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Effect of hardfaced interlayer thickness on ballistic performance of armour steel welds
Materials and Design 44 (2013) 59–68
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Technical Report
Effect of hardfaced interlayer thickness on ballistic performance of armour steel welds M. Balakrishnan a, V. Balasubramanian b,⇑, G. Madhusudhan Reddy c a
Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India Center for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India c Solidification Technology Division, Metal Joining Group, Defense Metallurgical Research Laboratory (DMRL), Kanchanbaugh, Hyderabad 500 058, India b
a r t i c l e
i n f o
Article history: Received 29 March 2012 Accepted 6 June 2012 Available online 2 August 2012
a b s t r a c t Due to weld thermal cycles and under matching fillers, the armour steel welded joints showed poor ballistic performance compared to the base metal. An attempt was made to enhance the ballistic performance of armour steel welded joints by depositing hardfaced (HF) interlayer between austenitic stainless steel weld metal. Though this method yielded marginal improvements in ballistic performance, cracks were observed in between base metal and HF layers. A novel method was proposed by the same authors to eliminate these cracks by depositing a soft buttering layer using austenitic stainless steel (ASS) consumable in between base metal and HF layer shows additional improvements on ballistic performance. Buttering layer enhanced the ballistic immunity by keeping the weld layer intact successfully at the time of impact, but the bullet was pierced through. In this investigation, an attempt has been made to stop the bullet completely by varying HF interlayer thickness. This paper reveals the effect HF layer thickness on ballistic performance of shielded metal arc welded armour steel welds. A HF interlayer thickness of 0.3 times, the plate thickness is found to be optimum from this investigation. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Armour grade quenched and tempered (Q & T) steel closely confirming to AISI 4340 is primarily used for construction of military and non-military vehicles, because of their high energy absorbing properties. The ballistic requirement of Q & T steels used for armour application requires high strength, greater notch toughness and high hardness [1–4]. The effect of heat-input on the heataffected-zone (HAZ) softening in Q & T steel and width of the soft zone and, in turn, its influence on hardness and ballistic performance was investigated by Reddy and Mohandas [5]. An increased heat-input resulted in a wide soft zone, leading to poor ballistic performance and this was attributed to softer microstructural constituents at this zone. Based on the work done by Alkemade [6] and bisalloy steels (high hardness armour steel) handbook it is recommended that heat input levels should be 1.0–2.5 kJ/mm and the preheat should be 50–75 °C. Q & T steels are prone to hydrogen induced cracking (HIC) in the heat affected zone (HAZ) after welding. The use of austenitic stainless steel (ASS) consumables to weld the above steel was the only available remedy because of higher solubility for hydrogen in austenitic phase [7]. ⇑ Corresponding author. Tel.: +91 4144 239734 (O), +91 4144 241147 (R), mobile: +91 9443412249; fax: +91 4144 238080/238275. E-mail addresses: [email protected] (M. Balakrishnan), [email protected], [email protected] (V. Balasubramanian), [email protected] (G. Madhusudhan Reddy). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.06.010
The resistance against projectile penetration of the various zones in the weldments of a high-strength low-alloy steel was investigated by Reddy et al. [8] through three welding processes, namely: shielded metal arc welding (SMAW); gas tungsten arc welding (GTAW); and flux cored arc welding (FCAW). In all the joints, the projectile passed through the weld metal region. However, the ballistic limit was found to be highest in the case of the shielded metal arc weld and least in respect of flux cored arc welds. HAZ of SMAW was resistant to penetration and exhibited the highest ballistic limit, whilst the HAZ of FCAW exhibited intermediate performance. The results of Choo et al. [9] indicated that the multi-layered hardfacing, i.e., the combination of hard and soft layers, might improve ballistic property through absorbing ballistic impact energy due to cracking of primary carbides and controlling the projectile traveling direction upright due to existence of the soft layer. But much remains to be learned concerning the hardfacing design and the improvement of the ballistic property of the armour plates. It was suggested that; therefore, future studies should include the new design of multi-layered armor plates and their evaluation, detailed parallel measurements of hardness, strength, and fracture toughness, and interpretation of microstructural factors by mutually related mechanical properties and hardfacing variables. An attempt was made to improve the ballistic immunity of austenitic welds through a hardfacing technique by SMAW process [10] and reported that sandwiching of the hardfacing alloy between austenitic
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cracking tendency of Q & T steel weldments [12]. Before making the joint fit up, the beveled edges of the double Vee groove was buttered with the ASS electrodes. The average buttering layer thickness was maintained to be 1.5 mm. The buttered layer was cooled to room temperature and the buttered edges were again beveled by grinding process as illustrated in Fig. 2. The joints were welded as per the sequence illustrated in Fig. 3. The welding parameters used to fabricate the joints are presented in Table 3. Necessary care was taken to avoid joint distortion and to get defect free welds. The preheating and interpass temperatures were maintained at 150 °C for both welding and hardfacing. Three different HF interlayer thicknesses (4 mm, 5.5 mm and 7 mm) were used in this investigation and the joints are named as AHA 4, AHA 5.5 and AHA 7 respectively. All the three joints were subjected to standard ballistic testing and their performances were evaluated. The standard ballistic test arrangement is shown in Fig. 4. The ballistic test procedure was dealt with elsewhere [5,8,10,11]. The ballistic performance was characterized by the depth of penetration of the projectile in the target plate. The depth of penetration of each bullet in the targets was measured by using vernier depth gauge (MAKE: MITUTOYO, Japan, 0–150 mm). The weldments were characterized by microstructure analysis and hardness measurement. The macrostructures were analyzed through stereo zoom macroscope (Make: Macscope Z Model CM 0646). The microstructure of the joint was analyzed at various locations using optical microscope (Make: MEIJI, Japan: Model ML 7100). The specimens were etched in 2% Nital for base metal and HAZ region. Aqua – Regia and Villella’s reagent was used to reveal the microstructure of ASS weld metal region and HF region respectively. The hardness measurement was carried out as per ASTM E-384-05 standards [13]. A Vickers microhardness testing machine (Model: HMV-2T and Make: Shimadzu, Japan) was employed to measure the hardness along the WCL and across the WCL with 500 g load for a dwell time of 15 s.
