Showing papers on "Austenite published in 1991"

Reversion Mechanism from Deformation Induced Martensite to Austenite in Metastable Austenitic Stainless Steels.

Abstract: Reversion mechanism from deformation induced martensite (α') to austenite (γ) has been investigated in two metastable austenitic stainless steels, 15.6%Cr-9.8%Ni (the 16Cr-10Ni) and 17.6%Cr08.8%Ni (the 18Cr-9Ni) steels, by means of magnetic analysis and transmission electron microscopy. Metastable γ almost completely transforms to lath α' by 90% cold rolling, and the α' again reverts to γ during annealing at temperatures above 700 K. Deformation induced α' in the 16Cr-10Ni steel undergoes a martensitic shear reversion during heating to 923 K annealing, while that in the 18Cr-9Ni steel does a diffusional nucleation-growth reversion on 923 K annealing. Grain refining processes are greatly influenced depending on the reversion mechanism. Martensitically reversed γ has a high density of dislocations immediately after the reversion and the γ grains are refined through recovery and recrystallization process just like that taking place in a deformed γ. On the other hand, diffusionally reversed γ is characterized by the nucleation of equiaxed γ grains within the α' matrix and the γ grains gradually grow during annealing.The reversion mechanism significantly depends on the chemical compositions of steels and annealing temperature. An increase in the Ni/Cr ratio causes an increase in the Gibbs free energy change between fcc and bcc structure, leading to a fall-down of austenitizing temperature for the martensitic shear reversion. The critical driving force required for the complete martensitic shear reversion is about -500 J/mol. To obtain the critical driving force in the 18Cr-9Ni steel, it should be heated to a high temperature above 1 023 K. However, the diffusional reversion can easily occur because the martensitic shear reversion temperature is too high in the 18Cr-9Ni steel. The 16Cr-10Ni steel also undergoes the diffusional reversion when it was annealed at low temperatures below the martensitic shear reversion, 923 K.

Mechanical Properties and Retained Austenite in Intercritically Heat-Treated Bainite-Transformed Steel and Their Variation with Si and Mn Additions

Abstract: Processing peculiarities and functions of alloying elements, such as Si and Mn, were studied for improving formability of steel sheets with mixed microstructures. Annealing a sheet steel with 0.2 pct C in the intercritical range produced very fine particles of retained austenite which were moderately stabilized due to C enrichment by subsequent holding in the bainite transformation range. Its strength-ductility balance is greatly superior to that of other dual-phase steels due to transformation-induced plasticity (TRIP). The holding time in the bainite transformation range varies with temperature, depending on the activation energy of C diffusion in austenite, and shifts to longer times with an increase of Si or Mn additions. The optimum cooling rate from the intercritical region is reduced with an increase of Mn content but is not influenced by Si content. Additional Mn makes the retained austenite content larger, although uniform elongation remains the same. In this case, the product of tensile strength and total elongation is increased due to an increase in the tensile strength. Contrary to Mn, Si does not affect retained austenite content but improves the uniform elongation by increasing its stability.

Stress induced transformation to bainite in Fe–Cr–Mo–C pressure vessel steel

Abstract: The kinetics of the bainitic transformation in a polycrystalline Fe–Cr–Mo–C alloy designed for applications in energy generation systems has been studied, with particular attention to the influence of mild tensile stresses on transformation behaviour. The steel was found to exhibit the incomplete reaction phenomenon, in which transformation to bainite stops well before the residual austenite acquires its paraequilibrium carbon concentration. It was found that even in the absence of an applied stress, the growth of bainitic ferrite caused anisotropic changes in specimen dimensions, consistent with the existence of crystallographic texture in its austenitic condition and, significantly, with the nature of the invariant-plane strain shape change that accompanies the growth of bainitic ferrite. Thus, transformation induced plasticity could be detected in fine grained polycrystalline samples, even in the absence of applied stress. The application of an external stress was found to alter radically the t...

