Acicular ferritic microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel

The transformations during continuous cooling and isothermal processes, the effects of hot deformation and the morphology of the final microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel designed for acicular ferrite microstructure were investigated. The results show that there are three independent “C” curves for isothermal phase transformation, i.e., TTT diagram, of low-carbon microalloyed steel, namely, polygonal ferrite–pearlite transformation “C” curve, the massive ferrite transformation “C” curve and the bainitic transformation “C” curve, respectively. Hot deformation accelerates acicular ferrite transformation and refines the steel's matrix. The microstructure of acicular ferrite for pipeline steels was discussed.     

The role of molybdenum additions and prior deformation on acicular ferrite formation in microalloyed Nb–Ti low-carbon line-pipe steels

Microstructures in Nb–Ti-microalloyed line-pipe steels with various molybdenum additions, consisted mostly of acicular ferrite plus polygonal ferrite after hot rolling and rapid cooling. Structure-sensitive surface relief after etching on shadowed extraction replicas, allowed quantification of the acicular and polygonal ferrite contents. Continuous cooling transformation diagrams of two alloys, one Mo-free and the other containing 0.22% Mo, were determined for cooling rates from 0.1 to 40 °C s^− 1 without and with prior deformation of the austenite below the nil-recrystallisation temperature. Molybdenum additions slightly enhanced the acicular ferrite formation in the strain-free austenite whereas prior deformation had a much greater effect, and strongly promoted acicular ferrite formation in both alloys. Thin foil electron microscopy of acicular ferrite in these low-inclusion content alloys showed a preference for parallel acicular ferrite laths with less “chaotically” nucleated laths.     

Isothermal decomposition of supercooled austenite in steels

Abstract

A brief rationalisation is made of the isothermal decomposition kinetics of supercooled austenite in steels. From the experimental evidence of the past several decades, it is concluded that each type of isothermal decomposition product, i.e. grain boundary allotriomorphs, Widmanstätten ferrite (and cementite), pearlite, upper bainite, lower bainite, lath martensite, and twinned martensite, has its own independent C-curve in time–temperature–transformation (TTT) diagrams. Special emphasis is placed on the isothermal transformation kinetics of martensite. It is demonstrated that martensite transformation follows C-curve kinetics in isothermal conditions; this is a general rule and holds for all steels. The reason why most experimental TTT diagrams fail to display separate C-curves for different products is briefly explained. A temperature–composition–product (TCP) diagram is constructed for Fe–C alloys (plain carbon steels) to depict the general pattern of the decomposition process and to display the conditions for the formation of the various decomposition products.

MST/1623     

Annealing Textures for Drawability: Influence of the Degree of Cold Rolling Reduction for Low-Carbon and Extra Low-Carbon Ferritic Steels

This work considers the optimization of deep drawing properties by studying the influence of hot rolling conditions, cold reduction rate, and final annealing on the evolution of steel sheet textures. Two steels have been selected: a low-C steel used for enameling applications, and an extra-low-C steel of the interstitial-free type. Results show that the intensity of {111} component—and, consequently, drawability—is considerably higher in the textures of cold-rolled and annealed sheets than in hot-rolled sheets. It is suggested that drawability of sheets annealed after cold rolling improves if greater than conventional reduction rates are used during rolling. Finally, it is shown that, contrary to what has sometimes been claimed, improvements of the “r” coefficient are not accompanied by a “pancake” morphology of the ferrite grains.     

INFLUENCE OF TWO-PHASE REGION ROLLING ON TRANSFORMATION AND RECRYSTALLIZATION BEHAVIOUR OF FERRITE IN COPPER-CONTAINING HSLA STEEL

Hot deformation of copper-containing microalloyed steels in the two-phase region was carried out to study the effect of copper on transformation and recrystallization behaviour of ferrite in HSLA steels. it was found that presence of copper could decrease the austenite to ferrite transformation temperature. The precipitation of epsilon-copper in ferrite could retard recovery and recrystallization by dislocation and grain boundary pinning in deformed ferrite. Retardation of transformation and ferrite recrystallization resulted in a less mixed structure consisting of fine transformed and recrystallized ferrite in the copper-containing steels.     

