Study of properties and mechanisms of backing weld of stainless steel piping with no backing gas

Study of properties and mechanisms of backing weld of stainless steel piping with no backing gasStudy of properties and mechanisms of backing weld of stainlesssteel piping with no backing gasLi guodong, LiWei Qi and Zheng Liji李国栋,栗卓新,魏琪,郑丽君*Abstract This work inestigates the proposed welding of stainless steels with no backing gas by using solid wire ER316L-Siand mixed gases( He +Ar +C02). The protection mechanism was also discussed from the view of are physics and the alloyelements in the solid wire. The result shows that when the proportion of helium in the above-noted mixed gases excesses 45percent, the mechanical properties and the intergranular corrosion tendency meet corresponding Chinese standards, Furthethe alloy elements in the solid wire can react with oxygen during the period of high temperature boosted by helium are; thisresult in the formation of a bed of thin slag, which can avoid the oxidation of the backing weld and eliminate the need ofbacking gaKey words stainless steel, solid wire, no backing gas, mixed gases0 Introduction00Crl7Nil4 Mo2. Different shielding gases were used andWelding stainless steel piping with and without back- the compositions of these gases were 7.5%Ar+90%Heing gas are the two main technologies being used in appli- 2. 5% C0, (MI), 22. 5% Ar +75% He+2. 5%COcations. The former is used widely because no slag forma- (M2), 37.5% Ar +60% He +2. 5% C0, (M3), 52.5%tion occurs, and the welder can easily observe the degree Ar+45% He +2.5% CO2(M4)and pure argon(M5)(a)of fusion and the shaping of the backing weld. Howaeve' In order to examine the properties of the joint, a 133 mmthe preparation work of filling argon into the backing welddiameter )x 10 mm( thickness)x 200 mm length)welding process without backing Bagly a newly developeis tedious. Flux cored wire weldingstainless steel pipe with a V-shaped groove were preparedvantage of this technology is that the backing welds are not The groove angle was 60, at a root gap of 2 mm and a 2protected by gases, and this can simplify the welding pro- mm root face. TIG welding was performed using solid wirecedure. Due to an inherent characteristic of flux-cored ER316L-Si, and multi-pass SMaW was carried ouwire, this technology also has disadvantages; for example, electrode A022, with diameters of 2 mm and 4 mm. Thethe melted slag can flow onto the surface of the backing current and voltage were checked using a Hannover analyweld, which causes interference with the welders judgzer, and the picking time was 3 s. Line scanning using aent. In order to answer the above questions, weldingscanning electron microscope SEM) was performed onstainlessusing solid wirethe cross-section of joint welded with solid wire ER316L-SiER316Iielded by different mixed gases(Ar+ Heand tri-mix shielding gas(7. 5% Ar+90% He+2. 5%CO2). The slag film in the backing weld was analyzed1 Experimental methodsing X-ray diffractionThe brial chosen for the investigation was中国煤化工This work was supported by Funding Project for Academic Human ReHigher Education under theJurisdiction of Beijing Municipality.CNMHGLi Guodong, Li Zhuoxin, Wei Qi and Zheng Lijun, College of Materials Science and Engineering, Beijing University of TechnologyBeijing,100124.LiGuodong,Correspondingauthor,E-mail:guodongli@bjut.edu.cnCHINA WELDING Vol 18 No. 3 September 2009gas meets GB/T17853-1999 the intergranular corrosion2. 1 Properties of the welded jointtendency meets GB4334. 5-2000. Of the two specimensMechanical properties and the intergranular corrosion cut from the joint shielded by the M4 mixed gases,onetendency of welded joints are given in Table 1. It can be was found to have cracked. This result shows that the ad-een that the mechanical properties of joint welded by solid dition of helium gas in the tri -mix gases should not be lesswire ER316L-Si and tri- mix shielding gas without backing than 45%Table 1 Mechanical properties and intergranular corrosion tendency of welded jointsMixedTensile strengthFace bendroot bend testFracture positionCorrosion tendencyg/MPaBend angel80°Bend angle I80°550WeldAccepted615Accepted610WeldFailure2.2 Effect of mixed gases on the protection of the voltage increased with the addition of helium in the mixedbacking weldgases. This evidently was caused by the different physicalThe effect of helium on weld penetration is shown in properties of argon and helium. Compared with argon, theFig. 1. It can be seen that the penetration of the weld in- mass of a helium atom is smaller. It is proposed that undercreased with increasing helium content. The thermal con- the action of the arc force, the speed of helium particlesductivity of helium is greater than that of argon. There- increases. This may result in an increase of heat loss duefore, with an increase of the volume percent of helium in to the increased diffusion of helium particle from are col-the mixed gases, the cooling effect of gases on the arc umn to the periphery. Therefore, the heat input powershould increase; this should serve to intensify the arc and namely IE, becomes larger. According to the minimumto increase the plasma jet force. According to Bermoulli's are voltage rule, the electric potential gradient increaseslaw, the kinetic energy of the plasma jet that moves in the given the current and surrounding condition. This meanshigh speed in arc column will transform to pressure, gen- that when arc length is fixed, arc voltage increases witherating additional electromagnetic dynamic pressure on the the increasing of helium content. Therefore, it is proposedweld pool. This effect can cause pool excavating, resulting that as heat input increases and the are column enlargesin a sagging of the weld pool, consequently increasing the this leads to a broader fusion widthpenetration of the weld. A higher helium volume contentshould generate a stronger effect of this action, as is indicated in Fig. 1. It can be seen that when the proportion of1.8helium was more than 75%, the penetration further im-proved. This may be related to the fact that the are dem-onstrates characteristics of helium arc, which can cause astronger plasma jet force.