Development and design of microstructure based ...

Materials and Design 103 (2016) 52–62

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Development and design of microstructure based coated electrode for ballistic performance of shielded metal arc welded armour steel joints A.K. Pramanick a, H. Das b, G.M. Reddy c, M. Ghosh d, G. Das d, S. Nandy e, T.K. Pal b,⁎ a

Forge Technology Department, National Institute of Foundry & Forge Technology (NIFFT), Ranchi 834003, India Metallurgical and Material Engineering Department, Jadavpur University, Kolkata 700032, India Defense Metallurgical Research Laboratory (DMRL), Hyderabad 500 058, India d National Metallurgical Laboratory (NML), Jamshedpur 831007, India e Rukhmani Electrode Pvt. Ltd, Badu, Barasat, Kolkata 700128, India b c

a r t i c l e

i n f o

Article history: Received 17 December 2015 Received in revised form 16 April 2016 Accepted 18 April 2016 Available online 20 April 2016 Keywords: Developed coated electrode SMAW Microstructure Ballistic performance and mechanical properties

a b s t r a c t Joining of high strength armour steel plate developed for lighter armour vehicles using both austenitic and ferritic type of electrode/filler wire could show inferior ballistic performance of the welds. Combination of hard facing electrode and softer Austenitic Stainless Steel electrodes, though provided improved ballistic performance, might not satisfy main design criteria such as strength and toughness due to non-homogeneous microstructure of weld metal. In order to satisfy improved ballistic performance as well as other design parameters, it is highly desirable to develop coated electrode which will attribute more homogeneous microstructure in weld metal similar to high strength armour steel plate. The present investigation has attempted to obtain martensite, bainite and some amount of retained austenite in the weld metal by proper design of alloying elements in the coating of SMAW electrode based on metallurgical index. Three different electrodes attributing different combination of microstructural constituents in armour steel weld metals exhibited excellent ballistic performance combined with other design criteria such as strength and toughness at −40°C. However, among the different microstructural constituents lower bainite along with retained austenite showing maximum toughness both at room temperature and at −40°C could be considered as most desirable microstructure in armour steel weld metal. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Armour grade quenched and tempered (Q & T) steel closely confirming to AISI 4340 is primarily used for highly stressed structures in many critical defence applications and non-military vehicles such as the construction of the hull and turret of combat vehicles and provides several potential advantages including their superior ballistic performance [1,2] lower weight, and manufacturing costs, ease of handling and transport etc. Furthermore, these highly mobile, metallic structures must be resistant to cracking, spalling and fracture upon multiple impacts from a wide range of projectiles selected for each class of vehicle from the full range of antitank weapons available today [3]. Thus ballistic performance which is defined as the maximum resistance against projectile penetration [4,5] is the most important criteria for armour grade Q& T steel. It is generally considered that the harder the steel, the better is the resistance to penetration [6]. Recent work by Maweja and Stumpf [7,8] on armour steel plate has shown that neither a higher hardness ⁎ Corresponding author at: Welding Technology Centre, Metallurgical and Material Engg. Dept., Jadavpur University, Kolkata 700032, India. E-mail addresses: [email protected] (A.K. Pramanick), [email protected] (T.K. Pal).

http://dx.doi.org/10.1016/j.matdes.2016.04.058 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

nor higher mechanical properties (Yield strength, ultimate tensile strength, impact energy, and % elongation) appears to be exclusive or even reliable criteria for predicting the ballistic performance. Alternatively, the microstructure based design has the advantage of addressing a direct relationship between the microstructure and ballistic performance. Predominantly twinned plate martensite with retained austenite was found superior in ballistic performance [7]. The percentage of retained austenite typically between 1% and 7% mainly governs the resistance to localised yielding of the armour steels and thus improves ballistic performance [5]. Shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) processes are widely used in the fabrication of combat vehicles [9]. The microstructural features in the weld and heat-affected zone (HAZ) have been shown to have a drastic influence on the ballistic performance of the joints. Several studies [10] were attempted to evaluate ballistic performance of the armour steel joints using different types of electrodes. For example, both austenitic and ferritic type of electrode/filler wire were attempted with varying heat input and weld joint only with lower heat input showed good ballistic performance. On the other hand, the weld joint with higher heat input having wider soft zone attributed inferior ballistic properties [10]. Again, for improved ballistic performance of armour steel plate joints, different weld procedure

