Development of a Welding Process to Improve Welded Microalloyed Steel Characteristics

  • Author / Creator
    Mohammadijoo, Mohsen
  • During the past several decades, the relationship between microstructure and mechanical properties of high strength microalloyed steels has been extensively studied in an effort to improve the fracture toughness and strength of these types of steels, without appreciably increasing steel manufacturing and pipeline fabrication (welding) costs. Fueled by the demand for developing pipeline steels with high strength, toughness and low cost by the construction, energy and transportation industries, it has always been essential to improve the properties of microalloyed steels and their weld and heat affected zone (HAZ). High strength, low-carbon microalloyed steels possess an excellent combination of strength and toughness obtained by appropriate alloying design and thermo-mechanical controlled processing (TMCP). However, this strength and toughness combination can be deteriorated by the high heat input and the thermal cycles that the steel experiences during welding. Since welding is an unavoidable stage in pipe manufacturing, it is essential to produce welded sections with as low a heat input as possible, while retaining adequate joint geometry and properties. From a metallurgical point of view, the property deterioration associated with welded microalloyed steels has always been a main challenge for manufacturing, specifically for pipeline manufacturers. The HAZ, particularly the coarse grain heat affected zone (CGHAZ), i.e., the region adjacent to the fusion line, typically has lower fracture toughness compared with the rest of the steel. The deterioration in toughness of the CGHAZ is attributed to the formation of martensite-austenite (M-A) constituents, local brittle zones (LBZ) and large prior austenite grains (PAG). Submerged arc welding (SAW) with more than one electrode, i.e., tandem submerged arc welding (TSAW), has been preferred over other welding process in the pipeline industry due to its inherent properties, such as deep penetration, high deposition rate and capability of welding thick sections. Nevertheless, the heat input in the TSAW process may be increased due to the increase in the overall welding current and voltage needed for a higher deposition rate, resulting in some adverse effects on the microstructure and toughness of the weld joint. In the present research, a welding process, tandem submerged arc welding with an additional cold wire (CWTSAW), has been developed for pipeline manufacturing to improve the weld and HAZ geometry, mechanical properties, particularly fracture toughness, and microstructure in the HAZ. Since an appropriate understanding of the welding conditions to guarantee requisite weld geometry, appearance and mechanical properties is always essential in the development of a welding process, the CWTSAW process was initially optimized and the effect of cold wire addition parameters on dilution, geometry and properties of the weld metal (WM) and the HAZ was investigated. As such, heat input, voltage and travel speed of both electrodes, along with three main cold wire parameters, were correlated with the geometry of the weld and HAZ, i.e., aspect ratio (AR), semi-penetration ratio (SPR), reinforcement area (RA) and CGHAZ area, dilution and the microhardness of the WM and CGHAZ. The results showed that the addition of a cold wire at a lagging position close to the trail electrode at 63 resulted in an overall improvement in the weld geometry, dilution and microhardness. Macrostructural analysis showed a decrease in the CGHAZ size by addition of a cold wire. Microstructural evaluation, using both tint etching optical microscopy (TEOM) and scanning electron microscopy (SEM), indicated the formation of finer PAGs and less fraction of M-A constituents with refined morphology within the CGHAZ when the cold wire was fed in TSAW. The low-temperature fracture toughness of the HAZ was improved by 38% when a cold wire was fed at 25.4 cm/min with a heat input of 22.2 kJ/cm. The improvements are attributed to lower actual heat introduced to the weldment and consequently faster cooling rate in the CGHAZ by cold wire addition. Incorporating a cold wire in TSAW essentially moderates the heat input by consuming the heat of molten pool and/or the energy of the trail electrode as the wire melts into the weld puddle. Analysis of the effective heat input of the CWTSAW process has indicated that the trail electrode heat input was reduced by 13% when cold wire was fed at 25.4 cm/min.

  • Subjects / Keywords
  • Graduation date
    Spring 2017
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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