Redistribution of the substitutional elements during σ and χ phase formation in a duplex stainless steel

While the kinetics of intermetallic phase formation in duplex stainless steel have been extensively studied for a wide range of compositions, relatively little research has been done on the mechanisms by which the phases nucleate and grow. In this research the emphasis was on the diffusional growth of the sigma (σ) and chi (χ) phases. Intermetallic phase precipitation in a standard DSS type EN 1.4462 was studied in detail at two temperatures: at 850 °C where large amounts of σ phase are formed, and at 650 °C where χ is the main intermetallic phase. The redistribution of the substitutional alloying elements Cr, Mo, Ni, Mn and Si between the ferrite, the austenite and the intermetallic phases was followed by means of scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDX). Mn and Si showed no distinct partitioning between the several phases. The diffusion paths of the other elements were found to be determined by two phenomena. On the one hand, the ferrite to austenite transformation causes Mo and Cr to enrich in the ferritic phase and Ni to enrich in the austenite. On the other hand, Mo (σ, χ) and Cr (σ) are removed from the ferrite by intermetallic phase precipitation. At 650 °C, strong Mo enrichments appear at the α‐γ phase boundaries indicating that grain boundary diffusion could be an important mechanism for Mo to diffuse to the growing χ phase.     

On Phase Equilibria in Duplex Stainless Steels

The equilibrium conditions of four duplex stainless steels; Fe‐23Cr‐4.5Ni‐0.1N, Fe‐22Cr‐5.5Ni‐3Mo‐0.17N, Fe‐25Cr‐7Ni‐4Mo‐0.27N and Fe‐25Cr‐7Ni‐4Mo‐1W‐1.5Cu‐0.27N were studied in the temperature region from 700 to 1000 °C. Phase compositions were determined with SEM EDS and the phase fractions using image analysis on backscattered SEM images. The results showed that below 1000 °C the steels develop an inverse duplex structure with austenite and sigma phase, of which the former is the matrix phase. With decreasing temperature, the microstructure will be more and more complex and finely dispersed. The ferrite is, for the higher alloyed steels, only stable above 1000 °C and at lower temperatures disappears in favour of intermetallic phases. The major intermetallic phase is sigma phase with small amounts of chi phase, the latter primarily in high Mo and W grades. Nitrides, not a focus in this investigation, were present as rounded particles and acicular precipitates at lower temperatures. The results were compared to theoretical predictions using the TCFE5 and TCFE6 databases.     

Influence of Isothermal Aging on the Embrittlement of 00Cr25Ni7Mo4N Super Duplex Stainless Steel

The effect of an isothermal aging treatment on the embrittlement of 00Cr25Ni7Mo4N super duplex stainless steel was studied by combining metallographic, SEM and TEM observation. The results showed that thermal aging treatment below 500°C and above 1050°C did not have any effect on the impact toughness. The observed embrittlement behaviour was mainly attributed to the generation of intermetallic phases in the steel matrix, which included the R phase formation below 600°C, the R phase and especially the σ phase formation at 700°C, and the formation of the σ phase in the range of 800‐1000°C. The impact toughness decreased to a minimum after a thermal aging for 30 minutes in the temperature range of 800°C to 900°C. A trend of decreasing impact ductility with increasing amount of precipitated σ phase was observed. The impact energy started to decrease as the amount of the σ phase reached about 10‐15 volume‐% and reached a minimum when the amount of the σ phase increased to about 30 volume‐%.     

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M₂₃C₆ was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti₄C₂S₂ which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M₂₃C₆ and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M₂₃C₆ and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.     

Microstructure Evolution during Recrystallization of Newly Developed Low Ni High Mn‐N Stainless Steels

Low cost stainless steels where nickel is replaced in a conventional Fe‐Cr‐Ni stainless steel by manganese and nitrogen were studied. In this work, three new steels based on the system (mass %) Fe‐18Cr‐15Mn‐2Ni‐2Mo‐XN were prepared and their microstructure after each treatment was evaluated by optical and scanning electron microscopy, and X‐ray diffraction. A good correlation between texture and microstructure evolution during annealing was established. A randomization of the texture during recrystallization of the austenite was observed. Recrystallization starts at temperatures above 850°C, and after annealing for 0.5 h at 900°C, the austenite is completely recrystallized, reaching the orientation density a value near 1. Precipitation of σ ‐ phase was observed in the samples annealed at temperatures ranging from 700 to 950°C.     