weld metal served the dual purpose of weld integrity and ballistic immunity due to the high hardness of hardfacing alloy and the energy absorbing capacity of austenitic weld metal. Welds with complete hardfacing shattered under ballistic impact. An overlay of hardfacing alloy over austenitic weld disintegrated due to extensive cracking in the hardfacing layer and in the interface between base metal and hardfacing. This is due to the brittle nature of the interface. However, to attain ballistic immunity equivalent to the base metal, a minimum thickness of hardfacing inter – layer is required. The primary aim of our early study [11] was to eliminate these cracks by depositing a soft buttering layer using ASS consumable in between base metal and HF layer above which a multi-layered structure was fabricated and evaluated. From the results, it was found that the buttering layer improved ballistic performance by the resulting microstructure and hardness distribution and thus kept the weld layers intact when the projectile was fired at interfaces and HAZ. However, at weld center line (WCL), further investigation was suggested with different HF layer thickness and joint configuration. Though numerous efforts was exerted in the past to increase the ballistic resistance of armour steel weldments by improving the hardness of materials with dubious success, but complete stopping of the bullet penetration is still a paramount goal. A novel technique to achieve this objective, believed to be investigated here for the first time, is the potential reduction of the momentum per unit area inherent in a perforating projectile by having the good combination of HF interlayers in between tough ASS weld layers. This paper reports the influence of HF interlayer thickness on ballistic performance of armour steel welds fabricated by SMAW process using ASS electrodes and chromium carbide forming hardfacing electrodes. 2. Experimental work The base metal used in this examination was 18 mm thick high strength low alloy Q & T steel closely conforming to AISI 4340 specification. The heat treatment applied was exposing the base metal to austenising temperature at 900 °C followed by oil quenching and subsequent tempering at 250 °C. This heat treatment is responsible for attaining high hardness, higher strength and good toughness in base metal. The chemical composition and mechanical properties of base metal and filler metals are presented in the Tables 1 and 2 respectively. In this investigation, unequal double vee joint configuration was prepared as shown in Fig. 1. SMAW process was selected because it offers highest ballistic limit and lower HAZ soft zone width [8]. ASS electrode was selected because it avoids time delayed
3. Results 3.1. Ballistic performance The ballistic test results are presented in Table 4. Figs. 5–7 shows the photographs of target plate AHA 4, AHA 5.5 and AHA 7 respectively after ballistic test. The ballistic test results are categorized as (i) bullet stopped without any damage at the rear side is referred as S1, while (ii) bullet stopped but makes a smooth bulge at the rear side is referred as S2. Fig. 5a is the front side view photograph and Fig. 5b is the rear side view photograph of AHA 4 joint. The corresponding locations of 1, 2, 3 and 4 in the front side view
Table 1 Chemical composition (wt%) of base metal and filler metals (all weld metal). Element
C
Mn
Si
S
P
Cr
Ni
Mo
V
Fe
Base metal (AISI 4340) ASS electrode AWS E 307-16 Hardfacing alloy AWS E FeCr–A7
0.35 0.08 4.0
0.54 3.3 1.0
– 0.90 1.50
– 0.015 –
– 0.04 –
1.25 20.30 30.00
1.75 8.50 –
0.52 1.5 2.0
– – 0.50
Bal Bal Bal
Table 2 Mechanical properties of base metal and filler metals (all weld metals). Base metal
Yield strength (MPa)
Tensile strength (MPa)
Elongation in 50 mm gauge length (%)
Hardness (HV0.5)
Charpy impact toughness at room temperature (J)
Base metal (AISI 4340) ASS electrode AWS E 307-16 Hardfacing alloy E FeCr–A7
1200 440 1420
1290 590 1510
12.5 30.0 4.0
450 280 710
49 65 39
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(a) Joint
(b) Sectionat A-A ALL DIMENSIONS ARE IN mm Fig. 1. Joint configuration.
ASS Buttering Layer
AISI 4340 STEEL
a. Before buttering
b. After buttering
ASS Buttering Layer AISI 4340 STEEL
(c) After grinding Fig. 2. Sequence of edge preparation.
Table 3 Welding conditions used for fabricating the joints.
Fig. 3. Schematic illustration for sequence (1–5) of welding. a.AHA JOINT (A-ASS filler, H – Hardfacing filler and B – ASS Buttering.
(Fig. 5a) are encircled and arrowed in Fig. 5b. The corresponding encircles shows the presence of smooth bulge at the rear side of the plate. In Fig 5a, the shot 1 was targeted at fusion line (FL), shot 2 was targeted at WCL, shot 3 was targeted at HAZ and shot 4 was targeted at base metal. The shot 1 and 2 was targeted at WCL, made
Parameters
Unit
Welding machine
Lincoln electric precision welding machine) mm 3.15 °C 150 °C 150 A 110 V 21 kJ/mm 0.6391
Filler wire diameter Preheat temperature Interpass temperature Welding current Arc voltage Heat input
Buttering
Hardfacing
ASS
tig™ 375 (multipurpose 4 150 150 160 23 1.05
4 150 150 153 22 0.9439
a smooth bulge (S2) at the rear side (Fig. 5b). Shot 3 was targeted at HAZ area, resulted S2 category result. The shot 4 targeted at base metal stopped the projectile and made a smooth bulge at the rear side (Fig. 5b). Fig. 5c–f are the magnified photographs of the corresponding locations as labeled in the plate itself. At these locations, the non-penetration of bullets is clearly visible from the
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Fig. 6a is the front side view and Fig. 6b is the rear side view of AHA 5.5 joint. Shot 1 was targeted at FL, stopped the projectile and allowed to a DOP of 16 mm only (Fig. 6a). Shot 2 was targeted at WCL and WCL successfully stopped the projectile (Fig. 6a). The shot 3 was targeted at base metal and it made a smooth bulge; encircled and arrowed at the rear side view of the target as shown in Fig. 6b. The shot 4 was targeted at HAZ area resulted in projectile shattering with a DOP of 16 mm. The weld layer offers higher resistance compared with other locations in the same target against the projectile penetration and made a DOP of 14 mm only. This can be clearly seen in rear side view photographs (Fig. 6b). Fig. 6c–f are the magnified views of the corresponding locations as labeled in Fig. 6a and is also evident of the non-penetration of the bullet at these locations. As in the previous case, all the shots at various locations of the target, fabricated with 5.5 mm HF interlayer successfully stopped the projectile. Fig. 7a is the front side view and Fig. 7b is the rear side view of AHA 7.0 joint. Shot 1 was targeted at WCL, stopped the projectile and allowing the bullet for a DOP of 16 mm only (Fig. 7a). Shot 2
Fig. 4. Ballistic testing arrangement.
photographs. The weld metal offered higher resistance than other locations in the same target against the projectile attack and allowed the projectile to enter a depth of penetration (DOP) of 14 mm only. All the shots at various locations of the target fabricated with 4 mm HF interlayer successfully stopped the projectile.
Table 4 Ballistic test results. Joint type
Shot no.
Velocity
Location
Result category
Depth of penetration (DOP) in mm
Illustration
AHA 4.0
1 2 3 4 1 2 3 4 1 2 3
821.90 828.09 823.86 839.46 826.31 832.81 827.40 827.82 817.96 825.12 821.02
Weld Weld HAZ Base metal Fusion line Weld Base metal HAZ Weld Weld HAZ
S2 S2 S2 S2 S1 S1 S2 S1 S1 S1 S2
14 15 15 16 16 14 16 16 16 16 15
Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped
AHA 5.5
AHA 7.0
S1 – bullet stopped and no bulge at rear side, S2 – bullet stopped and smooth bulge at rear side.
(a) AHA 4.0 joint front side
(c)
(d)
(b) AHA 4.0 joint rear side
(e)
(f)
Fig. 5. Photographs of various locations of ballistically tested AHA 4.0 joint (c–f–magnified views).
