Monitoring of the Process of Flash-Butt Welding

Transcription

1 Monitoring of the Process of Flsh-Butt Welding (Monitormento do processo de soldgem de topo por fiscmento) Yevgeni Chvertko, Mykol Shevchenko, Andriy Pirumov Ntionl Technicl University of Ukrine Kyiv Polytechnic Institute, Kyiv, Ukrine, Astrct Sttisticl methods of nlysis re currently widely used to develop control nd monitoring systems for different welding processes. These methods llow to otin informtion out the process including effect of ll fctors on its results, which is often difficult to evlute due to the complexity of the process. The uthors mde efforts to pply these methods to develop the system for monitoring the prmeters of flsh-utt welding in rel-time mode. The pper gives rief informtion out the fetures of flsh-utt welding of reinforcement rs nd some sic limittion of this process ppliction. The min resons of formtion of defects in welded joints re given s well s nlysis of possiility of ppliction of monitoring systems for their determintion. The on-line monitoring system sed on neurl networks ws developed for evlution of process devitions. This system is elieved to e dequte for determintion of process violtions resulting in disturnces of welding prmeter nd cn e used for prediction of possile defects in the welded joints. Key-words: Flsh-utt welding, Defects in joints, Neurl networks, Qulity monitoring Resumo: Análises esttístics são normlmente utilizds pr desenvolver sistems de controle e monitormento de diferentes processos de soldgem. Estes métodos permitem oter informção sore o processo, incluindo o efeito de todos os ftores sore o resultdo, os quis são difíceis de vlir devido complexidde do processo. Os utores do presente trlho tentrm plicr estes métodos pr desenvolver um sistem de monitorção dos prâmetros d soldgem de topo por fiscmento em tempo rel. O rtigo dá um idei resumid sore s crcterístics do processo de soldgem por fiscmento de verglhões, ssim como some limitções ásics d plicção do processo. As rzões principis pr formção de defeitos n junt soldd são presentds, ssim como nálise d possiilidde d plicção de um sistem de monitorção pr sus determinções. Um sistem em tempo rel sedo em redes neuris pr vlir s vrições do processo foi desenvolvido e presentdo. Este sistem é considerdo dequdo pr determinr s não conformiddes do processo, e pode ser usdo pr predizer possíveis defeitos n junt soldd. Plvrs-chve: Soldgem de topo por fiscmento, Defeitos, Redes neuris, Qulidde monitorção 1. Introduction At the present time the welding process is considered to consist of three opertions [1]: the welding procedure specifiction, welding itself nd fter-welding check. All the opertions mentioned require ppliction of different methods of control nd monitring. During the welding procedure specifiction the preprtion of prts to e welded is checked s well s preliminry clening, het-tretment, welding consumles nd process prmeters. The process of welding is followed y selective or continuous monitoring. The fter-welding check includes determintion of qulity prmeters, nlysis of posiile resons of formtion of defects nd s result updting (Receido em 02/04/2012; Texto finl em 18/09/2012). the welding procedure nd prmeters (if needed). Evlution methods sed on sttisticl nlysis nd sttisticl qulity control technologies re often used in welding, especilly for fter-welding check. Such qulity control methods llow to improve the system nd orgniztion of control [2]. Sttisticl methods do not replce the other methods of control, ut they significntly improve their effectiveness, dequcy nd economicl chrcteristics. Every welding process consists of two components: one is determined y the pre-set welding prmeters nd resources used, the second one ppers due to different physicl processes occuring during welding (they depend on the process nture, conditions in which welfing is run nd ll the internl nd externl disturnces). It is lmost impossile to determine ll such side processes, mening the exct influence of ech fctor on the qulity of the welded joint cnnot e definitely descried. Sttisticl nlysis llows to evlute the process in whole nd in such wy to tke into considertion the effect of ll fctors irreltively to their nture. Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr

