Showing papers on "Austenite published in 1975"

Kinetics of strain-induced martensitic nucleation

Abstract: Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of e’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.

The Volume Expansion Accompanying the Martensite Transformation in Iron-Carbon Alloys.

Abstract: A dilatometric investigation was conducted to determine the effect of carbon on the volume expansion accompanying the martensite transformation in iron-carbon alloys. It was found that the volume expansion at theM s temperature varies from 2.0 pct at 0.19 wt pct carbon to 3.1 pet at 1.01 pct carbon, largely due to the effect of carbon on lowering the temperature at which the transformation occurs. Also of importance is the solid solution effect of carbon on altering the lattice parameters of both the austenite and martensite phases at theMs.

Thickening kinetics of proeutectoid ferrite plates in Fe-C alloys

Abstract: Thermionic electron emission microscopy was used to measure directly the thickening kinetics of proeutectoid ferrite sideplates in Fe-C alloys. These kinetics were found to be exceedingly irregular. During the first few seconds of growth, the thickening rate is 5 xl0-5±1 cm/s; afterwards it usually diminishes to 1 - 30 × 10-6 cm/s. As predicted by a general theory of precipitate morphology, thickening was shown to occur only by the ledge mechanism, despite the relatively poor matching of the austenite and the ferrite lattices. Ledges were observed to lengthen at rates controlled by the diffusion of carbon in austenite. Tent-shaped and other more complex surface relief effects, rather than the invariant plane strain relief, were found to predominate. These features are shown to be the expected result of a diffusional transformation occurring by means of a ledge mechanism.

Recrystallization and Precipitation during Hot Working of a Nb-Bearing HSLA Steel

Abstract: A torsion-testing machine developed at IRSID has made it possible to study static and dynamic recrystallization of austenite over a wide range of temperatures, strains, and strain rates. Some of the results have already been published elsewhere (A. le Bon, J. Rofes-Vernis, and C. Rossard, Mem. Sci. Rev. Met., 1973, 70,, 577). Recent results are presented concerning precipitation during and after deformation, with particular emphasis on the influence of these phenomena on static and dynamic recrystallization of austenite.

Hydrogen performance of precipitation-strengthened stainless steels based on A-286

Abstract: Eleven alloys similar to commercial A-286 but with small variations in chemistry were tested to assess whether the performance of this type of stainless steel, which shows a ductility (RA) loss of about 48 pct when hydrogen charged, could be improved. It was found that removal of Mn resulted in anRA loss of only 15 pct; low Mn plus a 5 pct increase in Ni gave further improvement to 11 pctRA loss. Reductions in C, Si and B had little effect on theseRA losses. None of the above changes had significant effect on strength. Other chemistry changes were unsuccessful: additions of Al, Ti, Ti + Mo, or decreases in Ni, increased the ductility loss. Extensive examination of thin foils permitted a structural rationale for behavior in hydrogen in terms of the effects of second-phase particles on ductile fracture, particularly as affected by the misfit between the austenite and the γ’ precipitates. This rationale was consistent with fractographic observations.

Influence of boron on the decomposition of austenite in low carbon alloyed steels

Abstract: The influence of boron on the isothermal decomposition of Fe-Ni6-C0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history.

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Abstract: Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M23C6 was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti4C2S2 which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M23C6 and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M23C6 and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.

Crystallography of grain boundary proeutectoid ferrite

Abstract: A study has been made of the crystallography of proeutectoid ferrite precipitated at high angle austenite grain boundaries in an Fe-0.47 pct C alloy, isothermally transformed above the eutectoid temperature. Using the Kossel X-ray microdiffraction technique, ferrite orientations have been determined in relation to the orientations of both matrix grains at the grain boundary; the austenite orientations were derived indirectly. It has been observed that the ferrite, irrespective of morphology, possessed an orientation relationship with respect to at least one matrix grain which approximated to the Kurdjumov-Sachs and, to a lesser extent, the Nishiyama relationships. At several of the boundaries the ferrite was allowed to possess an orientation relationship with both matrix grains and it has been shown that the ferrite orientation at these boundaries was often influenced by the orientation of both matrix grains. Several instances have been observed in which preferential growth occurred into the austenite grain with which the precipitate did not have a specific orientation relationship. The results have been compared with the work of Ryder, Pitsch and Mehl,5 and factors governing the observation of Widmanstatten sideplates have been discussed.

Effect of Nitrogen Content in a 18Cr-5Ni-10Mn Stainless Steel on the Pitting Susceptibility in Chloride Solutions

Abstract: Anodic behavior of 18Cr-5Ni-10Mn steels containing 0.07 to 0.35% nitrogen was studied in a 0.1N H2SO4 solution with additions of 0.01 to 0.1 N NaCI. The nitrogen content in these steels does not significantly affect their passive behavior but increases their pitting resistance. The most susceptible sites for pit nucleation are boundaries between the austenite matrix and some second-phase precipitates (presumably carbides). The effect of nitrogen on the pitting resistance of Cr-Ni-Mn steels is discussed with respect to their phase compositions.