Interactions between recrystallization and precipitation in hot-deformed microalloyed steels

Abstract: The static recrystallization and precipitation characteristics of low alloy steels containing Nb were studied following high temperature compression in the austenite range. The recrystallization kinetics in the Nb-steels were slower than those in the plain carbon steels by approximately an order of magnitude when Nb was in solution. However, much greater retardation of recrystallization was observed in the Nb-steels when precipitates were formed. The initiation of both recrystallization and precipitation appeared to be localized in the early stage of their nucleation. Recrystallization initiated predominantely at the prior austenite grain boundaries. Boundaries were also the preferential nucleation sites for NbCN precipitation. Hence, the NbCN particles were distributed in a highly localized fashion, sometimes delineating what may have been the prior austenite grain or subgrain boundaries. The heterogeneous nature of the nucleation process for recrystallization and precipitation suggested that the critical factor which determined the retardation of recrystallization was the local pinning effect of precipitation. The local pinning force was estimated from the local distribution of precipitation and was found to be of magnitude comparable to that of the driving force for recrystallization. The precipitation pinning force increased in the beginning of precipitation, showed a peak in the intermediate stage, and finally decreased as particles coarsened and were distributed more uniformly.

Optimal Chemical Composition in Fe-Cr-Ni Alloys for Ultra Grain Refining by Reversion from Deformation Induced Martensite.

Abstract: A thermomechanical treatment, which applies reversion from deformation induced martensite (α') to ultra grain refining of austenite (γ), was proposed for metastable austenitic stainless steels. To determine optimal steels for the treatment, the effect of chamical composition on the γ-α' transformation behavior during cold rolling and the α'-γ reversion behavior by successive annealing was investigated in Fe-Cr-Ni ternary alloys. An ultra fine γ grain structure was obtained when steels satisfied the following three compositional conditions:(1) Metastable γ should be almost completely transformed to α' during cold rolling at room temperature. The amount of α' induced by 90% cold rolling can be estimated by the Ni equivalent (Ni+0.35Cr). For steels with the Ni equivalent of less than 16.0 mass%, over 90 vol% of γ transforms to α' during 90% cold rolling at room temperature.(2) Most of deformation induced α' must revert to γ again at relatively low temperatures where grain growth is difficult to occur. When the Cr equivalent (Cr-1.2Ni) is less than 4.0 mass%, most α' induced by the 90% cold rolling reverts to γ through 873K-0.6ks annealing. Retained α' is less than 10 vol%.(3) The Ms temperature of the reversed γ obtained through 873K-0.6ks annealing should be below room temperature. When the Ni equivalent (Ni+0.65Cr) of steels is more than 19.7 mass%, the reversed γ is stable at room temperature. For the Fe-Cr-Ni ternary alloys which satisfy these three conditions, an ultra fine γ grain structure might be obtained through the reversion from deformation induced α'. For example, γ grains of 0.5μm were observed in a 15.5%Cr-10%Ni steel which was subjected to 90% cold rolling and subsequent 873K-0.6ks annealing.

The thermal and metallurgical state of steel strip during hot rolling: Part III. Microstructural evolution

Abstract: A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill. The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening. The Institut de Recherches de la Siderurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature.

Low temperature aging behavior of type 308 stainless steel weld metal

Abstract: The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M23C6 carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.

Properties of sputtered stainless steel-nitrogen coatings and structural analogy with low temperature plasma nitrided layers of austenitic steels

Abstract: Structural analogies are revealed between 310 stainless steel-nitrogen deposits prepared by reactive sputtering and diffusion layers resulting from low temperature plasma nitriding of the same grade of stainless steel Both the coatings and the diffusion layers show high nitrogen supersaturations in the fcc lattice of austenite and present a rather favourable compressive stress and a good level of microhardness An interesting corrosion resistance is also expected With these properties, future applications can be considered

Effects of Silicon and Manganese Addition on Mechanical Properties of High-strength Hot-rolled Sheet Steel Containing Retained Austenite

