Showing papers on "Austenite published in 1988"

Welding Metallurgy of Stainless Steels

Abstract: Contents: Significance of Constitution Diagrams for the Understanding of Welding Phenomena * Metallurgical Processes During Solidification and Cooling in Stainless Steel Weld Metal * Metallurgical Phenomena in Secondary Crystallization of Stainless Steels and Weld Metals * Precipitation Phenomena in Stainless Steel and Weld Metals * Hot Cracking Resistance During the Welding of Austenitic Stainless Steels * Welding Metallurgy of Ferritic Stainless Chromium Steels with Carbon Contents Below 0.15 per cent * Welding Metallurgy of Low Carbon Chromium-Nickel Martensitic Stainless Steels (Soft Martensitic Steels) * Welding Metallurgy of Duplex Austenitic-Ferritic Stainless Steels * Welding Metallurgy of Austenitic Stainless Steels * General Instructions for the Welding and Post-Weld Surface Treatments of Fabrications and Welded Components Made from Austenitic Stainless Steel * Welding Metallurgy of Heat Resisting Steels * Welding Metallurgy of Austenitic-Ferritic Dissimilar Joints * Appendix: Abbreviations and Short Designations * References * Author Index * Subject Index.

Precipitation of the δ-Ni 3 Nb phase in two nickel base superalloys

Abstract: The precipitation of the equilibrium δ-Ni3Nb phase has been studied in two niobium bearing nickel base superalloys—INCONEL 718 and INCONEL* 625—both of which are hardenable by the precipitation of the metastableγ″-Ni3Nb phase. The morphology and the distribution of precipitates have been examined and the crystallographic orientation relationship between the austenite and theδ phases has been determined. The nucleation of theδ phase at stacking faults within pre-existing δ" precipitates has been discussed.

The tempering of iron- carbon martensite; dilatometric and calorimetric analysis

Abstract: The aging behavior of iron-carbon martensite (1.13 wt Pct C) between -190 °C and 450 °C was investigated by quantitative analysis of the corresponding changes in volume and enthalpy. A method to determine activation energies of the occurring solid-state transformations by performing non-isothermal measurements of some physical property of the specimen has been described. Martensitic specimens were prepared by carburizing pure iron and quenching in brine and liquid nitrogen. The dilatometric and calorimetric experiments were supplemented with microhardness measurements. At least five different stages of structural change can be distinguished, which are quantitatively analyzed in terms of their effects on volume and enthalpy: (i) transformation of retained austenite into martensite (between −180 and −100 °C); (ii) redistribution of carbon atoms (below 100 °C); (iii) precipitation of transition carbide (between 80 and 200 °C); (iv) decomposition of retained austenite (between 240 and 320 °C); and (v) conversion of transition carbide into cementite (between 260 and 350 °C).

A melting and solidification study of alloy 625

Abstract: The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M6C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M6C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m L , m S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.

Effects of aging on the microstructure of 17-4 PH stainless steel

Abstract: Microstructures developed in commercial 17-4 PH precipitation-hardened stainless steel after different heat treatments have been studied. The martensite formed in this alloy has been found to be of a lath morphology having a dislocated substructure. Transmission electron microscopy studies have revealed the formation of a copper-rich phase in the samples aged at temperatures below 550°C. However, at higher temperatures, in addition to the formation of these precipitates, austenite phase reversion has been noticed, primarily at the lath boundaries. The reversion process has been found to be accompanied by a sharp decrease in the hardness of the alloy. The morphologies of the transformation products and the crystallography of the transformation have been discussed.

Initial Development of Thermal and Stress Fields in Continuously Cast Steel Billets

Abstract: A mathematical model has been developed to compute the thermomechanical state of the shell of continuously cast steels in a round billet casting mold. The model determines the temperature distributions, the stresses in and the gap between the casting mold and the solidifying strand. The effect of variations in steel carbon content and mold taper on the thermal, displacement, and stress fields are examined. Comparisons with available experimental observations verify the predictions of the model. The model demonstrates that the thermal shrinkage associated with the phase change from delta-ferrite to austenite in 0.1 Pct C steel accounts for the decreased heat transfer observed in that alloy, as well as its susceptibility to cracking.

