Showing papers on "Austenite published in 1974"

The effect of austenitizing temperature on the microstructure and mechanical properties of as-quenched 4340 steel

Abstract: The effect of austenitizing temperature on both the plane strain fracture toughness,KIC, and the microstructure of AISI 4340 was studied. Austenitizing temperatures of 870 and 1200°C were employed. All specimens austenitized at 1200°C were furnace cooled from the higher austenitizing temperature and then oil quenched from 870°C. Transmission electron microscopy revealed an apparent large increase in the amount of retained austen-ite present in the specimens austenitized at the higher temperature. Austenitizing at 870°C resulted in virtually no retained austenite; only minor amounts were found sparsely scat-tered in those areas examined. A considerably altered microstructure was observed in specimens austenitized at 1200°C. Fairly continuous 100 to 200A thick films of retained austenite were observed between the martensite laths throughout most of the area exam-ined. Additionally, specimens austenitized at 870°C contained twinned martensite plates while those austenitized at 1200°C showed no twinning. Plane strain fracture toughness measurements exhibited an approximate 80 pct increase in toughness for specimens austen-itized at 1200°C compared to those austenitized at 870°C. The yield strength was unaffected by austenitizing temperature. The possible role of retained austenite and the elimination of twinned martensite in the enhancement of the fracture toughness of those specimens austen-itized at the higher temperature will be discussed.

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

Abstract: 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 theMs-Md 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 Ms. 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 - Md 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.

Stress-Assisted and strain-induced martensites in FE-NI-C alloys

Abstract: A metallographic study was made of the martensite formed during plastic straining of metastable, austenitic Fe-Ni-C alloys withMs temperatures below 0°C. A comparison was made between this martensite and that formed during the deformation of two TRIP steels. In the Fe-Ni-C alloys two distinctly different types of martensite formed concurrently with plastic deformation. The large differences in morphology, distribution, temperature dependence, and other characteristics indicate that the two martensites form by different transformation mechanisms. The first type, stress-assisted martensite, is simply the same plate martensite that forms spontaneously belowMs except that it is somewhat finer and less regularly shaped than that formed by a temperature drop alone. This difference is due to the stress-assisted martensite forming from cold-worked austenite. The second type, strain-induced martensite, formed along the slip bands of the austenite as sheaves of fine parallel laths less than 0.5μm wide strung out on the {111}γ planes of the austenite. Electron diffraction indicated a Kurdjumov-Sachs orientation for the strain-induced martensite relative to the parent austenite. No stress-assisted, plate martensite formed in the TRIP steels; all of the martensite caused by deformation of the TRIP steels appeared identical to the strain-induced martensite of the Fe-Ni-C alloys. It is concluded that the transformation-induced ductility of the TRIP steels is a consequence of the formation of strain-induced martensite.

Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel and an iron-nickel-carbon alloy

Abstract: The effects of austenitizing conditions on the kinetics at the start of martensite formation in Fe-31Ni and Fe-31 Ni-0.28C alloys have been studied using electrical-resistance measurements during cooling of the specimens to follow the course of the transformation. The primary object of the study was to decide whether or not a change in austenitizing temperature, in the absence of a change in austenite grain size, has any effect on the Ms temperature or the burst characteristics of athermal martensite. It is concluded that it does not, suggesting that the potential nuclei (embryos) of martensite are mechanically stable crystal defects. Another interesting observation is that when the austenite grain size is small, the Mb temperature increases with increasing grain size and the burst is always small. When the austenite grains are coarse, the Mb temperature is independent of the grain size and the burst is large. It is suggested that this phenomenon is a result of the elastic shear stress concentration being related to the size of the first martensite plate and, in turn, to the size of the austenite grain.

Thermodynamics of the carbides in the system Fe-Cr-C

Abstract: Phase relations in the Fe-Cr-C system in the temperature range 900 to 1150°C have been studied using metallographic and X-ray methods and the electron microprobe. An isothermal section of the phase diagram at 1000°C is shown. Lattice dimensions of the three carbides were determined for several values of the ratio Cr:(Fe +Cr). The solubilities of the carbides at each temperature were determined by metallographic study of quenched specimens. The distribution of Cr between austenite (γ) and the several carbides was determined by use of the electron microprobe. Data of Wada et al on the activity of carbon were used to calculate activities at the y-phase boundary and the free energy of the several carbides as a function of their chromium content. The data are treated thermodynamically on the basis of assumed random mixing of Cr and Fe atoms in each carbide. While this randomness was not definitely proved, the assumption was shown to be reasonable and the results useful. Extrapolation to 0 and 100 pct Cr gives values for the standard free energy of Cr7C3 and the hypothetical carbides Cr36C, and Fe7C3.

