M icrostructure and corrosion behaviour of gas tungsten arc welds of maraging steel

G.MADHUSUDHAN REDDY，K.SRINIVASA RAO*

aDefence Metallurgical Research Laboratory，Hyderabad，India

bDepartmentofMetallurgical Engineering，Andhra University，Visakhapatnam，India

Received 25 August2014；revised 22 September 2014；accepted 25 September 2014 Available online 27 November 2014

M icrostructure and corrosion behaviour of gas tungsten arc welds of maraging steel

G.MADHUSUDHAN REDDYa，K.SRINIVASA RAOb，*

aDefence Metallurgical Research Laboratory，Hyderabad，India

bDepartmentofMetallurgical Engineering，Andhra University，Visakhapatnam，India

Received 25 August2014；revised 22 September 2014；accepted 25 September 2014 Available online 27 November 2014

Superior properties of maraging steelsmake them suitable for the fabrication of components used formilitary applications likemissile covering，rocketmotor casing and ship hulls.Welding is themain process for fabrication of these components，while themaraging steels can be fusion welded using gas tungsten arc w elding（GTAW）process.A ll these fabricated com ponents require longer storage life and amajorproblem inweldsissusceptible to stress corrosion cracking（SCC）.Thepresentstudy isaimed atstudying the SCC behaviourofMDN 250（18%Ni）steel and itsweldsw ith respect tomicrostructural changes.In the presentstudy，5.2mm thick sheetsmadeofMDN 250 steel in the solution annealed conditionwaswelded using GTAW process.Post-weld heat treatments of directageing（480°C for3 h），solutionizing（815°C for1 h）followed by ageing and homogenizing（1150°C for1 h）followed by ageingwere carried out.A mixtureofmartensite and austenitewas observed in the m icrostructure of the fusion zone of solutionized and directaged weldsand onlymartensite in as-w elded condition.Homogenization and ageing treatment have elim inated reverted austenite and elemental segregation.Homogenized w elds also exhibited a marginal im provement in the corrosion resistance compared to those in the as-welded，solutionized and aged condition.Constant load SCC test data clearly revealed that the failure timeof homogenizedweld ismuch longer compared to other postweld treatments，and thehomogenization treatment is recommended to improve the SCC life of GTA welds of MDN 250Maraging steel.

18%Nimaraging steel；Gas tungsten arc welding；Post weld heat treatment；Solutionising；Ageing treatment；Pitting corrosion；Stress corrosion cracking（SCC）

1.In troduction

The high yield strength and excellent fracture toughnessof maraging steelmake it suitable for defence applications such asmissile，rocketmotor casing and ship hull.Agehardening is the main strengthening mechanism which develops yield strength of 2400 MPa，of low carbon martensite w ith nickel，cobaltand molybdenum precipitates［1］.The components for the defence applications are fabricated using variouswelding processes［2-4］.One of the primary requirements of fabricated components is long storage life.The corrosion isone of themajor problems during storage.It isnow well established that the high strength steels suffer from poor stress corrosion cracking（SCC）resistance and their SCC resistance decreases w ith increase in strength［5］.Previous investigations revealed that the threshold yield strength value for SCC in high strength steels is about 1400 MPa［6］.Hence it can be concluded that ultra high strength maraging steels also suffer from SCC problem.A lthough SCC behaviour of maraging steel in w rought condition has been studied in some detail，work available on the SCC behaviour of maraging steel welds is relatively scarce.Hence，any attemptmade to understand SCC behaviour ofmaraging steel welds used formaking defencecomponents is important.Themechanisms proposed for SCC of maraging steel are anodic path dissolution and hydrogen embrittlement［7］.Crackingwas found to occur in a plane and inclined to precracking［8］.Crack grow th velocity was found to bemuch faster inwater than in oil［9］.Jha etal.［10］studied SCC ofmaraging steel and found that an intergranularmode of cracking occurs.Studymadeon 250 grademaraging steelat elevated temperature in steam has shown that it offers better protection compared to manganese phosphate treatment［11］. SCC susceptibility of steel was caused by acid dip step in the pretreatment of phosphating process［12］.Brook et al. studied the role of oxygen in SCC ofm araging steeland found that cracking occurred along prior austenite grain boundaries［13］.Mellor et al.found themechanism of hydrogen embrittlement in SCC failure of maraging steel components［14］. SCC behaviour of w rought maraging steel was studied in detail，but the investigations on welds are very limited. Kenyon etal.revealed that themaraging steelweldshave poor SCC resistance compared to base metal［15］.One of the problems in fusion welding ofmaraging steel is the segregation of alloying elements in the interdendritic arm spacing. This may cause the formation of reverting austenite in the fusion zone of the welds.Both the segregation and reverted austenite affect the toughness and SCC resistance ofmaraging steel welds.Studies on the influences of reverting austenite and segregation on corrosion behaviour of maraging steel welds have not been available in the existing literature.Understanding of the corrosion behaviour of these welds is important in exploring a remedy to overcome the problem of cracking during storage of welded componentsmade ofmaraging steel.The present study isaimed at studying the pitting and stress corrosion behaviours of MDN 250（18%Ni）steel and its welds.

