11. Surfacing. and Shape Welding

Transcription

1 11. Surfacing and Shape Welding

2 11. Surfacing and Shape Welding 151 br-er11-01e.cdr DIN 1910 ( Welding ) classifies the welding process according to its applications: welding of joints and surfacing. According to DIN 1910 surfacing is the coating of a workpiece by means of welding. Dependent on the applied filler material a further classification may be made: deposition repair welding and surfacing for the production of a composite material with certain functions. Surfacing carried out with wearresistant materials in preference to the base metal material is called hardfacing; but when mainly chemically stable filler materials are used, the method is called cladding. In the case of buffering, surfacing layers are produced which allow the appropriate-to-thetype-of-duty joining of dissimilar materials and/or of materials with differing properties, Figure similar Figure 11.1 for repair welding base metal/ surfacing metal dissimilar hardfacing (wear protection) cladding (corrosion prevention) buffering (production of an appropriate-to-the-type-of-duty joint of dissimilar materials) wear caused by very high impact and compressive stress wear by friction (metal against metal) during high impact and compression stress strong sanding or grinding wear very strong wear caused by grinding during low impact stress A buffering, for instance, is an intermediate layer made from a relatively tough material between two layers with strongly differing thermal expansion coefficients. Figure 11.2 shows different kinds of stresses which demand the surfacing of components. Furthermore surfacing may be used for primary forming as well as for joining by primary forming. br-er11-02e.cdr cold forming tools hot forming tools cavitation wear parts (plastics industry) corrosion Figure 11.2 temperature stresses Components - Kinds of Stress

3 11. Surfacing and Shape Welding 152 In case of surfacing - as for all fabrication processes - certain limiting conditions have to be observed. For example, hard and wear-resistant weld filler metals cannot be drawn into solid wires. Here, another form has to be selected (filler wire, continuously cast rods, powder). Process materials, as for example SA welding flux demand a certain welding position which in terms limits the method of welding. The coating material must be selected with view to the type of duty and, moreover, must be compatible with the base metal, Figure For all surfacing tasks a large product line of welding filler metals is available. In dependence on the welding method as well as on the selected materials, filler metals in the form of wires, filler wires, strips, cored strips, rods or powder are applied, Figure The filler/base metal dilution is rather important, as the desired high-quality properties of the surfacing layer deteriorate with the increasing degree of dilution. component (material) wearing protection (armouring) hard facing on cobalt base nickel base stress compatibility coating manufacturing conditions availability iron base corrosion prevention ferritic to martensitic chromium steel alloys soft martensitic chromium-nickel steel alloys coating material (filler) consumable surfacing method austenitic-ferritic chromium-nickel steel alloys austenitic chromium-nickel steel alloys br-er11-03e.cdr br-er11-04e.cdr Boundary Conditions in Surfacing Materials for Surfacing Figure 11.3 Figure 11.4

4 11. Surfacing and Shape Welding 153 A weld parameter optimisation has the objective to optimise the degree of dilution in order to guarantee a sufficient adherence of the layer with the minimum metal dissimilation. A br-er11-05e.cdr Figure 11.5 F P A D= x 100% F P + FB Definition of Dilution surface built up by welding F B penetration area F P base metal (X-content surfacing layer - X-content FM) [% in weight] A D = x 100% (X-content - X-content ) [% in weight] FM: weld filler metal base metal A D: dilution FM ISF 2002 planimetric determination of the surfacing and penetration areas will roughly assess the proportion of filler to base metal. When the analysis of base and filler metal is known, a more precise calculation is possible by the determination of the content of a certain element in the surfacing layer as well as in the base metal, Figure Figure 11.6 shows record charts of an electron beam microprobe analysis for the elements nickel and chromium. It is evident that - after passing a narrow transition zone between base metal and layer the analysis inside the layer is quasi constant. As depicted in Figure 11.7 almost all arc welding methods are not only suitable for joining but also for surfacing. Cr percentages by mass Ni percentages by mass 30 % µm 500 distance 30 % br-er11-06e.cdr µm 500 distance Microprobe Analyses Figure 11.6