Modelling of materials properties in duplex stainless steels

Abstract

Work is being undertaken to develop a new multiplatform software programme for predicting a wide range of materials properties for various alloy types. These properties include thermophysical and physical properties, mechanical properties, time–temperature transformation (TTT)/continuous cooling transformation diagrams etc. and the calculations are being applied to a variety of multicomponent alloy types, such as Ni based superalloys, steels, Ti alloys, Al alloys. The current paper concentrates on duplex stainless steels and describes the scientific background used for the calculation of TTT diagrams and mechanical properties for these materials. It is shown that there is very good agreement between the calculated TTT diagrams and the observed results from the literature for the formation of the σ, χ, and the chromium rich α' phase, while the calculated proof stress, tensile stress and hardness are in good agreement with the available experimental data. The effects of solution treatment temperature and the volume fraction of σ phase on mechanical properties are also discussed.     

Structure–property relationships in intercritical heat affected zone of low-carbon microalloyed steels

Abstract

The conditions for martensite formation in the intercritical heat affected zone (HAZ) of two low-carbon microalloyed steels have been investigated using optical and transmission electron microscopy. Based on Charpy V-notch testing of a large number of thermally cycled specimens, it is concluded that embrittlement within the intercritical HAZ of such steels is closely related to the development of twinned martensite during the weld cooling cycle. The reduced HAZ toughness probably arises from the associated stress concentrations developed in the surrounding ferrite matrix, which give rise to the initiation of brittle fracture in the ferrite.

MST/634     

Internal friction during martensitic transformation in high manganese Mn–Cu alloys

Low-frequency internal friction and elastic modulus were studied for manganese-rich Mn–Cu alloys in the temperature range of martensitic transformation (20–300 °C). It is shown that the some special features of the transformation peak and its temperature are caused by the degree of the spinodal decomposition. The phenomenological model connecting an-elastic effects with the stages of evolution of the structure during martensitic transformation in manganese-rich Mn–Cu alloys (tweed structure–“parquet” structure–classical twinning martensite) is presented.     

A mechanistic model for the influence of stress ratio on the LEFM fatigue crack growth behavior of metals and alloys—I. Crack-ductile materials

Concentrating on local behavior of a highly stressed zone ahead of the crack tip, a recent mechanistic approach to analyse LEFM fatigue crack growth behavior in three stages at stress ratio R = 0 is extended here to include the effect of a positive stress ratio. This paper is limited to analysing primarily the stages I and II of “crack-ductile” materials, characterised by a purely “reversed shear” (or ductile “striation”) growth mechanism in stage II. It is shown that in these materials stage I is R-sensitive and stage II is insensitive, and these can, without invoking crack closure arguments, be rationalised alternatively by considering the dominance of a K_max-controlled “Submicroscopic Cleavage” and a ΔK-controlled “ reversed shear ” fracture mechanism, respectively. Assuming Paris type power relations to hold, a predictive model is developed that contains separate growth equations with R-effect for stages I and II and shows the existence of a characteristic “master shear-curve” and a “moving pivot-point” on this curve for a class of materials. Good agreement was found between quantitatively predicted growth curves at selected R-values and a relatively large volume of available experimental data for low strength steels, aluminum alloys and titanium alloys. Besides providing more physical explanations for the observed growth behavior, the model may also be useful as a convenient alternative to crack closure for obtaining fairly accurate and conservative estimates of fatigue life for design applications.     

Evaluation and review of simultaneous transformation model in high strength low alloy steels

Abstract

This work briefly describes and evaluates one of the most complete transformation models, which deals with the non-isothermal decomposition of austenite. The model, that does not consider the effect of precipitation on phase transformations, has been experimentally validated in high strength low alloy steels in order to evaluate how it works for microalloyed steels, where precipitation may play an important role. It has been found that the simultaneous transformation model is able to predict with an excellent agreement in microalloyed steels the formation of microstructures consisting of ferrite plus pearlite. However, the bainite formation is not successfully described by the model. The calculations incorrectly predict the formation of martensite instead of bainite in many situations.     

