Showing papers on "Bainite published in 1978"

Influence of martensite composition and content on the properties of dual phase steels

Abstract: A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when Fe-Mn-C alloys are quenched from the austenite plus ferrite phase field, so as to give a series of alloys with constant ferrite and martensite compositions but varying percent martensites. It is found that the strength of a dual phase structure is dependent on the ferrite grain size and the volume fraction of martensite, and is independent of the composition and strength of the martensite. In agreement with previous work the ductility of these steels is superior to that for standard HSLA steels at the same tensile strength. As shown in a previous paper the strength and ductility as a function of percent martensite are in agreement with Mileiko’s theory of composites of two ductile phases. This theory and the results indicate that the superior ductility of dual phase steels is largely a consequence of the high strength (fine grained), highly ductile (low interstitial content) ferrite matrix.

Mechanisms of tempered martensite embrittlement in low alloy steels

Abstract: An investigation into the mechanisms of tempered martensite embrittlement (TME), also know as “500°F” or “350°C” or one-step temper embrittlement, has been made in commercial, ultra-high strength 4340 and Si-modified 4340 (300-M) alloy steels, with particular focus given to the role of interlath films of retained austenite. Studies were performed on the variation of i) strength and toughness, and ii) the morphology, volume fraction and thermal and mechanical stability of retained austenite, as a function of tempering temperature, following oil-quenching, isothermal holding, and continuous air cooling from the austenitizing temperature. TME was observed as a decrease in bothKIc and Charpy V-notch impact energy after tempering around 300°C in 4340 and 425°C in 300-M, where the mechanisms of fracture were either interlath cleavage or largely transgranular cleavage. The embrittlement was found to be concurrent with the interlath precipitation of cementite during temperingand the consequent mechanical instability of interlath films of retained austenite during subsequent loading. The role of silicon in 300-M was seen to retard these processes and hence retard TME to higher tempering temperatures than for 4340. The magnitude of the embrittlement was found to be significantly greater in microstructures containing increasing volume fractions of retained austenite. Specifically, in 300-M the decrease inKIc, due to TME, was a 5 MPa√m in oil quenched structures with less than 4 pct austenite, compared to a massive decrease of 70 MPa√m in slowly (air) cooled structures containing 25 pct austenite. A complete mechanism of tempered martensite embrittlement is proposed involving i) precipitation of interlath cementite due to partial thermal decomposition of interlath films of retained austenite, and ii) subsequent deformation-induced transformation on loading of remaining interlath austenite, destabilized by carbon depletion from carbide precipitation. The deterioration in toughness, associated with TME, is therefore ascribed to the embrittling effect of i) interlath cementite precipitates and ii) an interlath layer of mechanically-transformed austenite,i.e., untempered martensite. The presence of residual impurity elements in prior austenite grain boundaries, having segregated there during austenitization, may accentuate this process by providing an alternative weak path for fracture. The relative importance of these effects is discussed.

The deformation behavior of a vanadium-strengthened dual phase steel

Abstract: A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when a V containing HSLA steel is cooled in a controlled manner from either the austenite or austenite plus ferrite phase fields Such a heat treatment results in the pearlite regions and carbide particles of the standard V steel being replaced by martensite; this leads to a decrease in the yield stress and an increase in ductility while the tensile strength is essentially unchanged The fatigue of dual phase steels is slightly superior in the high strain life (ductility controlled) region and slightly inferior in the low strain life (yield dominated) region when compared to standard V steel The replacement of the pearlite and cementite particles which can nucleate cracks, by more ductile martensite islands results in improved Charpy impact properties The strength and the ductility of the dual phase materials is shown to be in agreement with a theory of composites with two ductile phases This theory then allows one to understand the relative importance of various microstructural features in controlling strength and ductility In this way it is found that the key to the superior elongation (at a constant tensile strength) is largely due to the high strength (fine grained), highly ductile ferrite matrix

Retained austenite and tempered martensite embrittlement

Abstract: The problems of detecting the distribution of small amounts (5 pct or less) of retained austenite films around the martensite in quenched and tempered experimental medium carbon Fe/c/x steels are discussed and electron optical methods of analysis are emphasized. These retained austenite films if stable seem to be beneficial to fracture toughness. It has been found that thermal instability of retained austenite on tempering produces an embrittlement due to its decomposition to interlath films of M3C carbides. The fractures are thus intergranular with respect to martensite but transgranular with respect to the prior austenite. The temperature at which this occurs depends upon alloy content. The effect is not found in Fe/Mo/C for which no retained austenite is detected after quenching, but is present in all other alloys investigated.

