Low-temperature Transformation Research Articles

The Role of Retained Austenite and Its Carbon Concentration on Elongation of Low Temperature Bainitic Steels at Different Austenitising Temperature

The influence of austenitising temperature on the tensile properties of low temperature bainitic steel was investigated. With the increasing austenitising temperature, a significant change of elongation was found between 850 and 950 °C, which was changed from 1.0 ± 0.5 to 10.7 ± 2.0%; while there was a slight increase between 950 to 1050 °C (11.2 ± 1.5%). By characterising the retained austenite at necking and matrix, we found that the elongation is obviously correlated with the retained austenite content, and also determined by the volume change of retained austenite during the tensile test. The transformation induced plasticity (TRIP) effect, which contributes to the improve elongation, almost did not occur at 850 °C due to the relatively low volume percentage of retained austenite and its high carbon concentration, which resulted in a very low martensite transformation temperature. With the austenitising, the temperature was increased up to 950 and 1050 °C, and a large volume percentage of retained austenite was observed in the matrix. Meanwhile, a considerable amount of retained austenite has occurred by the TRIP effect because of a moderate carbon content.

Formation and optical properties of pure nano-sized anatase titania by low-temperature aqueous sol-gel route

Nanocrystalline anatase titania particles were synthesized by low-temperature aqueous sol-gel transformations from new types of precursors [Ti{OPri}4] (A), [{acac}2Ti{OPri}{ONC9H6}] (1), and [{acac}2Ti{ONC9H6}2] (2) {where, ONC9H6 = 8-hydroxyquinoline}. Formation of all the precursors was confirmed by elemental analysis, molecular weight measurements, FTIR, and NMR (1H and 13C). Titania samples (a), (b), and (c) were formed by aqueous sol-gel transformations of (A), (1), and (2), respectively. The resulting nano-materials were characterized using XRD, TEM, SEM, EDX, SAED, FTIR, and UV-Visible spectroscopic techniques. From the X-ray pattern, the phase purity of the synthesized powders was confirmed as anatase TiO2. Crystallite size of all the oxide samples was measured by XRD and TEM, found to be 26 nm (a), 11 nm (b), and 8 nm (c). Surface morphologies of all the samples were evaluated by SEM. Selected area diffraction (SAED) of (b) and (c) are also corroborated the XRD results. The absorption spectra of oxide samples, (a), (b), and (c) show energy band gap of 2.9 eV, 3 eV, and 3.1 eV, respectively. SAED-EDX analysis confirmed the formation of pure anatase-phase titania nanoparticles.

Study of Bauschinger effect of acicular ferrite and polygonal ferrite through ex-situ interrupted bending tests in API X80 linepipe steels

Linepipe steels complexly consisted of low-temperature transformation microstructures of bainitic ferrite, granular bainite, and acicular ferrite (AF) as well as polygonal ferrite (PF) which individually affect the Bauschinger effect occurring during the pipe-forming. In this study, microscopic analyses of electron back-scattered diffraction (EBSD) coupled with tension-compression and interrupted bending tests were performed for verification of the Bauschinger effect of AF and PF working as major microstructures in single-phase- and two-phase-rolled API X80 steels, respectively. With respect to microstructural effects on Bauschinger effect, the reduction in mobile dislocation density during the flattening was smaller in the AF than in the PF. However, the dislocation pile-up at low-angle substructures and high-angle grain boundaries was more frequently observed, thereby leading to the higher back stress and Bauschinger effect in the AF. Boundary kernel average misorientation (KAM) profile played a critical role in determining the Bauschinger effect because they were closely related with the back stress. Thus, the Bauschinger effect was higher in the single-phase-rolled steel than in the two-phase-rolled steel. The present ex-situ interrupted bending methods coupled with EBSD analyses are outstanding ones for the detailed explanation of Bauschinger effect and provide an important idea for the yield strength designs of linepipe steels.

