Metallurgical analysis of a failed maraging steel shear screw used in the band separation system of a satellite launch vehicle

S.V.S.Narayana MURTY*,Sushant K.MANWATKAR,P.Ramesh NARAYANAN

Material Characterization Division,Materials and Metallurgy Group,Vikram Sarabhai Space Centre,Trivandrum 695 022,India

Available online 24 May 2016

Metallurgical analysis of a failed maraging steel shear screw used in the band separation system of a satellite launch vehicle

S.V.S.Narayana MURTY*,Sushant K.MANWATKAR,P.Ramesh NARAYANAN

Material Characterization Division,Materials and Metallurgy Group,Vikram Sarabhai Space Centre,Trivandrum 695 022,India

Available online 24 May 2016

Maraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications.In one such critical application,this steel was used to fabricate shear screws of a stage separation system in a satellite launch vehicle.During assembly preparations,one of the shear screws which connected the separation band and band end block has failed at the first thread.Microstructural analysis revealed that the crack originated from the root of the thread and propagated in an intergranular mode.The failure is attributed to combined effect of stress and corrosion leading to stress corrosion cracking.

©2016 China Ordnance Society.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

M250 grade maraging steel;Shear screw;Separation system;Mermen band;Stress corrosion cracking

1.Introduction

Maraging steels are high strength steels widely used for aerospace applications where a combination of high specific strength and fracture toughness are the basis for selection. These steels are based on the discovery about the fact that controlled additions of cobalt and molybdenum to the ironnickel martensitic matrix result in a combined age hardening effect significantly higher than the additive effects of each of these elements[1].Further,minute additions of titanium and aluminum make the iron-nickel-cobalt-molybdenum matrix amenable to supplemental age hardening[2].Maraging steels of different grades have been designed to obtain high proof strength and optimum toughness catering to varied applications [3].Among the family of Ni-Co-Mo maraging steels,18Ni-8Co-5Mo(with a 0.2%yield strength of“1700 MPa”or“250 kilo pounds per square inch”)steel is widely used.Some of the applications in aerospace systems include solid rocket motor casings,merman bands for stage separation,fasteners requiring a good strength and toughness combination[4].In recent years,there is an increased interest in these materials in view of their uses for a range of applications[5–8].

In one of the applications,M250 grade maraging steel was used to fabricate shear screws used in the Mermen band separation system of a satellite launch vehicle.These Mermen band separation mechanisms are generally used in the upper stages of launch vehicles,where higher shock levels are not allowed due to proximity to satellite and other sensitive avionics system packages.These types of systems are also used to separate the payloads from the launch vehicles as well as for circumferential separation of the payload fairing.This system consists of a fore end ring attached to the upper stage and an aft end ring connected to the lower stage[9].The fore and aft end rings are clamped together by floating wedge blocks and are held together at the flanges by two preloaded semi-circular bands which are known Mermen bands.The separation system consists of 40 Nos.of wedge block integrated with two merman bands and band end block by wedge block holder and shear screws.The band assembly is connected by a bolt assembly and bolt cutter.The bolt cutter cuts the bolt and two band assemblies separate the stages.Failure of any of the components of separation system will lead to the mission failure.Therefore the fabrication and quality control are important aspects in the development of these critical systems.Separation mechanismsare designed to withstand ultimate structural load without failure and should separate only on command without causing damage to any other parts.

http://dx.doi.org/10.1016/j.dt.2016.05.001

2214-9147/©2016 China Ordnance Society.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Fig.1.(a)Photograph of the band with shear screws in place(except the failed one left with an empty slot)along with(b)drawing of the fastener and(c)photograph of the failed shear screw head(left)and shank(right).

Fig.2.Scanning electron micrograph(a)showing the fracture surface(head portion)of the shear screw having two distinctly separate regions(marked ductile region and brittle region)and(b)showing the fracture surface of the failed counterpart shank portion.Note the minor difference in the magnification of two photographs.

Fig.3.Scanning electron micrographs showing defects in thread regions of the shear screw showing local attack and cracks.

During assembly preparations,one of the shear screws(out of 40 Nos.)which connected the band and band end block has failed prematurely at the threaded portion approximately 7 mm from the end.The shear screw was fabricated with M250 grade maraging steel and was heat treated to 48–52 HRc.The failure has occurred during tightening of the M5 nut while applying the specified torque of 4.5 N-m.Failure was at the first thread of the shear screw.This paper presents the details of the studies carried out on the failed shear screw to identify the cause of failure.In view of the importance of failure to the criticality of the mission,detailed metallurgical analysis was carried out on the failed fastener.Remedial measures to avoid recurrence of the similar failures were suggested.Further one unused shear screw fabricated along with the failed one was also studied for the purpose of comparison.

