Rockwell hardness tester,optical microscope,scanning electron microscope and transmission electron microscope were used to study the effects of tempering cooling method on the hardness,microstructure and-20 ℃ low-temperature impact toughness of 80 mm thick Q690D steel.The results show that the tempering cooling method has little effect on the hardness distribution in the thickness direction of the quenched and tempered steel,compared with the quenched state hardness,the maximum hardness difference decreases from 13 HRC to 6 HRC.After tempering at 620 ℃ followed by air cooling,carbides precipitate along austenite grain boundary and lath boundary,the impact absorbed energy of the steel at-20 ℃ is 65 J,and the impact fracture morphology is quasi cleavage.After tempering followed by water cooling(5-8 ℃/s),the precipitation of carbide at grain boundary and strip boundary is restrained,the impact absorbed energy of the steel at-20 ℃ is increased to 210 J,and the fracture morphology is mainly dimples.After tempering followed by water cooling,the impact absorbed energy at-20℃ of industrial produced steel is 174-252 J,which is increased by 105％to 397％compared with air cooling.
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