Effect of Aging Heat Treatment H950 and H1000 on Mechanical and Pitting Corrosion Properties of UNS S46500 Stainless Steel

The effect of aging temperature on mechanical and pitting corrosion properties of UNS S46500 was investigated. Tensile and Hardness tests were carried out and the microstructure was analyzed by optical microscopy, scanning electron microscopy and X-ray diffraction; Thermo-Calc simulations helped in the phase identification. Pitting corrosion properties were investigated in 0.6M NaCl electrolyte with sulfate additions by Potentiodynamic Polarization (PP). Hardness, tensile and yield strength of the UNS S46500 steel after lower aging temperature, 510ºC (H950), are higher than the ones found in the 538ºC (H1000) aged steel.This result is explained by microstructure, X-ray diffraction and Thermo-Calc analysis, which indicated the presence of austenite, chi phase and probably Ni3Ti precipitates finely and uniform distributed throughout the martensite matrix. Pitting corrosion resistance is equivalent in both aging temperatures. The sulfate inhibitor effect on UNS S46500 steel was enhanced for 538ºC condition when the electrolyte reaches 1Cl-:1SO4 2- ratio, which is explained by Ni sulfate adsorption and the amount of interfaces in the microstructure resulting in smaller amount of adsorption sites, such as coarsened Ni3Ti precipitates, smaller fraction of chi phase and recovery of dislocations in martensite structure.本研究探究了时效温度对UNS S46500钢力学性能与点蚀腐蚀性能的影响。实验开展了拉伸试验与硬度测试，并通过光学显微镜、扫描电子显微镜及X射线衍射（X-ray diffraction）对显微组织进行分析；Thermo-Calc模拟辅助完成了物相鉴定。点蚀性能的测试在添加了硫酸盐的0.6M氯化钠电解液中开展，采用动电位极化法（Potentiodynamic Polarization, PP）进行表征。经510℃（H950）时效处理的UNS S46500钢，其硬度、抗拉强度与屈服强度均高于538℃（H1000）时效处理的试样。该结果可通过显微组织、X射线衍射及Thermo-Calc分析得到解释：分析结果显示马氏体基体中均匀弥散分布有奥氏体、χ相（chi phase），以及可能存在的Ni3Ti析出物。两种时效温度下的点蚀耐蚀性能相当。当电解液中Cl⁻与SO₄²⁻的比例达到1:1时，538℃时效处理试样的硫酸盐缓蚀效果更为显著，该现象可通过Ni硫酸盐吸附行为与显微组织界面特征得到解释：此时Ni3Ti析出物发生粗化、χ相占比降低，马氏体结构中的位错发生回复，导致吸附位点数量减少。