Abstract: An 80 kgf/mm2 grade high-strength hot rolled sheet steel with a significantly high product of tensile strength and total elongation (TS×EI=3 000) has been developed by utilizing transformation-induced plasticity of retained austenite in 0.2% carbon sheet steels by optimizing the silicon and manganese content and hot rolling conditions. Finish rolling temperature and coiling temperature are important factors in terms of introducing a large amount of retained austenite. Silicon addition over 1.0% results in a significant increase in the volume fraction of retained austenite due to the change in second phase from bainite+pearlite to bainitic ferrite. The maximum volume fraction of retained austenite and the consequential optimum combination of tensile strength and ductility is obtained in a 0.2%C-2.0%Si-1.5%Mn steel. The effect of retained austenite on ductility becomes small with further addition of manganese over 1.5%, because the retained austenite transforms in the early stage of the straining process due to the presence of martensite.

Influence of C Content and Annealing Temperature on Microstructure and Mechanical Properties of 400°C Transformed Steel Containing Retained Austenite

Abstract: In order to evaluate the influence of C content and annealing temperature on the mechanical properties of steels containing retained austenite, cold-rolled sheets containing 0.12 to 0.4 C, 1.2 Si, and 1.5 Mn have been intercritically annealed and isothermally transformed at 400°C. Annealing near AC1 temperature followed by the 400°C isothermal transformation for 100 to 300 sec results in the best combination of strength and ductility. The ultimate tensile strength ranges from 590 Mpa in the 0.12 C steel to 980 Mpa in the 0.4 C steel. The total elongation varies 39 to 33%, and is ranked well above that of conventional ferrite-martensite dual-phase steels at the comparable strength. Amounts of retained austenite in these specimens are 7 to 20% and linearly related to the C contents of the steel. Mechanical stability of the retained austenite is fairly improved compared to that found in conventional dual-phase steels, and enhances the ductility at high strength. Better combinations of strength and ductility are maintained even in the lower C steels due to the contribution of an increased amount of highly ductile ferrite.

Effect of prior deformation of austenite on the γ → ε martensitic transformation in Fe-Mn alloys

Abstract: The influence of prior deformation of austenite (γ) on the formation of hcp (∈) martensite in Fe-Mn binary alloys during subsequent cooling was investigated. The formation of bcc (α') lath martensite in deformed austenite was also examined for comparison. In both of the martensitic transformations to α'in the Fe-9%Mn alloy and to ∈ in the Fe-16%Mn and Fe-24%Mn alloys, the transformation start temperature (M s ) monotonously decreases with increasing the reduction of austenite at 773 K

Elemental partitioning and microstructural development in duplex stainless steel weld metal

Abstract: A thermodynamic analysis which is capable of estimating the austenite/ferrite equilibria in duplex stainless steels has been carried out using the sublattice thermodynamic model. The partitioning of alloying elements between the austenite and ferrite phases has been calculated as a function of temperature. The results showed that chromium partitioning was not influenced significantly by the temperature. The molybdenum, on the other hand, was found to partition preferentially into ferrite phase as the temperature decreases. A strong partitioning of nickel into the austenite was observed to decrease gradually with increasing temperature. Among the alloying elements, average nitrogen concentration was found to have the most profound effect on the phase balance and the partitioning of nitrogen into the austenite. The partitioning coefficient of nitrogen (the ratio of the mole fraction of nitrogen in the austenite to that in the ferrite) was found to be as high as 7.0 around 1300 K. Consequently, the volume fraction of austenite was influenced by relatively small additions of nitrogen. The results are compared with the experimentally observed data in a duplex stainless steel weld metal in conjunction with the solid state δ → δ + γ phase transformation. Particular attention was given to the morphological instability of grain boundary austenite allotriomorphs. A compariso between the experimental results and calculations indicated that the instability associated with irregular austenite perturbations results from the high degree of undercooling. The results suggest that the model can be used successfully to understand the development of the microstructure in duplex stainless steel weld metals.