5 11. Surfacing and Shape Welding 154 In the case of the strip-electrode submerged-arc surfacing process normally strips (widths: mm) are used. These strips allow high cladding rates. Solid wire electrodes as well as flux-cored strip electrodes are used. The flux-cored strip electrodes contain certain metal-arc welding - stick electrode - filler wire arc welding with self-shielded cored wire electrode - filler wire arc spraying - powder - wire br-er11-07e.cdr Figure 11.7 inert gas-shielded arc welding - MIG / MAG - MIG cold wire - filler wire TIG welding - TIG cold wire plasma welding - plasma powder - plasma hot wire plasma spraying submerged arc welding - wire electrode - strip electrode electroslag welding - wire electrode alloying elements. The strip is continuously fed into the process via feed rollers. Current contact is normally carried out via copper contact jaws which in some cases are protected against wear by hard metal inserts. The slag-forming flux is supplied onto the workpiece in front of the strip electrode by means of a flux support. The non-molten flux can be extracted and returned to the flux circuit. br-er11-08e.cdr power source drive rolls + - filler metal flux support flux application slag surfacing bead base metal Should the slag developed on top of the welding bead not detach itself, it will have to be removed mechanically in order to avoid slag inclusions during overwelding. The arc wanders along the lower edge of the strip. Thus the strip is melted consistently, Figure Figure 11.8

6 11. Surfacing and Shape Welding 155 Figure 11.9 shows the cladding of a roll barrel. The coating is deposited helically while the workpiece is rotating. The weld head is moved axially over the workpiece. br-er11-09e.cdr Figure 11.9 The macro-section and possible weld defects of a strip-electrode submergedarc surfacing process are depicted in Figure Electroslag surfacing using a strip electrode is similar to strip-electrode SA surfacing, Figure The difference is that the weld filler metal is not melted in the arc but in liquefied welding flux the liquid slag as a result of Joule resistance heating. The slag is held by a slight inclination of the plate and the flux mound to prevent it from running off. coarse grain zone lack of fusion mirco slag inclusions sagged weld crack formation in these areas of the coarse grain zone br-er11-10e.cdr Figure base metal gusset undercuts

7 11. Surfacing and Shape Welding 156 TIG weld surfacing is a suitable surfacing method for small and complicated contours and/or low quantities (e.g. repair work) with normally relatively low deposition rates. The process principle has already been shown when the TIG joint welding process was explained, Figure The arc is burning between a gas-backed nonconsumable tungsten electrode and the workpiece. The arc melts the base metal and the wire or rodshaped weld filler metal which is fed either continuously or intermittently. Thus a fusion welded joint develops between base metal and surfacing bead. molten pool br-er11-11e.cdr Figure rod/ wire-shaped filler metal arc shielding gas nozzle In the case of MIG/MAG surfacing processes the arc burns between a consumable wire electrode and the workpiece. This method allows higher deposition rates. Filler as well as solid wires are used. The wire electrode has a positive, while the workpiece to be surfaced has a negative polarity, Figure tungsten electrode base metal (- / ~) (+ / ~) surfacing bead br-er11-12e.cdr ISF 2002 Process Principle of TIG Weld Surfacing Figure 11.12

8 11. Surfacing and Shape Welding 157 contact tube shielding gas shielding gas nozzle weld filler metal arc surfacing bead workpiece feed direction br-er11-13e.cdr + - wire feed device power source shielding gas oscillation A further development of the TIG welding process is plasma welding. While the TIG arc develops freely, the plasma welding arc is mechanically and thermally constricted by a watercooled copper nozzle. Thus the arc obtains a higher energy density. Figure In the case of plasma arc powder surfacing this constricting nozzle has a positive, the tungsten electrode has a negative polarity, Figure Through a pilot arc power supply a non-transferred arc (pilot arc) develops inside the torch. A second, separate power source feeds the transferred arc between electrode and workpiece. The nontransferred arc ionises the centrally fed plasma gas (inert gases, as, e.g., Ar or He) thus causing a plasma jet of high energy to emerge from the nozzle. This plasma jet serves to produce and to stabilise the arc striking ability of the transferred arc gap. The surfacing filler metal powder added by a feeding gas flow is melted in the plasma jet. The partly liquefied weld filler metal meets the by transferred arc molten base metal and forms the surfacing bead. A third gas flow, the shielding gas, protects the surfacing bead and the adjacent hightemperature zone from the surrounding influence. The applied gases are mainly inert gases, as, for example, Ar and He and/or Ar- /He mixtures. tungsten electrode plasma gas conveying gas shielding gas surfacing bead br-er11-14e.cdr Figure filler metal workpiece HIG pilot arc welding arc oscillation UNTA power sources U TA