2 Some prospective studies re devoted to evlution of qulity chrcteristics y energetic prmeters of the welding process. This pproch is elieved to e the most pplicle in cse of mss production ecuse these prmeters re esy to mesure without dditionl equipment nd humn resources. The nlysis results re used minly for three purposes: to predict the qulity chrcteristics of the welded joint (possile defects, mechnicl properties nd geometry), to define the disturnces occuring during the process nd for continuous process monitoring. Such nlysis if often performed y mens of intellectul systems which llow to remove the humn from the procedure of control nd thus to reduce the sujectivity of its results nd the process cost. At the present time such monitoring systems re used for rc welding nd resistnce spot welding [3, 4]. The ojective of this work ws to investigte the posiility of ppliction of methods of sttisticl nlysis s well s rtificil intelligence systems for monitoring of flsh-utt welding process of concrete reinforcement rs. The monitoring will e sed on the nlysis of energetic prmeters of the welding process. 2. The monitoring of the process of flsh-utt welding of reinforcement rs 2.1. Technology of flsh-utt welding of reinforcement rs in construction of structures of monolithic reinforced concrete In construction nd repir of reinforced concrete structures the different methods of rc welding of concrete reinforcement re used. The mnul nd semi-utomtic electric rc welding, th-rc welding nd others hve found the widest spreding. It should e noted tht t plnts nd industril groups of sssemled reinforced concrete flsh-utt welding with continuous flshing is widely used, except the ove-mentioned methods. Now, it is one of min methods of producing the utt joints of concrete reinforcement under the shop conditions. The flsh-utt welding is chrcterized y high stle qulity of welded joints, lmost equl in strength to the prent metl tht mkes it possile to increse gretly the reliility nd Yevgeni Chvertko, Mykol Shevchenko, Andriy Pirumov 3 service life of reinforced concrete structures nd to provide the high productivity. The process of welding is performed in the utomtic conditions, comines the ssemly nd welding opertions in single cycle, does not require ppliction of uxiliry consumles (electrodes, welding wire, fluxes, gses, etc.). Moreover, specil requirements to the welder s qulifiction re not specified. At the present time, specil technologies nd equipment re eing developed to mke it possile to use this welding method in onsite nd semi-sttionry conditions [6]. The method ppliction is lso prtilly limited y existing qulity control procedures [5]. This rises the prolem of development of technologies of non-destructive rel-time monitoring. As rule, the concrete reinforcement, rs of up to 22 mm re joined y the flsh-utt welding with continuous flshing, nd the lrge-dimeter rs re joined y using flshing with preliminry resistnce preheting. The ltter is chrcterized y wide instle HAZ. The flsh-utt welding with pulsed flshing llows joining ll the ssortment of reinforced rs nd hs dvntges over the ove-mentioned methods. Mechnicl chrcteristics of these steels in hot-rolled stte re given in Tle 1, nd chemicl composition is given in Tle 2. The crried out technologicl investigtions of welding the hotrolled reinforcement rs mde it possile to estlish the vlues of min prmeters of welding conditions: djusting length L dj, tolernces for flshing L flsh nd upsetting L upset, open-circuit secondry voltge U 2o.-c rtes of flshing v flsh, nd upsetting v upset, time of welding nd time of upsetting with current t upseti [6, 8]. The incresed sensitivity to heting of het-hrdened reinforcement rs specifies the dditionl requirements to therml cycles of heting in welding nd increses gretly the requirements to the selection of its heting conditions during welding. Using the welding prmeters with smller tolernces for flshing usully cuses the rittle frcture ecuse of n intensive het dissiption into welding mchine electrodes eing cooled. The recent technologicl investigtions resulted in development of welding cycles for het-hrdened reinforcement rs which llow to produce welded joints lmost equl in strength to the prent metl [6]. Tle 1. Mechnicl chrcteristics of r reinforcing steels Steel grde σ 0.2, MP σ t, MP δ, % 35GS G2S St3Gsp Tle 2. Chemicl composition of r reinforcing steels, wt.% Steel grde C Mn Si Cr Ni S P Cu 35GS <0.30 <0.30 <0.045 <0.040 < G2S <0.30 <0.30 <0.045 <0.040 <0.30 St3Gsp <0.15 <0.30 <0.30 <0.050 <0.040 < Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr 2013

