Sigma Phase Formation Research Articles

RA 800H

Abstract RA 800H is an austenitic iron-base, heat-resistant alloy with excellent high-temperature strength and good resistance to oxidation in air up to 2000 F (1090 C). High creep-rupture properties are obtained by a solution-anneal treatment followed by a rapid cool; grain size is controlled in the range 1 to 5 on the ASTM scale. RA 800H is free from embrittlement by sigma-phase formation. This alloy is well suited for high-temperature processing in refining or petrochemical service. This datasheet provides information on composition, physical properties, elasticity, and tensile properties as well as creep. It also includes information on forming, heat treating, machining, joining, and surface treatment. Filing Code: SS-373. Producer or source: Rolled Alloys Inc..

Sigma phase formation in conventional and PM nickel-base superalloys

AbstractAgeing treatments of up to 5000 h at 705–980°C have been carried out on five chemically modified nickelbase superalloys based on Astroloy (No. I), MAR-M432 (No.2), IN-100 (No.3), PAI0l(No. 4), and AF-2-IDA (No.5). The kinetics and morphology of a-phase formation were examined in two unstable heats (Alloys 3 and 5) processed by powder metallurgy methods and by conventional casting and extrusion procedures. The morphologies of σ were similar in both forms of a given alloy; however, σ precipitation occurred sooner in the powder metallurgyform of Alloy 3 than in the cast and extruded form. Numerical methods based on electron-vacancy concepts predicted σ formation correctly when experimentally determined γ-phase (austenite) compositions were used. Standard methods for computing γ and γ′ contents and compositions were unsuccessful with these modified alloys, and therefore a new regression equation was developed for γ′ contents using a 71 alloy data base. Using this new equation, more accurate γ and γ′ c...

Metastable order-disorder transition and sigma phase formation in Fe-V binary alloys

The influence of binary and ternary alloying compositions and heating rate on theσ formation reaction are studied. Several ways are developed to supress the stableα phase formation and to understand the metastable CsCl-type order-disorder transition in Fe-V alloys. The Fe-V ordered phase has a metastable critical transition temperature of between 850 and 880° C at equiatomic composition, and the so called “650° C anomaly” corresponds to the 550° C anomaly in Fe-Co alloys. Theσ formation reaction does not commence most easily at equiatomic composition but at the iron-rich side of the 40 at. % V alloy. The Influence of ternary alloying additions on theσ formation in Fe-V alloys depends on the relative stabilities of the binaryσ phase with iron or vanadium.

Solution treatment behavior of Co‐Cr‐Mo alloy

Current practice in the manufacture of Co-Cr-Mo alloy total hip prostheses is the use of a solution treatment to increase the ductility of the as-cast alloy. This study is concerned with the reactions encountered during solution treatment at temperatures between 1165-1270 degrees C. These reactions, including incipient melting, a carbide transformation from M23C6 to M6C and sigma-phase formation, have been examined using both qualitative and quantitative metallographic techniques, and are shown to influence the production of a single phase microstructure. As a result, an optimum temperature for solution treatment of 1220 degrees C has been determined. It is further proposed that a reduction in the carbon content of this alloy would improve its solution treatment behavior.

Recrystallization and Sigma Phase Formation as Concurrent and Interacting Phenomena in 25%Cr-20%Ni Steel

Recrystallization kinetics of an austenitic steel containing nominally 25%Cr and 20%Ni has been investigated mainly by optical and transmission electron microscopy. The activation energy for recrystallization was found to be 4.6×105 J/mol which was excessively greater than the values previously observed in steel. It was surmised that a sigma phase formation accompanied the recrystallization phenomenon, and that this transformation acted as a barrier for growth of recrystallized grains. It is proposed that the sigma phase formation and recrystallization are concurrent and interacting phenomena and hence the activation energy described above is considered to be a sum of those for recrystallization, 2.3×105 J/mol, and for diffusion of chromium atoms, 2.3×105 J/mol, governing the sigma phase formation.

Workpiece damage produced by creep feed grinding

The nature and extent of workpiece damage induced by creepfeed grinding a Nimonic alloy have been investigated. Undersevere burn conditions considerable surface and internal oxidation occurs, together with the formation of cracks penetrating to depths of 2mm. A white unetchable subsurface layer is also formed in which the γ' precipitate is dissolved. Similar observations have been made for unburnt specimens, but here, the extent of the damage is much reduced. Heating after grinding causes reprecipitation of γ' in the white layer formed under both conditions, and the formation of the deleterious sigma phase in the white layer produced during burn. It is suggested that high-cycle fatigue and low-stress creep behaviour will be the properties most affected in service by grinding damage.

