DIN 1910 (“Weld-ing”) classifies the welding process according to its applications: weld-ing of joints and surfacing. Accord-ing to DIN 1910 surfacing is the coating of a work-piece 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. Sur-facing carried out with wear-resistant materials in preference to the base metal ma-terial is called hardfacing; but when mainly chemically stable filler mama-terials are used, the method is called cladding. In
the case of buffering, surfacing layers are produced which allow the appro-priate-to-the-type-of-duty joining of dis-similar materials and/or of materials with differing properties, Figure 11.1.
A buffering, for instance, is an interme-diate 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.
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base metal/ surfacing metal
similar
for repair welding l
dissimilar
hardfacing (wear protection) cladding (corrosion prevention) buffering (production of an appropriate-to-the-type-of-duty joint of dissimilar materials) l
l l
Figure 11.1
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Components -Kinds of Stress
m 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
cold forming tools hot forming tools cavitation
wear parts (plastics industry) corrosion
temperature stresses
m
Figure 11.2
In case of surfacing - as for all fabrica-tion processes - certain limiting con-ditions have to be observed. For ex-ample, 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 11.3.
For all surfacing tasks a large product line of welding filler metals is avail-able. In dependence on the welding method as well as on the selected ma-terials, filler metals in the form of wires, filler wires, strips, cored strips, rods or powder are applied, Figure 11.4.
The filler/base metal dilution is rather important, as the desired high-quality properties of the surfacing layer dete-riorate with the increasing degree of dilution.
A weld parameter optimisation has the objective to optimise the degree of dilu-tion in order to guarantee a sufficient
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Materials for Surfacing wearing protection (armouring)
corrosion prevention hard facing on
cobalt base nickel base iron base
ferritic to martensitic chromium steel alloys soft martensitic chromium-nickel steel alloys austenitic-ferritic chromium-nickel steel alloys austenitic chromium-nickel steel alloys q
q q
q q q q
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Boundary Conditions in Surfacing component
(material)
coating
coating material (filler)
surfacing method
stress compatibility
manufacturing conditions availability
consumable
Figure 11.3
Figure 11.4
adherence of the layer with the minimum metal dissimilation. A planimetric determi-nation 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 calcu-lation is possible by the determination of the content of a cer-tain element in the surfacing layer as well as in the base metal, Figure 11.5.
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 suit-able for joining but also for surfacing.
In the case of the strip-electrode submerged-arc surfacing process normally strips (widths: 20 - 120mm) are used. These strips allow high clad-ding rates. Solid wire electrodes as well as flux-cored strip electrodes are used.
The flux-cored strip electrodes contain
Definition of Dilution
© ISF 2002 br-er11-05e.cdr
surface built up by welding FB
penetration area FP
base metal
(X-content - X-content ) [% in weight]
(X-content - X-content ) [% in weight]
surfacing layer FM
base metal FM
Figure 11.5
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Cr percentages by mass
0 100 200 µm
Ni percentages by mass
0 100 200 300 µm 500
distance
Figure 11.6
certain alloying elements. The strip is continuously fed into the process via feed roll-ers. 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.
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 11.8.
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inert gas-shielded arc welding
metal-arc welding submerged arc welding
arc spraying
plasma spraying
electroslag welding - stick electrode
- filler wire
- MIG / MAG - MIG cold wire - filler wire
TIG welding
- TIG cold wire
plasma welding
- plasma powder - plasma hot wire
- wire electrode - strip electrode
arc welding with self-shielded cored wire electrode
- filler wire
- powder - wire
- wire electrode
Figure 11.7
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flux support
filler metal
slag surfacing bead
drive rolls +
-flux application
base metal
power source
Figure 11.8
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.
The macro-section and possible weld defects of a strip-electrode submerged-arc sur-facing process are depicted in Figure 11.10.
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Figure 11.9
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coarse grain zone lack of fusion mirco slag inclusions sagged weld
crack formation in these areas of the coarse grain zone
undercuts gusset
base metal
Figure 11.10
Electroslag surfacing using a strip electrode is similar to strip-electrode SA sur-facing, Figure 11.11. 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.
TIG weld surfac-ing is a suitable surfacing method for small and com-plicated contours and/or low quanti-ties (e.g. repair work) with normally relatively low
deposition rates. The process princi-ple has already been shown when the TIG joint welding process was ex-plained, Figure 11.12. The arc is burning between a gas-backed non-consumable tungsten electrode and the workpiece. The arc melts the base metal and the wire or rod-shaped weld filler metal which is fed either continuously or intermittently. Thus a fusion welded joint develops between base metal and surfacing bead.
In the case of MIG/MAG surfacing processes the arc burns between a consumable wire electrode and the
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molten pool
Figure 11.11
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Process Principle of TIG Weld Surfacing
rod/ wire-shaped filler metal
arc
shielding gas nozzle
base metal
(+ / ~) surfacing bead
tungsten electrode (- / ~)
Figure 11.12
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 11.13.
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 water-cooled copper nozzle. Thus the arc obtains a higher energy density.
In the case of plasma arc powder surfacing this constricting nozzle has a positive, the tungsten electrode has a negative polarity, Figure 11.14. 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 non-transferred arc ionises the cen-trally 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
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workpiece
+
-oscillation feed direction
shielding gas arc
surfacing bead shielding gas nozzle contact tube
weld filler metal power source
wire feed device shielding gas
Figure 11.13
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power sources HIG
U
workpiece oscillation
surfacing bead
pilot arc welding arc filler metal
shielding gas conveying gas plasma gas tungsten electrode
TA
UNTA
Figure 11.14
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
shield-ing gas, protects the surfacing bead and the adjacent high-temperature zone from the sur-rounding influence.