Low-Temperature Transformation of Amorphous Sol\u2013Gel TiO2 Powder to Anatase During Cold Spray Deposition

Nanocrystalline TiO2 coatings were successfully deposited on metallic substrate surfaces through low-pressure cold spray. Sol–gel amorphous TiO2 dry gel was for the first time used as a feedstock powder. Agglomerated morphology of the amorphous powder with high-density packing of particles and reduced pores number was an important factor for successful low-pressure cold spraying. However, in the present study one more factor contributing to anchoring of TiO2 particles by cold gas spraying onto a substrate was indicated. This factor enabling effective powder deposition and coating production is based on an exotic for conventional ceramic materials’ plastic deformation of amorphous powder. Furthermore, the detailed analysis of SEM, Raman and XRD measurements clearly demonstrated that amorphous titanium dioxide is transformed into crystalline anatase form during the process of low-pressure cold spraying.

Structural Aspect of Isothermal Bainitic Transformation in High-Carbon Manganese–Silicon Steel

Transmission and scanning electron microscopy and X-ray diffraction analysis were used to compare the structural states of high-carbon manganese–silicon steel which underwent low-temperature isothermal bainitic and martensitic transformations. It was shown that bainite consisting of thin α-phase plates and residual austenite interlayers forms during isothermal holding at 300°С. As the isothermal holding time increases, fine carbide particles precipitate within the bainite α-phase.

Effect of Washing on the Quality of Surimi-Like Preparation Obtained from Soft Tissue of Freshwater Mussel Sinanodonta woodiana (Lea, 1834)

ABSTRACT The research analyses the influence of the number of washings and different washing solutions, on the recovery of dry matter and protein in a preparation made from freshwater mussel Sinanodonta woodiana, (Lea, 1834). The study involved qualitative and quantitative characterisation of mussel protein preparation (MPP) obtained by electrophoretic separation and differential scanning calorimetry (DSC). The type of washing agent had the greatest influence on dry matter and protein content. Washing the homogenate with NaCl solutions and water was more effective on protein extraction than using only water. Two bands of proteins with molecular weights of 270–273 kDa and 42–43 kDa were characterized by the highest band intensity in electropherograms. The DSC analysis showed that on all denaturation curves of the samples there was one main peak of transformation ranging 56.69–62.94°C. The research revealed that washing homogenates with NaCl solutions caused the appearance of additional peaks at lower transformation temperatures, i.e. from 19.68°C to 36.45°C. The highest enthalpy value (4.53 Jg−1) was observed for the sample washed once NaCl solution and then water. The number of washings (1 or 2) and the use of a 0.5% NaCl solution or water were found to be the most favorable parameters for MPP production.

A Comparative Study on the Dynamic Tensile Behavior of Nanostructured Bainitic and Quenched-Tempered Martensitic Steels

In order to assess the dynamic tensile behavior of a newly developed three-step low-temperature-transformed bainitic microstructure, and a conventional one-step isothermal bainitic microstructure, and a quenched and tempered martensitic microstructure, a comparative research was conducted by using the dynamic tensile test. The results showed that more film-like retained austenite was obtained in the three-step low-temperature-transformed sample. The carbon content in the film-like retained austenite was found to be higher as compared to the blocky retained austenite. The dynamic tensile properties were all improved with an increase in strain rates (from 0.1 to 500 s−1). However, the three-step bainitic low-temperature-transformed sample showed the most remarkable dynamic tensile properties, including ultimate tensile strength and total elongation. This is attributed to a better transformation-induced plasticity effect that is exhibited by the higher stability of the film-like retained austenite. Therefore, the present study suggests that performing a three-step low-temperature bainitic transformation promotes grain refinement and the formation of film-like retained austenite and improves the dynamic tensile properties.

Effect of chemical dilution and the number of weld layers on residual stresses in a multi-pass low-transformation-temperature weld

Influences of chemical dilution and the number of weld layers on residual stresses in a multi-pass low-transformation-temperature (LTT) weld were investigated by finite element modelling and neutron diffraction. A coupled thermal-metallurgical-mechanical (TMM) model that took into account the chemical dilution effect was developed to simulate the complex LTT welding phenomena. The model was strictly validated by comparing the predictions with experimental measurements and the results found good agreement between them. The results showed that a transformation strain caused by LTT martensitic transformation introduced large compressive residual stresses (−500 MPa) into the weld zone and the quantities were closely related to the chemical dilution between welding layers. Simulation results revealed that the chemical dilution helped mitigate the large localized tensile residual stresses by modulating the phase transformation process and a single LTT layer was sufficient to generate high compression near the weld surface (~2.5 mm), which suggests a great importance for repair welding.

