Microstructure and mechanical properties of similar and dissimilar metal gas tungsten constricted arc welds:Maraging steel to 13-8 Mo stainless steel

C.V.S.Murthy ,A.Gopala Krishna ,G.M.Reddy

a Defence Research and Development Laboratory,Hyderabad,500 058,India

b Jawaharlal Nehru Technological University,Kakinada,India

c Defence Metallurgical Research Laboratory,Hyderabad,500 058,India

Keywords:Maraging steel(250)13-8 Mo stainless steel Mechanical properties Microstructure

A B S T R A C T Maraging steel(250)and 13-8 Mo stainless steel plates were joined by gas tungsten constricted arc welding(GTCAW)process in similar and dissimilar metal combinations using 13-8 Mo stainless steel filler wire.The similar and dissimilar metal welds made in solutionized condition were subjected to standard post weld hardening treatments direct ageing at 485°C,soaking for 3½ hours followed by air cooling(ageing treatment of maraging steel)and direct ageing at 510°C,soaking for 4 h followed by air cooling(ageing treatment of 13-8 Mo stainless steel).The joint characterization studies include microstructure examination,microhardness survey across the weldments and transverse weld tensile test.Similar and dissimilar metal weldments responded to both the post weld ageing treatment.After post weld aging,increase in yield strength,UTS and slight reduction in%elongation of similar and dissimilar metal were observed.The observed tensile properties were correlated with microstructure and hardness distribution across the welds.

1.Introduction

Nowadays a major trend in material research is to make products more functional,powerful and reliable.Another concern is towards optimum usage of material resources and its impact on the environment.Several situations arise in industrial practices which call for joining of dissimilar metals.The materials used are location dependent in same structure for effective and economical utilization of the special properties of each material.This has led to the development of joining of dissimilar metals,so as to meet both the needs of user and to enhance the performance capability.Although fusion and non-fusion techniques of joining have been effectively used for manufacturing components,a comprehensive scientific understanding of the joining process is lacking.

Maraging steels(250)are a class of very low carbon high alloy steels exhibiting a unique combination of ultra-high strength,excellent fracture toughness and good weldability[1,2].The alloy gains its strength from the precipitation hardening of its iron-nickel martensite microstructure.As a consequence,it possesses a combination of strength and toughness superior to other high strength steels by employing a relatively simple heat treatment[3-5].

13-8 Mo stainless steel is used in applications where a combination of tensile strength,fracture toughness,resistance to general corrosion and stress corrosion cracking is essential.The good mechanical behavior of 13-8 Mo stainless steel is commonly attributed uniform precipitation ofβ-NiAl ordered phase in the martensitic matrix[6-8].The important feature of these two steels is that they exhibit good weldability and they attain their strength after respective ageing treatments[2,6].These steels are therefore important candidate materials for critical applications such as rocket motor casings,high pressure bottles,sub-marine hulls,bridge layer tanks,petrochemical equipments etc[8-12].

In defence applications,Maraging steels(250)and 13-8 Mo stainless steels are widely used materials for fabrication of rocket motor casing,pressure vessels,booster casings, flexural pivot,drive shafts,valve parts etc.There are specific applications for different aerospace programs which call for welding between these two materials,wherein critical assemblies,one section requires higher strength with good toughness while another section requires adequate strength and toughness along with corrosion resistance.In recent research,the combination of assembly is such that the section made of 13-8 Mo PH stainless steel requires corrosion resistance as specific region of this part cannot be provided any protective coating while maraging steel can be painted for corrosion protection.

Extensive works has been carried out on welds of individual materials;little has been published concerning the applicability or use of maraging steels in dissimilar metal joints.No details study of the 13-8 Mo stainless steel to maraging steel appears to have been carried out.Therefore a systematic study of the microstructure and mechanical properties of similar and dissimilar metal welds of 13-8 Mo stainless steel and maraging steel was undertaken.Maraging steel(250)and 13-8 Mo stainless steel welds can be readily done in either the solution treated or any of the aged conditions.Smaller sections may be aged directly after welding;however for optimum post weld properties,the component should be solution treated and precipitation hardened after welding.The adoption of dissimilar metal combination provides possibilities for the flexible design of the component by using each material efficiently i.e.benefiting from the specific properties of each material to meet functional requirements.

