Showing papers on "Bainite published in 1997"

Influence of the amount and morphology of retained austenite on the mechanical properties of an austempered ductile iron

Abstract: High Si contents in nodular cast irons lead to a significant volume fraction of retained austenite in the material after the austempering treatment. In the present work, the influence of the amount and morphology of this phase on the mechanical properties (proof stress, ultimate tensile strength (UTS), elongation, and toughness) has been analyzed for different austempering conditions. After 300 °C isothermal treatments at intermediate times, the austenite is plastically stable at room temperature and contributes, together with the bainitic ferrite, to the proof stress and the toughness of the material. For austenite volume fractions higher than 25 pct, the proof stress is controlled by this phase and the toughness depends mainly on the stability of γ. In these conditions (370 °C and 410 °C treatments), the present material exhibits a transformation-induced plasticity (TRIP) effect, which leads to an improvement in ductility. It is shown that the strain level necessary to initiate the martensitic transformation induced by deformation depends on the carbon content of the austenite. The martensite formed under TRIP conditions can be of two different types: “autotempered” plate martensite, which forms at room temperature from an austenite with a quasi-coherent epsilon carbide precipitation, and lath martensite nucleated at twin boundaries and twin intersections.

Retained austenite characteristics in thermomechanically processed Si-Mn transformation-induced plasticity steels

Abstract: It is well known that a significant amount of retained austenite can be obtained in steels containing high additions (>1 pct) of Si, where bainite is the predominant microconstituent. Furthermore, retained austenite with optimum characteristics (volume fraction, composition, morphology, size, and distribution), when present in ferrite plus bainite microstructures, can potentially increase strength and ductility, such that formability and final properties are greatly improved. These beneficial properties can be obtained largely by transformation-induced plasticity (TRIP). In this work, the effect of a microalloy addition (0.035 pct Nb) in a 0.22 pct C-1.55 pct Si-1.55 pct Mn TRIP steel was investigated. Niobium was added to enable the steel to be processed by a variety of thermomechanical processing (TMP) routes, thus allowing the effects of prior austenite grain size, austenite recrystallization temperature, Nb in austenite solid solution, and Nb as a precipitate to be studied. The results, which were compared with those of the same steel without Nb, indicate that the retained austenite volume fraction is strongly influenced by both prior austenite grain size and the state of Nb in austenite. Promoting Nb(CN) precipitation by the change in TMP conditions resulted in a decrease in the V RA . These findings are rationalized by considering the effects of changes in the TMP conditions on the subsequent transformation characteristics of the parent austenite.

Influence of microstructure on fracture toughness of austempered ductile iron

Abstract: An investigation was carried out to examine the influence of microstructure on the plane strain fracture toughness of austempered ductile iron. Austempered ductile iron (ADI) alloyed with nickel, copper, and molybdenum was austenitized and subsequently austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and was correlated with the microstructure. The results of the present investigation indicate that the lower bainitic microstructure results in higher fracture toughness than upper bainitic microstructure. Both volume fraction of retained austenite and its carbon content influence the fracture toughness. The retained austenite content of 25 vol pct was found to provide the optimum fracture toughness. It was further concluded that the carbon content of the retained austenite should be as high as possible to improve fracture toughness.

Aging and Tempering Behavior of Iron-Nickel-Carbon and Iron-Carbon Martensite

Abstract: The aging at room temperature (RT) and the tempering behavior in the temperature range 293 to 973 K of ternary iron-nickel-carbon martensite (containing 14.4 at. pct Ni and 2.35 at. pct C) was investigated principally by using X-ray diffractometry to analyze changes in the crystalline structure and differential scanning calorimetry to determine heats of transformation and activation energies. These techniques also were used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pct C) in the temperature range 298 to 773 K. Analysis of the structural changes revealed that in both FeNiC and FeC the following processes occurred: (1) formation of carbon enrichments and development of a periodic arrangement of planar carbon-rich regions up to 423 K; (2) precipitation of e/η transition carbide and transformation of a part of the austenite into ferrite under simultaneous enrichment with carbon of the remaining austenite (between 423 and 523 K); (3) decomposition of the retained austenite into ferrite and cementite between 523 and 723 K (only partly for FeNiC); (4) precipitation of cementite between 523 and 723 K; and (5) for FeNiC, reformation of austenite from ferrite and cementite above 773 K. A short comparative discussion concerning the first stage of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martensites is given.