Fig.1.Weld joint preparation details.

2.Experimental

MDN 250（18%Ni）steelof 5.2mm thick sheetwasused in the present study.The preparation detailsof theweld jointare shown in Fig.1.Welding was carried out using gas tungsten arc welding process.Table 1 gives the welding variables. Table 2 gives the chem ical composition of the basemetal and fillerw ire.The stepsof postweld heat treatmentare（i）ageing at 480°C/3 h，followed by air cooling；（ii）Solutionizing at 815°C/1 h/air cooling+ageing at480°C/3 h/air cooling；and（iii）Homogenizing at 1150°C/1 h/air cooling+ageing at 480°C/3 h/air cooling in the study of corrosion behaviour. Optical microscopy was used to study the m icrostructural changes of MDN 250（18%Ni）steel during welding.Stress Tech 3000 X-ray system using CrKαradiation was used to measure the retained austenite contentand its fraction inwelds and basemetal.

A Constant load type SCC test rigwas used for SCC testing of both base metal and welds.Standard tensile and SCC specimens（Fig.2）weremachined out from the 5.2mm rolled plate of the maraging steel and its welds as per ASTM E8 standards.3.5%NaCl solutionwasused foralternate cyclesof immersion of specimen.A Constant load of 50 percent yield strength value of specimen for a given condition was applied. The Yield strength valuesare determ ined from tensile test data given in Table 3.The time taken for failure of the specimen at a constant load was used to compare the different conditions of steel.The longer the time to failure is，higherw ill be the SCC resistance.

Table 1 Gas tungsten arc welding variables.

3.Resu lts and discussion

3.1.M icrostructure

The Optical micrograph of the base metal is shown in Fig.3，which clearly indicates themartensitew ith grain sizeof about11μm.The opticalmicrographswere also recorded at different locations（A，B，C，D and E）on theweld and heat affected zone（HAZ）as shown in Fig.4.Martensite was formed at Location A and becomes coarser at Location B due to relatively high temperature atwelding interface.

The grain size ofmartensite at Location C is finer due to lower temperature in fusion zone.A t Location D，the m artensite phase experiences the peak tem peratures in the range of 590-730°C atwhich a reverted austenitemay form［16］and two-phase mixture is found.The unaffected base metal isat Location E.Themicrostructuresof the fusion zones of welds in the as-welded and postweld heat treated conditions are shown in Fig.5.Only themartensite can be seen in as-welded condition（Fig.5（a））.The mixture of martensite and austenite can be observed in themicrostructuresof fusion zones in the directaged and solutionized conditions（Fig.5（b）and（c））.

The pools or islands of austenite phase can be observed in the fusion zone of maraging steel welds w ith bright/white appearance.Itwas reported earlier that in a homogeneous18% Nimaraging steel（250 grade）composition only at heating temperature of above 650°C［17］.In the present investigation，these poolswere found to be formed even in direct ageing at 480°C.Since the austenite islands are present along the boundaries of dendritic cells，the alloying elements may segregate in these locationsand helps in lowering theaustenite reversion temperature.Homogenization treated fusion zone m icrostructure consists of low carbon martensitew ithout any reverted austenite（Fig.5（d））.Therefore the homogenization treatment can be used to m inim ize the segregation and avoid the formation of reverted austenite.Electron probe micro analysis（EPMA）was carried out to assess the segregation of alloying elements at dendrite boundary and center location through chem ical composition analysis.EPMA data are given in Table 4.

