Abstract
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.
Highlights
The mechanical properties of precipitation hardening steels can be attributed to the presence of a fine and uniformly distributed intermetallic particles in a martensitic matrix[1]
It is possible to observe that increasing the aging temperature leads to the reduction of the tensile and yield strengths, and a slight tendency to decrease of hardness, while elongation increased
The mechanical tests results showed that the tensile strength and elongation of the material is strongly depend to heat treatment applied
Summary
The mechanical properties of precipitation hardening steels can be attributed to the presence of a fine and uniformly distributed intermetallic particles in a martensitic matrix[1]. The UNS S46500 steel is strengthened by precipitation of Ni3Ti. The UNS S46500 steel is strengthened by precipitation of Ni3Ti It is known 2 that longer aging times and/or higher aging temperatures imply in growth and coarsening this precipitates, leading to mechanical properties reduction. Schnitzer et al.[3] proposed that austenite is formed during the aging of the material, and this austenite is called reverted austenite, which is formed at the same time of precipitates nucleation in the martensitic matrix, i.e. the austenite stabilizer element, Ni, is present in the precipitate nucleation Ni-X and simultaneously there is its diffusion into less ordered regions, e. Schnitzer et al.[3] proposed that austenite is formed during the aging of the material, and this austenite is called reverted austenite, which is formed at the same time of precipitates nucleation in the martensitic matrix, i.e. the austenite stabilizer element, Ni, is present in the precipitate nucleation Ni-X and simultaneously there is its diffusion into less ordered regions, e. g. grain boundaries or martensite lath boundaries, occurring its microsegregation and leading to austenite nucleation
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