Morphology of bainite and widmanstätten ferrite

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowB _s and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.     

Morphology and Crystallography of Bainite Transformation in a Single Prior-Austenite Grain of Low-Carbon Steel

To elucidate the difference in the microstructure development of the bainite and the martensite-austenite (M-A) constituents between blocks (groups with bainitic ferrite having a similar crystal orientation), in a single prior-austenite grain, the microstructural development and morphology of bainite in one coarse grain formed in a simulated heat-affected zone of low-carbon steel were analyzed by laser-scanning confocal microscopy and an electron back-scattering diffraction method. The correlation between the morphologies of bainitic ferrite and the M-A constituent was elucidated by analyzing blocks in a coarse and single prior-austenite grain. The possibility of M-A constituent formation was enhanced when the misorientation between bainite ferrite was large. Furthermore, it was found via in situ observations that bainitic ferrite units belonging to the same Bain group were formed synchronously, although they were apparently separated from each other by another Bain group.     

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.     

In-Situ Synchrotron Diffraction Studies on Transformation Strain Development in a High Strength Quenched and Tempered Structural Steel—Part I. Bainitic Transformation

In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)) structural steel during continuous cooling under different mechanical loading conditions has been studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the phase fractions and lattice parameters of the phases during heating and cooling cycles under different loading conditions. In addition to the thermal expansion behavior, the effects of the applied stress on the elastic strains during the formation of bainite from austenite and the effect of carbon on the lattice parameter of bainitic ferrite were calculated. The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation. The start temperature for the bainitic transformation decreases upon increasing the applied tensile stress. The elastic strains increase with increase in the applied tensile stress.     

贝氏体铁素体内的精细孪晶

用分析电镜和高分辨电子显微镜观察分析了贝氏体钢中贝氏体铁素体精细结构及其尺寸,研究了铸造状态贝氏体钢经正火的连续冷却和锻造后空冷且等温转变后贝氏体铁素体亚片条间的晶体结构。结果表明,贝氏体铁素体亚片条间存在明显的孪晶关系。贝素体铁素体孪晶亚片条宽度在2~30nm范围内,最小的孪晶亚片条仅2~5nm。孪晶片条间的孪晶界面并非是平直的孪生面(11 2),而是由一段段以孪生面(11 2)为平台的台阶组成。     

Brittle fracture initiation associated with the strain localization in a heat-affected zone of a low carbon steel

Brittle fracture initiation in the ductile-brittle fracture transition region in the heat-affected zone (HAZ) of weldments of a low carbon steel has been investigated. Consistent with the previous results from blunt notch Charpy tests, brittle fracture initiation was observed in the case of J-integral tests to take place at the intersection of small bainitic ferrite grains of different orientations within a mixed area of bainitic ferrite and quasipolygonal ferrite in proximity to the boundary between a coarse bainitic ferrite. Partial load drop during loading, pop-in phenomena, in fracture mechanics tests in the low-temperature region is caused by essentially the same mechanism as for unstable brittle fracture initiation. Inhomogeneous microstructure in the HAZ gives rise to intense strain localizations in the mixed area of bainitic ferrite and quasipolygonal ferrite due to the constraint of plastic deformation therein and may produce accumulated defects that form an incipient crack for the brittle fracture. Partial load drop proceeds in association with repetitive initiations of brittle facets and their ductile linking. The strong temperature dependence of the magnitude of partial load drop is likely to show that the temperature dependence of the brittle fracture initiation is controlled by the first initiation of a brittle facet and the ductile linking with the following induced facets. Existence of coarse bainitic ferrite grains is a prerequisite for the extension of an incipient crack.     