316L不锈钢金属间相固溶过程中的微结构表征

利用扫描电镜、X射线衍射仪和电子背散射衍射仪等设备研究了热轧态316L不锈钢金属间相(σ相和χ相)在固溶过程中的微结构演变规律,同时,利用高温激光共聚焦显微镜对金属间相回溶的全过程进行了原位在线观察,确定了最佳固溶温度。结果表明,固溶过程中发生了γ+σ+χ→γ+σ+χ+α/FeCr→γ+α/FeCr的相变过程。金属间相的回溶从1033.1℃开始到1149.5℃结束,回溶时间为21s,消除金属间相的最佳固溶温度约为1150℃。固溶处理前,基体中分布的带状组织形态不规则,χ相和σ相的面积比分别为0.46%和0.94%;固溶处理后,带状组织形态一致,主要分布铁素体和FeCr相,而χ相和σ相不可见。     

Influence of Laves Phase Precipitation and Coarsening on High‐Temperature Strength of Ferritic Stainless Steels

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high‐temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe? Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms – precipitation–hardening, grain refinement, and solid‐solution strengthening – were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high‐temperature strength about 40%, also after long‐term annealing. The contribution of the Laves phase precipitates on the high‐temperature strength is rather small due to their rapid coarsening.     

S31254超级奥氏体不锈钢析出相演变及微观表征

利用热力学计算了S31254超级奥氏体不锈钢在500~1 200 ℃温度范围内的平衡态析出相,并结合热模拟试验、扫描电镜、透射电镜等方法,对不同析出物的析出行为进行了表征和分析。结果表明,S31254不锈钢奥氏体基体中可存在的第二相包括σ、χ、Laves等金属间相,Cr₂N、π型氮化物相以及M₂₃C₆型碳化物相,高Mo、高N、高Cr含量是该钢析出相种类复杂的主要原因;试验钢具有高的第二相析出倾向,σ相开始析出温度约为1 150 ℃,而在900~800 ℃区间可发现χ相和σ相的转变,χ相更易作为一种稳定相存在;析出相的析出位置和形貌呈现不同特点,晶界析出主要为σ相、χ相和Laves相,而晶内主要有呈针状和块状分布的χ相和呈棒状析出的Cr₂N相。     

A study on precipitation kinetics of sigma phase in a hot-rolled super duplex stainless steel during isothermal aging based on the Johnson–Mehl–Avrami model

Isothermal aging treatment of 2507 super duplex stainless steel (SDSS2507) was conducted at 850°C after solution treatment at 1150°C. The characteristics of sigma (σ) precipitation kinetics in SDSS2507 were discussed and an improved JMA (Johnson–Mehl–Avrami) model had been developed. The results show that, the precipitation mechanism of σ phase in hot-rolled SDSS2507 was quite different to other ordinary DSSs. It can be found that σ phase precipitated via two mechanisms in this experiment. (1) The eutectoid decomposition δ?→?σ?+?γ₂. It dominated the precipitation kinetics of σ phase in aging 0–25?min, and at this time the volume fraction of σ phase increased rapidly. (2) After 25?min, the precipitation of σ phase absolutely relied on the transformation of γ?→?σ which is controlled by the diffusion of Mo and Ni in γ. In this period, the precipitation rate of σ phase was significantly decreased. In this case, the separate modeling on the basis of each transformation mechanism was adopted and then an improved JMA model was developed for the precipitation kinetics of σ phase in the whole aging process. As a result, a good agreement between the experimental data and this developed JMA model can be obtained.     