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(a) AHA 5.5 joint front side
(c)
(b) AHA 5.5 joint rear side
(d)
(f)
(e)
Fig. 6. Photographs of various locations of ballistically tested AHA 5.5 joint (c–f–magnified views).
(b) AHA 7.0 joint rear side
(a) AHA 7.0 joint front side
(c)
(d)
(e)
Fig. 7. Photographs of various locations of ballistically tested AHA 7.0 joint (c–e–magnified views).
located FL successfully stopped the projectile with a DOP of 17 mm only. The shot 3 was targeted at HAZ and it made a smooth bulge at the rear side is encircled in the rear side view photograph as shown in Fig. 7b. The DOP of bullet at this location
was 15 mm only. In this AHA 7 joint, the weld layer offers intermediate resistance as compared with other locations in the same target against the projectile penetration and made a DOP of 16 mm. This can be clearly seen in rear side view photograph
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(Fig. 7b). As in the previous two cases, all the shots at various locations of the target, fabricated with 7 mm HF interlayer also successfully stopped the projectile.
from defects such as lack of fusion, incomplete penetration and macro level cracks. The HAZ is also clearly visible. 3.3. Microstructure
3.2. Macrograph Fig. 8 shows the macrographs of three joints before ballistic testing. From these macrographs, it is clear that the joints are free
a. AHA 4 joint
The optical micrographs of various locations are presented in Fig. 9. The microstructure of base metal consists of acicular martensite structure with fine needles of lath martensite as shown in Fig. 9a [5]. The microstructures of various locations of the multi
b. AHA 5.5 joint
c. AHA 7 joint
Fig. 8. Macrographs of joint cross section.
(a) Base metal
(d) Interface- Buttering Hardfacing
(b) Undiluted Hard facing
(e) Interface-Hardfacing-ASSCap
(c) Interface-Buttering- BM
(f) g.ASS capping
(g) HAZ Fig. 9. Optical micrographs of base metal and various locations of welded joints.
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layered joint are shown in Fig. 9b–g. The HF region is made of the microstructure of homogeneous cast like structure as shown in Fig. 9b. It is composed of hard phase precipitates (Carbides) of
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hexagonal chromium carbides of different sizes on softer austenitic matrix [14]. Large amount of spine like carbides are also clearly visible in the microstructure of the HF layers in the joints. The weld
Fig. 10. Microhardness profile along the weld center line (D – diluted region).
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interface region of ASS buttering and base metal reveals that fine anchoring was obtained due to the formation of continuous epitaxial growth as shown in Fig. 9c. The weld interface region of ASS buttering and hardfacing reveals that the presence of fine continuous epitaxial growth of austenitic matrix and hexagonal carbides as shown in Fig. 9d. The interfaces between buttering and base metal and buttering and hardfacing have similar morphology in the joints. Interface microstructure of sandwiched joints has smaller portion of unmixed zone near to the periphery of the fusion boundary along with a softened layer of untempered martensite structure as shown in Fig. 9d.
The weld interface region of hardfacing and ASS capping reveals that epitaxial growth of d ferrite in austenitic matrix as shown in Fig. 9e. The undiluted weld metal microstructure in ASS root and ASS capping comprising of grain boundary d ferrite in a plain austenitic matrix is clearly visible from Fig. 9f. The HAZ invariably has the same microstructure in each combination (AHA 4.0, AHA 5.5 and AHA 7.0) as shown in Fig. 9g. The microstructure clearly depicts the presence of coarse martensite nearby the fusion line towards the base metal side and uniformly distributed martensite along with low temperature products like bainite [15–18].
Fig. 11. Microhardness profile across the weld center line (BM – base metal, HAZ – heat affected zone, B – buttering and ASS –austenitic stainless steel region).
M. Balakrishnan et al. / Materials and Design 44 (2013) 59–68 Table 5 Microhardness distribution of various locations of welded joints. Location
Hardness in HV (0.5 kg)
Base metal ASS root ASS buttering ASS capping Hardfacing Interface between Interface between Interface between Interface between
430–470 291–411 255–445 350–431 547–795 330–534 353–662 291–637 358–621
B-BM B-HF ASS Root-HF HF-ASS – capping
3.4. Hardness The hardness measurement was carried out in two different directions as shown in Fig. 10a to evaluate the hardness disparity both along and across the weld cross section. More than fifteen readings were taken at close proximity and the values are presented in graphs (Figs. 10 and 11) and the averages of three values at different locations are presented in Table 5. By comparing the hardness graphs as shown in Fig. 10b–d it is observed that there is a distinguished high hardness zone in between the soft zones. From the microhardness measurement, it is observed that the average buttering thickness is 1.5 mm including the dilution region. The average dilution is also observed as 0.7 mm on both sides of interfaces invariably in all the three joints. From Fig. 11 it is clear that a low hardness zone is in effect in between base metal and buttering layer in all the three joints. The width of the soft zone is measured to be 2–3 mm in all the joints and is clearly evident from the hardness profiles (Fig. 11a–c). The HAZ softening could be related to the microstructural evolution during the weld thermal cycle. The acicular martensitic structure of base metal is changed into low carbon martensite (tempered martensite) and e carbides [15,16] in addition with product like reverted austenite [17]. The retained austenite in the base metal would transform as bainite structure [15]. All the above mentioned microstructural transformation at HAZ region could be the reason for this softening effect at this zone.
