Research Of 445J2 Ultra-pure Ferritic Stainless Steel Stress Corrosion Cracking

In recent years,ferritic stainless steels that do not contain Ni or contain lower amounts of Ni have been developing rapidly due to frequent increases in Ni prices.In addition to being Ni-free,ferritic stainless steels also have remarkable characteristics such as good ductility,very low thermal expansion,high thermal conductivity and excellent corrosion resistance.Today,many ultra-pure ferritic stainless steels such as:445J2 has replaced 316L and other austenitic stainless steels gradually applied in the petroleum,chemical,marine and other fields of heat exchangers and has shown good prospects for application.The use of ultra-pure ferritic stainless steel as a raw material for heat exchangers,not only to take advantage of its excellent characteristics,but also to save Ni resources,so that production costs are reduced.However,although ultra-pure ferritic stainless steel 445J2 is successfully used in the key position of heat exchangers,but the use of various functional heat exchangers in increasingly harsh conditions,so that 445J2 steel in the process of processing and use also faced some problems.In the heat exchanger’s inner barrel and other processing,the inner barrel of the barrel body needs to be flanged with welds,the barrel body and between the barrel body and end caps also need to be welded into shape,so the heat exchanger often produces a number of service problems,concentrated in the weld seams prone to pitting,stress corrosion cracking,etc.Therefore,a systematic study of the stress corrosion cracking behavior of 445J2 ultrapure ferritic stainless steel was carried out in this paper using a slow strain rate tensile test.The effects of different sensitization temperatures(550℃,650℃,750℃),different chloride ion concentrations(1 wt.%,3.5 wt.%,5 wt.%),different solution temperatures(20℃,40℃,60℃)and different pH values(pH=7.6,pH=5.4)on the stress corrosion performance of 445J2 were investigated,and the laws of the stress corrosion behavior of 445J2 stainless steel under different environmental factors causing The main causes of stress corrosion cracking of 445J2stainless steel and its cracking mechanism under different environmental factors were investigated.The main research results obtained are as follows.(1)In the 3.5 wt.%NaCl solution,the stress corrosion cracking susceptibility of 445J2stainless steel showed a trend of increasing and then decreasing as the sensitization temperature increased,i.e.,the stress corrosion cracking of the specimen was greatest at the sensitization temperature of 650℃;with the increase of the solution temperature to 60℃,this trend was more obvious.650℃-60℃specimen stress corrosion sensitivity is the largest,the main reason is:when the solution temperature is higher,Cl~-easier to diffuse through the passivation film into the matrix,making the stainless steel stress corrosion sensitivity increased;sensitization temperature is lower,C atoms first diffuse to the grain boundary,the precipitation at the grain boundary is mainly carbide-rich,sensitization temperature is further increased,the diffusion rate of the elements reached a maximum,Nb and Ti elements have diffused to the grain boundary to replace the position of Cr in the carbide,making the Cr content at the grain boundary increased,thus making the 445J2 stainless steel stress corrosion sensitivity reduced.(2)The stress corrosion cracking susceptibility of 445J2 steel in 5 wt.%NaCl solution at a sensitization temperature of 650°C was the highest,and the correlation between stress corrosion susceptibility and chloride ion concentration was ranked from 5 wt.%>3.5 wt.%>1 wt.%,and the stress corrosion susceptibility increased more obviously as the solution temperature increased to 60°C.The main reason is:Cl~-will destroy the passivation film on the surface of 445J2 stainless steel,generating pitting pits,after the Cl~-concentration increases to5 wt.%,the number of pitting pits will increase,if the specimen is subjected to tensile stress,numerous pitting pits will become the origin of the crack making the specimen undergo stress corrosion cracking,the stress corrosion cracking mechanism in this condition is mainly anodic dissolution type.(3)When the solution pH=5.4,the specimen fracture time is greatly shortened,the fracture morphology has changed,basically showing icing lump and a large number of cracks,grooves,stress corrosion cracking degree is much greater than the specimen at pH=7.6.The main reason may be:with the decrease of pH value,the concentration of H~+in the solution increases,so that a large number of H atoms can enter the interior of the metal matrix,thus leading to a sharp increase in the degree of stress cracking of 445J2 stainless steel,the fracture mode also changed from anodic dissolution to anodic dissolution and hydrogen cracking together.