Effect of quenching and partitioning on the microstructure evolution and electrochemical properties of a martensitic stainless steel

Microstructure-property relationships of 420 stainless steel fabricated by laser-powder bed fusion

Fabrication and characterization of microstructure of stainless steel matrix composites containing up to 25 vol% NbC

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).

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

The aim of this work is to study the effects of austenitizing temperatures on the microstructure and electrochemical properties of a typical type of martensitic stainless steel (MSS) in 3.5 wt% NaCl aqueous solution. The microstructures of the experimental MSSs in different heat treatment states, such as annealed and quenched after different austenitizing temperatures, were characterized using SEM–EDS and XRD analysis. Electrochemical techniques, including potentiodynamic polarization and electrochemical impedance spectroscopy were also used to analyze the electrochemical behavior of the studied material in 3.5 wt% NaCl aqueous solution. The results show that the Cr-rich M23C6 carbides were precipitated during annealing, and then dissolved into the steel matrix during austenitization. An increase in the austenitizing temperature can reduce the amount of Cr-rich M23C6 carbides in the quenched steels, improving the homogeneity of Cr distribution. The pitting potential of the quenched samples increased with the austenitizing temperature, and the passive films that formed on the samples austenitized at higher temperatures were more protective against chloride corrosion.

Effects of austenitizing temperature on the microstructure and electrochemical behavior of a martensitic stainless steel

The martensitic stainless steel type AISI 420 is widely used for applications like cutlery, plastic molds, structural parts and medical devices [1]. This grade of steel is particularly important, because it is suitable to hardening after heat treatment such as quenching and tempering. After hardening it is possible to combine high strength, toughness and corrosion resistance. Regarding corrosion resistance it is well known that a minimum of 11% of chromium is necessary to attain corrosion resistance by the formation of the native protective oxide film [2], and for the martensitic grade the chromium must be dissolved into the matrix. Therefore, the corrosion resistance of martensitic stainless steels grade is sensitive to the carbide volume fraction dissolved on matrix after austenitizing for quenching and is close related to the carbide precipitation during tempering [3, 4]. Under such considerations, the heat treatment is an important processing step to control the corrosion resistance of this steel. Taking into account the importance on combining high strength and corrosion resistance, the present work present results concerning a detailed study on the influence of the hardening and tempering heat treatment cycles on the corrosion resistance of the martensitic stainless steel type AISI 420. The material used as reference was received as annealed bar with a ferritic matrix containing M23C6 chromium carbides with a homogeneous dispersion, as expected from the phase equilibrium [5]. The annealed state is considered here as reference for the highest volume fraction of chromium carbides. To study the corrosion resistance, the secondary M23C6 carbide fraction was varied, by dissolution, using oil quenching from austenitizing temperatures ranging from 900 ◦C to 1100 ◦C, for 1 h. Additionally tempering treatments were performed to study the influence of the carbide precipitation. The heat treatment response was evaluated by Rockwell C hardness. The corrosion resistance was evaluated by mass loss, by unit area, using a 0.5 M H2SO4 solution at room temperature. These tests were carried out between 10 to 180 min, and after the experiments the corroded surfaces were examined at a stereomicroscope. The influence of the austenitizing temperature on hardness after quenching is shown on Fig. 1. The hardness increases when the temperature raises up to 1050 ◦C and lowers for 1100 ◦C. The hardness increase is a consequence of the M23C6 carbide dissolution that increases the carbon supersaturation and the lattice distortion of the martensite [6]. The retained austenite fraction at 1100 ◦C is high enough to decrease the as quenched hardness [7]. Fig. 2 shows that the corrosion resistance is strongly influenced by the austenitizing temperature, and, therefore, by the carbide volume fraction. There is a decrease of the corrosion resistance with the increase of the austenitizing temperature up to 1075 ◦C for the temperature of 1100 ◦C the corrosion resistance increase. Considering that the corrosion resistance should enhance with the increase of the chromium content dissolved into the ferritic matrix, this is an unexpected result. Therefore, there must be another mechanism controlling the corrosion resistance that superimposes the beneficial aspect of the carbide dissolution. This behavior must be explained as a consequence of the increase of the internal martensite lattice stresses [8]

A. F. Candelária

Papers

Influence of the heat treatment on the corrosion resistance of the martensitic stainless steel type AISI 420