Groove Configurations and multi-pass Temper Bead in Flux Cored Arc Welding Process Used in Thick Steel Plate Welds of Large Structures: Mechanical properties and Metallurgical Studies

• Main Authors: Kuo-Hsia Ling, 凌國夏
• Other Authors: Yiin-Kuen Fuh
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/97kur4

博士 === 國立中央大學 === 機械工程學系 === 104 === Ships, bridges, pressure vessels, industrial machinery, automobile, rolling stock and many other fields are all produced by welding technology. The common problem in these fields is associated with welding process. The maximum hardness of the heat affected zone (HAZ) and the cold cracking susceptibility of welds are results in Weldability of steel. This paper utilized a flux-cored arc welding (FCAW) technique and investigated various groove configurations as well as multi-pass welding sequences to create welding specifications in the mechanical press industry. Experimental investigations were conducted to examine the influence of three different groove configurations of JIS SS400 structural steel welded joints on mechanical and metallurgical properties. Mechanical properties of the welded joints were evaluated by uniaxial tensile testing and Charpy V-notch (CVN) impact testing at room temperature. Simultaneously after a multi-pass welding sequence, various degrees of thermal treatments such as tempering or normalization inevitably occur in the heat affected zone (HAZ). The unfavorable microstructures in the HAZ and reheated zones can be intentionally modified via the above procedure such that the toughness and microhardness of the HAZ improves. The experimental results revealed that of the three types of groove configurations (C1, C and F), groove type C1 possessed the maximum yield strength (YS) and ultimate tensile strength (UTS) while groove type F possessed the highest CVN values tested at room temperature. The fundamental reason may be attributed to heat dissipation characteristics of each groove configuration and associated exertion of the multi-pass welding sequence. Microstructural and morphological features as revealed through an optical microscope also indicate a significant influential factor of these joints among the different groove configurations. Therefore, grain refinement of varying degrees can be obtained due to the variation of thermal characteristics of heat input/dissipation and thus, various mechanical and CVN impact properties can be obtained. It further examined the effects of temper bead welding (TBW) made with two different welding processes, namely with 4 Layers 4 Passes (4L4P) and 4 Layers 10 Passes (4L10P). Thermocouples were fixed to measure the thermal cycles, and the temperature distribution curves along the weld seams were measured by Infrared Radiation (IR) images. All welded samples were checked using nondestructive Phased Array Ultrasonic Testing (PAUT) to ensure defect-free samples before the experiments commenced. The Charpy impact tests were finished in 20 and -20°C respectively. Vickers microhardness measurement was carried out at room temperature. The 4L4P welding process was found to improve the impact toughness of the welding joints. In addition, it was found that the 4L4P welding process produced an overall lower hardness than the 4L10P welding process.