Effect of welding parameters on the mechanical and metallurgical properties of armor steel weldment

The effect of welding parameters on the mechanical and metallurgical properties of armor steel weldment was studied using gas tungsten arc welding process with two filler wires: carbon steel and austenitic stainless-steel filler wires (AWS A5.18 ER 70S-6 and AWS A5.9 ER 307, respectively). The joint configurations were mainly single V and single bevel grooves. Using both AWS A5.18 ER70S-6 carbon steel filler wire and AWS A5.9 ER307, austenitic filler wire for the two joints passed the required tensile strength by the military standard. The joints successfully regained the base metal hardness at a distance of less than 15 mm. The ultimate tensile strength of the joints welded in a single bevel groove is 906 MPa which is higher than the joints welded using a single V groove is 865 MPa. This could be attributed to the increase in dilution percentage with the single bevel joints. Both single V joint and single bevel joints passed the required Charpy V-notch impact test whether using both carbon and stainless-steel wires. The effect of heat input and cooling rate on the mechanical and microstructure of welded joints was studied. The reduction of heat input caused a narrow HAZ with a small reduction in its hardness values with much reduction in the width of softening microstructure zone. SEM observations show that the base metal has a martensitic structure, but the weld metal microstructures depend on the filler: carbon steel or austenitic steel types. Using a single bevel groove with an austenitic steel filler metal, the weld metal shows a martensitic/austenitic microstructure by SEM observation, and this was verified by the Schaeffler diagram which showed a high dilution percentage (about 35% dilution). This resulted in a significant increase in joint strength and a higher weld metal impact resistance.

different welding speeds and heat inputs. Furthermore, the results revealed that the weldments with greater heat input value show poor ballistic performance [22,23]. Therefore, the welding parameter should be adjusted to get welded joint without cold cracking (high heat input and apply preheat temperature, i.e., reduce the cooling rate). Also, the reduction of the width of an over-tempered zone by increasing the cooling rate should be adjusted to satisfy the width requirements by the military standards [7,21]. External factor such as cooling by compressed air at gentle pressure of 6 bars may be introduced after completion of the welding pass to reduce the width of the softening zone.
Military standards demonstrate that the ultimate tensile strength of welded joints should exceed 750 MPa to be succeeded using carbon steel filler metal. However, they should exceed a minimum ultimate tensile strength of 550 MPa using austenitic stainless steel filler metal. The retainment of the base metal hardness value must be achieved at a distance of 16 mm measured from the centerline of the weld metal. The Charpy V-notch impact test results of both weld metal and heat-affected zone are also important [7,14,18].
GTAW is preferred because of its better weld properties suited for joining advanced steel plates [24], Through control in welding parameters such as welding voltage, welding current, shielding gas, and the metal flow rate, that controls the penetration of the joint [25,26].
Always in the study of welded joint properties of steel, the cooling rate is determined in a temperature range between 800 and 500 °C. However, in welding of Armox steel, the cooling rate between 600 and 200 °C is also very important to reflect the phase transformation of the HAZ as investigated by Aleksander and Katarina [27].
Because of the difficulty in performing ballistic tests, the assessment of welding performance will be according to the HAZ thickness. These articles stated [7,18,22,28] that welds with a HAZ width of 16 mm or less withstood the bullets fired thereon, while if the HAZ increases, the performance of the plate will fail, and thus, the hardness distribution and recovery are examined in detail for each sample from the weld metal centerline to the base metal [7,18,28].
Requirements of quality for the welding of MIL-A-46100 steel are given by MIL-STD 1185 [28] and SD-X12140 [29] standards. The requirements are divided into three categories: (i) mechanical properties, (ii) weld soundness, and (iii) in-service performance.
Requirements for weld soundness are stated in military standards [28] and in AWS D1.1 [30] and ASME Section IX [31] standards for requirements of a sound weld.
The main aim of the research is to improve armor steel weldment properties using GTAW process by reducing the width of the heat-affected zone to meet the requirements of military codes. Therefore, two types of filler metals will be applied, and also two types of joint configuration will be used. The effect of heat input and cooling rate will be studied in order to optimize the welding conditions leading to improving the performance of welded joints and meeting the requirements of military standards [7,8,18]. The results will be discussed on the basis of the mechanical and metallurgical properties of the welded joints.

