Effect of Ti Content on the Microstructure and Properties of CoCrFeNiMnTix High Entropy Alloy.

The aim of this study was to investigate the effects of the Ti element addition on the microstructure and properties of CoCrFeNiMn high entropy alloys. The Ti element modified CoCrFeNiMnTix high entropy alloys were prepared by vacuum arc melting processing. The Ti rich body-centered cubic structure phase was observed in CoCrFeNiMnTi0.25 and CoCrFeNiMnTi0.55 instead of a simple face-centered cubic structure in CoCrFeNiMn. The amount of the Ti-rich phase depicted an increasing trend with increasing Ti content. Simultaneously, the mechanical properties of CoCrFeNiMnTix were obviously improved. When the Ti content is 0, 0.25 and 0.55, the microhardness is 175 HV, 253 HV and 646 HV, which has an obvious increasing trend, while the ductility decreased. The tensile properties show a trend of first strengthening and then decreasing, changing from 461 MPa to 631 MPa and then to 287 MPa. When x was 0.55, the solid-liquid transition temperature of the alloy decreased, and the melting temperature range increased.

The Evolution and Distribution of Microstructures in High-Energy Laser-Welded X100 Pipeline Steel.

High-energy beam welding was introduced for pipeline steel welding to reduce pipeline construction costs and improve the efficiency and safety of oil and gas transportation. Microstructures and their distribution in X100 laser-welded joints, which determine the joints' strength and toughness, are discussed in this paper. Welded joints were prepared by an automatic 10,000-watt robot-based disc laser-welding platform for 12.8 mm thick X100 pipeline steel. Then, the grain, grain boundary, orientation, and distribution pattern of each zone of the welded joints were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron backscattered diffraction (EBSD) analysis techniques. The results showed that the grain boundary density, contents of the high-angle and low-angle grain boundaries, distribution states, and evolution trends of coincident site lattice (CSL) grain boundaries were essentially the same in each zone from the base metal (BM) to the weld of the X100 pipeline steel laser-welded joint. The relative content of grain boundaries above 55°, which were composed of the Σ3 type CSL grain boundary, showed a considerable impact on the mechanical properties of the joint. The content of twin grain boundaries was closely related to the thermal cycles of laser welding, and the effect of the cooling rate was greater than that of the process of austenization.

Quantitative Correlation between Thermal Cycling and the Microstructures of X100 Pipeline Steel Laser-Welded Joints.

Due to the limitations of the energy density and penetration ability of arc welding technology for long-distance pipelines, the deterioration of the microstructures in the coarse-grained heat-affected zone (HAZ) in welded joints in large-diameter, thick-walled pipeline steel leads to insufficient strength and toughness in these joints, which strongly affect the service reliability and durability of oil and gas pipelines. Therefore, high-energy-beam welding is introduced for pipeline steel welding to reduce pipeline construction costs and improve the efficiency and safety of oil and gas transportation. In the present work, two pieces of X100 pipeline steel plates with thicknesses of 12.8 mm were welded by a high-power robot laser-welding platform. The quantitative correlation between thermal cycling and the microstructure of the welded joint was studied using numerical simulation of the welding temperature field, optical microscopy (OM), and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS). The results show that the heat-source model of a Gaussian-distributed rotating body and the austenitization degree parameters are highly accurate in simulating the welding temperature field and characterizing the austenitization degree. The effects of austenitization are more significant than those of the cooling rate on the final microstructures of the laser-welded joint. The microstructure of the X100 pipeline steel in the HAZ is mainly composed of acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF). However, small amounts of lath martensite (LM), upper bainite (UB), and the bulk microstructure are found in the columnar zone of the weld. The aim of this paper is to provide scientific guidance and a reference for the simulation of the temperature field during high-energy-beam laser welding and to study and formulate the laser-welding process for X100 pipeline steel.