Bainitic Type Research Articles

Microstructure characteristics and mechanical properties of deposited metals with different types of bainite

In this work, three types of bainite microstructure have been gained in the deposited metals by adjusting the Ni content (1 wt %, 2.5 wt %, and 4 wt %). The microstructure feature of different types of bainite has been characterized by optical microscope, scanning electron microscope, transmission electron microscope, and electron backscattering diffraction. Microstructure changed from granular bainite with different morphology to lath bainite with the increase of Ni content. The number of elongated Martensite-Austenite constituents in granular bainite increased with the increasing Ni content from 1 wt % to 2.5 wt % due to the increase in hardenability. The complex orientation in the deposited metal with 4 wt % Ni content attributed to the numerous autocatalytic nucleation occurrence during the bainite transformation. After crystallographic characterization, the bainite variant selection tendency decreased with the bainite type change from granular bainite to lath bainite with autocatalytic nucleation, and the density of high-angle grain boundaries increases obviously. The deposited metal with numerous autocatalytic nucleation has an outstanding match of high strength and excellent low-temperature toughness due to the effective grain-refinement strengthening. The large-sized elongated Martensite-Austenite constituents in the deposited metals with 1 wt % and 2.5 wt % Ni contents deteriorate low-temperature toughness in two aspects, promoting crack nucleation and reducing the crack propagation resistance. Interlaced bainite is the most optimized among different bainite types. This work guides the microstructure design in high-strength deposited metals.

Effect of Ni addition on bainite microstructure of low-carbon special bar quality steels and its influence on CCT diagrams

This work presents the effect of Ni content on the bainite morphology and in the mechanical properties of low-carbon special bar quality steels. Four steel ingots with the same base chemical composition, having only Ni content as their main difference, were melted in a pilot scale vacuum induction melting furnace, hot forged into square bars followed by cooling in still air to room temperature. Samples for Charpy-V notch tests of every steel were tempered to equalize their hardness level. Through dilatometry tests, it was found that Ni slows down the transformation kinetics of ferrite and pearlite phases, and, acts in favor of bainite and martensite formation. In low-carbon bainitic steels, Ni also favors the formation of lower bainite at the expanse of upper bainite. The steels with lower Ni contents, 0.03 % and 0.70 %, have their microstructures mostly composed of upper bainite, with clusters of alloy carbides precipitated in the boundaries of the bainitic ferrite packages. The clusters of hard carbides act as stress raisers, facilitating the propagation of cracks, and, reducing the toughness of the steels. The other two steels studied with higher Ni, 1.40 % and 2.75 %, showed superior hardenability and predominant microstructures of lower bainite and martensite, respectively. The steels composed of lower bainite and martensite have higher toughness than the steels with upper bainite due to their more refined microstructure and homogenous dispersion of the alloy carbides in the steel matrix. It was also observed in the bainitic steels that tempering does not influence the location where the alloy carbides precipitated during bainite transformation. After tempering, these carbides remain at the bainitic ferrite boundaries or dispersed in the bainitic ferrite packages depending on the type of bainite formed.

The influence of carbide formation in ferrite on the bainitic type transformation

The effect of carbide formation on the development of the bainitic type transformation was studied using phase field modeling. The phase field model incorporated the displacive structural transformation producing a misfitting product phase, long range elastic interactions, carbon partitioning from austenite and ferrite and the formation of carbides in ferrite modelled as carbon sinks. The results of the simulations performed for the systems with different carbide nucleation sequences demonstrate that initially, due to a slow nucleation of carbides and fast carbon partitioning, carbon diffusion to austenite was the dominant process leading to the decrease in the transformation rate. However, after some period of time, the effect of carbon depletion in the solid solution by carbides became more significant, which led to the reduction of the average concentration of carbon in the whole system and the acceleration of the structural transformation. The rate of carbide nucleation and the number of nucleated carbides controlled both the transformation rate and the period of time before the transformation acceleration.