The role of manganese and copper in the eutectoid transformation of spheroidal graphite cast iron

Abstract: The decomposition of austenite to ferrite plus graphite or to pearlite in spheroidal graphite (SG) cast iron is known to depend on a number of factors among which are the nodule count, the cooling rate, and the alloying additions (Si, Mn, Cu, etc.). This study was undertaken in order to deepen the understanding of the effect of alloying with Mn and/or Cu on the eutectoid reaction. For this purpose, differential thermal analyses (DTAs) were carried out in which samples were subjected to a short homogenization treatment designed to smooth out the microsegregations originating from the solidification step. The effect of various additions of copper and manganese and of the cooling rate on the temperature of the onset of the stable and metastable eutectoid reactions was investigated. A description of the conditions for the growth of ferrite and of pearlite is given and shows that these reactions can develop only when the temperature of the alloy is below the lower boundary of the ferrite/austenite/graphite or ferrite/austenite/cementite related three-phase field. The experimental results can be explained if the appropriate reference temperature is used. The cooling rate affects the temperature of the onset of the ferrite plus graphite growth in the same way as for the eutectic reaction, with a measured undercooling that can be extrapolated to a zero value when the cooling rate is zero. The growth undercooling of pearlite had values that were in agreement with similar data obtained on silicon steels. The detrimental effect of Mn on the growth kinetics of ferrite during the decomposition of austenite in the stable system is explained in terms of the driving force for diffusion of carbon through the ferrite ring around the graphite nodules. Finally, it is found that copper can have a pearlite promoter role only when combined with a low addition of manganese.

Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel

Abstract: Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20 °C to 200 °C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20 °C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.

Martensite and Bainite in Steels : Transformation Mechanism & Mechanical Properties

Abstract: Many essential properties of iron alloys depend on what actually happens when one allotropic form gives way to another, i.e. on the mechanism of phase change. The dependence of the mechanical properties on the atomic mechanism by which bainite and martensite grow is the focus of this paper. The discussion is illustrated in the context of some common engineering design parameters, and with a brief example of the inverse problem in which the mechanism may be a function of the mechanical properties.

Ultra high strength, secondary hardening steels with superior toughness and weldability

Abstract: High strength steel is produced by a first rolling of a steel composition, reheated above 1100 °C, above the austenite recrystallization, a second rolling below the austenite recrystallization temperature, water cooling from above AR to less than 400 °C and followed by tempering below the AC transformation point. The steel comprises predominantly martensite/bainite phase containing precipitates of the carbides, nitrides, or carbonitrides of vanadium, niobium, and molybdenum, wherein the concentrations of vanadium +niobium ⊃ 0.1 wt.%, the carbon content ranges from about 0.03 to 0.12 wt.%, and chromium is present in amounts ranging from 0.3-1.0 wt.%.

Mathematical Modelling of Bainite Transformation Kinetics

Abstract: Steels with a mixed microstructure of allotriomorphic ferrite, bainitic ferrite, retained austenite and martensite have potentially good combinations of high strength and formability. This work describes a kinetic theory for the formation of bainite in steels where the precipitation of carbides is suppressed. The theory is based on the mechanism of transformation. It includes, for example, a nucleation model based on the Magee/Olson/Cohen theory for martensite, growth by a diffusionless mechanism in which sheaves of bainite form by the repeated nucleation of sub-units. These latter platelets grow to a size limited by plastic accommodation of the shape deformation. The theory is compared against experimental data.

Bainite Transformation in a Silicon Steel

Abstract: The processes of upper bainite formation, the crystallographic aspects and the internal structure in a 1.83% silicon steel have been investigated. The T-T-T diagram is separated into two C-curves as in the case of steels containing strong carbide forming elements. In the early stage of upper bainitic transformation, very fine needlelike ferrite subunits with the parallelogram cross sections elongated in a γ // α direction form on a{111} γ sheet in a side-by-side fashion. In the later stage of transformation, these subunits are coalesced on a{111} γ plane and produces upper bainite laths. This bainite is always related to the parent austenite with the Kurdjumov-Sachs relationship and accompanies the surface reliefs. e carbide particles precipitate mainly after the ferrite subunits formation or after the coalescence of them. Therefore, the carbide precipitation is not a fundamental characteristic of bainite transformation but is a secondary effect. e carbide needles precipitating within a bainite lath by long time holding are aligned almost parallel to the α with almost constant intervals and are related to the ferrite by the Jack relationship. These aspects are exactly similar to those in tempered martensite. Such an e carbide configuration is thought to arise from the precipitation on the dislocations introduced during transformation.