Methods and experimental procedure
This section describes the used materials and experimental and testing methods.

Materials
The base metal was Armox 500T which is a commercial name for armor sheet metals with a dimension of 200 × 155 × 10 mm. The heat treatment includes austenitizing at 900 °C and then water quenching and followed by tempering at 200 °C [32]. The obtained base metal has high hardness and strength with acceptable toughness. The chemical composition and mechanical properties from the SSAB company specification of base metal are presented in Tables 1 and 2.

Filler metal types and their mechanical properties
Two filler metals are used with the GTAW process: one is a carbon steel AWS A5.18 ER70S-6, and the other is an austenitic type AWS A 5.9 ER 307. The selection was based on the results obtained in previous literature by [7,16,22,33] and filler metal manufacturers' specifications [19, 34,35] and requirements for the welding of armor steel according to MIL-STD 1185 standards [28]. Using austenitic filler metal is an advantage to decrease the susceptibility to cold cracking [16,33]. The welder tries to apply the lowest level filler wire welding current in order to reduce the deterioration of HAZ microstructure as a result of softening [6,7,16,18,22]. The chemical compositions and mechanical properties of all deposited weld metals are shown in Tables 3 and 4.

Welding joints fabrication and thermocouple location
Two different weld groove shapes namely single V and single bevel grooves were fabricated using machine CNC high-speed wire cut EDM (model FW 2U series -Switzerland). An    impermeable hole with a diameter of 1 mm was carried out in the centerline of specimens at a distance of 2.89 mm from the root edge of the groove as shown in Fig. 1 with a depth of 3 mm from the bottom of the sample, using CNC electrical discharge-forming machine (model DM 450 series -China). A K-type thermocouple is inserted in the hole to measure the temperature change in the heat-affected zone (HAZ).

Welding process and shielding gas
The GTAW process is applied using a welding machine type (Fronius Magic Wave 3000). The welding parameters and conditions are listed in Tables 5 and 6, respectively. A 100% argon gas is applied in the case of using AWS A 5.9 ER 307 austenitic stainless steel filler metal with a flow rate of 10 l/min. Argon back shielding is applied in the case of using AWS A5.9 ER307 filler metal with a flow rate of 8 l/min. And also, a 100% argon gas is used in the case of AWS A 5.18 ER 70S-6 carbon steel filler metal with a flow rate of 10 l/min.   represents the heat inputs at different welding conditions. Cooling is applied immediately after completion of each pass by using compressed air with a pressure of 6 bars. *The heat input is determined using Eq. (1) Hi: Heat input (kJ/mm) V: Arc voltage (Volts) I: Current (Amperage) μ: Thermal efficiency (GTAW = 0.6) S: Travel speed (mm/min)

Metallurgical properties of the joints
An optical emission spectrometry equipment is used to determine the chemical compositions of base metal and weld metals. Etching using a 2% nital solution was used   (2) Dilution (%) = A + C A + B + C * 100 Fig. 4 a Location of hardness distribution measurements of the welded specimen [16]. b Location of V-notch of the welded specimen for impact test [36]