Promising bending properties of a new as-rolled medium-carbon steel achieved with furnace-cooled bainitic microstructures

The bending properties of a new thermomechanically processed medium-carbon (0.40 wt% C) low-alloy steel, intended for the slurry transportation pipeline application, have been investigated. The studied material was hot-rolled to 10 mm thick strips followed by direct quenching to two different quench-stop temperatures (QST) of 560 °C and 420 °C. The samples were subsequently cooled slowly to room temperature in a furnace, producing two different bainitic microstructures. In general, the final microstructures on the centerline consisted of different bainitic features with yield strengths of a ~700 MPa and ~1200 MPa for QST 560 °C and 420 °C, respectively. To determine the factors affecting bendability, as determined by three-point brake press bending tests, local microstructural features and texture were characterized with transmission and scanning electron-microscoy and macrohardness tests. Detailed quantitative microstructural evaluation of both subsurface and mid-thickness regions revealed that the bainitic sample produced at the higher temperature (QST 560) consisted of almost equal amounts of bainite types B1 and B2, where B1 has bainitic sheaves with a low dislocation density and intralath cementite, and B2 a very dislocation-dense morphology with mainly interlath cementite. In the QST 420 sample, the high dislocation density components B2 bainite and martensite were dominant, although martensite was only present near the strip surfaces. Different post-rolling cooling conditions did not change the general crystallographic theme but resulted in a slight increase in the texture intensity of the QST 420 sample. Neither the concave nor convex subsurface regions showed significant changes in texture after bending. The more favorable distributions of microstructural components and textural component intensities in the QST 560 sample resulted in higher elongation to fracture and work-hardening capacity, resulting a smaller minimum usable punch radius than for the stronger QST 420 sample. Fractographic examination of the cracked surfaces revealed that cracks developed by shear band formation followed by surface roughening, which promoted subsequent void and micro crack nucleation and growth.

Interaction between carbon partitioning and carbide nucleation inside austenite during a bainitic type transformation

The interaction between the displacive transformation, carbon partitioning and carbide nucleation inside austenite during a bainitic type transformation was studied using a phase field model incorporating the displacive transformation and diffusion processes. The carbide nucleation was modeled as a formation of carbon sinks inside austenite with different nucleation sequences. The results obtained with different nucleation sequences demonstrate that carbide nucleation may play a secondary role controlling the transformation kinetics and microstructure evolution after the fast transformation stage completion; however, elastic interactions can control the ferrite morphology even at a later stage, thus leading to the change of the ferrite grain shape from a rod-like to plate-like. A rate of the carbide nucleation and the number of nucleated carbides controlled the extension of the stasis period separating the fast and slow transformation stages and the rate of the transformation after the stasis or during a temporary slowdown period. Results show that the transformation can proceed through several slowdown - acceleration periods.

The interaction between the displacive transformation and the diffusion process in the bainitic type transformation

The interaction between the displacive transformation and the diffusion process during the bainitic type transformation was studied using a phase field model incorporating both processes. A chemical free energy density function was derived to represent thermodynamic properties of low-alloy steels at high supercooling conditions at which the direct transformation from austenite to ferrite at a constant carbon concentration is possible. The results demonstrate that in such conditions both the displacive transformation and diffusional decomposition process may play important roles controlling the transformation kinetics and microstructure. A fast displacive transformation is dominant at the onset of the transformation; however, the diffusion-controlled decomposition can take control over the transformation kinetics and even the ferrite morphology at later stages as a result of the formation of a carbon enriched layer around ferrite grains. Both plate-like and rod-like shapes of ferrite grains can be obtained depending on the thermodynamic conditions and diffusion mobility.

DECAY OF SUPERCOOLED AUSTENITE OF LOW-CARBON PIPE STEEL WITH THE USE OF GLEEBLE 3500 COMPLEX

The promising direction in improving the strength properties of low-carbon steels is the use of controlled rolling providing formation of structures with prevalence of a bainite component. Analysis of the references has shown that now there are no detailed researches which allow to approve what morphological type of bainite provides the most optimal properties. In this regard in the present work the influence of cooling rate on the structure, properties, and structuralphase transformations of low-carbon complex-alloyed pipe steel containing 0.062 % C; 1.80 % Mn; 0.12 % Mo; 0.032 % Cr, 0.90 % Ni and other elements (Al, Cu, V, Nb, Ti) was studied. The dilatometric method was used to construct the CCT diagram of the decay of supercooled austenite of low-carbon complex-alloyed pipe steel. The qualitative and quantitative analysis of microstructure was carried out and hardness after various speeds of cooling was determined. There were identified the cooling rates providing bainite structures and increase in the strength properties of steel with specified composition. At a cooling rate since 0.05 to 6 °C/s, along with ferrite, a globular bainite is formed in the microstructure, consisting of bainitic α-phase and “islands” of martensite-austenite component ranging in size from 1 – 6 μm. At a cooling rate of 6 °C/s, conversion to reed bainite is observed, along with the borders of which the carbides and residual austenite are located. At cooling rates of more than 16 °C/s, bainite becomes bag-rack. With an increase in cooling rate from 50 to 150 °C/s, the average width of the bainite α-phase rails decreases from 2.22 to 1.32 μm.