The grain size dependence of flow and fracture in a Cr-Mn-N austenitic steel from 300 to 1300K

Abstract: The influence of polycrystal grain size in the range 18 µm to 184 µm on the tensile behavior of an austenitic stainless steel containing by wt pct 21 Cr, 14 Mn, 0.68 N and 0.12 C has been investigated over the temperature range 298 to 1273 K. Decreasing grain size has been shown to increase the flow stress at small strains in accordance with the Hall-Petch relationship at temperatures up to 873 K. The variation of the Hall-Petch constants with temperature is influenced by dynamic strain ageing between 575 and 775 K. Above 875 K, especially at low strain-rates a reversal of the Hall-Petch correlation occurs and the flow stress decreases with decreasing grain size. The relationship between ductility and temperature is marked by a minimum ductility at about half the absolute melting temperature and intergranular cavitation is observed. A decrease in grain size generally enhanced the ductility in this temperature regime whilst at fine grain sizes this trend was reversed. These results are explained in terms of a combination of a Griffith-Orowan type fracture criterion and an intergranular void sheet mechanism of fracture.

The Effect of Grain Size and Retained Austenite on the Ductile-Brittle Transition of a Titanium-Gettered Iron Alloy.

Abstract: The effect of microstructural changes on the ductile-brittle transition temperature (DBTT) was studied in a titanium-getter ed Fe-8Ni-2 Mn-0.15 Ti alloy. A fairly strong grain size dependence of the transition temperature, 8°C/mm−1/2, was found. Grain size refinement from 38 μm (ASTM #6.5) to 1.5 μm (ASTM #15.5) through a four-step thermal treatment lowered the transition temperature by 162°C. A small amount of retained austenite was introduced into this grain-refined microstructure, and the transition temperature was reduced by an additional 120 ~ 150°C. The reduction of the DBTT due to retained austenite was smaller when the austenite was in a large-grained structure (64°C). The distribution and stability of retained austenite were also studied.

A Thermodynamic Evaluation of Carbide Solubilities in the Fe–Mo–C, Fe–W–C, and Fe–Mo–W–C Systems at 1000°C

Abstract: The equilibria involving carbides and ferrite or austenite in the systems Fe–Mo–C, Fe–W–C, and Fe–Mo–W–C have been studied at 1000°e, using carefully selected alloys and well-controlled heat treatm...

Effect of microstructure on the temper embrittlement of Cr-Mo-V steels

Abstract: The effect of transformation product on the temper embrittlement susceptibility of a Cr-Mo-V steel doped with P and Sn has been investigated at different strength levels Results show that at low strength levels (< 10R C ) embrittlement susceptibilities of tempered bainite and ferrite-pearlite structures are comparable to each other, but lower relative to tempered martensite The lower susceptibility of tempered bainite relative to tempered martensite obtains up to about 40R C , above which the susceptibilities for the two structures are similar Variation of embrittlement susceptibility with microstructure is completely consistent with the degree of grain boundary segregation of P and Sn The segregation is smaller in bainite than in martensite at a given strength level and increases with increasing strength level for a given structure When compared at the same degree of embrittlement (ie, same shift in FATT), the amount of grain boundary segregate and the extent of intergranular fracture are lower in tempered bainite compared to martensite indicating that embrittlement of interfaces other than prior austenite boundaries might be an important factor in the embrittlement of bainite structures

Onset of recrystallization during the tensile deformation of austenitic iron at intermediate strain rates

Abstract: The onset of recrystallization during the tensile deformation of austenitic iron has been fully documented for the temperature range 950 to 1350°C and strain-rate range 2.8 × 10-5 to 2.3 × 10-2 s-1. Representative materials are zone-refined iron, electrolytic iron, Fe−0.05 C and Fe−5.2 Mn. In general, the strain at the onset of recrystallization decreases with increasing temperature of deformation and decreasing strain rate. The postponement of recrystallization is favored by prior annealing at temperatures above 1200°C and is greatest for the Fe−5.2 Mn alloy; however, for the range of strain rates used, it is difficult to completely eliminate recrystallization. The effects of test conditions on the onset of recrystallization are discussed in terms of a nucleation process that requires a critical amount of stored energy.

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

Abstract: The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raisedM S but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo2C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.

The effect of quench rate on the properties and morphology of ferrous martensite

Abstract: The effect of high quench rate on theMs temperature, percent transformed, martensite morphology and austenite hardness has been studied for several Fe-Ni-C steels. For these steels the quench rate was varied only in the austenite region. TheMs temperature was found to increase with increased quench rate for both high- and low carbon steels while the percent transformation increased or decreased depending upon the morphology of the steel. No variations in martensite hardness were found in the as-quenched condition, but a difference in tempering rate was found between fast and slow quenched specimens. Austenite hardness decreased slightly with increasing quench rate while the martensite morphology changed from lath to plate. Parallel aligned plate structures were observed which resemble a twinned lath morphology. It was demonstrated that the actual difference between this morphology and a true lath morphology is the self-accommodating nature of the lath structure. The morphology changes were compared to the measured changes in martensite properties in order to identify the mechanism of the morphology shift. It was concluded that for these alloys the morphology was controlled by the austenite shear mode.

