The Effect Of Heat Treatment Process On The Corrosion Resistance Of X65 Steel In Acidic Environment

Heat treatment technology plays a very important role in pipeline steel processing,casting,molding and subsequent use.Firstly,according to the terrain conditions laid along the pipeline along the road and the multi-directionality of the oil and gas delivery,the pipeline steel needs to be subjected to secondary heat treatment to assist the casting and forming of the elbow and the three-way pipe.Compared with the base metal,the structure and the performance of the bent pipe and the three-way pipeline subjected to the secondary heat treatment can greatly change,if the selected temperature and cooling mode are not reasonable,the performance of the bended pipe and the three-way pipe will deteriorate seriously,resulting in the failure of the pipeline.Therefore,the key to ensure the safe and stable operation of the pipe is to select appropriate heat treatment process parameters and improve the comprehensive quality of the bended pipe.Secondly,the choice of delivery state(final heat treatment process)of pipeline steel will vary according to the service environment.and the structure and performance of pipeline steel will also be different.The corrosion resistance of pipeline steel is an important index to evaluate the performance of pipeline steel.The better the corrosion resistance of pipeline steel is,the higher the safety and service life of pipeline steel is.However,with the deepening of petroleum exploitation,the quality of crude oil becomes worse and the acid corrosion becomes more serious.The corrosion resistance of pipeline steel with different final heat treatment to acid environment is different.Therefore,it is very important to select the appropriate final heat treatment process to improve the acid corrosion resistance of pipeline steel and to extend the service life of pipeline steel for the safe transportation of oil and gas.Through the stress relief annealing,normalizing,quenching and tempering heat treatment of pipeline steel,the relationship and difference between the microstructure and mechanical properties of pipeline steel after different heat treatment and the corrosion resistance in acid environment are studied.At the same time,the purpose of improving corrosion resistance of pipeline steel is to prolong the service life of pipeline steel,and provide basis for selecting the delivery status of pipeline steel in practical engineering,so as to provide theoretical basis for pipeline steel application and protection,and provide guarantee for safe transportation of oil and gas.Experimental methods:using tensile test,Charpy impact test,micro Vivtorinox hardness of different heat treatment process and different heat treatment parameters of the mechanical properties of X65 steel;electrochemical study on corrosion test,using different heat treatment the corrosion resistance of X65 steel with different heat treatment process and parameters;with time,temperature and applied stress as variables,using uniform corrosion test and electrochemical experiment time,temperature and applied stress on before and after heat treatment of X65 steel corrosion resistance;experimental study on effects of 6%ferric chloride pitting by different heat treatment process on pitting sensitivity of X65 steel.The results showed that:(1)after heat treatment.microstructure of X65 pipeline steel has changed dramatically,the TMCP microstructurc of acicular ferrite and quasi polygonal ferrite and polygonal ferrite,stress relief annealing due to the phase transformation temperature is still not reach,the organization of acicular ferrite and quasi polygonal ferrite + polygon ferrite,normalizing organization into polygonal ferrite + pearlite,quenching and tempering organization for bainitic ferrite and granular bainite;(2)7 days after corrosion in 5%NaCl+0.5%acetic acid solution,the corrosion resistance from high to low:Quenched and tempered state>stress annealed state>TMCP>normalizing,its corrosion resistance depends mainly on the organization and size of the potential difference,because the polygonal ferrite and the difference of the maximum potential,the corrosion resistance the worst;(3)with the prolongation of immersion time,the corrosion rate of the four kinds of X65 steel increased first and then decreased.The corrosion rate of the four states reached the highest at 21d;(4)the corrosion rate of X65 steel in the four states increased with the increase of temperature,and the corrosion rate of the four states at 60 degrees centigrade reached the highest;(5)X65 steel four states by constant strain four point bending experiments are no fracture,surface crack,four states in NACE solution at different temperatures showed good resistance to stress corrosion cracking performance;and stress corrosion rate are higher than without adding samples stress,stress corrosion of pipeline plays a great role;(6)After the pitting test of 6%ferric chloride,the pitting corrosion degree of the four states is from small to great:quenched and tempered state<TMCP<stress annealing state<the normalizing state.