MECHANICAL STABILITY OF RETAINED AUSTENITE IN TEMPERED 9Ni STEEL

Abstract: The behavior of the thermally stable austenite in the ductile fracture surface layer of a grain-refined and tempered 9Ni steel broken at 77 K was studied through use of Moss-bauer spectroscopy and transmission electron microscopy. Thin foils revealing the mi-crostructural profile of the fracture surface layer were prepared by electroplating a thick pure iron layer on the fresh fracture surface, then thinning a profile sample through a combination of conventional twin-jet electropolishing and ion milling techniques. The re-sults of both Mossbauer spectroscopy and TEM studies showed that the thermally stable austenite transforms to a dislocated martensite in the deformed zone adjacent to the duc-tile fracture surface. This result suggests that transformation of the retained austenite present in tempered 9Ni steel is compatible with low temperature toughness, at least when the transformation product is a ductile martensite.

A survey of eutectoid decomposition in ten Ti-X systems

Abstract: The mechanism of eutectoid decomposition in ten Ti-X binary systems, wherein X was successively Bi, Co, Cr, Cu, Fe, Mn, Ni, Pb, Pd and Pt, has been investigated. In hypo-eutectoid alloys, the bainite reaction (defined in the present context as a nonlamellar dispersion of intermetallic compound particles amongst proeutectoid a) predominated in all alloys studied but Ti-Cu, where both bainite and pear lite were formed. In alloys of near eutectoid composition pearlite was the dominant product in some systems and bainite was in others. These results are essentially independent of reaction temperature. They are quite different from analogous ones in Fe-C alloys, where pearlite is the principal eutectoid structure formed at high temperatures and bainite plays this role at low tem-peratures. The difference between the Ti-X and Fe-C behaviors was explained in terms of the much more pronounced tendency for proeutectoid α plate formation in Ti-X than in Fe-C alloys and, on a theory due to Hillert, of the need for disordered interphase boundaries in order to develop the cooperative growth mode that leads to the evolution of pearlite.

The Morphology and Mechanical Properties of Bainite Formed from Deformed Austenite

Abstract: Deformation of austenite containing 0.85 pct C is shown to significantly increase the ten-sile strength of bainite formed during subsequent transformation. Quantitative metallo-graphic measurements indicate that strengthening is due primarily to an increased dislo-cation density in the ferrite and reduced carbide size, with consequent finer distribution, compared with the bainite formed from undeformed austenite. It is also shown that de-formed austenite transforms to upper bainite at temperatures at least as low as 200°C due to enhanced nucleation and/or growth at slip band heterogeneities generated by the defor-mation process while the only effect on the formation of lower bainite is a retardation of the transformation and reduction of ferrite plate size.

The role of grain size on the ductile-brittle transition of a 2.25 Pct Cr-1 Pct Mo steel

Abstract: The ductile-brittle transition temperature of a 2 1/4 pct Cr-1 pct Mo steel has been meas-ured using ‘V’ notch Izod impact specimens for an unembrittled and embrittled 2 1/4 pct Cr-1 pct Mo steel with prior austenite grain sizes within the range 40 to 150 μm. The mi-crostructure of this steel was upper bainite. The variation of yield strength with grain size obeys a Hall-Petch relationship. The ductile-brittle transition temperature was found to have a pronounced grain size dependence for both unembrittled, 15 K mm1/2, and embrittled, 19 K mm1/2, specimens. The bainite colony size was found to vary as the prior austenite grain size. From the low temperature quasi-cleavage facet size, together with metallographic observations of crack path, it has been concluded that bainite colony size rather than prior austenite grain size is the effective grain size.

