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Effect of Cold Wire TSAW on Weld Geometry and CGHAZ Microstructure of Heavy Gauge X70
- Author / Creator
- Ren, Tailin
Heavy gauge (>11 mm wall thickness) X70 pipe is one of the products produced using submerged arc welding (SAW) and is the subject of this thesis. The SAW process offers substantial advantages, including a high deposition rate, deep penetration, and reduced welding times. These attributes are beneficial for welding thick-walled pipes. To increase the process productivity, tandem submerged arc welding (TSAW), containing two or more electrodes, has been developed for welding of line-pipes to achieve a high deposition rate. The use of multi-electrode TSAW may, however, result in an increase in the overall heat input which influences the metallurgical characteristics and mechanical properties of the weld metal (WM) and heat-affected zone (HAZ), especially the coarse grain heat-affected zone (CGHAZ). To minimize the detrimental effects of excessive heat input, cold wire tandem submerged arc welding (CWTSAW) has been developed. This technique can improve the WM and HAZ geometry, mechanical properties, especially fracture toughness, and the microstructure in the CGHAZ. An understanding of the effect of welding parameters on the welding geometry and properties is needed for the application of CWTSAW.
A series of CWTSAW trials were conducted on thick-wall (19.1 mm) American Petroleum Institute (API) X70 steel plates to investigate the effect of cold wire feed speed, heat input, voltage, travel speed, and bevel design on the resulting geometry (e.g., bead width, penetration depth, height of reinforcement area, reinforcement area, bead toe angle, and CGHAZ area), and micro-hardness of the WM and HAZ. The characteristics of weld geometry and hardness were analyzed using three statistical methods including analysis of variance, multiple regression analysis, and Taguchi’s signal-to-noise ratio. Charpy V-notch (CVN) impact tests of the weld CGHAZ were conducted. The microstructure of the weld was examined using optical microscopy, scanning electron microscopy and electron backscatter diffraction for phase identification and grain size measurement.
The statistical results showed that the bevel design had a significant effect on the height of reinforcement area, reinforcement area, bead toe angle, and CGHAZ area but only a minor effect on the weld metal and CGHAZ micro-hardness. Cold wire feed speed had the most dominant effect on the weld metal and CGHAZ micro-hardness profiles, since the cold wire addition altered the local thermal cycle by consuming heat from the molten pool.
The CVN results indicated that the averaged absorbed energy of the CGHAZ was improved by 60% (from 85 J to 137 J) for the sample with a cold wire feed speed of 40 in/min (16.9 mm/s), a travel speed of 50 in/min (21.2 mm/s) and a heat input of 2.9 kJ/mm. The fracture toughness improvement is attributed to a decrease in the actual heat input (2.6 kJ/mm) introduced to the weld and an intermediate cooling rate (36.3 °C/s) in the CGHAZ. However, inferior fracture toughness was obtained if the cold wire feed speed was increased to 80 in/min (33.9 mm/s), due to the relatively fast cooling rate (40.9 °C/s). A cold wire feed speed of 40 in/min (16.9 mm/s) was suitable for the welding process. Additionally, considering the thermodynamic based heat balance calculations for the CWTSAW process, a 20% increase in the welding travel speed was achieved (from 21.2 to 25.4 mm/s) by addition of a cold wire at a feed speed of 40 in/min (16.9 mm/s) while maintaining the improved fracture toughness.
Microstructure analysis showed that refinement of prior austenite grains and ferrite/bainite grains was achieved by cold wire addition. The sample with a cold wire feed speed of 40 in/min (16.9 mm/s) and a travel speed of 50 in/min (21.2 mm/s) had reduced martensite-austenite (MA) constituent fraction and finely dispersed MA constituents, which provided a favorable morphology in terms of fracture toughness. Elongated and massive block MA constituents formed in the sample with an 80 in/min (33.9 mm/s) of cold wire feed speed, as a result of the faster cooling rate; theses were detrimental to the fracture toughness in the HAZ of welded steel. The prior austenite grain size and MA morphology were significantly influenced by the peak temperature and cooling rate in the CGHAZ by additional cold wire during welding.
- Graduation date
- Fall 2021
- Type of Item
- Master of Science
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.