Impact On The Microstructure And Mechanical Properties Of Steel 9NiCrMo Different Heat Treatment Process

9NiCrMo produced by quenching and tempering may appear poor tempering stability, low percent of pass, high yield ratio and low temperature toughness. In order to solve these problems, through the alloy design and the adjustment of the heat treatment process, the effects of different heat treatment processes including intercritical quenching (QQ ’T), quenching and tempering treatment (QT) and cyclic hardening (QQT) as well as alloying component on the evolution of microstructure and properties had been systematically studied in this paper.The strength and yield ratio of9NiCrMo increased with increasing the temperature of the secondary quenching in the process of QQ ’T, but the toughness decreased. For9NiCrMo steel,710℃～780℃was the best appropriate secondary quenching temperature region, and the strength and the toughness of this steel matched very well when the secondary quenching temperature was750℃. The best appropriate tempering temperature region was500℃～590℃, the yield strength of the steel increased by0.14MPa/℃at500℃～550℃, and the toughness improved at the same time. The yield strength of the steel decreased by0.5MPa/癈under the process of870℃quenching+750℃quenching+550℃-590℃tempering. However, the steel obatined good toughness, and the impact energies of low-temperature were more than130J and110J at-20℃and-84℃respectively. The yield strength of the steel treated by quenching and tempering declined from1040MPa to930MPa (11MPa/℃) when the steel was tempered at600℃～610℃.The yield strength of the steel treated by870℃quenching+800℃quenching+570℃～600℃tempering (QQT) declined from1037Mpa to953Mpa when the tempering temperature increased from570℃to600℃, which imples tempering stability of the steel is poor under this heat treatment process and this process is not satisfied with the industrial production. The results stated above shows the steel treated by QQ ’T possesses the better matching of strength and toughness, and QQ’T is the best appropriate heat treating process for9NiCrMo steel.In order to study the effect of alloying element on the microstructure and properties of this kind of stell,7Ni,8Ni,10Ni steels with different Ni content were designed, the yield strength, tensile strength, low temperature toughness and the yield ratio of these steels were investigated and compared systematically. The results show the steel possesses excellent performance when the Ni content is9%. Moreover, Mo elements can improve the hardenability of the steel. Therefore, high Mo steel is suitable for the production of middle and thick plate while low Mo steel is suitable for the production of sheet.The excellent strength and toughness of the steel treated by QQ’T are closely related with the optimal microstructure. After QQ’T, the microstructure contained secondary martensite, ferrite and a little reverse transformed austenite. Partial austenite produced by the austenitizing process of some regions at higher secondary quenching temperature, which caused constituent fluctuations in the steel, and then the partial austenite transformed to secondary martensite caused during the quenching. The temperature of partial regions in the steel was higher than the critical poin due to the enrichment of C, Ni, Mn elements in these regions, which resulted in the producing of reverse transformed austenite in the process of tempering, and the reverse transformed austenite is beneficial to improving the toughness of the steel.