Sigma phase morphologies in cast and aged super duplex stainless steel

Solution annealed and water quenched duplex and super duplex stainless steels are thermodynamically metastable systems at room temperature.These systems do not migrate spontaneously to a thermodynamically stable condition because an energy barrier separates the metastable and stable states. However, any heat input they receive, for example through isothermal treatment or through prolonged exposure to a voltaic arc in the welding process, cause them to reach a condition of stable equilibrium which, for super duplex stainless steels, means precipitation of intermetallic and carbide phases. These phases include the sigma phase, which is easily identified from its morphology, and its influence on the material's impact strength.The purpose of this work was to ascertain how 2-hour isothermal heat treatments at 920 °C and 980 °C affect the microstructure of ASTM A890/A890M GR 6A super duplex stainless steel. The sigma phase morphologies were found to be influenced by these two aging temperatures, with the material showing a predominantly lacy microstructure when heat treated at 920 °C and block-shaped when heat treated at 980 °C.     

Thermal,magnetic and composition analyses of the reverse transformation of intermetallic sigma phase to ferrite

The reverse transformation of the sigma phase to ferrite in a duplex stainless steel upon heating has been studied by using differential thermal analysis (DTA), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), quantitative metallography and magnetic susceptibility measurement. It has been demonstrated that the reverse transformation of the sigma phase to ferrite is sensitively affected by the morphology of the sigma phase. However, EDS has shown that the composition of the sigma phase does not significantly affect its reverse transformation behaviour. Magnetic susceptibility measurements and TEM have revealed that untransformed, residual ferrite is present even after prolonged annealing between 873 and 1173 K and hence it does play a role in the sigma-to-ferrite reverse transformation. Quantitative metallography was employed to follow the coarsening of the sigma phase upon annealing and DTA was used to estimate its enthalpies of dissolution. As a potential application, a method for temperature monitoring using duplex stainless steels and DTA is suggested.     

The phase stability in Cr–Ni and Cr–Mn duplex stainless steels

The formation of secondary phases during isothermal treatments in the range 750–1000 °C and continuous cooling in 2205, 2507, 2304, and 2101 duplex stainless steels have been investigated. For all the steels herein considered, the Thermocalc calculations indicate the sigma and chi-phase precipitation which is confirmed by the experimental results only for the 2205 and 2507 grades. On the contrary, the secondary phases are very rarely observed after both isothermal aging (up to 750 h) and/or continuous cooling tests in the 2304 and 2101 Cr–Mn grades. This behavior could be justified by the different ferrite and austenite phase stability in the four grades, in the same temperature range of the sigma and chi precipitation, because these differences affect the dangerous phases precipitation mechanism and kinetic.     

Effect of post weld heat treatment on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel weldments

The effect of postweld heat treatment (PWHT) on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel joints with ER316LMn filler material was investigated. PWHT aging was performed for 1 h at four different temperatures of 600 °C, 760 °C, 870 °C and 920 °C, respectively. The microstructure revealed the sigma phase precipitation occurred in the weld metals heat-treated at the temperature of 870 °C and 920 °C. The PWHT temperatures have the less effect on the tensile strength, and the maximum tensile strength of the joints is about 630 MPa, reaching the 95% of the base metal, whereas the elongation is enhanced with the rise of PWHT temperatures. Meanwhile, the sigma phase precipitation in the weld metals reduces the impact toughness.     

Effect of alpha prime due to 475 °C aging on fracture behavior and corrosion resistance of DIN 1.4575 and MA 956 high performance ferritic stainless steels

The 475 °C embrittlement in stainless steels is a well-known phenomenon associated to alpha prime (α′) formed by precipitation or spinodal decomposition. Many doubts still remain on the mechanism of α′ formation and its consequence on deformation and fracture mechanisms and corrosion resistance. In this investigation, the fracture behavior and corrosion resistance of two high performance ferritic stainless steels were investigated: a superferritic DIN 1.4575 and MA 956 superalloy were evaluated. Samples of both stainless steels (SS) were aged at 475 °C for periods varying from 1 to 1,080 h. Their fracture surfaces were observed using scanning electron microscopy (SEM) and the cleavage planes were determined by electron backscattering diffraction (EBSD). Some samples were tested for corrosion resistance using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. Brittle and ductile fractures were observed in both ferritic stainless steels after aging at 475 °C. For aging periods longer than 500 h, the ductile fracture regions completely disappeared. The cleavage plane in the DIN 1.4575 samples aged at 475 °C for 1,080 h was mainly {110}, however the {102}, {314}, and {131} families of planes were also detected. The pitting corrosion resistance decreased with aging at 475 °C. The effect of alpha prime on the corrosion resistance was more significant in the DIN 1.4575 SS comparatively to the Incoloy MA 956.     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.     

