Microstructural evolution and mechanical properties of high strength 3–9% Ni-steel alloys weld metals produced by electron beam welding

The microstructures and mechanical properties of weld metals of high strength steels having 3–9% Ni content have been investigated with getting a better insight into the role of retained austenite. The weld metals were produced autogenously by electron beam welding (EBW) process. The results showed that once Ni content exceeded 4% prior austenitic grains of the weld metals were rapidly coarsened and solidified into a cellular dendritic structure. The content of retained austenite increased with Ni addition and was preferentially distributed along the lath boundary, edges of coalesced bainite, cellular dendritic boundaries and at prior austenite grain boundaries. Retained austenite morphology was also changed on increasing nickel content from a discontinuous film into a continuous one. The impact toughness for half-size specimens has shown a significant drop from 126 J to 40 J when Ni content increased from 3% to 5%, while further addition of Ni partially recovered the toughness. Thorough investigation of fracture surface of weld metals, after impact test, elucidated that retained austenite was beneficial and has an effective role in reducing the detrimental effect of coalesced bainite that formed in the microstructure.     

Acceleration of regeneration treatment for nanostructured bainitic steel welding by static recrystallisation

Abstract

In order to accelerate bainite transformation during regeneration treatment following nanostructured bainite steel welding, static recrystallisation mechanism is adopted to refine the coarse grain size during two-pass welding. Large mechanical deformation was introduced into the coarse grain zone of the first weld pass, where static recrystallisation would occur with thermal effect of the second weld pass. Static recrystallisation kinetics of nanostructure bainite was studied by two-pass hot deformation experiments. The explicit temperature function of two-pass welding was presented, and effect of welding parameters on static recrystallisation of the deformed coarse grain zone was analysed. Experimental results showed that grains in original coarse grain zone were greatly refined and that nanostructured bainite regeneration was remarkably accelerated.     

Influence of sulphur and vanadium additions on toughness of bainitic steels

Abstract

The Charpy impact toughness of bainitic steels, in the longitudinal direction, was increased by appropriate additions of sulphur and vanadium. The Charpy impact value in the longitudinal direction was highest when the sulphur content was 0.1 wt-%. A suppression of austenite grain growth by manganese sulphides was recognised when the austenitising temperature was higher than 1373 K. Furthermore, it was found that bainite packets were refined by an increase in sulphur content, because manganese sulphides existing in the austenite grains promoted bainite transformation. The main effect of the vanadium addition was probably to enhance the above effect by precipitation of vanadium carbonitrides on the manganese sulphides. Accordingly, an increase in toughness with additions of sulphur and vanadium may be attributed to bainite packet refinement resulting from the suppression of austenite grain growth and the promotion of intragranular bainite transformation by manganese sulphides.     

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

Abstract

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.     

Effect of austenite deformation on crystallographic texture during transformations in microalloyed bainitic steel

Abstract

The evolution of the texture of ferrite as a function of the coiling temperature has been studied in a hot rolled Nb alloyed CMnMoCrB complex phase steel by means of electron backscatter diffraction. Coiling that steel at 720 ° C led to ferrite and pearlite, and coiling at 550 ° C produced a bainite-martensite microstructure. The presence of residual austenite in the steels coiled at 680 and 550 ° C allowed for texture measurements in γ. Analyses of texture gave fundamental information on the decomposition of γ in both the recrystallised state and the deformed state. It was found that austenite, initially deformed below the non-recrystallisation temperature T_nr, recrystallised statically d partially during the γ α and the γ ^d α _b transformations. In the specimen coiled at 680 ° C, primary ferrite and bainite could be distinguished based on the confidence indexof the diffraction pattern. A clear variant selection was observed for the γ ^d α _b transformation, as arotation of ? ₁ = 30 ° occurred inthe austenite between the ferrite and the bainite formations. The bainite was found to result mainly from the decomposition of the brass {110} 〈 112 〉 and Goss {110} 〈 001 〉 orientations of deformed austenite. The residual austenite was found to be recrystallised γ γ austenite with the cube{001} 〈 100 〉 orientation. Coiling simulations were performed in a dilatometer starting from different austenite grains sizes and deformation states. In the most deformed specimens, the deformation state of the austenite and the combined effects between the different alloying elements presentin the steel were responsible for a solute drag like effect.     

