Relationship between Microstructure and Corrosion Behavior of Martensitic High Nitrogen Stainless Steel 30Cr15Mo1N at Different Austenitizing Temperatures

Abstract

The relationship between microstructure and corrosion behavior of martensitic high nitrogen stainless steel 30Cr15Mo1N at different austenitizing temperatures was investigated by microscopy observation, electrochemical measurement, X-ray photoelectron spectroscopy analysis and immersion testing. The results indicated that finer Cr-rich M2N dispersed more homogeneously than coarse M23C6, and the fractions of M23C6 and M2N both decreased with increasing austenitizing temperature. The Cr-depleted zone around M23C6 was wider and its minimum Cr concentration was lower than M2N. The metastable pits initiated preferentially around coarse M23C6 which induced severer Cr-depletion, and the pit growth followed the power law. The increasing of austenitizing temperature induced fewer metastable pit initiation sites, more uniform element distribution and higher contents of Cr, Mo and N in the matrix. In addition, the passive film thickened and Cr2O3, Cr3+ and CrN enriched with increasing austenitizing temperature, which enhanced the stability of the passive film and repassivation ability of pits. Therefore, as austenitizing temperature increased, the metastable and stable pitting potentials increased and pit growth rate decreased, revealing less susceptible metastable pit initiation, larger repassivation tendency and higher corrosion resistance. The determining factor of pitting potentials could be divided into three stages: dissolution of M23C6 (below 1000 °C), dissolution of M2N (from 1000 to 1050 °C) and existence of a few undissolved precipitates and non-metallic inclusions (above 1050 °C).

Influence of nitrogen on corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy

“…These meant that nitrogen promoted the formation of stable passive films [32] (Ecorr) of the 0N steel was distinctly lower than those of nitrogen-alloyed steels, which was in accordance with the variation of OCPs. In addition, the current transients relating to metastable pitting and repassivation [20,31,33] could be observed in the passive ranges. The addition of nitrogen significantly reduced the current fluctuations, i.e., reduced the metastable pitting susceptibility [27,34].…”

“…The high nitrogen martensitic stainless steels with different nitrogen contents were smelted under high nitrogen pressure, and the smelting, homogenization, hot forging and spheroidizing annealing processes were described in our previous work [20]. The chemical compositions of high nitrogen martensitic stainless steels are shown in Table 1.…”

“…To determine the prior austenite grain size, the specimens were etched in the solution containing 3 g KMnO4, 6 mL H2SO4 and 94 mL H2O, and then observed using DSX510 optical microscope (OM). To observe the distribution of precipitates, the specimens were etched using Vilella's reagent [20], and then analysed using Ultra Plus field emission scanning electron microscope (FE-SEM) equipped with energy dispersive spectroscope (EDS). To reveal the existing form of nitrogen, the atomic-scale chemical analysis of 0.41N steel was employed using CAMECA Instruments LEAP 4000 XHR.…”

“…The current and temperature were maintained around 1.0 A and 5℃, respectively, and the electrolytic dissolution of each specimen lasted for about 12 h. Then the X-ray diffraction (XRD) was performed on a Rigaku SmartLab 9kW X-ray diffractometer with Cu Kα radiation at 45 kV, 200 mA and 10°/min from 20° to 90°. Meanwhile, the precipitates in 0N and 0.41N steels were examined using FEI Tecnai F30 high resolution transmission electron microscope (HR-TEM), and the HR-TEM foils were prepared as described previously [20].…”

“…To some degree, such low solubility limits the improvement of corrosion resistance by the addition of nitrogen. The development of pressurized metallurgy, namely smelting and solidifying under high pressure, makes it possible to fully play the beneficial role of nitrogen on corrosion resistance of MSSs by effectively enhancing its content without inducing nitrogen pores simultaneously [18][19][20]. For example, high nitrogen martensitic stainless steel CRONIDUR30, manufactured by pressurized electroslag remelting (PESR), possesses excellent mechanical properties (hardness, wear-resistance, toughness) and superior corrosion resistance, which has been successfully used to ball screw bearings and nuts for aerospace and industrial applications [19,21].…”