The applied gases are mainly inert gases, as, for ex-ample, Ar and He and/or Ar-/He mix-tures.
The method is applied for surfacing small and medium-sized parts (car exhaust valves, extruder spirals). Figure 11.15 shows a cross-section of armour plating of a car exhaust valve seat. The fusion line, i.e., the region between surfacing and base metal, is shown enlarged on the right side of Figure 11.15 (blow-up). It shows hard-facing with cobalt which is high-temperature and hot gas corrosion resistant.
In plasma arc hot wire surfacing the base metal is melted by an oscil-lating plasma torch, Figure 11.16. The weld filler metal in the form of two parallel wires is added to the base metal quite
inde-br-er11-15e.cdr
section A
GW ZW
Figure 11.15
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workpiece
wires from spool plasma power
source shielding
gas
arc
plasma gas
tungsten
electrode
=
~
surfacing bead
hot wire power source weld pool
Figure 11.16
pendently. 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.
For surfacing purposes, besides the arc-welding methods, the beam welding meth-ods laser beam and electron beam welding may also be applied. Figure 11.17 shows the process principle of laser surfacing. The powder filler metal is added to the laser beam via a powder nozzle and the pow-der gas flow is, in addition, constricted by shielding gas flow.
Friction surfacing is, in principle, simi-lar to friction welding for the production of joints which due to the different materi-als are difficult to produce with fusion welding, Figure 11.18.
The filler metal is
“advanced” over the workpiece with high pressure and rota-tion. By the pressure and the relative movement frictional heat develops and
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Werkstück
laser beam
surfacing bead
shielding gas nozzle powder nozzle
powder flow
shielding gas direction of
the oscillation
Figure 11.17
© ISF 1998
feed direction
metal foil
metal foil feeding electron beam
surface layer base material
workpiece
Process Principle Electron Beam Surface Welding
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Figure 11.18
puts the weld filler end into a pasty condition. The advance motion causes an adher-ent, “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 high-quality filler metals are 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.
In thermal spraying the filler metal is melted inside the torch and then, with a high kinetic energy, discharged onto the unfused but preheated workpiece surface.
There is no fusion of base and filler metal but rather adhesive binding and mechani-cal 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. Plas-tics may be sprayed as well. The utilisation of filler metals in thermal spraying is rela-tively low.
The most important methods of thermal spraying are: plasma arc spraying, flame spraying and arc spraying.
In powder flame spraying an oxya-cetylene flame pro-vides the heating source where the centrally fed filler metal is melted, Figure 11.19. The kinetic energy for
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rotation
bulge force
surfacing layer
base metal
filler metal
advance
Figure 11.19
the acceleration and atomisation of the filler metal is produced by compressed gas (air).
In contrast to pow-der flame spraying, is for flame spray-ing a wire filler metal fed mechani-cally into the centre cone, melted, at-omised and accel-erated in direction of the substrate, Figure 11.20.
In plasma arc spraying an internal, high-energy arc is ignited between the tungsten cathode and the anode, Figure 11.21. This arc ionises the plasma gas (argon, 50 - 100 l/min). The plasma emerges from the torch with a high kinetic and thermal en-ergy 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 mate-rial properties, es-pecially a better toughness charac-teristic. The reason
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spraying material compressed
air
workpiece
flame cone fuel gas-oxygen
mixture
spray deposit
Figure 11.20
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compressed air
gas mixture
adjustable wire feed device
spraying wire spray deposit
spraying jet non-binding sprayed particles (loss in spraying)
fusing wire tip
Figure 11.21
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 tech-nique. Also in contrast with the shapes produced by forging, the workpieces pro-duced by shape welding show quality advantages, especially in the isotropy and the regularity of their toughness and strength properties as far as larger workpiece thick-nesses are
con-cerned. 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 cer-tain applications, a sensible techno-logical and eco-nomical alternative to the methods of primary forming, forming or joining, Figure 11.22.
Figure 11.23 shows an early application which is related to the field of arts.
Plasma Powder Spraying Unit
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powder injector
jet of particles copper anode
plasma gas cooling
water cooling water tungsten cathode arc anode carrier middle
frame gas
distributor isolation
ring back frame
Figure 11.22
Shape Welding - Integration
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shape welding
forming (forging)
joining (welding) primary forming
(casting)
Figure 11.23
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traction mechanism
joist turntable
phase 1 phase 3 phase 5
phase 2 phase 4 phase 6 phase 7
Figure 11.26
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 ac-cordingly higher,
Figure 11.24.
Figure 11.25 shows the principal proce-dure for the produc-tion of typical shape-welded parts. Cylindrical containers are pro-duced with the
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Shape Welded Goblet (1936)
Figure 11.24
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Shape Welding Procedures
+ 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
applications:
welding-on of connection pieces Baumkuchenmethode
Töpfermethode
Klammeraffe
Figure 11.25
thode” method: the filler metal is wel-ded by submerged-arc with helical movement in multiple passes into a tube which has the function of a trac-tion 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 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 11.26 and 11.27. The Klammeraffe method serves 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.
In the case of elec-tron beam surfac-ing the filler metal is added to the process in the form of a film, Figure 11.28.
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Production of a Pipe Bend by Shape Welding
te sting
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
Figure 11.27
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shafts boiler shell rings braces spherical caps pipe bends
forged products
shape-welded products
complexity of the parts
€/kg
Figure 11.28
2003