The Effect of Bainite Type on the Evolution of Carbide Constituent During an Accelerated Aging in Cr-Mo-V Steel

This paper aims to understand the effect of the type of bainite (i.e., the lower, the upper and the granular, developed by bainitic hardening at 550, 650 and 750 °C, respectively) on the evolution of carbide constituent due to a subsequent aging (at 700 °C for up to 500 h). The secondary carbides were identified by x-ray diffraction in the configurations of M3C, M7C3, M23C6, M2C, M6C and MC (M=Fe, Cr, Mo, Mn and V), which varied with the time of aging. Despite that the three types of bainites shared similar constituent of carbides after the aging for 2-100 h, the constituent of the upper bainite (M7C3 + M23C6 + MC) was different from that of granular bainite and lower bainite (M7C3 + M23C6 + MC + M6C) after the aging for 500 h. The concentration of the enrichment of alloying elements in ferrite matrix plays a dominant role in governing the precipitation of carbides in the aged steel.

Special features of the formation of welded joints in vertical laser-arc welding of low-alloy steels

ABSTRACTInvestigations were carried out into the special features of the technology of vertical laser-arc welding of large structures made of high-strength and low-alloy steels with a variable welding gap and a large root face.The parameters of the conditions of vertical laser-arc welding of root passes in the low-alloy steel 30–40 mm thick with a root face of 10 mm (welding speed, the amplitude and frequency of transverse oscillations, the intensity of welding current, arc voltage, laser radiation power) resulting in the formation of high-quality welded joints are determined. The experimental results show that the laser radiation power, required for producing continuous penetration of the root face, depends on the thickness of the parent metal: for the metal thickness of 14–30 mm the sufficient laser radiation power is 3.8 kW, for a thickness of 40 mm it is no less than 4.6 kW to penetrate a depth of 5 mm.The welded joints have a fine-dispersed acicular structure of the martensitic – bainitic type. The experimental results were used to develop a technology of welding of low-alloy steels of different thickness using robotic equipment.

Using the Principles of Image Analysis in the Assessment of the Proportion of Retained Austenite in the Case Hardened Layers

Abstract Structures of carburized layers after the surface saturation process in gaseous, liquid or solid medium and after subsequent heat treatment (hardening and low-temperature tempering) consist mainly of high carbon plate martensite with a certain portion of retained austenite. The presence of retained austenite (RA) in carburized layers is mostly considered as undesirable because it decreases hardness of the hardened layer and furthermore, a spontaneous conversion to a ferritic-carbide mixture of a bainitic type, accompanied by a change of properties, dimensional instability and the local increase in internal stress with the possible formation of cracks, can occur. The proportion of retained austenite is, therefore, a significant characteristics of the quality of hardened layers. This work deals with the evaluation of the volume fraction of retained austenite in carburized layers using image analysis on metallographic images.

Formation of Bainite in Advanced High-Strength Steels During Isothermal Transformation

Abstract The isothermal transformation of bainite in an advanced high-strength steel (AHSS) in the temperature range of 422°C to 482°C for transformation times of 10–6000 s has been examined using light optical microscopy (LOM), and orientation imaging microscopy (OIM). Thermodynamic calculations were done using equilibrium thermodynamic simulation software to produce a theoretical time-temperature-transformation (TTT) diagram, to which the experimental results of this study were compared. Advanced microstructural techniques, such as orientation-imaging-microscopy–electron-backscatter-diffraction (OIM-EBSD), image quality (IQ) phase identification, and polarized light microscopy, were used to observe and quantify the progress of the bainitic transformation during isothermal transformation. Orientation imaging microscopy using electron backscatter diffraction image quality (EBSD-IQ) was used with each sample to quantify the amount of bainite transformed. Results showed that the isothermal transformation at 422°C had 80.1 % bainite, whereas the isothermal transformation at 482°C had 85.5 % bainite transformed after 1200 s; while, after 6000 s, the amount of bainite increased to 90.1 % and 93.4 % respectively. As a result the chemical composition of the steel investigated and the microstructural characteristics of the bainite observed, it was not possible to discern the type of bainite formed at higher and lower temperatures because of the absence of carbides. The kinetics of transformation from the experimental results clearly indicated that the isothermal transformation is slower than that predicted by the theoretical thermodynamic program. The results of this study are presented and discussed.