An experimental and theoretical study of cementite dissolution in an Fe-Cr-C alloy

Abstract: The dissolution of cementite at 910 °C in an Fe-2.06Cr-3.91C (at. pct) alloy is investigated experimentally. The Cr concentration profiles in austenite and cementite are measured by means of the scanning transmission electron microscopy/energy dispersive spectrometry (STEM/EDS) technique at different dissolution times. The measurements show the Cr enrichment in the cementite during the dissolution process. The measurements suggest that the main part of the reaction for this alloy is controlled by Cr diffusion in the cementite or in the austenite matrix. This observation is in agreement with predictions of the local equilibrium hypothesis. The carbide fraction and average particle diameter are evaluated as functions of dissolution time. The Cr enrichment of the cementite results in a supersaturation and a possible decomposition of the cementite. Microstructural evidence for such a decomposition is found by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A new program package called DICTRA,[11] which is suitable for the simulation of diffusional reactions in multicomponent alloys, has been applied to the present case. The simulation is compared with the experimental data, and a good agreement between the two is found.

Low-temperature fatigue of 316L and 316LN austenitic stainless steels

Abstract: This article is concerned with the cyclic properties of 316L-type austenitic stainless at 300 and 77 K. The role of nitrogen alloying and of the temperature decrease is examined during low-cycle fatigue (LCF) and fatigue crack propagation. Fatigue resistance is enhanced by the addition of nitrogen in steel at both test temperatures. The results are discussed on the basis of micro-structural observations. Planar slip of dislocations is found in the nitrogen-containing steel and is favored by a decrease in test temperature. To some extent, the influence of interstitial nitrogen on the fatigue properties is related to its role in stabilizing austenite observed during cooling as well as during straining.

Effect of retained austenite on the hydrogen content and effective diffusivity of martensitic structure

Abstract: In this work, the effects of retained austenite on the hydrogen content and the effective hydrogen diffusivity of an otherwise fully martensitic structure have been studied. In the electrochemical permeation experiment, the results on the first permeation transient indicate that high-carbon as-quenched specimens have a lower effective diffusivity than those with an additional subzero treatment. This was due to the presence of retained austenite in the former specimens, which afforded more sites for hydrogen trapping throughout the specimen membrane, hence lowering the hydrogen diffusion during the first transient. As the hydrogen traps were filled up, however, the second permeation transients gave similar effective diffusivity for both as-quenched and quenched + subzero-treated specimens. After hydrogen charging in hydrogen sulfide solution, the hydrogen contents of the specimens were determined using the vacuum hot extraction method. The results show that the hydrogen contents of as-quenched specimens were higher than those of the specimens subjected to quenched + subzero treatment. This again was due to the existence of retained austenite in as-quenched martensitic matrix, where the interfaces between the retained austenite and martensitic plates provided extra sites for hydrogen trapping. The hydrogen content of the presenting retained austenite in the martensite was independent of the carbon content of the specimen, but only depended on the hydrogen-charging period.

Strain hardening and transformation mechanism of deformation-induced martensite transformation in metastable austenitic stainless steels

Abstract: Austenitic stainless steels with lower chromium or nickel contents can transform to martensite when they are plastically deformed at sufficiently low temperatures. The correspondingly obtained stress-strain curves show an unusual strain hardening behaviour, e.g. minima and maxima of the strain hardening coefficient dσ/d∅, in contrast with normal materials without transformation. A unified model is proposed on the basis of dislocation theory, thermodynamic and experimental observations, which can explain the strain hardening abnormality and give a quantitative relation between the flow stress and martensite transformation. It is shown that deformation-induced martensite formation closely correlates with the homogeneity and stability of plastic deformation. The martensite transformation is initiated by inhomogeneous deformation. The transformation proceeds by orientated rapid dislocation movement and multiplication. Its onset takes place spontaneously when a sufficiently high stress is reached, rather than cumulatively with increasing strain. The increase in the martensite volume fraction is linearly proportional to the square of the flow stress.