低碳钢超细晶铁素体的形成

将含碳量（质量分数）为0．057％和0．18％的低碳钢在不同过冷度、变形温度、变形速率和变形量的条件下进行热模拟实验,研究了含碳量和热变形条件对超细晶粒形成的影响．结果表明,变形前快速冷却（20℃／s）至Ar3以上附近温度并进行超过50％变形量的变形,能强烈促进过冷奥氏体形变诱发铁素体相变,铁素体在奥氏体晶内平行的变形带上形核,并发生动态回复和再结晶,从而使组织细化．形变诱发的相变过程由碳的扩散所控制,当钢的含碳量比较高时,小过冷度、大变形量和中等变形速率有利于铁素体相变,晶界碳化物的析出能够抑制铁素体晶粒的长大,因而高碳含量钢表现出更好的细化晶粒效果．     

Effect of V and V-N Microalloying on Deformation-Induced Ferrite Transformation in Low Carbon Steels

Deformation-induced ferrite transformation （DIFT） has been proved to be an effective approach to refine ferrite grains. This paper shows that the ferrite grains can further be refined through combination of DIFT and V or V-N microalloying. Vanadium dissolved in γ matrix restrains DIFT. During deformation, vanadium carbonitrides rapidly precipitate due to strain-induced precipitation, which causes decrease in vanadium dissolved in matrix and indirectly accelerates DIFT. Under heavy deformation, deformation induced ferrite （DIF） grains in V microalloyed steel were finer than those in V free steel. The more V added to steel, the finer DIF grains obtained. Moreover, the addition of N to V microalloyed steels can remarkably accelerate precipitation of V, and then promote DIFT. However, DIF grains in V-N microalloyed steel easily coarsen.     

Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Modelling of kinetics and dilatometric behaviour of austenite formation in a low-carbon steel with a ferrite plus pearlite initial microstructure

The compiled knowledge in literature regarding the isothermal formation of austenite from different initial microstructures (pure and mixed microstructures), has been used in this work to develop a model for non-isothermal austenite formation in low-carbon steels (C < 0.2 wt%) with a mixed initial microstructure consisting of ferrite and pearlite. Likewise, calculations of relative change in length have been made as a function of temperature, and the differences between theoretical and experimental results have been analysed in 0.1C–0.5Mn low-carbon low-manganese steel. Experimental kinetic transformation, critical temperatures as well as the magnitude of the overall contraction due to austenite formation are in good agreement with calculations.     

The knives of Frank J. Richtig as featured in Ripley's Believe It or Not!®

Unusual claims, resulting from “secret” heat treatment methods, surround the toughness and sharpness of the knives manufactured during the 20th century by a cutler named Frank J. Richtig. Richtig became famous because of his dramatic demonstrations of the toughness of his blades by hammering them through iron and steel objects and then illustrating their subsequent sharpness. Two of Richtig's blades have been examined and their microstructure and mechanical behavior determined. Comparisons with contemporary steels of similar composition, which are themselves related to ancient Damascus steels, have been made. As a result, proposals regarding Richtig's “secret” heat treatments are given. Some comments are made regarding historical methods of testing blades and thereby provide a context regarding the degree of difficulty in mimicking the remarkable toughness and cutting properties described by Richtig. Finally, some observations are made regarding the contemporary specialty knifemaking industry in the U.S.     

铁素体-珠光体型非调质钢的高周疲劳破坏行为

研究了三种碳和钒含量不同的铁素体-珠光型非调质钢的高周疲劳破坏行为,并与调质钢进行了对比.结果表明,铁素体-珠光体型非调质钢的高周疲劳性能与其微观组织特征有关.提高铁素体相硬度,其疲劳极限及疲劳极限比均提高,疲劳极限比最高可达0.60,远高于调质钢的0.50;热轧态粗大的网状铁素体-珠光体组织的疲劳性能较差,低于同等强度水平的高温回火马氏体组织。铁素体-珠光体型非调质钢疲劳破坏机制不同于调质钢,其疲劳裂纹基本上萌生于试样表面的铁素体/珠光体边界,并优先沿着铁素体/珠光体边界扩展;对于同等强度水平的调质钢,不存在像铁素体那样的软相,因而易在试样表层粗大的夹杂物处萌生疲劳裂纹.     

Critical assessment 29: TRIP-aided bainitic ferrite steels

Transformation-induced plasticity (TRIP)-aided bainitic ferrite steels developed for automotive applications have attractive mechanical properties such as ductility, formability, toughness, fatigue strength and delayed fracture strength. These mechanical properties are principally associated with a ductile lath-structure matrix and the strain-induced transformation of the metastable-retained austenite films of 3–20 vol.%. In this paper, data on the microstructural and mechanical properties of the low-carbon TRIP-aided bainitic ferrite steels are critically assessed, as well as their deformation mechanism.     