A.K. Pramanick et al. / Materials and Design 103 (2016) 52–62 Table 1 Composition of flux ingredients used for coating. Sl. No

Flux ingredient

Weight %

1 2 3 4 5 6 7 8 9 10 11 12 13

FeTi powder FeMn (HC) powder FeCr (HC) powder Fe-powder Rutile powder CaCo3 powder Fluorspar powder Ni-powder Mo-powder FeSi powder White TiO2 powder Feldspar powder Graphite powder

14 15

Extruding agent Na-alginate Powder 1.5 CMC Powder 0.5

16 17

K-silicate Na-silicate

E1

E2

E3

4.72 4.72 4.72 1.88 3.77 26.65 21.95 11.35 1.35 7.55 1.88 7.56 –

3.80 6.75 6.75 1 4.80 27 22 10 2.2 5.75 2 5.75 0.2

3.80 6.75 6.75 1 4.80 27 22 9.5 2.2 5.75 2 5.75 0.7

1.5 0.5

1.5 0.5

14 4

14 4

Binding agent 14 4

using different combination of coated electrodes for a given joint were attempted [11]. In one such a joint, three different types of coated electrodes such as austenitic stainless steel (ASS) electrode for capping front layer, chromium carbide hardfacing electrode as interlayer and the ASS electrode for root layer were used and other joint was made with similar layers except low hydrogen ferritic (LHF) electrode as capping front layer. It was reported that in both the joints projectile was stopped totally by high hardness hard facing interlayer. However, between two different types of joints authors reported that LHF electrode for capping front layer provided better ballistic performance than ASS for capping front layer. This is mainly due to presence of acicular ferrite with bainitic structure. Similar investigation using different types of hardfacing electrodes such as chromium rich carbide and tungsten carbide were performed by M. Balakrishnan et al. [12] and S. Babu et al. [13] improved ballistic performance was reported. Although the previous work showed improved ballistic performance of the joints by introducing harder hardface deposit layer in between soft ASS layer, the yield and ultimate strength, % elongation at room temperature

53

and transverse Charpy impact energy at − 40 °C, which are the main design parameters for most armour steel plates [4], have not been evaluated. Thus it is highly probable that previous approach of improving ballistic performance of weld metal with non-homogeneous microstructure containing harder constituents may not satisfy other design criteria. In order to satisfy the other design parameters as well as improved ballistic performance, it is highly desirable to develop coated electrode with more homogeneous microstructure such as martensite, bainite and retained austenite in the weld metal. It is to be mentioned here that formation of twinned martensite with retained austenite, which showed improved ballistic performance in high strength armour steel, is commonly associated with higher carbon content along with other alloying elements [14]. But, the conditions prevailing in making steel (armour steel plate) and producing weld metal differ significantly. In making steel, close control of composition, removal of undesirable elements such as sulpher and phosporous and control of cooling rate after thermo-mechanical treatment are possible and hence it is relatively easier to obtain desired microstructure. But in case of weld metal, close control of composition, refinement, mechanical working and accurate control of cooling rate cannot be performed. Furthermore, weld metal with relatively higher carbon content (0.25 wt% C) and segregation of alloying elements encountered during the solidification leads to cracking [15–17]. This imposes constraint on the enrichment of alloying elements, particularly the carbon content and thus the electrodes are being produced by trial and error method based on the conceptual knowledge on alloy design. Although recent studies on improved performance of armour steel plate have opened up an arena regarding a number of critical issues, but it appears that hardly any published work attempted to develop a coated electrode exclusively based on microstructural based design to improve the ballistic performance of weld metal along with fulfillment of other design criteria of high strength armour steel plate. The main focus of the present work is to develop SMAW electrode with a systematic variation of alloying elements through coating composition of the electrode for achieving the target composition of high strength armour steel plate [4]. Then the butt welded joints of high strength armour steel plate was produced by the developed electrodes and the performance of the welds has been evaluated using ballistic testing, tensile testing, impact toughness at room temperature and at −40 °C. Finally the microstructures have been exclusively characterized by OM (optical microscopy), SEM (scanning electron microscope), TEM (transmission electron microscope) with SAD (selected area diffraction) pattern and XRD (X-ray diffraction) and correlated with mechanical properties and ballistic performance of the weld metals. 2. Experimental procedure An experimental electrode formulation was made by varying the flux ingredients systematically in the coating to achieve the target composition (0.37–0.43 wt% C, 0.5–1.8 wt% Mn, 0.6–1.2 wt% Si, 0.8–1.5 wt% Cr, 0.5–0.6 wt% Mo and 1.8–4.0 wt% Ni) [4]. The flux ingredients used in the experiment is given in Table 1. Careful considerations have been made for each change that would have an influence on the metallurgical properties of the resultant weld metal in the form of chemical composition and microstructure. The constituents of the coating were first weighed in various proportions according to the weight requirements of the selected composition. The extrusion process has been illustrated by the flow chart shown in

