Submerged Arc Welding

Transcription

1 3. Submerged Arc Welding

2 3. Submerged Arc Welding 32 flux hopper electrode flux contact piece solid slag arc weld metal base metal liquid slag molten pool weld cavity Figure 3.1 br-er3-1e.cdr Process Principle of Submerged Arc Welding In submerged arc welding a mineral weld flux layer protects the welding point and the freezing weld from the influence of the surrounding atmosphere, Figure 3.1. The arc burns in a cavity filled with ionised gases and vapours where the droplets from the continuously-fed wire electrode are transferred into the weld pool. Unfused flux can be extracted from behind the welding head and subsequently recycled. Main components of a submerged arc welding unit are: the wire electrode reel, the wire feed motor equipped with grooved wire feed rolls which are suitable for the demanded wire diameters, a wire straigthener as well as a torch head for current transmission, Figure 3.2. Flux supply is carried out via a hose from the flux container to the feeding hopper which is mounted on the torch head. Depending on the degree of automation it is possible to install a br-er3-2e.cdr Figure 3.2 AC or DC current supply wire straightener wire feed rolls flux supply indicators power source wire reel welding machine holder Assembly of a Welding Equipment flux excess pickup behind the torch. Submerged arc welding can be operated using either an a.c. power source or a d.c. power source where the electrode is normally connected to the positive terminal. Welding advance is provided by the welding machine or by workpiece movement.

3 3. Submerged Arc Welding 33 Identification of wire electrodes for submerged arc welding is based on the average Mncontent and is carried out in steps of.5%, Figure 3.3. Standardisation for welding filler materials for unalloyed steels as well as for fine-grain structural steels is contained in DIN EN 756, for creep resistant steels in DIN pr EN 127 (previously DIN 8575) and for stainless and heat resistant steels in DIN pr EN 1272 (previously DIN ). The proportions of additional alloying elements are dependent on the materials to be welded and on the mechanical-technological demands which emerge from the prevailing operating conditions, Figure 3.4. Connected to this, most important alloying elements are manganese for strength, molybdenum for high-temperature strength and nickel for toughness. alloy type Mn MnMo Ni NiMo NiV NiCrMo commercial wire electrodes S1 S2 S3 S4 S2Mo S3Mo S4Mo S2Ni1 S2Ni2 S2NiMo1 S3NiMo1 main alloying elements Mn Ni Mo Cr V,5 1, 1,5 2, 1, 1,5 2, 1, 1, 1, 1,5 1, 2, 1, 1,,5,5,5,5,5 S3NiV1 1,5 1,,15 S1NiCrMo2,5 S2NiCrMo1 S3NiCrMo2,5,5 1, 1,5 2,5 1, 2,5,6,6,6,8,5,8 From a diameter of 3 mm upwards all wire electrodes have to be marked with the following symbols: S1 Si Mo S6: : : I _ IIIIII Example: S2Si: II _ S3Mo: III DIN EN 756 mat.-no. S S S S2Si S2Mo S2Ni1 S2Ni2 S3NiMo1 Reference analysis approx. weight % C =,8 Si =,9 Mn =,5 C =,1 Si =,1 Mn = 1, C =,11 Si =,15 Mn = 1,5 C =,1 Si =,3 Mn = 1, C =,1 Si =,15 Mn = 1, Mo =,5 C =,9 Si =,12 Mn = 1, Ni = 1,2 C =,1 Si =,12 Mn = 1, Ni = 2,2 C =,12 Si =,15 Mn = 1, Mo =,5 Ni = 1, Properties and application For lower welding joint quality requirements;in: boiler and tank construction, pipe production, structural steel engineering, shipbuilding For higher welding joint quality requirements; in: pipe production, boiler and tank construction, sructural steel engineering, shipbuilding. Fine-grain structural steels up to StE 38. For high-quality welds with medium wall-thicknesses. Fine-grain structural steels up to StE 42. Especially suitable for welding of pipe steels, no tendency to porosity of unkilled steels. Fine-grain structural steels up to StE 42. For welding in boiler and tank construction and pipeline production with creep-resistant steels. Working temperatures of up 5 C. Suitable for higher-strength fine-grain structural steels. For welding low-temperature fine-grain structural steels. Non-ageing. Especially suitable for low-temperature welds. Non-ageing. For quenched and tempered fine-grain structural steels. Suitable for normalising and/or re-quenching and tempering. br-er3-3e.cdr br-er3-4e.cdr Wire Electrodes for Submerged Arc Welding Properties and Application Areas for Wire Electrodes in Submerged Arc Welding Figure 3.3 Figure 3.4 The identification of wire electrodes for submerged arc welding is standardised in DIN EN 756, Figure 3.5. During manufacture of fused welding fluxes the individual mineral constituents are, with regard of their future composition, weighed and subsequently fused in a cupola or electric furnace, Figure 3.6. In the dry granulation process, the melt is poured stresses break the

