Research on Impact Property Performance Fluctuation of Hi-strength Oil Casing

In order to investigate the cause for impact property fluctuation, a series of samples are taken from the quenched and tempered seamless steel pipes of the same batch to analyze the fracture morphology, the microstructure and the inclusions of the specimens with 3.7 times of impact energy difference.It is identified via the metallographic observation and the analytic statistics the microstructures of the specimens with low and high impact properties are the same, i.e., the tempered sorbite plus little ferrite.The SEM result shows that a few carbides around the ferrite are larger in size and slightly poor in structural homogeneity, resulting in poor impact property.And it is also revealed via the TEM result and the quantitative statistics of carbides that the specimen with good impact performance has a carbon content of 0.137wt.% in the high-carbon part, and its A_c3=902 ℃ and grain is relatively fine.After high-temperature tempering, the structure homogeneity is good, and the carbides are uniformly distributed on the grain boundary and the matrix.The main reason for the low impact property is that the tempering temperature after quenching is too high, which causes the carbide to gather and grow up.