Pushing the Productivity Envelope with Strip Cladding
By Marc Purslow on Thursday, July 19th, 2012
Strip cladding is a high-productivity option for surfacing applications which uses a wide, flat strip in place of a wire electrode. Much like submerged arc welding (SAW), the process takes place beneath a granular flux which melts and forms a slag to protect the molten puddle. There are two variations of strip cladding; submerged arc strip cladding (SASC) and electroslag strip cladding (ESC). The primary difference between the two processes lies in the conductivity of the slag. ESC uses a flux which melts to create a conductive slag, resulting in a non-arc welding process whereby welding current is transferred through the slag to the work-piece. Since no arc is present, melting of the substrate is minimized, resulting in a low dilution level. Typical dilution rates for SASC are between 15 and 20%, while ESC dilution rates are as low as 8%. This is a significant advantage for cladding applications since dilution level often determines the number of layers required. Manufacturers often deposit 2 to 3 layers to achieve the required chemistry for corrosion and/or wear resistance. The use of strip cladding processes can often allow cladding to be completed with a single layer, which can have a significant positive impact on productivity.
While a specialized weld head is required (Figure 2), strip cladding uses much of the same equipment required for traditional SAW, making its implementation a low-cost solution for a significant increase in productivity. To achieve high deposition rates, an increase in current capacity may be required.
The maximum achievable deposition rates for SASC and ESC are dependent on strip width and are approximately 75 and 115 pounds per hour, respectively. Compared to SASC, ESC requires higher current levels due to the proportion of the current which is used to resistively heat and melt the strip. This results in a significant increase in deposition rate. For example, when operating SASC with 60mm strip, 800-1000 amps of current are sufficient, while ESC requires 1000 to 1500 amps for proper operation. ESC with 90mm strip requires up to 2000 amps.
EWI has recently conducted weld trials to determine whether the productivity of the SASC process could be increased by using DCEN (direct current electrode negative) current and AC (alternating current). SASC in DCEN mode resulted in a 24% higher deposition rate over DCEP mode at the same heat input. When producing SASC welds in CV/AC mode, an increased frequency resulted in an increased bead width.