Table 2 Chemical composition of base metal (wt%). Element Fig. 1. Flow process chart for electrode extrusion.

C

Mn

Si

S

P

Cr

Ni

Mo

Fe

Base metal (AISI 4340) 0.29 0.49 0.22 0.007 0.007 1.43 1.58 0.40 Bal.

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A.K. Pramanick et al. / Materials and Design 103 (2016) 52–62

Table 3 Mechanical Properties of base metal. Base metal

YS (MPa)

UTS (MPa)

EL (%)

Hardness (Hv)

Toughness (J at 27°)

Toughness (J at −40°)

AISI 4340 (Q&T) steel

905.06

1002.25

23.08

350.3 ± 7

166 ± 2

140 ± 2

Table 4 Chemical composition of weld metals (wt%). WELD ID W1 W2 W3

C

Mn

Si

Cr

Ni

Mo

Ti

N

S

P

Cu

W

Fe

0.20 0.3 0.382

1.41 1.81 1.91

2.14 1.55 1.53

1.37 1.98 1.90

3.39 4.17 4.22

0.38 0.57 0.58

0.038 0.039 0.04

0.014 0.014 0.016

0.011 0.011 0.009

0.015 0.022 0.022

0.008 0.009 0.006

0.041 0.031 0.024

Bal. Bal. Bal.

Fig. 1. A silicate binder was used to easily extrude the flux ingredients on to the steel core wire. The steel core wire used in the experimental electrodes was a EWNR IS 2879 bare solid wire of 450 mm long with a diameter of 4 mm. The ratio between diameter of coated electrode and diameter core wire is called coating factor (D/d) which is maintained at1.5. The typical chemical composition of the core wire is C = 0.05%, Mn = 0.48%, Si = 0.018%, S = 0.018% and P = 0.022%. The moisture was removed from the covering by backing of the electrode and formed a hard covering over the steel wire. After extrusion the electrodes were first air dried for 48 h and then backed in backing furnace at 60 °C for 60 min, at 90 °C for 10 min, at 120 °C for 10 min, at 200 °C for 5 min, at 250 °C for 5 min, at 300 °C for 5 min, at 350 °C for 5 min and finally at 420 °C for 60 min. Three different coated electrodes (E1, E2 and E3) were developed to join the armour steel plates. The base metal used in this investigation was armour steel plate close to AISI 4340 specification. The base metal chemical composition and mechanical properties is shown in Tables 2 & 3. The armour plates of 12.5 mm thick were sliced into the required dimensions (175 mm × 75 mm) by abrasive cutters and grinding. Single ‘V′ butt joints with included angle 60° were prepared to fabricate the joints. The SMAW (Shielded Metal Arc Welding) process was used to join the armour steel plate in multipass (6 passes) using three different developed coated electrodes (E1, E2 and E3) and corresponding joints are referred as W1, W2 and W3. Weld procedure was developed to produce crack free weld by applying only preheat using E1 and E2

Fig. 2. XRD pattern of different weld metals.

Electrodes and both preheat and post weld heat treatment (PWHT) using E3electrode. Welding parameters including preheat temperature (150 °C for 1 h), current (140 A), voltage (23 ± 1 V), speed (1.82 mm/s) and PWHT temperature (600 °C for 1 h) were kept constant in order to study performance weldmetal for each electrode. The chemical analysis of the weld metal was found out using an optical emission spectrometer from three different spots on each weld metal and the average of three measurements was used as given in Table 4. The cross-section of each joint was cut from transverse direction of welding to produce weld specimens for microstructural examination, tensile testing and Charpy impact toughness. For microstructure study the specimens were ground on silicon carbide “wet–dry” papers to

Fig. 3. Optical micrographs of top zone a1 and middle zone b1 and Scanning Electron Micrographs of top zone a2 and middle zone b2 of W1 weld metal without PWHT.