A stress relaxation method for following carbonitride precipitation in austenite at hot working temperatures

Abstract: Stress relaxation measurements were carried out on a plain carbon and four solution-treated Ti steels over the temperature range 850 to 1050 °C. The results show that the stress relaxation of plain carbon austenite after a 5 pct prestrain (i.e., in the absence of precipitation) can be described by the relation σ = σ0 -α ln(l + βt). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is completed. As a result, this mechanical method is particularly suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. A new model regarding the effect of precipitation on dislocation motion is proposed, on the basis of which, the phenomenology of the stress relaxation technique is clarified. Precipitation-time-temperature (PTT) diagrams were determined for the Ti bearing steels containing 0.05, 0.11, 0.18, and 0.25 pct Ti. The PTT curves obtained are C-shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the compositions of the steels tested.

Solid solution and grain size hardening of nitrogen-alloyed austenitic steels

Abstract: The influence of interstitially dissolved nitrogen on the yield strength of various austenitic steels is investigated. A Hall-Petch analysis of the yield strength is made and it is shown that nitrogen increases the yield strength by solid solution and grain size hardening.

Mechanism of σ-phase precipitation in Cr–Ni austenitic steels

Abstract: The mechanism of σ-phase precipitation in industrial Cr–Ni austenitic steels has been found to depend on the chemical composition of the austenite after precipitation of the interstitial phases whose formation always precedes nucleation of the σ-phase. Depending on the chemical composition of the austenite, the σ-phase may precipitate directly from the austenite and partially through the M23C6 carbide or through the ferrite, or only through the M23C6 carbide. In this last case the quantity of σ-phase precipitated does not exceed 0·5%.MST/529

Thermal conductivity and thermal expansion of stainless steels D9 and HT9

Abstract: Renewed interest in the use of metallic fuel in a liquid-metal fast breeder reactor has prompted study of the thermodynamic and transport properties of fuel and cladding materials. Two stainless steels are of particular interest because of their good performance under irradiation. These are D9, an austenitic steel, and HT9, a ferritic steel. Thermal conductivity and thermal expansion data for these cladding alloys are of particular interest in assessing in-reactor behavior. These two properties were measured for the two steels at temperatures to 1200 K. Of particular interest is the influence on these properties of a phase transition in HT9.

Manganese as an austenite stabilizer in FeCrMnC steels

Abstract: As part of an alloy development program for a nickel-free stainless steel, five FeMn, 13 FeCrMn and 15 FeCrMnC alloys were examined to determine the austenite-stable region in the FeCrMnC system. After various heat treatments, optical and electron microscopy studies and magnetic measurements were used to assess the microstructural constituents present. The results indicate that the Schaeffler diagram, which was developed to predict the phases present in FeCrNiC stainless steels, cannot be directly applied to FeCrMnC steels. Manganese has a lower austenite-stabilizing capability against δ-ferrite formation than the Schaeffler diagram predicts by simply considering it half as effective as nickel. However, its stabilizing capability against martensite formation is higher than predicted by the diagram on the same basis. The new results can be used to establish the austenite-stable composition region that should be suitable for alloying to develop a manganese-stabilized stainless steel.

