Showing papers on "Bainite published in 1996"

High resolution observations of displacements caused by bainitic transformation

Abstract: High resolution measurements of the displacements caused by the formation of bainite in a steel have confirmed that the surface relief due to each platelet in any given sheaf is identical to that of the others in a sheaf, and that the relief of each subunit conforms with the general features of an invariant plane strain. The maximum observed shear strain wasfound to be about 0·26, which is consistent with the magnitude expected from the phenomenological theory of martensite crystallography. Clear evidence has also been obtained of the plastic deformation induced in the adjacent austenite by the growth of bainite.MST/3270

Austenite decomposition during continuous cooling of an HSLA-80 plate steel

Abstract: Decomposition of fine-grained austenite (10-µm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanstatten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanstatten ferrite form. At cooling rates less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal and Widmanstatten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanstatten ferrite. Kinetic and microstruc-tural differences between Widmanstatten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products.

High-strength line-pipe steel having low yield ratio and excellent low-temperature toughness

Abstract: An ultrahigh-strength and low-yield-ratio line-pipe steel being excellent in HAZ toughness and field weldability and having a tensile strength of at least 950 MPa (exceeding the API Specification 100). The steel comprises a low-C-high-Mn-Ni-Mo-Nb-trace Ti steel, further selectively contains if necessary B, Cu, Cr and V, and has as the microstructure a hard-soft two-phase mixed structure comprising martensite/bainite and 20-90 % of ferrite, the ferrite containing 50-100 % of worked ferrite and having a grain diameter of 5 νm or less. It has thus become possible to produce an ultrahigh-strength and low-yield-ratio line-pipe steel (exceeding the API Specification 100) excellent in low-temperature toughness and field weldability. As a result, it has become possible to improve the pipeline safety remarkably and to improve the pipe-lining performance and conveying efficiency largely.

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

Abstract: High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M7Ca3 carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M23C6 precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M23C6 and M7C3 secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.

Age Hardening in a Cu-bearing High Strength Low Alloy Steel

Abstract: The copper age hardening response in a low alloy variant of the ASTM A710 type steel has been investigated at temperatures of 450, 500, and 550°C for three pre-treated conditions (as rolled ferrite, bainitic ferrite, and martensite). Transmission electron microscopy has been used to follow microstructural changes and their relation to the variations in the hardness curves. The results indicate that the age hardening responses of both the martensitic and bainitic ferrite structures are much higher than that of the as rolled condition, and this observation is rationalised in terms of higher solute Cu content, higher density of dislocations and greater uniformity of solute copper atoms in the pre-treated condition providing a multitude of nucleating sites for copper precipitation. Moreover, it has been found that the peak hardness in the martensitic and bainitic structures was attained when fine e-copper precipitates are predominantly observed on dislocations. Compared to the martensitic and bainitic structures, the presence of pre-existing interphase e-copper precipitates, as well as the formation of additional copper-rich clusters and precipitates from supersaturated ferrite contribute to the aging response in the hot rolled condition.

Microstructure of lower bainite formed at large undercoolings below bainite start temperature

Abstract: Metallographic observations of steel samples transformed isothermally to lower bainite at temperatures well below the bainite start temperature have revealed that it is possible for separately nucleated platelets to coalesce during prolonged growth. The circumstances conducive to the coalescence process are discussed, and a mechanism is proposed to explain why a bimodal size distribution of plate thickness is sometimes observed for lower bainite and martensite.MST/3370

Quantitative evidence for mechanical stabilization of bainite

Abstract: Metallographic observations have in the past demonstrated conclusively that the bainite transformation is susceptible to mechanical stabilisation. This means that the growth of bainite can be hindered when the transformation occurs in austenite which is plastically deformed, although the rate of heterogeneous nucleation may be accelerated. In the present work we report dilatometric measurements to confirm the earlier metallographic observations and include an interpretation of the effect of prestrain on the martensite thatformsfrom the carbon enriched residual austenite that remains after bainite.MST/3616

Mechanical stabilization of austenite against bainitic reaction in Fe-Mn-Si-C bainitic steel

Abstract: The influence of different amounts (5, 10, 20, 40%) of compressive deformation of austenite on the subsequent isothermal transformation of bainite has been investigated in a Fe-Mn-Si-C alloy steel. The results show that the prior deformation of austenite retards significantly the bainitic transformation. At the same isothermal transformation temperature, as the amount of prior deformation is higher, the quantity of bainite (which can be obtained after the isothermal transformation is completed) becomes less and it is also more difficult to develop the classical sheaf-like structure. The experiments reported here manifest that severe deformation causes mechanical stabilization of austenite against bainitic transformation.