Interdiffusion of Cr, Mo, and W in Iron

Abstract: The diffusion rates of chromium, molybdenum, and tungsten in α- and γ-iron have been determined by electron-probe microanalysis. Alloy concentrations up to 28·3 at.-% Cr, 5·05 at.-% Mo, 2·9 at.-% W have been investigated over the range 675 to 1485° C. At any temperature in either ferrite or austenite, D Mo > D Cr > D w. Interdiffusion in each alloy system was found to be independent of concentration for the composition ranges studied. Data from the present investigation have been combined with existing reliable data so that diffusion in these alloy systems may be described by the following values of frequency factor (Do ) and activation energy (Q), with errors given at the 95% confidence limits.

Carbide refining heat treatments for 52100 bearing steel

Abstract: The life of through-hardened 52100 anti-friction bearing components is improved if the excess carbides, undissolved during austenitization, are small and uniformly dispersed. One kind of carbide-refining heat treatment consists of 1) dissolving all carbides, 2) isothermally transforming the austenite to pearlite or bainite, and 3) austenitizing, quenching and tempering in the usual manner. Each step in this sequence of treatments was investigated, and the behavior of pearlitic and bainitic microstructures during subsequent austenitization was contrasted with the behavior of ferrite/spheroidized-carbide microstructures. It was shown that: 1) The usual hardening treatments given spheroidize-annealed bearing components result in an inhomogeneous microstructure, possibly due to the faster dissolution of carbides near austenite grain boundaries. 2) Austenitization of pearlite or bainite produces very uniform dispersions of ultra-fine carbides on the order of 0.1 µm diameter or less. 3) Specimens with ultra-fine carbides tend to have more retained austenite. 4) The rate of coarsening of ultra-fine carbides at austenitizing temperatures of 840°C and below, is slow enough so that conventional furnace heat treatments are satisfactory for producing this microstructure.

Increased fracture toughness in a 300 grade maraging steel as a result of thermal cycling

Abstract: Systematic changes in the fracture toughness of a 300 grade commercial maraging steel were obtained using a non-standard heat-treating process. Microstructures consisting of variable amounts of retained austenite in an aged martensitic matrix were produced. A mathematical model is presented relating these toughness values to the properties of the individual constituents. Increases in fracture toughness resulting from the non-standard heat-treatment were attributed to the nature of the distribution of the tough phase (retained austenite) in a brittle matrix of precipitation hardened martensite. In some cases, a strain-induced transformation to martensite was observed which greatly added to the toughness. Some improvements in fatigue crack propagation characteristics also resulted from this heat treatment.

The reversible martensite transformation in iron-platinum alloys near Fe3Pt

Abstract: A series of iron-platinum alloys containing 25 or 27 at. pct platinum and with ordering of the γ-phase varying from substantial disorder to nearly complete order have been thermally cycled between 25°C and - 196°C. The kinetics of the γ⇌α transformations, the hysteresis revealed by electrical resistanceJtemperature plots, the thermoelastic growth and the reappearance of an identical microstructure after thermal cycling (the microstructural memory effect) were studied as a function of the ordering of the γ-phase. Thermoelastic growth does not appear to be affected by changes in the degree of order of partially ordered specimens but the microstructural memory was imperfect in the most highly ordered specimen examined. In agreement with earlier observations by Dunne and Wayman,3 the difference between the As and Ms temperatures and the hysteresis decrease markedly as the order is increased. It is shown by transmission electron microscopy that in all but the most highly-ordered specimens the α- γ transformation produces plates of austenite with a high density of dislocations. These plates are separated from the surrounding untransformed parent austenite by arrays of dislocation loops lying in the interfaces between untransformed parent austenite and the original martensite plates. All the dislocations have a Burgers vector direction which is the same as that of the usual slip dislocation in austenite. Such dislocations lying in a habit plane must be sessile. In the well-ordered specimen dislocation pairs, typical of glide dislocations in a crystal with long-range order, were formed in the austenite formed by the reverse transformation. These dislocations were segregated into roughly plate-like clusters, but the number of clusters in unit volume was appreciably less than the number of original plates of martensite. In this case, no arrays of sessile loops of dislocations mark the locations of the original martensite-austenite interface. It is deduced from the microscopic and kinetic results that the inherited nuclei responsible for the microstructural memory effect are located in localized volumes of highly dislocated austenite formed by the α- γ transformation. No unique dislocation configurations which could be associated with specific nuclei were found. The effects of ordering on the various kinetic effects and the microstructural memory are discussed in terms of the concept of inherited nuclei, the change of the flow stress of the γ-phase with ordering and temperature and the variation of To and the transformation driving-force with ordering.