Destructive and nondestructive testing of welded joints
A dye penetrant test (PT) was applied to the welded joints to observe the surface defects. However, a magnetic particle test (MT) was applied to the joints welded with AWS A5.18 70S-6 only to observe the existence of surface defects. A radiographic test (RT) was applied to observe the existence of internal welding defects. Hardness distribution measurements were done using a Vickers hardness testing machine (DVK-2 Tokyo Japan) with a 20 kg load for 70 s. The hardness profile through weld metal, HAZ, and the base metal will be determined with a pitch of measurement position of 1 mm as shown in Fig. 4a; the test has been carried out according to ASTM standards [37]. Transverse tensile tests have been carried out using a tensile machine (universal testing machine WDW-300 China). The test has been carried out according to ASTM A370-12 [38], using two specimens. Charpy V-notch impact test was done using a Charpy V-notch impact testing machine using a low-temperature chamber at −40 °C and also at room temperature. Charpy impact tests in the weld metal and HAZ were performed as shown in Fig. 4b according to ASTM E23-12 c standard [39,40]. Five readings were taken in each location.

Base metal composition and their mechanical properties
The chemical analysis of base metal from SSAB company specification and actual analysis for base metal are presented in Table 7. The mechanical properties are shown in Table 8.

Effect of cooling rate of welded joints
As mentioned in "Introduction", the cooling rate between 600 and 200 °C is also very important to reflect the phase transformation of the HAZ as investigated by Aleksander and Katarina [27]. Figure 5 shows the cooling rate of welded samples without using compressed air after completion of welding. The cooling time (Δt 8/5) of the first pass is 35 s, and its cooling  rate (Δt 8/5) is 8.5 °C/s. The cooling rate (Δt 8/5) of the second, the third, and the fourth passes is 10 °C/s. The cooling time (Δt 6/2) of the first pass is 235 s, and its cooling rate (Δt 6/2) is 1.72 °C/s. The cooling rate (Δt 6/2) of the second, the third, and the fourth passes is 1.63 °C/s, 1.73 °C/s, and 1.73 °C/s, respectively. Without the application of cooling, the hardness distribution shows a value of 389 HV at a distance of 17 mm from the center of weld metal. This did not satisfy the requirements of military standards [7,8,14,18] (Fig. 17, "Hardness distribution results"). The same results were obtained with Morsy Amin Morsy et al. [7] in welding with the GMAW process using ferritic and austenitic wires. Visual, PT, and RT show crack-free welded joints without any defects. Figure 6 shows the cooling rate at the HAZ with the application of compressed air immediately after completion of welding. The cooling time (Δt 8/5) of the first pass is 25 s, and its cooling rate (Δt 8/5) is 12 °C/s. The cooling rate (Δt 8/5) of the second and the third passes is 12 °C/s. The cooling time (Δt 6/2) of the first pass is 110 s, and its cooling rate (Δt 6/2) is 3.63 °C/s. The cooling rate (Δt 6/2) of the second, the third, and the fourth passes is 3.53 °C/s, 3.63 °C/s, and 3.43 °C/s, respectively. With the application of cooling, the hardness distribution shows 450 HV at 14 mm from the center of welded metal, which satisfy the requirements of military standards (Fig. 15, "Hardness distribution results").
Visual, MT, and RT testing show crack-free welded joints without any defects. This indicated that the gentle cooling by compressed air after each pass with a pressure of 6 bars did not show any cracks and it is safe. The same results were obtained in the work of Aleksandar Cabrilo et al. [27] using GMAW with AWS A5.9 ER307 without application of compressed air. They left the joint to be cooled in atmospheric air after each pass. Figure 7 shows the cooling rate at the HAZ with the application of compressed air at the completion of welding. The cooling time (Δt 8/5) of the first pass is 50 s, and its cooling rate (Δt 8/5) is 6 °C/s. The cooling time (Δt 6/2) of the first pass is 100 s, and its cooling rate (Δt 6/2) is 4 °C/s. The cooling rate (Δt 6/2) of the second, the third, Visual, MT, and RT testing show crack-free welded joints without any defects. This indicated that the gentle cooling by compressed air after each pass with a pressure of 6 bars did not show any cracks and it is safe. Figure 8 shows the cooling rate at the HAZ with the application of compressed air at the completion of welding. The cooling time (Δt 8/5) of the first pass is 30 s, and its cooling rate (Δt 8/5) is 10 °C/s. The cooling rate (Δt 8/5) of the second and the third passes is 12 °C/s. The cooling time (Δt 6/2) of the first pass is 120 s, and its cooling rate (Δt 6/2) is 3.3 °C/s. The cooling rate (Δt 6/2) of the second, the third, and the  Visual, PT, and RT testing show crack-free welded joints without any defects. This indicated that the gentle cooling by compressed air after each pass with a pressure of 