Metallurgical characteristics of armour steel welded joints used for combat vehicle construction

G.Magudeeswaran,V.Balasuramanian,G.Madhusudan Reddy

aDepartment of Mechanical Engineering,PSNA College of Engineering and Technology,Kothandaraman Nagar,Muthanampatti,624 622,Dindigul,Tamilnadu,India

bCentre for Materials Joining&Research,Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608002,Tamil Nadu,India

c Outstanding Scientist&Associate Director,Defence Metallurgical Research Laboratory,Kanchanbagh,Hyderabad,500058,Telangana,India

Keywords:Armour grade Q&T steel Heat affected zone softening Shielded metal arc welding process Flux cored arc welding process Austenitic stainless steel Low hydrogen ferritic steel High nickel steel

ABSTRACT Austenitic stainless steel(ASS)and High nickel steel(HNS)welding consumables are being used for welding Q&T steels,as they have higher solubility for hydrogen in austenitic phase,to avoid hydrogen induced cracking(HIC)but they are very expensive.In recent years,the developments of low hydrogen ferritic steel(LHF)consumables that contain no hygroscopic compounds are utilized for welding Q&T steels.Heat affected zone(HAZ)softening is another critical issue during welding of armour grade Q&T steels and it depends on the welding process employed and the weld thermal cycle.In this investigation an attempt has been made to study the influence of welding consumables and welding processes on metallurgical characteristics of armour grade Q&T steel joints by various metallurgical characterization procedures.Shielded metal arc welding(SMAW)and flux cored arc welding(FCAW)processes were used for making welds using ASS,LHF and HNS welding consumables.The joints fabricated by using LHF consumables offered lower degree of HAZ softening and there is no evidence of HIC in the joints fabricated using LHF consumables.

1.Introduction

In recent years,there has been an increasing need for Q&Tsteels for use in highly-stressed structures,including many critical applications in defence such as construction of the hull and turret of combat vehicles.The major objective of any armour design and development is that to ensure superior ballistic performance.The ballistic performance of various armour steels is characterized based on their hardness.Generally,the harder the steel the better is the ballistic performance,hence high-hardness steels are used typically where penetration resistance and weight reduction are key consideration,low-hardness type steels being used where shock resistance is important in military applications[1].The performance of steels depends on the properties associated with their microstructures,that is,on the crystallographic arrangements,volume fractions,sizes,and morphologies of the various phases constituting a macroscopic section of steel with a given composition in a given processed condition.Steel microstructures are made up of various phases,sometimes as many as three or four different types,which are physically blended by solidification,solid-state phase changes,hot deformation,cold deformation,and heat treatment.Each type of microstructure and product is developed to characteristic property ranges by specific processing routes that control and exploit microstructural changes[2].The ballistic requirement of Q&T steels used for armour applications calls for high strength,greater notch toughness and moderate hardness.These low alloy steels are essentially nickel-chromiummolybdenum type and all the above properties are best obtained in fine(acicular)tempered martensitic structures produced by quenching and tempering heat treatments[3].

1.1.Armour grade Q&T steel welds

Q&T steels with yield strength upto 1500 MPa and having carbon equivalent in the rage 0.7-0.9 are generally used for the fabrication of combat vehicles which require resistance against projectile penetration.Welding is the major fabrication route adopted in armour vehicle fabrication.Hydrogen induced cold cracking after welding in HAZ and HAZ softening due to weld thermal cycle are two major problems that need to be addressed during welding of armour grade Q&T steels as they hinder the ballistic performance when they are used in armour applications[4].

1.2.Cracking in armour grade Q&T steel welds

Two types of cracking are expected to occur in the HAZ of armour grade Q&T steels.They are cracking at high temperature due to low ductility of HAZ and the second type of cracking which occurs at low temperature after a delay of time.The cracking that occurs at low temperature can be either due to hydrogen embrittlement or restraint cracking[5].Solution to overcome HIC are preheating and precautions during welding such as baking of electrodes and use of consumables which form austenitic phase.One of the main causes of hydrogen induced cracking is high hardness in the HAZ.For this reason,to maintain HAZ hardness below the cracking susceptible region,preheating is recommended[6].Inspite of taking all these precautions HAZ cracking is reported to prevail in the high hardness(470-550 Hv)condition of the base metal.Softening in the HAZ of the weldments of Q&T steel occurs due to the weld thermal cycle and is a characteristic of the welding process and consumables used in fabrication.The presence of a soft zone in welded structure may limit the design strength to a lower value and may also influence the ballistic behaviour of weldments[7].

1.3.Welding consumables and processes adopted for armour grade Q&T steels

Austenitic stainless steel(ASS)welding consumables and high nickel steel(HNS)consumables are being used for welding Q&T steels,as they have higher solubility for hydrogen in austenitic phase,to avoid hydrogen induced cracking(HIC).These consumables find application for the welding of high hardness Q&T steels to meet the service requirements of armoured vehicles.But use of ASS and HNS consumables for a non-stainless steel base metal must be avoided as they are more expensive.In recent years,the development of lowhydrogen ferritic steel(LHF)consumables with basic coatings that contain no hygroscopic compounds are attempted for welding Q&T steels[8-10]which are less expensive than the ASS and HNS consumables.