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P120 to P1200 grade, and polished consecutively on cloth wheels using alumina 0.05 μm grades. The polished weld metal samples were finally etched with 2% nital. The microstructures were evaluated using an OM, SEM (JEOL JSM −5510). For detailed microstructural analysis of the weld metal, thin foil specimens for TEM were prepared from 3 mm diameter cylinders removed from the middle region of each weld metal. The discs were then jet electro polished using 10% perchloric acid in methanol which was cooled to −35 °C. It was engaged as a complement to electron diffraction to clarify and quantify the presence of retained austenite. XRD analysis was performed to estimate the volume percentage of retained austenite (RA) using Cu-Kα radiation with 24 kV and tube current was 40 mA. The specified 2θ range was varied from 10° to 90° with a step size of 0.5°/min. The retained austenite fraction was estimated by collecting the peak intensities of (200)γ, (220)γ, (111)γ, (110)α, (200)α and (211)α [18]. Typical diffraction patterns of different weld metals are shown in Fig.2. Microhardness survey were carried out on the three metallographic specimens taken across the weld metal at three different positions by using 100gf load with 10 s dwell time. The hardness was taken simultaneously after a distance of 0.5 mm in all the weld metal using Leco Micro Hardness tester (Model LM248SAT) and the hardness data were plotted against the distance. Tensile properties of welded joints such as ultimate tensile strength (UTS), yield strength (YS) and % elongation were evaluated by using transverse tensile weld specimens which were prepared by keeping the weld at center of the gauge length as per ASTM E8M standard. The tests were performed under uniaxial loading at a crosshead speed of 5 mm/min in universal tensile testing m/c (Instron 8862). The Charpy V-notch (Cv) impact toughness test of

Fig. 5. Optical micrographs of top zone a1and middle zone b1 and Scanning Electron Micrographs of top zone a2 and middle zone b2 of W1 weld metal with PWHT.

Fig. 4. TEM (a) bright field image, with corresponding (b) selected area diffraction pattern (SAD), (c) bright field image and (d) and dark field image of lower bainite and retained austenite (RA) and (e) SAD pattern for RA for W1 weld metal without PWHT.

the base metal and weld metals was carried out at room temperature (27 °C) and − 40 °C using standard specimens of square cross section (10 × 10 mm) confirming the ASTM E23 specification. In the weld metal the notch was located in the transverse direction to the weld centre line. Fracture surfaces of the freshly impact toughness test specimens were examined using scanning electron microscope to understand the micro mechanism of fracture. The detail experimental setup used for ballistic testing is available in the previous paper [19]. For evaluating ballistic performance the targets (weld metals) were tested as per the military standard (JIS.0108.01) in a ballistic testing tunnel at Defence Metallurgical Research Laboratory (DMRL), Hyderabad under standardized testing conditions. Ballistic performance of the weld coupons was evaluated by 7.62 mm armour piercing incendiary (API). The projectiles were fired through a rifled gun from a distance of 10 m. The target plate was kept at 0 ° from the normal impact. The striking velocity of the projectile was measured using infrared light emitting diode photo voltaic cells by measuring the time interval between the interceptions caused by the projectile running across two transverse beams at fixed distance apart. The probes being placed at 6 m and 8 m distance from the nozzle of the gun barrel. The first probe activates the timer and the second probe de-activates it. Few numbers of preliminary experiments were performed and adjustments were made to obtain the required impact velocity of the projectile onto the target. The velocities of deformable projectiles were about 780 ± 10 m/s, which were measured using infrared light emitting diode photovoltaic cells. The weld zones of all the joints were tested to evaluate its ballistic performance. The photographs of the ballistic test specimens after testing are shown in Fig. 13. The ballistic performance was evaluated by the depth of projectile penetration and whether the joint was perforated or not. For each fabricated target, at least one shot