The Isothermal Decomposition of Austenite in Simple Chromium Steels

Abstract: Two high-purity steels containing 12 and 5% chromium with 0.2% carbon have been isothermally transformed in the temperature range 775-600°C. The various ferrite and carbide morphologies encountered...

Grain-Size Control in Steels

Abstract: An account has been given of the methods of grain-size control in steels. The importance of particle pinning in inhibiting grain coarsening has been described and some consideration given to the solubities and coarsening characteristics of the particles themselves. Grain-size control by hot working has also been considered, where the recrystallization characteristics of the austenite are important in connection. with the grain size of the ferrite-transformation product. The effects of second-phase particles on the recrystallization of cold-worked carbon steels have also been considered. The importance of the integrated nucleation rate on the final grain size has been demonstrated, independent of whether the ferrite grains are formed by transformation or by recrystallization.The mechanism of secondary recrystallization has been described, and it is suggested that this process can be caused by the presence of critically dispersed second-phase particles.

Influence of martensite formed during deformation on the mechanical behavior of Fe-Ni-C Alloys

Abstract: Tension tests were performed on metastable austenitic Fe-Ni-C alloys to study the influence of martensite formed during straining on mechanical behavior. Although both stressassisted, plate martensite and fine, lathlike strain-induced martensite were formed, only stress-assisted martensite occurred in sufficient amounts to influence mechanical behavior. The formation of stress-assisted, plate martensite raised the flow stress and the strain-hardening rate, but only after 25 to 40 pct martensite had formed. A TRIP effect was obtained,i.e., ductility was enhanced by concurrent martensite formation, but this occurred over only a narrow temperature range because of the strong temperature dependence of the austenite-to-plate martensite reaction. This is in contrast to the behavior of high-Cr austenitic steels, which exhibit a TRIP effect over a wide temperature range because of the formation of large amounts of fine strain-induced martensite, whose formation is less temperature-sensitive than that of plate martensite.

The Distribution of Boron in Pure Iron

Abstract: The solubility of boron in pure iron has been determined over the temperature range 500–1300° C using the technique of boron autoradiography, which can distinguish between boron in solution, boron-rich precipitates, and boron segregated to grain boundaries. However, the solubility limits determined should be regarded as upper limits, as the technique cannot differentiate between soluble boron and very fine boride particles (< 600A dia.). In γ-iron, the boron segregates uniformly to the grain boundaries, the degree of segregation reducing with increasing temperature. However, in α-iron precipitation of iron boride at grain boundaries was observed, with no detectable segregation between the particles. A Snoek peak was not detected in internalfriction measurements, indicating that boron is not in interstitial solid solution in α-iron. The segregation to austenite boundaries delays the nucleation of ferrite leading more readily to an acicular ferrite morphology. The structures obtained on quench-agein...

The high-temperature hardness of various phases in steel

Abstract: Apparatus constructed in the Department of Metallurgy and Materials Science, University of Cambridge, has enabled the hot hardness of the individual steel phases, ferrite, austenite, cementite, pearlite, Type I MnS, Mn–MnS eutectic, and silicates to be measured in situ with a high degree of accuracy. The work was undertaken in the context of studies of the deformation behaviour of non-metallic inclusions in steel, where the relative yield behaviour of inclusions and matrix is clearly a significant factor. Because of their wider interest the hot hardness results are presented separately. The high-temperature hardness of pearlite, of significance in its behaviour in ‘warm’ working, has been explained in terms of the hardness of its constituent phases.

The structure of splat-cooled Fe-20% Cr-25% Ni austenitic steel

Abstract: A controlled atmosphere splat quenching gun has been used to produce splats of Fe-20% Cr-25% Ni. Three types of structure were observed by thin foil electron microscopy, namely high-angle cellular, low-angle cellular, and linear arrays of dislocation loops, which were determined primarily by the heat transfer conditions. In the thin, most rapidly cooled areas (lift-off regions) high-angle cellular structures were observed which were largely free of defects. As the cooling rate decreases there was a greater tendency for low-angle cellular structures to form, but at intermediate cooling rates bands of dislocation loops were observed. These are explained in terms of solute segregation and vacancy coalescence along 〈100〉gg directions in the austenite.

