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Hydrogen embrittlement failure of hot dip galvanised high tensile wires
\ PERGAMON
Engineering Failure Analysis 5 "0888# 142Ð154
Hydrogen embrittlement failure of hot dip galvanised high tensile wires N[K[ Mukhopadhyay\ G[ Sridhar\ N[ Parida\ S[ Tarafder\ V[R[ Ranganath National Metallurgical Laboratory\ Jamshedpur 720 9906\ India Received 29 July 0887^ accepted 02 September 0887
Abstract A case of failure of high carbon eutectoid steel wires is investigated[ During the production stage\ i[e[ cold drawing and subsequent coiling:stranding operations\ the wires of ½3[9 mm diameter failed by central splitting along longitudinal planes[ Microscopic examination\ fractography and mechanical tests along with acoustic emission monitoring were carried out on the wire samples[ The experimental results con_rm that the failure is related to hydrogen embrittlement which has been characterised by fractography\ strain rate sensitivity and susceptibility to delayed fracture as indicated by acoustic activity[ It has been argued that the cohesive energy model for hydrogen embrittlement\ where hydrogen reduces the bond strength and cohesive strength ahead of pre!existing cracks\ can explain the failures observed in the present case[ It appears that improper pickling and subsequent baking processes\ during the _nal stages of drawing operations\ are responsible for the hydrogen related failures[ Keywords] Hydrogen embrittlement^ Galvanised wire^ High tensile wire^ Acoustic emission
0[ Introduction Several types of wire rod failures are reported in the literature\ mostly due to inclusions\ strain ageing\ hydrogen embrittlement\ liquid metal embrittlement\ improper control of microstructure etc[ resulting in bulging\ ~aking and splitting[ Splitting failures may occur spontaneously when the material is embrittled along a longitudinal plane and the residual stresses generated during the drawing and handling operations are relieved[ Cold drawn high carbon steel wires are amenable to being more weak along the longitudinal plane as compared to the transverse planes due to the _bre!like deformation of pearlitic phases in the longitudinal direction ð0Ł[ It has been observed that metallurgical embrittlement may also take place due to interfacial segregation of hydrogen "H# in high strength steels\ which leads to delamination:decohesion type failures ð1Ł[ A short discussion
Corresponding author[ Tel[] ¦80 546 315980^ fax] ¦80 546 315416^ e!mail] vrrÝcsnml[ren[nic[in 0249!5296:88:, ! see front matter Þ 0888 Published by Elsevier Science Ltd[ All rights reserved[ PII] S 0 2 4 9 ! 5 2 9 6 " 8 7 # 9 9 9 2 7 ! 6
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follows on hydrogen embrittlement theories and acoustic emission techniques used to detect hydrogen activity in metals under stress[ 0[0[ Hydrogen embrittlement The e}ect of H has been the subject of extensive studies to understand the mechanisms of degradation in mechanical properties of metals and alloys because industries often encounter failure of products due to hydrogen embrittlement "HE# ð2Ð5Ł[ Generally atomic H\ which is absorbed initially by the metal surface\ transforms to molecular H when the concentration of H is high[ Molecular H resides in voids\ pores and interfaces among many other defect sites[ If these defects are not present in the vicinity of a high H area\ then blisters or hair line cracks are formed to release the high pressure[ In the equilibrium situation\ both atomic and molecular H will be present inside the material at a high concentration of H[ Highly tensile stressed regions will be the most suitable sites for atomic H[ During welding the H picked up\ from residual H1O or damp electrodes\ di}uses to the base plate when the weld is hot and subsequently causes cold cracking[ During electroplating and pickling H can enter the lattice\ but is di}used!out by a subsequent baking treatment at 199Ð149>C[ Some of the important models of HE are discussed below[ One of the models\ the planar pressure mechanism\ predicts that the high pressure developed due to molecular H within gas pores inside the material causes cracking ð6\ 7Ł[ Although this model explains the embrittlement of H charged metal\ it is not accepted as a universal mechanism for HE as it cannot explain the delayed cracking phenomenon[ A model proposed by Troiano et al[ ð8Ł\ suggested that the cohesive strength is reduced due to the presence of H atoms[ Hydrogen atoms di}use easily to regions of triaxial tensile stress\ as at the tip of a crack\ and assist crack propagation by reducing the cohesive strength of the material ahead of it[ Thus the crack propagates discontinuously\ controlled by a critical concentration of atomic H built up near the tip of the crack[ One model based on the reduction of surface energy\ explains easy crack growth in the presence of H ð09\ 00Ł[ The model proposed by Beachem ð01\ 02Ł is based on enhancement of dislocation mobility which induces highly localised plastic ~ow at very low stress levels[ HE is also explained by metal hydride formation in materials such as titanium\ vanadium and zirconium "group IVa and Vb# ð03Ł[ It has also been suggested that hydride!induced embrittlement may occur in steels containing these metals[ A completely di}erent type of HE in the presence of H and C in plain carbon steel at high temperature and pressure occurs due to formation of methane inside the material[ The formation of methane and the development of high pressure causes blistering which gives rise to typical failures observed in the petroleum industry ð04Ł[ This process produced decarburisation and is somewhat di}erent from low temperature HE[ The process of H cracking is the result of one or more of the micro!mechanisms such as] "i# cleavage\ "ii# intergranular decohesion\ or "iii# microvoid coalescence[ All the three mechanisms in the same steel alloy\ when tested at di}erent strength levels\ have been reported in the literature ð5Ł[ Therefore\ depending on materials and operational parameters various characteristic fracture surfaces of HE will be generated[ 0[1[ HE and acoustic emission Acoustic emission "AE# is the name given to the elastic waves that are generated within a material as a consequence of deformation and fracture processes[ Acquisition and analysis of these signals