4 3. Submerged Arc Welding 34 crust into large fragments. During water granulation the melt hardens to form small grains with a diameter of approximately 5 mm. Wi re ele ctro de D I N EN S2Mo br-er3-5e.cdr Figure 3.5 DIN main no. Symbols of the chemical composition: S, S1...S4, S1Si, S2Si, S2Si2, S3Si, S4Si, S1Mo,..., S4Mo, S2Ni1, S2Ni1.5, S2Ni2, S2Ni3, S2Ni1Mo, S3Ni1.5, S3Ni1Mo, S3Ni1.5Mo Identification of a Wire Electrode in Accordance with DIN EN 756 As a third variant, compressed air is additionally blown into the water tank resulting in finely blistered grains with low bulk weight. The fragments or grains are subsequently ground and screened thus bringing about the desired grain size. lime quarz rutile bauxite magnesite rutile Mn - ore fluorspar magnesite alloys roasting kiln balance silos sintering furnace ball mill silos coke mixer balance raw material coke molten metal air dish granulator gas electrical furnace tapping coal-burning stove drying oven granulation tub cylindrical crusher screen foaming air heat treatment furnace screen drying oven cooling pipe balance balance br-er3-6e.cdr br-er3-7e.cdr Production of Fused Welding Fluxes Production of Agglomerated Welding Fluxes Figure 3.6 Figure 3.7

5 3. Submerged Arc Welding 35 Properties uniformity of grain size distribution grain strength homogeneity susceptibility to moisture storing properties resistance to dirt current carrying capacity slag removability high-speed welding properties multiple-wire weldability flux consumption Fused fluxes 1) --/+ 1) assessment : -- bad, - moderate, + good, ++ very good 2) core agglomerated flux -/+ -/++ -/+ Agglomerated fluxes 1) -/++ -/++ 2) -- /++ -/+ -/++ -/++ During manufacture of agglomerated weld fluxes the raw materials are very finely ground, Figure 3.7. After weighing and with the aid of a suitable binding agent (waterglass) a pre-stage granulate is produced in the mixer. Manufacture of the granulate is finished on a rotary dish granulator where the individual grains are rolled up to their desired size and consolidate. Water evaporation in the drying oven hardens the grains. In the annealing furnace the remaining water is subsequently removed at temperatures of between 5 C and 9 C, depending on the type of flux. br-er3-8e.cdr Figure 3.8 Properties of Fused and Agglomerated Welding Fluxes The fused welding fluxes are characterised by high homogeneity, low sensitivity to moisture, good storing properties and high abrasion resistance. An important advantage of the agglomerated fluxes is the relatively low manufacturing temperature, Figure 3.8. The technological properties of the welded joint can be improved by the addition of temperature-sensitive deoxidation and alloying constituents to the flux. Agglomerated fluxes have, in general, a lower bulk weight (lower consumption) which allows the use of components which are reacting among themselves during the melting process. However, the higher susceptibility to moisture during storage andprocessing has to be taken intoconsideration. MS CS ZS RS AR AB AS AF FB Z br-er3-9e.cdr Figure 3.9 MnO + SiO2 min. 5% CaO max. 15% manganese-silicate CaO + MgO + SiO2 min. 55% CaO + MgO min.15% calcium-silicate ZrO 2 + SiO 2 + MnO min. 45% ZrO2 min. 15% zirconium-silicate TiO 2 + SiO2 min. 5% TiO2 min. 2% rutile-silicate Al2O 3 + TiO2 min. 4% aluminate-rutilel Al2O 3 + CaO + MgO Al2O3 min. 4% min. 2% aluminate-basic CaF2 Al2O 3 + SiO 2 + ZrO2 CaF 2 + MgO max. 22% min. 4% min. 3% aluminate-silicate ZrO2 Al2O 3 + CaF2 min. 5% min. 7% aluminate-fluoride-basic CaO + MgO + CaF 2 + Mo SiO2 min. 5% max. 2% fluoride-basic CaF2 min. 15% other compositions Different Welding Flux Types According to DIN EN 76