Non-standard ROS-generating combination “theraphthal–ascorbic acid” in low-temperature transformations of carbon allotropes

Multi-wall carbon nanotubes and graphite are relatively stable carbon allotropes, which could be solubilized and/or unfolded by a series of physical and chemical methods. Here, we present a low-temperature formation of several nanocarbons starting from these precursors, such as nano-onions, graphene sheets, and carbon nanoribbons. The experiments were carried out under ultrasonic treatment of aqueous systems “carbon allotrope − theraphthal − ascorbic acid” at various ratios and concentrations. Resulting nanocarbons were studied by scanning electron microscopy, transmission electron microscopy, XPS, FTIR, and Raman spectroscopy. Based on the experimental results, the mechanisms for carbon nanotube unfolding and separation of graphite layers were proposed. These reactions are based on the formation of reactive oxygen species by theraphthal in ultrasonic conditions and their further interaction with carbon precursors. These transformations can be considered as greener methods for obtaining nanocarbons.

The Effect of Martensitic Phase Transformation Dilation on Microstructure, Strain–Stress and Mechanical Properties for Welding of High-Strength Steel

The application of low transformation temperature (LTT) wire can effectively reduce residual stress, without the need for preheating before welding and heat treatment after welding. The mechanism reduces the martensitic transformation temperature, allowing the martensite volume expansion to offset some or all of the heat-shrinking, resulting in reduced residual stress during the welding process. In this paper, commercial ER110S-G welding wire and LTT wire with chemical composition Cr10Ni8MnMoCuTiVB were developed to solve the problem of stress concentration. The microstructure of the LTT joint is mainly composed of martensite and a small amount of residual austenite, while the microstructure of the ER110S-G joint is mainly composed of ferrite and a small amount of granular bainite. The micro-hardness and tensile strength of the LTT joint is higher than that of ER110S-G joint; however, the impact toughness of the LTT joint is not as good as that of the ER110S-G joint. The martensitic phase transformation of LTT starts at 212 °C and finishes at around 50 °C, and the expansion caused by phase transition is about 0.48%, which is much higher than that of the base metal (0.15%) and ER110S-G (0.18%). The residual tensile stress at the weld zone of the ER110S-G joint is 175.5 MPa, while the residual compressive stress at the weld zone of LTT joint is −257.6 MPa.

Suggestion of welding procedure for compressive residual stress induction by using low transformation temperature welding materials

ABSTRACTTo understand the cause of compressive residual stress in welded joints, we analysed by numerical analysis the effect of welding pass sequence using low transformation temperature (LTT) welding materials on residual stress around the weld toe of boxing fillet welded joints. It was determined by numerical analysis that the produced compressive residual stress and the influence of the stiffeners are reduced in the equivalent position of the weld toe in a fillet welded joint because of the influence on the behaviour of the stiffener in the weld being due to residual stress distribution around the weld toe. The residual stress reduction method of extending the length of the welded bead and releasing the weld toe from the stiffener, similar to the concept of discarding a bead to reduce tensile residual stress, was effective in fillet welded joints. Numerical analysis of the relationship between residual stress around the weld toe and width of the weld bead in the bead-on-plate welding model clarified that compressive residual stress can be introduced around the weld toe by having a wide width weld bead. In addition, a fully penetrated welded joint was very effective for causing compressive residual stress around the weld toe.

Residual stress reduction of laser beam welds by use of low-transformation-temperature (LTT) filler materials in carbon manganese steels—In situ diagnostic: Image correlation