Evaluation of the effects of segregation on austenite grain boundary energy in Fe-C-X alloys

Abstract: A scanning Auger microprobe study has been made of the segregation of substitutional alloying elements to austenite grain boundaries in Fe-C-X alloys (where X = Mn, Ni, Si, Co, and Mo). The grain boundary enrichments in X are considerably smaller than those previously estimated from the thermal grooving method but appear consistent with other SAM results in the literature. Mo exhibits the highest enrichment factor, those for Si and Mn are intermediate, and no appreciable grain boundary enrichment of either Co or Ni is observed. In view of the special relevance of this information to nucleation kinetics of austenite decomposition products at austenite grain boundaries, the reductions in grain boundary energy attending the measured enrichments are evaluated using the model of interactive segregation of interstitial and substitutional solutes formulated by Guttmann and McLean. These calculations were performed under two different (limiting) conditions: (i) equilibrium segregation of both solutes is fully achieved at the isothermal reaction temperatures, and (ii) the boundary concentration of X is fully inherited from the austenitizing temperature and only paraequilibrium segregation of carbon is achieved. Various characteristics of interactive segregation are also discussed in terms of the interaction and binding energies of each solute.

Calculation of the Fe-C-Ti ternary phase diagram

Abstract: A thermodynamic analysis of phase equilibria in the Fe-Ti, Ti-C, and Fe-C-Ti systems has been carried out. The Gibbs energy has been expressed by the two-sublattice model being separated into the paramagnetic and ferromagnetic terms. In particular, the equilibrium between austenite and NaCl type Ti-carbide has been treated as a portion of the miscibility gap in the fcc phase. The thermodynamic parameters for each phase were evaluated on the basis of experimental phase equilibrium and activity data. The calculated phase boundary of austenite in equilibrium with Ti-carbide exhibited a characteristic shape, not only in the isothermal but also in the vertical section.

A remote laser system for ultrasonic velocity-measurement at high-temperatures

Abstract: The temperature dependence of the longitudinal ultrasonic velocity in iron and Dural has been obtained using a totally remote laser technique. The ultrasound is generated by irradiation of one face of a sample with a Q‐switched Nd:YAG laser pulse and is detected on the opposite face using a modified Michelson laser interferometer. The system has proved capable of measurements up to temperatures in excess of 1000 °C, with an absolute accuracy in velocity of ±1%, and relative accuracies of better than 0.1%. Anomalies in the data for iron have been discerned at the Curie temperature of ∼768 °C, due to the ferromagnetic to the paramagnetic phase transition, and at 910 °C due to the crystallographic phase transition from ferrite to austenite.

Swing back in kinetics near M s in hypereutectoid steels

Abstract: The kinetics and metallography of isothermal transformations in four hypereutectoid steels (0.85 to 1.80 wt pct C) have been studied in the temperature range between 623 and 333 K. Isothermal transformation diagrams for each steel were constructed by means of dilatometry and microscopy. It was found that the C curve, the reverse in kinetics, which has been called “Swing Back”, appeared at the temperature near the in each steel. This paper focuses on the swing back phenomena appearing in four steels. The nose temperatures of C curves nearin 0.85, 1.10, 1.45, and 1.80 wt pct C steels were determined to be about 520, 440, 400, and 375 K, respectively. It was clarified that the C curves near in 1.45 and 1.80 wt pct C steels were associated with the formations of lower bainite with midrib (LBm), thin-plate isothermal martensite (TIM), and leaf-like isothermal martensite (LIM), but those in 0.85 and 1.10 wt pct C steels were related to the formation of LBm. On the basis of the kinetics and metalography, the temperatureJcarbon-contentJtransformation diagram was constructed in the hypereutectoid range of steel.

Development of high toughness in austempered type ductile cast iron and evaluation of its properties

Abstract: In order to increase the toughness of austempered ductile cast irons, we attempted to strengthen the fracture initiation sites such as graphite-matrix interfaces and eutectic cell boundaries in a way of the microsegregation of alloying elements. For instance, the retained austenite which is stable under external stresses may be introduced preferentially into these sites by the addition of Ni, which segregates to a graphite periphery and of Mn, which partitions mainly to eutectic cell boundaries. Following this concept, the effects of various austempering processes on toughness are also in-vestigated. The cast iron alloying with Ni and Mn shows the best fracture toughness when it is heat-treated by either QB' or B' process; here, the QB' means the oil-quenching from an austenite γ phase range followed by austempering from a ferrite α plus γ range and the B' means austempering from a (α + γ) range. In the newly developed iron, there is a mixed microstructure composed of the ferrite, bainitic ferrite, and austenite. Abnormal elongation due to the TRIP effect in the austenite phase is found to have occurred at about 198 K. Moreover, it is shown that this TRIP effect may be caused by the formation of deformation twins.