Characterization of the intermetallic G-phase in an AISI 329 duplex stainless steel

Duplex austenite–ferrite stainless steels are susceptible to a variety of decomposition processes when aged within the intermediate range of temperatures (250–500 °C). One of these phenomena is the precipitation of the intermetallic G-phase. In the present investigation, the crystal structure and the chemical composition of the G-phase, precipitated in the ferritic phase of an AISI329 duplex stainless steel, is studied by electron microdiffraction and energy dispersive X-ray spectroscopy. It is determined that the space group of the G-phase is F 3 with a lattice parameter four times that of the ferritic matrix. The precipitation mechanism of the G-phase showed a synergetic relation with the ferrite decomposition in Cr-rich and Fe-rich domains. Based on the obtained results, the structural proximity of ferrite matrix and G-phase has been studied. Further analysis allows to suggest that the spinodal decomposition leads to an interdomain of a ferritic structure which is thermodynamically unstable and serves as a precursory site to the development of the G-phase by atomic position readjustments inferior to the atomic distances. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of Hf additions on phase transformation,microstructural stability,and hardness of nanocrystalline 304L stainless steels synthesized by mechanical alloying

304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 °C. The results showed that face-centered cubic (fcc) phase in 304L transformed to body-centered cubic (bcc) phase during MA. The in-situ studies revealed that bcc-to-fcc phase transformation completed after 105 min annealing at 900 °C for 304L, whereas Hf addition increased the required time and temperature for the complete transformation. The grain size of 304L stainless steel was ~10 nm after MA and remained ~167 and ~293 nm after annealing at 900 and 1100 °C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 °C. The hardness of 304L increased from ~200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 °C with Hf addition as opposed to 195 HV hardness of 304L.     

Degradation of mechanical and corrosion resistance properties of AISI 317L steel exposed at 550 °C

The use of austenitic stainless steel type AISI 317L has increased in the last years, in substitution to AISI 316L and other austenitic grades. The higher Mo content (3.0 wt.%. at least) gives higher corrosion resistance to AISI 317L. However, some concern arises when this material is selected to high temperature process services in refineries. Microstructural changes such as chromium carbide precipitation and sigma phase formation may occur in prolonged exposure above 450 °C. In this work, the microstructure evolution of AISI 317L steel during aging at 550 °C was analyzed. Thermodynamic calculations with Thermocalc® and detailed microstructural analysis were performed in steel plate base metal and in weld metal produced by GTAW process. The aging for 200, 300 and 400 h promoted gradual embrittlement and deterioration of corrosion resistance of both weld and base metal. The results show that the selection of AISI 317L steel to services where temperatures can reach 550 °C is not recommended.     

Formation of martensite in 304 grade stainless steels and their welds

The formation of martensite in metastable austenitic stainless steels was investigated. The results showed that the formation of martensite in 304 grade stainless steels due to the exposure to cryogenic temperatures is negligible. The amount of formed martensite is dependent on the chemical composition of the actual heat but for standard grades the amount is not expected to exceed a very few percent. In welds the formation of martensite is not promoted by the presence of δ‐ferrite. The formation of martensite due to cold forming at room temperature can reach around 20 %. Cold forming with subsequent exposure to cryogenic temperatures does not lead to additional formation of martensite due to the exposure to low temperatures. Cold forming at low temperatures leads to the highest amount of martensite formed in metastable stainless steels.     

