Influence Of Cold Treatment On Pitting Corrosion Behavior Of Super Martensitic Stainless Steel

With the development of the offshore and petroleum industries,the requirements for stainless steel used in the marine industry are becoming higher and higher.Considering the economic cost,martensitic stainless steel has better corrosion resistance than carbon steel,and its production cost is lower than duplex stainless steel.Its superior mechanical performance,reasonable corrosion resistance,and attractive cost make it widely use in oil and natural gas fields.15Cr super martensitic stainless steel has excellent mechanical properties performance,good corrosion resistance and attractive cost make it widely used in oil field.However,because of its high carbon content,it is easy to produce more carbides,which reduces its resistance corrosion performance,in order to ensure the comprehensive performance of super martensitic stainless steel is based on the principle of"low carbon",introduced 15Cr ultra-low carbon martensitic stainless steel.The cryogenic treatment process has the advantages of environmental protection and low cost,and can also improve the mechanical properties of steel.Therefore,cryogenic treatment was introduced during the heat treatment of steel.In this paper,a new type of 15Cr ultra-low carbon martensitic stainless steel was used as the research material,once and twice cryogenic treatments were used.X-ray diffraction,field emission scanning electron microscope,transmission electron microscopy and electrochemical methods were used to conduct microstructures of the test steel under different cryogenic treatment processes and the corrosion resistance was studied,and the formation mechanism of pitting corrosion of the test steel was explored.The main conclusions are as follows:1.The results showed that the microstructures of super martensitic stainless steel mainly consisted of reversed austenite,martensite and M₂₃C₆ carbides.Among them,the reversed austenite content in the quenched(Q)and quenched+cryogenic(QC)steel were less than 5%,and the reversed austenite content after once cryogenic treatment samples increased first and then decreased with the increased of tempering temperatures,and tempering temperature at 650?the reversed austenite reached the maximum about 36.67%.After once cryogenic treatment,the content of reversed austenite in the test steel tempering temperature at 700?for different times increased with the extension of the tempering times,and reached the maximum at about 1 h after tempering.After temper temperature at 700?for 2 h,the reversed austenite content began to decreased.The content of reversed austenite in the test steel after twice cryogenic treatment was similar,and ranging from 28%to 30%.2.The microstructure of the test steel after quenching(Q),quenching+cryogenic(QC)treatment is mainly martensitic lath.When tempering temperature below 550?,the reversed austenite is distributed in a thin film on the boundary of the martensitic lath and original austenite grain boundary,the cryogenic treatment promotes martensite lath refine.As the tempering temperature increased,the width of the martensite laths increased,and M₂₃C₆ carbides were observed between the martensitic laths and at the original austenite grain boundaries.The average short-axis size of the carbides in QCT600,QCT650 and sample E?were 49 nm,69 nm,and 71 nm,and the average long-axis size of QCT600,QCT650 and sample E?were 104 nm,120 nm,and80 nm.3.The pitting corrosion potential of quench(Q),quench+cryogenic(QC)treatment and tempering temperature at 500?after once cryogenic treatment were all higher than 0.2 V.The pitting corrosion potential of the test steel for once cryogenic treatment decreased with the increased of tempering temperatures,tempering temperature at 650?and 700?after once cryogenic treatment were 0.05V and 0.08 V.Although the pitting corrosion potential of the test steel after tempering temperature at 700?for different times after once cryogenic treatment decreased with the extended of the tempering times,and the lowest was about 0.14 V when tempering for 1 h.After the twice cryogenic treatment,the pitting corrosion potential of the samples were between 0.175?0.22 V,the pitting corrosion potential difference was within 0.05 V.The pitting potential of the test steel after the twice cryogenic treatment is higher than once cryogenic treatment.4.There is a Cr-depleted zone of about 25 nm around M₂₃C₆ carbide in QCT650,the existence of Cr-depleted zones intensifies the generation of pitting corrosion.The pitting resistance of 15Cr SMSS is mainly determined by the amount of carbides in the matrix and the size of carbides.The pitting resistance of twice cryogenic treatment is improved compared with once cryogenic treatment.