6 bars did not show any cracks and it is safe. The same results were obtained in the work of Aleksandar Cabrilo et al. [27] using GMAW with AWS A5.9 ER307 filler wire. They did not apply compressed air; however, they left the joint to be cooled in atmospheric air after each pass. Figure 9 shows the cooling rate at the HAZ with the application of compressed air at the completion of welding. The cooling time (Δt 8/5) of the first pass is 30 s, and its cooling rate (Δt 8/5) is 10 °C/s. The cooling time (Δt 6/2) of the first pass is 80 s, and its cooling rate (Δt 6/2) is 5 °C/s. The cooling rate (Δt 6/2) of the second, the third, and the fourth passes is 2.5 °C/s, 2.6 °C/s, and 2.7 °C/s, respectively. With the application of cooling, the hardness distribution shows a 455 HV hardness value at 13 mm from the center of welded metal with a value of 455 HV, i.e., which satisfy the requirements of military standards (Fig. 18, "Hardness distribution results").
Visual, PT, and RT testing show crack-free welded joints without any defects. This indicated that the gentle cooling by compressed air after each pass with a pressure of 6 bars did not show any cracks and it is safe. Figure 10 shows the dilution percentages of the single V and single bevel groove joints with the GTAW process using both AWS A5.18 ER70S-6 and AWS A5.9 ER 307 filler wires. The dilution in the single V joint was less than in the single bevel joint using two types of filler metals. The dilution greatly affects the mechanical and metallurgical properties of weld metal that deviate from the pure filler wire properties. This can be observed in the tensile strength of welded joint shown in Table 9 ("Tensile strength"), and the microstructure of weld metal is shown in Figs. 25 and 26 ("SEM characterization of welded joints"). The existence of martensite structure greatly affects the improvement of tensile strength. This can be explained using the Schaeffler diagram [40]. The same results were obtained in the study of Sun Y. L. et al. [41]. They discovered that the dilution in multi-pass steel welds using GTAW and SAW processes increases in dilution that favors martensite formation. The joint configuration has a significant effect on the increment of dilution rather than the increase of heat input (using AWS A5.9 ER307 filler wire) as shown in Fig. 10. Figure 11 shows the Schaeffler diagram indicating two different dilution levels using a single V groove (16.2% dilution) and single bevel groove (35% dilution) when applying AWS A 5.9 ER 307 as a filler metal. The figure indicates that the microstructure of the weld metal is mainly austenite and delta ferrite using a single V groove (16.2% dilution) and is mainly martensite and austenite using a single bevel groove (35 % dilution). This was confirmed with the microstructure observations shown in Figs. 22 and 23 and SEM observation shown in Fig. 26. Table 9 and Fig. 12a show the results of the tensile strength of welded joints. The higher tensile strength was obtained using carbon steel and austenitic stainless steel filler wires with a single bevel groove. The dilution of weld metal causes a great increase in the tensile strength of welded joints. All the welded joints were fractured at the weld metal as shown in Fig. 12b; Aleksandar Cabrilo et al. [27] studied the welding of PROTAC 500 armor steel, and they could obtain a tensile strength of about 830 MPa when they left the specimens cooled in the air after each pass. However, cooling by compressed air resulted in a higher ultimate tensile strength (906 MPa). Using a single bevel groove resulted in an increase in the ultimate tensile strength of the joint by about 5% more than using a single V-groove joint. However, using austenitic filler metal AWS A5.9 ER307 resulted in an increase in the joint strength by about 10% more than using carbon steel filler metal AWS A5.18 ER70S-6.  An application of cooling resulted in an increase in the joint strength which may reach to about 10%. Table 10 and Fig. 13a show the Charpy V-notch impact tests results that were conducted at room temperature and at −40 °C in the weld metal region. In addition, the test was performed in the heat-affected zone. These results show that the toughness energy values using austenitic weld metal are higher than that obtained using carbon steel. It can be noted that a single bevel groove for all specimens achieved the highest values for impact energy as shown in Fig. 13a and Table 10. All weld metal notched impact specimens were fractured at weld metals as shown in Fig. 13b. However, all HAZ notched impact specimens were fractured at HAZ as shown in Fig. 13c. The same results were obtained with Tekin Özdemir [42]. However, in our finding, the impact resistance results are higher than that of Tekin Özdemir's finding. At room temperature, using a single V groove resulted in an increase in weld metal CVN impact strength to about 20% with carbon steel filler metal and to about more than 30% with an austenitic filler metal.

