Showing papers on "Austenite published in 1972"

Tempering of steel

Abstract: Tempering of martensitic steels involves the segregation of carbon, the precipitation of carbides, the decomposition of retained austenite, and the recovery and recrystallization of the martensitic structure Because these several reactions overlap and occur on such a fine scale, it is only recently that our knowledge of the resulting structures has become reasonably complete Our present understanding of the processes involved in the tempering of iron-carbon martensites and how they are affected by alloying elements is reviewed

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Abstract: Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM s toM d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or e martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of e martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM s toM d range in these alloys.

Ultrafine-grained microstructures and mechanical properties of alloy steels

Abstract: Ultrafine-grained microstructures can be developed in a variety of alloy steels by coldworking followed by annealing in theα +γ region. Because the annealing temperatures are relatively low and the recrystallized structure is two-phase, grain growth is restricted. Specimens with grain sizes in the range 0.3 to 1.1 μ.m (ASTM 20 to 16) were obtained in manganese and nickel steels by annealing 1 to 400 hr at temperatures between 450° and 650°C (840° to 1200°F). The expected improvement in yield strength through grain refinement was observed in almost all alloys. Other tensile properties depend on factors such as grain size, austenite stability, and specimen geometry, that determine which of three types of plastic behavior will occur. Transformation of austenite during straining improves the mechanical properties of ultrafine-grained specimens.

A debate on the bainite reaction

Abstract: The authors debate three topics central to the controversies which have enveloped the bainite reaction ever since it was first recognized as a distinctive mode of austenite decomposition. These include: “what is bainite?”, “the growth mechanism of the ferritic component of bainite”, and “the sources of bainitic carbide precipitation.” RFH concludes that bainite is the product of a shear transformation. Individual bainite plates are suggested to grow substantially more rapidly than volume diffusion-control allows, but a constraint such as the build-up of volume strain energy limits the extent of their growth. This mechanism of growth ensures extensive supersaturation of bainitic ferrite with respect to carbon. Whether or not carbides precipitate in association with bainite plates and whether the carbide is cementite ore, however, is a complex question in competitive reaction kinetics. New experimental evidence is presented to demonstrate thate carbide precipitated in lower bainite dissolves upon heating above the kinetic-B stemperature in an alloy steel containing 1.5 pct Si. This result is taken to support the existence of the metastable eutectoid reactionγ ⇌ α + e atca 350°C. HIA and KRK define bainite as the product of a nonlamellar eutectoid reaction. On this view, carbide precipitation thus plays an essential, rather than an ancillary role. Development of the Widmanstatten morphology by the ferritic component of bainite is shown to be inessential to the classification of a eutectoid structure as bainite. When this morphology is present, however, it is concluded to grow by the ledge mechanism, without the participation of shear, at rates of the order of or less than those allowed by volume diffusion-control. New experimental evidence is presented to show that the lengthening and thickening kinetics of individual plates within sheaves of upper bainite are consistent with this description. The results of a new calculation indicate that the initial carbon content of bainite plates lies between theα/α + Fe3C) and the extrapolatedα/(α+ γ) phase boundaries, in agreement with expectation from the ledge mechanism of growth.

