Low-temperature Transformation Research Articles

Study on the thermal storage properties of Al-Si-Cu-Mg alloy

Based on thermodynamic calculation technology, the thermodynamic data of six alloys (inc. Al-Si and Al-Si-Cu-Mg systems) was calculated. The microstructure, phase transformation temperature and latent heat of the Al-4%Cu-12%Mg-7%Si alloy and Al-13%Si alloy were also verified by X-ray diffraction(XRD), scanning electron microscopy/Energy-dispersive system (SEM/EDS) and Differential scanning calorimetry (DSC). The results show that The enthalpy change value of Al-Si-Cu-Mg alloy is larger than that of Al-Si alloy from 800 °C to 400 °C, while its phase transformation temperature is lower. In particular, the enthalpy value of Al-4%Cu-12%Mg-7%Si alloy is 85% higher than that of Al-13%Si near eutectic alloy, and its initial temperature of phase transformation is about 74 °C lower. The former has relatively low phase transformation temperature and over-dimensioned latent heat of phase transformation, displaying the excellent thermal storage property. Therefore, the alloy is a good potential solar energy thermal storage material. The results in the paper also indicated that thermodynamic calculation is of value in seeking new potential solar energy thermal storage materials for solar thermal power generation system.

Elongated bead weld method for improvement of fatigue properties in welded joints of ship hull structures using low transformation temperature welding materials

To improve the fatigue properties in the welded joints of high strength steels, we developed an elongated bead method for low transformation temperature (LTT) welding materials. The proposed method intensifies the compressive residual stress and reduces the stress concentration. We investigated the length of the elongated bead in the boxing fillet portion of the gusset front, the effectiveness of inexpensive LTT wires with low Ni content and the potential applicability to repair welding. The most effective welding method, overlay of the elongated bead LTT weld metal on the conventional boxing fillet weld metal, extended the fatigue lifetime of the boxing fillet by seven to ten times. Residual stress distributions in joints welded with low-Ni metals of different martensite start temperatures (Ms) as well as the stress concentration in weld joints formed from elongated beads with different lengths were investigated in finite element simulations. Residual stress of weld and its toe was decreasing as the Ms of weld metal was lowering and the most compressive residual stress was obtained for weld metal with Ms temperature of 184 °C. The stress concentration reduces by about 30% of fatigue load, and the residual stress decreases down to compressive one of −685 MPa for the LTT overlay welding elongated bead treatment.

Phase formation sequence, magnetic and structural development during solid-state reactions in 72Pt/28fcc-Co (001) thin films

Abstract The phase formation sequence during the thermally induced solid-state reaction between polycrystalline Pt and epitaxial fcc-Co (001) films in the Pt/fcc-Co(001) bilayers are systematically examined using X-ray diffraction and magnetic measurements. The films have nominal atomic ratio Co:Pt = 28:72 and total thickness 300–400 nm. Annealing to the temperature of 375 °C does not change the structural and magnetic properties of the films; this is indicative of the absence of considerable mixing at the Co/Pt interface. With the subsequent increase of the annealing temperature, the phase formation in the Pt/fcc-Co(001) bilayers has been found to have two temperature (375 °C–575 °C and 575 °C–825 °C) intervals. Solid-state reaction between Pt and Co starts above 375 °C, and nanoclusters containing the ordered L10 phase epitaxially intergrow with the disordered A1 phase of the composition CoPt3 form and exist in the first temperature interval. The distinctive feature of the first interval is the formation of in-plane rotatable magnetic anisotropy. In the second temperature interval, the (L10 + A1) two-phase mixture grows into the ordered L12-CoPt3 phase leading to the disappearance of rotatable anisotropy. Possible origin of the rotatable magnetic anisotropy is discussed. The first magnetocrystalline anisotropy constant of L12-CoPt3 has the maximum value of −5.0·105egr/cm3 and order parameter 0.55 at 675 °C. A careful analysis of thin film solid-state reactions implies the existence of low-temperature transformation (∼375 °C) on the Pt-rich side of the Co-Pt system.