One of the widely employed fabrication process for high strength steels is fusion welding in general and gas tungsten arc weld of(GTAW)in particular where thin walled component are used.Consistency in weld quality,process control,economy and high weld jointefficiencies are the features of GTAW with respect to these steels employed in aerospace applications.The high arc density processes Gas tungsten constructed arc welds(GTCAW)has been especially developed and employed in aerospace applications.GTCAW is considered advantages over conventional GTAW process in joining thin walled specimen to minimize distortion.The Gas Tungsten Constricted Arc Welding process also known as InterPulse technique is a modification of conventional GTAW.It uses magnetic constriction and high frequency(20,000 Hz)modulation of the arc waveform to produce a constricted arc,which greatly reduces the overall heat input during welding[13].

In this investigation,maraging steel(250)and 13-8 Mo stainless steel plates were joined by GTCAW process.The welding can be done with both the filler wires i.e.maraging steel or 13-8 Mo stainless steel filler wire.13-8 Mo stainless steel filler wire was chosen over maraging steel filler due to high stress corrosion cracks resistance.However the present work has been restricted to welding with 13-8 Mo stainless steel filler wire only.A detailed study of microstructure and its correlation with the hardness and tensile properties in this investigation assumes significance due to non-availability of data on the subject in this dissimilar metal combination of welding.

2.Experimental procedures

The materials investigated are maraging steel(250)and 13-8 Mo stainless steel in the form of 2.5 mm thick sheets.Similar and dissimilar metal combinations were welded in their solutionized condition.Maraging steel(250)was solutionized at 820°C whereas 13-8 Mo stainless steel was at 925°C for 1 h followed by air cooling.Analyzed composition of the parent materials is given in Table 1.In order to investigate the in fluence of post weld hardening on the microstructure and mechanical properties of similar and dissimilar metals welds two standard post weld hardening treatments were employed.First treatment was direct ageing at 485°C soaking for 3½hours followed by air cooling(designated as HT1)and second treatment was direct ageing at 510°C soaking for 4 h followed by air cooling(designated as HT2).The microstructures of parent metals in the solution treated conditions and heat treatment conditions are shown in Figs.1 and 2.The tensile strength values of parent material in different heat treatment conditions are presented in Table 2 for ready reference.The weldments details and corresponding joint designations are given in Table 3.The welding was performed using Gas Tungsten Constricted Arc Welding process and the welding parameters are presented in Table 4.

The weldments microstructures were studied by metallography of various regions using Olympus optical microscope.Modified Fry's reagent(50 ml HCl,25 ml HNO3,1 gm CuCl2and 150 ml water)was used to etch weldments.The micro-hardness was measured at intervals of 500μm across the weldments at midsection on thickness both in as-welded and post weld precipitation hardened conditions for similar and dissimilar metals weld joints.All welded samples were tested for tensile properties in as weld and post weld hardened conditions.The testing procedures,geometry and dimensions of the specimens were as per the ASTM standards.The tests were carried out on 10 ton Servo hydraulic Universal-testing machine at a crosshead speed of 1 mm/min.The overall experimental plan and methodology adopted for the evaluation of welds are shown in Fig.3.

Fig.1.Optical microstructure of maraging steel(250)at 500×:(a)SA(b)HT1(c)HT2 condition.

Fig.2.Optical microstructure of 13-8 Mo stainless steel at 500×:(a)SA(b)HT1(c)HT2 condition.

3.Results and discussion

3.1.Microstructure

The microstructure of the maraging steel(250)in solutionised and aged conditions are shown in Fig.1,exhibits martensite with fine grain size corresponding to nominal ASTM grain size of 10μm(mean linear intercept grain size).Fig.2 shows the microstructure of 13-8 Mo stainless steel base metal in solutionised and aged conditions.Basically the microstructure contains lath type martensite.