4. Discussion In all the three joints, it is observed that the fully austenitic weld metal using ASS electrode having a microstructure of d-ferrite in a plain austenitic matrix is the beneficial microstructure for moderate strength welds with good crack resistance. Due to its high alloy content in ASS filler, it can tolerate up to 67% dilution by the base metal without the formation of weld metal martensite. This can be attributed to the low diffusivity of hydrogen in austenite, the fully austenite weld metal limit the amount of hydrogen diffusivity to HAZ and consequently reduces the likelihood of hydrogen induced cracking [6]. This microstructure can absorb more impact energy due to its high toughness. The nickel addition improves the toughness in two ways: one way nickel reduces d ferrite content in the weld metal and the other way it improves the toughness in fully austenitic composition by stabilizing the austenite. Thus the larger austenitic phase along with less d ferrite promotes higher toughness [12]. HF weld consisting of uniformly distributed hexagonal chromium carbides in an austenitic matrix possessing very high hardness can be beneficial to improve the ballistic immunity. The projectile penetration resistance of various armour materials is dependent on their hardness. Adequate toughness is called for, to avoid cracking tendency and consequent disintegration of the material. Generally harder the material better is the resistance to
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projectile penetration [4,19]. It is expected that the soft austenitic capping front layer will no longer offer resistance to projectile penetration. Capping front layer allows the bullet to puncture the front layer, when bullet reaches of HF layers; hardfacing alloy offers maximum resistance to penetration. The HF layer may change the direction of the projectile towards soft austenitic buttering layers and finally reduces the total velocity of the projectile. Sandwiching of the hardfacing alloy between ASS weld improves the ballistic resistance by the way of possible reduction of projectile velocity coupled with crack blending behavior. The thickness of the hardfacing layer dictates the level of resistance to projectile penetration, i.e., higher the thickness greater is the resistance to projectile penetration [12]. At WCL, for the HF layer thickness of 4 mm (AHA 4.0) the DOP was measured to be 14 and 15 mm with smooth bulge at the rear side, in AHA 5.5, the DOP was 14 mm without any bulge at the rear side and in AHA 7.0 the DOP was 16 mm without any bulge at the rear side. This can be attributed to the total reduction of bullet velocity by the ASS capping front layer before it hits the HF interlayer. From the literatures [12] it was expected that the ASS capping front layer will reduce as much as possible of the initial velocity of the projectile, while it is passing through it. Subsequently the high hardness HF interlayer will totally stop the projectile penetration. To do the reduction of initial velocity, the thickness of the HF interlayer determines the thickness of the ASS capping layer. Since, the HF interlayer thickness is more then ASS capping front layer thickness will be less. The thicknesses of ASS capping front layer is various from 8 mm, 6.5 mm and 5.0 mm for the joints AHA 4.0, AHA 5.5 and AHA 7.0 respectively. And from the microhardness graphs (Fig. 10) the dilution (D) also varies with respect to the HF interlayer thickness. For higher thickness HF interlayer higher dilution occurred whereas lower thickness HF interlayer lower amount of dilution was resulted. From this result it can be interpreted that, in low thick HF interlayer joints (AHA 4.0), the high thick ASS capping front layer successfully reduced the initial velocity to certain level and finally the bullet was stopped with less diluted HF interlayer (DOP15 mm). Whereas in high thick HF interlayer joints (AHA 7.0), the low thick ASS capping front layer successfully reduced the initial velocity to certain level and finally the bullet was stopped with highly diluted HF interlayer (DOP-16 mm). But in the in intermediate thick HF interlayer joints (AHA 5.5), the intermediate thick ASS capping front layer successfully reduced the initial velocity to maximum level and finally the bullet was stopped with DOP of 14 mm only. This could be attributed to the combined effect of the thickness of the undiluted HF interlayer and thickness of the ASS capping layer diluted with HF interlayer. While in AHA 7 joint results a DOP of 16 mm is maximum DOP among the three HF layer thicknesses considered. This could be attributed to the low thickness ASS capping front layer and higher thickness HF middle layer as the less impact could be absorbed by the front layer and highly diluted HF middle layer did not showed higher ballistic resistance than lower (AHA 5.5) and allows the projectile to a maximum DOP of 16 mm. The buttering layer provides high joint integrity by keeping the hardness level in between the base metal hardness and weld layers hardness. Interface microstructure of d-ferrite in a plain austenitic matrix and microhardness play an imperative role in keeping the joint more stable at the time of projectile attack. Microstructure in between buttering and base metal shows a clear anchoring between these two dissimilar metals and having the hardness value between 330 HV and 534 HV. Due to the presence of this hardness level in this location helps the joint against disintegration at the time of projectile attack. In the base metal locations, (location 4 in AHA 4 and location 3 in AHA 5.5) reveals the presence of typical crater (petal) failure of target plate. Whereas these feature is completely absent in other locations (WCL, FL, HAZ) of all the three targets. This could be
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attributed to the presence of acicular martensitic structure with some amount of retained austenite in base metal and which is assisting the solid state metal flow of target material at the time of ballistic impact [20]. The absence of petal type failure of target plate in other locations might be due to inhomogeneous microstructure due to the presence of multi-layer weld and the heterogeneity in the hardness level. These factors assist the solid state flow of the target at these locations and finally resulted in a small amount of fragmentations of the front layer as the intense of the impact at these points are severe. 5. Conclusions An attempt was made to improve the ballistic performance of armour steel welds by depositing an ASS buttering layer on the double Vee beveled base metal, above which a multi layered structure was fabricated with three different HF interlayer thickness (4.0, 5.5 and 7.0 mm). From this investigation the following conclusions are derived. (1) Buttering layer improved the ballistic performance of the armour steel welds due to the formation of preferable microstructure and required level of hardness at the weld metal region. This kept the weld layers effectively intact when the projectile is fired at WCL, interfaces and HAZ. (2) All three joints fabricated with 4 mm, 5.5 mm and 7 mm HF interlayer thickness (AHA 4.0, AHA 5.5 and AHA 7.0) successfully stopped the projectile by shattering it. (3) Of the three joints, the joint with 5.5 mm HF interlayer thickness offered a maximum resistance to the bullet penetration with a DOP of 14 mm without any bulge at the rear side. This HF layer thickness is turned to be 0.3 times of plate thickness. (4) At WCL, further investigation is required to understand the projectile shattering mechanism because the current study correlated the ballistic test results with the resultant microstructure and microhardness values only.
Acknowledgements The authors wish to record their sincere thanks to the Armament Research Board (ARMREB) New Delhi, Government of India for providing financial support to carry out this investigation through a R&D Project, No. ARMREB/MAA/2008/93. The authors also register their sincere thanks to Dr. K. Sivakumar, Scientist -F, Defense Metallurgical Research Laboratory (DMRL), Hyderabad
for granting permission to use ballistic testing facility. The authors wish to record their sincere thanks to the Mailam India Ltd., Pondicherry for their supply of welding consumables. References [1] Ade F. Ballistic qualification of armour steel weldments. Weld J 1991;70:53–4. [2] Madhusudhan Reddy G, Mohandas T. Ballistic performance of high-strength low-alloy steel weldments. J Mater Process Technol 1996;57:23–30. [3] Madhusudhan Reddy G, Mohandas T, Tagore GRN. Weldability studies of highstrength low-alloy steel using austenitic fillers. J Mater Process Technol 1995;49:213–28. [4] Woodward RL. Materials for projectile disruption. Mater Forum 1988;12:26–30. [5] Madhusudhan Reddy G, Mohandas T. Ballistic performance of high-strength low-alloy steel weldments. J Mater Process Technol 1999;57:23–30. [6] Alkemade SJ. The weld cracking susceptibility of high hardness armour steel. Defense Science and Technology Organization, Australia, AR no. 009–659; 1996. p. 1–17. [7] Magudeeswaran G, Balasubramanian V, Madhusudhan Reddy G. Hydrogen induced cold cracking studies on armour grade high strength, quenched and tempered steel weldments. Int J Hydrogen Energy 2008;33:1897–908. [8] Madhusudhan Reddy G, Mohandas T, Papukutty KK. Effect of welding process on the ballistic performance of high-strength low-alloy steel weldments. J Mater Process Technol 1998;74:27–35. [9] Choo Seong-Hun, Baek Eung-Ryul, Lee Sunghak. Ballistic impact behavior of multilayered armor plates processed by hardfacing. Metall Mater Trans A 1996;27A:3335–40. [10] Madhusudhan Reddy G, Mohandas T, Papukutty K. Enhancement of ballistic capabilities of soft welds through hardfacing. Int J Impact Eng 1999;22:775–91. [11] Balakrishnan M, Balasubramanian V, Madhusudhan Reddy G, Sivakumar K. Effect of buttering and hardfacing on ballistic performance of shielded metal arc welded armour steel joints. Mater Des 2011;32:469–79. [12] Magudeeswaran G, Balasubramanian V, Balasubramanian TS, Madhusudhan Reddy G. Effect of welding consumables on tensile and impact properties of shielded metal arc welded high strength, quenched and tempered steel joints. Sci Technol Weld Join 2008;13(2):97–105. [13] ASTM E-384-05. Standard test method for micro indentation hardness of materials; 2005. p. 1–33. [14] SFA 5.13. Specification for surfacing electrodes for shielded metal arc welding. ASME boiler and pressure vessel code (an International code), Sec II Part-C: Specification for welding rods, filler metals and electrodes. The American Society of Mechanical Engineers New York, New York; 2004. p. 303–324. [15] Sekhar PC, Schreiner ME, Richman MH. Tempering of martensite: a field ion microscope study. Metallography 1971;4:147–56. [16] Wells MGH. An electron transmission study of the tempering of martensite in a Fe–N&–C alloy. Acta Metall 1964;12:389–99. [17] Aismon N, Rosen A. Short communication, reverted austenite in maraging steel. Metallography 1981;14:163–7. [18] Unfried SJ, Garz´onb CM, Giraldoc JE. Numerical and experimental analysis of microstructure evolution during arc welding in armor plate steels. J Mater Process Technol 2009;209:1688–700. [19] Crouch IG. Metallic armour-from cast aluminium alloys to high strength steels. Mater Forum 1988;12:31–7. [20] Pizana C, Murr LE, Baquera MT, Anchondo IA, Putrevu A, Pina CY, et al. Solidstate flow, mechanical alloying, and melt-related phenomena for [0 0 1] singlecrystal W ballistic rod penetrators interacting with steel targets. Mater Sci Eng, A 2006;428:301–13.