56


was fired at weld metal (WM). The depth of penetration was measured using vernier depth gauge (MAKE: MITUTOYO, Japan, 0–150 mm). 3. Results and discussion In the present investigation, three coated electrodes were developed on the basis of microstructure based design and weld metals produced at constant welding parameters were tested as per design criteria for ballistic performance, hardness, tensile and Charpy impact at room and at − 40 °C. Microstructures were exclusively characterized using OM, SEM, TEM and XRD and correlated with mechanical properties. 3.1. Microstructure analysis Micro structural studies were performed using OM, SEM and TEM for three weld metals at two different positions (Top and middle zone). The optical microstructures of the top and middle zone of W1, W2 and W3 weld metals under different conditions (With and without PWHT) are shown in Figs. 3, 5, 7, 9 and 11 (a1 and b1). At the top of all weld metals, dendritic boundary region with finer lath-like microstructure at the prior dendrite boundaries developed during solidification, are clearly observed (Figs. 3, 5, 7, 9 and 11 a1). The microstructures of W1 and W2 (Without PWHT) show typical morphology of bainite/martensite with some inclusions which is characteristic of SMAW weld metals shown in Figs. 3 a1 and 7 a1. It is observed from optical micrographs

Fig. 6. TEM (a) bright field image, with corresponding (b) dark field image of Bainite and RA, (c) related SAD pattern and (d) bright field image of lower bainite and (e) SAD pattern for lower bainite of W1 weld metal with PWHT.

of W1, W2 and W3 weld metals (top portion) with PWHT shown in Figs. 5, 9 and 11 a1 that W1 and W2 mainly consist of bainite/martensite; whereas W3 appears to be only martensite. However, middle zone of both W1 and W2 without PWHT shows comparatively coarser dendritic structure with bainite/martensite (Figs. 3 and 7 b1) probably due to the effect of slow cooling experienced in the middle zone of multipass weld. Even after PWHT coarser dendritic structure with tempered bainite/martensite still exists in the middle zone of W1 and W2 (Figs. 5 and 9 b1). Whereas W3 consists of coarse dendrite with tempered martensitic type structure (Fig. 11 b1). The difference in microstructure with W3 compared to W1 and W2 suggests that relatively more hardenability elements and less Si content favour the formation of martensite. However, it is difficult to distinguish between martensite and bainite with optical microscopy as the morphology are very similar for both of them. That's why high resolution SEM/TEM images are performed for better identification. SEM micrographs of top and middle zone for all the three weld metals (W1, W2 and W3) with and without PWHT are shown in Figs. 3, 5, 7, 9 and 11 (a2 and b2). Both the zones of W1 weld metal however reveal lower bainite within the dendrites (Fig. 3 a2); whereas martensite lath within the dendrites is observed in W2 weld metal (Fig. 7 a2). After PWHT the microstructures of W1, W2 and W3 weld metals show comparatively coarser tempered bainite (Fig. 5 a2) and tempered martensite structure respectively (Figs. 9 a2 and 11 a2).Thus microstructural observation clearly shows that there is practically no difference in microstructure between top and middle position of the weld metals. TEM micrographs of W1 weld metal with and without PWHT are shown in Figs. 4 and 6. It is clearly seen from bright field and dark

Fig. 7. Optical micrographs of top zone a1 and middle zone b1 and Scanning Electron Micrographs of top zone a2 and middle zone b2 of W2 weld metal without PWHT.


57

Fig. 9. Optical micrographs of top zone a1 and middle zone b1 and Scanning Electron Micrographs of top zone a2 and middle zone b2 of W2 weld metal with PWHT. Fig. 8. TEM (a) bright field and (b) dark field image of Lath Martensite and RA, corresponding (c) selected area diffraction pattern and zone axis, (d) bright field and (e) dark field image of Twins of W2 weld metal without PWHT.

