Sigma Phase Precipitation Research Articles

Sprememba strukture in mehanskih lastnosti austenitnih jekel zaradi izločanja sigma faze pri visokih temperaturah

Sprememba strukture in mehanskih lastnosti austenitnih jekel zaradi izločanja sigma faze pri visokih temperaturah

Effect of Sigma Phase Precipitation on the Pitting Corrosion Mechanism of Duplex Stainless Steels

Effect of Sigma Phase Precipitation on the Pitting Corrosion Mechanism of Duplex Stainless Steels

Microstructural evolution and mechanical properties of 27Cr-4Mo-2Ni ferritic stainless steel during isothermal aging

The microstructural evolution and its effect on mechanical properties of 27Cr-4Mo-2Ni super ferritic stainless steels during intermediate temperature aging treatment were investigated. Experimental results demonstrated that Laves phase started to nucleate at dislocations and sub-grain boundaries before chi and sigma phase, while the chi phase and sigma phase precipitated at grain boundaries. Laves phase was partially dissolved into the matrix to provide additional Cr and Mo atoms for the rapid coarsening of sigma phase with increasing aging time during 800 °C aging treatment. Meanwhile, the average grain size increased from ~ 68 to ~ 111 µm. Mechanical properties such as RT impact toughness, tensile properties and micro hardness were significantly influenced by the brittle intermetallic and grain coarsening. When the alloy was aged at 800 °C, the rate of microhardness increase was accelerated by the formation and coarsening of sigma phase, and the value of Vickers hardness was positively correlated with the volume fraction of sigma phase. Impact toughness was much more sensitive to brittle precipitates than both the tensile properties and hardness. Precipitation of sigma phase induced brittle fracture during impact testing, especially in the situation where the grain boundaries were completely covered by sigma phase.

Interpretation for the fast sigma phase precipitation in the high intensity shot peened nanocrystallined Super304H stainless steel

Surface shot peening (SP) with an intensity of 0.5 MPa/12 min plus 650 °C aging was performed on the Super304H austenitic stainless steel with an attempt to decrease its intergranular corrosion susceptibility more effectively. It was found that the local stress/strain concentration with higher energy at some typical sites caused by over saturated cold deformation induced the early nucleation of sigma phase and the enhanced chromium diffusion in nanocrystallized microstructure promoted its fast growth in the material. The critical shot peening time to trigger fast precipitation of sigma phase is between 8 and 12 min under 0.5 MPa for the Super304H steel. The occurrence of massive sigma phase during aging brought a positive move of the reactivation potential of chromium depleted zones around sigma phase compared to M23C6.

STUDY OF EMBRITTLEMENT IN AUSTENITIC STAINLESS STEEL AISI 304H WITH 15 YEARS IN SERVICE EXPOSED HIGH TEMPERATURE

The present study is aimed at obtaining a model to predict the impact tenacity of austenitic stainless steel AISI 304H exposed to high temperature, in order to define the degree of embrittlement resulting from its metallurgical degradation due to Sigma/Chi phase precipitation. The element studied corresponds to a pipe of 60 inches of diameter associated to the top system of a regenerator that operates continuously at 1300 °F in a Catalytic Cracking plant of an Oil Refinery with 15 years in service. Twelve samples were taken along the studied pipe, taking a sample for each component of the pipeline, a sample for each mitred elbow and a sample for each straight section, of which 3 test-pieces were made per sample, for a universe of 36 test-pieces, complying with the dimensions defined in the ASTM E23-90. The chemical composition of the steel was determined by a portable analyzer based on positive identification of materials, the microstructure of the steel was observed by optical microscopy, the precipitate was sized to determine the shape factor and analyze its grouping, as well as the quantification of the Sigma phase precipitation using a digital image analyzer. The Vickers hardness measurement was performed on site. It was established that knowing the amount and shape of the precipitate, its grouping and hardness, the impact toughness of stainless steel is predicted, which provides an important tool for the study of mechanical integrity of components made of stainless steel AISI 304H exposed to operation continues at elevated temperature. The metallurgical degradation of stainless steel due to Sigma/Chi phase precipitation presents a logarithmic behavior indicating that the impact tenacity presents a sharp decrease in function of the acceleration of the deterioration, that is to say, it loses the ability to plastical deformation and therefore a high probability of occurrence of a fragile failure in operationally critical systems, therefore it should be monitored routinely and rigorously in the industrial field, highlighting its importance of monitoring through an "in situ" methodology based on non-destructive tests. Key Words: stainless steel, 304h grade, Sigma phases, intermetallic phase, embrittlement, impact toughness

Effect of Sigma Phase Morphology on the Degradation of Properties in a Super Duplex Stainless Steel.