3 Monitoring of the Process of Flsh-Butt Welding 2.2. Anlysis of possiility of ppliction of the system of monitoring for determintion of defects In generl the defects in the welded joints of reinforcement rs pper s result of three min resons [9]: violtion of technologicl process prmeters (including the results of different disturnces), violtion of technology nd rules of the welding equipment opertion nd internl prent metl defects. Violtion of technologicl process prmeters is often result of different disturnces (energetic, kinemtic nd technologicl). They my result in significnt difference etween the pre-set nd rel vlues of welding prmeters even in cse of proper welding equipment opertion nd preprtions of prts to e welded. In generl, two vrints of process re possile in this cse: one cusing the decrese of het input, the second leding to overheting of rs. In cse of low het input the plstic chrcteristics of the welded rs re reduced especilly if they were preliminrily het-hrdened. The joint is formed only on the prt of utt nd often discontinuities re detected. Mechnicl lods cuse rittle frcture of the weld itself. Typicl welded joint frcture nd its mcrosection re shown on Fig. 1. Overheting of the welded joint results in decresing of microhrdness in the HAZ due to locl het tretment occurring during the welding cycle. Appliction of the externl lod cuses formtion of two necks proving the hrdness loss (see Fig. 2, ). Further increse of het-input cuses formtion of hot crcks in the joint plne. The structure strength is reduced to MP. Violtion of ny technologicl process prmeter results in chnges of the electric prmeters, mening the monitoring system cn e effectively used to detect them. Figure 1. The reinforcement rs welded with low het-input: frcture fter tensile tests; mcrosection Figure 2. The overheted reinforcement rs: frcture fter tensile tests; mcrosection Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr

4 Yevgeni Chvertko, Mykol Shevchenko, Andriy Pirumov Violtion of technology nd rules of the welding equipment opertion in generl led to disturnces of the welding prmeters in comprison to their vlues specified in the welding procedure specifiction nd lso to disturnces relted to improper preprtion of prts to e welded. These disturnces re very similr to those cused y the violtion of technologicl process prmeters nd they result in formtion of the sme defects in the welded joints. Sometimes the urnt spots re detected on the sides of workpieces reducing the cross-section nd cusing the strength decresing (Fig. 3). As rule, the urnt spots pper in cse of low clmping force or hevy contmintion of current-crrying jws. In plce when the prt is not tightly djcent to the electrode the trnsient electric resistnce is incresed to the vlue exceeding the resistnce of flsh gp nd the process of locl flshing long the prtelectrode contct surfce egins. The formtion of urnt spots is lso cused y n incresed electricl resistnce etween the profiled lterl surfce of the concrete reinforcement nd electrode. Violtion of technology nd rules of the welding equipment opertion lso result in chnges of the welding process prmeters mking it possile to detect them y mens of the monitoring system. Figure 3. Frcture of reinforcement with urnt spot Internl defects of the prent metl (those cused y the process of reinforcement rs production) sometines re lso detected in the welded joints. Mostly they re lckfins which re formed during the r rolling process. They often pper on the edges of sls nd looms s thick oxide film inside the workpiece (see Fig. 4). It is completely impossile to detect such defects y mens of the monitoring system. To prevent their presence in the welded structure full-scle incoming control of rs should e pplied. Figure 4. Internl defect of the reinforcement r 2.3. Development of the technology of monitoring for flshutt welding process Most of defects which occur s result of the welding process cn e identified y mens of rel time monitoring of welding prmeters (without or in ddition to fter-welding mechnicl testing). In cse of flsh-utt welding of concrete reinforcement rs simple monitoring procedure ws used with single mesured prmeter secondry voltge. Welding voltge itself is one of the welding process prmeters tht is most sensitive to disturnces. The optiml conditions (s descried in [6]) s well s three cses of devition of the process prmeters deteriorting qulity chrcteristics of the joints, which most often occur during welding, such s decrese in workpiece feed speed v flsh, decrese in mchine open-circuit voltge U 2o.-c, nd chnge in workpiece extension L dj, were evluted. The joints produced s result of the experiments differed in the heting zone nd presence of defects, such s discontinuities (Fig. 5). It cn e seen from oscillogrms of the welding voltge otined under different welding conditions (Fig. 6) tht the sufficiently informtive signl reflecting the course of the explosivesprk process is high-frequency component. The low-pss Butterworth filter ws pplied to revel this component. The otined rrys were divided into locks equl to ten periods of the industril mins voltge (0.2 s), sed on permissile durtion of devitions during the flshing process [8]. Mthemticl expecttion of rndom quntity module reflecting the intensity of flshing t given stge of oscillogrm ws determined for ech lock. This resulted in the dt rrys reflecting the intensity of flshing (Fig. 7). 34 Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr 2013