Quantitative analysis of the dilatation by decomposition of Fe-C austenites; Calculation of volume change upon transformation

Abstract: A detailed analysis has been made of the length changes due to decomposition of iron-carbon austenites into ferrite and carbon enriched austenite above the A 1 temperature, and into ferrite and cementite below the A 1 temperature, using lattice-parameter data for the phases involved. The calculations have been performed as a function of carbon content, temperature and degree of transformation. A general definition for the total degree of transformation has been proposed for the case that more than one product phase develops. This definition also holds when the product phases develop consecutively, allowing an analysis of the kinetics of the austenite decomposition. Above the A 1 temperature, the relation between the degree of transformation and the dilatation is linear within good approximation. However, below the A 1 temperature a strong deviation from such a linear relationship occurs, as the result of subsequent decomposition of austenite into ferrite and carbon enriched austenite, and decomposition of the carbon enriched austenite into ferrite and cementite.

Lattice changes in the martensitic phase due to ageing in 18 wt% nickel maraging steel grade 350

Abstract: Changes in the lattice parameter of martensite phase in a maraging steel is measured as a function of ageing temperatures. A decrease in the lattice parameter is observed when aged between 400–650°C which is attributed to the depletion of alloying elements in the martensite matrix as a consequence of precipitation and austenite formation. An increase in the lattice from 650 to 710°C was characteristic of enrichment of alloying elements in the martensite matrix due to dissolution of austenite phase. A minimum value of the lattice parameter of martensite is observed at 650 °C which coincides with the maximum vol % of retained austenite. Electrical resistivity, hardness measurements, andin situ high temperature dilatometry was also carried out to delineate various precipitation reactions that occur in this steel.[/p]

Laser surface treatment of tool steels

Abstract: Laser surface treatment (LST) is a promising technique to improve the wear and corrosion resistance of materials. In die case of tool steels, laser surface treatment is carried out preferably in the liquid state to allow for complete dissolution of alloy carbides. In this paper, the main requirements for materials used in different types of tools and the advantages of using surface engineered materials for these applications are presented. The application of laser melting to the treatment of tool steels is exemplified for AISI 420 and 440C Cr steels and sintered AISI T15 HSS. Usually, the laser melted layers contain martensite, retained austenite and carbides. In steels containing large proportions of ferrite-forming alloying elements δ-ferrite may also be observed. The laser treatment of sintered steel leads to the elimination of residual porosity. The proportion of retained austenite in laser surface melted steels is much higher than in conventionally treated steels. However, the hardness of the steel is high because the austenite is strengthened by solid solution, dislocations and the small grain size. The high volume fraction of retained austenite usually prohibits the application of tool steels in the laser treated condition. Austenite may be eliminated by double or triple tempering treatments at temperatures in the range 550 to 650 °C. During tempering, carbides precipitate within austenite and martensite, and austenite transforms to martensite. Strong secondary hardening is often observed and the temperature of the secondary hardening peak of laser surface melted (LSM) steels is higher than after conventional heat treatment.

Isothermal Transformation Products in a Cu-bearing High Strength Low Alloy Steel

Abstract: A time-temperature-transformation (TTT) diagram has been determined for a copper-bearing steel, whicn is a low alloy TMCP variant of the ASTM A710 type of structural steel. Quantitative mesurements have been supplemented by optical microscopy and transmission electron microscopy to investigate the isothermal transformation behaviour as well as the associated precipitate morphologies. It is shown that the kinetics and product phases of the polymorphic transformation and precipitation reactions are sensitive to both temperature and time. The TTT-diagram shows a prominent transformation region for bainitic structures, at temperatures intermediate between those of polygonal ferrite and martensite. In the intermediate region, the microstructures were characterised by a ferritic matrix with a lath and/or plate shaped grains containing a high dislocation density, together with a minor dispersed "island" phase. For a short holding time (5 sec) at intermediate temperatures (580-430°C), the island phase was identified as untempered twinned and lath martensite, autotempered twinned and lath martensite, and martensite/austenite constituent, depending on the level of carbon partitioning in the remaining austenite before quenching in water. For a longer holding time, the carbon enriched austenite regions decomposed to carbide and ferrite by coupled growth. Polygonal and quasi-polygonal ferrite were formed at relatively high transformation temperatures and these microstructures contained a low dislocation density and were associated with interphase e-copper precipitates.