Microstructural instabilities in an industrial gas turbine engine vane

Microstructural changes occurring in a Udimet 500 turbine vane during service have been examined. The gamma prime (γ’) content and carbide contents determined experimentally were in close agreement with numerical estimates obtained by Decker’s4 method. The method of Barrows and Newkirk10 for predicting sigma phase formation was examined and numerical predictions compared with experimental observations. The γ matrix was correctly predicted to be sigma prone when these calculations were applied to experimentally determined γ phase compositions. When the calculations were applied to computationally determined compositions the γ phase was predicted to be stable if residual aluminum in the γ phase was considered to act as a stabilizing element, or unstable if aluminum was considered to promote the formation of sigma. These discrepancies are discussed in terms of the effects of carbide and sigma formation on matrix chemistry and the interdependence of these reactions.

ACI TYPE HP

Abstract Type HP is an iron-chromium-nickel alloy with a combination of elements that makes it resistant to both oxidizing and carburizing atmospheres at high temperatures. The alloy has good creep-rupture properties in the 1800-2000 F temperature range. It has an austenitic structure at all temperatures, thus it is not susceptible to embrittlement from sigma phase formation. Type HP was developed for rugged steel-mill furnace-roll service; no comparable wrought alloy exists. This datasheet provides information on composition, physical properties, elasticity, and tensile properties as well as creep. It also includes information on high temperature performance and corrosion resistance as well as casting, heat treating, machining, and joining. Filing Code: SS-307. Producer or source: Stainless steel foundries.

Prediction of Sigma-Phase Formation in Nickel-Base Superalloys

Experimental determinations of γ-matrix compositions in nickel-base superalloys have shown that small amounts of aluminium may be retained in solid solution after γ′ precipitation. Average electron-vacancy concentrations and critical electron-vacancy concentrations have been calculated for the residual γ-matrix compositions of 30 commercial nickel-base superalloy heats. The γ-matrix compositions were determined experimentally or by a computational procedure. Critical electron-vacancy concentrations were calculated by two methods in which aluminium is assumed to act as (a) a γ-stabilizer or (b) a sigma-promoter. Predictions of γ-phase stability based on these calculations are compared with experimental observations. Optimistic predictions of γ stability are obtained when aluminium is treated as a γ-stabilizer, while good correlation between observed and calculated stability is obtained by the alternative approach. The validity of this latter approach is discussed in terms of the known effects of aluminium on σ-phase stability and a third alternative approach is suggested.

Metallographic observations on the formation and occurrence of ferrite, sigma phase and carbides in austenitic stainless steels. Part III. Electron microscopy studies

Electron microscope techniques have been used to study the transformation characteristics of two austenitic stainless steels, an AISI Type 316 steel and an AISI Type 310 steel. In both, carbide precipitation preceded the formation of ferrite and/or sigma phase. In the Type 316 steel, ferrite nucleated at triple points and grew at the expense of the carbides formed earlier. At 800°C, the ferrite transformed in part to sigma phase after 1500 hrs. Intragranular sigma phase formed in this steel directly from the austenite after 600 hrs at 800°C. In the Type 310 steel, sigma phase precipitation followed carbide formation. The sigma phase formed at twin boundaries, along grain boundaries and intragranularly. Only occasional ferrite was detected. Initially, the carbides were uniquely oriented with the austenite matrix, { h 1 k 1 l 1} γ ‖; { h 1 k 1 l 1}M 23C 6, 〈 h 1 k 1 l 1〉 γ ‖; 〈 h 1 k 1 l 1〉 M 23C 6. This coherency was soon lost, however, and random orientations predominated. Unique orientation relationships were not detected between austenite and ferrite, austenite and sigma phase, or ferrite and sigma phase.

The effect of plastic deformation on the formation of sigma phase in an austenitic stainless steel

A 24% Ni, 25% Cr austenitic steel containing 0.27% Ti has been solution-treated and quenched from 1150°C. The effect of prior cold-rolling on the aging processes at 750°C has been studied by transmission electron microscopy, comparative tests having been made on a Ti-free steel. Acceleration of ferrite and sigma-phase formation on recrystallization of the matrix was observed. It is suggested that the effect arises from the segregation of Ti atoms to dislocations, which element would otherwise promote the formation of these phases. The presence of serrated stress-strain curves in tensile tests at 704°C in the Ti-containing alloy supports this hypothesis. Only when the dislocation density is reduced by recrystallization is the free Ti content sufficient to promote the formation of the ferrite and sigma phases.