9 11. Surfacing and Shape Welding 158 The method is applied for surfacing small and medium-sized parts (car exhaust valves, extruder spirals). Figure shows a cross-section of armour plating of a car exhaust valve br-er11-15e.cdr Figure seat. The fusion line, i.e., the region between surfacing and base metal, is shown enlarged on the right side of Figure (blow-up). It shows hardfacing with cobalt which is high-temperature and hot gas corrosion resistant. In plasma arc hot wire surfacing the base metal is melted by an oscillating plasma torch, Figure shielding gas arc br-er11-16e.cdr workpiece plasma gas plasma power source tungsten = electrode surfacing bead section A weld pool ZW GW wires from spool ~ hot wire power source The weld filler metal in the form of two parallel wires is added to the base metal quite independently. The arc between the tips of the two parallel wires is generated through the application of a separate power source. The plasma arc with a length of approx. 20 mm is oscillating (oscillation width between 20 to 50 mm). The two wires are fed in a V-formation at an angle of approx. 30 and melt in the high-temperature region in the trailing zone of the plasma torch. Figure 11.16

10 11. Surfacing and Shape Welding 159 For surfacing purposes, besides the arc-welding methods, the beam welding methods laser beam and electron beam welding may also be applied. Figure shows the process principle of laser surfacing. The powder filler metal is added to the laser beam via a powder nozzle and the powder gas flow is, in addition, constricted by shielding gas flow. Figure Friction surfacing is, in principle, similar to friction welding for the production of joints which due to the different materials are difficult to produce with fusion welding, Figure The filler metal is advanced over the workpiece with high pressure and rotation. By the pressure and the relative movement frictional heat develops and puts the weld filler end into a pasty condition. The advance motion causes an adherent, spreaded layer on the base metal. This method is not applied frequently and is mainly used for materials which show strong differences in their melting and oxidation behaviours. A comparison of the different surfacing methods shows that the application fields are limited - dependent on the welding method. A specific method, for example, is the low filler/base metal dilution. These methods are applied where highquality filler metals are rotation advance base metal br-er11-18e.cdr Figure force filler metal bulge surfacing layer

11 11. Surfacing and Shape Welding 160 welded. Another criterion for the selection of a surfacing method is the deposition rate. In the case of cladding large surfaces a method with a high deposition rate is chosen, this with regard to profitability. compressed air spraying material workpiece In thermal spraying the filler metal is melted inside the torch and then, with a high kinetic energy, dis- fuel gas-oxygen mixture flame cone spray deposit charged onto the unfused br-er11-19e.cdr but preheated workpiece surface. Figure There is no fusion of base and filler metal but rather adhesive binding and mechanical interlocking of the spray deposit with the base material. These mechanisms are effective only when the workpiece surface is coarse (pre-treatment by sandblasting) and free of oxides. The filler and base materials are metallic and non-metallic. Plastics may be sprayed as well. The utilisation of filler metals in thermal spraying is relatively low. The most important methods of thermal spraying are: plasma arc spraying, flame spraying and arc spraying. In powder flame spraying an oxyacetylene flame provides the heating source where the centrally fed filler metal is melted, Figure The kinetic energy for the acceleration and atomisation of the filler metal is produced by compressed gas (air). compressed air gas mixture adjustable wire feed device br-er11-20e.cdr Figure spraying wire spraying jet fusing wire tip non-binding sprayed particles (loss in spraying) spray deposit

