African Fusion August 2018

Figure 2: Dilution in weld cladding, where A is the area above the plate and B is the area penetrated in the plate thickness.

 

  × 100

B A B +

(%) =

D

Welding process

Typical dilution (%)

SMAW FCAW

25-30 20-25

Figure 3: Evolution of strip cladding.

corrosive service fluids. While C-Mn, low-alloy steels and other material substrates provide strength and other mechanical properties, cladding provides the desired corrosion and wear resistance. The result is fabrication flexibility and cost savings. There are many ways to apply this corrosion-resistant layer – either by using roll-bonded or explosive-bonded clad plates or by applying more flexible weld cladding on a C-Mn or low alloy steel substrate. In the field of weld cladding, fabricators always look for solutions that will help themto achieve desired clean chemical and other properties of the clad material in least number of layers at faster speeds and higher deposition rates. To realise the above goals, welding consumable manu- facturers have come up with continuous upgrades in welding techniques. Over time, the more productive electro slag weld cladding (ESW) process was introduced as an alternative to standard submerged-arc (SAW) strip cladding. With the intro- duction of the ESW process, it was possible to offer a single layer cladding solution instead of the traditional two-layer cladding technique. As a further improvement to the process, newly designed high-speed fluxes were developed to substan- tially increasewelding speed in ESWcladding, froma standard speed of 16 to 19 cm/min to 25 to 40 cm/min. The latest innovation in the welding technique, H-ESC, is nowcapable of single layer high-speedandhighly-productivity cladding for Ni-625, which meets the maximum 5% Fe con- tent and the process is also suitable for other Ni-based and stainless steel alloys. This is achieved by introducing in a 3rd dimension to the ESW process in the form of added hot metal cored wires (MCWs). Weld cladding Typical CPE is large in size and vessels that require internal cladding on all the internal surfaces can involve the deposi- tion of five and 30 Mt of weldmetal. As a result, fabricators are always looking for weld cladding processes that can provide highest possible deposition rateswithminimumpossible dilu- tion, so that the desired undiluted cladding chemistry could be achieved by the thinnest possible thickness. While most of the existing arc and electro slag welding processes canbe utilised for weld-cladding, strip claddingwith submerged arc and electro slag welding process are the most attractive choices due to substantially higher deposition rates and, more importantly, higher surface area coverage rates. Table 1 shows a comparison of deposition rates for the most 25-35 SAW strip: 60×0.5 mm 18-20 ESW strip: 60×0.5 mm 7-12 Table 2: A comparison of the dilution percentages for different welding processes. SAW wire

commonly used weld cladding processes. From Table 1 it is clear that ESW is the most productive welding process for cladding applications and therefore the most preferred option for fabricators across the world. Since corrosion-resistant cladding material such as Ni- based alloys or stainless steel are appliedon ferritic C-Mn steel, low alloy steel or other types of base material, there is always the issue of dilution with the base material that needs to be considered as an important factor when selecting the correct weld cladding process. Typically, dilution (D%) is calculated as shown in the equa- tionand its associated schematicdiagrampresented inFigure2 from which it is clear that the lower the dilution percentage, the easier it is to achieve the desired weld cladding chemistry. In simple terms, it is only possible to achieve desired cladding chemistry in fewer layers if the dilution percentage is low. Table 2 provides a comparison of the typical dilution per- centages for different welding processes. Interestingly, traditional ESW is the most attractive clad- dingprocess fromadilutionperspectiveaswell, hence itswide- spread application claddingwork for oil and gas applications. Strip cladding processes The strip cladding process has been evolving ever since its introduction (Figure 3). The target has always been to achieve chemical and mechanical requirement of the surface alloy in a single layer and at high speed. There are two conventional strip cladding processes – submerged arc (SAW) and electro slag (ESW) strip cladding. Submerged arc (SAW) strip cladding This process (Figure 4) utilises an arc that runs back and forth at high speed along the strip, depositing weld metal onto the base material. Because this is an arc welding process, there will be penetration into the basematerial resulting in dilution levels of around 20%. Deposition rates are in the region of 12 to 14 kg/hr for 60×0.5 mm consumable strip.

Figure 4: The conventional SAW strip cladding process.

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August 2018

AFRICAN FUSION