The Establishment Of Part Of The Iron-based Thermodynamic Database And Its Application In S135 Drill Pipe

With the development of science and technology,the consumption of petroleum and natural gas has rapidly increased,and the number of deep wells and ultra-deep wells has increased rapidly,which leads to the higher requirements on the performance indicators of drill pipes.Researchers have been trying to obtain high-quality drill steel with better strength and toughness.However,"attempts" was often used in the study of new materials in the past,which requires researchers to obtain composition and process parameters through extensive experiments and experiences.In this paper,the partial iron-based thermodynamic database was established by using phase diagram thermodynamic calculation method(CALPHAD).The S135 drill pipe steel with 0.31wt.%C was selected as the experimental material.In order to obtain a drill pipe material with good strength and toughness matching,the heat treatment process was designed based on the calculated phase diagram,and the microstructure and mechanical properties of the drill pipe were studied.By collecting the thermodynamic parameters of the low-component systems and testing the calculated phase diagrams and thermodynamic properties,the partial iron-based thennodynamic database(Fe-Al-C-Cr-Cu-Mn-Mo-N-Nb-Ni-Si-Ti-V)was established.The thermodynamic calculation of Nd-Sn and Sn-Tb binary systems was completed to supplement the established database.According to the database,the longitudinal phase diagram of the S135 drill rod steel composition was calculated,and the calculated values of A1 and A3 of the phase diagram were 800 ? and 890 ?,respectively.The temperatures of A1 and A3 of the calculated phase diagram were verified by the alloy method.Based on the observation and analysis of the structure,the experimental values of A1 and A3 were determined to be 730 ?and 825 ?,respectively.According to the experimental data reported in the literature,the rationality of the database was verified from two aspects,which are design of the heat treatment process and the precipitation analysis of the second-phase particles of car-bon and nitrogen compounds.Based on the calculated phase diagram,the duplex microstructure of ferrite and martensite of S135 drill pipe steel was studied.The volume ratio of the different phases in the duplex microstructure was calculated according to the lever law,and the sub-temperature quenching temperature of the sample was determined at 770 ?.In order to test whether the mechanical properties obtained from quenching at 770 ? are the best,the heat treatment process was finally formulated as annealing at 880 ? for 3 h + quenching at 750-790 ? +tempering at 250 ? for 1 h.The experimental results showed that with the increase of quenching temperature,the martensite embossing phenomenon became more and more obvious,and volume ratio of martensite gradually increased,the volume ratio of ferrite gradually decreased.The hardness and the strength of S135 drill pipe increased,and the plastic property increased firstly and then decreased and reached the maximum value at 770 ?.With tempering at 250 ?,the strength of duplex microstructure slightly declined,but the plastic property was significantly improved.The heat treatment process formulated according to the calculated phase diagram(annealing at 880 ? for 3 h + quenching at 770 ?+ tempering at 250 ? for 1 h)can make the duplex microstructure of S135 drill pipe steel get the best match of strength and plasticity.Based on the calculated phase diagrams,the duplex microstructure of bainite and martensite of S135 drill pipe steel was studied.Auste280-310 ?,the duplex microstructure of low bainite and martensite was obtained.After mpering in a temperature range of tempering at 500 ?,as the austmpering temperature increased,the hardness of duplex microstructure decreased but still exceeded the original value,and the impact toughness and strength were significantly improved.When austempering in the range of 280 ? to 310 ?,the austempering at 310 ? could get the best performance and a good match of high strength and toughness in the low bainite and martensite duplex microstructure.The results showed that,under the premise of ensuring the hardness requirement and the absence of high bainite,the bainite transformed from the higher temperature is more conducive to the improvement of toughness of the duplex microstructure.According to the above results of the study,the partial iron-based thermodynamic database established in this study reflect a certain guiding function in heat treatment process design studies,but it needs to be further optimized,and then could accurately guide the design of heat treatment processes,and determine the composition of the materials and process parameters,etc.