Electron microscope study on martensitic transformations in Fe- Pt alloys: General features of internal structure

Abstract: Martensitic transformations of Fe-Pt alloys near the composition of Fe3Pt with various degrees of order were examined systematically mainly by transmission electron microscopy. This alloy system exhibits two types of martensitic transformations, fcc-bct (bcc) and fcc-fct. The present experimental results indicate that each transformation is independent and compctitive. As the degree of order of austenite (L12 superlattice) increases, the dislocation density of bct (bcc) martensite is remarkably reduced and the shape of internal twins is changed, which is closely related to the thermoelastic behavior and the reversibility of the transformation.

Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel

Abstract: Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain fracture toughness (K IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows: in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural factor controllingK IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls theK IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase bainite have a significant effect on theK IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite, dramatically improves theK IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the parent martensite is associated with tempered martensite, it significantly lowers the KIC.

Expression for solubility product of niobium carbonitride in austenite

Abstract: An expression for the solubility product of niobium carbonitride in the austenite is obtained that agrees reasonably well with published data. A generalisation of such an expression is also given which provides a simple thermodynamical estimate for the solubility product of the multicomponent fcc carbonitride of the form (Nb, Ta, V, Ti, Zr) (C, N).MST/576

Laser transformation hardening of iron-carbon and iron- carbon- chromium steels

Abstract: The laser transformation hardening response of Fe-0.5C-0.8Mn and Fe-0.5C-0.8Mn-0.8Cr steels was examined. A 2 kW CO2 laser was used to scan the steel surfaces at various rates. Complete transformation of pearlite to austenite, and hence to martensite, occurred in the laser heated surface layer of the Fe-C-Mn steel. During equivalent heat treatment of the Fe-C-Mn-Cr steel, incomplete austenitization of the pearlite colonies left the cementite plates largely undissolved. However, the maximum surface hardness was approximately the same for both alloys. Comparison of calculated and measured hardened depths yielded values of the effective coupling coefficient of the laser beam to the steel which varied as a function of beam interaction time. Modeling the process allowed a dis-tinction to be made between the effects of alloying elements and of pearlite spacing upon the depth of complete austenitization. In this case, the effect of the difference in pearlite spacing between the two steels was negligible. In the alloy steel, Cr and Mn were strongly partitioned to the cementite before heat treatment, and remained so after laser processing. Incomplete austenitization of that steel is attributed to partitioning of alloying elements to the cementite and their retarding influence on the diffusion controlled dissolution kinetics of the alloyed carbide.

Cracking kinetics of two-phase stainless steel alloys in hydrogen gas

Abstract: The kinetics of hydrogen-induced slow crack growth (SCG) under constant load was studied in two stainless steel alloys containing mixtures of bcc and fcc phases. FERRALIUM 255, a duplex stainless steel, consisting of ∼50 pct austenite in a ferrite matrix, was tested in hydrogen gas at 0 to 100 °C with the loading axis both perpendicular and parallel to the rolling direction. In addition, specimens of AISI 301 were deformed in air in different ways to produce various amounts of bcc phase in an austenite matrix prior to testing in H2 gas at room temperature. The kinetics of subcritical slow crack growth (SCG) in these alloys was compared with that for austenitic and for ferritic stainless steels. The SCG rates were rationalized in terms of differences in hydrogen permeation in the two phases. The results confirm that a higher rate of supply and accumulation of hydrogen in the region ahead of the crack tip allows a higher cracking velocity.