Weldable high-tensile steel excellent in low-temperature toughness

Abstract: A high-tensile steel that comprises a low-C-high-Mn-Ni-Mo-trace-Ti steel, further contains Cu, B, Cr, Ca, V and other elements, and has as the microstructure either a mixed tempered martensite/bainite structure containing at least 60 % of tempered martensite resulting from the transformation of nonrecrystallized austenite having an average austenite grain diameter (dη) of 10 νm or less, or at least 90 % of tempered martensite resulting from the transformation of nonrecrystallized austenite. By regulating the P value to be within the range of 1.9 to 4.0, the steel is excellent in low-temperature toughness, HAZ toughness and field weldability in, e.g., the cold district and has a tensile strength of at least 950 MPa (exceeding the API Specification 100).

Modeling of Transformation Behavior and Compositional Partitioning in TRIP Steel

Abstract: Transformation behavior and compositional partitioning in TRIP (Transformation Induced Plasticity) steel was investigated by means of microstructural observation and computer modeling. Studies were made on each of three stages of the continuous annealing process applied to TRIP steel. Ortho-equilibrium partitioning of alloying elements of Si and Mn was attained even in short intercritical annealing time. A transformation model, in which transformation is controlled by carbon diffusion, well described the volume fractional change of ferrite and pearlite during the cooling to austempering temperature. Slower cooling rates significantly increased carbon concentration enriched in untransformed austenite and caused pearlite transformation. Ultimate bainite volume fraction obtained by austempering increased with austempering temperature. Analysis with computer modeling revealed that transformation kinetics above 350°C followed a model based on the diffusional mechanism, while it complied with a model based on the displacive mechanism below 350°C.

Experimental investigation of the transformation texture in hotrolled ferritic stainless steel using single orientation determination

Abstract: Two ferritic stainless steels (≈16.5 mass pct Cr) were hot-rolled using seven subsequent passes. The first sample was rolled within the range 1280 °C to 750 °C,i.e., the deformation started in the ferritic region. The second sample was rolled within the range 1080 °C to 770 °C,i.e., the deformation started in the ferritic-austenitic region. In both cases, up to 40 vol pct of the ferrite transformed into austenite during hot rolling. During the last passes, the austenite transformed into cubic martensite. After hot rolling, these former austenitic regions were identified using a selective etching technique and examined using single orientation determination in the scanning electron microscope. The regions which remained ferritic throughout the hot-rolling process were investigated as well. Whereas the texture of the martensite considerably depended on the hot-rolling conditions, especially on the temperature and on the intervals between the rollings, the texture of the ferrite was less affected. The textures of the martensite were interpreted in terms of the crystallographic transformation rules between austenite and martensite. The textures of the ferrite were discussed in terms of recovery and recrystallization.

Microstructural evolution during simulated OLAC processing of a low-carbon microalloyed steel

Abstract: The microstructural evolution during simulated on-line accelerated cooling (OLAC) of a commercial Grade 80 pipe steel was studied using a quench deformation dilatometer. The transformed matrix microstructure contains various amounts of polygonal ferrite, granular bainite and acicular ferrite, depending mainly on the accelerated-cooling interrupt temperature. The final microstructure is predicted well by drawing the OLAC schedule on the appropriate CCT diagram. Three distinct groups of precipitates are found in the final microstructure, which form during reheat, austenite deformation, and cooling, respectively. The distribution and composition of the precipitates varies widely with steel composition and processing schedule. The microstructure of industrially processed plate agrees well with that of corresponding laboratory simulations.

Dual phase steel for cold rolled sheet or strip - contg. manganese@, aluminium@ and silicon

Abstract: Cold rolled sheet or strip is made from a steel contg. in wt.%: 0.05-0.3 C; 0.8-3 Mn; 0.4-2.5 Al; 0.01-0.2 Si; balance Fe plus usual impurities. The steel is free from pearlite and has a predominantly ferritic structure with inclusions of martensite and opt. bainite and/or residual austenite. -

Stress-affected transformation to lower bainite

Abstract: Experiments have been conducted on the formation of lower bainite under the influence of an externally applied stress. It was found that both the ferritic and cementite components of the microstructure respond to the stress. The ferrite plates that grow appear to comply best with the applied stress, leading to an aligned microstructure. Within any given ferrite plate, the stress was found to favour the formation of a single variant of cementite. The results are consistent with a displacive mechanism of transformation, and provide an explanation for the frequent observation that cementite precipitates in a single orientation within any given plate of lower bainite ferrite.

Effect of bainite transformation and retained austenite on mechanical properties of austempered spheroidal graphite cast steel

Abstract: Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness.

The influence of diffusion of carbon in ferrite as well as in austenite on a model of reaustenitization from ferrite/cementite mixtures in Fe-C steels

Abstract: The kinetics of formation of austenite from ferrite and cementite mixtures has been modelled by assuming the local equilibrium at the planar phase interfaces. The exact solutions to the diffusion equations governing the volume diffusion of carbon in austenite and ferrite are presented. The concurrent motions of the two interfaces are calculated via solving a set of transcendental equations derived from the flux balance conditions. At low isothermal transformation temperatures, it is found that the time required for reaustenitization is slightly greater than the time previously calculated with no diffusion of carbon in ferrite.