Iron‐Rich Iron‐Titanium‐Silicon Alloys with Strengthening Intermetallic Laves Phase Precipitates

Two‐phase ternary Fe‐Ti‐Si alloys with Si contents from 2 to 16 at.% and Ti contents from 2 to 28 at.% were studied with respect to room temperature hardness, fracture strain and yield stress at room and higher temperatures up to 1150 °C. In addition oxidation was checked at temperatures between 400 and 1150 °C. The alloys are strengthened by precipitation of the stable Laves phase (Fe,Si)₂Ti which is a hard and brittle intermetallic phase. The yield stress as well as the brittle‐to‐ductile transition temperature (BDTT) increase with increasing Ti content. Yield stresses up to about 1400 MPa and BDTT between 100 °C and 600 °C with fracture strains of the order of 1 % below BDTT were achieved. The observed short‐term oxidation performance at temperatures up to 1150 °C compares favourably with that of Fe‐AI alloys with high Al contents.     

Structural stability of super duplex stainless weld metals and its dependence on tungsten and copper

Three different superduplex stainless weld metals have been produced using manual metal arc welding under identical welding conditions. The concentration of the alloying elements tungsten and copper corresponded to the concentrations in commercial superduplex stainless steels (SDSS). Aging experiments in the temperature range 700 °C to 1110 °C showed that the formation of intermetallic phase was enhanced in tungsten-rich weld metal and also dissolved at higher temperatures compared with tungsten-poor and tungsten-free weld metals. It could be inferred from time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams produced in the present investigation that the critical cooling rate to avoid 1 wt pct of intermetallic phase was 2 times faster for tungsten-rich weld metal. Microanalysis in combination with thermodynamic calculations showed that tungsten was accommodated in χ phase, thereby decreasing the free energy. Experimental evidence supports the view that the formation of intermetallic phase is enhanced in tungsten-rich weld metal, owing to easier nucleation of nonequilibrium χ phase compared with σ phase. The formation of secondary austenite (γ₂) during welding was modeled using the thermodynamic computer program Thermo-Calc. Satisfactory agreement between theory and practice was obtained. Thermo-Calc was capable of predicting observed lower concentrations of chromium and nitrogen in γ₂ compared with primary austenite. The volume fraction of γ₂ was found to be significantly higher in tungsten-rich and tungsten + copper containing weld metal. The results could be explained by a higher driving force for precipitation of γ₂ in these.     

Influence of Strain Rate on Hot Ductility of a V‐Microalloyed Steel Slab

The hot ductility and malleability of a vanadium‐microalloyed steel is investigated by means of tensile and compression tests at temperatures ranging from 700 to 850°C and strain rates of 3 × 10^?4 to 0.3 s^?1. The deformation tests are performed after austenitization and cooling to test temperature. The so‐called second ductility minimum is located around 750°C for all strain rates except for the highest one, where no ductility trough is observed. Ductility steadily increases with strain rate at a given temperature, and the fracture mode progressively changes from intergranular to transgranular. In the region of minimum ductility, intergranular cracking occurs at low strain rates by void nucleation, growth and coalescence within thin layers of deformation induced ferrite covering the austenite grain boundaries. Cracking is favoured by V(C,N) precipitation associated with the γ/α phase transformation. Ductility remains low above the temperature of minimum ductility, where no apparent ferrite formation is observed (790 °C). Void formation takes place as a result of grain boundary sliding in combination with matrix and grain boundary precipitation. These voids are able to grow and link up forming intergranular cracks. Ductility increases with strain rate mainly due to the short time available for precipitation as well as for intergranular void growth and coalescence.     

Reverse Martensitic Transformation and Resulting Microstructure in a Cold Rolled Metastable Austenitic Stainless Steel

The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with A_f′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing M_s‐temperature and new α′‐martensite formation upon cooling.     