Table 2 Composition of basemetal and fillermetal.

Table 3 Tensile properties of basemetal and maraging steelwelds.

Fig.2.Schematic diagram of the specimen used for tension and SCC testing.

Fig.3.M icrostructutre of basemetal.

EPMA results of as welded condition clearly reveals that the segregation of alloying elements such as nickel，molybdenum and titanium at the boundaries is high.However，the segregation of cobalt and aluminium was very low at the boundaries compared to bulk composition.Solutionized treatment isable to reduce thesegregation of elements to some extent，butnot complete.

EPMA X-ray maps of the fusion zones are given in Figs.6-8 and reveal the distribution of elements.Homogenization treatment is able to eliminate the segregation of Mo and Ti due to dissolution in the materialmatrix.The distribution of alloying elements was found to be more uniform after hom ogenization treatment compared to the as-welded and solutionized conditions.An X-ray method was used tomeasure the austenite fraction to confirm the effect of segregation elimination on the formation of reverted austenite，xraymethod of austenite fractionmeasurementwas carried out. The results of X-ray determ ination of austenite volume percentage are given in Table 5.It is very clear that the homogenization treatment could avoid the formation of reverted austenite.The above results confirm that the elimination of segregation during ageing could avoid the formation of reverted austenite.

Fig.4.M icrostructures of（A）.weld，（B，C，D）.Heat affected zones and（E）.Basemetal ofmaraging steel in as-welded condition.

Fig.5.M icrostructure of the fusion zone.

3.2.General corrosion

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Maraging steel is prone to general corrosionmainly due to themartenistematrix w ith intermetallic precipitatesw ith local strain fields.Galvanic coupling is formed between the precipitates and thematrix leads to continuous dissolution in the NaClenvironment.Potentiodynamic polarization testsused for corrosion evaluation did not result in passive behaviour and led to cathodic polarization and anodic dissolution w ithout any passivation.Thepolarization curvesofweldsaregiven in Fig.9 .Corrosion resistancemainly dependson the typeofageingand the treatment prior to ageing.Corrosion potential（Ecorr）is used to compare the corrosion behaviours ofmaraging steel welds in different heat treatment conditions as there is no possibility of passivity andmeasuring pit potential value.The greater the positive value of Ecorr is，the higher w ill be the general corrosion resistance.The effectof postweld treatment ongeneralcorrosion ofmaragingsteelweldsmadeofmaragingsteel fillerwas studied，and the corrosion potentials and rates are given in Table 6.The corrosion potential and rate data clearly reveals that thegeneral corrosion resistance ofwelds is improved after homogenization treatment.Thismay be attributed to the possible microstructural changes that occur after homogenization treatment.The interface between the reverted austenite and themartensite isan area inwhich the nucleation of corrosion pits occurs.Segregation is also a source of corrosion nucleation and decreases corrosion resistance.From them icrostructural studies，itwas found that homogenization treatment reduces segregation significantly and elim inates reverted austenite in the fusion zone.Hence the general corrosion resistance，improved significantly after post weld heat treatmentof homogenization and ageing.

Fig.6.EPMA X-ray maps of fusion zone in aswelded condition.

Fig.7.EPMA X-raymapsof fusion zone in aswelded condition solutionized condition.

Fig.8.EPMA X-raymaps of fusion zone in as homogenized condition.

Table 4 EPMA data of dendrite center and boundary in the fusion zone.