6 3. Submerged Arc Welding 36 The welding fluxes are, in accordance with their mineralogical constituents, classified into nine groups, Figure 3.9. The composition of the individual flux groups is to be considered as in principle, as fluxes which belong to the same group may differ substantially with regards to their welding or weld metal properties. In addition to the groups mentioned above there is also the Z-group which allows free compositions of the flux. The calcium silicate fluxes are recognized by their effective silicon pickup. A low Si pickup has low cracking tendency and liability to rust, on the other hand the lower current carrying AB AS AF FB Z br-er3-1be.cdr - medium manganese pickup - good weldability - good toughness values in welding by the pass/ capping pass method - application field:unalloyed and low alloyed structural steels - suitable for a.c. and d.c. - applicable for multilayer welding or welding by the pass/ capping pass method - mainly neutral metallurgical behavior - manganese burnoff possible - good weld appearance and slag removability - to some degree suitable for d.c. - recommended for multi layer welds for high toughness requirements - application field: high-tensile fine grain structural steels, pressure vessels, nuclear- and offshore components - suitable for welding stainless steels and nickel-base alloys - neutral behaviour as regards Mn, Si and other constituents - mainly neutral metallurgical behaviour - however, manganese burnoff possible - highest toughness values right down to very low temperatures - limited current carrying capacity and welding speed - recommended for multi layer welds - application field: high-tensile fine-grain structural steeels - all other compositions Figure 3.1b Classification of Fluxes for Welding According to DIN EN 76 ( II) MS CS ZS RS AR br-er3-1ae.cdr - high manganese and silicon pickup - restricted toughness values - high current carrying capacity/ high weld speed - unsusceptible to pores and undercuts - unsuitable for thick parts - suitable for high-speed welding and fillet welds acidic types - highest current carrying capacity of all fluxes - high silicon pickup - suitable for welding by the pass/ capping method of thick parts with low requirements basic types - low silicon pickup - suitable for multiple pass welding - current carrying capacity decreases with increaseing basicity - high-speed welding of single-pass welds - high manganese pickup/ high silicon pickup - restricted toughness values of the weld metal - suitable for single and multi wire welding - typical: welding by the pass/ capping pass method - average manganese and silicon pickup - suitable for a.c. and d.c. - single and multi wire welding - application fields: thin-walled tanks, fillet welds for structural steel construction and shipbuilding Figure 3.1a Classification of Fluxes for Welding According to DIN EN 76 (I) capacity of these fluxes has to be accepted. A high Si pickup leads to a high current currying capacity up to 25 A and a deep penetration. Aluminate-basic fluxes have, due to the higher Mn pickup, good mechanical properties. With the application of wire electrodes, as S1, S2 or S2Mo, a low cracking tendency can be obtained. Fluoride-basic fluxes are characterised by good weld metal impact values and high cracking insensitivity. Figures 3.1a and 3.1b show typical properties and application areas for the different flux types.