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Technical Report
Effect of hardfaced interlayer thickness on ballistic performance of armour steel welds M. Balakrishnan a, V. Balasubramanian b,⇑, G. Madhusudhan Reddy c a
Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India Center for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India c Solidification Technology Division, Metal Joining Group, Defense Metallurgical Research Laboratory (DMRL), Kanchanbaugh, Hyderabad 500 058, India b
a r t i c l e
i n f o
Article history: Received 29 March 2012 Accepted 6 June 2012 Available online 2 August 2012
a b s t r a c t Due to weld thermal cycles and under matching fillers, the armour steel welded joints showed poor ballistic performance compared to the base metal. An attempt was made to enhance the ballistic performance of armour steel welded joints by depositing hardfaced (HF) interlayer between austenitic stainless steel weld metal. Though this method yielded marginal improvements in ballistic performance, cracks were observed in between base metal and HF layers. A novel method was proposed by the same authors to eliminate these cracks by depositing a soft buttering layer using austenitic stainless steel (ASS) consumable in between base metal and HF layer shows additional improvements on ballistic performance. Buttering layer enhanced the ballistic immunity by keeping the weld layer intact successfully at the time of impact, but the bullet was pierced through. In this investigation, an attempt has been made to stop the bullet completely by varying HF interlayer thickness. This paper reveals the effect HF layer thickness on ballistic performance of shielded metal arc welded armour steel welds. A HF interlayer thickness of 0.3 times, the plate thickness is found to be optimum from this investigation. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Armour grade quenched and tempered (Q & T) steel closely confirming to AISI 4340 is primarily used for construction of military and non-military vehicles, because of their high energy absorbing properties. The ballistic requirement of Q & T steels used for armour application requires high strength, greater notch toughness and high hardness [1–4]. The effect of heat-input on the heataffected-zone (HAZ) softening in Q & T steel and width of the soft zone and, in turn, its influence on hardness and ballistic performance was investigated by Reddy and Mohandas [5]. An increased heat-input resulted in a wide soft zone, leading to poor ballistic performance and this was attributed to softer microstructural constituents at this zone. Based on the work done by Alkemade [6] and bisalloy steels (high hardness armour steel) handbook it is recommended that heat input levels should be 1.0–2.5 kJ/mm and the preheat should be 50–75 °C. Q & T steels are prone to hydrogen induced cracking (HIC) in the heat affected zone (HAZ) after welding. The use of austenitic stainless steel (ASS) consumables to weld the above steel was the only available remedy because of higher solubility for hydrogen in austenitic phase [7]. ⇑ Corresponding author. Tel.: +91 4144 239734 (O), +91 4144 241147 (R), mobile: +91 9443412249; fax: +91 4144 238080/238275. E-mail addresses: [email protected] (M. Balakrishnan), [email protected], [email protected] (V. Balasubramanian), [email protected] (G. Madhusudhan Reddy). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.06.010
The resistance against projectile penetration of the various zones in the weldments of a high-strength low-alloy steel was investigated by Reddy et al. [8] through three welding processes, namely: shielded metal arc welding (SMAW); gas tungsten arc welding (GTAW); and flux cored arc welding (FCAW). In all the joints, the projectile passed through the weld metal region. However, the ballistic limit was found to be highest in the case of the shielded metal arc weld and least in respect of flux cored arc welds. HAZ of SMAW was resistant to penetration and exhibited the highest ballistic limit, whilst the HAZ of FCAW exhibited intermediate performance. The results of Choo et al. [9] indicated that the multi-layered hardfacing, i.e., the combination of hard and soft layers, might improve ballistic property through absorbing ballistic impact energy due to cracking of primary carbides and controlling the projectile traveling direction upright due to existence of the soft layer. But much remains to be learned concerning the hardfacing design and the improvement of the ballistic property of the armour plates. It was suggested that; therefore, future studies should include the new design of multi-layered armor plates and their evaluation, detailed parallel measurements of hardness, strength, and fracture toughness, and interpretation of microstructural factors by mutually related mechanical properties and hardfacing variables. An attempt was made to improve the ballistic immunity of austenitic welds through a hardfacing technique by SMAW process [10] and reported that sandwiching of the hardfacing alloy between austenitic
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cracking tendency of Q & T steel weldments [12]. Before making the joint fit up, the beveled edges of the double Vee groove was buttered with the ASS electrodes. The average buttering layer thickness was maintained to be 1.5 mm. The buttered layer was cooled to room temperature and the buttered edges were again beveled by grinding process as illustrated in Fig. 2. The joints were welded as per the sequence illustrated in Fig. 3. The welding parameters used to fabricate the joints are presented in Table 3. Necessary care was taken to avoid joint distortion and to get defect free welds. The preheating and interpass temperatures were maintained at 150 °C for both welding and hardfacing. Three different HF interlayer thicknesses (4 mm, 5.5 mm and 7 mm) were used in this investigation and the joints are named as AHA 4, AHA 5.5 and AHA 7 respectively. All the three joints were subjected to standard ballistic testing and their performances were evaluated. The standard ballistic test arrangement is shown in Fig. 4. The ballistic test procedure was dealt with elsewhere [5,8,10,11]. The ballistic performance was characterized by the depth of penetration of the projectile in the target plate. The depth of penetration of each bullet in the targets was measured by using vernier depth gauge (MAKE: MITUTOYO, Japan, 0–150 mm). The weldments were characterized by microstructure analysis and hardness measurement. The macrostructures were analyzed through stereo zoom macroscope (Make: Macscope Z Model CM 0646). The microstructure of the joint was analyzed at various locations using optical microscope (Make: MEIJI, Japan: Model ML 7100). The specimens were etched in 2% Nital for base metal and HAZ region. Aqua – Regia and Villella’s reagent was used to reveal the microstructure of ASS weld metal region and HF region respectively. The hardness measurement was carried out as per ASTM E-384-05 standards [13]. A Vickers microhardness testing machine (Model: HMV-2T and Make: Shimadzu, Japan) was employed to measure the hardness along the WCL and across the WCL with 500 g load for a dwell time of 15 s.