Welded joint shapes using different wires and their joint configurations
The welded joints are all homogenous without the occurrence of defects, inclusions, or cracks. Penetration of all weld passes is sufficient, and no lack of fusion was noticed in all samples. This was confirmed by a radiographic test for all the samples. All the samples were accepted. Also, a magnetic particle test (MT) was applied for specimens welded with ferritic filler wire and dye penetrant test (PT) is applied to the specimens welded with austenitic filler wire. No cracks were observed in all the specimens welded with or without the application of compressed air. Figure 14a, b, c, and d shows the welded joint weld faces and their macrostructures. Table 11 and Fig. 15 show the hardness distribution of the joint welded using AWS A5.18 ER70S-6 filler wire and applying a single V-groove joint. Without cooling, the hardness distribution at the mid-depth of the cross section of the specimen shows the lower base metal hardness values. The application of cooling resulted in the significant retainment of base metal hardness at a distance of 14 mm (450 HV) from the centerline of weld metal. At the root, the application of cooling resulted in high weld metal hardness. This could be attributed to the higher dilution of weld metal from base metal and the high cooling rate.

Hardness distribution results
In the previous papers [7], the plate thickness was 6 mm (SEWBOR 500), and the retained hardness was obtained at 15 mm without application of cooling. Aleksandar Cabrilo et al. [27] applied GMAW on an 11-mm-thick plate (PROTAC 500). The retainment of hardness was obtained at 14 mm from the centerline of weld metal. Cooling was not applied; however, the specimen was left to cool after each pass to about 150 °C (interpass temperature was 150 °C) [27]. Magudeeswarana V. G. et al. [16] used a 14-mm plate (AISI 4340) thickness with different welding processes and filler metals. Reduction of heat input to 1.33 kJ/mm resulted in the retainment of base metal hardness after about 13 mm [16]. Table 12 and Fig. 16 show the hardness distribution of the joint welded using AWS A5.18 ER70S-6 filler wire and applying a single bevel groove joint. Without cooling, the hardness distribution at the mid-depth of the cross section of the specimen shows a slight increase in the hardness values at the HAZ and an abrupt decrease in the values at the base metal. With cooling, the base metal hardness was retained at a distance of 13 mm (455 HV) from the weld metal center. This is attributed to higher dilution and higher cooling rates. At the root, application of cooling resulted in a significant increase in the hardness values at the root that reaches to 510 HV at a distance of 7 mm from the weld metal centerline.
Our finding proves that the cooling by using gentle compressed air after each pass resulted in a retainment of base metal hardness after a distance of 7 mm at the root and at 13 mm at the mid-depth compared with the finding of Aleksandar Cabrilo et al. [27] and Magudeeswarana V. G. et al. [16]. Table 13 and Fig. 17 show the hardness distribution of the joint welded using AWS A5.9 ER 307 filler wire and applying a single V-groove joint. Without cooling, the hardness distribution at the mid-depth of the cross section of the specimen shows an increase in the hardness values without retainment of the base metal hardness at a distance less than 16 mm. With the application of cooling, the base metal hardness was retained at a distance of 15 mm (460 HV) from the weld metal center.
At the root, the application of cooling resulted in the high hardness values at the root and the retainment of base metal hardness at a distance of 4 mm (473 HV) from the weld metal centerline. This could be attributed to the higher cooling rates and the high dilution of weld metal.
In the previous paper [7], the retainment of hardness value at 16 mm without cooling could be attributed to the low heat input (about 0.8 kJ/mm) applied to 6-mm thick plate compared with 1.3 kJ/mm in this research work applied to 10-mm-thick plate. The application of compressed air is beneficial to compensate for the high heat input applied in this thick plate (10 mm).  Table 13 Hardness distribution of the joint welded using AWS A5.9 ER 307 filler wire and applying a single V-groove joint  Table 14 and Fig. 18 show the hardness distribution of the joint welded using AWS A5.9 ER 307 filler wire and applying a single bevel groove joint. Without cooling, the hardness distribution at the mid-depth of the cross section of the specimen shows an increase in the hardness values that occurred in the HAZ with a sudden decrease in the hardness at the base metal region. Application of cooling resulted in the significant retainment of base metal hardness at a distance of 13 mm (455 HV) from the centerline of weld metal.
At the root, the application of cooling resulted in a significant increase in the hardness values at the root that reaches to 522 HV at a distance of 5 mm from the weld metal centerline.