Low-Transformation-Temperature (LTT) materials are used for residual stress reduction in weld seams and, subsequently, for the prevention of distortion. Typically, LTT materials are highly alloyed Fe-based materials with levels of chromium and nickel that ensure that austenite transforms to martensite at reduced temperatures. The dilatation associated with the transformation is prevented by the surrounding material. Consequently, compressive stresses develop within the transformed region, and these counteract the accumulation of stresses due to thermal contraction. The precise chemistry of the weld metal determines the transformation temperature as well as the magnitude of the volume expansion. The chemical composition of the weld metal can, in turn, be influenced by the energy input during welding. In recent years, LTT-filler materials have been applied successfully in arc welding processes for residual stress reduction. Digital image correlation provides one avenue for the detection of surface displacements or deformations, as they occur during the cooling down process, in the vicinity of a weld. Observation of the movement of surfaces using a stereo camera setup allows the recording of displacements in three dimensions. Technically, this is achieved by identifying recognizable points on the surface under observation and by tracking their movement via comparisons between several time-shifted images. Visualization of surface contractions, as well as the effects of solid-state phase transformations during the joining process, especially during cooling, is key to understanding and influencing the residual stress state in welds as well to reducing distortion. In this study, the used installation setup, and the methodology for sample preparation, shows the production of information relating to surface displacements in the direct vicinity of the laser weld during the cooling down stage. Different welding parameters were investigated, resulting in different dilution levels for the LTT-Filler material, thereby influencing the distances to the weld pool and temperatures at which transformations took place. In order to provide a point of reference, comparable welds, made with conventional filler wires, were also investigated. The displacements after welding are always lower when using an LTT filler wire when compared to a conventional wire, proving that LTT wires can be used to mitigate distortion during laser beam welding.

Features of Isothermal Formation of Carbide-Free Bainite in High-Carbon Manganese-Silicon Steel

Change in the stability of overcooled austenite of high-carbon manganese-silicon steel relative to decomposition via the intermediate stage upon isothermal holding and martensitic γ → α transformation upon subsequent cooling has been studied using dilatometry and X-ray diffraction. The kinetics of low-temperature bainite transformation has been studied. The tetragonality of the crystal lattice of bainite α-phase caused by the superequilibrium content of carbon was found.

A Novel Method of Reducing Residual Stress: Low Transformation Temperature (LTT) Weld Filler

A Novel Method of Reducing Residual Stress: Low Transformation Temperature (LTT) Weld Filler

Effects of low-temperature transformation and transformation-induced plasticity on weld residual stresses: Numerical study and neutron diffraction measurement

In this study, the weld residual stresses (RS) in a 25mm thick ferrite steel plate with newly developed low-temperature transformation (LTT) welding wire were investigated by finite element method and neutron diffraction (ND) measurement. A thermo-elastic–plastic finite element model coupled with solid-state phase transformation (SSPT) was developed to investigate the distribution and formation mechanism of RS, which has been verified by ND measurement. The results demonstrate that the developed LTT alloy can significantly reduce the RS and even generate compressive RS in the weld zone, due to the interrupted cooling shrinkage caused by the LTT. The higher inter-pass temperatures related to the microstructure evolution result in an increased region of compressive stress within the weldment. Moreover, the longitudinal RS in the weld zone gradually changes to tension as the initial temperature of martensitic transformation increases. Notably, the relaxation effect of transformation-induced plasticity on RS and its influence on model accuracy were discussed.

Processing and Characterization of Liquid-Phase Sintered NiTi Woven Structures

Porous NiTi is of interest for bone implants because of its unique combination of biocompatibility (encouraging osseointegration), high strength (to prevent fracture), low stiffness (to reduce stress shielding), and shape memory or superelasticity (to deploy an implant). A promising method for creating NiTi structures with regular open channels is via 3D weaving of NiTi wires. This paper presents a processing method to bond woven NiTi wire structures at contact points between wires to achieve structural integrity: (i) a slurry consisting of a blend of NiTi and Nb powders is deposited on the surface of the NiTi wires after the weaving operation; (ii) the powders are melted to create a eutectic liquid phase which collects at contact points; and (iii) the liquid is solidified and binds the NiTi woven structures. The bonded NiTi wire structures exhibited lower transformation temperatures compared to the as-woven NiTi wires because of Nb diffusion into the NiTi wires. A bonded woven sample was deformed in bending and showed near-complete recovery up to 6% strain and recovered nearly half of the deformation up to 19% strain.