Splat cooling of iron-molybdenum-carbon alloys

Abstract: Two molybdenum alloy steels, which normally undergo the austenite → martensite phase transformation during solid state quenching, have been rapidly cooled from the melt in a controlled atmosphere “gun” splat cooling device. The matrix phases produced wereδ-ferrite, martensite, and austenite; the carbide Mo2C was also present in the as-quenched condition in the higher alloy composition studied. The amount of austenite retained to room temperature was found to be inversely related to the cooling rate. The morphology of the martensite in the splat-cooled alloys exhibited a marked change compared with its characteristic appearance in the conventionally solid-state quenched material. This was attributed to the dual effect of increased cooling rate on carbon segregation in the parent austenite and of decreased section thickness in which the martensite forms. The degree of solute segregation observed in the microstructures of the matrix phases was shown to depend on the extent of the equilibrium liquidus-solidus temperature range. The precipitation of Mo2C during ageing in the range 600 to 700° C paralleled the behaviour of conventionally quenched and tempered alloys, although local inhomogeneities did produce precipitation phenomena not encountered in solid-state quenched material.

Heat treatment effects on creep and rupture behavior of annealed 2.25 Cr-1 Mo steel

Abstract: The strength of 2 1/4 Cr-1 Mo steel depends on the microstructure, which, in turn, de-pends on the heat treatment. In the fully annealed and isothermally annealed conditions, the microstructure is primarily proeutectoid ferrite with varying amounts of bainite and pearlite. The relative amounts of the latter constituents depend on the cooling rates during the anneal. The creep and rupture properties were determined for steel plates (from a single heat) given three different annealing treatments: two were fully annealed, but cooled at different rates from the austenitizing temperature, and the third was iso-thermally annealed. Properties were determined at 454, 510, and 566°C. At 454 and 510°C, the cooling rate had a significant effect on the creep and rupture properties, with the ma-terial cooled fastest being the strongest. Although at 510°C strengths at short rupture times differed widely, the properties approached a common value at longer rupture times. The properties differed very little at 566°C, even for short rupture times. The effect of heat treatment was concluded to be the result of interaction solid solution hardening, a dislocation-drag process. This process gave rise to nonclassical creep curves (as op-posed to classical curves with single primary, secondary, and tertiary stages). By examining the creep-curve shape, it was possible to interpret the heat treatment effects on the creep-rupture properties.

Schematic transformation diagrams for steel

Abstract: This paper examines the diffusional transformations of austenite and concludes that separateC-curves are required for pearlite, upper bainite, lower bainite and isothermal martensite. A schematic isothermal transformation diagram incorporating the four curves is presented for a plain carbon eutectoid steel and used to develop a schematic continuous cooling transformation diagram. These diagrams are shown to be more compatible with the available experimental information than are the usual diagrams based on a single transformation curve.

An investigation of the effects of austenite strength and austenite stacking fault energy on the morphology of martensite in Fe-Ni-Cr-0.3C alloys

Abstract: The relative effects of austenite stacking fault energy and austenite yield strength on martensite morphology have been investigated in a series of three Fe-Ni-Cr-C alloys. Carbon content (0.3 wt pct) andM 6 temperature (− 15°) were held constant within the series. Austenite yield strength atM s was measured by extrapolating elevated temperature tensile data. Austenite stacking fault energy was measured by the dislocation node technique. Martensite morphologies were characterized by transmission electron microscopy and electron diffraction techniques. A transition from plate to lath martensite occurred with decreasing austenite stacking fault energy. The austenite yield strength atM s for the low SFE, lath-forming alloy was found to be higher than previously reported for lath-forming alloys. The relative effects of these variables on martensite morphologies in these alloys is discussed.