Low-temperature transformation to bainite in a medium-carbon steel

Abstract The consequences of reducing the carbon concentration from 0.8 to 0.6 wt.% in a bulk nanostructured bainitic-steel have been investigated with the aim of enabling processing. We observed that the average thickness of the product phase plates increased from below 20–40 nm to about 100 nm after isothermal transformation at the same temperature. The increase in plate size led to decrease in hardness as expected. However the hardness reduction was not as large as might be expected, a transition to lower bainite at temperatures below 250°C may have provided additional strengthening. Transformation just below the martensite start temperature resulted in a mixture of martensite and bainite, with no observed increase in the rate of bainite transformation.

Confirmation of tensile residual stress reduction in electron beam welding using low transformation temperature materials (LTT) as localized metallurgical injection – Part 1: Metallographic analysis

Abstract For the reduction of the distortion and the residual tensile stresses in welded seams on carbon manganese steels, low transformation temperature materials (LTT) were developed. These materials use the volume expansion effect during martensitic transformation. The volume expansion counteracts volume shrinkage during cooling. The positive effects of the low transformation temperature alloys on the residual tensile stresses were demonstrated in various investigations. The low transformation temperature materials were, so far, used as filler material in arc welding processes in large volumes. The use of modular thermal fields of the electron beam welding processes offers the potential of a temporally activated use of compressive stress induction by phase transformation of the low transformation temperature alloys. The aim is to exert influence in situ on the welding residual stress state and thus on the distortion of complex parts. The metallographic analysis of an electron beam welded seam in unalloyed steel with low transformation temperature filler material is demonstrated. The evaluation is made via electron backscatter diffraction (EBSD) and is shown in the first part. The second part will show the effect on near surface residual stresses examined by hole-drilling method in combination with an optical evaluation by electronic speckle pattern interferometry.

Thermodynamic properties of Dy2O3 · 2ZrO2 and Ho2O3 · 2ZrO2 in the range 10–340 K

The low-temperature heat capacity of Dy2O3 · 2ZrO2 and Ho2O3 · 2ZrO2 has been determined by adiabatic calorimetry in the temperature range 10–340 K. The results have been used to calculate the entropy, enthalpy increment, and reduced Gibbs energy of the zirconates without taking into account their low-temperature magnetic transformations.

Fatigue strength improvement at attachement side boxer weld by low temperature transformation welding material

Fatigue strength improvement at attachement side boxer weld by low temperature transformation welding material

Thermal stability and latent heat of Nb–rich martensitic Ti-Nb alloys

Ti-Nb-based alloys are candidate materials for load-bearing and for functional biomedical implant components. For these applications alloy compositions with relatively low martensitic transformation (MT) temperatures are of highest interest. In this work we examine the thermal stability and the temperature-induced β ↔ α″ MT of Nb-rich martensitic and partially martensitic Ti-Nb alloys exhibiting martensite start temperatures Ms < 300°C. The first part of this article investigates the phase transformations and precipitation reactions occurring during isochronal cycling in dependence of the Nb content using differential scanning calorimetry and dilatometry in combination with X-ray diffraction. The second part studies the temperatures, thermal hysteresis and transformation interval of the β ↔ α″ MT. The latent heat, elastic and irreversible energy contributions of the thermoelastic energy balance are quantified in dependence of Nb content in the remaining parts. All energy contributions decrease with increasing Nb content and the latent heat becomes very small (< 5 J/g) for the Nb-richest martensitic compositions. Above a critical Nb concentration the β → α″ MT is incomplete.

Applicability of low transformation temperature welding consumables to increase fatigue strength of welded high strength steels

Application of Low Transformation Temperature (LTT) consumables in welding is a recent approach to increase the fatigue strength of welds. In this paper high strength steels with yield strengths ranging from 650 to 1021MPa were fillet and butt welded using different LTT and conventional consumables. The effects of weld metal chemical composition on phase transformation temperatures, residual stresses and fatigue strength were investigated. Lower transformation start temperatures and hence lower tensile or even compressive residual stresses were obtained close to the weld toe for LTT welds. Fatigue testing showed very good results for all combinations of LTT consumables and high strength steels with varying strength levels. For butt welds, the characteristic fatigue strength (FAT) of LTT welds at 2million cycles was up to 46% higher when compared to corresponding welds made with conventional filler materials. In fillet welds, a minimum FAT improvement of 34% and a maximum improvement of 132% was achieved when using LTT wires. It is concluded that different LTT consumables can successfully be employed to increase fatigue strength of welds in high strength steels with yield strength up to 1021MPa. Weld metals with martensite transformation start temperatures close to 200°C result in the highest fatigue strengths.

