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Super martensitic stainless steels are the most useful steels in the oil gas industries and automobile industries due to their excellent mechanical properties and corrosion resistance. This paper focuses on the microstructure and mechanical & metallurgical properties of super martensitic stainless steels. The main objective has been to review the affect of alloy additions and also different heat treatment processes on microstructure and mechanical properties of super martensitic stainless steels. Additionally, difference in properties and microstructure of martensitic stainless steels and super martensitic stainless steels has been discussed.
Phase Transformations in Cast Superaustenitic Stainless Steels
Journal of Materials Engineering and Performance, 2009
The goal of this investigation was to study phase transformations in cast superaustenitic stainless steels. Experiments were performed to determine the phase transformation behavior for alloys CN3MN and CK3MCuN. Samples were taken from keel bars that were heat treated between 1160 and 1230 °C and then isothermally held for times ranging from 1 min to 2040 h at temperatures in the range of 700-900 °C. The resulting microstructures were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy and wavelength dispersive spectroscopy. The microstructures, composed primarily of sigma-and Laves-phases within a purely austenitic matrix, showed relatively slow transformation kinetics, with transformation not completed even after the longest anneals. SEM and TEM analysis of superaustenitic samples reveal that precipitation takes place initially on grain boundaries and proceeds intragranularly. The results of these SEM and TEM investigations, along with volume fractions and number densities as a function of time and temperature, will be presented and discussed.
Effect of Heat Treatment on Reversed Austenite in Cr15 Super Martensitic Stainless Steel
The effect of different heat treatments on the reversed austenite in Cr15 super martensitic stainless steel was investigated. The experimental results indicate that the microstructure of the steel is composed of tempered mar-tensite and diffused reversed austenite after quenching at 1050 *C and tempering from 550 to 750 'C. The volume fraction and size of reversed austenite increase with increasing tempering temperature and both of them reach the maximum value at 700 'C. The volume fraction and size of reversed austenite decrease when the temperature is above 700 ' C. The transmission electron microscope (TEM) results indicate that the orientation relationship between tempered martensite and reversed austenite belongs to Kurdjmov-Sach (K-S) relationship.
Tempering of a martensitic stainless steel: Investigation by in situ synchrotron X-ray diffraction
Tempering of a martensitic stainless steel is investigated by means of in situ X-ray diffraction using high-energy synchrotron radiation. The simultaneous evolutions of the fractions of martensite/ferrite, retained austenite, M 23 C 6 and M 2 X (X = N,C) precipitates are determined during a continuous heating, and show that precipitation of M 2 X can be observed directly only above 650°C, concomitantly to the transformation of austenite into ferrite. However, a careful cross-analysis of the evolution of the lattice parameters of all phases shows the precipitation of metastable carbides/nitrides at low temperatures, followed by some stress relaxation. Between 500 and 650°C, the lattice parameters of M 2 X feature changes associated with the evolution of their composition, as supported by the analysis of additional isothermal agings, and consistently with the literature. Finally, the complex variations of the lattice parameters of austenite are analyzed thanks to a simple micromechanical model, supporting the previous conclusions and highlighting the subtle interplay between precipitation and stress relaxation.
The second phases in the S31254 super austenitic stainless steel is easily precipitated, and the solution treatment plays an important role on the microstructure and performance. In this work, the solution treatment is performed on S31254 steel at 1180, 1200, and 1220 C for 1, 2, and 4 h, respectively. Given the grain size and precipitation of second phase, the 1200 C for 4 h or 1220 C for 2 h may be suitable as the solid-solution parameters. The corrosion resistance and impact toughness of the steel after solution treatment at 1200 C are further investigated by means of microstructural analysis and electrochemical experiments. The sample after solution treatment for 4 h, where no second phase can be observed, exhibits a lower corrosion rate and a better mechanical properties, compared to that after annealed for 1 or 2 h. The electrochemical impedance spectroscopy, Mott-Schottky, and XPS results indicate that the passive films of the samples, with a significantly decreased donor and acceptor densities and increased Cr 2 O 3 and Mo-riched oxides, may be responsible for the improvement of corrosion resistance. The impact toughness results show that the samples after solution treatment for 4 h has the highest impact energy among the studied samples.