中国煤化工2. 3 Effect of helium on the are voltageCNMHGThe probability density distribution as a function ofare voltage is shown in Fig. 2. It can be seen that theFig 1 Influence of helium gas on the weld penetrationStudy of properties and mechanisms of backing weld of stainless steel piping with no backing gas2. 4 The effect of alloy elementshe protection ofThe result of line scanning measurements of a crosssection of the weld is illustrated in Fig. 4. The figureshows that the content of oxygen, nitrogen, silicon, andnickel changed little across the weld, and the contents of0.liron and chromium also do not vary. This means that theweld is not oxidized00Are length: 4 mr16Fig. 2 Probability density distributionfunction of are voltageThe influence of volume percentage of heliuMVolume percent of helium (%)are voltage is shown in Fig 3. It can be seen that whenFig 3 Influence of volume percentage of helium on are voltagee volume percent of helium was between 45% and60%, the arc voltage was lowest, and the arc presented140the characteristics of argon arc. When the volume percetO Kof helium increased from 60% to 90%, the are voltage80creased, indicating that helium in the mixed gases beginsCrkto ionize. When the added helium attains 75%, the arc20voltage was 20. 83 V and has exceeded the excitation voltage of the helium atom. This means that the physical char-0861.91172382585.73447.7acteristic of mixed gases display the characteristic of helid/μmum. The reason is that inert gases can exist in met-stableand ionize in the accumulative way once its minimum ioni-Fig 4 Line scan of ER316L-Si single weld seamzing voltage is obtainedAs noted above, helium gases deepened the penetThe results of X-ray diffraction measurements of slagtion of the weld. Therefore, using this effect, it is pro- produced by solid wire ER316L-Si is presented in Fig. 5.posed that in order to improve the quality of one side weld- It can be seen that the main phases of slag in the backinging, the spacing between groove should be decreased and weld are MnCr2O4, Mn, SiO,, and MnSiO,, having ththe welding speed should be increased; this would be ben- content of 56. 2%, 18. 3%, 25.4%, respectively.Thisficial in reducing the dwell time at high temperatures and suggests that the alloy elements in the wire have reacteddiminishing the time interval of reaction between the weld with oxygenand that this has prevented the oxi-and air. As a consequence, the metal oxidized could bdation中国煤化工reduced and the possibility of oxidizing of the weld metalN MH Gses high silicon con-could be reducedtent that improves the oxidation resistance 2.The chemi-cal bond of silicon is covalent bond that is lowCHINA WELDING Vol 18 No. 3 September 2009MnCr800Cr,0)+(Mn0)=(Mn,O4)According to equations (1)-(8), the slag com-posed of MnCr, 04, Mn2 SiO4, and MnSiO,, on the surfaceof a backing weld can prevent the weld from oxidation3 Conclusi(1)TIG welding of stainless steels is achieved without backing gas using solid wire ER316L-Si and with26(°)mixed gases( He + Ar+CO2). The mechanical propertiesFig 5 X-ray diffraction of slag produced by solidof the weld meet GB/T17853-1999 standards, and thewire ER316L-Siintergranular corrosion resistance meets GB4334. 5-2000standards when the addition of helium in the tri- mix gasescan reduce the surface tension of molten metal. therefore, exceed 45%%improving its fluidity -4. As the density of silicon is lower than that of iron and chrome it can float on the surfac(2)When the helium content in the mixed gases isof a weld pool during the period of high temperature crea-increased, the penetration depth of the weld deepens. It isted by the helium arc. It can then react preferentially with proposed that, with this behavior, the spacing betweenthe oxygen in air, as:grooves can be reduced and the welding speed should beincreased; this will be beneficial by reducing the dwell[Si]+02=(Sio2)( 1) time at high temperature and will diminishtime Inter-al of reaction between weld and air.The above reaction can become part of a chain that(3)In TiG welding with tri- mix gases, the siliconoccurs during welding. When stainless steel is welded the ER316L-Si solid wire can reduce the surface tension ofwithout backing gas, the oxidation ability of chromium and molten metal, improving the fluidity of molten metal. It ismanganese is stronger than that of iron; therefore, they are proposed that during the period of high temperature crea-easier to combine with oxygen in air and make the iron-ox- ted with a helium arc, the alloy elements such as siliconide reduced. The chemical reactions are as followshrome and manganese can react with oxygen preferential-ly. As a consequence, a bed of slag film is generated2[Fe]+02=2[FeO](2) the surface of the weld, avoiding the oxidation of the back2[Mn]+O2=2(MnO)(3)References(Fe0)+[Mn]=[Fe]+(MnO)(4) [1] LiZX, Huangpu P, Wu H X. Progress in bacingnology of stainless steel tubes. Weapon Material Science and( MnO)+(SiO2)=( MnSiO,)(5)Technology,2001,24(6):54-57[2] Messer B, Lawrence G. Welding stainless steel piping with2(Mn0)+(Si0,)=(Mn, SiO4)no backing gas. Welding Journal, 2002, 81(12): 32-34[3]Kim, Park J, Chulgyu. Flux cored wire for gas-shielded areCr203)中国煤化工-tive element effects on theCNMHG4[Cr]+302=2Cr2O3am welds. Welding Journal,1983,62(3):72s-77[ Cr203]