Effects of coiling temperature and pipe-forming strain on yield strength variation after ERW pipe forming of API X70 and X80 linepipe steels

Since pipes are undergone repeated tension and compression strains during pipe-forming, and flattening, flattened sheets often show too higher or lower yield strength than hot-rolled coils, which poses to difficulties in satisfying yield strength standards. In this study, effects of microstructure and pipe-forming strain (thickness/diameter (t/D)) on yield strength variation were investigated in X70 (483MPa) and X80 (552MPa) linepipe steels fabricated by controlling Mo content and coiling temperature, and their yield strength, strain hardening exponent, and Bauschinger stress parameter were measured by tension-compression tests with varying tensile-pre-strain. In the X80 steels whose Mo content was higher than that of the X70 steels, the higher Mo content promoted the formation of low-temperature transformed microstructures such as acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent, which played a role in decreasing Bauschinger effect. The reduction in yield strength was smaller in the X80 steel than in the X70 steel. As the coiling temperature decreased, the volume fractions of AF, BF, and pearlite increased, while those of QPF, GB, and MA decreased, and led to the increase in yield strength by about 30MPa. The yield strength slightly increased after the pipe forming at higher coiling temperature, while it was largely reduced at lower coiling temperature. When the steels having different t/D were compared, the yield strength after the pipe forming increased largely by 65MPa under the higher t/D as the strain hardening effect overrode the Bauschinger effect. In order to prevent or minimize the large reduction in yield strength after the pipe forming, low-temperature transformation microstructures, coarse grain size, and high t/D were desirable.

Simulations in multipass welds using low transformation temperature filler material

Transient thermal and residual stress fields in flux-cored arc welds were examined using a finite element (FE) model. Experimental multipass welds were produced using both conventional and low transformation temperature (LTT) filler metals. Temperature-dependent material properties and both convective and radiant heat loss boundary condition have been considered in the FE model. The effects of the transformation temperature and interpass intervals on residual stresses were examined. It was found that compressive longitudinal residual stresses were developed at the weld centreline in the LTT filler metal. A short-time interpass interval causes the weld fusion zone to be above the martensite start temperature allowing the optimal use of the phase transformation effect. The FE model is sensitive to alteration in welding parameters and can satisfactorily predict the residual stress distribution in welded parts.

Formation of abnormal structures and their effects on the ductility of eutectoid steel

The formation of abnormal structures and their effects on reduction of area (RA) were investigated in eutectoid steels transformed at different temperatures ranging from 560 °C-650 °C. The occurrence of abnormal structures, such as upper bainite, degenerate pearlite, free ferrite, and grain boundary cementite, was confirmed. The volume fraction of upper bainite and degenerate pearlite decreased on increasing the transformation temperature, while the amount of free ferrite increased. As the transformation temperature increased, RA increased, reached a maximum, and then decreased, while the tensile strength continuously decreased. The crack formations during the tensile test could be classified into three types: tearing, shear cracking, and void formation/ coalescence. The decrease of the ductility at low transformation temperatures was attributed to the increased amount of upper bainite and degenerate pearlite, since the formation of cracks occurred by tearing interfaces or by void formation at abnormal structures during the tensile test. Meanwhile, the decrease in RA at high transformation temperatures was attributed to the occurrence of shear cracking rather than the presence of abnormal structures.

Low-Temperature Transformation of Methane to Methanol on Pd1 O4 Single Sites Anchored on the Internal Surface of Microporous Silicate.

Direct conversion of methane to chemical feedstocks such as methanol under mild conditions is a challenging but ideal solution for utilization of methane. Pd1 O4 single-sites anchored on the internal surface of micropores of a microporous silicate exhibit high selectivity and activity in transforming CH4 to CH3 OH at 50-95 °C in aqueous phase through partial oxidation of CH4 with H2 O2 . The selectivity for methanol production remains at 86.4 %, while the activity for methanol production at 95 °C is about 2.78 molecules per Pd1 O4 site per second when 2.0 wt % CuO is used as a co-catalyst with the Pd1 O4 @ZSM-5. Thermodynamic calculations suggest that the reaction toward methanol production is highly favorable compared to formation of a byproduct, methyl peroxide.