Sigma phase is commonly considered to be the most deleterious secondary phase precipitating in duplex stainless steels, as it results in an extreme reduction of corrosion resistance and toughness. Previous studies have mainly focused on the kinetics of sigma phase precipitation and influences on properties and only a few works have studied the morphology of sigma phase and its influences on material properties. Therefore, the influence of sigma phase morphology on the degradation of corrosion resistance and mechanical properties of 2507 super duplex stainless steel (SDSS) was studied after 10 h of arc heat treatment using optical and scanning electron microscopy, electron backscattered diffraction analysis, corrosion testing, and thermodynamic calculations. A stationary arc was applied on the 2507 SDSS disc mounted on a water-cooled chamber, producing a steady-state temperature gradient covering the entire temperature range from room temperature to the melting point. Sigma phase was the major intermetallic precipitating between 630 °C and 1010 °C and its morphology changed from blocky to fine coral-shaped with decreasing aging temperature. At the same time, the average thickness of the precipitates decreased from 2.9 µm to 0.5 µm. The chemical composition of sigma was similar to that predicted by thermodynamic calculations when formed at 800–900 °C, but deviated at higher and lower temperatures. The formation of blocky sigma phase introduced local strain in the bulk of the primary austenite grains. However, the local strain was most pronounced in the secondary austenite grains next to the coral-shaped sigma phase precipitating at lower temperatures. Microstructures with blocky and coral-shaped sigma phase particles were prone to develop microscale cracks and local corrosion, respectively. Local corrosion occurred primarily in ferrite and in secondary austenite, which was predicted by thermodynamic calculations to have a low pitting resistance equivalent. To conclude, the influence of sigma phase morphology on the degradation of properties was summarized in two diagrams as functions of the level of static load and the severity of the corrosive environment.

Finite Element Wear Behaviour Modeling of Super duplex stainless steel AISI 2507 Using Ansys

In this paper the finite element simulation approach has been employed for the work that has been recently carried out by Davanageri et al.(2017). The super duplex stainless steel AISI 2507 material was heat treated at 850°C with different ageing time (30, 60, 90 minutes), followed by water quench. The heat treatment was carried out to encourage the precipitation of inter-metallic secondary sigma phase (σ). The study of dry sliding wear behavior was carried out with the pin-on-disc equipment. The heat treatment and ageing time increases the hardness of the duplex steel, resulting in improved wear resistance. The Archard wear model and finite element software ANSYS WORKBENCH-16 was used to determine the wear volume loss. The specific wear rate or dimensional wear coefficient is the most significant factor in the wear volume calculation and it varies with material and operating parameters. The study shows that frictional coefficient varies with material properties (hardness) at similar operating condition. Finally the results reveal that there is a good agreement that exists between the simulated (FE) values and those of the experimental values.

Effect of Cold Rolling on the Transformation Kinetics of Sigma- and Chi- Phases in UNS S32205 Duplex Stainless Steel

Abstract Duplex stainless steels have very good properties including good ductility, moderate toughness and good stress corrosion cracking resistance. These properties can be severely affected by the precipitation of sigma (σ) and chi (χ) phases, in particular ductility and toughness. The effect of cold deformation on the isothermal transformation kinetics of UNS S32205 steel was investigated. The steel was cold rolled up to 64 % reduction in thickness and annealed at 850 °C from 10 min up to 4320 min (3 days). Microstructural characterization used light optical and scanning electron microscopy, energy dispersive spectroscopy, and magnetic measurements. Formation of intermetallics is much faster in the cold-rolled samples. After 4320 min, samples show the same amounts of each phase for all rolling strains. Results have been validated by thermodynamic calculations.

