Research On High Efficient Quenching And Tempering Treatment Of E690 Offshore Platform Steel

By means of Differential Scanning Calorimetry(DSC), the continuous heating austenitizing behavior of E690 steel under different heating rate was investigated,and based on the JMAK model, the isochronal austenitic transformation of E690 steel was described; the minimum time of carbide completely dissolution during the continuous heating austenitizing process was analysised by measuring the internal friction-temperature curve of the quenched sample under different austenitizing temperature and austenitizing time treatment, and also the minimum time of carbon completely dissolution precipitation during the tempering process; And the tempering equation was established by the P parameter method to study the relationship between mechanical property and the P parameter value; The influence of tempering temperature on microstructure and properties was also analysised in this dissertation.The continuous heating austenitizing behavior of E690 steel analysis showed that the continuous heating austenitizing process is mainly composed of two stages, one is transformation from ferrite to austenite, another stage is cementite dissolution; Variation of heating rates affects the austenitic transformation starting temperature and finishing temperature. Both AC1 and AC3 increased as the heating rate applied is raised, which means that austenitic transformation is retarded to the higher temperature. the relation between the peak of transformation rate curve and the heating rate accords with the formulation: 14.0118.010943.6/24max??????VVdtdf.The study of the ferrite to austenite transformation kinetics during continuous heating austenitizing process based on the JMAK model showed that: according to the calculated austenitizing kinetic parameters, using these parameters to predict the relationship of austenite transformation fraction with austenitizing time of E690 steel.The predicted results are verified by the internal friction method, the measured results agree well with the prediction results.The internal friction research results shows that: in the process of continuous heating austenitizing, part of the carbides were dissolved at first in the ferrite to austenite transformation, the residual carbides dissolution need a higher temperature; the carbide will completely dissolved within 5 minutes at 875 ℃austenitizing temperature, and the austenitic homogenization and austenitic grain are in good condition, prolong insulation time has little significance to increase the amount of dissolved carbide.The study about solid solution carbon precipitation at different tempering temperature indicated that the higher tempering temperature, the lower SKK strength of internal friction peak, the more solid solution carbon precipitation. After tempering at 650 and 690 and ℃ ℃holding for 20 min and 40 min respectively, the hardness will keep stable and have no connect with the holding time. Tempering at 690℃, the carbide can nearly completely precipitate within 20 min. The structure and property of the sample at different tempering temperature analysis indicated that the structure of the quenched E690 steel mainly made up of lath bainite. The beam of lath bainitic will gradually merge, the lath interface will fuzzy, polygonal ferrite and separated carbide will increased with the increase of tempering temperature. the microhardness is decreased steadily.The tempering equation established according to the P method have a good fitting effect can accurately reflect the combination of process parameters on the mechanical behavior of tempering. The research results above provide technical support to make high efficient quenching and tempering process, and provide that it is feasible that establish a more efficient quenching and tempering process through shorten the austenitizing time and tempering time. Optimized quenched and tempered process process as follows: austenitizing at 875 ℃ for 10 min and tempered at 650 ℃ for 200 min, the microstructure after the optimized treatment process was tempered sorbite, The mechanical performance indexes accord with hull steel specification requirements, compared with the before heat treatment process, Heat treatment efficiency increased by 30%.