The pearlite reaction

Abstract: A critical appraisal of theory and experiments for both isothermal and forced velocity pearlite is presented. It is concluded for binary systems that both the theoretical models for volume diffusion and boundary diffusion control are well-advanced and adequate for the purposes of experimental test. However, some ambiguity remains in the boundary diffusion model with respect to the thermodynamics of the boundary ”phase” region, so it is still not possible to predict absolute rates of transformation. The theoretical problem for ternary pearlites is also well understood, although rigorous theory seems intractable. A new perturbation procedure for definition of the optimal steady-state spacing is presented and amplified for both isothermal and forced velocity pearlite, and for both volume and boundary diffusion models. In terms of the critical spacing Sc for isothermal pearlite and the spacing at minimum undercooling Sm for forced velocity pearlite the predicted stability points are as follows: {fx2777-1} For isothermal pearlite these perturbation results correspond closely to the state of maximum entropy production rate while for forced velocity pearlite the correspondence is also satisfactory. A detailed analysis of the data leads us to reaffirm the author’s conclusions that the eutectoid reactions in Cu-12 pct Al and some related ternary alloys reported by Asundi and West are controlled by volume diffusion and that the eutectoid reaction in Al-78 Zn reported by Cheetham and Ridley is controlled by boundary diffusion. We conclude further after careful analysis that the pearlite reaction in Fe-0.8 C is controlled for the higher temperatures by volume diffusion of carbon in austenite. We are also led to state that the pearlite transformations in Fe-C-Mn and Fe-C-Ni occur for the most part in a nopartition regime and are therefore controlled by volume diffusion of carbon in austenite, while the transformations in Fe-C-Cr and Fe-C-Mo, being forced by thermodynamics to sustain partition of chromium and molybdenum, are controlled by phase boundary diffusion of the latter elements. nt]mis|M. P. PULS, formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada

Chemistry of Grain Boundaries and Its Relation to Intergranular Corrosion of Austenitic Stainless Steel

Abstract: The basis of intergranular corrosion in austenitic stainless steels is examined by relating the grain boundary composition to the corrosion properties. The technique of Auger electron spectroscopy has been used to obtain the chemistry of intergranular fracture surfaces. It was found that the chromium depleted zones exist and that the depletion theory was valid for tests in weakly oxidizing solutions. In highly oxidizing solutions, however, the impurity segregation and not chromium depletion best explains the deterioration of corrosion properties. Impurity elements such as sulfur, silicon, nitrogen, and phosphorous were observed in the various steels examined. An attempt is made to explain the observed corrosion properties on the basis of chromium depletion and solute segregation theories combined with an electrochemical mechanism.

The solidification sequence in an 18-8 stainless steel, investigated by directional solidification

Abstract: The directional solidification technique was applied in order to investigate the complicated solidification sequence in a commercial austenitic stainless steel which was known to yield a primary precipitation of § ferrite when cast into a 5 tons ingot. Three stages of solidification were found. The first precipitation of § ferrite was interrupted by precipitation of austenite and at the end of the solidification there was a transition back to precipitation of § ferrite. The competition between the first two stages is affected by the cooling rate and the nitrogen content. The precipitation of austenite from the melt results in the usual coring whereas o ferrite forms with a very homogeneous composition, presumably due to rapid diffusion in this phase. On cooling austenite forms from the § ferrite and this reaction also results in coring, presumably due to rapid diffusion in § ferrite.

Activity of carbon and solubility of carbides in the FCC Fe-Mo-C, Fe-Cr-C, and Fe-V-C alloys

Abstract: The activity of carbon in austenitic Fe-Mo-C, Fe-Cr-C, and Fe-V-C alloys has been studied by equilibration with controlled CH4-H2 atmospheres at temperatures in the range 850° to 1200°C. The observations included a number of compositions in the two-phase fields, γ + carbide. Equations are given for the activity coefficient of carbon as a function of temperature and composition in the austenite field and from these the other thermodynamic properties of the solution may be computed as desired. The phase boundaries γ/γ + carbide were determined by breaks in the isoactivity lines. This was supplemented in the case of Fe-Mo-C alloys by metallographic linear analysis of equilibrated samples. The results confirm certain published phase diagrams and discredit others.

Ferrite and bainite in alloy steels

Abstract: The addition of alloying elements even in small concentrations can alter the properties and structure of ferrite and bainite. The various morphologies of ferrite-carbide aggregates are surveyed including alloy pearlite, fibrous carbide eutectoids and precipitation of fine alloy carbides atγ-α interfaces. Modern ideas on the morphology and growth kinetics of ferrite and upper and lower bainite are also summarized. Using this information, an attempt is made to rationalize subcritical transformations of austenite in low alloy steels. Basic factors influencing the strength of alloy ferrites are discussed, leading to an examination of structure-mechanical property relationships in ferrite and bainite. Finally the exploitation of the ferrite and bainite reactions to produce useful alloy steels by direct transformation of austenite is explored.