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can be used to detect\ with high sensitivity\ deformation and fracture in a material ð05Ł[ As HE is characterised by the tendency to crack leading to failure at reduced ductility\ acoustic emission techniques "AET# can be applied to detect hydrogen embrittlement failure[ It has been shown that AE as a result of HE and stress corrosion cracking "SCC# provides information on the crack growth process in greater detail than is otherwise possible ð06Ð08Ł[ Hartbower et al ð07Ł monitored crack initiation and propagation during SCC of maraging steel\ and their results could be excellently correlated with the cumulative counts of the AE signals[ Dunegan and Tetelman ð08Ł have also used this technique to monitor the onset of unstable fracture in hydrogen charged 3239 steel test specimens and bolts under constant load[ They proposed a power law relationship between count rate and stress intensity factor and suggested a critical AE rate at onset of rapid fracture[ Similar studies by Parida and Bhattacharya ð19Ł\ on bend specimens of the same steel\ have shown that the incubation time to nucleate microcracks can be monitored in terms of some directly observable AE parameters[ 1[ Investigation High strength steel wire rods which are used for manufacturing steel ropes\ in a local industry\ have been found to be failing at the _nal stage of production ð10Ł[ Stelmor cooled billets of 019×019 mm1 having a nominal composition of C*9[71\ Mn*9[6\ Si*9[1\ S*9[91 max[ and P*9[91 max[\ are hot!rolled in stages to 01 mm diameter wire rods\ which are _rst pickled and baked at 049>C for 04 min\ followed by ~ux coating[ The wires are then pre!drawn to 09 mm diameter before patenting[ A gap of 09 h is allowed between coating and pre!drawing and patenting[ Subsequent to the patenting process\ the wires are again pickled and dipped in hot water and drawn through a ~ux to 5 mm diameter[ This is followed by galvanising and _nal drawing to 3[04 mm diameter followed by stranding:spooling[ The wire rods\ which have failed at various stages during spooling and stranding\ typically show a split along the drawing direction "Fig[ 0#[ The longitudinal splitting into layers during the rolling
Fig[ 0[ Wire rod showing a typical split failure[
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operation\ termed delamination\ is one of the qualitative indication of the decrease in ductility[ The fresh fracture surfaces were observed to be shiny\ indicating the possibility of HE[ No other distinctive features were observed on the surfaces of failed wire rods[ The present investigation was carried out to con_rm embrittlement either due to metallurgical reasons or due to the presence of H[ An attempt has also been made in this investigation to identify HE by means of AE monitoring during delayed cracking experiment[
2[ Experimental details Metallographic examinations\ after polishing with diamond paste in the _nal stage\ in the unetched and etched condition\ were carried out on the longitudinal and transverse cross sections of the wire[ The failed surfaces were examined with the help of a scanning electron microscope "S[E[M[# equipped with EDX facility[ Delayed cracking tests were conducted on partially split wire samples\ by hanging a dead weight "799 g#\ resulting in approximately 69) of the yield stress of the material\ to one of the split ends[ During these tests\ the AE signals were monitored on partially split wire samples using a 049 kHz resonant piezoelectric sensor placed on the wire surface[ The AE signals were ampli_ed by a 39 dB per!ampli_er and fed to a computerised AE system to record and analyse the signals[ A schematic of the experimental set!up is shown in Fig[ 1[ Since there is no single theory to prove H attack conclusively\ recourse may be had to con_rm or eliminate the presence of H in the steel wire rod samples by conducting mechanical tests ð0\ 11Ł[ For the tensile tests\ the samples were cut such that the parallel length between the grip ends was maintained at 099 mm[ The tests were conducted on a servohydraulic test system equipped with digital controls[ For obtaining comparative estimates of the toughness of the wire rod material\ samples of 44 mm length were employed and tested on an impact test machine[ Since it is believed that the deleterious e}ects of H would diminish if the suspected components are baked\ all the tests as described above were conducted both on the as!received as well as the baked specimens[ The baking treatment given to the specimens\ after removing the galvanised layer by polishing\ was 29 min at a temperature of 149>C\ to eliminate any atomic hydrogen[
Fig[ 1[ Schematic view of the experimental set up to study delayed cracking using AE[
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3[ Results 3[0[ Microscopy Microscopic examination was conducted to observe both the inclusions and the microstructural features[ Typical features are shown in Fig[ 2"a and b#[ In addition to the few inclusions that are observed\ a number of pores approximating to 0Ð1) volume fraction were present in most of the areas that were examined[ It may be noted that the inclusions and the pores could be distinguished from each other by varying the focusing plane\ as their planes of foci are di}erent\ and this has been demonstrated in Fig[ 2"a and b#[ It is also observed that some of the pores have coalesced to form crack like defects[ Figure 3"a and b# presents the typical features that are expected in a cold drawn steel wire rod\ viz[\ a _brous appearance comprising elongated ferrite and cementite lamella "pearlite#[ A crack along the drawing direction has been marked in Fig[ 3"a#[ 3[1[ Fractography The longitudinal fracture surfaces of the failed surfaces\ examined under the SEM\ showed a _brous appearance with secondary cracks "marked by arrows in Fig[ 4#[ The presence of secondary cracks suggests H attack of some form or other[ A detailed look at a crack embedded in the material supports the classi_cation of this failure as delamination[ The presence of some non! inclusion hard particles "marked by an arrow head# segregated randomly on the fracture surface is observed[ EDX survey carried out on these particles indicated them to be cementite[ The formation of globular cementite during drawing operations occurs by the densi_cation of cementite through wrinkling or buckling\ aided by accelerated di}usion of C during deformation ð1\ 12Ł[ 3[2[ Tensile and impact tests It is believed that a material a}ected with atomic H would show similar toughness values as that of the same material without any H in it\ when tested under high strain rate "such as in the impact
