NM550 wear-resistant steel is widely used in large-scale engineering and mining machinery under extremely harsh working conditions. In NM550 steel, the addition of Ti can cause the formation of micron-scale TiN and TiC inclusions, easily triggering cleavage fractures. The fracture behavior and precipitation rule of micron-scale TiN and TiC inclusions on the tensile process in NM550 steel was investigated by scanning electron microscopy, transmission electron microscopy, and energy spectrum analysis combined with thermodynamic theory. The TiN precipitated in the solid–liquid two-phase region at a precipitation temperature of 1710 K, whereas that of TiC was 1158 K along the austenite grain boundary. The sizes of the TiN precipitated in the liquid phase and the TiC precipitated in austenite were both at the micron scale, which is prone to cleavage fracture during the stretching process. Under tensile stress, microcracks were first initiated at the TiN inclusion, which were further separated forming a hole, whereas the TiC inclusion was divided into two sections with a long and narrow gap formed between the substrates. The sizes of the TiN and TiC were related to the cooling rate, Ti, and N contents. The larger the cooling rate and the lesser the Ti and N content, the smaller the TiN and TiC sizes.

The microstructure and mechanical properties of X70 pipeline steel with a ferrite/martensite dual-phase microstructure produced by thermo-mechanical controlled processing were investigated by tensile tests, Charpy V-notched (CVN) impact tests, drop-weight tear tests, guided-bend tests, scanning electron microscopy and transmission electron microscopy combined with thermodynamic simulation analysis. All the mechanical properties met the strength, ductility, toughness and deformability properties requirements of X70 grade pipeline steel with strain-based design. The shear fracture area and absorbed energy of CVN at −10 °C were >97% and >205 J in base metal (BM), weld metal (WM) and heat affected zone (HAZ) with low transition temperature, indicating adequate resistance to propagating fracture. The microstructure of WM was mainly intragranular acicular ferrite that can guarantee high strength, toughness and over matching requirements of the welded joint. Because of being exposed to successive heat inputs, the ferrite plus martensite/bainite microstructure of BM was heated between A_c1 and T_s forming the HAZ. However, a high CVN impact toughness of 345 J at −10 °C in HAZ was obtained, which indicated that the excellent mechanical properties of BM would not be seriously deteriorated during the welding thermal cycles with the reasonable addition of Ti and Nb.

This work investigates the correlation between TiN inclusions and microstructural properties of HARDOX 450 steel using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscattered diffractometer (EBSD) methods. Some amount of microsized TiN inclusions were formed in the temperature range of the solid–liquid zone; however, they exhibited fracture features of deep dimples rather than a cleavage plane, which is closely related to the ability of the microstructure to arrest cracks. Upon tensile loading, a single microcrack first appeared inside the microsized TiN inclusions, and then, multiple microcracks formed, parts of which widened in the direction of tensile stress. A schematic mechanism map was plotted to reveal the propagation behavior and fracture mechanism of the microcracks in the TiN inclusions.

The relationship between the microstructure and the mechanical and corrosion properties of a welded joint of 620-grade marine steel was studied using metallographic microscopy, scanning electron microscopy, an energy dispersive spectrometer, transmission electron microscopy, and microhardness and tensile tests. The results showed that the strength and hardness of the weld center area (WM_mid) were higher than those of the inner and outer welding surface region (WM_in and WM_out) because the volume fraction of the martensite-austenite (MA) constituents (21.6%) was higher than that in WM_in and WM_out (18.0% and 14.3%, respectively). There were numerous MnO-Al₂O₃-SiO₂-TiO₂-type inclusions located at the bottom of dimples in the fracture surface; however, the MA constituents took precedence over this kind of inclusion in inducing pitting corrosion. In contrast, pitting corrosion can be initiated by Al₂O₃-MgO-CaO-CaS inclusions in the heat-affected zone (HAZ) and base metal (BM). The corrosion resistance of the welded joint was in the order of weld metal > HAZ > BM. The WM_mid with smaller dendrite spacing and a larger size of MA constituents had better corrosion resistance compared with the WM_in and WM_out. The corrosion resistance of the HAZ decreased in the sequence of coarse grain HAZ, fine grain HAZ, and intercritical HAZ.

The 5% Ni steel is often used to make steel storage tanks to store liquefied natural gas (LNG). Herein, the microstructure and mechanical properties of 5% Ni steel samples during bending were studied through combining scanning electron microscopy, energy dispersive spectroscopy, optical microscopy, X-ray diffraction, and electron backscattered diffractometer methods with tensile tests. The outer and inner arcs underwent tensile and compressive stress, respectively, resulting in a severely deformed microstructure with a high density of dislocation, improving both the tensile and yield strengths. The ductility of the 5% Ni steel samples decreased significantly after bending due to the work hardening and dislocation accumulation. During bending, the shear bands occurred at the surface or subsurface, which were caused by strain localization. Amounts of “harder” grains with high TF and more orange and red KAM areas with high local strain at the outer and inner arcs produced a greater stress concentration than that of the mid-thickness, which can induce crack initiation and propagation due to the large deformation during bending.

This paper aims to study the relationship between the microstructure and the mechanical properties of X70 submarine pipeline steel with 40.5 mm thickness. The microstructure was examined by using optical microscopy, scanning electron microscopy and an electron backscattered diffractometer, while the mechanical properties were examined by using a hardness test, a tensile test, a Charpy impact test and a drop weight tear test (DWTT), respectively. The results show that the base metal (BM) of the pipe has a low yield ratio of 0.83 and an excellent elongation of more than 45%. The DWTT shear area of the steel plate reaches 87%, showing excellent low-temperature toughness. The Charpy impact energy increases when the distance from the fusion line increases, and it reaches a maximum at the BM near the heat-affected zone (HAZ) due to the small martensite-austenite (MA) constituents and fine grains. The concentrated distribution of blocky/slender MA constituents along the prior austenite grain boundaries of the intercritically reheated coarse-grained HAZ and the large MA constituents are the main reasons for the deteriorating impact toughness. Delamination cracks in the DWTT fracture surface only occurred in the midthickness of a sample with a small opening width that spread about 2.1 mm perpendicular to the DWTT fracture surface and were finally arrested at the acicular ferrite clusters containing a high density of high-angle boundaries.