Characterization of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: microstructural observation at different scales, mechanism of formation and growth

Abstract: The martensitic transformation induced by traction at room temperature in a Fe–16Mn–9Cr–5Si–4Ni (%mass) has been studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory properties. The martensitic microstructure and the fine structure of the stacking faults are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out; these bands correspond to a mixture of thin martensite plates and extremely thin austenitic zones. Inside a grain, the monopartial nature of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.

Ferrite formation characteristics in Si-Mn TRIP steels

Abstract: If the austenite-to-proeutectoid ferrite reaction is the first phase transformation which occurs during cooling of a hypoeutectoid steel, growth of ferrite takes place by rejection of carbon into the untransformed austenite, such that the carbon content of the austenite increases with increasing ferrite volume fraction. The carbon enrichment of austenite affects the state of retained austenite in the final microstructure. Such an effect is important when designing microstructures of transformation induced plasticity (TRIP) steels, which are a new class of high formable steels. These grades of TRIP steels, which are in excess of Si and Mn, make use of ferrite and bainite as the major micro-components. This work details the results of a systematic investigation on the ferrite formation characteristics and their effects on the state of the retained austenite. The results showed that the retained austenite volume fraction increases with the amount of ferrite and, after a plateau, decreases. Furthermore, the presence of acicular ferrite resulted in a significant increase in the quantity of the retained austenite.

Wear-resistant sintered alloy, and its production method

Abstract: The invention provides a valve seat having a suitable degree of wear resistance, which can be produced without recourse to expensive elements represented by cobalt and at a cost lower than ever before. This valve seat is formed of a wear-resistant sintered alloy having a general composition consisting essentially of, in weight ratio, 0.736 to 9.65% of nickel, 0.736 to 2.895% of copper, 0.294 to 0.965% of molybdenum, 0.12 to 6.25% of chromium, and 0.508 to 2.0% of carbon with the balance being iron, and inevitable impurities, and having a metallic structure in which there are dispersed (1) a martensite, (2) a bainite having a nucleus of sorbite and/or upper bainite and surrounding said nucleus, (3) an austenite having a high nickel concentration, and (4) a hard phase surrounding with a ferrite having a high chromium concentration and composed mainly of a chromium carbide.

Bainitic chromium-tungsten steels with 3 Pct chromium

Abstract: Previous work on 3Cr-1.5MoV (nominally Fe-3Cr-2.5Mo-0.25V-0.1C), 2.25Cr-2W (Fe-2.25Cr-2W-0.1C), and 2.25Cr-2WV (Fe-2.25Cr-2W-0.25V-0.1C) steels indicated that the impact toughness of these steels depended on the microstructure of the bainite formed during continuous cooling from the austenitization temperature. Microstructures formed during continuous cooling can differ from classical upper and lower bainite formed during isothermal transformation. Two types of nonclassical microstructures were observed depending on the cooling rate: carbide-free acicular bainite at rapid cooling rates and granular bainite at slower cooling rates. The Charpy impact toughness of the acicular ferrite was considerably better than for the granular bainite. It was postulated that alloying to improve the hardenability of the steel would promote the formation of acicular bainite, just as increasing the cooling rate does. To test this, chromium and tungsten were added to the 2.25Cr-2W and 2.25Cr-2WV steel compositions to increase their hardenability. Charpy testing indicated that the new 3Cr-W and 3Cr-WV steels had improved impact toughness, as demonstrated by lower ductile-brittle transition temperatures and higher upper-shelf energies. This improvement occurred with less tempering than was necessary to achieve similar toughness for the 2.25Cr steels and for high-chromium (9 to 12 pct Cr) Cr-W and Cr-Mo steels.