Fig.4 shows the optical microstructures of the weld zone and heat affected zones of similar weldments of maraging steel(250)inas-welded conditions.The center of weld zone shows fine grain martensite(zone ‘a’),whereas the interface(zone ‘b’),shows dendritic structure.The region adjoining the weld represents the coarse grain martensitic phase(zone ‘c’).During welding the parent material adjacent to the weld interface(zone ‘c’)is heated to high temperature in the austenite zone where considerable grain growth occurs.On cooling,the metal transforms to martensite and inherits the coarse prior austenite grain size.The zone ‘d’shows dark band region of HAZ.It is a dark etching zone where martensite phase experiences peak temperatures in the range of 595-621°C,where some reverted austenite will therefore form in a martensitic matrix.This zone exhibits two-phase microstructure in which white pools of reverted austenite is formed in the martensite matrix[2,14-17].

Table 2 Tensile properties of base metals.

Fig.5 shows the optical microstructures of the weld zone and heat affected zones of similar weldments of 13-8 Mo stainless steel in as-welded condition.Similar to maraging steel welds,13-8 Mo steel welds also show distinct band in the regions:zone ‘a’to zone‘d’.The structure observed at weld zone is coarser martensite(zone‘a’)compare to maraging steel(250)weld zone(zone ‘a’of Fig.4).The interface(zone ‘b’)is does not shows much variation in structure as the filler wire is of same composition.In this case also dark band region is found(zone d).The dark band region of 13-8Mo stainless steel weld is wider compare to maraging steel weld(zone ‘d’of Fig.4).

Table 3 Joint designation.

Table 4 Welding parameters.

Fig.3.Schematic flow chart showing the experimental details.

Fig.4.Microstructure of similar weld of maraging steel(250)in as-welded condition at 200×:(a)WZ-HAZ interface(b)Weld zone(c)Coarse grain HAZ(d)Dark band region of HAZ.

Different zones of dissimilar metal weldments of as-welded condition are shown in Fig.6.The center of weld zone i.e.zone ‘c’depicts fine grain structure.At the interface of maraging steel(250)side,the side of fusion zone shows clearly dendritic structure(zone‘d’)whereas 13-8 Mo stainless steel side,there is no evidence of clearly distinguished line(zone ‘b’).The zone ‘a’is a dark etching zone where martensite phase experiences peak temperatures in the range of 595-621°C,where some reverted austenite will therefore form in a martensitic matrix.The dark etching zone exhibits two-phase microstructure in which white pools of reverted austenite is formed in the martensite matrix[2,14-17].The dark band region of 13-8 Mo stainless steel side is wider(～1.8 mm)compare to maraging steel side(～1 mm),which could be due to difference in thermal conductivity.

Fig.5.Microstructure of similar weld of 13-8 Mo SS in as-welded condition at 200×:(a)Weld zone(b)WZ-HAZ interface(c)Coarse grain HAZ(d)Dark band region of HAZ.

Fig.6.Microstructure of dissimilar weld in as-welded condition at 200×:(a)Dark band region of HAZ(b)interface towards 13-8 Mo SS side(c)Weld zone(d)interface towards Maraging steel(250)side.

Post weld aging treatments of similar and dissimilar metal welds resulted smooth microstructure at optical as indicated in Figs.4-6.

3.2.Hardness

Hardness surveys across the similar weldments of maraging steel(250)and 13-8 Mo stainless steel are shown in Figs.7 and 8 respectively.Hardness variations across the interfaces as a result of welding could be attributed to weld thermal cycle.For as-welded condition,the hardness at the weld zone(zone A)is nearly equal to the parent metal(zone D)for both the similar weldments i.e.for MM-AWand 13-13-AW condition[16,17].There is significant increase in overall hardness in post weld heat treatment conditions i.e.HT1 and HT2 for both the weldments.

