Experimental Investigation on Advanced Gas Metal Aro Welding Techniques on Low Alloy Steels using Metal Cored Filler Wire

Abstract

Regulated metal deposition (RMD) welding technology is a relatively novel modified short circuit technology developed by Miller. Precise control over the weld parameter and ultimately weld puddle enables this technique to be exploited for the root pass of the critical weld joints especially involving pressure application. The idea behind the development of the method is to harness the benefits of the short circuit mode, i.e., lesser heat input due to lower welding parameters and enhanced weld deposited due to accurate control. The RMD technique employs systematic steps wherein the current waveform and cycle are modified in the power source to achieve the same. The gas metal arc welding (GMAW) process is widely used in the fabrication of over-dimensional equipment such as ethylene oxide reactor, fractional distillation column, and other equipment for petroleum refining. This is because the GMAW employs a continuous consumable wire feeding, and hence the deposition efficiency is good. The process uses solid wire during welding, which poses a challenge of limited current density and hence limited deposition. An alternative was implemented known as flux-cored arc welding (FCAW), which employs cored wire with flux inside the core. The current efficiency w increased drastically, and hence the deposition. However, the wire consumption was raised as flux does not participate in the weld deposit. A new variant of the consumable wire involving cored wire filled with consumable instead of flux is called metal-cored wire. The process is known as metal-cored arc welding (MCAW). These wires impart increased current density and enhanced weld deposition for the joints. Currently, limited literature has been reported on the use of metal-cored wires in the GMAW process for welding due to certain limitations. The primary limitation in using the technique is the weld properties. The equipment for pressure vessels demands a robust mechanical and metallurgical property and a variety of postprocessing, i.e., heat treatment. Thus in the current study, a well-defined investigation has been carried out to establish the process parameters for RMD in root pass and GMAW in subsequent passes. In addition to this, the PWHT and step cooling heat treatment has been attempted and mechanical and metallurgical properties. Temper embrittlement susceptibility and ductile to brittle transition temperature testing have also been carried out to investigate the suitability of the process for 2.25 Cr – 1.0 Mo steels. For the initial studies, a different combination of process parameters such as current, wire feed speed, and voltage was used to deposit the bead-on-plate trials for RMD and GMAW process using metal-cored wires. 11 different BOP were attempted, and subsequently, macrostructures were developed. The macrographs were analyzed for the weld bead characteristics such as bead width and height in addition to the depth of penetration. It was observed that the weld bead characteristics varied on increasing the process parameters and depending on the passes being deposited, process parameters were identified. The selected parameters were also used for depositing the root pass on a 10mm thick weld joint of 2.25 Cr - 1.0 Mo base material, and the fill up passes for joint were endeavored with the GMAW technique by means of metal-cored wires and further analyzed. The entire weldments were further subjected to post-weld heat treatment, particularly stress-relieving. Mechanical characterization is further carried out wherein tensile properties (transverse and all weld), toughness and bend test of the weld joint were evaluated and found to be acceptable. The ductile-to-brittle transition temperature (DBTT) testing was performed by testing a sequence of impact specimens tosub zero temperatures. The tensile value of the welded joint after PWHT was achieved was 560 MPa, which was well above required as per construction codes. The impact value of 265 J after PWHT at -30° C test temperature was achieved, which is one of the most noticeable outcomes of the study. Additionally, DBTT values for the weld zone after PWHT also came around -70°C, which is well below the room temperature and indicates the ability of the weld to sustain extreme environments. Another important consideration for the weld joint was investigating the temper embrittlement susceptibility of the welded joint. The same has been attempted in the current study subjecting a 25 mm thick 2.25 Cr – 1.0 Mo weld joint to step cooling heat treatment was prepared using a combination of the RMD and GMAW processes incorporating metal-cored wires to step cooling heat treatment prescribed API standards. The temper embrittlement susceptibility of the weld joint was also ascertained by Bruscato X-factor and by formulating DBTT curves by carrying out the impact toughness testing at various temperatures. Detailed microscopy and hardness studies were also carried out. It was established from the study that the X-factor value for the welded joint was 15.4. The DBTT for the weld joint was found to occur at -37°C, which was well below 10°C. Optical microscopy and scanning electron microscopy indicated the presence of carbides. The Energy-dispersive X-ray spectrometry studies indicated the presence of chromium and manganese-rich carbides and Sulphur near the grain boundaries. The study establishes a base for using metal-cored wires, particularly in high temperature and pressure application of Cr-Mo steels for higher thickness joints Keywords: ductile-to-brittle transition temperatures (DBTT); gas metal arc welding process (GMAW); metal-cored; regulated metal deposition (RMD); step cooling heat treatment (SCHT); temper embrittlement; welding, Metal cored wire (MCAW)