7 3. Submerged Arc Welding 37 Figure 3.11 shows the identification of a welding flux according to DIN EN 76 by the example of a fused calcium silicate flux. This type of flux is suitable for the welding of joints as well as for overlap welds. The flux can be used for welding of unalloyed and low-alloy welding f l ux DIN E N 76- S F C S 1 67 AC H 1 DIN main no. flux/ welding method of manufacture F fused A agglomerated M mechanically mixed flux flux type (figure 3.9) br-er3-11e.cdr Figure 3.11 Identification of a Welding Flux According to DIN EN 76 hydrogen content (table 4) type of current metallurgical behaviour (table 2) flux class 1-3 (table 1) steels, as, e.g. general structural steels, as well as for welding high-tensile and creep resistant steels. The silicon pickup is.1.3% (6), while the manganese pickup is expected to be.3.5% (7). Either d.c. or a.c. can be used, as, in principle, a.c. weldability allows also for d.c. power source. The hydrogen content in the clean weld metal is lower than the 1 ml/1 g weld metal. The flux classes 1-3 (table 1) explain the suitability of a flux for welding certain material groups, for welding of joints and for overlap welding. The flux classes also characterise the table 1 flux class unalloyed and low-alloyed steel general structural steel high-tensile & creep resistant steels stainless and heat resistant steels Cr- & CrNi steels welding of joints hardfacing pickup of elements as C, Cr, Mo br-er3-12e.cdr Figure 3.12 table 4 identification H5 H1 H15 table 2 metallurgial behaviour burnoff pickup or burnoff pickup identification figure Parameters for Flux Identification According to DIN EN hydrogen content ml/1g all-weld metal max proportion flux in all-weld metal % over,7,5 up to,7,3 up to,5,1 up to,3 up to,1,1 up to,3,3 up to,5,5 up to,7 over,7 metallurgical material behaviour. In table 2 defines the identification figure for the pickup or burn-off behaviour of the respective element. Table 4 shows the gradation of the diffusible hydrogen content in the weld metal, Figure 3.12.

8 3. Submerged Arc Welding 38 Figure 3.13 shows the identification of a wire-flux combination and the resultant weld metal. It is a case of a combination for multipass welding where the weld metal shows a minimum yield point of 46 N/mm² (46) and a minimum metal impact value of 47 J at 3 C (3). The flux type is aluminate-basic (AB) and is used with a wire of the quality S2. wire-flux c om bina t i on D I N EN S AB S2 standard no. wire electrode and/or wire-flux combination for submerged arc welding chemical composition of the wire electrode type of flux (figure 3.1) The tables for the identification of the tensile properties as well as of the impact energy are combined in Figure br-er 3-13e.cdr Figure 3.13 strength and fracture strain (table1 and 2) Identification of a Wire-Flux Combination According to DIN EN 756 impact energy (table 3) The chemical composition of the weld metal and the structural constitution are dependent on table 1 identification table br-er3-14e.cdr table 2 identification 2T 3T 4T 5T identification temp. for minimum impact energy 47J C Figure 3.14 Identification for strength properties of multipass weld joints minimum yield point n/mm minimum base metal yield strength N/mm tensile strength N/mm 2 44 up to up to 6 5 up to up to up to 72 minimum fracture strain % Identification for strength properties of welding by the pass/ capping pass method welded joints minimum tensile strength N/mm Identification for the impact energy of clean all-weld metal or of welding by the pass/ capping pass method welded joints Z no demands Parameter for Weld Metal Identification According to DIN EN A the different metallurgical reactions during the welding process as well as on the used materials, Figure The welding flux influences the slag viscosity, the pool motion and the bead surface. The different combinations of filler material and welding flux cause, in direct dependence on the weld parameters (current, voltage), a different melting behaviour and also different chemical reactions. The dilution with the base metal leads to various strong weld pool reactions, this being dependent on the weld parameters. The diagram of the characteristics for 3 different welding fluxes assists, in dependence of the used wire electrodes, to determine the pickup and burn-off behaviour of the element