The Bainite Reaction in Fe-Si-C Alloys: The Secondary Stage

The de composition of austenite following the initial rapid formation of bainitic ferrite in silicon alloyed high-carbon steels isothermally transformed at 380 °C has been studied. The reaction is initiated by the formation of a triclinic carbide, closely relaled to cementite. The carbide is highly symmetric with {952} martensite, and crossings between carbide plates and faulting on the (010)_c planes suggest that the carbide formation takes place by shear. The carbide formation is followed by the decomposition of the remaining austenite to ferrite, which is thought to be due to the lowering of the austenite carbon content resulting from the carbide formation. B. P. J. SANDVIK, formerly with Laboratory of Physical Metallurgy at Helsinki University of Technology, Finland and Ovako OyAb, Imatra, Finland.     

低碳钢中贝氏体组织在A1温度以下重加热过程中的演化与热稳定性

通过原位追踪金相观察、维氏硬度测试、透射电子显微术、电子背散射衍射等实验手段研究了低碳钢中贝氏体组织在550~675℃范围内重加热过程中的演化与热稳定性.实验结果表明:贝氏体组织通过回复与再结晶方式演化为多边形铁素体,在该过程中粒状贝氏体首先演化为多边形铁素体,然后多边形铁素体再吞噬贝氏体铁素体,贝氏体铁素体表现出了高于粒状贝氏体的热稳定性;在回复过程中,贝氏体铁素体中相邻铁素体板条之间的小角度晶界部分撤除,铁素体板条发生倾转与合并;贝氏体组织在重加热过程中的演化存在一个稳定阶段,处于回复与再结晶之间,其持续时间随温度的降低而显著延长.      

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.     

Variant Selection in Grain Boundary Nucleation of Upper Bainite

The crystallography of bainitic ferrite nucleated at austenite grain boundaries was studied in an Fe-9Ni-0.15C (mass pct) alloy. The relationship between bainitic ferrite orientations (variants) and grain boundary characters, i.e., misorientation and boundary orientation, was examined by electron backscatter diffraction analysis in scanning electron microscopy and serial sectioning observation. Bainitic ferrite holds nearly the Kurdjumov–Sachs (K-S) orientation relationship with respect to the austenite grain into which it grows. At the beginning of transformation, the variants of bainitic ferrite are severely restricted by the following two rules, both advantageous in terms of interfacial energy: (1) smaller misorientation from the K-S relationship with respect to the opposite austenite grain and (2) elimination of the larger grain boundary area by the nucleation of bainitic ferrite. As the transformation proceeds, variant selection establishing plastic accommodation of transformation strain to a larger extent becomes important. Those kinds of variant selection result in formation of coarse blocks for small undercooling. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASMI Phase Transformations Committee.

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Thermodynamics and kinetics of the formation of widmanstätten ferrite plates in ferrous alloys

Experimental data on the formation of Widmanstätten/bainitic ferrite in ferrous alloys(i.e., the Widmanstätten start temperature, partition of alloying elements, incomplete transformation, lengthening kinetics,etc.) are examined on the basis of thermodynamic calculations and kinetic analyses. A morphological change of ferrite from grain-boundary allotriomorph to Widmanstätten plate occurs well above theT ₀ temperature, except in high Mn and Ni alloys, but does so in the regime of carbon diffusion control in all alloys. Under the assumption that the plate tip consists of a pair of ledges of the height equal to the tip radius, the reported lengthening kinetics of ferrite plates can be accounted for very well by the diffusion-controlled motion of these ledges in a wide range of carbon supersaturation. It is also shown that the transformation stasis (incomplete transformation) observed below the kinetically definedB _s in some iron alloys cannot be unequivocally attributed to either the completion of the precipitation of no-partitioned ferrite or the loss of the driving force for subsequent shear transformation.     