Fig[ 2[ Optical micrographs of unetched wire drawn sample[ "a# Transverse section shows abundance of pores in focused condition[ "b# Over!focused condition showing pores as illuminated spots and inclusions as dark spots[ The correspondence between "a# and "b# can be noted[
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Fig[ 3[ "a# Optical micrograph and "b# SEM photograph showing microstructures featuring typical _brous nature comprising elongated ferrite and cementite lamella[ The cracks along the drawing direction can be observed in "a#[
test#\ and that under a slow strain rate test "such as in a standard tensile test# would show a signi_cant di}erence in toughness[ The impact toughness results from both as!received and baked specimens are shown in Table 0[ It may be noted that although the specimen geometry was non! standard for CVN toughness tests\ the impact toughness values from identical specimens would be useful for making comparisons[ It was observed that both the lots showed an average value of 22 J under identical test conditions[ The nature of the stress!strain plots does not show any evidence of strain ageing[ Figure 5\ and also Table 0\ show the tensile properties of the baked as well as the as!received samples[ A noticeable increase may be observed in the ) elongation and yield stress of the baked specimens\ suggesting hydrogen embrittlement to be operative in the as!received wire[ A quantitative estimate of the residual hydrogen in the wires could not be attempted because atomic hydrogen would
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Fig[ 4[ SEM fractographs of the failed wire samples[ "a# Subsurface secondary cracks indicative of hydrogen damage^ "b# the fracture features at high magni_cation appear to correlate well with interlamellar spacing^ "c# globule shaped cementite particles segregated in a colony[
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Table 0 Tensile and impact test results Specimen
) Elongation
9[1) YS "MPa#
UTS "MPa#
Impact toughness "J#
As!received Baked "149>C#a
2[91 4[21
0322 0432
1193 1067
22[6 22[2
a
Values reported are the average of three specimens[
Fig[ 5[ Tensile behaviour of high tensile wires in as!received and baked condition[ Signi_cant improvement in elongation of the baked specimen can be noted[
di}use during the specimen preparation stage itself[ The fracture surfaces of the tensile test specimens were also observed under the SEM and are reported in Fig[ 6"a\ b and c#[ Figure 6"c# shows the dimple network in the centre of the fracture surface of the as!received sample[ Similar features were observed on the fracture surfaces of the baked samples[ 3[3[ Delayed cracking test with AE monitoring The AE signals collected from a wire rod sample that was kept under constant load for a period of 4[4 h are presented as cumulative AE events vs time in Fig[ 7 and as amplitude of the AE events vs time in Fig[ 8[ It is seen from these _gures that even though the wire rod was under constant load\ a number of AE signals "28 AE events# were generated during the test and it may also be
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Fig[ 6[ SEM fractographs of "a# as!received specimen^ "b# baked specimen^ "c# dimple fracture observed at higher magni_cation\ location as at "a#[
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Fig[ 7[ Cumulative number of AE events generated during delayed cracking test of the wire[
Fig[ 8[ Peak amplitude of AE events generated during delayed cracking test of the wire showing intermittent nature of the events[
noted that these signals were generated at intervals and not continuously[ This indicates that some dynamic substructural phenomenon is taking place in the material during the test\ and the process is time dependent and therefore is di}usion controlled[ Moreover\ as can be seen from Fig[ 8\ the peak amplitude of these signals is between 24Ð59 dB[ Signals having an amplitude of this nature are attributed to H evolution or due to H induced microcrack formation by di}usion of H to the most stressed region "encircled area in Fig[ 1# ð03Ł[ All these observations indicate that the atomic hydrogen present in the material di}used to the highly stressed region and formed microcracks which resulted in the generation of intermittent AE signals[
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4[ Discussion The present material has undergone nearly 79) cold working resulting in a _ne and elongated pearlitic microstructure[ The strength level is more than 1999 MPa[ Therefore this material has a tendency to absorb H and retain the same at the interfaces of the pearlitic microstructures[ The cold working by way of the drawing operation also leads to a residual stress pattern where high tensile stress exists in the central region[ It is well known that H has a tendency to stay in a high tensile stress region[ Hydrogen evolved during pickling operations "Fe¦HCl : FeCl1¦ðHŁ# is adsorbed by the wire rod[ In industrial practice the ingress of H is retarded by employing a proper inhibitor in the pickling solution[ In spite of the inhibitor\ any trace of hydrogen that may have remained in the products would then be removed by a suitable baking operation[ If any one of the above processes is ine}ective\ retention of H even of the order of 0 ppm would render the product prone to hydrogen related failure ð13Ł[ An attempt is made to explain\ by decohesion model\ as to how even a low level of H can cause brittle fracture\ by invoking Gri.th|s theory ð11Ł[ sf
X
1Egs pc
"0#
