Optimization Of Heat Treatment Process For Micro-alloyed Wellhead Steel And Its Performance Evaluation

With the exploitation of marine resources gradually developed from shallow to deep sea areas,the environment in which deep-sea oil recovery equipment is located is becoming more and more demanding,and the requirements for such equipment with steel are becoming higher and higher.Among them,the subsea wellhead is a key component of marine engineering equipment,and such components have not been localized in our country,basically dependent on imports.Therefore,this paper independently designed and prepared a micro-alloyed 4140 steel,and analyzed its thermal deformation process through Gleeble hot compression simulation experiments;determined the optimal heat treatment process based on orthogonal experimental design;evaluated the corrosion resistance of the experimental steel in different corrosive media,and tested the stress corrosion cracking resistance of the experimental steel in the H₂S environment.The specific research contents and results are as follows.Hot compression experiments were conducted on the experimental steel to analyze the stress-strain curves and tissue changes at different temperatures(850?1200°C)and strain rates(0.1?10 s^-1),to establish the heat deformation instantonal equation of the experimental steel,and to establish the thermal processing diagram based on the DMM model to determine the optimal thermal processing range of the experimental steel:temperature 1025?1200°C and strain rate 0.1?1 s^-1.A four-factor(normalizing temperature,quenching temperature,tempering temperature,tempering time)three-level orthogonal experiment was designed,and the results of the extreme difference analysis showed that the effect of tempering temperature on the mechanical properties of the experimental steel was the most significant,followed by tempering time,and the effect of normalizing temperature and quenching temperature on each mechanical property index was not significant.The tempered steel was tempered sothitic,and the presence of spherical and elongated(Fe,Mn,Si,Cr)₃C alloy carburite,which could significantly improve the strength,was observed by TEM.After comprehensive analysis,the optimal heat treatment process was determined:normalizing at 920°C(holding time 2 h),quenching at850°C(holding time 1 h)and tempering at 670°C for 4 h.After heat treatment by this process,the performance indexes of the experimental steel met the material standards for forgings for deep-sea high-pressure wellhead devices.The electrochemical behavior of the experimental steel in 3.5%Na Cl solution and simulated oilfield recovery fluid was investigated,and it was found that different heat treatment processes had less influence on the electrochemical behavior of the experimental steel;the immersion test showed that the corrosion rate in 3.5%Na Cl solution was faster than that in the simulated oilfield recovery fluid;the average corrosion rate of the experimental steel was significantly accelerated in the dynamic solution and salt spray environment.The corrosion products formed in the 3.5%Na Cl solution and salt spray environment are mainly?-Fe₂O₃ and Fe₃O₄,and the products in the simulated oilfield recovery fluid are mainly Ca CO₃,?-Fe₂O₃ and Fe₃O₄.Fe(OH)₃ and?-Fe OOH are more likely to exist stably in the static solution.SEM observation found that the corrosion morphology as a whole showed the characteristics of uniform corrosion,with the local presence of corrosion micropores.The rust layer formed in 3.5%Na Cl solution was found to be enriched with Cr elements in the inner layer,while the specimen in the simulated oilfield recovery fluid was wrapped by a layer of Ca CO₃,and the rust layer in the salt spray environment was formed by multi-layer stacking,with many large-sized holes;the experimental steel after the optimal heat treatment process meets the H₂S stress corrosion test standard and meets the service requirements,and the experimental steel has good resistance to hydrogen sulfide stress corrosion cracking ability.The room temperature tensile fracture of the corroded specimens still exhibits the characteristics of plastic fracture.