Evaluation of Fracture Toughness of Welded High Strength Low Alloy Line-pipe Steel

Abstract

The present investigation revolves around evaluation of fracture toughness of welded API X65 steel plate. API X65 is a high strength low alloy (HSLA) steel which is used in linepipes for transporting oil and natural gas. In the above application, catastrophic failure due to fatigue is reported to cause sudden damage and loss to human-life as well as property. Hence, from end application point of view, knowledge on fracture toughness especially in welded plates is of immense importance. Through the present work, an effort has been made to add to the limited number of reports on the above subject. In this work, first, the selected steel was characterized in terms microstructure, hardness and tensile properties. Thereafter, the welding of steel plates was performed with sub merged arc welding (SAW) with root pass of shielded metal arc welding (SMAW). The microhardness profile in base material (BM), heat affected zone (HAZ), weld zone (WZ) and weld nugget were evaluated along with micro structural characterization of the above zones using optical and transmission-electron microscopy. The fracture toughness of various zones of welded plate, was evaluated in terms of J-R curve following the concept of EPFM. The samples for fracture toughness tests were prepared as per ASTM E1820. Fractography of the failed specimens were also carried out. The evaluation of fracture toughness of welded API X65 plates indicates that, JQ value of heat affected zone was slightly lower than the base material, but for the weld zone it dropped significantly by 53% due to the non-uniform crack front and crack propagation. The phenomenon is attributed to presence of different defects in the weld region. JQ value for fusion line samples showed significant scatter with an average reduction of 40% in JQ value as compared to base material. The scatter in fusion line value can be attributed to error in precise placement of notch or deviation of crack path during testing. Presence of Nb and V imparts precipitation hardening effect to the API X65 steel, such that it has a satisfactory combination of strength and toughness parameters fulfilled by API service requirements.