New approach for the bainite start temperature calculation in steels

Abstract

The bainite start temperature B _s is defined as the highest temperature at which ferrite can transform by a displacive transformation. A common observation is that the bainite start temperature is very sensitive to the chemical composition, indicating that the influence of solutes is more than just thermodynamic. Empirical linear regression models have long been used to calculate the B _s in a limited range of compositions. This paper attempts to create an empirical model of wider applicability and higher accuracy by means of neural networks. The results are compared with those calculated using the thermodynamic theory for bainite transformation, revealing that in general this theory agrees with the experimental results, but some discrepancies can still be found when the alloys are heavily alloyed.     

Cyclic strain hardening of polygonal and acicular ferrite/bainite microstructures in microalloyed steels in the temperature range − 150°C to 27°C

Cyclic strain hardening has been observed to be markedly sensitive to microstructural changes in microalloyed steels. Two significantly different microstructures - polygonal ferrite grains of average grain size 10–120 μm and acicular ferrite/upper bainite colonies of dimensions 200–625 μm - were examined in order to determine the influence of each on cyclic strain hardening and related properties. Tests were conducted at temperatures between ?150 and 27°C. The cyclic strain hardening exponent, β_c, was significantly more sensitive to changes in the size of the polygonal ferrite grains than to changes in the acicular ferrite/upper bainite colony size.     

Bainite formation influenced by large stress

Abstract

By using an iron alloy that transforms slowly to bainite at a low temperature, it has been possible to study the development of the upper bainite under the influence of exceptionally large stresses, which are nevertheless below the yield strength of the parent austenite. It is found that a uniaxial stress whose magnitude is below the elastic limit, strongly favours the growth of compliant variants, leading to an organised microstructure. It also accelerates the overall rate of reaction. A comparison between samples transformed with and without an applied stress revealed significant changes in the crystallographic texture, consistent with the observed microstructures. Stress assisted transformation resulted in large blocks of bainite in identical orientation.     

Isothermal transformations in advanced high strength steels below martensite start temperature

Abstract

The transformation products in advanced high strength steels have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Rapid cooling to various temperatures below martensite start was carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite, followed by isothermal holding. The transformation kinetics was monitored by means of dilatometry and microstructural characterisation by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below M_S are mainly composed of different fractions of tempered martensite, isothermal bainite with carbide precipitation and retained austenite.     

Transformation texture of allotriomorphic ferrite in steel

Abstract

Many aspects of the crystallographic texture which develops when austenite transforms into martensite or bainite are well established because the process by which the parent lattice is transformed into that of the product is mathematically defined. This is not the case when the ferrite forms by a reconstructive mechanism. The allotriomorphic ferrite nucleates heterogeneously at austenite grain boundaries, and although a reproducible, low energy orientation relationship is expected to exist between the ferrite and one of the austenite grains with which it is in contact, there are reports that the ferrite can simultaneously adopt this orientation with more than one austenite grain. The authors examine this possibility using crystallographic theory in order to assess the probability of such events as a function of the strength of the texture within the austenite before its transformation.     

Effect of austenite grain size and bainite morphology on overall kinetics of bainite transformation in steels

Abstract

An attempt is made to rationalise some contradictory observations on the effect of austenite grain size on the overall kinetics of the bainite transformation in steels. Experiments have been carried out on two steels which show opposite effects of austenite grain size on the reaction rate. General equations describing the reaction rate are derived by taking into account the morphology of the bainite in each steel. The equations derived can explain the contradictory effects of the austenite grain size on the overall reaction rate. The results are also found to be in good agreement with the published data.     

Transformation of austenite remaining after intercritical rolling to ferrite

Abstract

The impact of austenite deformation in the intercritical range on the rate of transformation in continuous cooling to ferrite, pearlite, bainite or martensite has been studied. The austenite associated with the rolled ferrite is much higher in carbon content, which does not influence the pearlite transformation but retards bainite and martensite. Furthermore, in comparison with rolling of stable austenite the increased strain hardening of the intercritically cooled austenite accelerates the formation of ferrite and pearlite (+ 10–30°C) and refines them but retards the bainite and martensite transformations (?20–40°C). At the intermediate cooling rate near 16 K s^?1, these several influences combined with near doubling of the ferrite production give rise to the suppression of bainite formation and to maximum increased delay of martensite start.     