For similar welds of maraging steel(250),the hardness at weld zone(zone A)is lower than parent metal(zone D)for M-M-HT1 and M-M-HT2 due to usage of low strength filler wire.In addition to weld region,zone C which represents dark band region,shows low hardness due to the presence of fine dispersion of reverted austenite in martensite.The overall hardness for 13-13-HT1 to 13-13-HT2 condition is not varying much for 13-8 Mo stainless steel weldments except in dark band region i.e.zone C.The hardness values of 13-8 Mo stainless steel weldments are lesser than the maraging steel weldments for both posts weld heat treatment conditions.

Fig.9 shows hardness survey across the weldments of dissimilar weldments,in the as welded and post weld precipitation hardened conditions.For as welded condition i.e.M-13-AW,at the center of the zone B of both side,hardness is increases probably due to marginal amount of aging of the martensite microstructure.The hardness value in this zone is higher in maraging steel(250)side.Afterwards hardness is decreased probably due to softening of these regions by the high temperatures generated during welding leading to grain coarsening.For unaffected base metal,(zone D)the hardness of 13-8 Mo stainless steel is slightly less compared to maraging steel(250).

In the case of post weld precipitation hardened samples i.e.M-13-HT1 and M-13-HT2 conditions it is found that there is considerable amount of increase in the hardness compared to that of as welded condition which is due to the formation of precipitates i.e.Ni3Ti,Ni3Mo andβNiAl during aging treatment in all the zones[2,3,8,12,13].For unaffected base metal(zone D)the hardness is much higher on maraging steel(250)side.The hardness at zone C(dark band region)for both sides is low compare to adjacent zone due to presence of reverted austenite[2,7,11].

3.3.Tensile strength

The tensile properties of transverse weld specimens in as-weld and various post weld hardening treatments of similar and dissimilar welds are presented in the Table 5.The failure zones for similar welds of maraging steel(250)and 13-8 Mo stainless steel are shown in Figs.10 and 11 respectively.From Table 5 it is evident that the strength of the similar welds of 13-8 Mo stainless steel in as-welded condition is slightly higher than similar weld of maraging steel(250),whereas in post weld aged condition,similar welds of maraging steel(250)is higher in strength and slightly lower in ductility compared to similar welds of 13-8 Mo stainless steel.

Fig.7.Hardness survey of maraging steel(250)weldments.

Fig.8.Hardness survey of 13-8 Mo Stainless Steel weldments.

Fig.9.Hardness survey of dissimilar weldments.

The strength of similar welds of maraging steel(250)and 13-8 Mo stainless steel in post weld aged condition is high and ductility is low,compared to that of joints in as-weld condition.The weld joint in as-weld condition fractured in HAZ in case of 13-8 Mo stainless steel and weld zone in maraging steel(250)weldments due to presence of low hardness.In the similar metal welds of post weld aged condition,the transverse tensile specimen fracture occurred in the weld.This may be due to pronounced segregation of elements aged on titanium and molybdenum along the interdendritic and inter cellular boundaries in fusion zone[10].The fracture occurred in the weld region though there is a low hardness region in heat effected zone(Figs.7 and 8),as this region is very narrow compared to the width of weld and also it is of least significance[2].

For similar welds of maraging steel post weld aging treatment of heating at 510°C soaking for 4 h followed by air cooling provides highest strength even though this aging treatment is related to 13-8 Mo stainless steel(i.e.1601 MPa at 485°C/3Hr/AC to 1621 MPa at 510°C/4Hr/AC).The reason for this behavior could be the increase in formation of Ni3Ti and Ni3Mo precipitates increases as the temperature increases from 485°C to 510°C[18].Whereas for similar welds of 13-8 Mo stainless steel,both the post weld aging treatments provide similar strengths(i.e.1445 MPa at 485°C/3Hr/AC and 1449 MPa at 510°C/4Hr/AC).