Modelling transformation plasticity during the growth of bainite under stress

Abstract: A model is presented which is capable of reproducing the trends observed in radial and longitudinal transformation strains when steel transforms to bainite under the application of a uniaxial compressive stress. The predictions of the model are tested against experimental dilatometric measurement of the bainite transformation. In view of the complexity of the modelling problem, good agreement is found between predicted and experimental values of transformation plasticity and kinetics.

The effect of stress on the Widmanstätten ferrite transformation

Abstract: Transformation of Widmanstatten ferrite has been considered to be displacive with a large shear component (larger than martensite and bainite) It is expected that the external stress exerts a profound effect on Widmanstatten ferrite formation Optical metallographs show that under the influence of an applied compressive stress (below the yield strength of austenite) the number of crystallographic variants of Widmanstatten ferrite is reduced drastically and there is a strong tendency for the plates that do grow to align Transformation strains (including longitudinal and radial strains) during Widmanstatten ferrite formation are shown to be consistent with the microstructural changes Furthermore, transmission electron micrographs clearly reveal that the applied stress leads to the decoupling of the adjacent accommodating plates of Widmanstatten ferrite All the experimental results are explained in terms of the invariant plane-strain displacive mechanism for Widmanstatten ferrite

Fatigue crack growth behaviour of a Si–Mn steel with carbide-free lathy bainite

Abstract: An investigation has been performed to examine the fatigue crack propagation (FCP) rate and fatigue threshold of an Si–Mn steel containing carbide-free lathy bainite. Compact tension specimens prepared from this steel were given four different heat treatments to produce four different austenite contents. The fatigue test was carried out at stress ratio of −1 in a room temperature ambient atmosphere. The results show that the FCP threshold of the steel increases with an increase in the volume fraction of carbon-saturated austenite. The crack growth behaviours show that the deformation strengthening ability of the austenite has a significant effect on the FCP in the threshold region. The effect of austenite on the FCP threshold is seven times that of the bainite.

Aging effects in a Cu-12Al-5Ni-2Mn-1Ti shape memory alloy

Abstract: The isothermal aging effects in an as-quenched Cu-11.88Al-5.06Ni-1.65Mn-0.96Ti (wt pct) shape memory alloy at temperatures in the range 250 degrees C to 400 degrees C were investigated. The changes in the state of atomic order and microstructural evolutions were traced by means of in situ X-ray diffraction and electrical resistivity measurements, as well as transmission electron microscopy (TEM) and optical observations. The kinetics of the aging process, i.e., the temperature and time dependence of the properties including hardness, resistivity, martensitic transformation temperatures, and shape memory capacity were characterized, and at least three temperature-dependent aging stages were distinguished: (1) DO3 or L2(1) atomic reordering, which causes the martensitic transformation temperatures to shift upward and leads the M18R martensite to tend to be a N18R type structure; (2) formation of solute-depleted bainite which results in a drastic depression in martensitic transformation temperatures and loss of the shape memory capacity, accompanied by the atomic disordering in both the remaining parent phase and bainite; and (3) precipitation of the equilibrium cu and gamma(2) phases and destruction of the shape memory capacity.

High-strength steel plate having high dynamic deformation resistance and method of manufacturing the same

Abstract: A high-strength steel plate used by being molded to parts which absorb impact energy at a collision, such as a front side member, and displaying a high impact energy absorption power, and a method of manufacturing such a steel plate are disclosed. In this high-strength steel plate displaying a high impact energy absorption power, the microstructure of a finished steel plate is a dual-phase structure containing ferrite and/or bainite, and having a main phase of either ferrite or bainite, and the third phase containing 3-50 vol. % of residual austenite. An average value σdyn (Mpa) of deformation stress in a 3-10 % equivalent strain range of the microstructure deformed in a strain rate range of 5x102-5x103 after predeformed to a degree of over 0 % to not more than 10 % in terms of equivalent strain satisfies the expression σdyn ≥ 0.766xTS+250 where TS(MPa) is the maximum stress measured by a static tension test in a strain rate range of 5x10?-4-5x10-3?(l/s) prior to the application of the predeformation. The steel plate has a high dynamic deformation resistance satisfying a work hardening index at 1-5 % strain of not less than 0.080.