Thermal Parameters Influence Assessed by Electrochemical Impedance Spectroscopy on a Ni50Ti48Nb2 Shape Memory Alloy

This paper presents some electrochemical impedance spectroscopy research results concerning the corrosion resistance of a shape memory Ni50Ti48Nb2 alloy. This one was previously studied by SEM and DSC [1,2] but some new research features had to be made clear in order to be able to explain its electrochemical corrosion behavior. The chemical composition Ni50Ti48Nb2 was chosen in order to obtain a shape memory alloy having a wider hysteresis than equiatomic NiTi, for the purpose of achieving a better thermomechanical stability. Cryogenic applications are aimed. After processing the cast ingot, two samples, S1 and S2, were further annealed at 800�C/12 h and, respectively, at 900�C/12h. Scanning electron micrographs together with the chemical elements mapping results were obtained. They were related to the previous results concerning the informations on the structure of the different phases found in this NiTiNb alloy: austenite, martensite and secondary phases, as well as some primary compounds [1,2]. Considering the size and shape of the complex precipitate particles of NiTiNb in the two differently heat treated samples, these were found responsible for some changes in the transformation temperatures [3] but the electrochemical corrosion behavior of the alloy seems to be influenced to a lesser extent by the heat treatments. Both samples exhibit good values of corrosion resistance, however S2 shows better values than S1. Thus lower transformation temperatures and a slightly better corrosion resistance make the Ni50Ti48Nb2 alloy annealed at 900�C/12h subsequently submitted to thermal cycling to be the right choice for producing couplings in the cryogenic industry.

Highly dispersed Pd on macroporous SmMn2O5 mullite for low temperature oxidation of CO and C3H8.

The catalytic behavior of a palladium catalyst supported on macroporous SmMn2O5 mullite (Pd/SMO-EG&M) for CO and C3H8 oxidation was measured under lean-burn conditions. Different analytical techniques including XRD, Raman, BET, CO chemisorption, SEM, FTEM, XPS, TPD, TPR and CO + O2 pulse were undertaken to evaluate its physical and chemical properties. It was concluded that the crystal structure, morphology and specific surface area (SSA) of SmMn2O5 remained unchanged after Pd addition. The Pd/SMO-EG&M exhibited a low complete transformation temperature for CO (105 °C) and C3H8 (350 °C) oxidation. Such remarkable oxidation activity was attributed to high Pd dispersion (38.4%), which improved the reducibility and mobility of oxygen species, as revealed by TPR and TPD measurements. The high activity of oxygen species for Pd/SMO-EG&M above 250 °C accelerated the oxidation capacity as well. In a word, our study indicates that the macroporous Pd–mullite catalyst has potential applications in exhaust purification for gasoline vehicle.

低変態温度溶接材料を用いた溶融金属積層造形材の変形低減法に関する基礎的検討

Current work deals with investigation of the effect of the use of low transformation temperature welding wire (LTT welding wire) on distortion reduction of parts made by wire and arc additive manufacturing technique (WAAM). LTT welding wire and SUS308L wire, which was austenitic stainless steel wire, were used for making WAAM test pieces. Distortion of base plates of test pieces were evaluated by 3D non-contact coordinate measurement system. As a result, longitudinal bending distortion of the base plates of test pieces in which LTT welding wire was partially or fully used was smaller than those of the test piece which was made only by SUS308L wire. Especially, longitudinal bending distortion of the base plates of test pieces which was made only by LTT welding wire was reduced drastically from those of the test piece which was made only by SUS308L wire. However, angular distortion of the base plates were not reduced drastically even if the test piece was made only by LTT welding wire. The difference of the reduction effect of the use of LTT welding wire was discussed by considering the position of neutral plane of each distortion mode.

Characterisation of quasi‐stationary temperature fields in laser welding by infrared thermography

AbstractIn this work, high‐speed thermography is shown to effectively capture quasi‐stationary temperature fields during the laser welding of steel plates. This capability is demonstrated for two cases, with one involving the addition of a ferritic‐bainitic filler wire, and the other involving the addition of a low‐transformation‐temperature (LTT) filler wire. The same welding parameters are used in each case, but the temperature fields differ, with the spacing between isotherms being greater in the case where the low‐transformation‐temperature filler material is added. This observation is consistent with the differences in the extent of the heat‐affected zone in each sample, and the shape of the weld pool ripples on the weld bead surfaces. The characterization of temperature fields in this way can greatly assist in the development of novel methods for reducing residual stresses, such as the application of low‐transformation‐temperature filler materials through partial‐metallurgical injection (PMI). This technique reduces or eliminates tensile residual stresses by controlling the temperature fields so that phase transformations take place at the optimum times, and success can only be guaranteed through precise knowledge of the temperature fields in the vicinity of the welding heat source in real time.