3.3.Stress corrosion cracking behaviour

Passivity is generally not observed in 18%Nimaraging steelswhich normally undergo uniform corrosion in themost common environments.A major problem is stress corrosion cracking tendency during long storage，and the type of loading and exposed environment are the im portant factors. Crack initiation occurs at the pits，and crack propagationtakes place due to stress application.Crack propagationmay take place due to any one of the threemechanisms，namely anodic dissolution of the crack tip，hydrogen embrittlement and failure of the corrosion product layer.The intergranular mode of cracking wasmost common mode of SCC failure［18］.Ageing temperature influences the precipitate morphology and was found to strongly affect the SCC resistance［19］.

SCC test data of the welds is given in Table 7.Tim e to failure at50%of yield strength of the steelweld is taken as the criteria to assess the resistance to SCC under sea water environment.SCC occurs due to the combined action of general corrosion and stress.Prediction of SCCmechanism ismainly related to the identification of activepath in themicrostructure for the rapid crack propagation.The test data clearly reveals that post weld homogenizing and ageing heat treatments resulted in the improvement of SCC resistance of welds and the results was found to be similar to general corrosion behaviour of thewelds.

In the present study，the possible active paths of crack initiation and propagation arem icro-segregation and reverted austenite in the m atrix of martensite.The enhancem ent in tensile and yield strength and the reduction in the segregation and the amount of reverted austenite are themain causes of improvement in the SCC resistance of the welds after homogenization treatment.Scanning electron micrography was used to observe the fractured surfaces of failed SCC test specimens of welds in different conditions to reveal the possible reasonsandmechanism of SCC.Themultiple cracks and intergranularcracksareobserved in the fractographsof aswelded specimen（Fig.10）.

The reverted austenite/marteniste interface may be the reason for complete intergranular mode of failure and segregation ofmolybdenum and titanium，which result in the formation of multiple cracks.Segregation and reverted austenite are identified to be the active path sources for the crack initiation and propagation for the SCC of maraging steel welds in the present investigation.The intergranularity of cracking is reduced after the direct ageing of welds（Fig.11）.Quasi-cleavage mode of crack propagation was observed in the fractograph（Fig.12）of solution treated welds，which may be attributed to the reduced amount of reverted austenite and the segregation.The post weld treatm ent of hom ogenization resulted in a cleavage free fracture surface（Fig.13），which is a clear indication of improvem ent in the SCC resistance.

Table 5 Volume percentage of austenite in fusion zone after postweld heat treatment.

Table 6 Corrosion potential and rate values of maraging steel gas tungsten arc weldments.

Table 7 Stress Corrosion cracking test data of marageing steel gas tungsten arc weldments.

Hence thepostweld heat treatmentsof homogenization and a ging of themaraging steelweldsmadew ith basemetal filler w ire are recommended to improve the SCC resistance of maraging steels used for the fabrication ofm issiles and other defence applications.

Fig.9.Polarization curves ofmaraging steel GTA welds.

Fig.10.SEM fractograph of failed SCC testspecimen in aswelded condition.

Fig.11.SEM fractograph of failed SCC test specimen in directly aged condition.

Fig.13.SEM fractograph of failed SCC test specimen in homogenized condition.

Fig.12.SEM fractograph of failed SCC test in solutionized condition.

4.Conclusions

Thepostweld heat treatmentof homogenization eliminated the segregation of alloying elements and avoided the formation of reverted austenite in thematrix of the gas tungsten arc welds ofmaraging steel.

The homogenized and aged welds exhibited higher general corrosion resistance compared to the directly aged，solutionized and aged conditionsand it isattributed to absence of austenite/martensite interface.

The Stress corrosion cracking resistance was found to be highest for maraging steel welds after the post weld heat treatment condition of homogenization.

The post weld heat treatment of homogenization is recommended to have better stress corrosion cracking resistance for the welds of maraging steel used for the fabrication of m issiles and other defence com ponents which require long storage life.

Acknow ledgements

The authors would like to thank Dr.Amol Ghokale，Director，Defence Metallurgical Research Laboratory，Hyderabad for his continued encouragement and permission to publish this work.Financial assistance from Defence Research Development Organization（DRDO）is gratefully acknow ledged.

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E-mail addresses：gm reddy_dm rl@yahoo.com（G.MADHUSUDHAN REDDY），arunaraok@yahoo.com（K.SRINIVASA RAO）.

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