The application of cooling is necessary because trying to decrease the heat input to a lower value in the welding of this 10-mm-thick plate failed. Increase of heat input to about 1.4 kJ/mm and application of compressed air resulted in the success of the welding procedure and attainment of the base metal hardness value at a distance of 13 mm. The low heat input applied in welding of 6-mm-thick plate [7] resulted in the success of welding procedure without cooling by any means such as compressed air.
The same results were obtained by DAVID ROBLEDO et al. [18]. The increase in the rate of cooling was conducted by applying copper backing using GMAW process with a heat input of about 1 kJ/mm. This resulted in a reduction in the heat-affected zone and retainment of base metal hardness at a distance of about 9 mm from the center of the weldment. Microstructure analysis of welded joints Figure 19 shows the microstructure of the base metal. It is tempered martensite with retained austenite. Figure 20a and b shows the microstructure of the weld metal at the upper pass using carbon steel wire in the single V-joint groove with the cooling application. It is a columnar structure with ferrite and pearlite with coarse grain. However, at the root pass, the microstructure is a cellular structure with a bainitic structure as shown in Fig. 20b. Figure 20c shows the existence of an over-tempered  microstructure, and that indicated the rise of this region's temperature to a high value lower than the austenitizing temperature yet over-tempering temperature. Figure 20d shows also the existence of the martensite structure in the HAZ, which  indicates that this zone reached a temperature of about 1250 °C and cooled rapidly to form these phases. Figure 21a shows the microstructure of the weld metal at the upper pass using carbon steel wire in a single bevel joint groove with the application of cooling. It is a columnar structure. Figure 21b shows the weld microstructure of the weld metal at the root pass. It is a bainitic structure with some acicular ferrite. Also, this figure shows microstructure at the HAZ in the root area. It is a martensitic structure. Figure 21c shows the weld metal and HAZ at the upper pass, the weld metal shows the ferrite-pearlite structure, and the HAZ shows the over-tempered structure. Figure 21d shows the existence of over-tempering microstructure near the base metal zone, and this indicates the rise of this region's temperature to a high value lower than the austenitizing temperature yet over-tempering temperature. Figure 22a shows the microstructure of the weld metal at the upper pass using AWS A5.9 ER 307 filler wire and a single V-joint groove with the application of cooling. It is an austenite structure. Figure 22b shows the existence of a martensitic microstructure in the area of weld metal too close to the base metal indicating the high percentage of dilution of the weld metal. Figure 22c shows the microstructure of the HAZ grain coarsening martensitic structure. The grain coarsening region reached a temperature of 1250 °C which causes grain coarsening. This was proven by the measuring of hardness in this region that reached 324 HV. Figure 22d shows the existence of an overtempering microstructure in the HAZ. Figure 23a and b shows the microstructure at the upper pass of the weld metal and heat-affected zone using a single bevel groove with the application of cooling. The weld metal at the center shows an austenitic structure with a hardness value of 272 HV as shown in Fig. 23a. Figure 22b shows the existence of a martensitic microstructure in the area of weld metal too close to the base metal indicated the high percentage of dilution of the weld metal. Figure 23c shows the microstructure at the HAZ in the grain coarsening zone containing a martensitic structure. Figure 23d shows the over-tempering zone (softening zone) in the HAZ.
In all specimens, the degree of softening of the HAZ depends of the weld thermal cycle, which is a function of heat input and cooling rate. This degree of softening in HAZ depends on the kinetics of phase transformation in welded steel [43,44]. Figure 24 shows SEM observations of joint welded using AWS A5.18 ER70S-6 filler wire and applying a single V-groove joint with cooling. Figure 24a shows an SEM micrograph of the base metal which shows the tempered martensite structure. Figure 24b shows the micrograph of the HAZ. It is a martensitic structure. Figure 24c and d shows SEM micrographs of weld metal region that consists of ferrite and pearlite. Figure 25 shows SEM observations of joint welded using AWS A5.18 ER70S-6 filler wire and applying a single bevel-groove joint with cooling. Figure 25a shows the micrograph of the HAZ that shows the over-tempered zone. Figure 25b and c shows SEM micrographs of weld metal region that consists of ferrite and pearlite. Figure 25d shows the weld metal too close to the base metal. It shows the martensitic structure. This was formed as a result of high dilution from base metal and the high cooling rate. Figure 26a shows the microstructure at the center of the weld metal using AWS A5.9 ER307 filler wire with a single V groove with cooling. Figure 26b shows the microstructure of the weld metal close to the base metal using a single bevel groove with cooling. It is a martensitic structure that was formed as a result of high dilution and high cooling rate.