field image of Fig. 4 c and d that mainly lower bainite with some retained austenite (RA) between the bainite lath is present in W1 weld metal with PWHT. Bright and dark field images reveal tempered bainite lath after PWHT as shown in Fig. 6 a and b. Presence of bainite along with retained austenite has also been confirmed from the selected area diffraction pattern (Figs. 4 b, e and 6 c, e) with indices (110)α and (200)γ respectively. Whereas W2 weld metal without PWHT shows martensitic lath of around 100–200 nm width, twinning and presence of retained austenite between the martensite lath. Bright field and dark field image of lath martensitic with RA and twinning are shown in Fig. 8 a, b and d, e respectively. Selected area diffraction pattern with (220)γ, (111)γ and (110)α and zone axis conforms the presence of RA and martensite respectively (Fig. 8c). Weld metal W2with PWHT shows tempered and coarser martensite lath of around 250–350 nm width and retained austenite between the martensite lath. Bright and dark field images of lath martensite and RA are shown in Fig. 10 a and b respectively. Selected area diffraction pattern with (220)γ and (110)α identified the presence of RA and martensite respectively (Fig. 10 c). Weld metal W3 shows coarser lath martensite structure (Fig. 12 a) of around 500 nm width, twinning and retained austenite. Bright and dark field images of RA are shown in Fig. 12 c and d respectively. Selected area diffraction pattern with (220)γ and (110)α confirms the presence of RA and martensite respectively (Fig. 12 b). The microstructure of weld metal mainly depends on the chemical composition and cooling rate. In the present investigation welding

parameters were kept constant and thus cooling rate can be considered same for all the three weld metals. Essentially alloying elements in the weld metals have a significant role on the final microstructure. Each of the alloying elements have an effect; but the individual effect is

Fig. 10. TEM (a) bright field and (b) dark field image of Lath Martensite and RA, corresponding (c) selected area diffraction pattern and zone axis.

58


sometime difficult to determine due to the synergistic effects of other additions. Therefore, attempts have been made to provide calculated metallurgical index to characterize the weld metal microstructure. Interestingly, the Bs, Bf temperature as well as Ms and Mf temperature are all related to the chemical composition of the steel. The Ms and Bs temperatures were found out from the chemical composition using the following formula and are given in Table 5.

Ms ð °CÞ ¼ 539−423 ðC Þ−30:4 ðMnÞ−17:7 ðNiÞ−12:1 ðCrÞ−7:5 ðMoÞ

[20]

Bs ð °CÞ ¼ 830−270 ðC Þ−90 ðMnÞ−37 ðNiÞ−70 ðCrÞ−83 ðMoÞ

[21]

Interestingly as the percentage of carbon content in weld metals increases from W1 to W3, Bs and Ms temperature decrease in that respective manner and consequently the driving force for bainitic transformation also decreases [22].

Fig. 11. Optical micrographs of top zone a1and middle zone b1 and Scanning Electron Micrographs of top zone a2 and middle zone b2 of W3 weld metal with PWHT.

Fig. 12. TEM (a) bright field image of Lath Martensite and RA, corresponding (c) selected area diffraction pattern, (c) bright field and (d) dark filed image of RA, and (e) Twins W3 weld metal with PWHT.

Fig. 13. Photograph after ballistic test (a) W1 weld metal Without PWHT (b) W1 weld metal with PWHT, (c) W2 weld metal without PWHT, (d) W2 weld metal with PWHT and (e) W3 weld metal with PWHT.


59

Fig. 14. Hardness across weld centerline of (a) and (b) W1 weld metal, (c) and (d) W2 weld metal, (e) and (f) W3 weld metal.

From Table 5 it is clearly seen that W1 weld metal shows maximum difference between Bs and Ms temperature (87.66 °C) and thus attributes more amount of bainite. TEM micrograph depicts the enhancement of lower bainite. It is generally considered that bainite grows without diffusion, but because of the relatively high transformation temperatures involved, the excess carbon is partitioned into the residual austenite [23–25]. The carbon enrichment of the austenite can result in the subsequent formation of lower bainite. The remaining carbon stabilizes the austenite [26] and enhances the formation of retained austenite. According to Pickering's model, the only difference between upper and lower bainite is that lower bainite contains internal carbide with definite morphological texture [27] which is clearly seen in Fig. 4(c-d) and d indicating lower bainite. Also from the kinetics point of view face to face formation of carbide observed in W1 weld metal microstructure (Fig. 4c and d) confirms the presence of lower bainite [28]. On the other hand, W2 and W3 weld metals having very close difference between Bs and Ms temperature (7.92 °C and 17.2 °C) have favoured the formation of martensite over bainite. The formation of martensite, as expected, increases with the increase in carbon