Lower bainite alloy steel article and method of making same

Abstract: A lower bainite alloy steel article particularly suitable for gears, bushings and the like includes carbon in the range of 0.60 to 0.80 Wt.%, manganese in the range of 0.45 to 1.00 Wt.%, silicon in the range of 0.15 to 2.20 Wt.%, molybdenum in the range of 0.40 to 0.70 Wt.%, and the balance substantially iron. A process of heat treatment of the article includes the steps of quenching it from a first temperature at a preselected rate and of holding it at a second temperature for less than about two hours to complete transformation of the alloy directly from an austenite morphology to a substantially complete low temperature bainite morphology.

Carburization of ferrous alloys

Abstract: A method for carburizing low alloy steel having carbon content of no more than 0.5% employing two-step carburizing process. The method comprising pre-carburizing the steel in a carburizing atmosphere having a carbon potential of less than Acm to produce a carburized case having hypereutectoid composition, cooling the steel to transform the case into a bainite, pearlite or martensite structure, and carburizing the steel by heating thereof to a carburizing temperature for a sufficiently long time to produce carbides in a spheroidal or spherical form, the carburizing being carried out in such a manner that the heating speed of the steel is being controlled under 20° C. per minute in a region between Ac1 temperature and 750° C.-950° C. and the carbon potential in the carburizing atmosphere is being maintained over Acm when the temperature is over Ac1.

Manufacture of high tensile steel sheet with superior cold workability

Abstract: PURPOSE:To obtain the title sheet with low yield ratio by treating a steel sheet consisting of a specified amt. of C, Mn, Si, and the balance Fe and inevitable impurities while specifying the temp. in hot rolling and the cooling rate and by coiling the treated sheet. CONSTITUTION:A steel sheet consisting of C 0.05-0.25%, Mn 0.1-2%, Si <=1%, and the balance Fe and inevitable impurities is treated under the following conditions and coiled: at the time of hot rolling the temp. is regulated to below the equilibrium ferrite transformation start temp. (T1), and a reduction ratio of 20- 80% is applied. The hot rolled sheet is then cooled from T1 to the ferrite transformation start temp. (T2) at an average cooling rate above 30 deg.C/sec and from T2 to above the pearlite transformation start temp. at an average cooling rate below 30 deg.C/sec for above 2 sec. The cooled sheet is further cooled at a cooling rate above 50 deg.C/sec and coiled at below the bainite transformation start temp. Thus, a high tensile steel sheet is obtd. having a mixed structure of bainite phase and/or martensite phase and ferrite phase and an yield ratio below 80%.

Kinetics of the martensite transformation in athermal Fe-C-Ni-Cr alloys

Abstract: An investigation was conducted to study the martensite transformation kinetics of athermal Fe-C-Ni-Cr alloys. The transformation rate was found to be dependent on alloy composition and was a function of i) the rate of change of chemical free energy with temperature, and ii) the energy expended in deforming the austenite surrounding a growing martensite embryo in order to accommodate the transformational volume and shear strains. Increasing either of the two factors leads to an increase in transformation rate. Also, it was found that the magnitude of the contribution of the deformation energy to the overall nonchemical free energy associated with the transformation depends on the alloy system and specific composition.

The influence of microstructure on creep crack growth in 0.5 Pct Cr, 0.5 Pct Mo, 0.25 Pct V steel

Abstract: The effect has been determined of changes in bainite content and prior austenite grain size on the creep fracture behavior of a low alloy ferritic steel. Increasing the bainite content is found to increase both crack growth and crack opening displacement rates whereas a simultaneous decrease in both the prior austenite grain size and bainite content is found to lower these rates. The results, coupled with metallo graphic evidence, are interpreted in terms of creep crack growth being controlled by the accumulation of displacement at a crack tip.

Hot bar cooling

Abstract: A steel rod or bar comprises an outer surface layer of ferrite-pearlite type structure, an annulus immediately below said surface layer having an acicular micro-structure comprising martensite and/or bainite and a core of bainite or martensite or ferrite and carbides or a combination of two or more of these constituents.