Conclusions
In this investigation, a detailed mechanical and metallurgical characterization was carried out on Armox 500T armor steel welded joints using the GTAW process, and the following results can be concluded.
1. Using AWS A5.18 ER70S-6 filler wire, both single V joint and single bevel joints passed the required tensile strength by the military standard. Also, the joints successfully regained the base metal hardness (about 450 HV) at a distance of 14 mm and 13 mm respectively from the center of weld metal. 2. Using AWS A5.9 ER307 filler wire, both single V joint and single bevel joints passed the required tensile strength by the military standard. Both successfully regained the base metal hardness (about 455 HV) at a distance of 15 mm and 13 mm respectively from the center of weld metal. 3. The ultimate tensile strength of joints welded using a single bevel is 906 MPa which is higher than that of joints welded using a single V groove is 865 MPa. This could be attributed to the increase in dilution percentage in the single bevel joints. 4. Both single V joint and single bevel joints passed the required Charpy V-notch impact test whether using carbon or stainless-steel wires. 5. The effect of heat input and cooling rate on the mechanical and microstructure of welded joints was studied. Reduction of heat input caused the narrow HAZ with a small reduction in its hardness values with less softening of its microstructure. 6. Using a single bevel groove with an austenitic steel filler metal, the weld metal at the area close to the base metal shows the formation of martensitic/austenitic microstructure as shown by SEM observation and verified by Schaeffler diagram (35% dilution).
Abbreviations CVN Charpy V-notch Δt 8/5 The difference in temperature between 800 and 500 °C Δt 6/2 The difference in temperature between 600 and 200 °C SEM Scanning electron microscope PT Dye penetrant testing RT Radiographic testing MT Magnetic particle testing