Effect of different welding energy on microstructure and toughness of HAZ of low carbon bainitic steel

Abstract

Low carbon bainitic steel (LCBS) with excellent combined properties is the first choice for materials of pipeline transiting geological disaster. However, welding will worsen its toughness. In this paper, three kinds of welding heat input were designed to study the relationship between the toughness and microstructure in the coarse grain zone of welding heat affect zone (CGHAZ) of LCBS. The evolution characteristics of the microstructure of LCBS and the CGHAZ, and impact fracture were investigated by optical microscopy (OM), scanning electron microscopy (SEM). The results indicated that microstructure of LCBS consists mainly of bainite ferrite (BF) and granular bainite (GB). Heat input for 22 kJ/cm, the original austenite grains become coarsening, the microstructure is a small amount of quasi polygonal ferrite (QF) and polygonal ferrite (PF), which exhibits low Charpy impact toughness. However, for heat input of 19 kJ/cm, the degree of grain coarsening is small and distribution of martensite-austenite (M-A) constituents is the chain. The statistics of image software show that with the increase of heat input (16–22 kJ/cm), the average grain size of original austenite is basically the same (25 um), which is mainly due to Nb solute drag restraint the growth of austenite grain.

Effect of the Cooling Rate of Thermal Simulation on the Microstructure and Mechanical Properties of Low-Carbon Bainite Steel by Laser-Arc Hybrid Welding

“…The C had no time to diffuse, while the island-shaped carbon-rich austenite was surrounded by the ferrite. They became martensite, and another part remained to form retained austenite [7]. When t 8/5 = 4 s, the cooling rate was fast and the diffusion time of carbon atoms was shorter, so the formed fine carbon-rich austenite region was the granular M-A phase, which was characterized indistinctly.…”

“…However, in the LAHW process of low-carbon bainite steel, the welded joint of the base material (BM) cannot achieve the original mechanical properties attributed to the welding thermal cycle of the heating speed, which is responsible for the characteristics of the local uneven cooling speed. As a result, the HAZ in the joint turns into the main weak area, especially the coarse-grained heat-affected zone (CGHAZ), limiting its application [7,8].…”

Fracture behavior of S355J2W steel and weld at low temperature

“…10 The heat-affected zone (HAZ) is considered the weakest zone in joints because of the different phase distribution and chemical components. 11 Wang 12 observed that the heat input during the welding process has a great influence on HAZ toughness. In addition, Vojvodic 13 reported that for high-strength low-alloy steel, the welded joints are critical regions for structural integrity because of the possible brittle fracture behavior.…”