Alloy Steels for High-Strength, As-Rolled Rails

Abstract: A laboratory study was conducted on a broad range of compositional variations of chromium--molybdenum steels for application as high-strength, as-rolled rails. A series of experimental heats was prepared and processed to simulate commercial production of as-rolled 65-kg/m (132-lb/yd) rail, with regard to cooling rate and transformation characteristics. Test specimens were evaluated in terms of hardness, tensile properties, and microstructure. The mechanical properties are related to the observed microstructures. Increasing the molybdenum content of the steel is highly effective in increasing yield strength by refining the pearlite; the appearance of coarse bainite reduces the yield strength. Vanadium contributes additional strengthening, but only under certain conditions where the pearlite is refined. Lowering carbon and adding columbium produces microstructural changes that are not beneficial to the mechanical properties. These results make possible some tentative conclusions regarding the optimization of composition for the commercial production of as-rolled rails in the 760 to 1170 N/mm/sup 2/ (110 to 170 ksi) yield strength range.

Notched Toughness of Medium Carbon Ni-Cr-Mo Steels Having Mixed Structure of Martensite and Residual Ferrite

Abstract: Synopsis: Slow bending and Charpy impact bending tests have been performed with V-notched and fatigue-precracked specimens to study notched toughness of medium carbon Ni-Cr-Mo steels having mixed structure of martensite and residual ferrite. It has been found that residual ferrite, when it remained in lower tempered martensite , had detrimental effects on the slow bending and Charpy impact bending toughness even though characteristic relationship between the toughness and volume fraction of residual ferrite was found by the variation in the notched radius and loading speed. When the residual ferrite appeared in higher tempered martensite, harmless effect was found on the Charpy impact bending toughness in a state of upper shelf beyond room temperature, but the toughness became to be significantly deteriorated with a decrease in test temperature. From the analyses of microfractographs, the detrimental effect on the toughness primarily results from the fact that residual ferrite under V-notch and fatigue-precrack fractures in a brittle manner.

Analysis of Fracture Morphology of Hydrogen-Assisted Cracking in Steel and Its Welds

Abstract: The relationships between fracture morphology of hydrogen-assisted cracking, microstructure, and crystallographic orientation were investigated using the scanning electron microscope. In the fracture morphology of hydrogen-assisted cracking, quasicleavage fracture, intergranular fracture, and dimple rupture were observed. In the quasicleavage fracture caused by hydrogen, subcracks often were observed along the boundary of martensite lath or ferrite lath, or both, and at the interface between the matrix and carbides. The unit fracture facet for hydrogen-assisted cracking, therefore, was defined as the region between subcracks. The unit fracture facet for hydrogen-assisted cracking was smaller than that for cleavage fracture of martensite, upper bainite, and bainitic ferrite. Using the etch pit method the crystallographic orientation of the fracture morphology of hydrogen-assisted cracking was found to be the {110} plane. The fracture morphologies of hydrogen-assisted cracking can be categorized as one of four types, each of which can be explained by the microscopic diffusion behavior of hydrogen.

The effect of temperature and microstructure on the creep damage found in low alloy ferritic steels

Abstract: Constant strain rate tests at 10-5 s-1 have been carried out in the temperature range 723 to 973 K on two 1 1/2 pct Cr · 1/2 pct V ferritic steels, the first steel with a 20 pct bainite, 80 pct ferrite microstructure and the second with a fully ferritic structure. Measurements of the quantitative strain, egb, due to grain boundary sliding (gbs), were made and in both steels the γ values (where γ = egb/eT) increased with increasing temperature. In both structures, sliding was found to occur on all boundaries. A qualitative study of cavitation damage and final fracture mechanisms was also made. It is suggested that in the mixed structure, cavities are nucleated by gbs at carbides whereas in the fully ferritic structure, cavity nucleation is by the interaction of intragranular slip with a grain boundary. Optical observations showed that the large scale cavitation behavior was superficially very similar in both steels, but scanning electron microscope observations showed remarkable differences in the fine scale cavitation damage. The implications of these results are discussed in terms of the relationship between matrix deformation, grain boundary deformation and creep fracture.