Fig.1(a)shows the photograph of the band with shear screws in place except the failed one left with an empty slot.The schematic of the fastener is presented in Fig.1(b)and c shows the photograph of the failed shear screw head(left)and shank (right).In Fig.1(c),the fracture surfaces are on the top.A closer look at Fig.1(b)reveals that the shank diameter is 6 mm with k6 tolerance.Hence it can vary from 6.001 to 6.009 mm.This will be assembled to a hole with a diameter of 6 mm with H7 tolerance,which can vary from 6.000 to 6.012 mm.Hence the clearance ensured is-0.009 to+0.011 mm.This is a transition fit shearscrewand if anormalscrew isused on aclearancehole, obtaining a 180 degree bearing surface is not possible.Further, the screw may bend resembling a three point bending test.

Maraging steel used for fabrication of the failed shear screw was processed through a double step vacuum melting process which includes an initial vacuum induction melting(VIM)followed by vacuum arc remelting(VAR).The resulting VAR ingot was thermomechanically processed to obtain bar stock. This bar stock was used for the fabrication of the fasteners.The heat treatment procedure involves solution treatment at 820°C for 1 hour followed by air cooling.The fabricated shear screws were subsequently aged at 480°C for 3 hours and were air cooled to achieve the desired mechanical properties.The threaded region of the shear screw was processed by thread rolling.The chemistry of the material(in weight percent)was 17.7 Ni,7.9 Co,4.8 Mo,0.47 Ti,0.12 Al,0.006 C,0.02 Si,Sand P each<0.005 with the balance being iron.The hydrogen content of the steel was measured to be was 1 ppm.The assembly operations on the sub-system included proof loading of the structure using the shear screws followed by die penetrant testing,reassembly and hand tightening and storing in that condition for about 180 days.

Fig.4.Scanning electron micrographs showing(a)brittle regions on the fracture surface of the head portion of the shear screw close to the outer surface,(b)higher magnification of region from(a)showing corrosion products and(c,d)features of fracture surface at high magnification showing brittle intergranular features.

2.Observations

The head and threaded portions of the failed shear screw were subjected to scanning electron microscopy after thorough ultrasonic cleaning.Fig.2(a)shows the low magnification scanning electron micrograph of the fracture surface of the head portion of the shear screw having two distinctly separate regions(marked ductile region and brittle region)and Fig.2(b)showing the fracture surface of the failed counterpart shank portion.Fig.3(a)and (b)show the scanning electron micrographs revealing defects in thread regions such as local corrosion attack(Fig.3(a))seen as small blisters and cracks along the threads(Fig.3(b)).The presence of corrosion products could be seen in almost all the threads at the location of failure.Similar observations were made on shank portion of failed screw.Fig.4(a)and c shows the scanning electron micrographs of brittle regions on the fracture surface of the head portion of the shear screw close to the outer surface of the failed fastener(Fig.4(a)).The higher magnification photomicrograph of region from Fig.4(a)showing corrosion products is presented in Fig.4(b).Fig.4(c)and d shows the photomicrograph of the fracture surface at high magnification.Fracture features were predominantly intergranular in nature and many secondary cracks could be seen(Fig.4(c)and(d)).Separation of grain boundaries was noticed at many locations on the fracture surface.Corrosion products were seen within the region of grain separation(Fig.4(c)).This is a clear evidence for the anodic dissolution of second phases present at grain boundaries during crack propagation.This leaves behind products within region of grain separation.

Fig.5 shows the scanning electron micrographs taken from the ductile region of the fracture surface of the head portion of the shear screw at two different magnifications showing dimples on the fracture surface.Fig.6 shows the scanning electron micrographs of the shank portion of the shear screw showing brittle regions in the fracture surface taken at high magnifications.The photomicrographs reveal mud cracks (center of Fig.6(a))and corrosion products(Fig.6(b)).The corrosion products with dried mud pattern resulting due to anodic dissolution were seen.The observations made on the shank portion(Fig.6)are similar to those made from the head portion of the failed shear screw(Fig.4).

The corrosion products noticed on the head(Fig.4(c))and on the shank(Fig.6(b))were subjected to energy dispersive spectroscopy(EDS)in a scanning electron microscope.Fig.7(a)and b show the EDS spectra taken on the corrosion products on the head and thread portions of the cracked shear screw respectively. In both spectra,apart from the alloying elements of maraging steel,chlorine peak can be clearly seen.Fig.8 shows scanning electron micrographs taken at higher magnification showing brittle intergranular cracks with the grains separated.Fig.9 presents the scanning electron micrograph of the surface of the head portion of failed shear screw.Surface roughness(Fig.9(a))and scratch marks(Fig.9(b))can be clearly noticed.

