African Fusion August 2018

Hybrid electro slag cladding

The process is restricted by the amount of current that can be applied without increasing dilution. Care must be taken for the overlap area as any residual slag will not be re-melted and may result in lack of fusion. For this reason, it is often not recommended for lane closures of rolled clad plate in the area where the roll cladding meets the base material. Since the dilution percentage is higher in SAW strip clad- ding, it is always necessary to apply a buffer or barrier layer in this process using strips of richer chemistry before applying second and subsequent layers using a strip that matches final required chemistry. Due to higher levels of dilution, this pro- cess always involves a minimum two or more layers to reach desired undiluted surface cladding material chemistry and it is, therefore, a costlier option. This process utilises a conductive flux and is an arcless process (Figure 5). The process initiates with an arc between continu- ously fed filler strip and the basemetal, which results in initial melting of high fluoride (CaF 2 ) containing flux. The molten slag, which is electro-conductive in nature due to the presence of special ingredients in the flux, supports current carrying at lower voltages, resulting in the arc being extinguished. The heat, which is required tomelt the strip and continue the process, is generated via the Joules heating effect (H = I 2 R) where I is the welding current and R is the electrical resistance generated in the electro-conductivemolten slag as a result of the high current flowing through it. Since the concentrated arc force is absent in this process, there is no deep penetration (digging) effect on the base ma- terial. As a result, the process is inherently characterised by low dilution level (7 to 12%). The ESW process, therefore, has significant advantages over its SAW counterpart. Using this process, cladding in single layer is a very com- mon practice due to the relatively low dilution levels. Single layer cladding is achieved either by using over-alloyed strip or by using a combination of over-alloyed strip coupled with alloyed flux. The former of these two options is more popular, however, since use of alloyed flux is not permitted by many process licensors. Table 3 provides a comparison between cladding using ESW and SAW processes. While conventional ESW strip clad- ding enjoys significant benefits over SAW strip cladding, this technique still has limitations in meeting the expectation of fabricators when there is a need to: (I) Deposit Ni-625 cladding to achieve <5% Fe in single layer using neutral flux. Electro slag strip cladding (ESW) Conventional ESW technique

Figure 5: The conventional electro-slag (ESW) strip cladding process.

Figure 6: The H-ESC electro-slag strip cladding technique.

(II) Achieve guaranteed >40% Ni in a single layer Ni-825 de- posit using neutral flux. These limitations encouraged Lincoln Electric to develop and introduce an innovative solution (ESW using the H-ESC tech- nique) into the market as described below. Hybrid electro-slag strip cladding (H-ESC) H-ESC is a new variant of ESW process developed and first in- troduced in 2016. In addition to the consumable strip and flux being used in the conventional ESW strip cladding processes, H-ESC adds multiple hot metal cored welding wires (MCWs) to the molten pool as a third constituent in this technique (Figure 6). Only neutral flux need be used in this technique. The role of the additional multiple hot metal-cored wires to the molten pool include: • Metal-cored wire offers a higher current density as com- pared to solidwire, therefore requiring lessmelting energy. TheMCWabsorbs the balance energy required for melting from the molten pool, thereby controlling heat input and reducing the dilution of the base material. Over-alloyed MCW wire can also compensate for the loss of alloying elements. This technique can achieve desired chemistry

Parameter Flux feeding Heat source

SAW strip cladding Both Front & Rear side

ESW strip cladding

Only Front side

Joule/resistance heating: H = I 2 R

Arc Energy

Flux consumption

High: typically 1:1.3

Low: typically 1:0.6

Dilution (%)

18-20 10-14 12-14

7-12

Welding speed (cm/min)

15-18 / 24-35

Deposition rate (kg/hr) 60×0.5 mm strip

22-30

content in weld metal

Cleaner chemistry with lower O 2

content

Weld metal quality

Higher O 2

Number of layers required to achieve undiluted surface chemistry

Minimum two layers

Single or two layers

Table 3: A comparison between SAW and ESW cladding processes.

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August 2018

AFRICAN FUSION