where sf is the fracture stress necessary to cause the propagation of an elliptical crack of length 1c^ E is the Young|s modulus^ gs is the surface energy[ When H is absorbed\ it decreases the bond strength and the surface energy ð00Ł[ From eqn "0# it can be seen that the fracture strength is reduced appreciably due to the reduction of surface energy[ This reduction in fracture strength can also be expressed as a function of H concentration at the root of a pre!existing crack ð14Ł] sf−sH bcl
"1#
where sH is the fracture strength of material containing H^ c is the concentration of H^ b and l are appropriate constants which can be determined by experiments[ As this material is heavily cold worked\ a number of defects\ primarily dislocations\ interfaces\ globular shaped cementite\ pores\ microcracks etc[ are generated and these defects are the most potential sites for H entrapment[ In the presence of any external stress\ the microcrack tips act as stress raisers and would further attract H from the surrounding region[ When this H concentration reaches a critical limit\ the fracture stress is reduced drastically and the crack extends to the limit of accumulated H before it stops[ Again\ H di}uses more readily to the fresh crack tip and assists in further propagation of the crack[ The data obtained from the AET during delayed cracking tests con_rms the general understanding\ as explained above\ of the way the cracks propagate under stress\ below the yield stress\ in presence of H[ It is interesting to note that baking for half an hour has improved the ductility and yield strength of the material and reduced the cracking tendency as evidenced in the fractograph "Fig[ 6#[ The early yielding in the as!received sample\ as compared to the baked sample\ can be explained by H assisted plasticity[ As H reduces the cohesive strength\ there will be a corresponding reduction in resistance to localised dislocation mobility in the as!received sample as compared to the H free sample "baked sample#[ It has been reported that hydrogen can di}use out even at room tem! perature if the materials are left in fresh air for a lengthy period[ However\ H trapped in the lattice
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can not be removed completely once the galvanising treatment has been carried out ð13Ł[ The baking operation may not have removed all the H\ particularly the molecular H which caused the pores in the microstructure[ Therefore\ the baking operation has to be carried out immediately after pickling\ before galvanising\ in order to avoid entrapment of H by defects present in the material[ The profusion of pores observed during microstructural examination "Fig[ 2# indicates that molecular H has evolved in the material[ The fracture surfaces showing dimples in tensile tests should not be taken as a signature of ductile fracture[ The pores are in fact responsible for the presence of dimples on the fracture surface and reduction in the ductility of the material[ It is reported in the literature that H assisted cracking can also occur by microvoid coalescence alone and H a}ects the rate of nucleation of voids and reduces plasticity at later stages of fracture ð14Ł[ The split wire surface\ however\ does not show dimples but shows major features of delamination: decohesion type of fracture with interspersed hard particles "cementite# and longitudinal cracks[ These defects are e}ective during bending and twisting\ but not during tensile testing\ and lead to a delamination or decohesion type of cracking[ From the microstructural features "Fig[ 3"b##\ it appears that the delamination has taken place either along the prior austenite grain boundaries\ i[e[\ along the interface of the pearlite colonies or along ferrite:cementite interfaces\ which have been weakened by the presence of H[ In the case of tensile tests these defects do not play a role as they are parallel to the tensile axis of the test sample[ The reason for failures of the wire rods has thus been attributed to hydrogen induced cracking[ After the H has been trapped inside the material due to improper pickling and baking operations\ it has initiated the cracking in the presence of a high tensile residual stress at the axis of the wire and led to a delamination type failure[ During coiling or twisting\ plastic bending is induced in a plane which\ along with residual stress\ causes the wires to fail by splitting along the weakest plane i[e[\ the direction of drawing[
5[ Concluding remarks Based on the investigations carried out\ and the observations thereon\ it appears that the cohesive strength of the boundaries:interfaces was reduced by the presence of atomic H[ This ultimately led to abrupt splitting during subsequent twisting and bending because the symmetric residual stress pattern developed during the drawing operation is relieved[ It appears that a proper inhibitor during the pickling process can prevent ingress of H into the wire rods[ Further\ the baking operation after pickling can be optimised to avoid interaction of H with the materials leading to H assisted cracking[
Acknowledgements The authors thankfully acknowledge useful discussions with Dr A[ Biswas "UMIL\ Ranchi#\ Dr R[ N[ Ghosh "NML\ Jamshedpur# and Dr D[ K[ Bhattacharya "NML\ Jamshedpur#[ Appreciation is also due to Mr S[ Das "NML\ Jamshedpur# for his help in conducting the SEM examinations[ The authors are grateful to Prof[ P[ Ramachandra Rao\ Director\ NML\ for his support and encouragement to publish this work[
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Colangelo J\ Heiser FA[ Analysis of metallurgical failures[ NY] John Wiley + Sons[ 0863\ p[ 023[ Base CM\ Nam WJ\ Lee CS[ Scripta Materialia 0885^24]530[ Hirth JP\ Ashby MF[ Perspective in hydrogen in metals[ Oxford] Pergamon Press\ 0873[ Gibala R\ Hebemann RF[ Hydrogen embrittlement and stress corrosion cracking[ Metals Park\ OH] ASM\ 0873[ Thompson AW\ Bernstein IM[ Advances in corrosion science and technology 0879^6]034[ Hertzberg RW[ Deformation and fracture mechanics of engineering materials[ John Wiley + Sons\ 0865\ p[ 314[ Zapfree C\ Sims C[ Trans[ AIME 0830^034]114[ Tetlman AS\ Robertson WD[ Trans[ AIME 0851^113]664[ Troiano AR[ Trans ASM 0859^41]43[ Oriani RA\ Josephic PH[ Acta Met 0866^41]868[ Petch NJ\ Stables P[ Nature 0841^058]731[ Beachem C[ Stress corrosion cracking and hydrogen embrittlement of iron base alloys[ Houston] NACE\ 0866\ p[ 265[ Lynch SP[ Metals Forum 0868^1]078[ Gahr S\ Grossbeck ML\ Birnbaum HK[ Acta Met 0866^14]014[ Viswanathan R[ Damage mechanisms and life assessment of high temperature components[ ASM International\ 0878\ p[ 241[ Wadley HNG\ Scruby CB\ Speak JH[ Int Met Rev 0879^1]30[ Chaskelis HH\ Cullen WH\ Kraft JM[ ASTM!STP 448[ Philadelphia] American Society for Testing and Materials[ 0862\ p[ 20[ Hartbower CE\ Renter WG\ Morris CF\ Crimmins PP[ ASTM!STP 494[ Philadelphia] American Society for Testing and Materials[ 0860\ p[ 076[ Dunegan HL\ Tetelman AS[ Eng Fract Mech 0860^1]276[ Parida N\ Bhattacharya AK[ Advances in structural testing analysis and design[ New Delhi] Tata McGraw!Hill\ 0889\ 0220[ Ranganath VR\ Mukhopadhyay NK\ Sridhar G\ Tarafder S[\ Parida N[ Internal report on failure analysis of high tensile galvanised wires\ component integrity evaluation programme "CIEP#[ NML\ Jamshedpur\ December\ 0885[ Deiter GE[ Mechanical metallurgy[ NY] McGraw!Hill\ 0877\ p[ 389[ Nam WJ\ Base CM[ Material Science + Engg 0884^192]167[ Dopler T\ Nistelberger M\ Jeglistsch F\ Hampejs G[ Wire 0885^35]04[ Lancaster JF[ Metallurgy of welding[ London] Chapman + Hall\ 0883\ p[ 195Ð196[
Engineering Failure Analysis 5 "0888# 142Ð154
Hydrogen embrittlement failure of hot dip galvanised high tensile wires N[K[ Mukhopadhyay\ G[ Sridhar\ N[ Parida\ S[ Tarafder\ V[R[ Ranganath National Metallurgical Laboratory\ Jamshedpur 720 9906\ India Received 29 July 0887^ accepted 02 September 0887
Abstract A case of failure of high carbon eutectoid steel wires is investigated[ During the production stage\ i[e[ cold drawing and subsequent coiling:stranding operations\ the wires of ½3[9 mm diameter failed by central splitting along longitudinal planes[ Microscopic examination\ fractography and mechanical tests along with acoustic emission monitoring were carried out on the wire samples[ The experimental results con_rm that the failure is related to hydrogen embrittlement which has been characterised by fractography\ strain rate sensitivity and susceptibility to delayed fracture as indicated by acoustic activity[ It has been argued that the cohesive energy model for hydrogen embrittlement\ where hydrogen reduces the bond strength and cohesive strength ahead of pre!existing cracks\ can explain the failures observed in the present case[ It appears that improper pickling and subsequent baking processes\ during the _nal stages of drawing operations\ are responsible for the hydrogen related failures[ Keywords] Hydrogen embrittlement^ Galvanised wire^ High tensile wire^ Acoustic emission
0[ Introduction Several types of wire rod failures are reported in the literature\ mostly due to inclusions\ strain ageing\ hydrogen embrittlement\ liquid metal embrittlement\ improper control of microstructure etc[ resulting in bulging\ ~aking and splitting[ Splitting failures may occur spontaneously when the material is embrittled along a longitudinal plane and the residual stresses generated during the drawing and handling operations are relieved[ Cold drawn high carbon steel wires are amenable to being more weak along the longitudinal plane as compared to the transverse planes due to the _bre!like deformation of pearlitic phases in the longitudinal direction ð0Ł[ It has been observed that metallurgical embrittlement may also take place due to interfacial segregation of hydrogen "H# in high strength steels\ which leads to delamination:decohesion type failures ð1Ł[ A short discussion
Corresponding author[ Tel[] ¦80 546 315980^ fax] ¦80 546 315416^ e!mail] vrrÝcsnml[ren[nic[in 0249!5296:88:, ! see front matter Þ 0888 Published by Elsevier Science Ltd[ All rights reserved[ PII] S 0 2 4 9 ! 5 2 9 6 " 8 7 # 9 9 9 2 7 ! 6
143
N[K[ Mukhopadhyay et al[ : En`ineerin` Failure Analysis 5 "0888# 142Ð154
follows on hydrogen embrittlement theories and acoustic emission techniques used to detect hydrogen activity in metals under stress[ 0[0[ Hydrogen embrittlement The e}ect of H has been the subject of extensive studies to understand the mechanisms of degradation in mechanical properties of metals and alloys because industries often encounter failure of products due to hydrogen embrittlement "HE# ð2Ð5Ł[ Generally atomic H\ which is absorbed initially by the metal surface\ transforms to molecular H when the concentration of H is high[ Molecular H resides in voids\ pores and interfaces among many other defect sites[ If these defects are not present in the vicinity of a high H area\ then blisters or hair line cracks are formed to release the high pressure[ In the equilibrium situation\ both atomic and molecular H will be present inside the material at a high concentration of H[ Highly tensile stressed regions will be the most suitable sites for atomic H[ During welding the H picked up\ from residual H1O or damp electrodes\ di}uses to the base plate when the weld is hot and subsequently causes cold cracking[ During electroplating and pickling H can enter the lattice\ but is di}used!out by a subsequent baking treatment at 199Ð149>C[ Some of the important models of HE are discussed below[ One of the models\ the planar pressure mechanism\ predicts that the high pressure developed due to molecular H within gas pores inside the material causes cracking ð6\ 7Ł[ Although this model explains the embrittlement of H charged metal\ it is not accepted as a universal mechanism for HE as it cannot explain the delayed cracking phenomenon[ A model proposed by Troiano et al[ ð8Ł\ suggested that the cohesive strength is reduced due to the presence of H atoms[ Hydrogen atoms di}use easily to regions of triaxial tensile stress\ as at the tip of a crack\ and assist crack propagation by reducing the cohesive strength of the material ahead of it[ Thus the crack propagates discontinuously\ controlled by a critical concentration of atomic H built up near the tip of the crack[ One model based on the reduction of surface energy\ explains easy crack growth in the presence of H ð09\ 00Ł[ The model proposed by Beachem ð01\ 02Ł is based on enhancement of dislocation mobility which induces highly localised plastic ~ow at very low stress levels[ HE is also explained by metal hydride formation in materials such as titanium\ vanadium and zirconium "group IVa and Vb# ð03Ł[ It has also been suggested that hydride!induced embrittlement may occur in steels containing these metals[ A completely di}erent