Fig.5.Scanning electron micrographs taken from the ductile region of the fracture surface of the head portion of the shear screw.

The head and thread portions of the failed shear screw were sliced off along a plane parallel to their axis and were mounted in a bakelite mount along their axial plane for optical microscopy. The specimens were polished using conventional polishing techniques using a series of emery papers and finally with a one micron diamond paste.The freshly polished specimens were etched with 2%nital to reveal the microstruture.Fig.10(a)and (c)show the optical photomicrographs of failed shear screw showing cracks originating from the surface initiated at several locations taken at two different magnifications(Fig.10(a)and (b)).At higher magnifications(Fig.10(b)),the cracks show extensive branching.The path of crack propagation was preferentially along the grain boundaries with the presence of multiple secondary cracks(Fig.10(b)).The secondary cracks propagated inwards leaving behind a wide gap.Minor branching of the secondary cracks with having a blunt tip is a typical evidence for corrosion assisted crack propagation.This crack initiated at the thread root,propagating inwards with a number of branchesthroughout its propagation(Fig.10(b)).Fig.10(c)shows the lath martensitic microstructure of the shear screw parent material, typical of M250 grade maraging steel in aged condition.

Fig.6.Scanning electron micrographs showing brittle regions in the fracture surface of the shank portion of the shear screw.

Fig.11 shows the optical photomicrographs of shank portion of the failed shear screw showing crack initiation near the thread root(Fig.11(a)).Fig.11(b)shows the morphology of cracks with extensive branching that has initiated from a pit. Fig.12(a)and b show the scanning electron micrographs showing surface of unfailed shear screw taken from the same batch stored under identical conditions to that of the failed one. Scanning electron microscopic observations revealed the surface of the unfailed shear screws having pit throughout their surfaces.Rough patches were also observed just below the head portion and at the threads.Optical microscopic observations did not reveal the presence of any defects in the thread portion. Microstructure of the shear screw revealed tempered martensite,typical of M250 maraging steel.

3.Discussion

Fig.7.(a)Energy dispersive spectroscopy(EDS)of the failed M250 shear screw showing presence of chlorine taken in the head portion of the failed fastener;(b)energy dispersive spectroscopy(EDS)of the failed M250 shear screw showing presence of chlorine in the shank portion of the failed fastener.

High strength and ultrahigh strength fasteners are broadly characterized by tensile strengths,and are often used for number critical aerospace applications.In the aerospace industry,high strength and ultrahigh strength(ranging from 1000 to 2000 MPa)mechanical fasteners are used in such critical applications such as load bearing structures in the airframes. Mechanical fasteners and bolted joints are deceptively simple elements and damage resulting in loss of structural integrity can occur by a complex interaction of material characteristics,environmental conditions,manufacturing flaws and service conditions.The consequences of failure can range from minimal to catastrophic,even resulting in loss of life.The prevention of failure is therefore a fundamental preoccupation for aircraft designers,fastener manufacturers and application engineers alike.

Failure was at the first thread of the shear screw.The band in assembled condition was stored for about 180 days in marine environment.Intergranular mode of fracture was observed under scanning electron microscope.Further,multiple secondary cracks were noticed on the fracture surface with the evidence of anodic dissolution leaving behind corrosion products at crack tip and within grain separation gap.This clearly indicates failure under simultaneous action of stress and corrosion. The stress resulted from the assembly stresses during the band assembly and corrosion from the severe marine environment to which it was exposed.

Fig.8.Scanning electron micrographs taken at higher magnification showing brittle intergranular cracks with the grains separated.

Service failures of maraging steel components and parts have been reported by many researchers[10–13].Stress corrosion cracking(SCC)is a serious problem in high strength steels. SCC is the cracking/failure induced to the susceptible material by the combined effect of tensile stress and a corrosive environment.Thus for SCC to occur,three conditions must be simultaneously satisfied:a susceptible material,a corrosive environment and tensile stress.In the absence of either tensile stress above some minimum threshold value or corrosive environment,cracking does not occur.It is reported that the SCC resistance of high strength steels decreases as strength increases in many corrosive media.Kenyon et al.[14]reported that steels with yield strength of above 1400 MPa are susceptible to cracking in corrosive environments[14].Diwakar et al.[15]have studied the KISCCof maraging steel in water.They found that that the KISCCdeteriorates to as low as 8 MPa m1/2in water.This implies that exposure to moist environment can drastically deteriorate the mechanical properties of M250 maraging steel. Further,SCC of maraging steel fabricated parts and components has been studied by Jha et al.[10–13].They established that the cracking in this material was intergranular in nature.In the present case too,the cracking was found to be typically intergranular in nature.