Titanium-containing hot-rolled steel sheet with high strength and high drawability and its manufacturing processes

Abstract: The subject of the invention is a hot-rolled steel sheet with high strength and high drawability, whose composition, expressed in percentages by weight, is: C≦0.12%; 0.5≦Mn≦1.5%; 0≦Si≦0.3%; 0≦P≦0.1%; 0≦S≦0.05%; 0.01≦Al≦0.1%; 0≦Cr≦1%; 0.03≦Ti eff ≦0.15%, Ti eff being the content of titanium not in the form of nitrides, sulfides or oxides; 0≦Nb≦0.05%; and whose structure comprises at least 75% of ferrite hardened by precipitation of Ti or Ti and Nb carbides or carbonitrides, the remainder of the structure comprising at least 10% of martensite and possibly bainite and residual austenite. The subject of the invention is also processes for manufacturing such sheets.

Formation of bainite in ferrous and nonferrous alloys through sympathetic nucleation and ledgewise growth mechanism

Abstract: The subunits constituting a bainitic sheaf in an Fe-C-Cr-Si alloy were discovered by scanning tunneling microscopy (STM) to consist of sub-subunits, and sub-subunits were also composed of sub-sub-subunits. Detailed investigation shows that a bainitic relief is composed of many smaller reliefs, which correspond to a different structure of bainite,i.e., subunits, sub-subunits, and sub-sub-subunits. It is determined by STM that the surface relief arising from the formation of bainite in an Fe-C-Cr alloy istent shaped rather than an invariant plane strain (IPS) type of surface relief. Careful observation shows that the relief obtained from a sub-sub-subunit is also tent shaped. It is discovered by STM that an α1 plate,i.e., bainite formed in Cu-Zn-Al alloys, is composed of subunits. This is also demonstrated by transmission electron microscopy (TEM). The preceding results indicate that bainitic plates in Cu-Zn-Al alloys and bainitic subunits in steels are not the smallest structural units. Based on the preceding results on the ultrafine structure and the nature of surface relief accompanying bainite, it is proposed that the bainitic structure forms through a sympathetic nucleation and ledgewise growth (SNLG) mechanism. This article shows that the SNLG mechanism can be successfully applied to interpret the complicated structure of bainite.

High strength steel excellent in low temperature toughness and fatigue strength and its production

Abstract: PROBLEM TO BE SOLVED: To simultaneously improve the low temp. toughness and fatigue strength in a high strength steel having 570 to 950MPa class tensile strength by superrefining the α grain size in a steel having a specified compsn. and dispersing secondary phases having a wide difference in strength with the αby the suitable size and quantity. SOLUTION: A slab having a compsn. contg., by weight, 0.01 to 0.20% C, 0.03 to 1.0% Si, 0.30 to 2.0% Mn, 0.002 to 0.1% Al and 0.001 to 0.01% N, contg. ≤0.02% P and ≤0.01% S as impurities, and the balance Fe, and in which carbon equivalent Ceq shown by the formula is regulated to 0.35 to 1.0% is subjected to rolling in two phase regions to form α into fine grains. At this time, the refining of the structure is previously executed together with the optimization of the rolling in two phase regions. Then, the high strength steel having a structure contg. ferrite having ≤3μ average grain size by 10 to 70% volume fraction, and the balance bainite or martensite or mixed one of bainite and martensite having 10μ average grain size and 300 to 700 Vickers hardness can be obtd. COPYRIGHT: (C)1998,JPO

The effects of microstructure, strength level, and crack propagation mode on stress corrosion cracking behavior of 4135 steel

Abstract: The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.

Coupled Simulation of Heat Transfer and Phase Transformation in Continuous Casting of Steel

Abstract: A computer software package has been developed to simulate temperature, shell growth and phase transformations in continuous casting of steel The package includes two earlier developed models, a heat transfer model and a thermodynamic–kinetic phase transformation model The phase transformation model takes into account the effect of alloying and cooling on the phase transformation temperatures and on this basis, on important, solidification related thermophysical properties, enthalpy and thermal conductivity, generated by the model Hence, while affecting these properties, the phase transformations have a special influence on the heat transfer in the strand The output of these coupled calculations are temperature distribution, phase distribution (liquid, delta ferrite and austenite, and approximately, proeutectoid ferrite, pearlite, bainite and martensite) and volume distribution through the strand, and also, hardness distribution on the cross-section of the strand at room temperature Characteristics of the coupled model and some results of calculations are presented