Microstructural effects on high-cycle fatigue properties of microalloyed medium carbon steel 38MnVS

High-cycle fatigue properties of medium carbon microalloyed (MA) steel 38MnVS with different microstructural characteristics were studied by rotating bending fatigue test. Low alloy steel 40Cr with quenched and tempered (QT) microstructure was also used for comparison. The results show MA steel in the as-forged condition has the best fatigue property mainly owing to the significant precipitation strengthening of fine V(C,N) precipitates and much fine ferrite–pearlite microstructure, which is even better than that of the QT steel 40Cr. The formation of film-like ferrite along coarse prior austenite grain boundary and coarse bainitic type microstructure as well as the coarsening of the initial fine precipitates significantly deteriorated the MA steel's fatigue properties. SEM examination of fatigue fracture surfaces revealed that the fatigue fracture mechanism of MA steel is different from that of QT steel. It is concluded that microstructure of ferrite–pearlite type MA steels should be carefully controlled after rolling and/or forging to obtain improved fatigue properties.

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.

Transmission Electron Microscopy of Longitudinal Double Submerged Arc Weld (LDSAW) of X-120M Steel Line Pipe

This paper deals with the development of submerged arc welding wires for longitudinal double submerged arc welded (LDSAW) line pipe weld through transmission electron microscopy (TEM). The experimental procedure in the paper describes the test coupons preparation for submerged arc welding (SAW) with various combinations of wire and flux with varying level of alloying element. Microstructural characterization by transmission electron microscopy has been carried out to establish the desired microstructure in the weld of LDSAW for manufacturing the API-5L X-120M line pipes. The TEM micrographs for shows the lath type ferrite and bainitic type ferrite with high dislocation density. The lath type ferrite and bainitic type ferrite with high dislocation density also have fine precipitates in the ferrite matrix having orientation relationship. The weld metal suitable for X-120M have the microstructure of mainly bainitic and martensitic with high dislocation density and coarse precipitates in the matrix. The bainitic and martensitic microstructure have excellent fracture toughness down to -20 °C at this strength level (X-120M).

Bainite Transformation in Deformed Austenite

This study documents the characteristics of bainite transformations in deformed austenite during continuous cooling of a low-carbon microalloyed steel. In particular, it describes the distinguishing features of the two types of bainite observed: conventional bainite (CB) and acicular ferrite (AF). CB nucleates at prior austenite grain boundaries and grows as packets of parallel ferrite laths. Growth of CB packets is limited by either the deformation substructure (“mechanical stabilization”) or austenite grain boundaries. AF nucleates at intragranular sites and grows as individual ferrite laths or groups of parallel laths. In an AF group of parallel laths, some neighboring laths have the same orientation, but a significant number have dissimilar orientations. Neighboring laths having dissimilar orientation are two variants of either Kurdjumov–Sachs (KS) or Nishiyama–Wasserman (NW). It is proposed that the AF nucleation sites are dislocation boundaries having a relatively high misorientation (i.e., microbands (MBs) and minimized by the variant selection mechanism).

Structure–property correlation in austempered alloyed hypereutectic gray cast irons

The austempering behavior of a series of hypereutectic alloyed gray iron compositions with carbon equivalent from 4.37 to 5.14 was studied to understand the influence of microstructure on its mechanical and wear properties. The alloying elements in the alloys included Ni, Mo, Cr and inoculation by micro-constitution of Ti, Nb and Ce. The alloys were austempered at 360 °C and upper bainitic type feathery ferrite was observed in the matrix. While the graphite content determined by optical metallography varied between 16 and 24 vol%. The volume of austenite determined by XRD analysis showed values between 20 and 26%. The ferrite lath size was determined using XRD peak broadening. The tensile property varying between 188 and 270 MPa, showed no significant variation with volume percentage of carbon or austenite in the ausferrite. However the wear rate varying between 0.5 and 2.6 × 10 −7 g/Nm, showed a decreasing trend with graphite content attributed to the higher lubricating effect of released carbon during sliding wear. The specific wear rate of hypereutectic alloys, increased with increasing ferrite lath size due to enhanced softer ferrite phase on the sliding surface. The wear rate was found to increase with volume of austenite, austenite carbon content and austenite lattice parameter, which is attributed to increased stability of austenite against strain induced martensite formation and the increased formation of bainitic carbides in the second stage tempering. The various technical aspects in correlating the microstructure with the mechanical and wear properties of hypereutectic austempered gray iron are described.