weld metal served the dual purpose of weld integrity and ballistic immunity due to the high hardness of hardfacing alloy and the energy absorbing capacity of austenitic weld metal. Welds with complete hardfacing shattered under ballistic impact. An overlay of hardfacing alloy over austenitic weld disintegrated due to extensive cracking in the hardfacing layer and in the interface between base metal and hardfacing. This is due to the brittle nature of the interface. However, to attain ballistic immunity equivalent to the base metal, a minimum thickness of hardfacing inter – layer is required. The primary aim of our early study [11] was to eliminate these cracks by depositing a soft buttering layer using ASS consumable in between base metal and HF layer above which a multi-layered structure was fabricated and evaluated. From the results, it was found that the buttering layer improved ballistic performance by the resulting microstructure and hardness distribution and thus kept the weld layers intact when the projectile was fired at interfaces and HAZ. However, at weld center line (WCL), further investigation was suggested with different HF layer thickness and joint configuration. Though numerous efforts was exerted in the past to increase the ballistic resistance of armour steel weldments by improving the hardness of materials with dubious success, but complete stopping of the bullet penetration is still a paramount goal. A novel technique to achieve this objective, believed to be investigated here for the first time, is the potential reduction of the momentum per unit area inherent in a perforating projectile by having the good combination of HF interlayers in between tough ASS weld layers. This paper reports the influence of HF interlayer thickness on ballistic performance of armour steel welds fabricated by SMAW process using ASS electrodes and chromium carbide forming hardfacing electrodes. 2. Experimental work The base metal used in this examination was 18 mm thick high strength low alloy Q & T steel closely conforming to AISI 4340 specification. The heat treatment applied was exposing the base metal to austenising temperature at 900 °C followed by oil quenching and subsequent tempering at 250 °C. This heat treatment is responsible for attaining high hardness, higher strength and good toughness in base metal. The chemical composition and mechanical properties of base metal and filler metals are presented in the Tables 1 and 2 respectively. In this investigation, unequal double vee joint configuration was prepared as shown in Fig. 1. SMAW process was selected because it offers highest ballistic limit and lower HAZ soft zone width [8]. ASS electrode was selected because it avoids time delayed
3. Results 3.1. Ballistic performance The ballistic test results are presented in Table 4. Figs. 5–7 shows the photographs of target plate AHA 4, AHA 5.5 and AHA 7 respectively after ballistic test. The ballistic test results are categorized as (i) bullet stopped without any damage at the rear side is referred as S1, while (ii) bullet stopped but makes a smooth bulge at the rear side is referred as S2. Fig. 5a is the front side view photograph and Fig. 5b is the rear side view photograph of AHA 4 joint. The corresponding locations of 1, 2, 3 and 4 in the front side view
Table 1 Chemical composition (wt%) of base metal and filler metals (all weld metal). Element
C
Mn
Si
S
P
Cr
Ni
Mo
V
Fe
Base metal (AISI 4340) ASS electrode AWS E 307-16 Hardfacing alloy AWS E FeCr–A7
0.35 0.08 4.0
0.54 3.3 1.0
– 0.90 1.50
– 0.015 –
– 0.04 –
1.25 20.30 30.00
1.75 8.50 –
0.52 1.5 2.0
– – 0.50
Bal Bal Bal
Table 2 Mechanical properties of base metal and filler metals (all weld metals). Base metal
Yield strength (MPa)
Tensile strength (MPa)
Elongation in 50 mm gauge length (%)
Hardness (HV0.5)
Charpy impact toughness at room temperature (J)
Base metal (AISI 4340) ASS electrode AWS E 307-16 Hardfacing alloy E FeCr–A7
1200 440 1420
1290 590 1510
12.5 30.0 4.0
450 280 710
49 65 39
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(a) Joint
(b) Sectionat A-A ALL DIMENSIONS ARE IN mm Fig. 1. Joint configuration.
ASS Buttering Layer
AISI 4340 STEEL
a. Before buttering
b. After buttering
ASS Buttering Layer AISI 4340 STEEL
(c) After grinding Fig. 2. Sequence of edge preparation.
Table 3 Welding conditions used for fabricating the joints.
Fig. 3. Schematic illustration for sequence (1–5) of welding. a.AHA JOINT (A-ASS filler, H – Hardfacing filler and B – ASS Buttering.
(Fig. 5a) are encircled and arrowed in Fig. 5b. The corresponding encircles shows the presence of smooth bulge at the rear side of the plate. In Fig 5a, the shot 1 was targeted at fusion line (FL), shot 2 was targeted at WCL, shot 3 was targeted at HAZ and shot 4 was targeted at base metal. The shot 1 and 2 was targeted at WCL, made
Parameters
Unit
Welding machine
Lincoln electric precision welding machine) mm 3.15 °C 150 °C 150 A 110 V 21 kJ/mm 0.6391
Filler wire diameter Preheat temperature Interpass temperature Welding current Arc voltage Heat input
Buttering
Hardfacing
ASS
tig™ 375 (multipurpose 4 150 150 160 23 1.05
4 150 150 153 22 0.9439
a smooth bulge (S2) at the rear side (Fig. 5b). Shot 3 was targeted at HAZ area, resulted S2 category result. The shot 4 targeted at base metal stopped the projectile and made a smooth bulge at the rear side (Fig. 5b). Fig. 5c–f are the magnified photographs of the corresponding locations as labeled in the plate itself. At these locations, the non-penetration of bullets is clearly visible from the
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Fig. 6a is the front side view and Fig. 6b is the rear side view of AHA 5.5 joint. Shot 1 was targeted at FL, stopped the projectile and allowed to a DOP of 16 mm only (Fig. 6a). Shot 2 was targeted at WCL and WCL successfully stopped the projectile (Fig. 6a). The shot 3 was targeted at base metal and it made a smooth bulge; encircled and arrowed at the rear side view of the target as shown in Fig. 6b. The shot 4 was targeted at HAZ area resulted in projectile shattering with a DOP of 16 mm. The weld layer offers higher resistance compared with other locations in the same target against the projectile penetration and made a DOP of 14 mm only. This can be clearly seen in rear side view photographs (Fig. 6b). Fig. 6c–f are the magnified views of the corresponding locations as labeled in Fig. 6a and is also evident of the non-penetration of the bullet at these locations. As in the previous case, all the shots at various locations of the target, fabricated with 5.5 mm HF interlayer successfully stopped the projectile. Fig. 7a is the front side view and Fig. 7b is the rear side view of AHA 7.0 joint. Shot 1 was targeted at WCL, stopped the projectile and allowing the bullet for a DOP of 16 mm only (Fig. 7a). Shot 2
Fig. 4. Ballistic testing arrangement.
photographs. The weld metal offered higher resistance than other locations in the same target against the projectile attack and allowed the projectile to enter a depth of penetration (DOP) of 14 mm only. All the shots at various locations of the target fabricated with 4 mm HF interlayer successfully stopped the projectile.