Stability and mechanical properties of some metastable austenitic steels

Abstract: The relation between austenite stability and the tensile properties, as affected by testing temperature and processing, was studied for a series of alloys of increasing compositional complexity, viz., the Fe-Ni, Fe-Ni-C, and Fe-Ni-Cr-Mn-C systems. The “stress” and “strain induced” modes of transformation to martensite differed significantly in their influence on the shape of the stress-strain curve. Under certain testing conditions, unusually low yield strengths and high work hardening rates were observed in some of these alloys. Maxima in yield strengths were observed for all austenitic alloys containing carbon that were processed at deformation temperatures between 200° and 300°C. Evidence gleaned from electron microscopy and magnetic and mechanical testing suggested that the maxima were due to the formation of carbon atmospheres on dislocations during processing. The influence of austenite stability on the mechanical properties of steels, varied by systematic changes in test temperature (22° to -196°C), composition (8 pct, 12 pct, 16 pct, and 21 pct Ni) and deformation temperature (25° to 450°C), was evaluated quantitatively.

Factors influencing the strength differential of high strength steels

Abstract: Room temperature tensile and compressive true stress-true strain curves of various high strength steels (quenched and tempered 4340 steel, 410 martensitic stainless steel, and H-11 steel; and aged 300-grade 18 Ni maraging steel) were analyzed to determine the effect of the various microstructures, on what has been termed the strength differential (SD),i.e., the strength level difference between the tensile and compressive flow curves. Care was taken to insure that the compressive deformation was homogeneous. Regardless of the amount of plastic deformation, the quenched and tempered steels exhibited a higher flow stress in homogeneous compressive deformation than for tensile deformation. The extent of the SD was dependent on tempering temperature. This observation is consistent with what others have observed regarding yield strength behavior of quenched and quenched-and-tempered steels. Despite the low carbon content, aged maraging steel also showed a greater resistance to homogeneous compressive deformation. Metallographic examination of the maraging steel revealed the banding that is indicative of segregation. However, homogenization had little effect on the SD despite a change in austenite grain size, reverted austenite content, and the austenite-to-martensite transformational strains shown by Goldberg to be present in segregated material.

Solidification of M2 high speed steel

Abstract: The freezing process in AISI type M2 high speed tool steel (6 pct W, 5 pct Mo, 4 pct Cr, 2 pct V, 08 pct C) was studied by metallographic and thermal analysis techniques Unidirectional solidification of small laboratory melts in a modified crystal growing apparatus was employed to provide metallographic sections of known macroscopic growth direction Also cooling curves were obtained on 40 g specimens solidified in thimble crucibles X-ray microradiography, electron probe scanning techniques, and quantitative microanalysis of dendrites and interdendritic carbides were extensively used to supplement conventional metallography Carbon and vanadium contents of M2 were varied in order to observe the effect of an austenite and ferrite stabilizer on the thermal analysis curves and microstructure The nonequilibrium freezing process in M2 includes three major liquid-solid reactions: 1) Liquid → Ferrite, 1435°C; 2) Liquid + Ferrite → Austenite, 1330°C; 3) Liquid → Austenite + M6C + MC, 1240°C These reactions account for the as-cast structure of the commercial alloy The addition of carbon depresses the liquidus (1) and solidus temperatures (3) and narrows the gap between the liquidus (1) and peritectic transformation (2) This gap is eliminated at > 139 wt pct C, where the initial freezing reaction is the crystallization of austenite The accompanying microstructural change is the elimination of σ eutectoid dendrite cores The addition of vanadium promotes ferrite formation by strongly depressing the peritectic reaction and thus widening the gap between the liquidus and the peritectic

Aging and brittleness in an Fe-Ni-Mn alloy

Abstract: A study of the aging behavior of an Fe-12 Ni-6 Mn alloy has been made. Rapid hardening was found to occur over a wide temperature range. Extraction of the overaged precipitate has been carried out and the precipitate identified as fct θ NiMn by electron diffraction. Studies of the effect of aging on the toughness and tensile ductility have shown that severe embrittlement occurs over a wide temperature range. The relationship between the hardening and the embrittlement was studied by comparison of “activation energies” determined from Arrhenius plots. It is suggested that embrittlement in this alloy is the result of segregation of manganese to prior austenite grain boundaries resulting in intergranular failure. To avoid embrittlement, aging was performed at temperatures above the embrittlement range which resulted in good toughness with moderate hardness.