Investigation on the solution treated behavior of economical 19Cr duplex stainless steels by Mn addition

Effect of Mn on microstructure, mechanical property and pitting corrosion of 19% Cr economical duplex stainless steels with solution temperatures ranging from 1040 to1220 °C has been investigated. The austenite content increases with more Mn addition, but decreases by increasing solution temperature, which can be inferred by trend of partition coefficient K_Mn with solution temperature. Meanwhile, a balanced austenite-ferrite duplex structure of solution-treated specimens was obtained with Mn addition. The impact energy at 20 °C increased with decreasing solution temperature from 1220 °C to 1040 and 1120 °C, and improved by more Mn addition due to more ductile austenite phase formation. These toughness variations were consistent with fracture morphology characteristic changing. The effect of more Mn addition and solution treatment of 1120°Con decreasing of tensile strength and 0.2% offset yield strength were slight. However, the elongation to fracture (%) fell greatly with Mn addition up to 8.1 wt.% for as-rolled and solution treated specimens due to larger deformation strains of austenite than that of ferrite. The decreasing trend of pitting corrosion potential became slower with Mn addition from 3.6 to 8.1 wt.%. The pitting corrosion resistance was lowered by increasing solution temperature due to more weakened repassivation ferrite phase formation.     

Influence of heat treatments on toughness and sensitization of a Ti-alloyed supermartensitic stainless steel

Supermartensitic steels are a new class of martensitic stainless steels developed to obtain higher corrosion resistance and better toughness through the reduction of carbon content, and addition of Ni and Mo. They were developed to more critical applications or to improve the performance obtained with conventional grades AISI 410, 420, and 431. In this study, the influences of the tempering parameters on the microstructure, mechanical properties (hardness and toughness), and sensitization of a Ti-alloyed supermartensitc stainless steel were investigated. The material showed temper embrittlement in the 400–600 °C range, as detected by low temperature (−46 °C) impact tests. The degree of sensitization measured by double loop reactivation potentiodynamic tests increased continuously with the increase of tempering temperature above 400 °C. Healing due to Cr diffusion at high tempering temperatures was not observed. Double tempered specimens showed high amounts (>20%) of reverse austenite but their toughness were similar to specimens single tempered at 625 and 650 °C.     

Phase decomposition of liquid-quenched Al-14 at. % Ag: small-angle X-ray scattering study

Small-angle X-ray scattering was used to study phase decomposition of liquid-quenched Al-14 at. % Ag. Ageing was carried out at temperatures from 50 to 175° C. Though it is clear that ideal linear spinodal behaviour was not obtained, the general trend points to the early unmixing of this alloy as being characterized by spinodal decomposition. The activation energy for the process is that for equilibrium volume diffusion, and it is therefore concluded that quenched-in vacancies play no role in phase decomposition following liquid quenching.     

Aging at 400–600°C of submerged arc welds of 22Cr–3Mo–8Ni duplex stainless steel and its effect on toughness and microstructure

Abstract

Submerged arc welds of a 22Cr–3Mo–8Ni (wt-%) duplex stainless steel were aged in the temperature range 400–600°C to simulate stress relieving conditions of mild steel. Particular attention was paid to the relationship between toughness and microstructure. It was concluded that sufficient toughness could be obtained on aging in the range 500–550°C provided that the aging time did not exceed 10 h. At temperatures >550°C, rather rapid embrittlement occurred as a result of the precipitation of essentially R phase and to some extent π phase. At ~≤500°C, spinodal decomposition of ferrite caused embrittlement, albeit more slowly than the precipitate induced embrittlement. Precipitation of the Mo rich phases Rand π was found to cause depletion of Mo in solid solution in ferrite. This offers a possible explanation for the decrease in pitting corrosion resistance observed in previous work.

MST/1426     

Modeling and experimental study of long term creep damage in austenitic stainless steels