Table 4 Ballistic test results. Joint type
Shot no.
Velocity
Location
Result category
Depth of penetration (DOP) in mm
Illustration
AHA 4.0
1 2 3 4 1 2 3 4 1 2 3
821.90 828.09 823.86 839.46 826.31 832.81 827.40 827.82 817.96 825.12 821.02
Weld Weld HAZ Base metal Fusion line Weld Base metal HAZ Weld Weld HAZ
S2 S2 S2 S2 S1 S1 S2 S1 S1 S1 S2
14 15 15 16 16 14 16 16 16 16 15
Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped Stopped
AHA 5.5
AHA 7.0
S1 – bullet stopped and no bulge at rear side, S2 – bullet stopped and smooth bulge at rear side.
(a) AHA 4.0 joint front side
(c)
(d)
(b) AHA 4.0 joint rear side
(e)
(f)
Fig. 5. Photographs of various locations of ballistically tested AHA 4.0 joint (c–f–magnified views).
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(a) AHA 5.5 joint front side
(c)
(b) AHA 5.5 joint rear side
(d)
(f)
(e)
Fig. 6. Photographs of various locations of ballistically tested AHA 5.5 joint (c–f–magnified views).
(b) AHA 7.0 joint rear side
(a) AHA 7.0 joint front side
(c)
(d)
(e)
Fig. 7. Photographs of various locations of ballistically tested AHA 7.0 joint (c–e–magnified views).
located FL successfully stopped the projectile with a DOP of 17 mm only. The shot 3 was targeted at HAZ and it made a smooth bulge at the rear side is encircled in the rear side view photograph as shown in Fig. 7b. The DOP of bullet at this location
was 15 mm only. In this AHA 7 joint, the weld layer offers intermediate resistance as compared with other locations in the same target against the projectile penetration and made a DOP of 16 mm. This can be clearly seen in rear side view photograph
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(Fig. 7b). As in the previous two cases, all the shots at various locations of the target, fabricated with 7 mm HF interlayer also successfully stopped the projectile.
from defects such as lack of fusion, incomplete penetration and macro level cracks. The HAZ is also clearly visible. 3.3. Microstructure
3.2. Macrograph Fig. 8 shows the macrographs of three joints before ballistic testing. From these macrographs, it is clear that the joints are free
a. AHA 4 joint
The optical micrographs of various locations are presented in Fig. 9. The microstructure of base metal consists of acicular martensite structure with fine needles of lath martensite as shown in Fig. 9a [5]. The microstructures of various locations of the multi
b. AHA 5.5 joint
c. AHA 7 joint
Fig. 8. Macrographs of joint cross section.
(a) Base metal
(d) Interface- Buttering Hardfacing
(b) Undiluted Hard facing
(e) Interface-Hardfacing-ASSCap
(c) Interface-Buttering- BM
(f) g.ASS capping
(g) HAZ Fig. 9. Optical micrographs of base metal and various locations of welded joints.
M. Balakrishnan et al. / Materials and Design 44 (2013) 59–68
layered joint are shown in Fig. 9b–g. The HF region is made of the microstructure of homogeneous cast like structure as shown in Fig. 9b. It is composed of hard phase precipitates (Carbides) of
65
hexagonal chromium carbides of different sizes on softer austenitic matrix [14]. Large amount of spine like carbides are also clearly visible in the microstructure of the HF layers in the joints. The weld
Fig. 10. Microhardness profile along the weld center line (D – diluted region).
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interface region of ASS buttering and base metal reveals that fine anchoring was obtained due to the formation of continuous epitaxial growth as shown in Fig. 9c. The weld interface region of ASS buttering and hardfacing reveals that the presence of fine continuous epitaxial growth of austenitic matrix and hexagonal carbides as shown in Fig. 9d. The interfaces between buttering and base metal and buttering and hardfacing have similar morphology in the joints. Interface microstructure of sandwiched joints has smaller portion of unmixed zone near to the periphery of the fusion boundary along with a softened layer of untempered martensite structure as shown in Fig. 9d.
The weld interface region of hardfacing and ASS capping reveals that epitaxial growth of d ferrite in austenitic matrix as shown in Fig. 9e. The undiluted weld metal microstructure in ASS root and ASS capping comprising of grain boundary d ferrite in a plain austenitic matrix is clearly visible from Fig. 9f. The HAZ invariably has the same microstructure in each combination (AHA 4.0, AHA 5.5 and AHA 7.0) as shown in Fig. 9g. The microstructure clearly depicts the presence of coarse martensite nearby the fusion line towards the base metal side and uniformly distributed martensite along with low temperature products like bainite [15–18].
Fig. 11. Microhardness profile across the weld center line (BM – base metal, HAZ – heat affected zone, B – buttering and ASS –austenitic stainless steel region).
M. Balakrishnan et al. / Materials and Design 44 (2013) 59–68 Table 5 Microhardness distribution of various locations of welded joints. Location
Hardness in HV (0.5 kg)
Base metal ASS root ASS buttering ASS capping Hardfacing Interface between Interface between Interface between Interface between
430–470 291–411 255–445 350–431 547–795 330–534 353–662 291–637 358–621
B-BM B-HF ASS Root-HF HF-ASS – capping
3.4. Hardness The hardness measurement was carried out in two different directions as shown in Fig. 10a to evaluate the hardness disparity both along and across the weld cross section. More than fifteen readings were taken at close proximity and the values are presented in graphs (Figs. 10 and 11) and the averages of three values at different locations are presented in Table 5. By comparing the hardness graphs as shown in Fig. 10b–d it is observed that there is a distinguished high hardness zone in between the soft zones. From the microhardness measurement, it is observed that the average buttering thickness is 1.5 mm including the dilution region. The average dilution is also observed as 0.7 mm on both sides of interfaces invariably in all the three joints. From Fig. 11 it is clear that a low hardness zone is in effect in between base metal and buttering layer in all the three joints. The width of the soft zone is measured to be 2–3 mm in all the joints and is clearly evident from the hardness profiles (Fig. 11a–c). The HAZ softening could be related to the microstructural evolution during the weld thermal cycle. The acicular martensitic structure of base metal is changed into low carbon martensite (tempered martensite) and e carbides [15,16] in addition with product like reverted austenite [17]. The retained austenite in the base metal would transform as bainite structure [15]. All the above mentioned microstructural transformation at HAZ region could be the reason for this softening effect at this zone.