Modifying transformation pathways in high entropy alloys or complex concentrated alloys via thermo-mechanical processing

Often the experimentally-observed, single-phase high entropy alloy (HEA) is the result of second-phase precipitation constrained by thermodynamic and kinetic factors. Using Al0.3CoCrFeNi as a candidate HEA, this paper demonstrates the strong influence of thermo-mechanical processing on the transformation pathway adopted for isothermal second-phase precipitation. A traditional thermo-mechanical processing route comprised of homogenization cold-rolling solution treatment in the single fcc phase region, followed by a precipitation anneal at a lower temperature, results in a homogeneous distribution of nanometer scale-ordered L12 (gamma prime-like) precipitates within the fcc matrix. In contrast, if cold-rolling is followed directly by annealing at the precipitation temperature, then the resulting microstructural evolution pathway changes completely, with concurrent recrystallization of the matrix fcc grains and precipitation of B2 and sigma phases, largely at the grain boundaries. These experimentally observed variations in transformation pathway have been rationalized via the competition between the thermodynamic driving force and activation barrier for second-phase nucleation in this alloy, coupled with the kinetics of the process. The microstructural variations that result from these dramatically different phase transformation pathways can lead to some rather exceptional mechanical properties that can be varied over a large range even for a single Al0.3CoCrFeNi HEA composition.

Effect of ageing time on microstructure and mechanical properties of SAF 2205 duplex stainless steel

Purpose: SAF 2205 duplex stainless steels (DSSs) are materials characterized by a favourable combination of the properties of ferritic and austenitic stainless steels. This type of stainless steel presents good weldability, corrosion resistance especially for stress corrosion cracking (SCC). However, this steel presents an unavoidable disadvantage that is its potential microstructural instability. Although duplex stainless steels design idea is to present two main types of microstructure, other phases and carbides or nitrides can precipitate. In the case of DSS SAF 2205, in addition to austenitic and ferritic microstructure, during heat treatment processing, welding or use may occur precipitation of undesirable intermetallic phases such as chi, Widmanstätten austenite, sigma besides carbides and nitrides. The precipitation of s-phase is associated with effects that cause both reduction of toughness and decreases the corrosion resistance on austenitic, ferritic and duplex stainless steels. Design/methodology/approach: This study evaluated the aging treatment effect on hardness, impact toughness and ferrite content of a SAF 2205 duplex stainless steel. Samples were solubilized at 1150°C, quenched in water and aged at 850°C during 1, 5, 10, 30, 60 or 180 minutes. After aging, cooling was to room temperature in air. Findings: Aging time promoted s-phase precipitation and hardness increase. Hardness and ferrite volume measurements, microscopy and the prediction of sigma phase bases the discussion. Impact toughness decreased with time aging and intermetallic phase precipitation. Research limitations/implications: As future work could be performed some corrosion test, vary the cooling rate after aging, and using other techniques to identify phases. Focus the research at lower aging times to try the describe Cr partitioning process to form sigma phase. Practical implications: High aging time should be avoided for SAF 2205 DSS. Originality/value: Usually sigma-phase precipitation on DDS is correlated to welding process. This paper correlates it to aging heat treatment.

Effect of sigma phase precipitation on microstructure and properties of cast ZG0Cr26Ni5Mo3Cu3 duplex stainless steel under different heat treatments

ZG0Cr26Ni5Mo3Cu3 Duplex Stainless Steel (DSS) was solution treated at 1,060 ºC for 3 h, followed by water cooling. Tempering treatments were conducted at 720, 750 and 780 ºC, respectively, for 16 h, followed by air cooling. The microstructures of ZG0Cr26Ni5Mo3Cu3 duplex stainless steel samples treated at different tempering temperatures were observed by scanning electron microscope (SEM) and energy dispersal spectroscopy (EDS), and the phase consitutions were analyzed using X-ray diffraction (XRD). The effects of the precipitation of sigma (σ) phase on the duplex phase percentage, hardness, impact toughness and corrosion resistance of the DSS were studied. Results showed that microstructures of ZG0Cr26Ni5Mo3Cu3 after solution treatment consists of ferrite (α) phase and austenite (γ) phase; after being tempered at different temperatures, σ phase appeared due to a eutectoid-type reaction of α→σ+γ2 during tempering treatment. It was observed that σ phase distributed along the grain boundary. The volume fraction of σ and γ phases increased with increasing tempering temperature in the range of 720 to 780 ºC, whereas the volume fraction of α phase showed the opposite trend. When the percentage of σ phase increased, the hardness of steel also increased. In the solution treated steel, hardness was measured to be only 244.0 HB, because σ phase did not appear. However, it increased to 391.8 HB when the DSS was tempered at 780 ºC because a great of deal of σ phase appeared. The impact toughness and corrosion resistance of DSS decreased when the percentage of σ phase increased.