type of HE in the presence of H and C in plain carbon steel at high temperature and pressure occurs due to formation of methane inside the material[ The formation of methane and the development of high pressure causes blistering which gives rise to typical failures observed in the petroleum industry ð04Ł[ This process produced decarburisation and is somewhat di}erent from low temperature HE[ The process of H cracking is the result of one or more of the micro!mechanisms such as] "i# cleavage\ "ii# intergranular decohesion\ or "iii# microvoid coalescence[ All the three mechanisms in the same steel alloy\ when tested at di}erent strength levels\ have been reported in the literature ð5Ł[ Therefore\ depending on materials and operational parameters various characteristic fracture surfaces of HE will be generated[ 0[1[ HE and acoustic emission Acoustic emission "AE# is the name given to the elastic waves that are generated within a material as a consequence of deformation and fracture processes[ Acquisition and analysis of these signals
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can be used to detect\ with high sensitivity\ deformation and fracture in a material ð05Ł[ As HE is characterised by the tendency to crack leading to failure at reduced ductility\ acoustic emission techniques "AET# can be applied to detect hydrogen embrittlement failure[ It has been shown that AE as a result of HE and stress corrosion cracking "SCC# provides information on the crack growth process in greater detail than is otherwise possible ð06Ð08Ł[ Hartbower et al ð07Ł monitored crack initiation and propagation during SCC of maraging steel\ and their results could be excellently correlated with the cumulative counts of the AE signals[ Dunegan and Tetelman ð08Ł have also used this technique to monitor the onset of unstable fracture in hydrogen charged 3239 steel test specimens and bolts under constant load[ They proposed a power law relationship between count rate and stress intensity factor and suggested a critical AE rate at onset of rapid fracture[ Similar studies by Parida and Bhattacharya ð19Ł\ on bend specimens of the same steel\ have shown that the incubation time to nucleate microcracks can be monitored in terms of some directly observable AE parameters[ 1[ Investigation High strength steel wire rods which are used for manufacturing steel ropes\ in a local industry\ have been found to be failing at the _nal stage of production ð10Ł[ Stelmor cooled billets of 019×019 mm1 having a nominal composition of C*9[71\ Mn*9[6\ Si*9[1\ S*9[91 max[ and P*9[91 max[\ are hot!rolled in stages to 01 mm diameter wire rods\ which are _rst pickled and baked at 049>C for 04 min\ followed by ~ux coating[ The wires are then pre!drawn to 09 mm diameter before patenting[ A gap of 09 h is allowed between coating and pre!drawing and patenting[ Subsequent to the patenting process\ the wires are again pickled and dipped in hot water and drawn through a ~ux to 5 mm diameter[ This is followed by galvanising and _nal drawing to 3[04 mm diameter followed by stranding:spooling[ The wire rods\ which have failed at various stages during spooling and stranding\ typically show a split along the drawing direction "Fig[ 0#[ The longitudinal splitting into layers during the rolling
Fig[ 0[ Wire rod showing a typical split failure[
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operation\ termed delamination\ is one of the qualitative indication of the decrease in ductility[ The fresh fracture surfaces were observed to be shiny\ indicating the possibility of HE[ No other distinctive features were observed on the surfaces of failed wire rods[ The present investigation was carried out to con_rm embrittlement either due to metallurgical reasons or due to the presence of H[ An attempt has also been made in this investigation to identify HE by means of AE monitoring during delayed cracking experiment[
2[ Experimental details Metallographic examinations\ after polishing with diamond paste in the _nal stage\ in the unetched and etched condition\ were carried out on the longitudinal and transverse cross sections of the wire[ The failed surfaces were examined with the help of a scanning electron microscope "S[E[M[# equipped with EDX facility[ Delayed cracking tests were conducted on partially split wire samples\ by hanging a dead weight "799 g#\ resulting in approximately 69) of the yield stress of the material\ to one of the split ends[ During these tests\ the AE signals were monitored on partially split wire samples using a 049 kHz resonant piezoelectric sensor placed on the wire surface[ The AE signals were ampli_ed by a 39 dB per!ampli_er and fed to a computerised AE system to record and analyse the signals[ A schematic of the experimental set!up is shown in Fig[ 1[ Since there is no single theory to prove H attack conclusively\ recourse may be had to con_rm or eliminate the presence of H in the steel wire rod samples by conducting mechanical tests ð0\ 11Ł[ For the tensile tests\ the samples were cut such that the parallel length between the grip ends was maintained at 099 mm[ The tests were conducted on a servohydraulic test system equipped with digital controls[ For obtaining comparative estimates of the toughness of the wire rod material\ samples of 44 mm length were employed and tested on an impact test machine[ Since it is believed that the deleterious e}ects of H would diminish if the suspected components are baked\ all the tests as described above were conducted both on the as!received as well as the baked specimens[ The baking treatment given to the specimens\ after removing the galvanised layer by polishing\ was 29 min at a temperature of 149>C\ to eliminate any atomic hydrogen[
Fig[ 1[ Schematic view of the experimental set up to study delayed cracking using AE[