4. Discussion In all the three joints, it is observed that the fully austenitic weld metal using ASS electrode having a microstructure of d-ferrite in a plain austenitic matrix is the beneficial microstructure for moderate strength welds with good crack resistance. Due to its high alloy content in ASS filler, it can tolerate up to 67% dilution by the base metal without the formation of weld metal martensite. This can be attributed to the low diffusivity of hydrogen in austenite, the fully austenite weld metal limit the amount of hydrogen diffusivity to HAZ and consequently reduces the likelihood of hydrogen induced cracking [6]. This microstructure can absorb more impact energy due to its high toughness. The nickel addition improves the toughness in two ways: one way nickel reduces d ferrite content in the weld metal and the other way it improves the toughness in fully austenitic composition by stabilizing the austenite. Thus the larger austenitic phase along with less d ferrite promotes higher toughness [12]. HF weld consisting of uniformly distributed hexagonal chromium carbides in an austenitic matrix possessing very high hardness can be beneficial to improve the ballistic immunity. The projectile penetration resistance of various armour materials is dependent on their hardness. Adequate toughness is called for, to avoid cracking tendency and consequent disintegration of the material. Generally harder the material better is the resistance to
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projectile penetration [4,19]. It is expected that the soft austenitic capping front layer will no longer offer resistance to projectile penetration. Capping front layer allows the bullet to puncture the front layer, when bullet reaches of HF layers; hardfacing alloy offers maximum resistance to penetration. The HF layer may change the direction of the projectile towards soft austenitic buttering layers and finally reduces the total velocity of the projectile. Sandwiching of the hardfacing alloy between ASS weld improves the ballistic resistance by the way of possible reduction of projectile velocity coupled with crack blending behavior. The thickness of the hardfacing layer dictates the level of resistance to projectile penetration, i.e., higher the thickness greater is the resistance to projectile penetration [12]. At WCL, for the HF layer thickness of 4 mm (AHA 4.0) the DOP was measured to be 14 and 15 mm with smooth bulge at the rear side, in AHA 5.5, the DOP was 14 mm without any bulge at the rear side and in AHA 7.0 the DOP was 16 mm without any bulge at the rear side. This can be attributed to the total reduction of bullet velocity by the ASS capping front layer before it hits the HF interlayer. From the literatures [12] it was expected that the ASS capping front layer will reduce as much as possible of the initial velocity of the projectile, while it is passing through it. Subsequently the high hardness HF interlayer will totally stop the projectile penetration. To do the reduction of initial velocity, the thickness of the HF interlayer determines the thickness of the ASS capping layer. Since, the HF interlayer thickness is more then ASS capping front layer thickness will be less. The thicknesses of ASS capping front layer is various from 8 mm, 6.5 mm and 5.0 mm for the joints AHA 4.0, AHA 5.5 and AHA 7.0 respectively. And from the microhardness graphs (Fig. 10) the dilution (D) also varies with respect to the HF interlayer thickness. For higher thickness HF interlayer higher dilution occurred whereas lower thickness HF interlayer lower amount of dilution was resulted. From this result it can be interpreted that, in low thick HF interlayer joints (AHA 4.0), the high thick ASS capping front layer successfully reduced the initial velocity to certain level and finally the bullet was stopped with less diluted HF interlayer (DOP15 mm). Whereas in high thick HF interlayer joints (AHA 7.0), the low thick ASS capping front layer successfully reduced the initial velocity to certain level and finally the bullet was stopped with highly diluted HF interlayer (DOP-16 mm). But in the in intermediate thick HF interlayer joints (AHA 5.5), the intermediate thick ASS capping front layer successfully reduced the initial velocity to maximum level and finally the bullet was stopped with DOP of 14 mm only. This could be attributed to the combined effect of the thickness of the undiluted HF interlayer and thickness of the ASS capping layer diluted with HF interlayer. While in AHA 7 joint results a DOP of 16 mm is maximum DOP among the three HF layer thicknesses considered. This could be attributed to the low thickness ASS capping front layer and higher thickness HF middle layer as the less impact could be absorbed by the front layer and highly diluted HF middle layer did not showed higher ballistic resistance than lower (AHA 5.5) and allows the projectile to a maximum DOP of 16 mm. The buttering layer provides high joint integrity by keeping the hardness level in between the base metal hardness and weld layers hardness. Interface microstructure of d-ferrite in a plain austenitic matrix and microhardness play an imperative role in keeping the joint more stable at the time of projectile attack. Microstructure in between buttering and base metal shows a clear anchoring between these two dissimilar metals and having the hardness value between 330 HV and 534 HV. Due to the presence of this hardness level in this location helps the joint against disintegration at the time of projectile attack. In the base metal locations, (location 4 in AHA 4 and location 3 in AHA 5.5) reveals the presence of typical crater (petal) failure of target plate. Whereas these feature is completely absent in other locations (WCL, FL, HAZ) of all the three targets. This could be
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attributed to the presence of acicular martensitic structure with some amount of retained austenite in base metal and which is assisting the solid state metal flow of target material at the time of ballistic impact [20]. The absence of petal type failure of target plate in other locations might be due to inhomogeneous microstructure due to the presence of multi-layer weld and the heterogeneity in the hardness level. These factors assist the solid state flow of the target at these locations and finally resulted in a small amount of fragmentations of the front layer as the intense of the impact at these points are severe. 5. Conclusions An attempt was made to improve the ballistic performance of armour steel welds by depositing an ASS buttering layer on the double Vee beveled base metal, above which a multi layered structure was fabricated with three different HF interlayer thickness (4.0, 5.5 and 7.0 mm). From this investigation the following conclusions are derived. (1) Buttering layer improved the ballistic performance of the armour steel welds due to the formation of preferable microstructure and required level of hardness at the weld metal region. This kept the weld layers effectively intact when the projectile is fired at WCL, interfaces and HAZ. (2) All three joints fabricated with 4 mm, 5.5 mm and 7 mm HF interlayer thickness (AHA 4.0, AHA 5.5 and AHA 7.0) successfully stopped the projectile by shattering it. (3) Of the three joints, the joint with 5.5 mm HF interlayer thickness offered a maximum resistance to the bullet penetration with a DOP of 14 mm without any bulge at the rear side. This HF layer thickness is turned to be 0.3 times of plate thickness. (4) At WCL, further investigation is required to understand the projectile shattering mechanism because the current study correlated the ballistic test results with the resultant microstructure and microhardness values only.
Acknowledgements The authors wish to record their sincere thanks to the Armament Research Board (ARMREB) New Delhi, Government of India for providing financial support to carry out this investigation through a R&D Project, No. ARMREB/MAA/2008/93. The authors also register their sincere thanks to Dr. K. Sivakumar, Scientist -F, Defense Metallurgical Research Laboratory (DMRL), Hyderabad
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