12 11. Surfacing and Shape Welding 161 In contrast to powder flame back frame isolation ring gas distributor middle frame anode carrier powder injector spraying, is for flame spraying a wire filler metal fed mechanically into the copper anode centre cone, melted, atomised and accelerated in direction of the substrate, jet of particles Figure cooling plasma water gas cooling water tungsten cathode arc br-er11-21e.cdr Figure Plasma Powder Spraying Unit In plasma arc spraying an internal, high-energy arc is ignited between the tungsten cathode and the anode, Figure This arc ionises the plasma gas (argon, l/min). The plasma emerges from the torch with a high kinetic and thermal energy and carries the side-fed powder along with it which then meets the workpiece surface in a semi-fluid state with the necessary kinetic energy. In the case of shape welding, steel shapes with larger dimensions and higher weights are produced from molten weld metal only. In comparison to cast parts this method brings about essentially more favourable mechano-technological material properties, especially a better br-er11-22e.cdr Figure forming (forging) primary forming (casting) shape welding Shape Welding - Integration joining (welding) ISF 2002 ISF 2002 toughness characteristic. The reason for this lies mainly in the high purity and the homogeneity of the steel which is helped by the repeated melting process and the resulting slag reactions. These properties are also put down to the favourable fine-grained structure formation which is achieved by the repeated subsequent thermal treatment with the multi-pass technique. Also in contrast with the shapes produced by forging, the workpieces produced by shape welding show quality advantages,

13 11. Surfacing and Shape Welding 162 especially in the isotropy and the regularity of their toughness and strength properties as far as larger workpiece thicknesses are concerned. In Europe, due to the lack of expensive forging equipment, very high individual weights may not be produced as forged parts. Therefore, shape welding is, for certain applications, a sensible technological and economical alternative to the methods of primary forming, forming or joining, Figure Figure shows an early application which is related to the field of arts. br-er11-23e.cdr Figure Shape Welded Goblet (1936) The higher tooling costs in forging make the shape welding method less expensive; this applies to parts with certain increasing complexity. This comparison is, however, related to relatively low numbers of pieces, where the tooling costs per part are accordingly higher, Figure /kg forged products shape-welded products shafts boiler shell rings braces spherical caps pipe bends complexity of the parts br-er11-24e.cdr Figure 11.24

14 11. Surfacing and Shape Welding 163 Figure shows the principal procedure for the production of typical shape-welded parts. Cylindrical containers are produced with the Baumkuchenmethode method: the filler metal is welded by submerged-arc with helical movement in multiple passes into a tube which has the function of a traction mechanism (for the most part mechanically removed later). This brings about the possibility to produce seamless containers with bottom and flange in one working cycle. Elbows are mainly manufactured with the Baumkuchenmethode Töpfermethode Klammeraffe + several weld heads possible + no interruption during weld head failure - core made of foreign material necessary applications: shafts, large boiler shell rings, flanges + free rotationally-symmetrical shapes + several weld heads possible + weld head manipulation not necessary + each head capable to weld a specific layer + small diameters possible - component movement must correspond with the contour - number of weld heads limited when smaller diameters are welded applications: spherical caps, pipe bends, braces + transportable unit - limited welding efficiency Töpfer method. On the traction mechanism a rotationally symmetrical part with a semicircle cross-section is produced which is later separated and welded to an elbow, Figures and The Klammeraffe method serves phase 7 phase 6 phase 4 phase 2 traction mechanism phase 5 phase 3 phase 1 br-er11-25e.cdr Figure applications: welding-on of connection pieces Shape Welding Procedures ISF 2002 the purpose to weld external connection pieces onto pipes. A portable unit which is connected with the pipe welds the connection pipe in a similar manner to the Töpfer method. joist turntable br-er11-26e.cdr Figure 11.26

15 11. Surfacing and Shape Welding 164 testing 1. welding of the half-torus 2. stress relief annealing 3. mechanical treatment 4. seperating/ halving 5. folding 6. welding togehter 7. stress relief annealing 8. testing In the case of electron beam surfacing the filler metal is added to the process in the form of a film, Figure br-er11-27e.cdr Production of a Pipe Bend by Shape Welding Figure electron beam surface layer base material metal foil feed direction workpiece metal foil feeding ka11-18.cdr ISF 1998 Process Principle Electron Beam Surface Welding Figure 11.28