9 3. Submerged Arc Welding 39 welding flux welding filler metal manganese, Figure For example: A welding droplet reaction welding data flux with the mean charac- base metal teristic and when a wire electrode S3 is used, has a slag dilution welding data neutral point where neither pickup nor burn-off occur. weld pool reaction welding data br-er 3-15e.cdr Figure 3.15 weld metal The pickup and burn-off behaviour is, besides the filler material and the welding flux, also directly dependent on the welding amperage and welding voltage, Figure By the example of the selected flux a higher welding voltage causes a more steeply descending manganese characteristic at a constant neutral point. Silicon pickup increases with the increased voltage. The influence of current and voltage on the carbon content is, as a rule, negligible. Metallurgical Reactions During Submerged Arc Welding Mn-pickup Mn-burnoff br-er 3-16e.cdr Figure 3.16 S1 1,% 2,% 3,% Mn in wire S2 S3 S4 S5 S6 Manganese-Pickup and Manganese-Burnoff During Submerged Arc Welding Inversely proportional to the voltage is the rising characteristic as regards manganese in dependence on the welding current, Figure Higher currents cause the characteristic curve to flatten. As the welding voltage, the welding current also has practically no influence on the location of the neutral point. Silicon pickup decreases with increasing current intensity.

10 3. Submerged Arc Welding 4 pickup/ burnoff X in weight % weld flux LW 28 (DIN EN 76 SF CS 1 76 AC H 1) current intensity 58 A welding speed 55 cm/min,6 % Mn,2 -,2 -,4 -,6,6 % Si,2 -,2 -,4,5 -,5 -,1 -,15 33 V 36 V 25 V % Mn wire % Si wire neutral point,5 1, 2, 2,5 25 V 27 V 29 V 36 V,5,1,15,2,25,3,35,4,45,5,15,2, V % C wire pickup/ burnoff X in weight % weld flux LW 28 (DIN EN 76 SF CS 1 76 AC H 1) arc voltage 29 V welding speed 55 cm/min,6 45 A % Mn 65 A 58 A neutral point,2 7 A,5 1, 2, 2,5 -,2 % Mn wire 8 A -,4 -,6,6 % Si,2 -,2 -,4,5 -,5 -,1 -,15 8 A,5,1,15,2,25,3,35,4,45 % Si wire 45 A 8 A % X SZ 45 A,15,2,25 % C wire br-er3-17e.cdr ISF 26 br-er3-18e.cdr ISF 26 Pickup and Burnoff Behaviour in Dependence on Welding Voltage and Wire Electrode Pickup and Burnoff Behaviour in Dependence on Welding Current and Wire Electrode Figure 3.17 Figure 3.18 br-er3-19e.cdr flux diagramm LW 28 (DIN EN 76 SF CS 1 76 AC H 1), manganese wire electrode 4 mm Ø acc. to Prof. Thier example: I = 58 A U = 29 V Mn SZ1 =.48 % Mn Mn = 1.69 % Mn Welding Flux Diagramm for Determination of the Mn Content in the Weld Metal SZ2 The Mn-content of the weld metal can be determined by means of a welding flux diagram, Figure In this example, the two points on the axis which determine the flux characteristic are defined for the parameters 58A welding current and 29V welding voltage, with the aid of the auxiliary straight line and the neutral point curve (Mn NP ). In this case, the two points are positioned at.6% Mn and 1.25% Mn SZ. Dependent on the manganese content of the used filler material, the pickup or burn-off contents can be recognized by the reflection with respect to the characteristic line (.38% Figure 3.19

11 3. Submerged Arc Welding 41 Mn-pickup with a wire containing.5%mn,.2% Mn-burnoff with a wire containing 1.75%Mn). flux diagramm LW 28 (DIN EN 76 SF CS 1 76 AC H 1), silicon wire electrode 4 mm Ø acc. to Prof. Thier example: I = 58 A U = 29 V Si =.16 % Si SZ auxiliary straight line The structure of the characteristic line for the determination of the silicon pickup content, is, in principle, exactly the same as described above, Figure 3.2. As silicon has only pickup properties and therefore no neutral point exists, the second auxiliary straight line must be considered for the determination of the second characteristic line point. auxiliary straight line br-er3-2e.cdr ISF 26 Welding Flux Diagram for Determination of the Si Content in the Weld Metal Weld preparations for multipass fabrication are dependent on the thickness of the plates to be welded, Figure If no root is planned during weld preparation and also no support of the weld pool is made, the root pass must be welded using low energy input. Figure 3.2 from both sides, the double-u butt weld may be applied, Figure Before the opposite side is welded, the root must be milled out (gouging/sanding). This type of weld cannot be produced by flame cutting and is, as milling is necessary, more expensive, although exact weld preparation and correct selection of the welding parameters lead to a high weld quality. When welding very thick plates which are accessible preparation geometry br-er 3-21e.cdr Figure 3.21 weld buildup manual metal arc welding manual metal arc welding manual metal arc welding Welding Procedure Sheets for Single-V Butt Welds, Single-Y Butt Welds with Broad Root Faces and Double-V Butt Welds and