Phenomena of carbon atom redistribution in martensite

Abstract: Phenomena related to the redistribution of carbon atoms prior to carbide precipitation (tempering) are considered in this paper. 1) Order-disorder processes are described, and the influence of several factors is considered. It is shown that the lattice conjugation and coherence between different variants of martensite plates as well as between martensite plates and austenite leads to a disappearance of the tetragonal cubic phase transition. 2) A possible mechanism for the formation of “abnormal” martensite having a comparatively low axial ratio is considered to be (011)M transformation twinning. 3) Diffuse maxima observed in electron diffraction patterns near reciprocal lattice points are discussed. The diffuse scattering is related to the short range order of carbon atoms and a spinodal decomposition of the Fe-C solid solution because of the C-C elastic strain interaction. 4) The influence of irradiation on the carbon atom distribution in martensite is considered, and it is shown that the observed decrease in martensite tetragonality after cooling below 0°C and its restoration upon subsequent heating may be explained by a phase transition involving the condensation of carbon atoms on irradiation-induced defects. 5) A short review on experimental results related to the redistribution of carbon atoms because of the presence of dislocations is given.

Untempered Ultra-high Strength Steels of High Fracture Toughness

Abstract: ULTRA-HIGH strength steels now in use were developed decades ago by trial and error methods and they all have some undesirable characteristics, such as low fracture toughness at high levels of yield strength. So far there has been little effort to use the new concepts of alloying and micromechanics of fracture to improve existing alloys, or to find new ones with better combinations of properties. We are studying the factors that contribute to notch brittleness in high strength steels and we have learned how to increase the fracture toughness of steels having yield strength in excess of 200,000 pound inch−2 by as much as 70%. We use treatments that differ significantly from the normal commercial practice for quenched and tempered low alloy steels, which involves heating to the lower end of the austenite temperature range (to minimize grain size), quenching fast enough to produce martensite and tempering at a temperature that will optimize mechanical properties.

The effect of reverted austenite on the mechanical properties and toughness of 12 Ni and 18 Ni (200) maraging steels

Abstract: A study has been made of the effects of reverted austenite on the mechanical properties and toughness of three maraging steels. It is found that reverted austenite has no detrimental effects on the mechanical properties and toughness and even improves these properties when precipitated along martensite lath boundaries. This occurs for underaged specimens. A detrimental effect on toughness is found when reverted austenite precipitates at prior austenitic grain boundaries which occurs for over aged specimens. Overaged precipitates are also responsible for the decrease in toughness in the overaged condition.

Adiabatic instability in the orthogonal cutting of steel

Abstract: The reversion of Fe-18.5 Ni-0.52 C tempered martensitic steel to austenite under shear was used to study the formation of discontinuous chips by orthogonal cutting. For certain combinations of cutting speed, depth of cut, and tool rake angle, chips with bands of reverted austenite along their sheared edges were formed. Tensile tests on the same material exhibited transformed austenite on the specimen fracture surfaces for tests conducted below 200°C. Metal cutting theory predicts that continuous plastic deformation during chip formation cannot heat the material to its reversion temperature. Analysis of the machine-sample interaction before chip separation shows that adiabatic instability can occur, resulting in localized shearing and a temperature rise to at leastAs. Only those chips which are heated during continuous deformation to temperatures between 100° and 200°C undergo adiabatic instability and reversion.