content from 0.2%C in W1 to 0.3% C in W2 and 0.38%C in W3·This indicates that the driving force for the displacive decomposition of austenite decreases leading to the formation of martensite. The presence of higher concentrations of hardenability elements like Mn, Ni, V, Cr, and Mo in W2 and W3 also favoured the formation of martensite. Interestingly with increasing carbon content, Ms temperature decrease and more twinning is observed in W3 as shown in Fig. 12e. 3.2. Ballistic performance The ballistic limit is basically the velocity which is required for a particular projectile to reliably penetrate into a particular piece of armour or in other words, a given projectile which will not defeat a given target when the projectile velocity is lower than the ballistic limit. The utmost aspect and consideration of ballistic performance is whether the joints are perforated or not under the huge impact of 7.62 AP projectiles out of shots fired. The ballistic test results are mainly categorized into two forms i.e. bullet stopped without any damage at the rear side and bullet stopped but makes a smooth bulge at the rear side. Ballistic tests have

60


fact that more enrichment of alloying elements including carbon in weld metal W3 attributed greater extent of segregation during solidification of molten weld pool and the hardness results clearly indicate that even after PWHT such non- homogeneity still exists. 3.3.2. Tensile The transverse tensile test was carried out for each welded joint. It is clearly shown from the Fig. 15 that failure took place from the base metal for all the three welded joints with and without PWHT indicating that weld metal is stronger than base metal. Interestingly in spite of variation in microhardness value, which is an indication of strength, among the weld metals, all the weld metals show higher strength than base metal. These results clearly indicate the importance of microstructure based design of coated electrode for welding armour steel plate.

Fig. 15. Failure location in tensile test of welded joints (a) W1 weld metal without PWHT, (b) W1 weld metal with PWHT, (c) W2 weld metal without PWHT (d) W2 weld metal with PWHT and (e) W3 weld metal with PWHT.

been done on five butt weld coupons for each case. Fig. 13 shows the photographs of targeted plate after ballistic test and the results are accumulated in Table 6. All the targets (W1, W2 and W3) are successfully stopped the impact of 7.62 mm AP projectile with very shallow erosion of 0.5–1 mm and have been considered under first category with an excellent ballistic performance. From the ballistic performance test results it shows that the energy absorbed by the target weld metals with microstructures containing martensite, bainite and retained austenite is quite high. Also it can be concluded that microstructure based design coated electrode of one type can enable excellent ballistic performance in service. The new results are interesting, but it is necessary for similar experiments to be carried out on large pieces before the design engineer will be convinced.

3.3.3. Impact toughness The Charpy impact toughness results of weld metals at 27 °C and − 40 °C are presented in Table 7. The maximum toughness of 36 J at room temperature (27 °C) and 30 J at − 40 °C were obtained in weld metal W1without PWHT. It has been reported that lower bainite has the capability to play as a crack arrester and simultaneously the cracks will be blunted due to the presence of fine carbide particles [29]. Effectively crack propagation will be inhibited enhancing higher impact energy. Furthermore, W2 weld metal with and without PWHT shows higher toughness than W3 weld metal containing higher amount of retained austenite (Table 5). Many researchers have claimed that an increase in the amount of retained austenite produces an increase in both tensile strength and ductility, which enhances the toughness [30]. However, few studies simultaneously explored that the formation

3.3. Mechanical properties To satisfy the design criteria of armour steel, mechanical properties of three different weld metals have been evaluated and correlated with microstructures. 3.3.1. Hardness The micro hardness measurement across the three welded joints with and without PWHT was carried out in two different positions (Top and middle zone) as shown in Fig. 14. A significant variation in hardness is observed among the welds and this variation has been correlated well with weld metal microstructures. Comparing the hardness values of W1, W2 and W3 in Fig. 14 it shows that higher hardness is observed in weld metals having without PWHT compare to weld metals with PWHT at two different positions. Also from Fig. 14 it is clear that, W3 weld metal shows maximum hardness at two different positions compared to W2 and W1. Furthermore, as expected scatter of hardness showing the non-homogeneity decreases after PWHT. Interestingly less scattered in hardness distribution indicating more homogeneity is observed in weld metal W1. Since hardness is directly correlated with microstructure, the microstructure containing higher amount of bainite with lesser retained austenite in W1 should provide more homogeneity. It also appears that higher amount of martensite and retained austenite in the weld metal W3 is largely responsible for not only the higher magnitude of hardness but also the greater extent of non-homogeneity. This is mainly due to the

Fig. 16. SEM fractographs of Charpy specimen (a) W1 weld metal Without PWHT (b) W1 weld metal with PWHT, (c) W2 weld metal without PWHT, (d) W2 weld metal with PWHT and (e) W3 weld metal with PWHT tested at −40 °C.