Susceptibility of Cr-Ni-Mn Stainless Steels to Pitting in Chloride Solutions

Abstract: Anodic behavior of 18Cr-5Ni steels containing 0.03 to 15% Mn was studied in 0.1N H2SO4 solution with additions of 0.01 to 0.1N NaCl. The presence of 5.7 to 15% Mn in those steels decreases their ability to passivate and their resistance to pitting. The most susceptible locations for pit nucleation are boundaries between the austenite matrix and the carbide particles. The pitting resistance of chromium-nickel-manganese steels is discussed with respect to their phase compositions.

Comparative fracture toughness of an ultrafine grained Fe-Ni alloy at liquid helium temperature

Abstract: The fracture behavior of an Fe-12 Ni-0.25 Ti alloy, grain refined through thermal cycling was studied down to liquid helium temperature using a simple method of cryogenic fracture toughness testing. Comparison tests were also made with two other common cryogenic alloys. Ultrafine grained Fe-12 Ni-0.25 Ti alloy, which is 100 pct ferritic, behaved in a ductile manner similar to 304 austenitic steel. It was shown that the ductile-brittle transition temperature of the ferritic steel specimen can be suppressed below liquid helium temperature by grain refinement even in the presence of a sharp crack. Yield strength of 149 ksi with fracture toughness of 307 ksi√in. at liquid nitrogen temperature, and yield strength of 195 ksi with fracture toughness of 232 ksi√in. at liquid helium temperature were obtained.

Role of hydrogen in stress corrosion cracking of austenitic stainless steels

Abstract: The tensile properties of Types 304L and 310 austenitic stainless steels have been studied at 20 and 150 C (68 and 302 F) after cathodically charging with hydrogen at 100 C (212 F). The ch...

Some effects of retained austenife on the fatigue resistance of carburized steel

Abstract: Cyclic deformation properties have been measured for high carbon steels that simulate the surface layers on carburized AISI 4027 steel. The influence of retained austenite is manifested in cyclic hardening and the development of mean compressive stresses. These responses, which are found to vary in direct proportion to the initial retained austenite content, are attributable to deformation-induced transformation. The importance of such cyclic changes is demonstrated by strain-life relations for carburized 4027; specimens with large amounts of retained austenite in the surface layers possess superior fatigue resistance to otherwise identical specimens that are more completely transformed. It is also shown that the benefits of retained austenite diminish as the intrinsic ductility of the fully transformed material increases.

Nickel and/or cobalt-coated steel with carburized interface

Abstract: A case-hardened, corrosion-resistant steel article is provided which comprises, applying a coating of nickel and/or cobalt to the surface of the article and then subjecting said coated article to carburization by heating said coated article to and maintaining it at an austenitic temperature under carburizing conditions for a time sufficient to effect carbon diffusion into the surface of the steel article. The corrosion resistance of the steel may be further enhanced by applying a thin layer of a final metal coat selected from the group consisting of Cr, Sn, Pb, Zn, Cu and Cd.

Plastic deformation of austenitic iron at intermediate strain rates

Abstract: The plastic deformation of austenitic iron, represented by a zone-refined iron, an electrolytic iron, an Fe−0.05 C alloy, and an Fe−5.2 Mn alloy, has been documented for the temperature range 950 to 1350°C (1740 to 2460°F) and the strain-rate range 2.8 × 10−5 to 2.3 × 10−2 s−1. The intrusion of recrystallization during deformation restricts the documentation to initial periods of strain usually less than 0.10. The general problem of retaining grain structures representative of polycrystals in specimens annealed at temperatures above 0.95T m is recognized, and a basis for its solution is presented. Chemical composion appears to influence the plastic-flow behavior of austenitic iron primarily through its effect on the grain structure. Thus, the large-grained zone-refined iron is relatively weak, and the difference in behavior between the Fe−0.05 C alloy and the Fe−5.2 Mn alloy is small.

Martensitic alloy conditioning

Abstract: A two-stage process for conditioning an annealed martensitic alloy of titanium and nickel to improve its service life and provide enhanced elongation activity under high operating stress. In the first stage of the process, the alloy is maintained under a tensile stress sufficient to strain it beyond its plastic yield point while it is repeatedly thermally cycled in a primary temperature range between a lower temperature limit below the temperature at which conversion of martensite to austenite commences on heating and an upper temperature limit at least about equal to the temperature at which essentially all the martensite is converted to austenite on heating. In the second stage of the process, the alloy is maintained at a tensile stress sufficient to strain it beyond its plastic yield point while it is repeatedly thermally cycled in a secondary temperature range between a lower temperature limit equal to or higher than the temperature at which conversion of martensite to austenite commences on heating and an upper temperature limit equal to or lower than the temperature at which conversion of austenite to martensite commences on cooling. A novel product having enhanced service life and elongation activity is obtained.