Pitting and stress corrosion cracking studies on AISI type 316N stainless steel weldments

Abstract Pitting and stress corrosion cracking (SCC) studies on the 316N stainless steel (SS) weldments in as-welded (AW) and thermally aged (solution annealing (SA), 550 °C/4 h and 750 °C/1 h) conditions were carried out in acidic and acidic chloride media. Pitting corrosion and SCC resistance was the highest for SA weldment, which was attributed to homogenization of microstructural and microchemical heterogeneities during solution annealing. SA specimen showed the most stable passive film as compared to AW weldment because of higher amount of delta-ferrite in AW weld metal, which resulted in an increased heterogeneity and inferior SCC resistance in AW. Stability of passive film was found to be adversely affected due to heat treatments (at 550 °C/4 h and 750 °C/1 h) because of the precipitation of carbide and sigma phases.

Correlation between process parameters, microstructure and properties of 316 L stainless steel processed by selective laser melting

High-density 316 L specimens were fabricated by selective laser melting (SLM). Different processing parameters, including laser power (100, 200 W) and scanning strategies (alternating stripes without and with re-melting after every layer) were employed to evaluate their impact on microstructure and texture of the specimens. Microstructures of the specimens in as-built condition were characterised by columnar grains of austenite with intercellular segregation of Mo, Cr and Si, resulting in creation of non-equilibrium eutectic ferrite. It was found that laser energy density and scanning strategy strongly affect cellular substructure of austenite and amount of ferrite, as well as kind and degree of texture. Specific microstructure of austenite in as-built condition is the cause of almost double increase of yield strength accompanied by much smaller improvement of ultimate tensile strength and 1.4 times reduction of elongation at fracture in comparison of properties of hot-rolled SS316L sheet. Moreover, features of this substructure determine kind of the changes occurring during stress relieving at 800 °C for 5 h (among others, precipitation of sigma-phase strongly activated by presence of ferrite and residual stresses), demonstrated by decreased yield strength value with no significant changes of ultimate tensile strength and elongation. At the same time, an attempt was made to explain some unclearly interpreted observations in the literature related to a correlation between process parameters, microstructure and properties of SLM-processed steel 316 L.

The sigma phase precipitation during high temperature deformation of the X2CrNiMoN25-7-4 duplex steel

The sigma phase precipitation during high temperature deformation of the X2CrNiMoN25-7-4 duplex steel

The dilatometric technique for studying sigma phase precipitation kinetics in F55 steel grade

Sigma phase precipitation occurring during the exposure of duplex stainless steels in the temperature range from 800 to 900 °C deeply affects the material toughness and corrosion resistance. σ-Phase precipitation process is strongly influenced by many physical parameters, such as the specific chemical composition, the ferrite amount and its average grain size, and the entity of plastic deformation due to the previous technological process. The strong dependencies of σ-phase precipitation on all these factors justify the continuous study of the process kinetics. This paper focuses on the σ-phase precipitation kinetics in F55 steel grade. The investigation has been performed by an innovative experimental method, such as the anisothermal dilatometric technique. The application of the Kissinger’s method has been used for deriving the process activation energy and kinetics. The results have been compared with the ones obtained by metallographic analysis and hardness tests performed on isothermally aged samples, heat-treated in a laboratory furnace at 850 °C.

Influence of Si on sigma phase precipitation and pitting corrosion in superaustenitic stainless steel weld metal

In this study, we investigated the effect of Si on the precipitation of the sigma phase in super austenitic stainless steel welds. The Scheil module solidification, partition coefficient and activity were calculated using the thermo-calc software to study the thermodynamics of the precipitation behavior of the sigma phase during solidification of the weld metal. The fraction and average size of the sigma phases significantly increased with increasing Si content in the welds, which consisted of γ-dendrite and inter-dendritic sigma phases. Scheil solidification showed that the formation temperature range of the sigma phase increased with increasing Si content. Also, partition coefficient of Cr and especially Mo and Si substantially decreased under the L-γ solidification with addition of Si. And it resulted in increase of Cr, Mo and Si contents in the sigma phases. The activity of Mo rose sharply with increasing Si, which caused an increase in precipitation due to the fundamental elements of the sigma phase. Corrosion test results showed that pitting was sensitive to an increase in the Si assisted sigma phase precipitation.