The majority of armour fabrication is performed by fusion welding process and they demand for the highest welding quality.Shielded metal arc welding(SMAW)and the flux cored arc welding(FCAW)processes are widely used in fabrication of combat vehicle construction[11].The welding consumables and processes have a substantial influence on hydrogen induced cracking and heat affected zone softening in armour grade Q&T steels.Armour grade Q&T steel has well established weld compatibility with ASS and HNS consumables and they are used in construction of armour vehicles.Thus,there is a need to utilize LHF consumables for welding armour grade Q&T steel used in construction of military vehicles.The use of LHF consumables may reduce heat affected zone softening and may offer equal resistance to HIC as that of ASS and HNS consumables.From the above discussion,it is anticipated that welding consumables and welding processes may have considerable effect on the performance of the armour grade Q&T steel joints metallurgical.The properties and integrity of the weld metal depend on the variant microstructural features in the various zones of the joint[12,13].The use of ASS,HNS and LHF consumables for welding armour grade Q&Tsteel by SMAWand FCAW processes will lead to the formation of distinct microstructural features in the weld and heat affected zone and will have a significant influence on the performance of the joints.Hence,in this investigation,an attempt has been made to study the effect of welding consumables(ASS and LHF)and welding processes(SMAW and FCAW)on metallurgical characteristics of armour grade Q&Tsteel weldments.The aim of this investigation is to assess the feasibility of using LHF consumables for fabrication of armour grade Q&T steels and its metallurgical compatibility with the base metal.

2.Experimental work

2.1.Base material

The base metal used in this investigation is typically a low alloy quenched and tempered steel used in the construction of combat vehicle construction.The heat treatment of the base metal used in this investigation comprises of austenitising at 900oC followed by oil quenching and subsequent tempering at 250oC.This combination of heat treatment is responsible for high hardness,higher strength and toughness and hence it imparts superior resistance against any ballistic attack and they are used in the construction of military vehicles[14,15].The base metal used in this investigation is armour grade quenched and tempered steel closely confirming to AISI 4340 specifications.

2.2.Filler materials for joining

The filler metals used in this investigation confirming to the following specifications as given below:

(1)Basic coated ASS electrode for SMAW process closely confirming to AWS E 307 specifications with Cr and Ni as main alloying elements.

(2)Basic coated LHF steel electrode for SMAW process confirming to AWS E 11018-M 307.It is a typically low alloy ferritic electrode.

(3)Basic coated HNS steel electrode for SMAW process conifrming to AWS E NiCrFe3with greater proportion of nickel content along with chromium and manganese.

(4)Basic coated ASS flux cored wire for open arc FCAW process closely confirming to AWS E 307 T1-1 specifications with Cr and Ni as main alloying elements.

(5)Basic coated LHF steel flux cored wire for FCAW process with Co2shielding confirming to AWS E 110 T5-K4.It is a typically low alloy ferritic steel flux cored wire.

The chemical composition of the base metal and filler materials are presented in Table 1.

2.3.Fabrication of single ‘V’butt joints

Rolled plates of 14mm thick base metal were sliced into the required dimensions(300mm×100 mm)by abrasive cutters and grinding.Single ‘V’butt joint configuration,as shown in Fig.1 was prepared to fabricate the joints using ASS,LHF and HNS consumables by SMAW and FCAW processes.A preheating temperature of 100οC was used in this investigation as per the guidelines cited elsewhere in the literature[15-17].In order to ensure complete side wall fusion,the welding assembly was placed at 25 to 30οinclined to the work table surface.About 4 to 5 passes were deposited to complete the welding and deposited slag was completely removed between passes.A temperature of 150οC was maintained between subsequent passes in accordance with guidelines cited elsewhere[5].The direction of welding was parallel to the rolling direction.All necessary care was taken to avoidjoint distortion and the joints were made with applying clamping devices.The welding conditions and process parameters used in the fabrication of the joints are given in Table 2.The soundness of all the welded plates was checked using ultrasonic testing and radiography examination.

The joint fabricated using ASS consumable and SMAW process is referred as SA joint;the joint fabricated using LHF consumable and SMAW process is referred as SF joint;the joint fabricated using HNS consumable and SMAW process is referred as SN joint.Similarly,the joint fabricated using ASS consumable and FCAW process is referred as FA joint;the joint fabricated using LHF consumable and FCAW process is referred as FF joint.The base metal is referred as BM hereafter.These notations have been followed while displaying the various experimental results and subsequent discussions.

Table 2 Optimised welding conditions and parameters.

2.4.Characterization of armour grade Q&T steel welds

A transverse cross section of the specimen was extracted from the joint and was subjected to conventional metallographic preparations to reveal the various features of the joints fabricated in this study.The details of the various metallurgical characterization are detailed in the following sections.

2.4.1.Macrostructure

Macrostructural features of the transverse cross section of the weld specimen were carried out using light optical microscope(Make:MEIJI,Japan:Model)under a low magnification of 20×to detect cracks(if any)in the weld metal or in the heat affected zone(HAZ).The filler material for acceptance must not show crack or micro fissures either in the weld or in the HAZ.Absence of cracks was interpreted as the resistance of the weld metal to the HIC.