One of the main challenges for some reactors components in austenitic stainless steels at high temperature in-service conditions is the demonstration of their behavior up to 60 years. The creep lifetimes of these stainless steels require on the one hand to carry out very long term creep tests and on the other hand to understand and to model the damage mechanisms in order to propose physically-based predictions toward 60 years of service. Different batches of austenitic stainless steels like-type 316L with low carbon and closely specified nitrogen content, 316L(N), are subjected to numerous creep tests carried out at various stresses and temperatures between 525 °C and 700 °C up to nearly 50 • 10³ h.Interrupted creep tests show an acceleration of the creep deformation only during the last 15% of creep lifetime, which corresponds to macroscopic necking. The modeling of necking using the Norton viscoplastic power-law allows lifetime predictions in fair agreement with experimental data up to a transition time of about ten thousand hours which is temperature dependent. In fact, one experimental result together with literature ones, shows that the extrapolation of the ‘stress–lifetime’ curves obtained at high stress data leads to large overestimations of lifetimes at low stress. After FEG–SEM observations, these overestimates are mainly due to additional intergranular cavitation as often observed in many metallic materials in the long term creep regime. The modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation proposed by Riedel is carried out. For each specimen, ten FEG–SEM images (about 250 observed grains) are analyzed to determine the rate of cavity nucleation assumed to be constant during each creep test in agreement with many literature results. This measured constant rate is the only measured parameter which is used as input of the Riedel damage model. Lifetimes for long term creep are rather fairly well evaluated by the lowest lifetime predicted by the necking model and the Riedel model predictions. This holds for experimental lifetimes up to 200,000 h and for temperatures between 525 °C and 700 °C. A transition time as well as a transition stress is defined by the intersection of the lifetime curves predicted by the necking and Riedel modelings. This corresponds to the change in damage mechanism. The scatter in lifetimes predicted by the Riedel model induced by the uncertainty of some parameter values is less than a factor of three, similar to experimental scatter. This model is also validated for various other austenitic stainless steels such as 304H, 316H, 321H (creep rupture data provided by NIMS). A transition from power-law to viscous creep deformation regime is reported in the literature at 650 °C–700 °C for steel 316H. Taking into account the low stress creep rate law, it allows us to predict lifetimes up to 200,000 h at very high temperature in fair agreement with experimental data.     

Failure Investigation of a Furnace Tube Support

A furnace tube support failed after 6 months service at 850 °C. The support was an HK alloy, a member of the heat-resistant cast alloy family (H-Series) steels. The H-series steels are widely used in the petrochemical industry for components requiring enhanced high-temperature properties. Microstructural changes occurring at high temperature clearly affect the mechanical properties. The property degradation in HK-40 steel furnace tube support subjected to high temperature was caused by the formation of sigma phase. The investigation included metallurgical analysis, materials characterization, and mechanical analysis.     

Some Characteristics of Automotive Exhaust Valve Alloys

The PH stainless steels and the nickel-base superalloys, can be evaluated for exhaust valve applications by considering their metallurgical, environmental and high temperature strength properties. The PH stainless steels are characterized by their good sulfidation and high temperature strength. Good PbO corrosion resistance is achieved with the low silicon, nickel containing alloys. Stable alloys show the greatest high temperature strength which can be improved further by a solution and age treatment. Aging below the optimum temperature of 760°C results in grain boundary sensitization and low impact properties while higher temperatures produce more of the discontinuous phase. The addition of refractory elements can be detrimental to the oxidation resistance of these alloys. The highest elevated temperature strength and best PbO and oxidation resistance is achieved with the nickel-base superalloys. These alloys are completely stable and highest strength at elevated temperatures is achieved with a solution and age treatment. These alloys show lower sulfidation corrosion resistance relative to the PH stainless steels, however this can be improved with higher chromium contents.     

Fe-Cr-Ni系不锈钢在热老化和退火过程中铁素体调幅分解的相场法研究

用基于Cahn-Hilliard方程的相场法研究了Fe-Cr-Ni系不锈钢中的铁素体在热老化和后续退火过程中调幅分解的演化过程，结果表明：在热老化过程中调幅分解生成相连的网络状α'相，调幅分解引起的Cr成分波动的波长和振幅都随着热老化时间的延长而增大；在随后的退火过程中α'相逐渐溶解而Cr成分波动的振幅迅速减小，但是波长继续增大。还讨论了热老化时的调幅分解对铁素体纳米压痕硬度的影响以及退火温度对调幅分解回复(α'相溶解)所需时间的影响，结果表明：铁素体的纳米压痕硬度主要与调幅分解的振幅有关，且随着振幅的增大而提高。同时，提高退火温度能显著缩短调幅分解回复所需的时间，退火回复时间与退火温度之间有Arrhenius形式的关系。     

Study of the spinodal decomposition of an Fe-28Cr-2Mo-4Ni-Nb alloy by small-angle neutron scattering

Superferritic stainless steels become embrittled when aged in a temperature interval around 475 °C, This phenomenon has been studied in the alloy Fe-28Cr-2Mo-4Ni-Nb by measuring the decrease in the energy absorbed in the impact test as a function of ageing time at 475 °C. The change which occurs in the microstructure has been studied by small-angle neutron scattering. It is shown that ageing at 475 °C produces the decomposition of the alloy in zones rich in chromium via spinodal decomposition. The high rate of embrittlement observed, compared with that which occurs in Fe-Cr binary alloys of similar chromium content, is related to a faster development of the spinodal decomposition in the steel studied.