Evaluation of Mechanical Properties and Microstructure of X70 Pipeline Steel with Strain-Based Design

Abstract

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 Ac1 and Ts 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.

Effect of Microstructure on Mechanical Properties of X65MS Welded Pipe Joint

“…In addition, GB contained a high carbon amount and large amount of lattice deficiencies resulting in potential low CVN impact toughness. [ 45 ] And during the phase transformation process, carbon atoms in the bainite diffuse into the austenite, making the carbon rich austenite more stable and forming M/A components at lower temperatures. With the increase of cooling time, the size and volume fraction of M/A components increased, and their morphology changed to elongated blocks.…”

Current Status and Trends of Low-Temperature Steel Used in Polar Regions

“…Additionally, Mo raises the activation energy of carbon diffusion in austenite, thereby reducing the dispersion coefficient of carbon and inhibiting the formation of proeutectic ferrite. It is worth noting that increasing the content of Mo may improve the alloy's strength, but its toughness will decrease [99][100][101][102][103]. Cr, known as a ferrite stabilizer, enhances the strength of steel by forming chromium carbide.…”

Study on Fracture Behavior and Toughening Mechanisms of Ultra-High-Strength Pipeline Steel

“…The toughening mechanism below the X100 level has been well studied. There, a good match of strength and toughness was achieved [8][9][10][11][12][13][14]. The contradiction between strengthening and toughening becomes more prominent when the steel grade reaches X100 [7,[15][16][17][18][19].…”