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Table 5 Calculated the metallurgical index and % of retained austenite of different weld deposit. Weld ID

Weld condition

Martensite start temp. (Ms°C)

Bainitic start temp. (Bs°C)

(Bs–Ms) °C

% of retained austenite

W1 W1 W2 W2 W3

Without PWHT With PWHT Without PWHT With PWHT With PWHT

308.57 308.57 237.98 237.98 200.48

396.23 396.23 245.90 245.90 217.68

87.66 87.66 7.92 7.92 17.20

14.32 15.51 15.43 17.96 20.46

Table 6 Ballistic performance results. Weld ID W1 W1 W2 W2 W3

Condition

Velocity (m/s)

Location

Probability of perforation (%)

Ballistic performance


720 ± 10 720 ± 10 720 ± 10 720 ± 10 720 ± 10

Weld Weld Weld Weld Weld

0 0 0 0 0

Passed Passed Passed Passed Passed

of strain-induced martensitic from retained austenite during fracturing of notched specimens has an adverse effect on impact toughness [31]. The results of toughness values of the two weld metals (W2 andW3) probably indicate similar strain-induced martensitic transformation from retained austenite leading to lower toughness of W3 than W2 weld metal. Thus the three microstructure constituents developed in three different weld metals namely lower bainite + RA, twinned martensite + RA, coarser twinned martensite + RA have shown excellent ballistic performance and acceptable strength and low temperature toughness. However, among the three microstructures lower bainite + RA attributed maximum low temperature toughness. In the presence of internal twinning, the martensite is more likely to be deformed by mechanical twinning rather than slip [32,33]. It is well known that the plastic deformation is restricted when the deformation mode in mechanical twinning which subsequently results in low toughness. On the other hand, lower bainite having finer and more uniform distributed carbides, higher amount of dissolved carbon in bainite ferrite and fine bainite ferrite grains exhibited superior toughness [34]. Since the impact energy provides a good indication of the ability of armour steels to absorb the kinetic energy of the projectile, lower bainite structure could provide better ballistic performance than twinned martensite. The freshly fractured surfaces of the Charpy impact samples (tested at −40 °C) were examined under SEM to identify the mode of fracture. The SEM fractrographs of fractured W1, W2 and W3 weld metals impact specimens are shown in Fig. 16 (a and b) (SEM images of W1) shows mainly ductile dimples type of fracture surface indicating ductile behaviour. Whereas, Fig. 16 c shows mixed mode of dimple and quasicleavage type of fracture surface. On the other hand, Fig.s 16 d and e (W2 and W3 with PWHT) show cleavage type of failure justifying the presence of higher amount of brittle martensite.

4. Conclusions 1. The present investigation has shown that microstructure based design of coated electrode can exhibit excellent ballistic performance combined with other design criteria such as strength and toughness at −40 °C of armour steel weld metal. 2. It has been possible to achieve carbon content in the weld metal varying from 0.20 to as high as 0.38% along with other alloying elements close to the target composition by changing the basic flux composition. However, additional PWHT was performed in weld metal to avoid cracking when carbon content exceeded greater than 0.3%. 3. Three different weld metals produced by three developed coated electrodes attributed different microstructural constituents such as bainite, martensite and retained austenite which have been well correlated with chemical composition through microstructural indices such as Ms and Bs. A predominantly martensite microstructure is observed as the difference between Ms and Bs temperature decreases; whereas increase in difference between Ms and Bs temperature favours the formation of bainite. 4. Among the different microstructural constituents lower bainite along with retained austenite showing maximum toughness both at room temperature and at − 40 °C could be considered as most desirable microstructure in armour steel weld metal as the other performances remain similar. Acknowledgements The authors wish to record their sincere thanks to Dr. S. M. Shariff, Scientist — E, ARCI, Hyderabad, for granting permission for SEM characterization of the samples. References

Table 7 Result of Charpy impact test. Weld Metal W1 W1 W2 W2 W3

Condition

Avg. toughness (J at 27°) Avg. toughness (J at −40°)


36 33 32 29 26

30 28 26 24 21

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