Metallographic observations on the formation and occurrence of ferrite, sigma phase, and carbides in austenitic stainless steels: Part II: Studies of AISI Type 316 Stainless Steel

The precipitation effects occurring in a commercial AISI Type 316 steel in the temperature range 600° to 850°C have been studied for times up to 900 hours by means of optical metallographic techniques. Carbide precipitation along grain boundaries precedes the formation of ferrite and sigma phase, and growth of the latter precipitates appears to depend on re-solution of the carbides. Sigma appears to form directly from the austenite and not from the ferrite formed at shorter times.

Metallographic observations on the formation and occurrence of ferrite, sigma phase, and carbides in austenitic stainless steels: Part I: Studies of AISI Type 310 Stainless Steel

The structural changes taking place in a commercial AISI Type 310 steel at temperatures in the range 650° to 950°C have been examined for times of up to 900 hours by means of optical metallographic techniques. The precipitation of sigma phase, ferrite, and carbides has been examined in terms of their distribution and morphology. The appearance of ferrite as a transformation product has not been reported previously for Type 310 stainless steel, but no association between its formation and that of sigma was found.

The effect of Thermomechanical History on the Stability of Alloy 718

Previous studies showed that Ni3Cb precipitation behavior appears to follow a predictable pattern with increased strain in the precipitation range for this phase. Additional investigation was initiated in order to further the understanding of Ni3Cb precipitation under similar strain conditions and to develop some information on the effects of such conditions on the stability of gamma prime and the formation of sigma or other phases often considered detrimental to performance. Samples from a production heat were treated above the Ni3Cb precipitation range and subsequently strained between 0% RA and 35% RA at 1500°F. Light and electron microscopy and X-ray diffraction studies were performed on samples in the as-strained condition; after a 1750°F gamma prime solution treatment; after a 1750°F treatment followed by a double aging, gamma prime precipitation treatment and after a fully aged plus 1250°F-500 hour exposure. It was found that the quantity of Ni3Cb which precipitated during the 1750°F treatment step increases markedly with the degree of strain, that gamma prime stability is not significantly affected by strain or the quantity of Ni3Cb precipitate, and that sigma and similar phases do not form during prolonged exposure at the upper end of the useful temperature range for this alloy, even at the highest strain level investigated.

Zirconium Gettering Effect of Stainless Steel in Sodium

During the study of the stress-rupture properties of austenitic stainless steels in high-purity static liquid sodium, zirconium foil was placed in the sodium to getter oxygen and maintain a high-purity sodium environment throughout the test. Results of the biaxial stress-rupture tests on thin-walled Type-304 and Type-316 stainless-steel tubing in the range 1200°F (649°C) to 1400°F (760°C) showed that the rupture strength of the tubing was significantly lower in the zirconium-gettered static sodium than in non-gettered static sodium or helium. This reduction in rupture strength was greater with higher test temperature and longer exposure time. In the presence of sodium, the zirconium foil decarburized the stainless steel; the decarburization process also resulted in an extensive sigma-phase formation on the region of the tubing exposed to sodium. The decarburization increased with test temperature and time of exposure. It was concluded that the decreased stress-rupture strength of specimens tested in zirco...

The formation of ferrite and sigma-phase in some austenitic stainless steels

An austenitic steel of composition wt.% 24Ni, 25Cr, 0.5Ti has been solution treated at 1150°C., and aged at 750°C for periods of up to 2000 hr. The precipitation processes have been followed by transmission electron microscopy. Intragranular precipitation. Ferrite precipitates (with a Kurdjumov-Sachs orientation relationship with the matrix) nucleate on residual M 23C 6 and chromic oxide particles, and grow along 〈110〉 γ to form a Widmannstatten pattern of precipitates with two parallel {111 } γ interfaces and two curved interfaces along their length. Some ferrite particles undergo an in situ transformation to sigma phase, which then grow by the dissolution of neighbouring ferrite. Precipitation in grain, and incoherent twin boundaries. The growth of ferrite in these regions is accompanied by the dissolution of the M 23C 6 which formed there initially. The formation of sigma follows, and during its growth the ferrite is consumed. The role of ferrite in nucleating sigma. By studying a wider range of alloy compositions, and by comparing the data of other workers, it is concluded that sigma is precipitated directly from austenites of low (Ni + Cr) content, whereas in alloys of high (Ni + Cr) content sigma forms via the b.c.c. metastable ferrite phase.

