The presence of acicular ferrite (AF) in the heat-affected zone (HAZ) of steels used offshore is generally seen as beneficial for toughness. In this study, the effects of varying fractions of AF (0–49 vol.%) were assessed in the simulated, unaltered and coarse-grained heat-affected zones (CGHAZ) of three experimental steels. Two steels were deoxidized using Ti and one using Al. The characterization was carried out by using electron microscopy, energy-dispersive X-ray spectrometry, electron backscatter diffraction and X-ray diffraction. The fraction of AF varied with the heat input and cooling time applied in the Gleeble thermomechanical simulator. AF was present in one of the Ti-deoxidized steels with all the applied cooling times, and its fraction increased with increasing cooling time. However, in other materials, only a small fraction (13–22%) of AF was present and only when the longest cooling time was applied. The impact toughness of the simulated specimens was evaluated using instrumented Charpy V-notch testing. Contrary to the assumption, the highest impact toughness was obtained in the conventional Al-deoxidized steel with little or no AF in the microstructure, while the variants with the highest fraction of AF had the lowest impact toughness. It was concluded that the coarser microstructural and inclusion features of the steels with AF and also the fraction of AF may not have been great enough to improve the CGHAZ toughness of the steels investigated.

Comparison of Impact Toughness in Simulated Coarse-Grained Heat-Affected Zone of Al-Deoxidized and Ti-Deoxidized Offshore Steels

Abstract The research objective was to determine the effect of current on microstructure, hardness values, and tensile strength of A36 steel before (raw material) and after welding using SMAW using electrodes E7016 diameter 3.2 mm. The research adopted an experimental method and utilized Olympus Metallurgical Microscope, Micro Vickers Hardness Tester, and Universal Testing Machine for data analysis. The results revealed that welding caused a change in the microstructure of the weld area into Grain boundary Ferrite (GF), Widmanstatten Ferrite (WF), and Acicular Ferrite (AF). The hardness test showed that higher current leads to greater hardness values. The specimens with a current of 130 Ampere had a hardness level of 181.39 VHN, which was higher than specimens with 120 Ampere (176.21 VHN) and 110 Ampere (159.56 VHN). The raw material had a hardness value of 125.4 VHN. The welding resulted in a difference in tensile stress, with specimens using 130 Ampere having the highest tensile strength level of 520.20 MPa, followed by 120 Ampere with 504.61 MPa, and 110 Ampere with 483.03 MPa. The research concludes that varying the electric current during welding affects the microstructure, tensile strength, and hardness value of Steel ASTM A36.

The effect of variation of the electric current on physical and mechanical properties in A36 steel welding

Intercritically reheat-coarse grained heat-affected zone (ICCGHAZ), as a brittle zone in double-pass welding, its toughness is affected by the peak temperature of the secondary thermal cycle. To elucidate the effect of secondary peak temperature on the microstructure and impact toughness of ICCGHAZ in laser-arc hybrid welded X100 pipeline steel joints, thermal simulation studies were carried out. The experimental results indicated that the microstructure of ICCGHAZ consisted of tempered martensite and products of secondary thermal cycle along the prior austenite grain boundaries. The products of secondary thermal cycle were necklace-type M-A constituent, necklace-type lath martensite and granular bainite + acicular ferrite at 740 °C, 790 °C and 840 °C, respectively. Based on experimental observations, transformation mechanism of ICCGHAZ at different secondary peak temperatures was proposed. The higher half zone(790℃∼Ac3) in ICCGHAZ of the laser-arc hybrid welded joint has a higher impact energy than the base metal. Consequently, microhardness difference between the products of secondary thermal cycle and matrix was reduced such that the stress concentration and crack nucleation rate was decreased during impact. The lattice orientation was more dispersed and fraction of high angle grain boundaries was increased at 840 °C, which suppressed crack propagation. The method of pipeline steel by laser-arc hybrid welding can effectively reduce the width of low toughness zone in ICCGHAZ and increase the impact energy of the welded joints.

Effect of secondary peak temperature on microstructure and toughness in ICCGHAZ of laser-arc hybrid welded X100 pipeline steel joints

In the present work, continuous cooling transformation (CCT) of coarse-grained heat-affected zone (CGHAZ) and simulation of Charpy-sized impact specimens were performed using a Gleeble 3800 thermomechanical simulator. Results obtained from the dilation studies show significant effect of cooling rates on microstructure and low-temperature (–20 °C) Charpy impact toughness. Phase transformation temperatures (Ar3 and Ar1) and impact toughness decreased while hardness and amount of bainite increased with increasing cooling rates. At slow cooling condition (< 5 °Cs–1) quasi-polygonal ferrite and pearlite phases were observed in the microstructure. At medium cooling rate (5–25 °Cs–1), bainite and quasi-polygonal ferrite were obtained in the microstructure. For still faster cooling rates, microstructure was completely bainitic in nature. The microstructures were confirmed by hardness measurement where the hardness value for lower, medium and high cooling rates were 191–196, 213–214 and 234–253 HV, respectively. Charpy impact toughness increased with decrease in cooling rate due to the presence of softer ferrite phase.

Continuous cooling transformation diagram and mechanical properties in weld coarse-grain heat-affected zone of API X70 steel

In order to further optimize welding process of Nb–Ti–Mo microalloyed steel, welding thermal cycles on coarse-grained heat-affected zone (CGHAZ) of welded joints were simulated using Gleeble 1500 The microstructure and low-temperature impact fracture were investigated using a scanning electron microscope and a pendulum impact machine, respectively Moreover, the relationship between cooling time t8/5 and the microstructure of CGHAZ was discussed, and the effect of microstructure on impact toughness was also studied As cooling time increased, martensite fraction decreased from 978% (3 s) to 30% (60 s) The fraction of martensite/austenite (M/A) constituent increased from 22% (3 s) to 390% (60 s), its shape changed from granular to strip, and the maximum length increased from 24 μm (3 s) to 70 μm (60 s) As cooling time increased, the prior austenite grain size increased from 340 μm (3 s) to 490 μm (60 s), the impact absorption energy reduced from 1018 J (5 s) to 72 J (60 s), and the fracture mechanism changed from quasi-cleavage fracture to cleavage fracture The decreased toughness of CGHAZ was due to the reduction of lath martensite-content, coarsening of original austenite grain, and increase and coarsening of M/A constituent The heat input was controlled under 7 kJ cm−1 during actual welding for these steels

Effect of cooling time t8/5 on microstructure and toughness of Nb–Ti–Mo microalloyed C–Mn steel

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Effect of cooling time t8/5 on microstructure and toughness of Nb–Ti–Mo microalloyed C–Mn steel