The chemistry of micrometeoroid and space debris remnants captured on hubble space telescope solar cells

Prior to the retrieval in 1993 from low Earth orbit (LEO), the “—V2” Solar Array wing of the Hubble Space Telescope was exposed to hypervelocity impacts (micrometre to millimetre scale) from both micrometeoroids and space debris. The initial survey of the damage (100–3500μm diameter sized craters) identified that micrometeoroid remnants dominated the flux in the 100–1000μm size regime, with debris dominating>1000μm. These residues were composed of remnants of silicate minerals, calcite, metal sulfides and metals that often appeared as complex poly-mineralic melts within melt pits. A further survey of 10–100μm diameter craters identified that the most common chemistry was space debris with the crossover from meteoroids to debris being at around 30μm D_CO. Residues include remnants of specialised steels and paint fragments but the dominant type is aluminium and aluminium oxide, which are almost certainly remnants of solid rocket motor operations. It is found that the relative contribution of debris as a function of size, agrees remarkably with a prediction derived using flux data from Long Duration Exposure Facility and a meteoroid model.     

A mechanistic model for the effect of stress ratio on the LEFM fatigue crack growth behavior of metals and alloys—II. Crack-brittle materials

A recently proposed mechanistic model for the effect of stress ratio, R, on the LEFM (long) fatigue crack growth behavior of “crack-ductile” materials is extended here to explain and predict similar behavior under similar conditions of “crack-brittle” materials characterised by the presence of “static” modes of fatigue fracture in stages II and III. It is shown that in these materials the stage I behavior is similar, but the stages II and III behave differently from crack-ductile materials. Mechanism-based existence of two types of stage II curves characterised respectively by “ pure shear mode ” (SM-II) and “mixed-mode” (MM-II), both plotting linear but having different slopes, is introduced. It is shown that while stage SM-II is insensitive, stage MM-II is significantly sensitive to R, in the same material. Similar to stage I, another “ moving pivot-point ” exists at the transition from SM-II to MM-II, which slides down the “ master shear-curve ” with increasing R. Assuming a critical K_max for the initiation of static modes, a critical R for saturation of these modes, and Paris-type growth relations, a quantitative predictive model containing growth equations for stages SM-II and MM-II, has been developed. Stage III is discussed only qualitatively. Reasonably good agreement was found between predicted curves at selected R-values and a relatively large volume of experimental data for steels, Al-alloys and Ti-alloys. This simple, alternative model may be used for obtaining quick, fairly accurate and conservative estimates of R-influenced crack growth rates for design applications in preference to crack-closure which frequently requires elaborate and tedious experimental procedures.     

On the possible generalizations of the Kitagawa–Takahashi diagram and of the El Haddad equation to finite life

The celebrated Kitagawa–Takahashi (KT) diagram, and the El Haddad (EH) equation, have received great attention since they define quite successfully the region of non-propagation (or the condition of self-arrest) for short to long cracks. The EH equation can be also seen as an “asymptotic matching” between the fatigue limit and the threshold of crack propagation. Above this curve, finite life is expected, since cracks propagate and eventually lead to final failure. In this paper, possible extensions of the EH equation to give the life of a specimen with a given initial crack as a function of the applied stress range, using only “asymptotic matching” equation between known regimes, namely the Wöhler SN curve (or some simplified form, like Basquin law), and the crack propagation rate curve (or just the Paris’ law). This permits an extension of the so-called “intrinsic crack” size concept in the EH equation for infinite life. The generalized El Haddad equation permits to take into account approximately of some of the known deviations from the Paris regimes, for short cracks, near the fatigue threshold or fatigue limit, or to the static failure envelope. The new equations are also plotted as SN curves, showing that power-law regimes seem very limited with many possible deviations and truncations, even when the crack propagation law has a significant power-law regime. The diagram remains partly qualitative (for example, we neglect geometric factors), and can be considered a first attempt towards more realistic maps. Particularly interesting are the cases with the Paris exponent m < 2, in which propagation tends to be very slow until very close to the toughness failure, making the maps qualitatively different.