2.4.2.Microstructure

The microstructure of the joint was analysed at various locations using light optical microscope(Make:MEIJI,Japan:Model),and scanning electron microscope(SEM)(Make:Philips,UK:Model).The specimens were etched with 2%inital reagent to reveal the microstructure of theweld region of LHF weld,base metal(BM)and HAZ regions.Aqua-regia and Kalling's reagent was used to reveal the microstructure of the ASS weld region and HNS weld region respectively.

Thin foils were prepared from the base metal and weld metal regions of the joints to analyze the microstructure using transmission electron microscope(TEM)(Make:Philips,UK:Model).Foils were made using window technique at 10-12V DC at a temperature below-40οC using electrolyte containing perchloric acid and methanol.Thin foils after electrolytic thinning were cleaned with methanol and dried.The amount of delta ferrite present as secondary phase in the austenitic stainless steel weld metal was measured using ferritoscope(Make:Helmoltz Ficher GMBh,Germany:Model)as per AWS A 4.24-74 guidelines,quantitatively as ferrite number(FN).

2.4.3.Microhardness

Vicker's microhardness testing machine(Make:Shimadzu,Japan;Model HMV-T1)was employed with 0.5kg load for measuring the hardness across the weld as shown in Fig.2 as per ASTM E-384-05 guidelines(ASTM,2005).Five readings were taken in at close proximity distance in each zone and mean values are used for further analysis and discussion.

2.4.4.Microchemistry using electron probe micro analysis(EPMA)

The micro chemistry along the weld-HAZ interface of all the joints was analysed using CAMEBAX MICRO-439 unit.The concentration of Cr,Ni and Mn at intervals of 5μm was obtained over a length of 350μm across the weld-HAZ interface on both the sides to study the diffusion of elements from weld metal to base metal across the interface as shown in Fig.3.

2.5.Residual stresses

The residual stress measurement was carried out using X-ray stress analyzer(Make:Stresstech OY,Finland;Model:XStress 3000)employing Cr Kαradiation in a single pass weld as like the same deposited for implant testing.Residual stresses were evaluated in this analyzer with multiple exposure sin2ψtechnique based on the diffraction from(300)planes in ASS welds and from(211)planes in ferritic welds.The residual stresses experiment comprise of several measurements of lattice spacing over a range ofψorientations(-45oto+-45o)to the surface of the specimen.Residual stress measurements were carried out across the weldment(i.e.,perpendicular to the welding direction).The accuracy of the measurements being approximately±20 MPa.For computation of stresses from strain data,appropriate X-ray elastic constants were used.Prior to the measurements of stress,the surfaces were cleaned with acetone solution.Following this,the spots were electropolished using electro polishing kit with 20%perchloric acid in ethanol,cooled to 0oC prior to measurements.

3.Results

3.1.Macrostructure

Macrostructure of the joints are displayed in Fig.4 and there is no evidence of delayed cracking(due to HIC)in all the joints.Due to the minor variations in the heat input of welding processes(ref Table 2),an appreciable variation in the width of the HAZ region is evident from the macrostructure of the welds.It is also observed that the ASS joints(Fig.4(a)&(b))have a wider HAZ region while the LHF joints(Fig.4(c)&(d))contain narrow HAZ.However,the HAZ width of the SN joint(Fig.4(e))is wider than SA and SF joints.

Moreover,the joints fabricated using SMAW processes have a narrow HAZ width compared to their respective FCAW counterparts.

3.2.Microstructure

The microstructure of the base metal consists of an acicular martensite(Fig.5(a)&(b))structure with fine needles of lath martensite as shown in Fig.5(c).Microstructure of the joints was examined at different locations and optical micrographs taken at different regions of welded joints are displayed in Figs.5-7.From the micrographs,it is understood that all the joints invariably contain three distinctive regions.They are:(i)weld metal(WM)region,(ii)fusion zone(FZ)(ii)heat affected zone(HAZ).The distinct microstructural features of the various regions are presented in the following sections.

3.2.1.Weld metal region

A duplex microstructure of fine skeletal delta ferrite in a plain austenitic matrix is revealed in the weld metal region of ASS joints(Fig.6).However,the morphology of the delta ferrite in the SA and FA welds are not the same.The FA weld exhibits much widely spaced delta ferrite(coarse)in a plain austenitic matrix(Fig.6(b)&(d))where as the SA welds exhibit much closely embedded( fine)delta ferrite in a plain austenitic matrix(Fig.6(a)&(c))and the above delta ferrite morphology is evident from the TEM micrographs of the respective joints(Fig.6(e)&(f)).The delta ferrite content is lower in SA joint(5.8 FN)while the FA joint weld metal contains larger proportion of delta ferrite(6.7 FN).The weld metal region LHF joints are characterized by ferritic microstructure(Fig.7).But a distinct ferrite microstructural morphology is revealed in the weld metal region of SF and FF joints.The weld metal region of SF joint exhibits fine acicular ferrite morphology(Fig.7(a)&(c))where as FF weld metal shows coarse polygonal ferrite matrix(Fig.7(b)&(d)).The above features are clearly evident fromthe TEM micrographs(Fig.7(e)&(f)).A fine needle like acicular ferrite morphology is evident from the TEM micrograph of weld metal region of SF joint(Fig.7(e))while a coarse plate like polygonal ferrite is revealed in the weld metal region of FF weld metal region(Fig.7(f)).A larger proportion of plain austenitic phase with a scattered delta ferrite matrix is revealed in the weld region of the SN joint(Fig.8(a)&(b)).The delta ferrite morphology(Fig.8(c))is quite different from the ASS welds and is much finer and thin as evident from Fig.8(c).