Evaluation of machinable, acid-reconstituted alumina for electron tubes

At the present time 9–12% Cr ferritic/martensitic (F/M) steels with target operating temperatures up to 650 °C and higher are being developed in order to further increase thermal efficiency so as to reduce coal consumption and air pollution. An 11% Cr F/M steel was prepared by reference to the nominal chemical composition of SAVE12 steel with an expected maximum use temperature of 650 °C. The precipitate phases of the 11% Cr F/M steel normalized at 1050 °C for 0.5 h and tempered at 780 °C for 1.5 h were investigated by transmission electron microscopy. Except for Cr-/Cr-Fe-Co-rich M23C6, Nb-/V-/Ta-Nb-/Nd-rich MX, Fe-rich M5C2, Co-rich M3C and Fe-Co-rich M6C phases previously identified in the steel, two types of sigma phases consisting of σ-FeCr and σ-FeCrW were found to be also present in the normalized and tempered steel. Identified σ-FeCr and σ-FeCrW phases have a simple tetragonal crystal structure with estimated lattice parameters a/c = 0.8713/0.4986 and 0.9119/0.5053 nm, respectively. The compositions in atomic pct of the observed sigma phases were determined to be approximately 50Fe-50Cr for the σ-FeCr, and 30Fe-55Cr-10W in addition to a small amount of Ta, Co and Mn for the σ-FeCrW. The sigma phases in the steel exhibit various blocky morphologies, and appear to have a smaller amount compared with the dominant phases Cr-rich M23C6 and Nb-/V-/Ta-Nb-rich MX of the steel. The σ-FeCr phase in the steel was found to precipitate at δ-ferrite/martensite boundaries, suggesting that δ-ferrite may rapidly induce the formation of sigma phase at δ-ferrite/martensite boundaries in high Cr F/M steels containing δ-ferrite. The formation mechanism of sigma phases in the steel is also discussed in terms of the presence of δ-ferrite, M23C6 precipitation, precipitation/dissolution of M2X, and steel composition.

高クロム-ニッケル系耐熱鋳鋼のセメントクリンカー中における高温酸化

This paper deals with the result of high temperature oxidation tests on the heat resisting cast steels for use in cement clinker at high temperatures. The samples used in this study were 28% Cr-20% Ni-Si-Al steel and commercial 25% Cr- 12% Ni steel. These steels were oxidized continually in cement clinker at 1, 100°C for 100, 250 and 500 hours; and at 700°C also for 100, 250, 500 hours and further for 1, 000 hours. At 1, 100°C the weight loss of 28% Cr-20% Ni-Si-Al steel was much lower than that of 25% Cr-12% Ni steel, while at 700°C the difference between the weight losses of the two steels was rather slight. The effect of heat treatment on the oxidation resistance of these two steels was also investigated and it was confirmed that the water cooling from 1, 150°C for 28% Cr-20% Ni-Si-Al steel caused increase of the weight loss at 1, 100°C. However, for 25% Cr-12% Ni steel, the same heat treatment reversely caused decrease of the weight loss at 1, 100°C. The effects of heat treatment on each steel were discussed in the relation with the microscopic changes.Furthermore, the effect of addition of silicon and aluminum on the oxidation resistance of 28% Cr-20% Ni-Si-Al steel was examined. In this experiment the steels of various silicon and aluminum contents, ranging from 1 to 3% for silicon and from 0.35 to 1.05% for aluminum, were used. Under the condition that the silicon content of the steels. does not exceed 2%, the weight loss at 1, 100°C slightly decreased as aluminum content increased. It was considered that the most suitable content of aluminum plus silicon for decrease in weight loss was between 3 and 4%.We concluded that 28% Cr-20% Ni-Si-Al steel had excellent oxidation resistance in cement clinker at 1, 100°C, but for practical use it had a serious weakness, that is, the formation of sigma phase at about 700°C.

The Effect of Chemical Composition on the Formation of Sigma Phase of High Carbon 25Cr-20Ni Heat Resistant Alloy Castings

The Effect of Chemical Composition on the Formation of Sigma Phase of High Carbon 25Cr-20Ni Heat Resistant Alloy Castings

25%Cr鋼のσ相生成におよぼすNiの影響

25%Cr鋼のσ相生成におよぼすNiの影響