Morphology and constitution of the phases in as-welded microstructure of austempered ductile iron

It was found by optical and electron microscopic examination of the microstructure of as-weld austempered ductile iron that the weld matrix is composed of austenite and bainite, the volume fractions of which were determined. In addition, the carbon content of austenite was measured and therefore the average carbon content of the matrix was calculated. In the matrix of the weld metal two types of bainite, bainite ferrite and lower bainite, were found. According to the morphology and distribution of the bainite plates, the nucleation and growth modes of bainite was inferred.

Modelling Upper and Lower Bainite Trasformation in Steels

Bainite is of considerable importance in the design of high strength steels. There are two types of morphologies, upper and lower bainite. In upper bainite, cementite forms between adjacent bainitic ferrite plates. In certain steels, however, the cementite reaction is suppressed so that carbon-enriched austenite remains untransformed between bainitic ferrite plates. In lower bainite, cementite also has the opportunity to precipitate within bainitic ferrite plates. In order to model the development of these microstructures, it is necessary to treat the simultaneous formation of both the ferritic and carbide components of the microstructure. A theory has been developed to do exactly this, enabling the estimation of the phase fractions, the cementite particle size and the transition from upper to lower bainite. The results have been compared against experimental data.

Time-Temperature-Transformation Diagram within the Bainitic Temperature Range in a Medium Carbon Steel

The time-temperature-transformation (TTT) diagram within the medium temperature range of medium carbon steel has been determined. A single type of C-curve is found within the bainite temperature range for the studied steel. Distinct reaction C-curves were not observed for both types of microstructure, upper bainite and lower bainite in the TTT diagram. Experimental results on the kinetics of the isothermal formation of bainite at different temperature have demonstrated that both type of microstructure, upper and lower bainite, possesses similar overall transformation kinetics. Some applications of phase transformation theory towards the formation of bainitic microstructures are discussed, with particular emphasis on the bainite start temperature, transition temperature from upper to lower bainite, martensite start temperature and the thickness of bainitic plates. Bainite can be regarded as a non-lamellar mixture of ferrite and carbides, but within this broad description, it is possible to identify two classical morphologies, traditionally named upper and lower bainite. Both upper and lower bainite consist of plates of ferrite, known as sub-units, separated by cementite. The plates of ferrite grow in aggregates called sheaves of bainite. Within each sheaf, the plates of ferrite are parallel and share a common crystallographic orientation. The essential difference between upper and lower bainite is with respect to the carbide precipitation. In upper bainite, the bainitic ferrite is free of precipitation; carbides grow from the regions of carbon enriched austenite, which are trapped between the sub-units of ferrite. By contrast lower bainitic ferrite contains a fine dispersion of plate-like carbides within the bainitic ferrite plates. Both upper and lower bainite are formed by the propaga- tion of displacive ferrite sub-units followed by the partition of the excess carbon into the residual austenite. Cementite can then precipitate from the enriched austenite between the ferrite plates. The growth of the bainitic sub-units is accompanied by a change in the shape of the transformed region, which can be described as an invariant-plane strain with a large shear component. The parent austenite cannot accommodate the large shape deformation elastically and relaxes by plastic deformation in the region adjacent to the bainite. This plastic deformation stifles the growth of bainite sub-units before they hit any obstacle, and the transformation proceeds with the formation of a new sub-unit. 1) The mechanism of transformation continues until the carbon concentration of the residual austenite reach the To curve, at which ferrite and austenite with identical chemical compo-

Characterization of Austempered Ductile Iron Through Barkhausen Noise Measurements

The outstanding mechanical properties of austempered ductile irons (ADI) are linked to the microstructure of the matrix obtained by subjecting a ductile iron with an appropriate composition to a heat treatment called austempering. Then the microstructure of the matrix consists of bainitic ferrite with different volume fractions of retained austenite. The aim of this work is to use the magnetic Barkhausen noise (MBN) as a nondestructive method for characterizing the microstructure of ADI. First, it is shown that the amplitude and position of the peak-shaped MBN response is quite sensitive to the microstructure of the matrix of ductile irons. Thus each type of constituent (equiaxial ferrite, pearlite, martensite or bainite) exhibits a typical response and, in turn, it can be identified from the MBN response. Furthermore, a good correlation is found between MBN signal parameters and ADI heat treatment parameters, indicating that MBN is also quite sensitive to fine evolutions of the microstructure of ADI. MBN peak position is especially sensitive to the type of bainite, whereas peak amplitude is linked to the progress of the bainite reaction. Hence MBN measurements appear to be a powerful tool to assess some important microstructural features of ADI castings.