Solidification modeling and solid-state transformations in high-energy density stainless steel welds

Abstract: The solidification and solid-state transformations which occur during the high-energy density (HED) welding of austenitic stainless steel were studied. Comparisons were made between structures observed in gas tungsten arc (GTA) welds and those of electron beam (EB) and laser welds using Fe-Ni-Cr ternary alloys with Cr/Ni ratios ranging from 1.5 to 1.85. Weld solidification and microsegregation was modeled using a finite difference analysis and compared with experimental results. These calculations were also used to help interpret the origin of the observed microstructures. Calculations showed that little solid-state diffusion occurs during the solidification and cooling of primary austenite solidified welds, whereas structures which solidify as ferrite may become almost completely homogenized as a result of diffusion. A change in solidification mode from primary austenite to primary ferrite was found to occur at higher Cr/Ni ratios with the HED welds than with GTA welds and is attributed to dendrite tip undercooling. A nearly segregation-free, single-phase austenite structure which appears to be unique to the rapid solidification velocities and cooling rates of HED welds was also observed. It is suggested that this structure is a product of ferrite solidification which transforms to austenitevia a massive transformation.

Property enhancement in Type 316L stainless steel by spray forming

Abstract: Type 316L stainless steel was processed by rapid solidification, liquid dynamic compaction (LDC) and subjected to a series of simple thermomechanical treatments (TMT) to produce a fine austenitic grain structure. The as-deposited, fine grained structure was rollable in excess of 80% at liquid nitrogen temperature (77 K) without annealing or edge cracking. High tensile and yield values resulted from the cold worked austenitic structure at 298 K and from martensite formation on rolling at 77 K. Ductility values were on the low side but at useful levels for these high strengths.

Effect of titanium additions on austenite grain growth kinetics of medium carbon V–Nb steels containing 0·008–0·018%N

Abstract: An investigation has been carried out into the effects of multiple additions of the microalloying elements V, Ti, and Nb on the austenite grain growth characteristics of 0·4C–1·6Mn (mass-%) steels. It has been shown that simultaneous additions of V, Ti, and Nb inhibit grain growth and increase the grain coarsening temperature markedly. In both V and Ti steels, increased Nb contents refine further the austenite grain size. Using a thermodynamic model, good agreement has been obtained between the measured and calculated volume fractions of carbonitride particles at the solidus temperature, although such particles are too large for effective grain boundary pinning. Afeature of interest is that increased Nb contents in Ti–V steels decrease the size of the larger Ti rich carbonitride particles and inhibit their formation during solidification of the steel. The stability of the carbonitrides increases with increasing Ti content especially. The N dissolved in the austenite decreases with simultaneous ad...

Scaling of Austenitic Stainless Steels and Nickel-Base Alloys in High-Temperature Steam at 973 K

Abstract: Scaling of commercial high-strength austenitic stainless steels and nickel-base alloys, possible candidates for ultra-supercritical (USC) boiler-tube materials, was studied in atmospheric ...

Characterization of TiN coatings deposited on plasma nitrided tool steel surfaces

Abstract: Wear-resistant TiN coatings deposited on tool steels are used frequently in industry. There is a trend towards further optimizing these coatings, e.g. by plasma nitriding the tool surface prior to TiN deposition. In this work the influence of the nitriding conditions on the surface properties of AISI 304 and ASP 23 tool steels was investigated. The plasma nitriding was carried out in a triode ion plating configuration normally used to deposit TiN coatings. At the surface of AISI 304 stainless steel, only a thin compound layer (Fe4N, Fe3N) was found, probably as a consequence of the rather slow nitrogen diffusion in the austenite matrix. For ASP 23 high speed steel, the different nitriding behaviour of the martensitic matrix causes the formation of a diffusion layer which results in an increasein hardness at the surface. On an analogous set of specimens the TiN deposition was started immediately after the plasma nitriding. To optimize this combined treatment, the influence of the nitrogen content of the nitriding medium and that of the thickness of an intermediate titanium layer were examined. By means of X-ray diffraction only an influence on the lattice parameter of TiN and the nitriding layers was found, as compared with the lattice parameter for the separate treatments. Bad adhesion of TiN coatings on plasma nitrided AISI 304 was observed and was explained in terms of the high internal stresses in the compound layer on the nitrided surface. For the ASP 23 substrate a better coating adhesion was observed, probably due to a different structure of the nitriding layer and to the enhanced substrate harness. At this stage of the research, none of the above-mentioned nitriding treatments results in a sufficient adhesion of the TiN layer, although some useful facts could be established for further research.