Fig.9.Scanning electron micrograph showing surface of failed shear screw (head portion).

The shear screw in the present study was held under assembly loads and was exposed to the atmosphere of marine coast for a period of about 180 days.Surface corrosion was noticed at threaded region of the screw near the fracture edge.The presence of cracks at thread roots near the fracture edge and their propagation into the screw core confirmed that cracks were electrochemically active at thread roots.The presence of corrosion products at crack tip and its morphology suggested simultaneous role of corrosion during crack propagation.Fracture surface of the failed screw revealed predominantly intergranular mode of fracture,a typical feature of SCC of maraging steel.

Taking into consideration the evidences obtained from failed shear screw and based on the above detailed discussions,the following points emerge as conclusive outcomes.(a)The conditions of storage of the shear screws were poor,thus resulting in general corrosion at several locations.This caused crack initiation at roots of threads which propagated further under the influence of assembly loads.This is further corroborated by the fact that corrosion was noticed even in unused shear screws;(b) crack propagation was intergranular in nature and(c)presenceof corrosion products at the crack tip as well as within grain separation areas was clear evidence of anodic dissolution during crack propagation.

Fig.10.Optical photomicrographs of failed shear screw showing(a,b)cracks originating from the surface initiated at several locations taken at two different magnifications and(c)typical lath martensitic microstructure of the shear screw parent material.

Fig.11.Optical photomicrographs of failed shear screw showing(a)crack initiation near the thread root and(b)morphology of cracks with extensive branching initiated from a pit.

Even though the fasteners were stored under controlled conditions,periodic inspection and monitoring are essential to ensure part quality.Calcium based grease can be used for storing the maraging steel fasteners.

4.Conclusions

Based on optical microscopy and(intergranular cracks with secondary crack branching)scanning electron microscopic (brittle intergranular features and corrosion products)observations,it is concluded that the shear screw failed due to“Stress Corrosion Cracking”.The fasteners can be stored in calcium based grease and the grease can be periodically changed to avoid the recurrence of such failures.

Acknowledgment

The authors wish to thank Director,VSSC for his permission to publish this work.

Fig.12.Scanningelectronmicrographsshowingsurfaceofunfailedshearscrew taken from the same batch stored under identical conditions to that of the failed one.Notegeneralcorrosionofthesurfacewithsmallpitsandcorrosionproducts.

References

[1]Decker RF.NPL symposium no.15;1963,648–71.

[2]Frank WA.Metals handbook,vol.1.10th ed.Material Park(OH):ASM International;1990.p.793–800.

[3]18 percent nickel maraging steels.In:INCO data book.Publication No. 4419.London:INCO Europe;1976.

[4]Weiss BZ.Comous NR,Clark JB,editors.Speciality steels and hard materials.Oxford:Pergamon Press;1983.p.35.

[5]Nageswara Rao M.18%nickel maraging steels:science and technology. Saarbruecken:Lam Lambert Academic Publishing;2013.

[6]Nageswara Rao M.Int J Mater Res 2006;97:1594–607.

[7]Nageswara Rao M,Chatterjee M,Sivasubramanian K.HTM J Heat Treat Mater 2012;67:195–201.

[8]Sha W,Ye A,Malinov S,Wilson EA.Mater Sci EngA 2012;536:129–35.

[9]Singaravelu J,Sundaresan S,Nageswara Rao B.J Mater Eng Perform 2013;22:926–35.

[10]Jha AK,Reddy KG,John KM,Ramesh Narayanan P,Natrajan A, Lakshamanan TS.Pract Metallogr 1994;31:144–50.

[11]Reddy KG,Jha AK,Diwakar V.Eng Fail Anal 2001;8:263–9.

[12]Jha AK,Kiranmayee MS,Manwatkar SK.Eng Fail Anal 2013;27:308–13.

[13]Jha AK,Kiranmayee MS,Sreekumar K,Sinha PP.Eng Fail Anal 2011;18:1128–33.

[14]Kenyon N,Kirk WW,Van Rooyen D.Corros 1971;27:390–400.

[15]Diwakar V,Arumugham S,Lakshmanan TS,Sarkar BK.J Mater Sci 1986;21:1927–31.

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E-mail address:susarla.murty@gmail.com(S.V.S.N.MURTY).

13 January 2016;revised 4 May 2016;accepted 13 May 2016