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3[ Results 3[0[ Microscopy Microscopic examination was conducted to observe both the inclusions and the microstructural features[ Typical features are shown in Fig[ 2"a and b#[ In addition to the few inclusions that are observed\ a number of pores approximating to 0Ð1) volume fraction were present in most of the areas that were examined[ It may be noted that the inclusions and the pores could be distinguished from each other by varying the focusing plane\ as their planes of foci are di}erent\ and this has been demonstrated in Fig[ 2"a and b#[ It is also observed that some of the pores have coalesced to form crack like defects[ Figure 3"a and b# presents the typical features that are expected in a cold drawn steel wire rod\ viz[\ a _brous appearance comprising elongated ferrite and cementite lamella "pearlite#[ A crack along the drawing direction has been marked in Fig[ 3"a#[ 3[1[ Fractography The longitudinal fracture surfaces of the failed surfaces\ examined under the SEM\ showed a _brous appearance with secondary cracks "marked by arrows in Fig[ 4#[ The presence of secondary cracks suggests H attack of some form or other[ A detailed look at a crack embedded in the material supports the classi_cation of this failure as delamination[ The presence of some non! inclusion hard particles "marked by an arrow head# segregated randomly on the fracture surface is observed[ EDX survey carried out on these particles indicated them to be cementite[ The formation of globular cementite during drawing operations occurs by the densi_cation of cementite through wrinkling or buckling\ aided by accelerated di}usion of C during deformation ð1\ 12Ł[ 3[2[ Tensile and impact tests It is believed that a material a}ected with atomic H would show similar toughness values as that of the same material without any H in it\ when tested under high strain rate "such as in the impact
Fig[ 2[ Optical micrographs of unetched wire drawn sample[ "a# Transverse section shows abundance of pores in focused condition[ "b# Over!focused condition showing pores as illuminated spots and inclusions as dark spots[ The correspondence between "a# and "b# can be noted[
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Fig[ 3[ "a# Optical micrograph and "b# SEM photograph showing microstructures featuring typical _brous nature comprising elongated ferrite and cementite lamella[ The cracks along the drawing direction can be observed in "a#[
test#\ and that under a slow strain rate test "such as in a standard tensile test# would show a signi_cant di}erence in toughness[ The impact toughness results from both as!received and baked specimens are shown in Table 0[ It may be noted that although the specimen geometry was non! standard for CVN toughness tests\ the impact toughness values from identical specimens would be useful for making comparisons[ It was observed that both the lots showed an average value of 22 J under identical test conditions[ The nature of the stress!strain plots does not show any evidence of strain ageing[ Figure 5\ and also Table 0\ show the tensile properties of the baked as well as the as!received samples[ A noticeable increase may be observed in the ) elongation and yield stress of the baked specimens\ suggesting hydrogen embrittlement to be operative in the as!received wire[ A quantitative estimate of the residual hydrogen in the wires could not be attempted because atomic hydrogen would
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Fig[ 4[ SEM fractographs of the failed wire samples[ "a# Subsurface secondary cracks indicative of hydrogen damage^ "b# the fracture features at high magni_cation appear to correlate well with interlamellar spacing^ "c# globule shaped cementite particles segregated in a colony[
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Table 0 Tensile and impact test results Specimen
) Elongation
9[1) YS "MPa#
UTS "MPa#
Impact toughness "J#
As!received Baked "149>C#a
2[91 4[21
0322 0432
1193 1067
22[6 22[2
a
Values reported are the average of three specimens[
Fig[ 5[ Tensile behaviour of high tensile wires in as!received and baked condition[ Signi_cant improvement in elongation of the baked specimen can be noted[
di}use during the specimen preparation stage itself[ The fracture surfaces of the tensile test specimens were also observed under the SEM and are reported in Fig[ 6"a\ b and c#[ Figure 6"c# shows the dimple network in the centre of the fracture surface of the as!received sample[ Similar features were observed on the fracture surfaces of the baked samples[ 3[3[ Delayed cracking test with AE monitoring The AE signals collected from a wire rod sample that was kept under constant load for a period of 4[4 h are presented as cumulative AE events vs time in Fig[ 7 and as amplitude of the AE events vs time in Fig[ 8[ It is seen from these _gures that even though the wire rod was under constant load\ a number of AE signals "28 AE events# were generated during the test and it may also be
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Fig[ 6[ SEM fractographs of "a# as!received specimen^ "b# baked specimen^ "c# dimple fracture observed at higher magni_cation\ location as at "a#[
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Fig[ 7[ Cumulative number of AE events generated during delayed cracking test of the wire[
Fig[ 8[ Peak amplitude of AE events generated during delayed cracking test of the wire showing intermittent nature of the events[
noted that these signals were generated at intervals and not continuously[ This indicates that some dynamic substructural phenomenon is taking place in the material during the test\ and the process is time dependent and therefore is di}usion controlled[ Moreover\ as can be seen from Fig[ 8\ the peak amplitude of these signals is between 24Ð59 dB[ Signals having an amplitude of this nature are attributed to H evolution or due to H induced microcrack formation by di}usion of H to the most stressed region "encircled area in Fig[ 1# ð03Ł[ All these observations indicate that the atomic hydrogen present in the material di}used to the highly stressed region and formed microcracks which resulted in the generation of intermittent AE signals[