From Table 5,it is evident that the dissimilar metal welds of maraging steel(250)and 13-8 Mo stainless steel in post weld aged condition(HT2)has high strength and low ductility compared to that of dissimilar weld metals in as-weld and post weld aged(HT1)conditions.The failure locations of the tensile samples of dissimilar metal welds in as-weld and post weld aged(HT1&HT2)conditions shown in Fig.12.In the as-welded condition and post weld aged condition(HT1),the fracture occurred in weld zone where as in HT2 condition,fracture occurred in the dark band region of 13-8 Mo stainless steel.In both the zones,hardness values are also found to below compared to adjacent zones(Fig.9,zone A of M-13-AW&M-13-HT1 and zone C of M-13-HT2).

The highest strength is attained in M-13-HT2 condition i.e.given post weld hardening treatment of heating at 510°C soaking for 4 h followed by air cooling for all the similar and dissimilar welds.It could be due to Ni3Ti,Ni3Mo andβ-NiAl precipitates increases as the temperature increases from 485°C to 510°C.The response of post weld aging on dissimilar welds is significant i.e.1467 MPa at 485°C/3Hr/AC and 1528 MPa at 510°C/4Hr/AC.

A detailed scanning electron microscopic study of the fracture surfaces of the tensile specimens were carried out with a view to understand the nature of fracture.Figs.13-15 shows the fractography of the tensile samples of similarand dissimilar metal welds in as-welded and post weld aged conditions.The predominant failure exhibit ductile fractures with fine dimples in as-welded and in coarse/shallow dimples in post weld aged conditions.The observed fractures are in tune with observed tensile properties.

4.Conclusion

·Systematic investigations were carried out to study the similar and dissimilar welding of Maraging steel(250)and 13-8 Mo stainless steel by GTCAW(Interpulse)welding process.The effects of post weld hardening treatment on weldments were studied.

·A comparison of base metal properties has revealed that the tensile strength of maraging steel is higher than 13-8 Mo stainless steel in both the aged condition i.e.HT1(485°C/3½hr/AC treatment)&HT2(510°C/4hr/AC treatment).

·For similar welds of maraging steel post weld aging treatment of heating at 510°C soaking for 4 h followed by air cooling provides highest strength even though this aging treatment is related to 13-8 Mo stainless steel.Whereas for similar welds of 13-8 Mo stainless steel,both the post weld aging treatments provide similar strengths.The response of post weld aging on dissimilar welds is significant and highly effective i.e.1467 MPa at 485°C/3Hr/AC and 1528 MPa at 510°C/4Hr/AC.

Table 5 Tensile properties of similar and dissimilar welds.

Fig.10.Failure zones of similar welds of maraging steel(250).

Fig.11.Failure zones of similar welds of 13-8 Mo stainless steel.

Fig.12.Failure zones of dissimilar welds in different condition.

Fig.13.Fractographs of Similar welds of Maraging steel(250)at 1000×(a)AW(b)HT1(c)HT2.

Fig.14.Fractographs of Similar welds of 13-8 Mo stainless steel at 1000×(a)AW(b)HT1(c)HT2.

Fig.15.Fractographs of dissimilar welds at 1000×(a)AW(b)HT1(c)HT.

·For similar welds of 13-8 Mo stainless steel also,dark band region is found.For dissimilar welds the dark band region of 13-8 Mo stainless steel is wider compared to maraging steel side.

·The highest strength is attained in M-13-HT2 condition i.e.given post weld hardening treatment of heating at 510°C,soaking for 4 h followed by air cooling for all the similar and dissimilar welds because of the presence of high amount of Ni3Ti,Ni3Mo andβNiAl precipitates.

Acknowledgements

Financial assistance from Defence Research and Development Organisation is gratefully acknowledged.The authors would like to thank Director,Defence Research&Development Laboratory,Hyderabad for his continued encouragement and permission to publish this work.The authors also thank Materials Development Division for help in metallography and mechanical testing.The authors also thank DMRL for extending their facility of SEM.