Hot Cracking of Austenitic Steel Weld Metal

Abstract: It is well known that a small amount of ferrite in austenitic stainless steel weld metal is very effective for the prevention of hot cracking. But the reason seems to be not quite clear. This experiment was carried out to know the relation between the solidification phases, sulphur microsegregation and hot cracking susceptibility of Ni-Cr alloyed steels. Because it has been confirmed by previous experiment that γ iron crust of primary solidification phase through the peritectic reaction of Fe-Ni system is very responsible to the hot cracking of 3.5% nickel alloyed steel weld metal. Circular groove hot cracking test specimens were prepared by three series of Ni-Cr alloyed cast steels, which contain about 0.15% sulphur. The groove of specimen was welded by TIG and MIG welding. The experimental results obtained are as follows;(a) The weld metals which crystallize as primary γ iron are much more sensitive to hot cracking than those which crystallize as primary α iron and transform to γ iron. This result concides with the tendency of sulphur microsegreagation of weld metals, which was detected by EPMA.(b) The hot cracking susceptibility of Ni-Cr alloyed steel weld metals relates to the boundary line of primary γ or α solidification surfaces, while the ferrite amount of weld metal at the room tenperature by Schaeffler diagram seems to have no direct relation with the susceptibility.

Pitting of Molybdenum Bearing Austenitic Stainless Steel

Abstract: The effect of temperature over the range 10 to 90 C (50 to 194 F) on the pitting corrosion of a series of molybdenum bearing austenitic stainless steels in an acid chloride medium is demonstrated. The role of molybdenum in improving the corrosion resistance of these alloys is clearly shown.

Long-time isothermal temper embrittlement in Ni-Cr-Mo-V steels

Abstract: Four commercial purity Ni-Cr-Mo-V steels of closely comparable bulk chemistry and grain size, but tempered to various strength levels, were embrittled by exposure at 600°, 750°, and 850°F for times up to 35,000 hr. Maximum temper embrittlement occurred at 850°F in all steels. Severe cases of embrittlement resulted in a marked decrease in tensile ductility and an intergranular tensile fracture. Auger electron emission analysis showed that P, Sn, Ni, and Cr were segregated at prior austenite boundaries in the steels exposed to 750° and 850°F. Increased segregation of phorphorus and tin was always accompanied by increased segregation of nickel and chromium. The severity of grain boundary segregation increased with increasing values of fracture transition temperature. Despite comparable bulk chemistry and grain size, the degree of segregation was different in different steels. Under exposure conditions causing severe embrittlement, the FATT values displayed a strong dependence on the strength level of the steel. In a giyen steel, while the composition and morphology of carbides at austenite boundaries were the same as in the matrix, the density and size of carbides were much higher at the austenite boundaries. The preference of these boundaries as fracture sites would seem to arise from two considerations, namely, a high degree of impurity and alloy element segregation and the fact that the density and size of carbides at these boundaries is higher than that in the matrix.

Fine structure in quenched Fe-Al-C steels

Abstract: The origin of “extra” spots in electron diffraction patterns taken from retained austenite and martensite in Fe-7 pct Al-2 pct C has been investigated. The present interpretation differs from that of previous investigators. It is concluded that a high density of fine carbide precipitates 50A in diameter is responsible for the extra reflections. The precipitates have the perovskite structure and grow epitaxially in the austenite during quenching. When martensite forms at a lower temperature, the precipitates are transformedin situ.

On the ferrite-austenite equilibrium in the Fe-Cu system

Abstract: The thermodynamics of ferrite containing dissolved copper is analyzed theoretically, taking into account the effect of copper on the Curie temperature. By combination with experimental information on the solubility of copper in austenite, this model is used to predict the solubility of copper in ferrite. The result shows good agreement with experimental data but indicates that the data should be extrapolated in a manner different from that which has been previously suggested. The effects of copper and cobalt on the relative stability of the α and γ phases in iron-rich alloys are compared. It is concluded that their difference is mainly caused by the difference in their effects on the Curie temperature of α-iron.

