Metallurgical Phase Transformations Research Articles

Distribution and Characteristics of Residual Stresses in Super Duplex Stainless Steel Pipe Weld

This paper explores the distribution and features of residual stresses formed by super duplex stainless steel pipe welding. Experimental investigations, which encompass an elevated temperature tensile test and metallographic observation along with a hardness test and residual stress measurement, were first conducted to obtain the mechanical properties at high base metal temperatures and to confirm whether or not the duplex stainless steel undergoes martensitic phase development during the welding process. Finally, experiments were performed to scrutinize the residual stress evolution through the metallurgical phase transformation in the weld region and its vicinity. A sequentially coupled 3D thermal, mechanical and metallurgical finite element (FE) model capable of incorporating the experimental consequences was established next. A 3D FE simulation of the girth welding process was conducted, and the axial and hoop residual stress profiles along the girth were evaluated. The results substantiate that martensitic phase evolution occurs in the process of cooling during the welding of super duplex stainless steel, and they also highlight the significance of taking the metallurgical phase transformation into account in the numerical reproduction of the girth welding process for the accurate expression of weld-induced residual stresses, which is especially important for precisely predicting hoop residual stresses.

Alloying strategies for additive manufacturing of Ti6Al4V based alloys, composites and functionally graded materials: Microstructure and phase evolution of intra and inter-layer

The further developments of additive manufacturing (AM) provides a possibility for Ti6Al4V and its functionally graded materials (FGMs), but limits further applications due to its low ductility, low microhardness and poor wear resistance. The key lies in the presence of microstructures such as epitaxial columnar β-grain orientation and heterogeneous phase distribution in AM Ti6Al4V, as well as the interfacial bonding problems (intermetallic phases, delamination, cracking) in AM Ti6Al4V FGMs. In order to address these fundamental intra and inter-layer problems, firstly the microstructure and phase evolution process of AM Ti6Al4V columnar-to-equiaxed transformation (CET) promoted by some alloying strategies was summarized in this paper according to alloying elements and reinforcing particle types. Subsequently, controlled changes in the composition and microstructure of AM Ti6Al4V FGMs were achieved by adding interlayers/introducing gradient interfaces, and the metallurgical reactions and phase transformation mechanisms that inhibit the formation of intermetallic compounds and interfacial defects were explored. Finally, the difficulties and future research directions for alloying strategies to achieve high strength and ductility of AM Ti6Al4V were presented. This review aims to improve the reliability of AM Ti6Al4V and its FGMs for applications in critical load-bearing structures through in-situ alloying and multi-material for the microstructure design of AM Ti6Al4V.

Influence of preheating on residual stresses in ultra-high strength steel welded components

Preheating is generally required in the welding of ultra-high strength steels (UHSSs) for avoiding cold cracking. The present work systematically explores the influence of preheating on welding residual stresses (WRS) in components made of UHSS by means of both experiments and numerical simulations. The results reveal that, no matter what UHSS is with or without metallurgical phase transformation, preheating has a great influence on the magnitude of transverse residual stress (σT-RS). In general, the higher preheating temperature (TPH) the lower magnitude of σT-RS is. Preheating would change the longitudinal residual stress (σL-RS) sign in the weld area and σL-RS amount obviously diminishes with the increase of TPH for transforming UHSS, while that has nearly no influence on the maximum σL-RS but has an impact on the area of tensile σL-RS for non-transforming UHSS. The effect of entire preheating and partial preheating on WRS in UHSS welded components could be nearly the same. Nevertheless, TPH in the partial preheating case (about 300 °C here) should be higher than that in the entire preheating one (corresponding 100 °C here) for that same effect. The weld dimensions, especially the size of the heat-affected zone (HAZ), and cooling time Δt8/5 significantly increase with TPH. Preheating has almost no influence on the restraint degree of welded components.

