Phase transitions in rapidly solidified stainless steels cathodically hydrogen charged

The effects of cathodic hydrogen charging and subsequent aging on phase transitions and microstructures of rapidly solidified (RS) austenitic stainless steels (types 310 RS, 316 RS, and 316TiM RS) were investigated. The behavior of the martensitic phases,α′(bcc) andε(hcp), as well as the austenite phase,γ (fcc), of the RS steels during aging after charging was compared to the behavior of these phases of equivalent conventionally processed commercial solution-treated (ST) austenitic stainless steels (types 310 ST, 316L ST, and 316TiM ST) following identical cathodic-charging conditions by means of X-ray and electron diffraction techniques. The behavior of theα′ phase of both RS and ST steels (that formα′ phase) during aging was found to be very similar, while the behavior of bothγ andε phases during aging of all of the RS steels studied, as compared to the equivalent ST steels, was different. The development of lower internal stresses and minor lattice expansion of the RS steels, as compared to the ST steels, is probably due to a different distribution of hydrogen within the near-surface layer of the RS steels than that of the ST steels, which appears to be related to the markedly different microstructural characterizations of the RS steels from the ST steels. Scanning and transmission electron microscopy (SEM and TEM) observations indicated that the tendency toward cracking along the columnar-like structure is typical of all of the charged RS steels studied. Formerly Research Associate, Department of Materials Engineering, Ben Gurion University of the Negev     

Influence of strain-induced phase transformation on the surface crystallographic texture in cold-rolled-and-aged austenitic stainless steel

Stainless steels (SSs) having a stable and metastable austenitic phase were studied to see the influence of strain-induced phase transformation in the metastable austenitic stainless steel on the evolution of texture during cold rolling and aging. AISI 304L and 316L SS plates were unidirectionally cold rolled up to a 90 pct reduction and aged at different aging temperatures. The strain-induced transformation of austenite to α′-martensite phase and the evolution of texture in both the phases were studied as a function of rolling reduction as well as aging temperature in the metastable 304L austenitic stainless steel. The X-ray diffraction (XRD) technique was employed to quantify the volume fractions and characterize the texture of austenite and martensite phases in the rolled and aged conditions. Results are compared with the texture evolution in the stable austenitic 316L SS.     

Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels

An investigation of phase transformations in hydrogen-charged metastable austenitic stainless steels was carried out. Solution-annealed, high-purity, ultralow-carbon Fel8Crl2Ni (305) and laboratory-heat Fel8Cr9Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm², at room temperature for 5 minutes to 32 hours, in an lN H₂SO₄ solution with 0.25 g/L of NaAsO₂ added as a hydrogen recombination poison. Changes in microstructure and hydrogen damage that resulted from charging and subsequent room-temperature aging were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrides from hydrogen charging (hep ε* in 305 SS and fcc γ* and hcp ε* in 304 SS) were observed. The evidence suggests the following mechanisms for hydride formation during charging: (1)γ → ε → ε* hydride and (2) γ → γ* hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1)ε* hydride (hcp) → expanded ε (hcp) phase →α′ (bcc) phase and (2) γ* hydride →γ phase. The transformation from ε* toα′, however, was incomplete, and a substantial fraction of ε was retained. A kinetics model for hydride decomposition and the accompanying phase transformation during aging is proposed.     

Phase transformation of austenitic stainless steels as a result of cathodic hydrogen charging

The effects of cathodic hydrogen charging and aging on surface phase transformations were studied in solution treated and cold worked specimens of two austenitic stainless steels. Quantitative phase evaluation using an X-ray technique has shown that cathodic hydrogen charging and aging can result in a considerable amount of surface transformation toε andα ′ martensites. The extent of this surface transformation differs significantly from deformation-induced transformation at the same temperature, and abnormally high volume fractions ofε martensite are produced by the charging process. A minimum charging current density is necessary to induce transformation. In cold-worked samples, further surface transformation due to hydrogen charging and aging is inhibited by high volume fractions of pre-existing martensite. A. P. BENTLEY, formerly with the Department of Metallurgy and Materials Science, University of Cambridge     

Damping properties of austenitic stainless steels containing strain-induced martensite

Damping properties of two austenitic stainless steel grades, EN 1.4318 and EN 1.4301, were investigated. The test materials were cold rolled to different reductions and damping capacity was measured as a function of temperature with an internal friction method. Microstructures of the test materials were studied by means of X-ray diffraction (XRD) and magnetic measurements. The results showed that damping capacity of the studied materials depended on the amounts of strain-induced ε- and α′-martensite phases. At temperatures around 0 °C, highest damping capacity was achieved with cold rolling reduction of 10 to 15 pct. This behavior is related to the existence of ε-martensite and stacking faults. Internal friction peak due to α′-martensite phase was present at the temperature of 130 °C. Strain aging heat treatment at 200 °C for 20 minutes decreased the damping capacity in the entire studied temperature range.     

