Study Of Mechanical Properties And Corrosion Resistance Of Ferritic Stainless Steel For Automobile Exhaust System

Facing the global crisis in nickel supply, ferritic stainless steel saving nickel are more advantageous than its counterparts-austenitic stainless steel containing nickle. The ferritic stainless steel has been taken place of carbon steels and has been used extensively in automobile exhaust system over the last decades because of its excellent high temperature properties and lower thermal expansion coefficient. The strength, thermal fatigue and corrosion problems of ferritic stainless steel is inevitable when the material is producted and used. This paper is to evaluate the room temperature strength, thermal fatigue, pitting corrosion resistance and intergranular corrosion resistance of ferritic stainless steel prepared in the lab, which is very important to promote the development of products and application for the resource-saving ferritic stainless steel applied in automobile exhaust system.After annealing at1100℃of cold deformed ferritic stainless steel, with prolonging the annealing time, the tensile strength and the yield strength decrease. The elongation presents no significant change with increasing the annealing time. The fracture surfaces present ductile fracture.Almost all the primary fatigue cracks of ferritic stainless steel are initiated at the tips of the V-notches. The inclusions along the grain boundaries become the prior initiation of the fatigue cracks. With increasing the experimental temperature, the length and propagated rate of cracks increase. At high temperature, crack propagation is an oxidation dominant process.The anodic polarization and electrochemical impedance system are used to evaluate the pitting corrosion resistance of ferritic stainless steel. With increasing the cold deformation grade, the pitting corrosion resistance of the steel firstly decreases and then increases. The grain size of ferritic stainless steel with60%cold deformation grade increases with increasing the annealing time at1100℃. The pitting corrosion resistance of ferritic stainless steel decreases with prolonging the annealing time. The effect of chloride ions concentration of solution on the ferritic stainless steel with60%cold deformation grade annealing at1100℃for5minutes shows that the chloride ions concentration of solution has an obvious effect on the steel, and the pitting corrosion resistance of the steel decreases with increasing the chloride ions concentration of solution. The effect of the pH value ranging from5.61to7on the pitting corrosion resistance of the steel is not remarkable. However, the obvious effect of the pH value ranging from1to5.61on the pitting corrosion and the pitting corrosion resistance of the steel decreases remarkably with decreasing the pH value of the solution.The Fe-Cr-Nb-Mo ferritic stainless steel for capacitance measurement and Mott-Schttky analysis is60%cold deformed and annealed at1100℃for5minutes. The results show that the passive films formed on steel in the solution with different chloride ions concentration present n-type semiconducting properties. With increasing the chloride ions concentration of the solution, the doping densities ND, and the flat bland potential EFB increase, and the thickness of space charge layer W decreases. Thus the stability of the passive films formed on the steel decreases which results in the deterioration of the pitting corrosion resistance. The susceptibility of pitting corrosion of the ferritic stainless steel and semiconducting properties of passive films formed on the steel is closely interrelated.Electrochemical potentiokinetic reactivation (EPR) test is conducted to examine intergranular corrosion of the ferritic stainless steel. With increasing the sensitization temperature, the values of Ir/Ia and Qr/Qa firstly increase then decrease, and arrive at the maximum values at700℃. After sensitizing from400℃to700℃, the size and content of grain boundary precipitates increase. The precipitation is identified as Nb(C, N) with partial clustering of Cr which results in the depletion of Cr in the grain boundary and brings obvious intergranular corrosion susceptibility. With the sensitizing treatment from700℃to900℃, the precipitates uniformly distributes in the grain boundaries. The critical Ir/Ia value is determined to be above3%to measure the occurrence of intergranular corrosion.With increasing the sensitization time, the values of Ir/Ia and Qr/Qa increase. The reason is that the size and content of precipitates along grain boundaries increase with the sensitization time increasing. The precipitation is Nb(C, N) with partial clustering of Cr which results in the depletion of Cr in the grain boundary and brings obvious intergranular corrosion susceptibility for the steel. Intergranular corrosion sensitive properties of the ferritic stainless steel can be regarded as the combined action of the metastable precipitation phase and Cr depletion along the grain boundaries.