Effect Of Heat Treatment On Impact Toughness Of A333Gr8 Cryogenic Pipe At Low Temperature

A steel company in China adopts AT heat treatment to have the trial production of A333Gr8 steel pipe which suffers the problems of low temperature impact toughness. The enterprise is eager to solve this problem. Therefore, it studied the influences of the process parameter change in the process of QT heat treatment on the A333Gr8 pipeline steel low temperatures impact toughness, analyzed the mechanism of heat treatment. The experiment took the rolling status A333Gr8 produced by this steel factory as the object, adopted the KSW-4D-11 resistance furnace to make QT and QLT as heat treatment, German Leika research-level complete automatic metallographic microscope to observe the microscopic structure, TH-320 Rockwell apparatus to measure the surface hardness, JB-500 tensile testing machine to carry out the tensile test and the WAW-300 impact toughness tester to test the impact toughness under the condition of-196?.The experiment found: After QT and QLT heat treatment, the tissue was tempered sorbite. Compared with the original state, its grain refinement and tissue distribution are more average and the conventional mechanics performance was sound. As the reverted austenite was generated, the low temperature impact toughness of A333Gr8 increased obviously. Under the condition of-196?, the maximum absorption was bigger than 200 J in average, which was much bigger than the original status of 27 J. However, the QT heat treatment is sensitive to the temper temperature and time changes. When the temper temperature was �40 ?and the temper time was �0.5h, the absorbed energy of A333Gr8 experienced great changes within the scope of 44.42-217.83 J and the temper parameter changes tend to caused the instability of the low temperature toughness of A333Gr8 easily,The subcritical quenching in the QLT heat treatment process generated fine austenite and changed the grain size of original tissue. Meanwhile, the existence of residual ferrite did not only limit the growth of austenite, but also increased the alloy composition in the austenite, which is beneficial for the improvement of A333Gr8 low temperature toughness of low temperature steel. Compared with QT heat treatment, QLT heat treatment features better low temperature impact toughness under the-196?, featuring the average absorbed energy of over 200 J and the minimized absorbed energy of 187.17 J, which was bigger than the requirements of the steel factory.