Phase transformation of Zn-4Al-3Cu alloy during heat treatment

The phase transformation in Zn-4 Al-3 Cu alloy employing various solution-treatment temperatures (230 °C to 325 °C) was studied by means of microhardness, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The starting microstructure of the as-cast Zn-4Al-3Cu alloy consists of an α phase (aluminum-rich, fcc structure) in the η matrix (zinc-rich, h.c.p. structure) prior to solution-treatment. A platelike ε phase with 3-μm length and 0.5-μm thickness was found in the η phase matrix after solution-treating the as-cast material at 240 °C for 1 hour. The ε phase was then dissolved gradually back into the η matrix above that temperature. A four-phase transformation, α + ε → T′ + η, was observed from the temperature 250 °C to 310 °C, wherein the T′ phase formed at the interface of ε platelet and η phase matrix. This T′ phase was further identified as a rhombohedral structure. As the solution-treatment temperature was increased to above 310 °C, the ε phase was completely dissolved back into the η matrix and numerous β phase particles were distributed uniformly in the η matrix. The β phase subsequently decomposed at room temperature to a fine α phase embedded in the η matrix. For the materials solution-treated above 250 °C, the microhardness of the η matrix increased in 40 minutes during natural aging, which was associated with the formation of fine ε phase of 0.15-μm diameter. The orientation relationship between this fine ε phase and η phase was determined as .     

Low-temperature extension of the lehrer diagram and the iron-nitrogen phase diagram

Iron layers are nitrided in mixtures of ammonia and hydrogen at low temperatures, using a thin nickel caplayer as a catalyst. In the coordinate field of inverse temperature vs nitriding potential, we determined the boundaries between areas in which the α, γ′, or ε phases are in thermal equilibrium. Using these data, the Fe-N phase diagram is extended from 350 °C to 240 °C and extrapolated down to 200 °C. The α, γ′, and ε phases probably coexist in a triple point in the Lehrer diagram around 214 °C.     

A fine γ′+α cellular structure in Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C and its influence on high-temperature tensile properties

Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C alloy, which reveals an excellent combination of high strength and good elongation endowed by formation of homogeneously dispersed fine γ′ precipitates in the matrix during aging at 823 K, has been investigated by means of transmission electron and optical microscopies, electron diffractions, and tensile tests. The influence of unique γ′+α cellular products on the mechanical properties has also been studied. Because of low elastic mismatch between the austenitic γ matrix and isomorphic γ′ precipitate phases, the homogeneously distributed precipitate particles, which formed at the early stage of aging, were observed to persist even after long-term aging. After very lengthy aging, the fine γ′ phase particles were changed to coarser γ′ lamellae at the grain boundary reaction front, which were alternately arranged with fine α lamellae that were estimated to have been transformed from the austenite-stabilizing-solute(Ni, C)-depleted γ lamellae. The fine duplex γ′+α cellular product did not affect deleteriously the room-temperature tensile properties of the alloy. However, the cellular structure was observed to cause the grain boundary embrittlement of the aged alloy at elevated temperatures higher than 681 K.     

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part I. derivation of flow curve equations

A semi-mechanistic model for predicting the flow behavior of a typical commercial dual-phase steel containing 20 vol pct of ‘as quenched’ martensite and varying amounts of retained austenite has been developed in this paper. Assuming that up to 20 vol pct of austenite with different degrees of mechanical stability can be retained as a result of certain thermomechanical treatments in a steel of appropriate low carbon low alloy chemistry, expressions for composite flow stress and strain have been derived. The model takes into account the work hardening of the individual microconstituents(viz., ferrite-@#@ α, retained austenite- γ _r, and martensite -α′) and the extra hardening of ferrite caused by accommodation dislocations surrounding the ‘as quenched’ as well as the strain-induced(γ _r→ α′) martensite. Load transfer between the phases has been accounted for using an intermediate law of mixtures which also considers the relative hardness of the soft and the hard phases. From the derived expressions, the flow behavior of dual phase steels can be predicted if the properties of the individual microconstituents are known. Versatility of the model for application to other commercial steels containing a metastable phase is discussed.     