12 3. Submerged Arc Welding 42 preparation geometry br-er3-22e.cdr Figure 3.22 Another variation of heavyplate welded joints is the so-called steep single-v butt weld, Figure The very steep edges keep the welding volume at a very low level. This technique, however, requires the application of special narrow-gap torches. The geometry during slag detachment and also during reworking weld-related defects may cause problems. Here, high demands are made on torch manipulation and process control. Special narrow-gap welding fluxes facilitate slag removal. br-er3-23e.cdr Figure 3.23 The most important welding parameters as regards weld bead formation are welding current, voltage and speed, Figure A higher welding current causes higher deposition rates and energy input, which leads to reinforced beads and a deeper penetration. The weld width remains roughly constant. The increased welding voltage leads to a longer arc which also causes the bead to be wider. The change in welding speed causes - on both sides of an optimum - a decrease of the penetration depth. At lower weld speeds, the weld pool running ahead of the welding arc acts as a buffer between arc and base metal. At high speeds, the energy per unit length decreases which leads, besides lower penetration, also to narrower beads. weld buildup side 1 side 2 Welding Procedure Sheet for Double-U Butt Welds Welding Procedure Sheet for Square-Edge Welds manual metal arc welding turning and sanding manual metal arc welding turn turn turn GMA welding GMA welding welding welding oscillated

13 3. Submerged Arc Welding 43 penetration depth t p in mm br-er3-24e.cdr constant: plate thickness: 25 mm wire electrode: 4 mm flux: MS-Typ Amp. welding current ( I) constant: constant: w U = 32 Volt v = 6 cm/min t p I = 6 Amp. v = 6 cm/min t p Volt arc voltage (U) w I = 6 Amp. U = 32 Volt t e welding speed (v) weld width w in mm cm/min w consumption kg flux / kg wire consumption kg flux/ kg wire br-er3-25e.cdr 2,4 2,2 2, 1,8 1,6 1,4 1,2 1,,8,6,4,2 1,6 1,4 1,2 1,,8,6,4,2 4 4 A) flat weld - I square butt joint fused composition fluxes agglomerated fluxes current intensity (A) B) fillet weld fused composition fluxes agglomerated fluxes current intensity (A) Influence of the Weld Parameters on Penetration Depth and Weld Width Welding Flux Consumption in Dependence on Current Intensity and Seam Shape Figure 3.24 Figure 3.25 Weld flux consumption is dependent on the selected weld type, Figure Due to geometrical shape, the flux consumption of a fillet weld is significantly lower than that of a butt weld. Because of their lower bulk weight, the specific consumption of agglomerated fluxes is lower than that of fused direction of welding fluxes Two different control concepts allow the regulation of the arc length (the prin- L 1 L 2 L 3 ciple is shown in Figure 3.26). The application of the appropriate control system is dependent on the br-er3-26e.cdr available power source Control of the Arc Length characteristics. Figure 3.26