The Bainite reaction in Fe-Si-C Alloys: The primary stage

The morphology, crystallography, and substructure of bainitic ferrite formed in silicon alloyed high-carbon steels in the temperature range 290 to 380 °C have been studied. The bainite exhibits a plate shaped morphology, an irrational habit plane, and an irrational orientation relationship. The bainitic ferrite is heavily dislocated, while the surrounding austenite contains thin twins, the density of which is highest in the austenite between closely spaced ferrite plates. The bainite plates can cross these twins in such a manner that the twinned region remains in a crystallographic orientation, which is quite different from that of the other regions of the bainitic ferrite plates. Epsilon carbide subsequently precipitates on the austenite twin/bainitic ferrite boundaries. The bainitic ferrite shape strain direction and magnitude are estimated from displacements of austenite twins inherited in the ferrite. All results, including measurement of the austenite carbon content, are consistent with a shear mode of transformation. B.P.J. SANDVIK, formerly with Laboratory of Physical Metallurgy at Helsinki University of Technology, Finland and Ovako OyAb, Imatra, Finland     

The isothermal decomposition of austenite in a high purity Iron-Chromium binary alloy

The transformation characteristics of a series of high purity iron-chromium alloys, within the γ-loop composition range, have been studied using continuous-cooling dilatometry. An Fe-10 wt pct Cr alloy, which exhibited a relatively slow γ → α transformation, was chosen for detailed investigation by isothermal dilatometry, and by optical and transmission electron microscopy. The TTT diagram exhibited a high-temperature Ccurve in which the transformation products were equiaxed ferrite and Widmanstätten ferrite, the latter developing from perturbations on the α:γ interface. In this range, a ledge mechanism was the predominant mode of ferrite growth. A simple activation energy analysis suggests that the γ→ α reaction is interface controlled, and supports the existence of a “solute-drag” effect by carbon even at low concentrations. Direct quenching of the alloy produced martensite. In the intermediate temperature range, it is proposed that the γ→ α reaction is bainitic in character.     

High-temperature transmission electron microscopyin situ study of lower bainite carbide precipitation

In situ observation of the bainite carbide precipitation processes in 40CrMnSiMoV steel by means of high-temperature transmission electron microscopy (TEM) is conducted. It is evident that carbides can precipitate either in bainitic ferrite or from austenite when carbide-free bainite (meta-bainite) obtained by isothermal transformation is tempered at higher temperatures. In view of the quantity of carbides precipitated from ferrite in combination with the result of an X-ray diffraction analysis of the bainitic ferrite carbon content, it can be concluded that bainitic ferrite growth involves supersaturation of carbon content to some degree. Formerly with Northwestern Polytechnical University Formerly with Northwestern Polytechnical University This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Metallography of bainitic transformation in silicon containing steels

The formation of carbide in lower bainite was studied in two silicon containing carbon steels by transmission electron microscopy and diffraction techniques. Epsilon carbide was identified in the low temperature isothermally transformed bainite structure. The crystallographic relationship between epsilon carbide and bainitic ferrite was found to follow the Jack orientation relationship,viz, (0001)ε l l(011)α, (101l)ε l 1(101)α. The cementite observed in lower bainite was in the shape of small platelets and obeyed the Isaichev orientation relationship with the bainitic ferrite,viz, (010) cl 1(1-11)α, (103) cl 1 (011)α. Direct evidence showing the sequence of carbide formation from aus-tenite in bainite has also been obtained. Based on the observations and all the crystallo-graphical features, it is strongly suggested that in silicon containing steels the bainitic carbide precipitated directly from austenite instead of from ferrite at the austenite/fer-rite interface as has been proposed by Kinsman and Aaronson (Ref. 1). The uniformity of the carbide distribution is thus envisaged to be the outcome of precipitation at the aus-tenite-ferrite interphase boundary. DER-HUNG HUANG, formerly with the Department of Materials Science and Mineral Engineering, University of California     

Isochronal Phase Transformations of Low‐Carbon High Strength Low Alloy Steel upon Continuous Cooling

Aimed to acquire optimum comprehensive properties for the oil and gas pipeline steels, thermal treatment should be controlled to achieve ideal microstructures. Effects of cooling rates on transformation kinetics and microstructures of the low‐carbon high strength low alloy (HSLA) steel were investigated to obtain an optimized thermal treatment technology. Dilatometric measurements, light microscopy, scanning electron microscopy, and transmission electron microscopy were employed in present work. The transformed microstructures contained polygonal ferrite + pearlite, acicular ferrite (AF), and bainitic ferrite (BF) due to the cooling rates increasing from 5 to 3000°C min^?1, in present investigated HSLA steel. The result shows that, the increase of cooling rate accelerates AF transformation and refines the steel's matrix. The morphology of martensite/austenite structures transformed from islands in AF to films in BF with the increase of cooling rate.     