Phase Transformations on ASTM a 744 Gr. CN3MN Superaustenitic Stainless Steel after Heat Treatment
Defect and Diffusion Forum, 2011
The superaustenitic stainless steel ASTM A 744 Gr. CN3MN (22Cr-25Ni-7Mo-0.2N) has as mainly characteristic high corrosion resistance in severe environment. As the corrosion resistance depends on the microstructure, it was investigated the phase transformations after a solution treatment at 1200°C. Thermocalc calculation for 53Fe-25Ni-22Cr alloy indicates austenitic phase between 1300 and 800°C and austenite + sigma phase below 800°C. The as-cast steel studied presented 2.7 % of precipitates volume fraction and the precipitates were located on the grain boundaries and inside the austenitic grains. X-ray diffraction confirmed the presence of sigma phase in as-cast sample. Scanning electron microscopy showed that the level of Cr and Mo was higher in the precipitates than in the austenitic matrix and the Ni content was higher in matrix compared to precipitates. After heating at 1200°C during 90 minutes, the precipitate volume fraction was reduced to 2.1 % and the grain boundaries precip...
Materials Characterization, 2008
Three austenitic steels (18Cr-8Ni, 18Cr-10Ni, 21Cr-30Ni), used for long-term applications at temperatures between 600 and 800°C were investigated. In the investigation, metallography, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, and scanning electron microscopy were used. In additional to the experimental measurements, thermodynamic predictions were done using the ThermoCalc software and the non-commercial database STEEL16F. Various combinations of M 23 C 6 , sigma, and MC phases were identified in the austenite matrix of these steels. It was confirmed experimentally that extra large particles (up to 10 μm) observed in the 21Cr-30Ni steel are M 23 C 6 , even though this carbide was not predicted as the equilibrium carbide at service temperature (800°C). The analytical-experimental approach, combining thermodynamic predictions and experimental measurements, was found to be reliable for the characterization of austenitic steels.
Materials
A 16Cr5NiMo supermartensitic stainless steel was subjected to different tempering treatments and analyzed by means of permeation tests and slow strain rate tests to investigate the effect of different amounts of retained austenite on its hydrogen embrittlement susceptibility. The 16Cr5NiMo steel class is characterized by a very low carbon content. It is the new variant of 13Cr4Ni. These steels are used in many applications, for example, compressors for sour environments, offshore piping, naval propellers, aircraft components and subsea applications. The typical microstructure is a soft-tempered martensite very close to a body-centered cubic, with a retained austenite fraction and limited δ ferrite phase. Supermartensitic stainless steels have high mechanical properties, together with good weldability and corrosion resistance. The amount of retained austenite is useful to increase low temperature toughness and stress corrosion cracking resistance. Experimental techniques allowed us t...
Journal of Magnetism and Magnetic Materials, 2019
An original feature of this work is the proposal of two equations to fit the volume fraction of ferromagnetic (α + α′) phases that can be applied in the measurement of magnetic saturation of comminuted duplex and superduplex stainless steels (for example, powders or filed chips). Duplex stainless steels contain similar volume fractions of austenite (γ) and ferrite (α) in their microstructure. Two steels exemplify this class, namely the most widely used duplex UNS S31803 and superduplex UNS S32520 stainless steels. The phenomena of work hardening, formation, and reversion of strain-induced martensite (α′) in austenite were compared in both stainless steels. Samples were work-hardened and annealed under identical conditions, and their behavior was evaluated mainly through X-ray diffraction and magnetic measurements. Notably, the volume fraction of strain-induced α′ in duplex stainless steel was as high as 32%, which indicated that this steel had a greater tendency to form α′ than superduplex stainless steel, for which the corresponding value equaled 15%. Annealing at 650°C for 2 h promoted the reversion of strain-induced α′ into γ, decreasing the volume fraction of the former phase from 32 to 2% (duplex) and from 15 to 6% (superduplex).