Study of the effect of sigma phase precipitation on the sliding wear and corrosion behaviour of duplex stainless steel AISI 2205

The effects of σ-phase on the corrosion and unidirectional sliding wear behaviour of wrought duplex stainless steel (DSS) 2205 were investigated. The precipitation of σ-phase was induced by an isothermal aging at 850°C during 5, 15, 30 and 60min. Microstructural characterisation shows that the volume of δ-phase decreases as σ-phase increases. Dry sliding wear tests were carried out in a tribometer (model TRB, Anton Paar, Switzerland) with pin on disc technique at 7 and 15cm/s sliding rate with a load of 1N and an α-Al2O3 ball as counterpart. Results shown an important increase on wear resistance under dry sliding conditions after 5min of heat treatment due to the hardness provided by σ-phase on the DSS compared to base metal (BM). Nonetheless, cracks by Hertzian contact stress due to the difference of plastic deformation of matrix and σ-phase were observed at 15cm/s sliding rate in heat treated samples, induced by stress fatigue during the abrasion wear mechanism on the surface. Also there was observed that δ-phase is prone to dissolution in acidic media as σ-phase volume increases. Thus, results shown an improvement on wear resistance on samples aged for 5min in the starting critical intergranular corrosion susceptibility.

The Kinetic diagram of sigma phase and its precipitation hardening effect on 15Cr-2Ni duplex stainless steel

The kinetics of sigma phase precipitation at the temperature range of 750–950°C in a rapidly solidified 15Cr-2Ni-2Al-11Mn resource-saving duplex stainless steel was investigated. After fitting the experimental results with the Avrami equation, the TTT diagram of sigma phase was established. It is found that the precipitation rate of sigma phase maximizes at about 850°C and that the precipitation hardening effect sharply peaks at about 1.5vol% sigma phase content, which is obtained by aging at 850°C for 180min. With the further increase of sigma phase content from 1.5vol%, the strength reduces and the ductility increases again.

Phase-Field Modeling of Sigma-Phase Precipitation in 25Cr7Ni4Mo Duplex Stainless Steel

Phase-field modeling is used to simulate the formation of sigma phase in a model alloy mimicking a commercial super duplex stainless steel (SDSS) alloy, in order to study precipitation and growth of sigma phase under linear continuous cooling. The so-called Warren–Boettinger–McFadden (WBM) model is used to build the basis of the multiphase and multicomponent phase-field model. The thermodynamic inconsistency at the multiple junctions associated with the multiphase formulation of the WBM model is resolved by means of a numerical Cut-off algorithm. To make realistic simulations, all the kinetic and the thermodynamic quantities are derived from the CALPHAD databases at each numerical time step, using Thermo-Calc and TQ-Interface. The credibility of the phase-field model is verified by comparing the results from the phase-field simulations with the corresponding DICTRA simulations and also with the empirical data. 2D phase-field simulations are performed for three different cooling rates in two different initial microstructures. A simple model for the nucleation of sigma phase is also implemented in the first case. Simulation results show that the precipitation of sigma phase is characterized by the accumulation of Cr and Mo at the austenite-ferrite and the ferrite-ferrite boundaries. Moreover, it is observed that a slow cooling rate promotes the growth of sigma phase, while a higher cooling rate restricts it, eventually preserving the duplex structure in the SDSS alloy. Results from the phase-field simulations are also compared quantitatively with the experiments, performed on a commercial 2507 SDSS alloy. It is found that overall, the predicted morphological features of the transformation and the composition profiles show good conformity with the empirical data.

Double Kinetics of Intermetallic Phase Precipitation in UNS S32205 Duplex Stainless Steels Submitted to Isothermal Heat Treatment

One of the most studied deleterious phases in stainless steels is the sigma phase, due to its high potential to decrease the toughness and corrosion resistance of these steels. Eight samples of as-received cold rolled UNS S32205 duplex stainless steel were submitted to isothermal heat treatments at 850 oC during 10, 15, 30, 60, 120, 180, 240, and 300 minutes in order to study the precipitation kinetics of the sigma and chi deleterious phases. Several complementary microstructural analysis techniques were used to determine the volume fraction of the intermetallic phases, including optical microscopy and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Evaluation was made of the kinetics of isothermal formation of sigma and chi phases in commercial duplex stainless steels (UNS S32205). The results indicated two different mechanisms for sigma phase precipitation: sigma phase formed from the chi phase for shorter isothermal heat treatment times, and sigma phase precipitated at the ferrite-austenite interface for longer isothermal heat treatment times, using a temperature of 850 oC. The phase transformation kinetics determined using the JMA equation indicated that chi phase precipitation caused faster sigma phase formation.