3.2.2.Fusion zone(Weld-HAZ interface)

The fusion zone microstructures of the joints are shown in Fig.9.The weld-HAZ interface region of the ASS joints reveals a white phase(WP)(Fig.9(a)&(b)).The width of white phase is larger for FA joint(Fig.9(b))compared to that of SA joint(Fig.9(a)).Unlike the ASS joints,the LHF joints do not have any white phase region but they are characterized by untempered martensiticstructure(Fig.9(c)&(d)).A finer untempered martensitic structure is revealed in the fusion zone of SF joint(Fig.9(c))compared to the fusion zone of FF joint(Fig.9(d)).On the other hand the interface microstructure of the SN joint does not have grain boundary phase as in the case of SA and FA joints but has a smaller proportion of unmixed zone near to the periphery of the fusion boundary along with a softened layer of untempered martensite structure as shown in Fig.9(e).

3.2.3.Heat affected zone(HAZ)

The micrographs of fine grain HAZ(FGHAZ)region show softened region of untempered martensite in all the joints(Fig.10).Similarly,all the joints reveal tempered maretensite structures in their respective coarse grain HAZ(CGHAZ),away from the fusion boundary(Fig.11).It is evident from Figs.10 and 11 that there are minor variations in the martensitic microstructural features in the HAZ regions of the joints and this may be due to the differences in heat input of the welding process.It is evident from the micrographs of the HAZ region that a finer martensitic structure is revealed in the HAZ regions of the LHF joints(Fig.10(c)&(d))and(Fig.11(c)&(d))compared to ASS joints(Fig.10(a)&(b))and Fig.11(a)&(b))irrespective of the welding process used.Similarly the joints fabricated using SMAW process revealed a finer martensitic structure(Fig.10(a)&(c))and Fig.11(a)&(c))compared to their FCAW counterparts(Fig.10(b)&(d)and Fig.11(b)&(d))among the joints fabricated using ASS and LHF consumables.The micrograph of the SN joint is much coarser than SA and SF joints as revealed from Figs.(10(e)&11(e)).

3.3.Hardness

The hardness across the weld cross section was measured using Vicker's micro-hardness testing machine.Five readings were taken in at close proximity distance in each zone and mean values are presented in Table 3 and the hardness traverse across each weldment is displayed in Fig.12.The hardness of the unwelded base metal is 455HV.SA joint exhibits a hardness of 261 Hv in the weld metal region,while FA joint recorded 245 HV.Similarly the weld metal hardness is found to be 311 Hv and 294 Hv for SF and FF joints,respectively.The SN weld metal region exhibited only 202 Hv and is lower than all other joints.However,the SF joint shows higher hardness in the weld metal region compared to other joints.

The hardness in the region close to the weld/HAZ interface in the weld region side(white phase region in case of ASS joints;Region of hard untempered martensite in case of LHF joints;region of unmixed zone in case of SN joint)of the SA,SF,SN,FA and FF joints are 400 Hv,466 Hv,385 Hv,390 Hv and 450 Hv respectively.Similarly the hardness in the fusion boundary of SA,SF,SN,FA and FF joints are 480 Hv,520 Hv,460 Hv,465 Hv and 502 Hv respectively.The SN joint exhibit a lower hardness in the above regions while the SF joints exhibit a maximum hardness compared to other joints.

The hardness value in the HAZ region is found to be lower than that of the base metal hardness invariably in all the joints.The hardness in the FGHAZ region(beside boundary)of SA,SF,SN,FA and FF joints are 437 Hv,443 Hv,431 Hv,424 Hv and 427 Hv respectively.The hardness in the CGHAZ region(away from the fusion boundary)of SA,SF,SN,FA and FF joints are 425 Hv,430 Hv,418 Hv,465 Hv and 502 Hv respectively.The above variations are due to the differences in the heat input(ref Table 2).It is evident from the above Fig.and Table 3,that there is a larger degradation in hardness values in the CGHAZ region away from the fusion boundary.This indicates that a soft zone does exist in the CGHAZ region invariably in all the joints.This softening effect is very much less in case of LHF joint,compared to ASS joints.Also,the SMAW joints showed lesser softening in CGHAZ region than their FCAW counterparts irrespective of the process used.However,the SF joint show the least degree of HAZ softening compared to all other joints.