Microstructure Evolution and Mechanical Behavior of a CMnSiAl TRIP Steel Subjected to Partial Austenitization Along with Quenching and Partitioning Treatment

The present study investigated the microstructure evolution and mechanical behavior in a low carbon CMnSiAl transformation-induced plasticity (TRIP) steel, which was subjected to a partial austenitization at 1183 K (910 °C) followed by one-step quenching and partitioning (Q&P) treatment at different isothermal holding temperatures of [533 K to 593 K (260 °C to 320 °C)]. This thermal treatment led to the formation of a multi-phase microstructure consisting of ferrite, tempered martensite, bainitic ferrite, fresh martensite, and retained austenite, offering a superior work-hardening behavior compared with the dual-phase microstructure (i.e., ferrite and martensite) formed after partial austenitization followed by water quenching. The carbon enrichment in retained austenite was related to not only the carbon partitioning during the isothermal holding process, but also the carbon enrichment during the partial austenitization and rapid cooling processes, which has broadened our knowledge of carbon partitioning mechanism in conventional Q&P process.     

The morphology,crystallography, and mechanism of carbide precipitation in an Fe-0.12 Pct C-3.28 Pct Ni alloy

Carbide precipitation during the eutectoid decomposition of austenite has been studied in an Fe-0.12 pct C-3.28 pct Ni alloy by transmission electron microscopy (TEM) supplemented by optical microscopy. Nodular bainite which forms during the latter stages of austenite decomposition at 550 °C exhibits two types of carbide arrangement: (a) banded interphase boundary carbides with particle diameters of about 20 to 90 nm and mean band spacings between 180 and 390 nm and (b) more randomly distributed (“nonbanded”) elongated particles exhibiting a wide range of lengths between 33 and 2500 nm, thicknesses of approximately 11 to 50 nm, and mean intercarbide spacings of approximately 140 to 275 nm. Electron diffraction analysis indicated that in both cases, the carbides are cementite, obeying the Pitsch orientation relationship with respect to the bainitic ferrite. The intercarbide spacings of both morphologies are significantly larger than those previously reported for similar microstructures in steels containing alloy carbides other than cementite (e.g., VC, TiC). Both curved and straight cementite bands were observed; in the latter case, the average plane of the interphase boundary precipitate sheets was near {110}_α//{011}_c consistent with cementite precipitation on low-energy {110}_α//{111}_γ ledge terrace planes (where α,β, andc refer to ferrite, austenite, and cementite, respectively). The results also suggest that the first stage in the formation of the nonbanded form of nodular bainite is often the precipitation of cementite rods, or laths, in austenite at the α:γ interfaces of proeutectoid ferrite secondary sideplates formed earlier. Although these cementite rods frequently resemble the “fibrous” microstructures observed by previous investigators in carbide-forming alloy steels, they are typically much shorter than fibrous alloy carbides. The bainitic microstructures observed here are analyzed in terms of a previously developed model centered about the roles of the relative nucleation and growth rates of the product phases in controlling the evolution of eutectoid microstructures.     

Direct Observation that Bainite can Grow Below MS

In situ simultaneous synchrotron X-ray diffraction and laser scanning confocal microscopy have confirmed that bainite in steels can grow below the martensite start temperature. This observation suggests that the formation curves for bainite in time-temperature-transformation diagrams should be extended below the martensite start temperature. Furthermore, the implication of this observation on the growth mechanism of bainitic ferrite is discussed.