Effect of arc on radiation thermometry in welding process

38CHINA WELDING Vol.11 No.1 May 2002Effect of arc on radiation thermometry in welding processLi Liangyu，Wangyan and Wu Baolin李亮玉王燕武宝林AbstractThe efecet of arc on radiation thermometry is analyzed in a field close to the arc during the uelding process , and the ratioof signal to noise and other factors are obtained for a small current arc . The method of the temperature measurement is feasible whenthe are current is decreased to a smallr value in the rvelding process .Key words arc , radiation , thermometry0 Introduction1 The features of radiation of small current arcThe information obtained from the top temperature fieldIn fact , the radiation intensity of arc is composed of twoplays an important role in quality control during welding pro-parts : the spectrum intensity and continuous spectrum inten-cess. The method of radiation thermometry is mainly used duesity. When the current is small , the radiation of arc is closeto the particularity of top temperature field. But this methodto the complete line spectrum and band spectrum ,and be-is affected by many factors ,of which , the effect of arc is thetween them there is a big gap ,that is to say , spectrum inten-most serious and difficult to overcome because of the intensesity is very small ,the continuous spectrum intensity almostradiation and wide spectrum. At present , the main steps toapproximates zero and the temperature of arc is low. So theovercome it are as follows :radiated energy of arc is smaller than that of blackbody at the( 1 ) Utilizing long wavelength in radiation thermometry.same temperature. When the current increases , the electronicAs shown in material[ 1 ], the radiation intensity of arc ap-continuous spectrum will be generated and the line spectrumproximate zero when the wavelength exceeds4 pum. So we canwill expand due to the Doppler Effect and the Compton Ef-obtain the information of the temperature field by thermometryfect. As a result , the gap will reduce , viewing the arc as ausing monochromatic infrared rays( 8~ 12 pum ) generated bywhole , the radiation distribution is similar to the blackbodycut-off filter. But the devices of test and measurement are ex-s. This is the reason why the effect of arc is very intensepensive.when the curent is big. As shown in material[ 2 ] , when the(2) Putting out arc instantly. As shown in materialcurrent is small and temperature is below 7 500 K , the radia-[2],we can detect the top temperature field by tuming offtion intensity of arc can be approximately denoted :the power , but this will affct the stability of welding pro-E(λ ,T:)=eC{(ex.-1)-5(1)cess , and the devices are strictly demanded.( 3 ) Small current for instant. We can reduce the radia-λ is the wavelength ;tion intensity of arc by greatly decreasing the current instant-C1 and C2 are the first plank constant and the secondly. At present , this method is mainly used in vision imagedisposal and single point measurement 31plank constant respectively ;The third method is easy to carry out using moderm de-Tk is the, temnerature of arc :vices ,so this paper aims to analyze the effect of arc on radia-中国煤化工。rface( ε≈0.001 ).YHCNMHGtion thermometry.u万友数据Yan and Wu Baolin ,Tanjin Polytechnic University , Tianjin , 300160 E-mail : liangu 2001 @ yahoo. comEfect of arc on radiation thermometry in welding process392 Effect of arc on inspected points in a field close to theexchanged , that is to say :arcAj Fauk=Ai FldWhen the current is small , the are is very weak , the lo-so formula( 2 ) can be denoted :cation of the inspected point and the detector has a great ef-Ea=exE(λ ,T)+(1-ε;) E(λ ,T。) Fa (3)fect on measurement. As shown in Fig.1 ,the shape of are iswhere :a taper( abc ) , the inspected point is d , the angle includedFak is the radiation-interchange angle factors on d tobetween the incident ray of detector and the normnal line ofworkpiece plane including d is β , the angle included be-From formula( 3 ), we can see that the bigger is thetween the da( a is the top of the arc ) and the workpiecespecific enmission of inspected point , the smaller is the valueplane is θ.of the second part , that is to say , the smaller is the effect ofarc. As far as the practical temperature measurement systemshowed in Fig.1 is concerned , the workpiece obeys LanbetLaw , so the active power of radiation at unit-stereo-angle de-tected from d can be denoted :~ DetectorεxE(λ ,T)ld=π'cos β(4)Arc.Now analyze the distribution of energy rllected by d in-to global space centering around d. Obviously ,a large por-tion of energy distributes over the space ,which is symmetri-bCdcal about d and makes an angle of θ with workpiece , the re-main enters the detector miscellaneously. Then the interferen-Workpiece'tial radiation power entering the detector from unit-stereo- an-gle can be denoted :Fig.1 Location of temperature measurement system(1-ελ )E(λ T) Fu(1-f)m ρ(5)π-20Presuming that the temperature of the workpiece surfaceis T ,specific emission is ελ , then the radiation intensity ofspectrum is ελ E6( λ ,T) ,at the same time , arc also radi-f is the fficient cofficient of rflcted level of wok-ates energy on d , one part of the energy is absorbed , thepiece surface ,0≤f≤1 ;other exits into the global space centering around d. So theρ is the efficient coefficient of diffuse reflection of work-effective radiation of inspected point can be denoted :piece surface ,0≤ρ≤1 ;Eix Au=e;EKλ ,T)Aa+(1-εx E(λ T:)Ai Fa(2)m is the distribution cofficient of are location in halfglobal space ,0≤m≤l.Ad is the dimensionof d ;When the location of are in the global space centeringAk is the dimension of effective radiation of arc on in-around d varies ，level of the effect is different,so m isspected point ;used中国煤化Tergy entering are surfaceFad is the radiation- interchange angle factors on arc tofrom dMHC N M H Gm the half global space.d;Fig.2 shows energy exitance from inspected point into halfglobal space.EK λ ,T。 )is efective radiation of arc.Becadsd璐diation-interchange angle factors may be4(CHINA WELDING Vol.11 No.1 May 2002fective radiation area of arc. But the key factors are tempera-ture of inspected point , effective temperature of arc andwavelength used.According to the temperature measurement systemshowed in Fig.1 and Fig.2 ,use formula( 8 ) to analyze theinterrelation between the ratio of signal to noise and wave-length. When welding on low carbon steel with small curentTIC ,θ=φ= π/4 β = π/6 , temperature of inspected pointis1000K,arctemperatureare4000Kand7500Kre-Fig.2 Energy radiation from inspected point intospectively , the speific emission of workpiece( ελ )is 0.85 ,half global spaceand according to material[4],f =0.8, ρ=0.001( ap-proaching practice ). Then Fig.3 can be obtained.According to Lanbet Law ，applying the theory ofbeamed radiation , referring the formula of material[ 4 ], theratio can be denoted :0^φ「晋4000K( w-θ+ " )cos oJ sind dwx1026Fak=πsimplifying it :102 ET500KFak=(一θ-文sin20 )(6)0'Obtaining m ,the ratio of arc surface area receiving ra-diated energy from d to surface area of half global must be00.511.522.533.54concerned , so does the arc location in half global space. Ac-Wavelength J/μmcording to Fig.2 , m can be denoted :Fig.3 Interrelation between S/N ratio K and wavelength λφ日」县_ (w-θ+2 )sinadwAccording to Fig.3 , the longer is wavelength , the big-m=2π- sinθger is the ratio of signal to noise. When wavelength exceeds0.7 pμm , the temperature of small current arc is 7 500 K ,=，办( 1- coso0 )in0(7)2π 6ratio K approximates 400. If measurement precision isn' tSo the ratio of signal to noise can be denoted :strictly demanded , the effect can be ignored.e,E(λ ,T)cos β_πK=FF1-ea)(入T)(1-1)ρ m'ak3 Analyzing the resultsπ-2θAccording to particularity of welding , the effect of arcon radiation thermometry can be verified through continuouslyE、_ . π-2θ_ ∞sβC[e示-1]l λ-S 1(1-6.) π (1-f)0m? eC[eo -1-x-5Pasampling temperature and making comparisons before and af-ter arc中国煤化工dition( using temperature1. Ex.π-2θε 1-εx π (1-f)ρm Fakosβ去切)(8)measurer:MYH. CNMHGI ) are : the thick of low-carbon steel plate is 1.5 mn( using bead welding in TIC pro-cess), the flow of argon is 5 L/min , the resolution of ad-From formula( 8 ), we can see , the ratio of signal tonoise is relate to not only surface state of workpiece , locationvanced temperature measurement system with prism spectro-of inspeta存鰲据nd detector , but also the location and ef-graph exceeds 0. 5C and signals of different wavelengthEffect of arc on radiation thermometry in welding process(0.7 pum and 1.0 pum) from one point can be obtained si-In Fig.4 , there are 250 sampling dots( before arc ismultaneously. It takes no more than 1 ms to sample signalsout ) at left of the imaginary line , at right , there are 100 con-and process them. Inspected point is in front of arc and thetinuous sampling dots( after arc is out ). From Fig.4 , we candistance is 6 mm. Incident ray of detector makes an angle ofsee that there isn' t sudden change of temperature after arc is30°with normal line of workpiece plane including inspectedout ,so the effect of arc can be ignored. The temperature be-point. Workpiece runs at a speed of 1.2 mm/s before the arcetween 0.7 μm and 1.0 pμm is different due to the differentis out. Current must be decreased from 50 A to 5 A andsurface radiant emittance .maintains for 10 ms before sampling. Sampling interval is0.3 s. Put out arc ,at the same time workpiece stops , then4 Conclusionscontinuously sample. Fig. 4 can be obtained .If wavelength and the location of detector are appropri-ate , the effect of small current arc on radiation thermometry is1 250very weak in welding process. It is feasible that applying this1 200method to measure temperature from the front side of work-1 150piece.兰11001 050References[1] Chin B A , Madsen N H . Infrared thermography for sensing1 000| 1.0 pum0.7μmthe are welding poess. Welding Joumal , 1983 ,62(9):227 ~950234900[2] Rider G. Control of welding sizre and position. Robotic Welding.85New York : IFS( Publication) Lud , 1987.8C[3] Kozono Y. Control system of reverse side bead with surface tem-750perature monitoring of TIG weld. Quarterly Jourmal of the Japan5(100 150 200 250 300 350Welding Society ,1986 ,4(2)57~58Sampling dots N[4] Xie Zhi. Industrial radiation thermometry. Shenyang :Dongbei U-niversity Press , 1993. ( in Chinese )Fig.4 The curve of the measured temperature tothe sampling dots中国煤化工MHCNM HG