5 Monitoring of the Process of Flsh-Butt Welding c d Figure 5. Mcrosection of the joints produced under different welding conditions: here nd elow in Figures 6 nd 7: optiml conditions, decresed speed, c decresed voltge, d incresed extension c d Figure 6. Typicl voltge oscillogrms t different welding prmeters Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr

6 Yevgeni Chvertko, Mykol Shevchenko, Andriy Pirumov c Figure 7. Typicl dt rrys reflecting the intensity of flshing for groups of joints d The prolem of utomtic clssifiction of the dt rrys cn e successfully solved y using neurl networks. The Proility Neurl Network (PNN) nd Lerning Vector Quntiztion (LVQ) were used for clssifiction sed on the criterion of the presence of devition of one of the process prmeters (v flsh, U 2o.-c, L dj ). The dt otined during experiments were used for network trining, the sme trining sequence ws pplied for oth types of networks. The error in identifiction of disturnces ws not in excess of 8 % for LVQ nd 13 % for PNN (Fig. 8). This proves possiility of the developed system ppliction for most determining of typicl distunces of the welding process. To determine the generl qulity chrcteristics for ech group of welded joints the mechnicl testing of full-scle specimens of joints of hot-rolled reinforcement rs of steel 35GS were performed in ccordnce with requirements [5]. Frcture of ll specimens occurred in the prent metl t lrge distnce from the welded joint nd HAZ (Fig. 9). Mcrosection of the joint nd distriution of microhrdness (Vickers hrdness test, lod 0.05 MP) re shown in Fig. 10, microstructure of zone of joint nd prent metl is given in Fig. 11. Similr results were otined lso on steels St3Gps nd 25G2S in het-hrdened stte. The frcture of most specimens (out 90 %) occurred in prent metl eyond HAZ. Mcrosection of such joint nd distriution of hrdness in it re given in Fig. 12. The mechnicl tests hve proved tht devitions of the prmeters of welding process result in the reducing of the joint qulity: the tensile strength reduces, res with significntly lowered microhrdness pper in HAZ. Figure 8. Results of opertion of PNN nd LVQ networks 36 Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr 2013