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

Abstract

In the present study, wedge-shape sa mples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.     

Acoustic emission monitoring of bainitic and martensitic transformation in medium carbon steel during continuous cooling

Abstract

This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermomechanical simulator. After austenising at a certain temperature, the studied specimen was cooled down and the root mean square (RMS) value of the continuous AE signal was measured. During cooling two distinct peaks in the RMS data were observed at temperatures of 200-300°C and 500-600°C, which have been attributed to martensite and bainite formation respectively. The observed bainite peak strongly indicated that the mechanism of bainite growth is displacive. The AE monitoring of bainite and martensite formation was supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainitic and martensitic transformation was studied by varying the austenising temperature T _a. It was found that upon lowering T _a, i.e. with decreasing austenite grain size, the bainite peak increases while the martensite peak decreases.     

Lattice compatibility at interfaces after recrystallisation

Abstract

The nature, distribution, and implications of local, i.e. intragrain, lattice rotations have been explored in recrystallised nickel using electron backscattered diffraction. The data processing methodology for such methods is discussed in some detail. The results reveal that orientation perturbations with a periodicity of ～1 μm occur frequently within grains, and that there is a strong tendency for two grains which feature either both large perturbations or both small perturbations to be neighbours. The results are interpreted in terms of lattice compatibility, potential sites for instigation of secondary recrystallisation, and the meaningful specification of grain boundary misorientation.     

Microstructure and properties of carbide free bainite railway wheels produced by programmed quenching

Abstract

A kind of carbide free bainite steel was used to produce railway wheels. The alloy design, manufacture process, microstructure and behaviours are summarised. The novel steel is distinctively superior in excellent combination of strength and toughness compared with the traditional pearlite type wheel steels. The details of the microstructure of novel steel have been investigated in nanoscale.     

Opening previously impossible avenues for phase transformation in innovative steels by atom probe tomography

Abstract

After decades of debate on the mechanism for the formation of bainite, it is accepted that bainite grows via a displacive mechanism, i.e. as plate shaped transformation products exhibiting an invariant plane strain surface relief effect. But there is still much discussion on the diffusion or diffusionless nature of bainite. The purpose of this atom probe tomography study was to track the atom distributions during the bainite reaction in steels with different carbon and silicon contents. The steels were transformed over a wide range of temperatures (200–525°C) to elucidate the role of reaction rate and diffusion in the formation of bainite with and without cementite precipitation.     

Modification of microstructure to increase impact toughness of nanostructured bainite–austenite steel

Abstract

To improve impact toughness of the nanostructured bainite–austenite steel, a heat treatment operation was developed to divide prior austenite grains by plates of martensite directly before isothermal transformation. In the investigation, nanostructured steel containing 0·55%C, 1·95%Mn, 1·82%Si, 1·29%Cr and 0·72%Mo was used. It was found that a partial transformation to martensite achieved by cooling to 160°C followed by direct isothermal transformation to bainite at 225°C was the most promising treatment to improve Charpy impact energy of the investigated steel. For each testing temperature: ambient, 0, ?20, ?40 and ?60°C, the specimens subjected to the developed treatment showed a higher averaged impact energy than the specimens subjected to the standard treatment.     

Carbon partitioning during bainite transformation in low alloy steels

Abstract

Carbon partitioning in untransformed austenite during bainite transformation has been studied using high speed dilatometry. It was found that in specimens partially transformed to bainite, during subsequent quenching to ambient temperature two martensite start temperatures M _s can be registered. Because M _s depends directly on a carbon content in austenite, the obtained results may indicate that the carbon concentration trapped in films of austenite between parallel subunits of bainitic ferrite is much larger than in the blocks of austenite. It would indicate the necessity of a substantial modification of bainite and martensite regions on the time–temperature–transformation (continuous cooling) diagrams.     

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.