14 3. Submerged Arc Welding 44 The external regulation of the arc length by the control of the wire feed speed requires a power source with a steeply descending characteristic, Figure In this case, the shortening of the arc caused by some process disturbance, entails a strong voltage drop at a low current rise. As a regulated quantity, this voltage drop reduces the wire feed speed. Thus, the initial arc length can be regulated at an almost constant deposition rate. In contrast, the internal regulation effects, when the arc is reduced, a strong current rise at a low voltage drop (slightly descending characteristic). At a constant wire feed speed the initial arc length is independently regulated by the increased burn-off rate which again is a consequence of the high current. U U U S U U U S br-er3-27e.cdr I A A I S I K I external regulation ( U-regulation) U Figure 3.27 U A I S internal self regulation I Control System for Constant Length of Arc A ( I-regulation) I I I I I backing flux The reaction of the internal regulation to process disturbance is very fast. This process is self regulating and does not require any machine expenditure. br-er3-28e.cdr Figure 3.28 ceramic backing bar flux copper backing Examples of Weld Pool Backups In submerged arc welding of butt joints, it is, depending on the weld preparation, necessary to support the liquid weld pool with a backing, Figure This is normally done with either a ceramic or copper backing with a flux layer or by a backing flux. Dependent on the shape of the backing bar, direct formation of the underside seam can be achieved. When welding circumferential tubes, the inclination angle of the electrode has a direct

15 3. Submerged Arc Welding 45-3 influence onto the formation of the weld bead, Figure For external as well as for internal tube welds, the best weld shapes may be obtained with an adjusted angular position of the torch. If the advance is too low, the molten bath runs ahead and produces a narrow weld = with a medium-sized ridge, too high an advance causes the flowback of the molten bath and a wide seam with a formed trough in the centre. The processes described here for external tube welds are, the other way round, b 1 b 2 b 3 also applicable to internal tube welds. t 1 t 2 t 3 br-er3-29e.cdr Figure 3.29 To increase the efficiency of submerged arc welding, different process variations are applied, Figure 3.3. In multiwire welding, where up to 6 wires are used, each welding torch is operated from a separate power source. In twin wire welding, two wire electrodes are connected in one torch and supplied from one power source. Dependent on the application, the wires can be arranged in a parallel or in a tandem. inclusion Wire Position in -Welding for Circumferential Tube Welds In submerged arc welding with iron powder addition can the deposition rate be single wire tandem substantially increased at constant electrical parameters, Figure The increased deposition rate is realised by either the addi- parallel twin wire tandem, twin wire tion of a currentless wire (cold wire) or of a preheated filler wire (hot wire). The use of a rectangular br-er3-3e.cdr Process Variations of Submerged-Arc Welding Figure 3.3

16 3. Submerged Arc Welding 46 iron powder/ chopped wire cold wire strip instead of a wire electrode allows a higher current carrying capacity and opens the method also for the wide application range of surfacing. hot wire br-er3-31e.cdr Figure 3.31 strip Process Variations of Submerged-Arc Welding However, the mentioned process variations can be combined over wide ranges, where the electrode distances and positions have to be appropriately optimised, Figure Current type, polarity, geometrical co-ordination of the individual weld heads and the selected weld parameters also have substantial influence on the weld result. 1. WH 2. WH tandem welding three-wire welding = ~ WH = WH WH 3. WH ~ ~ HW 2. WH 3. WH deposition rate 1 kg/h single wire+ metal powder single wire+ hot wire double wire three-wire four-wire 1 single wire tandem A 35 current intensity three-wire, hot wire welding four-wire welding = ~ 8 ~ ~ ~ ~ ~ weld metal 12 kg/h 9 6 4, mm 5, mm 3, mm 3 ~ A 8 current intensity voltage = 3 V speed = 4 cm/min wire protrusion = 1d length br-er3-32e.cdr br-er3-33e.cdr Position of Wire Electrode in Submerged-Arc Multi-Wire Welding Application Fields for Submerged-Arc Process Variants Figure 3.32 Figure 3.33

17 3. Submerged Arc Welding 47 The description of these individual process variations of submerged arc welding shows that this method can be applied sensibly and economically over a very wide operating range, Figure It is a high-efficiency welding process with a deposition rate of up to 1 kg/h. Due to large molten pools and flux application positional welding is not possible. When more than one wire is used in order to obtain a high deposition rate, arc interactions occur due to magnetic arc blow, Fig- ( _ ) (_ ) + + elektrode + _ + ~ ure Therefore, the selection of the current ( ) + ( + ) _ + _ type (d.c. or a.c.) and also sensible phase displace- arc ments between the indi- workpiece vidual welding torches are very important. br-er3-34e.cdr Magnetic Interaction of Arcs at Tandem Welding Figure 3.34