3.4.Electron probe microanalysis

The electron probe microanalysis(EPMA)profiles taken across the joints are displayed in Figs.13-15.From the EPMA profiles,it is inferred that the diffused zone or intermixed zone is minimum in all the joints and it is only few microns width resulting in a good metallurgical bonding.The diffused or intermixed zone width is very small in SN joint compared to other joints.However,the width of diffusion or intermixed zone in LHF joints is marginally narrower compared to ASS joints.Similarly,the width of the diffusion or intermixed zone of the joints fabricated using SMAW process is narrow by few microns than their FCAW counterparts irrespective of the consumables used.

3.5.Residual stress

The transverse residual stress distribution measured across the weldments is shown in Fig.16.In all cases,the peak(maximum)residual stress is observed in fusion boundary.The ASS joint exhibits a lower magnitude of residual stress distribution than their LHF counterparts irrespective of the welding processes used.Similarly,the magnitude of residual stress distribution in FCAW joints is lower than SMAW counterparts irrespective consumables used.However,the SN joints show a lower magnitude of residual stress compared to other joints.

4.Discussion

It is inferred from the above results of various analysis carried out that the armour grade Q&T steel joints fabricated using ASS,LHF and HNS consumables by SMAW and FCAW processes exhibit variant metallurgical features.These metallurgical features will have a directin fluence in the performance of the joints.The reasons for the above results are discussed in detail in the following sections.

4.1.Macrostructure of the joints

The macrostructure of the welded joints(Fig.4)reveals non existence of cracks or defects in all cases.The risk of delayed cracking due to diffusible hydrogen does not prevail with the joints fabricated using LHF consumables as the macrostructure(Fig.4(c)&(d))reveal no evidence of cracks in the weldment.Thus,it is inferred that the joints fabricated using LHF consumables offer required resistance to HIC similar to the joints fabricated using ASS and HNS consumables and the major reason being the lower level of diffusible hydrogen present in weld metal[18,19].

The most interesting feature from the macrostructure of the joints is that there exists a considerable difference in the width of the HAZ.The minor difference in the width of the HAZ region is attributed to the difference in the weld thermal cycle(heat input)which is a characteristic feature of the welding consumables and processes used.The width of the HAZ for the joints fabricated with lower heat input is smaller while it is higher for the joints fabricated with a higher heat input.A joint with smaller width of HAZ is most preferred for structural applications especially in combat vehicle construction as the width of the HAZ is related to HAZ softening phenomenon which has a direct influence on the ballistic performance[20].In the present case,it is evident from Table 2,the heat input of the ASS joints are higher than their LHF counterparts irrespective of the process used.Similarly,the heat input of the joints fabricated using SMAW joints is lower than their FCAW counterparts irrespective of the consumables used.However,the heat input of the SN joint fabricated using HNS consumables are characterized by higher heat input than SA and SF joints.The influence of heat input is evident from width of the HAZ revealed in the macrostructure presented in Fig.4.The heat affected zone of the SF joints(Fig.4(c))exhibited a narrow width than other joints,fabricated forthis investigation,duetolowerheat input.This canbe attributed to a better structural integrity of the joints compared to all other joints considered in this investigation.

4.2.Microstructure of the weld metals

The weld metal chemistry of SN joint shows the presence of nickel 63(wt%),chromium 15.95(wt%)and Manganese 6.45(wt%).However,the weld metal of SA joint has 9.18(wt%)nickel and 19.614(wt%)of chromium and the FA joint weld metal has 8.36(wt%)of nickel and 19.854(wt%)of chromium.Also the SA joint weld metal has 6.59(wt%)of manganese against of 6.04(wt%)in the FA joint weld metal.Thus SA joint weld metal has higher nickel and manganese content than FA joint.But FA weld metal has slightly higher chromium content than the SA joint.This difference in composition of nickel,chromium and manganese in the SA and FA joints weld metals has larger influence on the microstructural features.Although the microstructural features observed in the weld metal region of SA and FA joints are similar,the difference is attributed to the presence of second phase,namely,the delta ferrite.Thus the delta ferrite content is lower in SA joint(5.8 FN)while the FA joint weld metal contains larger proportion of delta ferrite(6.7FN).Approximately 4-8%of delta ferrite in austenitic matrix has been considered effective in controlling micro-cracking in austenitic stainless steel.More than 10%of delta ferrite is not desirable since it causes a reduction of ductility and impact toughness at low temperatures and forms harmful sigma phase at higher temperatures[21].

It is understood that the solidification behaviour of austenitic weld metal is mostly dependent on the thermal history.Heat input and cooling rates associated with SMAW process and FCAW process have been identified to be the factors responsible for resulting features delta ferrite morphology and content in the austenitic stainless steel weld metal[22].Substantial variations in the delta ferrite content from weld to weld were observed for welds produced under identical conditions.Coarseness of the delta ferrite distribution is attributed to substructure size,which in turn is governed by relative weld energy heat-input utilized[23].Variation in amount and distribution of the ferrite phase is due to the inherent heterogeneity of the weld-metal solidification process.The microstructures and mechanical properties of stainless steel were studied using a directional solidification technique and it is found that the cooling rates associated with weld deposits have a strong influence on the characteristics yielding different microstructures and showing a marked difference in mechanical behaviour[24].The solidification behaviour and subsequent solid state transformations are influenced by cooling rates associated with welding.