Influence of the Heterogeneous Nucleation Sites on the Kinetics of Intermetallic Phase Formation in Aged Duplex Stainless Steel

The aim of this work is to study the influence of the heterogeneous nucleation site quantity, observed in different ferrite and austenite grain size samples, on the phase transformations that result in intermetallic phases in a UNS S31803 duplex stainless steel (DSS). Solution treatment was conducted for 1, 24, 96, or 192 hours at 1373 K (1100 °C) to obtain different ferrite and austenite grain sizes. After solution treatment, isothermal aging treatments for 5, 8, 10, 20, 30, or 60 minutes at 1123 K (850 °C) were performed to verify the influence of different amounts of heterogeneous nucleation sites in the kinetics of intermetallic phase formation. The sample solution treated for 1 hour, with the highest surface area between matrix phases, was the one that presented, after 60 minutes at 1123 K (850 °C), the smaller volume fraction of ferrite (indicative of greater intermetallic phase formation), higher volume of sigma (that was present in coral-like and compact morphologies), and chi phase. It was not possible to identify which was the first nucleated phase, sigma or chi. It was also observed that the phase formation kinetics is higher for the sample solution treated for 1 hour. It was evidenced that, from a certain moment on, the chi phase begins to be consumed due to the sigma phase formation, and the austenite/ferrite interface presents higher S _V for all solution treatment times. It was also observed that intermetallic phases form preferably in austenite-ferrite interfaces, although the higher occupation rate occurs at triple junction ferrite-ferrite-ferrite. It was verified that there was no saturation of nucleation sites in any interface type nor triple junction, and the equilibrium after 1 hour of aging at 1123 K (850 °C) was not achieved. It was then concluded that sigma phase formation is possibly controlled by diffusional processes, without saturation of nucleation sites.     

High temperature phase chemistries and solidification mode prediction in nitrogen-strengthened austenitic stainless steels

Nitronic 50 and Nitronic 50W, two nitrogen-strengthened stainless steels, were heat treated over a wide range of temperatures, and the compositions of the ferrite and austenite at each temperature were measured with analytical electron microscopy techniques. The compositional data were used to generate the (γ + δ phase field on a 58 pct Fe vertical section. Volume fractions of ferrite and austenite were calculated from phase chemistries and compared with volume fractions determined from optical micrographs. Weld solidification modes were predicted by reference to the Cr and Ni contents of each alloy, and the results were compared with predictions based on the ratios of calculated Cr and Ni equivalents for the alloys. Nitronic 50, which contained ferrite and austenite at the solidus temperature of 1370 °C, solidified through the eutectic triangle, and the weld microstructure was similar to that of austenitic-ferritic solidification. Nitronic 50W was totally ferritic at 1340 °C and solidified as primary delta ferrite. During heat treatments, Nitronic 50 and Nitronic 50W precipitated secondary phases, notably Z-phase (NbCrN), sigma phase, and stringered phases rich in Mn and Cr.     

Iron‐Rich Iron‐Aluminium‐Molybdenum Alloys with Strengthening Intermetallic μ phase and R phase Precipitates

Single‐phase and two‐phase ternary Fe‐Al‐Mo alloys with Al contents of usually 10 ‐16 at.% and Mo contents up to 42 at.% have been studied with respect to hardness at room temperature, yield stress and fracture strain at room temperature and higher temperatures up to 1000 °C and oxidation at temperatures of 400 ‐ 1000 °C. Thse alloys are strengthened by precipitation of the metastable R phase and/or the stable m phase depending on composition and heat treatment; both are hard and brittle intermetallic phases. The yield stress as well as the brittle‐to‐ductile transition temperature increases with increasing Mo content to reach yield stresses above 1400 MPa with, however, fracture strains below 1 % at temperatures below 800 °C. The observed short‐term oxidation is similar to that of other Fe‐Al alloys.     

The effect of nitrogen on precipitation and transformation kinetics in vanadium steels

The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstätten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstätten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen.     

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo,and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M₂₃C₆ carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.     

The microstructure of an Fe-Mn-Si-Cr-Ni stainless steel shape memory alloy

The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of austenite, δ-ferrite, and the (Fe,Mn)₃Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the Fe₅Ni₃Si₂ type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium phases, the austenite grains show the presence of athermal ɛ martensite. The athermal α′ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the γ-ɛ-α′ transformation sequence.