatment. A reduction of 7.6%, 0.2%, 6.1%, and 7% in tensile strength was observed in sample 2, sample 3, sample 4, and sample 5, respectively, although the sample 3 (quick water sprinkle + sand cooling process) has a comparable result with that of the original sample.(ii)The tempering process has significantly improved the ductility of the sample. It has been established by virtue of 23% enhancement in elongation of sample 3. However, other treatments have shown a negative result on elongation.(iii)As per the hardness test results, all the heat treatments have increased the hardness of the welded joint which is nearly 9%, 10%, 5%, and 1% improvement in sample 2, sample 3, sample 4, and sample 5, respectively.(iv)The heat treatment has made significant changes in the microstructural appearance of the samples. As soon as the cooling becomes faster, the δ-ferrite gets precipitated in the form of dendrites. The third sample, which was primarily cooled in sand, has shown very coarse γ-grains with δ-ferrites dispersed as dots throughout it. The heat treatment has created a widely spaced lathy structure and a less dendritic structure of δ-ferrite in the HAZ. In base metal parts, the heat treatment has made the γ-matrix coarse and equiaxed. Also, twin formation is restricted due to heat treatment.
Data Availability
All the data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.