The cooling rates associated with SA and FA joints in the present study are not quantified.However,FCAW process which is characteristic of higher deposition rate with relatively higher heat input(2.37 kJ/mm)as compared with the SMAW process that has lower heat input(1.60kJ/mm)seems to have altered the cooling rates in the FA joint weld metal resulting differences in the primary austenitic phase and secondary delta ferrite phase respectively.The minor variations in weld metal chemistry of nickel,chromium and manganese together with smaller difference in heat input has yielded a significant influence on delta ferrite content and morphology of the weld metal region of SA and FA joints.Thus the microstructure in the weld metal region of FA joint reveals widely spaced delta ferrite(coarse)in a plain austenitic matrix(Fig.6(b)&(d))where as the SA weld metal region exhibit much closely embedded( fine)delta ferrite in a plain austenitic matrix(Fig.6(a)&(c))and the above delta ferrite morphology is evident from the TEM micrographs of the respective joints(Fig.6(e)&(f)).However,the microstructure of the SN joint weld metal region reveals higher proportion of plain authentic matrix with a scattered delta ferrite(Fig.8)due to higher nickel and lower chromium content compared to ASS welds.

It is a common practice to correlate the various weld metal properties with heat input.The difference in the ferrite morphology of high strength steel welds is due to the difference in heat input[25].The formation of acicular ferrite is controlled by weld heat input.Thus,if the heat input is higher,the content of the acicular ferrite will be very less and vice versa.On the other hand,higher heat input will enhance the formation of coarse pro-eutectoid ferrite or polygonal ferrite in the weld metal region [26].Different weld bead morphologies are likely to lead to different weld cooling rates that will affect the microstructure(Harrison and Farrar,1987).Weld cooling rate plays the decisive role in determining weld microstructure in high strength steels.The general effect of increasing the cooling rate is to lower transformation temperatures.When cooled at sufficiently low rates,the microstructure predominantly tends to become polygonal ferrite[27].In the present investigation,heat input of 1.33 kJ/mm was recorded during the fabrication of SF joint and 2.03kJ/mm was recorded during fabrication of FF joint.The weld metal region of SF joint exhibits a fine acicular ferrite morphology(Fig.7(a)&(c))where as FF weld metal shows coarse polygonal ferrite matrix(Fig.7(b)&(d)).The above features are clearly evident from the TEM micrographs(Fig.7(e)&(f)).A fine needle like acicular ferrite morphology is evident from the TEM micrograph of weld region of SF joint(Fig.7(e))while a coarse plate like polygonal ferrite is revealed in the weld region of FF weld metal region(Fig.7(f)).

4.3.Microstructure of the fusion zone(weld/HAZ interface)

In case of similar welds,the microstructure of the weld metals and HAZ will be unique and essentially the zone adjacent to the fusion boundary is a characteristic feature of the base metal and weld metal.But this is not true in the case of dissimilar welds.The fusion boundary microstructure in dissimilar welds often possesses some unique features.Normal epitaxial nucleation during solidification along the fusion boundary gives rise to grain boundaries that are continuous from the base metal into weld metal across the fusion boundary.These boundaries are roughly perpendicular to the fusion boundary and have been referred to as“Type I”boundaries.In dissimilar welds,where an austenitic weld metal and ferritic base metal exist,a second type of boundary that runs roughly parallel to the fusion boundary is often observed.This has been referred to as a “Type II”boundary[28].These boundaries typically have no continuity across the fusion boundary to grain boundaries in the base metal.Several investigators have reported that hydrogen-induced disbonding typically follows Type II grain boundaries[9,29]and this is a great disadvantage in using ASS consumables for welding Q&T steels in view of structural integrity.

The fusion zone microstructure of the ASS welds(SA and FA)exhibits a soft white phase of the interface similar to that of the type II boundaries as described above.The region of white phase and fusion boundary are located in close proximity distance to each other.The formation of the white phase(rich in carbon and chromium)in the FA and SAwelds is due to the diffusion of carbon from base metal region to weld metal region and migration of chromium from weld metal region to base metal region[30]and it depends upon the weld thermal cycle employed for fabricating the joints.In this study,the rate of diffusion of elements(carbon and chromium)across the fusion zone is higher for FA joint due to relatively higher heat input(2.37 kJ/mm)compared to SA joint that recorded a lower heat input(1.60 kJ/mm).The width of the white phase in FA weld(Fig.9(b))is relatively larger compared to SA welds(Fig.9(a)).The microhardness values(Table 3)reveal that FA weld has a softer white phase region compared to SA joints.The above variations in the grain boundary phase features are due to the difference in the heat input employed for fabricating the welds and the rate of diffusion of elements.However,interface region of the SN joints is characterized by a smaller proportion of unmixed zone near to the periphery of the fusion boundary(Fig.9(e)).This is due to the fact that high nickel content acts as a diffusion barrier obstructing the diffusion of elements across the fusion boundary[9]which results in a much lower hardness(385 Hv)in the unmixed zone.