Low Temperature Welding Test and Numerical Simulation of Metallurgical Phase Transformation of Q460GJC Thick Plate

Low Temperature Welding Test and Numerical Simulation of Metallurgical Phase Transformation of Q460GJC Thick Plate

Methods to reduce residual welding stresses in mining excavator metal structures

Residual stresses in metal structures of mining machinery caused by welding occur due to simultaneous uneven heating and cooling of local sections of structures, variable cooling rates in the different areas of the weld and deformations caused by metallurgical phase transformations. Residual stress in the welded joint can significantly increase the external load that can lead to structural failure. Research on the calculation, measurement and relief of residual stresses under welding is an important issue when predicting the service life of metal structure units of mining machinery and equipment. An accurate quantitative estimation of residual stresses in welded products, repair welds included, as well as the search for the fundamental principles of the elimination methods of residual deformations are of considerable practical interest, which is relevant for the authors of this paper. The study involved a general review of the latest researches in the field of estimation and measurement of residual stresses caused by electrogas welding. It has been proposed to use various techniques and analytical methods for quantitative estimation of welded joint residual stresses on the basis of fracture mechanics, which enabled to take preventative measures at early stages to reduce the cost of repair and maintenance of welded metal structures of mining excavators. The conducted study resulted in formulating the proposals on relief of residual welding stresses in the metal structures of mining excavators.

Phase decomposition of Ni5Pb2Te3 in diffusion bonded PbTe-Ni interfaces – A TEM and thermodynamic study

The study of metallurgical phase transformations is essential for developing reliable interconnects in thermoelectric devices. PbTe-based devices are used in mid-temperature range applications, with Ni being the most preferred contact metal. Several studies have reported the direct joining of PbTe and Ni through rapid hot-pressing routes. However, the study of equilibrium phase transformations in these processes is complex owing to many external variables (applied pressure, electrical current, and simultaneous sintering of PbTe). Furthermore, the long-term phase transformation behavior of Ni5Pb2Te3 and (Ni3±xTe2) β2 phases generated from PbTe/Ni interaction has not been examined. This paper reports the phase decomposition of the high-temperature Ni5Pb2Te3 phase in diffusion bonded PbTe/Ni heterointerfaces and presents a general thermodynamic framework to predict such transformations. The phase decomposition is studied through advanced analytical TEM/STEM techniques. The Ni5Pb2Te3 phase transforms into monoclinic β2 and PbTe phases with a lamellae-like arrangement accompanied by the formation of extrinsic stacking faults in the β2 lamellae. Our work highlights an effective experimental technique (solid-state diffusion bonding) that can be applied to study a metal-contact problem in thermoelectrics and outlines a framework to derive essential thermodynamic data in the absence of any experimental measurements and crystallographic information.

Prediction of welding residual stress and distortion in multi-layer butt-welded 22SiMn2TiB steel with LTT filling metal

The objective of this study is to investigate the influence of low transformation temperature (LTT) filling metal on residual stress and distortion in multi-layer butt-welded 22SiMn2TiB steel. A three-dimensional thermal elastic–plastic finite element model is developed for LTT filling metals, and metallurgical phase transformation is taken into account to predict the martensite fraction of the weld metal. A thermodynamic-based method is employed to calculate martensite start temperature (Ms) and the influence of dilution on Ms is also considered. Volume change due to austenite transformation during heating and martensite transformation during cooling are coupled with the numerical simulation of welding process. The evolution of welding residual stress and welding distortion are both predicted. The simulation results revealed that the distortion of welding plate using LTT filling metal is much smaller than that using austenitic metal. In the weld zone, transverse residual stress is lower using austenitic filling metal, but longitudinal stress is much smaller using LTT filling metal. Longitudinal stress distribution for both LTT and austenitic filling metals is very high in the heat-affected zone, which is close to the yield stress of base metal. The LTT filling metal can not only guarantee the strength of welded joint for armor steel, but also reduce the welding distortion obviously, and the residual stress is also very low at the same time. The predicted temperature profiles and welding distortion are generally in good agreement with the experimental ones.