Corrosion resistant austenitic stainless steel

Abstract: An austenitic stainless steel containing 0.25 percent Max. carbon, 15-20 percent manganese, 1% Max. silicon, 16-22 percent chromium, 3 percent Max. nickel, 0.5-3 percent molybdenum, 0.5-2 percent copper, 0.2-0.8 percent nitrogen, and the balance iron which in its annealed condition is substantially fully austenitic and is capable of an ultimate tensile strength in its annealed condition of about 125 ksi, and which can be cold worked to strength levels in excess of 200 ksi in which condition it is substantially fully austenitic and nonmagnetic.

The effect of austenite grain size on microcracking in martensite of an Fe-1.22C alloy

Abstract: Austenitic grain sizes of ASTM No. 9 and coarser were produced in an Fe-1.22 pct C alloy austenitized by immersion in molten lead at 1640†F (893°C), a temperature just above theA cm for this alloy, for periods between 20 s and 1 h. Microcracking sensitivity,Sv, measured as crack area/unit volume martensite, was determined as a function of grain size in brine quenched specimens. Two locations of microcracks were observed in this investigation: 1) intragranular, resulting from the impingement of one martensite plate with another, and 2) grain boundary or intergranular resulting from the impingement of martensite plates at prior austenite grain boundaries. Intragranular microcracking sensitivity, the subject of previous investigations, increased and became the dominant type of cracking with increasing grain size, and reached a constant level for grain sizes of ASTM No. 4.5 and coarser. Total microcracking sensitivity, consisting of both intragranular and grain boundary microcracks, also increased with increasing grain size, then decreased to approach the intragranular value for grain sizes coarser than ASTM No. 3.5. On the other end of the scale, grain boundary microcracking made up a much larger proportion of the total microcracking in the fine grained specimens.

The transformation to austenite in a fine grained tool steel

Abstract: The transformation to austenite in a fine grained tool steel has been investigated quantitatively until the disappearance of the ferrite. The initial structure of the steel consisted of ferrite and globular carbides. The nucleation starts at carbides which lie at the ferrite grain boundaries. The kinetics are in good agreement with Cahn's theory of grain boundary nucleated transformation. A constant growth rate was found up to 30 pct transformation. Site saturation occurs early in the reaction; this was confirmed by metallographic examination. The rate law is controlled by growth and is independent of the nucleation rate. The mechanism which is controlling this growth is the advancing ferrite-austenite interface reaction. The alloying elements influence the atomic mobilities, increasing the necessary activation energy to about 110 kcal/mol.

Low cycle fatigue of a high strength metastable austenitic steel

Abstract: The low cycle fatigue (LCF) behavior of a high strength, metastable austenitic steel called TRIP steel has been studied High strain LCF experiments on cylindrical, well-polished specimens under diametral strain control were carried out To study the effect of a mixed austenite-martensite matrix, LCF tests were also done on the TRIP steel after inducing significant amounts of martensite in the austenite matrix by means of a very high unidirectional prestrain To establish the role played by the martensite transformation, tests were also run above the MD The amount of martensite induced was magnetically measured by means of a “permeameter” built specifically for this purpose It was found that the LCF life of the TRIP steel, both at room temperature (in the presence of martensitic transformation) and at 200°C (in the absence of the transformation), was related to the plastic strain range, ePR, by the Manson-Coffin law Either cyclic hardening or softening occurred at room temperature, depending primarily upon the plastic strain range used in cycling Hardening was observed below 3 pct plastic strain range For LCF tests at 200°C, cyclic softening was observed in all cases The hardening and softening behavior has been found to depend on the martensitic transformation taking place in these steels during cycling However, the LCF life correlated best to the percent reduction in area, independent of the extent of the martensite transformation

Effect of prior austenitic grain boundary composition on temper brittleness in a Ni-Cr-Sb steel

Abstract: Grain boundary segregation during temper embrittlement of an Sb-containing, Ni-Cr steel has been examined both by Auger electron analysis and by chemical analysis by neutron activation of residues of surface layers dissolved by etching intercrystalline fracture surfaces. No grain boundary segregation of either alloying additions or impurities was detected during austenitization or tempering. Redistribution of Cr, Ni, and Sb between carbide and ferrite was observed during tempering, but no grain boundary segregation was noted. Both Ni and Sb were observed to segregate to the boundaries during embrittling. The segregated Sb was shown to be uniformly distributed along the prior austenitic grain boundaries and to control the ductile brittle transition temperature of the alloy studied. Ni segregating to the prior austenitic boundaries during embrittling was shown to be localized in a phase other than the ferritic portions of the boundaries. A possible location was shown to be the ferritecarbide interfaces in the grain boundaries. Weakening of these normally tenacious carbide and ferrite interfaces could account for the change in mode of brittle failure from transcrystalline cleavage to intercrystalline along the prior austenitic grain boundaries that is observed in temper brittle steels.