On the other hand,the fusion zone microstructure of the LHF joints(SF and FF)has hard untempered martensite and no type II boundary exists adjacent to the fusion boundary.The crack followed this region of untempered martensite(Fig.7(c)&(d))in LHF welds.The formation of untempered martensite is due to the diffusion of carbon from the base metal to the weld metal region and is greatly influenced by the heat input employed for fabricating the LHF joints.FF welds recorded relatively a higher heat input(2.03 kJ/mm)than the SA welds(1.60 kJ/mm).Thus,the rate of diffusion of carbon from the base metal region to the weld metal region in the FF welds is relatively higher compared to SF welds.This resulted in minor variations in fusion zone characteristics(region of untempered martensite and fusion boundary)and it is clearly evident from the micro hardness values in the above region(Table 3).Thus,the hardness in the region of untempered martensite and the fusion boundary is higher for SF joint compared to FF joint.

4.4.Fusion boundary characteristics

In fusion welding,the bulk weld metal is usually well mixed,so that if the composition is known,or can be predicted from the depletion of elements from each side of the joint,the bulkproperties canbe reasonably well predicted.Atthe fusion boundary there are,however,two zones between the bulk of the weld metal and the HAZ.The first of these consists of the parent metal which has melted but has mixed with only a small proportion of the weld metal by diffusion.This is because weld pool motion,although vigorous,is laminar and next to the unmelted metal the liquid,i.e molten parent metal,is stagnant.The second zone is unmelted HAZ into which smaller proportion of weld metal has diffused in the solid state at high temperatures[30,31].

Table 3 Microhardness(HV)values(0.5kg load)a

These features can be clearly seen in the micrograph of the interface region of the SN joint(Fig.10(e))and EPMA traces(Fig.15).The unmelted HAZ was enriched with alloying elements but a retained martensitic structure(Fig.12(e))and is of smaller variations in hardness to the remainder of the HAZ.High nickel content acts as a diffusion barrier and thus not allowing the diffusion of elements on either side of the fusion boundary and hence,the diffused/intermixed zone is very narrow in case of SN joint than other joints and is clearly revealed in the EPMA traces(Fig.15).The melted(WM region)and unmixed zone are sufficiently enriched with alloying elements for it to become austenitic,but the content of the alloying elements was not enough to enhance the hardness and strength of the bulk of the weld metal which is characterized by high austenitic phase(Fig.8).The hardness of the fusion boundary of the SN joint is found to be relatively higher(465 VHN)than other regions,is possibly because it was slightly rich in carbon,which could have diffused from the HAZ region.However,the carbon depletion is not quantified in this investigation and corresponds to similar observations made by Madhusudhnan Reddy et al.,[9].

In case of SA joint,the difference in weld metal chemistry is very small compared to FA joint.However,the interface microstructure of SA joint(Fig.9(a))and FA joint(Fig.9(b))reveals a white phase region consisting of highly alloyed martensite.However,the width of the white phase region is larger in case of FA joint(Fig.9(b))compared to SA joint(Fig.9(a)).Despite the surrounding austenite in WM region which is capable of holding hydrogen safely in solution,it is also liable to suffer HIC,which is a disadvantage of using ASS filler for Q&T steel joints and welding has to be carried out in a protective atmosphere using a thoroughly investigated process parameters.Though a well defined white phase region does exist in the micrograph of the ASS joints(Fig.9(a)&(b)),there is no evidence of cracking after welding due to negligible amount of diffusible hydrogen present in weld metal[18,19].

The hardness of fusion boundary in ASS joints is much higher compared to WM region and HAZ region(Table 3).The major reason for the same may be that the fusion boundary is enriched with high alloying elements due to diffusion of elements across the fusion boundary on either side and is clearly evident form the EPMA traces(Fig.13).It is also evident from the EPMA traces(Fig.13)that the diffused/intermixed zone is higher in case of FA joint compared to SA joint and is due to the minor variations in the heat input used to fabricate the SA and FA joints.Thus,hardness in the fusion boundary of the SA joint is 480 Hvand higher than the FA joint(460 Hv).

The chemistry of SF weld metal does not show much variation compared tothe base metal and both(WM and BM)are of low alloy type.It is evident from the EPMA traces that the extent of diffusion of elements on either side of the fusion zone is marginally smaller in LHF joints(Fig.14)compared to ASS joints(Fig.13).Hence,defect free,good metallurgical bonding are obtained in LHF joints as evident from the macrostructure(Fig.3(c)&(d)).The extent of diffusion of elements in SF and FF joints are notsimilardue to minor variations in heat input.Thus,the intermixed or diffused zone is smaller in SF joint than FF joint and it is clearly evident from the EPMA traces(Fig.14).Unlike ASS joints there is no white phase region or unmixed zone in LHF joints(Figs.9(c)&6(d)).The fusion boundary of LHF joints are characterized by untempered martensitic region and are extremely harder than the fusion boundary hardness of the ASS joints.The fusion boundary hardness of SF joint is 520 Hv and that of the FF joint is 502 Hv which is relatively higher than other regions.This is possibly because of slightly rich carbon content,which could have diffused from the HAZ region[9].