7 Monitoring of the Process of Flsh-Butt Welding Figure 9. Welded specimens of hot-rolled () nd het-hrdened () concrete reinforcement fter tensile tests Figure 10. Mcrosection () nd distriution of microhrdness () in the zone of welded joint of reinforcement r of steel St3Gps Figure 11. Microstructure of welded joint (), HAZ () nd prent metl (c) of reinforcement r of St3Gps The developed method of monitoring cn e used to predict the defects cused y the violtion of technologicl process prmeters. It is lso possile to detect disturnces cused y violtion of technology nd rules of the welding equipment opertion. But in this cse the monitoring system will definitely fil to determine the reson of these disturnces. It will clssify them s if they were cused y the violtion of the process prmeters. In ny cse, these disturnces will result in formtion of the sme defects in the joint, so the results of dt nlysis will e useful to predict them. The internl defects of the prent metl re completely impossile to e detected y Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr 2013 mens of the monitoring procedure developed. To prevent their presence in the welded structure full-scle incoming control of rs should e pplied. Appliction of technologies of joining of hot-rolled nd hethrdened concrete reinforcement will mke it possile to produce utt joints equl in strength to the prent metl under the site nd semi-sttionry conditions, to increse the service life of reinforced concrete structures, to increse their reliility nd to gurntee the high service life. The results otined proved the fesiility of using of rtificil neurl networks for qulity monitoring in flsh-utt welding. 37

8 Yevgeni Chvertko, Mykol Shevchenko, Andriy Pirumov Figure 12. Mcrosection () nd distriution of hrdness () in the zone of welded joint of reinforcement r of steel 25G2S 3. Conclusion 1 Appliction of the system of monitoring of the welding prmeters llows to determine disturnces cused y violtion of technologicl process prmeters nd of technology nd rules of the welding equipment opertion. Therefore such systems llow to predict relted defects in the welded joints. The internl defects of the prent metl cnnot e detected y electricl prmeters of the welding process. 2. The monitoring of the welding process is sed on the nlysis of high-frequency component of the welding voltge. The PNN nd LVQ rtificil neurl networks re successfully used for utomtic monitoring. Such monitoring systems cn e effectively used together with or insted of mechnicl tests in industril conditions to determine the possiility of defect formtion in the joint depending on devinces occuring during the joining process. Welding Journl, n 11, 2007, p [5] GOST : Reinforced nd emedded welded pieces of concrete structures. Generl specifiction. Moscow: Stndrt. [6] CHVERTKO, P. N. Flsh-utt welding of reinforcement rs of A400s A600S clsses in construction of structures of monolithic reinforced concrete. Pton Welding Journl, n 8, 2010, p [7] DSTU 3760:2006: Reinforced r for concrete structures. Generl requirements. Generl specifiction. Kyiv: Derzhspozhyvstndrt. [8] KUCHUK-YATSENKO, S. I. (1992) Flsh utt welding. Kiev: Nukov Dumk. [9] CHVERTKO, P. M., CHVERTKO, Ye. P. Fetures of flshutt welding of concrete reinforcement rs. Bulletin of Admirl Mkrov Ntionl University of Shipuilding, n. 4, 2010, p Acknowledgements The uthors wish to thnk reserchers from the Welding Fculty of the Ntionl Technicl University of Ukrine Kyiv Polytechnic Institute nd from the Deprtment of Pressure Welding of the E.O.Pton Electric Welding Institute for support. 5. References [1] M. XIE, G. BOLMSJO Qulity ssurnce nd control for rootic GMA welding Prt 1: QA model nd welding procedure specifiction. Joining Sciences, n. 1(4), 1993, р [2] PIRUMOV, A. E., SHEVCHENKO, M. V., SKACHKOV I.O. Qulity monitoring of welding y electric chrcteristics of process. Scientific Bulletin, n. 5, 2011, p [3] SKACHKOV, I.O., PIRUMOV, A. E., MAKSIMOV, S. Yu., PRILIPKO, E. A. On neurl network ppliction for welded joint qulity control in underwter welding. Pton Welding Journl, n 6, 2006, p. 21 [4] RUDENKO, P. N., GAVRYSH, V. S. System of utomtic control of resistnce spot welding process KSU KS 02. Pton 38 Soldg. Insp. São Pulo, Vol. 18, Nº. 01, p , Jn/Mr 2013