| Low carbon martensitic stainless steel is widely used in machinery industry for its high strength,good corrosion resistance and cavitation resistance as well as good welding performance,such as the manufacture of hydraulic turbine runners,compressor impellers and petrochemical high-pressure pipelines and other key components.With the expansion of engineering fields such as oil and gas exploration into environmentally harsh areas,the service environment faced by low carbon martensitic stainless steels has become more sophisticated and tough,which makes the service performance requirements of low carbon martensitic stainless steels become more demanding.In order to further improve the service performance of the steel.In this paper,three low carbon martensitic stainless steels without Cu and with 1%and 3%Cu addition,named as 0Cu steel,1Cu steel and 3Cu steel,were designed for the study of low carbon martensitic stainless steel 0Cr13Ni4Mo.Cu precipitation phase and different content of the reversed austenite were obtained through adjustment the heat treatment process and using various techniques such as SEM,XRD,TEM,APT and potentiodynamic polarization,electrochemical impedance spectroscopy,electrochemical testing and other means were used to study the effect of Cu precipitation phase,reversed austenite and the combined action of the two phases on the corrosion resistance of this stainless steel in 3.5%Na Cl solution,respectively.The corrosion behavior of Cu precipitated phase and reversed austenite and the related corrosion mechanism were discussed.The main research contents and conclusions are as follows:（1）The organization of 0Cr13Ni4Mo steel with 3%Cu（3Cu）and without Cu（0Cu）after different heat treatments and the corrosion performance were compared and analyzed to investigate the effect of Cu precipitation on the corrosion performance of the stainless steel.The results showed that after solid solution treatment at 1050°C,Cu is uniformly solidly dissolved into the slatted martensitic matrix;after tempering at 400°C,Cu forms very small nanoclusters,which are enriched with a large number of Fe atoms;after tempering at 500°C,the Cu nanoclusters grow into Cu-rich precipitated phases with a size of 5-10 nm,with Cu atoms mainly segregated at the core and in a coherent relationship with the matrix,while the carbides grow from the Fe-rich nanoclusters into Cr-rich precipitated phases.3Cu low-carbon martensitic stainless steel exhibits excellent corrosion resistance after tempering at 500°C,which has the smallest icorrof 42.3n A·cm2,Eb of 0.109 V and Ecorr of-0.147 V,the largest capacitive arc radius value,the highest Rf of 48.35 kΩ·cm2 and Rct of 447.2Ω·cm2.Therefore,it has the optimal corrosion resistance.This is because the growth of Cu-rich precipitated phases is accompanied by the discharge of Cr atoms to the surrounding matrix,reducing the Cr-depleted caused by the Cr-rich carbides,thus reducing the corrosion susceptibility of 3Cu low carbon martensitic stainless steel.（2）Microstructures without/with reversed austenite were obtained by tempering 0Cu steel at 500°C and 600°C,respectively,and the effect of reversed austenite on the corrosion behavior of stainless steel was investigated.The results showed that the potentiodynamic polarization curves of 0Cu steels tempered at600°C exhibited a more positive Ecorr of-0.170 V,a minimum icorr of 50 n A·cm2and a higher Eb of 0.122 V.The electrochemical impedance spectra showed better uniform corrosion resistance and surface passivation film stability,and0Cr13Ni4Mo steels with reversed austenite in the microstructure showed better corrosion resistance.This is due to the reversed austenite always precipitates in the vicinity of the carbides,compensate for the carbide-depleted Cr zone,thus improving the corrosion resistance of stainless steel.（3）0Cu steel,1Cu steel and 3Cu steel were subjected to two-stage tempering treatment at 640°C+600°C to obtain a composite organization of Cu precipitation phase and reversed austenite,and the effect of the joint action of the two phase on the corrosion behavior of the stainless steel was investigated.The results show that the content of reversed austenite increases with the increase of Cu content,3Cu low carbon martensitic stainless steel exhibits excellent corrosion resistance after two-stage tempering treatment to obtain a maximum reversed austenite content of 10.9%.This may be due to the fact that the organization of the reversed austenite formed during tempering to encapsulate the crystal structure fcc ofε-Cu precipitated phase to reduces electrochemical corrosion caused by galvanic effect,thus enhancing the corrosion resistance of the stainless steel overall. |