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4[ Discussion The present material has undergone nearly 79) cold working resulting in a _ne and elongated pearlitic microstructure[ The strength level is more than 1999 MPa[ Therefore this material has a tendency to absorb H and retain the same at the interfaces of the pearlitic microstructures[ The cold working by way of the drawing operation also leads to a residual stress pattern where high tensile stress exists in the central region[ It is well known that H has a tendency to stay in a high tensile stress region[ Hydrogen evolved during pickling operations "Fe¦HCl : FeCl1¦ðHŁ# is adsorbed by the wire rod[ In industrial practice the ingress of H is retarded by employing a proper inhibitor in the pickling solution[ In spite of the inhibitor\ any trace of hydrogen that may have remained in the products would then be removed by a suitable baking operation[ If any one of the above processes is ine}ective\ retention of H even of the order of 0 ppm would render the product prone to hydrogen related failure ð13Ł[ An attempt is made to explain\ by decohesion model\ as to how even a low level of H can cause brittle fracture\ by invoking Gri.th|s theory ð11Ł[ sf
X
1Egs pc
"0#
where sf is the fracture stress necessary to cause the propagation of an elliptical crack of length 1c^ E is the Young|s modulus^ gs is the surface energy[ When H is absorbed\ it decreases the bond strength and the surface energy ð00Ł[ From eqn "0# it can be seen that the fracture strength is reduced appreciably due to the reduction of surface energy[ This reduction in fracture strength can also be expressed as a function of H concentration at the root of a pre!existing crack ð14Ł] sf−sH bcl
"1#
where sH is the fracture strength of material containing H^ c is the concentration of H^ b and l are appropriate constants which can be determined by experiments[ As this material is heavily cold worked\ a number of defects\ primarily dislocations\ interfaces\ globular shaped cementite\ pores\ microcracks etc[ are generated and these defects are the most potential sites for H entrapment[ In the presence of any external stress\ the microcrack tips act as stress raisers and would further attract H from the surrounding region[ When this H concentration reaches a critical limit\ the fracture stress is reduced drastically and the crack extends to the limit of accumulated H before it stops[ Again\ H di}uses more readily to the fresh crack tip and assists in further propagation of the crack[ The data obtained from the AET during delayed cracking tests con_rms the general understanding\ as explained above\ of the way the cracks propagate under stress\ below the yield stress\ in presence of H[ It is interesting to note that baking for half an hour has improved the ductility and yield strength of the material and reduced the cracking tendency as evidenced in the fractograph "Fig[ 6#[ The early yielding in the as!received sample\ as compared to the baked sample\ can be explained by H assisted plasticity[ As H reduces the cohesive strength\ there will be a corresponding reduction in resistance to localised dislocation mobility in the as!received sample as compared to the H free sample "baked sample#[ It has been reported that hydrogen can di}use out even at room tem! perature if the materials are left in fresh air for a lengthy period[ However\ H trapped in the lattice
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can not be removed completely once the galvanising treatment has been carried out ð13Ł[ The baking operation may not have removed all the H\ particularly the molecular H which caused the pores in the microstructure[ Therefore\ the baking operation has to be carried out immediately after pickling\ before galvanising\ in order to avoid entrapment of H by defects present in the material[ The profusion of pores observed during microstructural examination "Fig[ 2# indicates that molecular H has evolved in the material[ The fracture surfaces showing dimples in tensile tests should not be taken as a signature of ductile fracture[ The pores are in fact responsible for the presence of dimples on the fracture surface and reduction in the ductility of the material[ It is reported in the literature that H assisted cracking can also occur by microvoid coalescence alone and H a}ects the rate of nucleation of voids and reduces plasticity at later stages of fracture ð14Ł[ The split wire surface\ however\ does not show dimples but shows major features of delamination: decohesion type of fracture with interspersed hard particles "cementite# and longitudinal cracks[ These defects are e}ective during bending and twisting\ but not during tensile testing\ and lead to a delamination or decohesion type of cracking[ From the microstructural features "Fig[ 3"b##\ it appears that the delamination has taken place either along the prior austenite grain boundaries\ i[e[\ along the interface of the pearlite colonies or along ferrite:cementite interfaces\ which have been weakened by the presence of H[ In the case of tensile tests these defects do not play a role as they are parallel to the tensile axis of the test sample[ The reason for failures of the wire rods has thus been attributed to hydrogen induced cracking[ After the H has been trapped inside the material due to improper pickling and baking operations\ it has initiated the cracking in the presence of a high tensile residual stress at the axis of the wire and led to a delamination type failure[ During coiling or twisting\ plastic bending is induced in a plane which\ along with residual stress\ causes the wires to fail by splitting along the weakest plane i[e[\ the direction of drawing[
5[ Concluding remarks Based on the investigations carried out\ and the observations thereon\ it appears that the cohesive strength of the boundaries:interfaces was reduced by the presence of atomic H[ This ultimately led to abrupt splitting during subsequent twisting and bending because the symmetric residual stress pattern developed during the drawing operation is relieved[ It appears that a proper inhibitor during the pickling process can prevent ingress of H into the wire rods[ Further\ the baking operation after pickling can be optimised to avoid interaction of H with the materials leading to H assisted cracking[
Acknowledgements The authors thankfully acknowledge useful discussions with Dr A[ Biswas "UMIL\ Ranchi#\ Dr R[ N[ Ghosh "NML\ Jamshedpur# and Dr D[ K[ Bhattacharya "NML\ Jamshedpur#[ Appreciation is also due to Mr S[ Das "NML\ Jamshedpur# for his help in conducting the SEM examinations[ The authors are grateful to Prof[ P[ Ramachandra Rao\ Director\ NML\ for his support and encouragement to publish this work[
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