The chemistry and structure of wear-resistant, iron-base hardfacing alloys

Abstract: The chemistry and microstructure of iron-base alloys resistant to galling wear were determined by using optical microscopy and scanning electron microscopy (SEM). Castings and weld overlays, deposited by the gas tungsten arc and plasma arc welding (GTAW and PAW, respectively) processes, were evaluated. The microstructure typically consisted of a primary austenitic matrix, eutectic carbides (M7C3 type), and noneutectic carbides. Processing techniques that resulted in high cooling rates yielded microstructures with finer features, less complete partitioning of alloying elements to the carbides, and improved resistance to galling wear. Carbon and manganese appeared to improve resistance to galling wear. Nickel was detrimental to galling wear resistance.

Acicular ferrite transformation in alloy-steel weld metals

Abstract: In this paper, the morphology of acicular ferrite in alloy-steel weld metals has been investigated. The effect of the grain size of prior austenite on acicular ferrite transformation has also been studied. It is found that acicular ferrite can form in reheated weld metals when the austenite grain size is relatively large. On the other hand, classical sheaf-like bainite will form at the same temperature if the austenite grain size is kept small. Further results strongly suggest that acicular ferrite is in fact intragranular bainite rather than intragranular Widmanstatten ferrite.

Recrystallization controlled rolling and accelerated cooling for high strength and toughness in V-Ti-N steels

Abstract: Optimum thermomechanically controlled process parameters have been established for the production of Ti-V-N microalloyed high-strength low-alloy (HSLA) steels. On the basis of laboratory simulation and full-scale processing, it has been shown that nitrogen is an essential alloying element addition and full appreciation of its effects leads to the ability to utilize high nitrogen steel in connection with hot rolling in a high-temperature regime to produce HSLA products with very favorable combinations of yield strength and toughness. The effects of reheating temperature, rolling reduction, cooling rate, and finish-cooling temperature (FCT) on the ferrite grain size and mechanical properties have been examined. It has been shown that the potential for precipitation strengthening is dependent on vanadium, nitrogen, and cooling parameters. Accelerated cooling (ACC) prevents precipitation of vanadium nitrides in austenite and enhances both grain refinement and precipitation strengthening. By adjusting nitrogen content and processing parameters, a yield strength of 500 MPa and impact transition temperature (ITT) below -60 ‡C can be obtained in the as-hot-rolled condition in Ti-V-N steels, using high finish-temperature hot rolling and accelerated cooling.

Microstructural engineering applied to the controlled cooling of steel wire rod: Part II. Microstructural evolution and mechanical properties correlations

Abstract: In the second part of this paper, the microstructural evolution and mechanical properties of plain-carbon steel rods which have been subjected to known cooling conditions are described. Specifically, the isothermal phase transformation kinetics for the decomposition of austenite into ferrite and pearlite have been determined with a diametral dilatometer and characterized in terms of empirical coefficients in the Avrami equation. The continuous cooling transformation (CCT) start time, fraction ferrite, ferrite grain diameter, and pearlite interlamellar spacing have been quantified and correlated with steel composition and cooling rate. Tensile tests have been conducted to obtain yield strength (YS) and ultimate tensile strength (UTS), which, with literature data, have been related to the microstructure and composition of the steels. These correlations, which apply to both hypoeutectoid and eutectoid steels, have been incorporated in a mathematical model of the Stelmor process, to be described in Part III of this article.[441]