Structural and martensitic transformation of MnNiSn shape memory alloys

Ferromagnetic shape memory alloys are characterized by both the structural austenite to martensite transformation and also by the magnetic transition from ferromagnetic to paramagnetic. The set of properties makes them candidates for use in several applications such as sensors, actuators, or magnetic refrigeration systems. Among the Heusler-type alloys that exhibit this behavior, the most studied system is the Ni–Mn–Ga. However, to overcome the high cost of Gallium and the generally low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing Sn. The martensitic transformation and the solidification structures of Mn50Ni50−xSnx (x = 7, 8.7 and 10.5) ribbons prepared by melt-spinning were investigated by means of scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. While the As-spun alloys Mn50Ni43Sn7 and Mn50Ni41.3Sn8.7 displayed a single-phase (14-M monoclinic martensite) structure at room temperature, the As-spun and Mn50Ni39.5Sn10.5 displayed a single-phase cubic Heusler L21. The martensitic transformation temperatures were noted to decrease with the increase of Sn concentration.

In-situ load analysis in multi-run welding using LTT filler materials

Modifying the level of mostly detrimental welding residual stresses already during the welding process would be highly attractive as time- and cost-consuming post processing may be prevented. The nature of stress buildup during welding-associated cooling is highly affected by phase transformations. Up to now, it is not clear in which way this is applicable to real component welding exhibiting high shrinkage restraint and complex heat input. In this study, two different low transformation temperature (LTT) alloys have been investigated concerning the stress development in restrained multi-run butt welding in order to evaluate the potential of stress reduction. Pulsed gas metal arc welding (P-GMAW) welding was executed on a testing facility designed to simulate real lifelike restraint conditions of component weldments. The effect of reduced MS-temperatures and the heat control on the globally acting stresses was monitored by in-situ measurement of the reaction forces during welding fabrication. Additional local residual stress measurements allowed analyzing global as well as local loading of the welded construction. Although phase transformation has a significant influence on unloading the joint during each weld pass, the reaction stress upon cooling to room temperature seems to be determined mainly by the heat input. On the surface, low longitudinal residual stresses were observed in case of LTT whereas transverse residual stresses are less affected.

The Impact of Retained Austenite Characteristics on the Two-Body Abrasive Wear Behavior of Ultrahigh Strength Bainitic Steels

In the current study, a high-carbon, high-alloy steel (0.79 pct C, 1.5 pct Si, 1.98 pct Mn, 0.98 pct Cr, 0.24 pct Mo, 1.06 pct Al, and 1.58 pct Co in wt pct) was subjected to an isothermal bainitic transformation at a temperature range of 473 K to 623 K (200 °C to 350 °C), resulting in different fully bainitic microstructures consisting of bainitic ferrite and retained austenite. With a decrease in the transformation temperature, the microstructure was significantly refined from ~300 nm at 623 K (350 °C) to less than 60 nm at 473 K (200 °C), forming nanostructured bainitic microstructure. In addition, the morphology of retained austenite was progressively altered from film + blocky to an exclusive film morphology with a decrease in the temperature. This resulted in an enhanced wear resistance in nanobainitic microstructures formed at low transformation temperature, e.g., 473 K (200 °C). Meanwhile, it gradually deteriorated with an increase in the phase transformation temperature. This was mostly attributed to the retained austenite characteristics (i.e., thin film vs blocky), which significantly altered their mechanical stability. The presence of blocky retained austenite at high transformation temperature, e.g., 623 K (350 °C) resulted in an early onset of TRIPing phenomenon during abrasion. This led to the formation of coarse martensite with irregular morphology, which is more vulnerable to crack initiation and propagation than that of martensite formed from the thin film austenite, e.g., 473 K (200 °C). This resulted in a pronounced material loss for the fully bainitic microstructures transformed at high temperature, e.g., 623 K (350 °C), leading to distinct sub-surface layer and friction coefficient curve characteristics. A comparison of the abrasive behavior of the fully bainitic microstructure formed at 623 K (350 °C) and fully pearlitic microstructure demonstrated a detrimental effect of blocky retained austenite with low mechanical stability on the two-body abrasion.

