Effect of microstructure synergism on cryogenic toughness for CGHAZ of low-carbon martensitic steel containing nickel

Varying chemical composition are used to investigate the influence of martensite-austenite (M-A) constituents on the cryogenic toughness of coarse grain heat affected zone (CGHAZ) for low-temperature steel containing nickel. The tensile tests, Charpy impact tests and crack tip open distance (CTOD) tests are carried out at −196 °C using the thermal simulated samples to evaluate the cryogenic properties of CGHAZ. The microstructure feature is studied with Optical microscope (OM), Scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) detector, Electron backscatter diffraction (EBSD), Transmission electron microscopy (TEM) and X-ray diffraction (XRD). Results show that the dislocation density and grain boundary density in martensite increase significantly by slightly increasing the carbon content. There is no obvious change on the content of M-A components caused by decreasing Ni content. With the refinement of martensite substructure, the occurrence probability of locally densely-distributed M-A constituents increase. In the case of small strain, the strengthened martensite reduces the stress concentration on M-A constituents. The synergism of martensite matrix and M-A constituents is helpful to reduce microcracks on their interfaces. With the strain increase, the microstructure synergism effect decreases and microcracks initialize. In the area of densely-distributed of M-A constituents, the coalescence of microcracks speed up the crack propagation rapidly. The resulting brittle instability accelerates the failure and reduces the cryogenic fracture toughness.