Hydrogen Induced Slow Crack Growth in Stable Austenitic Stainless Steels

The behavior of hydrogen induced slow crack growth in type 310 and type 16-20-10 stable austenitic stainless steels along with type 321 unstable austenitic stainless steel were investigated. It was found that slow crack growth could occur in all three types of stainless steels, and the threshold values wereK _H/K_c = 0.55, 0.7, and 0.78 for type 321, 310, and 16-20-10 stainless steel respectively, when charged under load. Slow crack growth could also occur if the precharged specimens were tested under constant load in air. No slow crack growth occurred in the precharged and then out-gassed specimens. This indicates that delayed cracking in stable austenitic stainless steels is induced by hydrogen. Since there is no hydrogen induced α’ martensite in type 310 and 16-20-10 stainless steel, the existence of a’ martensite is not necessary for the occurrence of slow crack growth in the austenitic stainless steels, although it can facilitate slow crack growth. The mode of hydrogen induced delayed fracture in either the stable or unstable austenitic stainless steel is correlated with theK, value; the fracture surface is changed from ductile to brittle asK ₁ is decreased.     

Structure and properties of high-temperature austenitic steels for superheater tubes

The structure and properties of high-temperature austenitic steels intended for superheater tubes are analyzed. Widely used Kh18N10T (AISI 304) and Kh16N13M3 (AISI 316) steels are found not to ensure a stable austenitic structure and stable properties during long-term thermal holding under stresses. The hardening of austenitic steels by fine particles of vanadium and niobium carbides and nitrides and γ′-phase and Fe₂W and Fe₂Mo Laves phase intermetallics is considered. The role of Cr₂₃C₆ chromium carbides, the σ phase, and coarse precipitates of an M ₃B₂ phase and a boron-containing eutectic in decreasing the time to failure and the stress-rupture strength of austenitic steels is established. The mechanism of increasing the stress-rupture strength of steels by boron additions is described. The chemical compositions, mechanical properties, stress-rupture strength, and creep characteristics of Russian and foreign austenitic steels used or designed for superheater tubes intended for operation under stress conditions at temperatures above 600°C are presented. The conditions are found for increasing the strength, plasticity, and thermodeformation stability of austenite in steels intended for superheater tubes operating at 700°C under high stresses for a long time.     

Hydrogen-related phase transformations in austenitic stainless steels

The effect of hydrogen and stress (strain) on the stability of the austenite phase in stainless steels was investigated. Hydrogen was introduced by severe cathodic charging and by elevated temperature equilibration with high pressure H₂ gas. Using X-ray diffraction and magnetic techniques, the behavior of two “stable” type AISI310 steels and an “unstable” type AISI304 steel was studied during charging and during the outgassing period following charging. Transformation from the fcc γ phase to an expanded fcc phase, γ*, and to the hcp ε phase occurred during cathodic charging. Reversion of the γ* and e phases to the original γ structure and formation of the bcc α structure were examined, and the kinetics of these processes was studied. The γ* phase was shown to be ferromagnetic with a subambient Curie temperature. The γ⇆ε phase transition was studied after hydrogen charging in high pressure gas, as was the formation of a during outgassing. These results are interpreted as effects of hydrogen and stress (strain) on the stability of the various phases. A proposed psuedo-binary phase diagram for the metal-hydrogen system was proposed to account for the formation of the γ* phase. The relation of these phase changes to hydrogen embrittlement and stress corrosion cracking of stainless steel is discussed.     

Effect of strain rate on the strain-induced <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis>′-martensite transformation and mechanical properties of austenitic stainless steels

The effect of strain rate on strain-induced γ → α′-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3×10⁻⁴ and 200 s⁻¹. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γ → α′ transformation. A correlation between the work-hardening rate and the rate of γ → α′ transformation was found. Therefore, the changes in the extent of the α′-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the α → α′ transformation.     

Martensitic Transformation During Cold Rolling Deformation of an Austenitic Fe-26Mn-0.14C Alloy

A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), α′-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to ε-martensite. However, a small fraction (~4 pct) of α′-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The stability of ε-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m² because of the increase in temperature during the cold rolling deformation.     