Study on residual stress in socket weld by numerical simulation and experiment

In this paper, welding residual stress in socket weld of 304L stainless steel pipe was investigated using numerical simulation and validated by X-ray stress measurement. From the simulation results, the maximum tensile residual stresses were located at weld root and weld toe on both sides of the weld along pipe, which led to the fatigue failure. Pre-bevelling and low transformation temperature (LTT) dressing could decrease tensile residual stress both in hoop and axial direction at weld root and weld toe. After LTT dressing, compressive residual stress was generated throughout weld toe. Compressive stress can delay fatigue crack initiation and propagation. Therefore, pre-bevelling and LTT dressing can improve the fatigue life of socket weld.

Evolution of bainitic microstructure in vanadium-bearing microalloyed steel with two-step cooling

In the present work, a medium carbon vanadium-bearing microalloyed steel with low-alloy contents was subjected to air cooling followed by quenching (two-step cooling) which resulted in multiphase structures that consist of ferrite, bainite, martensite and austenite. The characterisation of the samples obtained by various two-step cooling methods was done using a SEM, a X-ray diffractometer and a magnetometer. Volume fractions of retained austenite (RA) and its carbon contents were determined and tensile properties were also evaluated. The RA content along with its carbon content has been found to increase to a maximum value and quenching at that point improves the ductility. Also, the lower transformation temperature of bainite enhances the toughness.

Transformation-rate maxima during lath martensite formation: plastic vs. elastic shape strain accommodation

Recently, a modulated formation behaviour of lath martensite in Fe–Ni(-based) alloys was observed, exhibiting a series of transformation-rate maxima. This peculiar transformation behaviour was explained on the basis of the hierarchical microstructure of lath martensite, minimising the net shape strain associated with martensite formation, by a block-by-block formation of martensite packages occurring simultaneously in all packages. In the present work, the martensitic transformation upon slow cooling of two Fe–Ni alloys, containing 22 and 25 at.% of Ni, respectively, was investigated by high-resolution dilatometry with the aim of identifying the influence of alloy composition on the modulated transformation behaviour. The differences observed for the two alloys, a more rapid sequence of the transformation-rate maxima and a narrower temperature range in case of Fe-25 at.% Ni, can be explained consistently as a consequence of the lower transformation temperatures in Fe-25 at.% Ni, highlighting the role of temporary accommodation of the shape strain during formation of the lath martensite microstructure: the depression of the transformation toward lower temperatures leads to a higher strength of the austenite, hence resulting in a more elastic (less plastic) temporary accommodation of the shape strain upon block formation and thereby in a more effective mutual compensation of the shape strain by neighbouring blocks. A kinetic model on the basis of energy-change considerations is presented which is able to describe the observed modulated transformation behaviour.

Interface Induced Growth and Transformation of Polymer-Conjugated Proto-Crystalline Phases in Aluminosilicate Hybrids: A Multiple-Quantum (23)Na-(23)Na MAS NMR Correlation Spectroscopy Study.

Nanostructured materials typically offer enhanced physicochemical properties because of their large interfacial area. In this contribution, we present a comprehensive structural characterization of aluminosilicate hybrids with polymer-conjugated nanosized zeolites specifically grown at the organic-inorganic interface. The inorganic amorphous Al-O-Si framework is formed by alkali-activated low-temperature transformation of metakaoline, whereas simultaneous copolymerization of organic comonomers creates a secondary epoxide network covalently bound to the aluminosilicate matrix. This secondary epoxide phase not only enhances the mechanical integrity of the resulting hybrids but also introduces additional binding sites accessible for compensating negative charge on the aluminosilicate framework. This way, the polymer network initiates growth and subsequent transformation of protocrystalline short-range ordered zeolite domains that are located at the organic-inorganic interface. By applying an experimental approach based on 2D (23)Na-(23)Na double-quantum (DQ) MAS NMR spectroscopy, we discovered multiple sodium binding sites in these protocrystalline domains, in which immobilized Na(+) ions form pairs or small clusters. It is further demonstrated that these sites, the local geometry of which allows for the pairing of sodium ions, are preferentially occupied by Pb(2+) ions during the ion exchange. The proposed synthesis protocol thus allows for the preparation of a novel type of geopolymer hybrids with polymer-conjugated zeolite phases suitable for capturing and storage of metal cations. The demonstrated (23)Na-(23)Na DQ MAS NMR combined with DFT calculations represents a suitable approach for understanding the role of Na(+) ions in aluminositicate solids and related inorganic-organic hybrids, particularly their specific arrangement and clustering at interfacial areas.