4.5.Heat affected zone softening characteristics

Welding of high strength and high hardness Q&T steel involves HAZ softening and it is a characteristic feature of the welding processes and welding consumables used(Rodrigues et al.,2004).The degree of softening in the HAZ is a function of weld thermal cycle,which is a characteristic of the welding process employed.The softening characteristics also depend on the kinetics of the phase transformation of the steel and are a function of the chemistry of the steel utilized[32,33].The presence of a soft zone in a welded structure maylimit the design strength to a lower value and may also influence the ballistic behaviour of weldments when put into armour applications[20].In the present work,a soft zone with a tempered martensite does exist in the CGHAZ of all the joints but the softening is not uniform.The SF joint has a lesser degree of softening in CGHAZ than other joints.The above difference in the HAZ softening phenomenon is due to the difference in the heat input involved during welding.

If the heat input is higher,then the extent of softening is more and hence a coarse tempered martensite is found in all the joints.The,lower heat input(1.33 kJ/mm)of SF joints favored higher hardness in CGHAZ(430 Hv)and thus lesser degree of softening in the CGHAZ regionwhen compared to SA and SN joints.Similarly,FF joint recorded a lesser heat input(2.03kJ/mm)during fabrication,showed higher hardness in CGHAZ region(410 Hv)revealing lesser degree of CGHAZ softening than FA joint(403 Hv).Thus,it is evident that the joints fabricated using LHF steel consumables exhibited lesser degree of softening than ASS consumables,irrespective of the welding process.From this investigation,it is found that,the CGHAZ softening is lower in SMAW joints than FCAW joints irrespective of the welding consumables.A similar observation has been made by Madhusudhan Reddy et al.,[20].

4.6.Residual stress

Residual stresses in weldments are caused by non-uniform expansion and contraction of weld metal,HAZ and base parent metal.Usually,residual stresses are caused by three components,viz.,residual stresses due to shrinkage,residual stress due to quenching and residual stress due to phase transformation[34].In practice,welds show a superposition of all these three components and the form of residual stress distribution is therefore determined by the most dominant process among these three.Residual stress due to quenching arises because of differential cooling rates by different locations over plate thickness.This is significant only for a very thick plate sections and is negligible for low thickness plates.Thus the residual stress pattern depends on one of the other two components,viz.,the shrinkage of the weld metal by the adjacent colder regions.Tensile residual stress occurs in regions,which are last to cool.Clearly,shrinkage develops tensile residual stresses at the weld center and compressive elsewhere.Phase transformation stresses are caused by phase transformation of austenite to ferrite or martensite.Tensile residual stresses are produced in regions where the transformation occur first and compressive stresses in regions where the transformations occur at last.

In the present investigation,the there is no phase transformation or quenching effect in the weldments.The residual stresses are inbuilt in the weldments only due to shrinkage effect.The higher coefficient of thermal expansion in nickel steels contributes for a lower magnitude of residual stress in SN joints compared to the SA and SF joints.Similarly,the coefficient of thermal expansion of the austenitic stainless steel is much larger than the low alloy ferritic steels and hence SA joint exhibits a lower magnitude of residual stress than SF joint.Recent investigations by Ref.[34]revealed that the magnitude of the residual stress was found to vary with respect to welding processes.Their findings suggested that FCAW process was associated with lower residual stress as compared to SMAW process[7].Thus,the magnitude of the residual stress is inversely proportional to heat input,i.e.,if the heat input is higher,then magnitude of residual stress is lower and vice versa.Thus FCAW joints have shown lower residual stress than SMAW joints.There is an appreciable variation in heat input of SA and FA joints and the same trend is also observed with SF and FF joints.Thus the FCAW joints recorded higher heat input during fabrication than SMAW joints.The difference in heat input caused appreciable variations in the magnitude of residual stresses are shown in Fig.16.

It is evident from the above illustrations,it is found that LHF consumables are morefittest for joining armour gradae Q&T steel and thus facilitating for a costeffective combatvehicle construction.

5.Conclusions

In this investigation,a detailed metallurgical characterization was carried out on the armour grade Q&T steel welds and was analysed in detail.The following conclusions are derived from the above analysis and illustrations.

(1)No evidence of delayed cracking due to hydrogen was observed in all the joints.

(2)The heat input played an important role to alter the morphology of the microstructure of the various zones of the joints.

(3)The ASS joints are characterized by formation of white phase at the interface zone and the SN joints are characterized by unmixed zone at the interface.

(4)The LHF joints exhibited less degree of HAZ softening due to lower heat input than all other joints.

(5)The residual stress distribution does show appreciable variation in all the joints.

(6)The LHF consumables arefit to be used for joining armour grade Q&T steel joints and thus paves way for cost effective fabrication of combat vehicles.

Acknowledgements

The authors are thankful to Armament Research Board(ARMREB),New Delhi for funding this project work(Project no.MAA/03/41),M/s Combat Vehicle Research Development Establishment(CVRDE),Avadi,Chennai for providing base material and Department of Manufacturing Engineering,Annamalai University for providing testing facility and M/s Defence Metallurgical Research Laboratory(DMRL),Hyderabad for providing the facility to carry out metallurgical characterization for this investigation.