Visualization of Hydrogen Diffusion in a Hydrogen-Enhanced Fatigue Crack Growth in Type 304 Stainless Steel

To study the influence of hydrogen on the fatigue strength of AISI type 304 metastable austenitic stainless steel, specimens were cathodically charged with hydrogen. Using tension-compression fatigue tests, the behavior of fatigue crack growth from a small drill hole in the hydrogen-charged specimen was compared with that of noncharged specimen. Hydrogen charging led to a marked increase in the crack growth rate. Typical characteristics of hydrogen effect were observed in the slip band morphology and fatigue striation. To elucidate the behavior of hydrogen diffusion microscopically in the fatigue process, the hydrogen emission from the specimens was visualized using the hydrogen microprint technique (HMT). In the hydrogen-charged specimen, hydrogen emissions were mainly observed in the vicinity of the fatigue crack. Comparison between the HMT image and the etched microstructure image revealed that the slip bands worked as a pathway for hydrogen to move preferentially. Hydrogen-charging resulted in a significant change in the phase transformation behavior in the fatigue process. In the noncharged specimen, a massive type α′ martensite was observed in the vicinity of the fatigue crack. On the other hand, in the hydrogen-charged specimen, large amounts of ε martensite and a smaller amount of α′ martensite were observed along the slip bands. The results indicated that solute hydrogen facilitated the ε martensitic transformation in the fatigue process. Comparison between the results of HMT and EBSD inferred that martensitic transformations as well as plastic deformation itself can enhance the mobility of hydrogen.     

Early stages of decomposition in iron-carbon and iron-nitrogen martensites: Diffraction analysis using synchrotron radiation

The early stages of decomposition of iron-carbon and iron-nitrogen martensites were studied by means of diffraction analysis. After aging at room temperature (RT) for 3. 5 years and after tempering at 405 K for 1 hour,ε/η transition carbide reflections but noα′-type (superstructure) reflections were detected for FeC martensites. However, for FeN martensites, theα′ superstructure reflections were observed on aging at RT for 3. 5 years and on tempering at 405 K for 1. 5 hour. For short aging times at RT (up to 60 hours),α′ reflections (not of the superstructure type) were observed for FeN martensites. Upon tempering, changes in theα′ crystal structure occur, which were discussed in terms of annihilation of structural vacancies on the nitrogen sublattice and stress relaxation. In the diffraction pattern recorded from FeC martensite (for short aging times at RT), weak reflections occurred which could not be identified conclusively yet and which disappeared after tempering for 1 hour at 405 K.     

Phase transformations and phase equilibria in the Fe-N system at temperatures below 573 K

The phase transformations of homogeneous Fe-N alloys of nitrogen contents from 10 to 26 at. pct were investigated by means of X-ray diffraction analysis upon aging in the temperature range from 373 to 473 K. It was found that precipitation of α″-Fe₁₆N₂ below 443 K does not only occur upon aging of supersaturated α (ferrite) and α′ (martensite), but also upon transformation of γ′-Fe₄N_1-z and ɛ-Fe₂N_1-x (<20 at. pct N). No α″ was observed to develop upon aging of γ(austenite). Therefore, it is proposed that γ′ is a stable phase at temperatures down to (at least) 373 K. Phase formation upon annealing at low temperatures is apparently governed by the (difficult) nucleation and (slow) growth of new Fe-N phases: α″ forms as a precursor for α because of slow nitrogen diffusion, and nitrogen-enriched ɛ develops as a precursor for γ′ because of a nucleation barrier.     

Influence of strain-induced martensitic transformations on fatigue crack growth rates in stainless steels

The fatigue crack growth rates (FCGR) of two unstable austenitic stainless steels (Fe-16 Cr-13Ni) and (Fe-18Cr-6.5Ni-0.19C) were determined in theM_s-M_d temperature range where a strain induced μ → α′ martensitic transformation occurs near the crack tip. These FCGR were compared to the rates measured in the stable austenitic phase of a Fe-31.5Ni and a Fe-34 Ni alloy and in the martensitic phase obtained by quenching the Fe-31.5 Ni alloy below M_s. In the Fe-31.5 Ni, the FCGR are an order of magnitude higher in the martensitic than in the austenitic structures for ΔK ≤ 40 ksi in. The FCGR of the stainless steels decrease markedly when the test temperature approachesM _s in theM _s - M_d range. The FCGR for the alloy Fe-18Cr-6.5 Ni-0.19 C in a warm-worked condition are consistently higher than for the same alloy in the annealed condition for ΔK ≤ 40 ksi √in.. The results are discussed in terms of the influence of phase structures, stacking fault energy and work hardening exponent on the FCGR.