The development of martensitic microstructure and microcracking in an Fe-1.86C alloy

Abstract: The development of the martensitic microstructure in a 1.86 wt pct C steel has been followed by quantitative metallographic measurements over the transformation range of 0.12 to 0.50 fraction transformed (f). The transformation kinetics are described by the equationf = 1 − exp [−0.008 (M s − Tq)] where Ms and Tq are the martensite start and the quenching temperatures respectively. Fullman’s analysis shows that the average volume per martensite plate decreases by almost an order of magnitude over the transformation range studied, but this decrease is less than that predicted by the Fisher analysis for partitioning of austenite by successive generations of martensite. Microcracking increases with increasingf up to 0.3, but does not increase forf above 0.3 where transformation proceeds by the nucleation of large numbers of small martensite plates. These observations indicate that a critical size of martensite plate is necessary to cause microcracking.

New High-strength Stainless Steels

Abstract: A COMMON method of producing high creep strength in metals is to introduce a second, dispersed, phase to act as an obstacle to the motion of dislocations. At large volume fractions of the second phase this simple method gives very large increases in strength. Another important feature of high-temperature alloys, however, is that their strength should be retained for the duration of their service life, and this implies that the second phase particles must be resistant to coarsening. This is a very stringent condition and the most effective way of meeting it seems to be to use a dispersed phase with a very large negative free energy of formation. This is the basis of alloys such as sintered aluminium powder (SAP) and thoria dispersed nickel (TD nickel).

Recrystallization following hot-working of a high-strength low-alloy (HSLA) steel and a 304 stainless steel at the temperature of deformation

Abstract: The recovery and recrystallization behavior of two commercial quality steels, a Cb(Nb) strengthened high-strength low-alloy (HSLA) steel and a 304 stainless steel, was studied following hot-working. Specimens were deformed in tension at a constant head velocity of 2 in.Js to reductions-in-area of 30 to 50 pct at temperatures in the austenite range from 1600° to 1900°F. The subsequent annealing behavior was observed at the temperature of deformation. Decreasing recrystallization rates with decreasing temperature andJor deformation were observed. It is suggested that CbC precipitation occurred during annealing of the HSLA steel and accounted for an arrest in the softening behavior. For the 304 stainless steel it is concluded that dynamic recrystallization took place during deformation, that thermal microtwinning was an active recovery mechanism during annealing, and that there was a preference for grain boundaries as nucleation sites for recrystallized grains. These conclusions regarding the annealing behavior of 304 stainless steel were supported by metallographic analysis of specimens water quenched from the temperature of deformation.

The distribution of boron in austenite

Abstract: The partitioning of boron between grain boundary and austenite matrix sites in steel has been examined by the use of boron autoradiography. Results show that inalloyscontainingO.0037at.pctB and 0.68 at. pct C, boron segregates to the austenite grain boundary with a binding energy of 9.6 ±1.0 kcal/mole. In addition to this experimental observation, the effect of carbon concentration on the boron distribution is considered theoretically.

Superplasticity in eutectoid steel

Abstract: A eutectoid steel exhibited abnormally large tensile extensibility in three quite dissimilar circumstances: i) micrograin plasticity, defined by a strain-rate sensitivity exponent m = 0.42, was found at 716°C for straining the ferrite-cementite aggregate at the rate e = 4.4 x 10-3 min-1, giving 133 pct elongation; ii) at a much greater strain rate, e = 25 min-1, superplasticity appeared in austenite strained at 927°C, giving 142 pct elongation; iii) a new type of transformation plasticity was predicted and experimentally verified: 490 pct elongation resulted from thermal cycling 21 times across Ae1. Plastic stability analysis distinguishes it from micrograin plasticity by showing that it owes to strain-hard ening during plastically stable flow; hence, there are no restrictions e or m. Furthermore, it is not necessary to perform the straining during the transformation, since the strain-hardening capacity can be regenerated by thermal cycling through the phase transformation if the transformation serves to recover the flow stress. Additional work showed that straining during austenitizing fails to increase m above pretransformation levels.