Surface Composition of Stainless Steel during Active Dissolution and Passivation

Abstract: Anodic segregation of selected alloy constituents is found by ESCA analysis to occur as a result of active dissolution and during the passivation of stainless steel in deaerated acid. The resulting surface composition of the steel is radically different from the bulk and conditions seem to favour the formation of surface phases of intermetallic compounds. The resulting surface phases appear to control the kinetics of active dissolution and the composition of the passive film. In this respect we propose that in the more complex stainless steels passivation may be due to the formation of a duplex kinetic barrier consisting of an outer oxide based passive film and an inner intermetallic surface phase having a higher lattice binding energy than the bulk alloy. This paper will focus on the influence of anodically segregated Ni, Mo and N in a range of austenitic stainless steels exposed to deaerated HCl solutions.

Comparison of Dynamic Recrystallization and Conventional Controlled Rolling Schedules by Laboratory Simulation

Abstract: A seamless tube mill rolling process was simulated with the aim of optimizing the ferrite grain refinement by decreasing the finish rolling temperature during the stretch reducing mill (SRM) stage. Tests were performed on one Nb-V and two Ti-V micralloyed steels. For the Ti-V steels, maximum grain refinement is achieved when the SRM exit temperature is just above the Ar3; at a cooling rate of 3.5°C/s during transformation, ferrite ASTM grain size numbers of #13 and higher are produced. When the reductions are applied below the no-recrystallization temperature or Tnr, the strains are accumulated at first as a result of the absence of static recrystallization during the short interpass times. At large strains, dynamic recrystallization is initiated, due to the absence of strain-induced precipitation during the brief unloading intervals. Under these conditions, the rolling loads are lower than when a conventional controlled rolling schedule is employed. In the latter case, the interpass times are sufficiently long for the occurrence of appreciable strain-induced precipitation, which is responsible for the prevention of dynamic recrystallization, and the consequent accumulation of strain and pancaking of the austenite. For the Nb-V steels, when the SRM cooling curve passes through the nose of the CCP (continuous-cooling-precipitation) PS curve, austenite pancaking takes place. By contrast, when the cooling curve passes above or below the CCP curve, the dynamic recrystallization of austenite is initiated, leading to ferrite grain sizes which are generally finer than those produced by the pancaking process.

Continuous heating transformation of bainite to austenite

Abstract: The transformation of a bainitic FeMnSiC alloy into austenite has been studied using dilatometry, transmission electron microscopy and microanalytical techniques. The formation of austenite was investigated using two different starting microstructures, the first consisting of a mixture of bainitic ferrite and residual austenite, and the second of a mixture of tempered bainitic ferrite and carbides. Results from isothermal austenitization experiments confirm earlier work on a different alloy, that because of the incomplete reaction phenomenon associated with bainite growth, there is a large temperature hysteresis before the reverse transformation to austenite becomes possible. Continuous heating experiments revealed an identical austenization behaviour for both initial microstructures when the heating rate utilized was small. This is because any residual austenite then tends to transform into pearlite or to decompose into ferrite and discrete particles of carbides before the sample reaches a temperature where austenite growth becomes thermodynamically feasible. Consequently, the two initial microstructures become identical by the time T γ is reached. At faster heating rates the residual austenite remains stable during heating and then commences to grow as the appropriate elevated temperature is reached. A slightly higher degree of superheating is found to be necessary in the absence of residual austenite in the starting microstructure, since austenite nucleation is then necessary prior to growth. Since the excess superheating is rather small, the results indicate that nucleation does not appear to be a major hurdle to the formation of austenite in the alloy studied.

A thermodynamic assessment of the Fe-N-Ti system

Abstract: A thermodynamic analysis of phase equilibria in the Fe-N-V system has been carried out on the basis of the established three binary descriptions and selected ternary experimental data. The two-phase equilibrium between austenite and VN phase was treated as a portion of the miscibility gap in the fcc phase. A set of isothermal sections was presented in the whole range of compositions. Some thermodynamic analysis was carried out on the solubility products of VN in ferrite and austenite.