Comparison of surface integrity, tool wear and chip morphology in CO2 cryogenic and dry milling of 304 stainless steel

In this study, dry and CO2 cryogenic milling conditions on 304 stainless steel is investigated. The cutting speeds of (60, 100, 140 m min−1), feed rates of (0.1, 0.13, 0.16 mm tooth−1) and axial depth of cuts of (0.5, 1, 1.5 mm) are considered in the L9 Taguchi array to design both dry and cryogenic slot milling tests. In cryogenic milling, defects in surface topography and tool wear (flank wear, built up edge and tool chipping) reduced significantly compared to dry machining conditions. Cryogenic milling improved the surface roughness from 4% to 71% compared to the corresponding dry conditions. In dry milling, more serrated chips and more discontinuities are seen in the lateral sides of the chips than cryogenic. In cryogenic compared to dry milling, the surface hardness of the workpiece decreased from at least 8% to a maximum of 12%. Also, the hardness of dry milling surface, increased from 43% to 102% compared to the bulk material. The maximum depth of hardening in cryogenic milling is 23% lower than that of dry condition. Increasing cutting parameters or dry machining will intensify the plastic deformation and compressing the microstructural grains adjacent to the machining surface, as in the deformed surface zone, the grain boundaries are indistinguishable. The maximum reduction in the thickness of the deformed layer was about 16% in the cryogenic milling compared to the corresponding dry condition. Considering XRD diagrams for dry and cryogenic milling compared to non-machined condition, no significant metallurgical phase transformation has occurred in the workpiece material.

溶接過程での状態変化の理解と溶接金属部の特性制御

溶接過程での状態変化の理解と溶接金属部の特性制御

Analysis of the reaction runaway in Al/Ni multilayers with combined nanocalorimetry and time-resolved X-ray diffraction

Self-sustaining runaway reactions in reactive multilayers exhibit heat-up with over 106 K/s to temperatures of higher than 1000 °C, which defines unprecedented kinetic regimes for metallurgical phase transformations. The latter allows for developing alternative concepts for microstructure design. In order to explore the phase transformations in these kinetic regimes, we combine nanocalorimetry with time-resolved synchrotron X-ray diffraction. Nanocalorimetry allows us to perform thermal analysis of ignition as well as the reaction runaway and to develop necessary and mandatory quantitative criterions for ignition. In order to trace the temporal phase evolution, we use time-resolved synchrotron X-ray diffraction. We heat the Al/Ni multilayers with 5000 K/s and find that Ni starts to diffuse into the Al layer at 271 °C. Ignition occurs, dependent on the bilayer thickness, at about 400 °C in the solid state and atomic diffusion is revealed as the dominating mechanism. During the runaway, samples heat up in four stages with maximal 106 K/s to 1100 °C. Ni2Al3 is the first phase to form which starts to nucleate once Al melts. The majority of the intermetallic phase grows after the runaway reaction in the fourth stage and reaches its maximum during cooling. This trend of the temporal phase evolution eventually enables us to propose a mechanism exhibiting conceptual similarities with the exothermic dissolution mechanism recently suggested for self-sustaining reaction fronts in Al/Ni multilayers.

Metallurgical Phase Transformation and Behavior of Steels Under Impact Loading

This study deals with the mechanical behavior of the austenitic stainless steel, AISI 304, under quasi-static and impact loadings. In particular, the evolution of the transformation-induced plasticity (TRIP) is investigated. Several compression tests have been performed at various strain rates under both static and impact conditions (crash tests). In order to show the influence of the strain rate on the phase transformation, the microstructures of as-received and deformed specimens have been observed with a scanning electron microscope. Then, the newly formed ferromagnetic martensitic phase is analyzed with the help of the vibrating sample magnetic (VSM) method and by X-ray diffraction (XRD). The results obtained from the mechanical testing indicate that the TRIP effect is actually occurring since a significant change of the curve slope is visible during the hardening regime in the stress–strain curve. The VSM and XRD techniques, on the other hand, show that the proportion of the martensitic phase is reduced when the strain rate is increased. The quantity of martensite formed is greater when quasi-static loadings are applied to the material.

A metallurgical phase transformation framework applied to SLM additive manufacturing processes

While significant progress has been made in the last few years, the reliability of Additively Manufactured (AM) parts is often less than desirable as they suffer from manufacturing defects and hence subpar strength and fatigue life. To address this challenge, numerical methods are sought to provide insight into the process and help accelerate progress in raising the quality of AM parts. In metal AM applications, assessing the amount of unfused powder, melt pool volumes, and metallurgical phase transformations is often of interest. In this work, we introduce a generic framework for assessing metallurgical phase transformations, building on a previously-developed general simulation framework for predicting temperature evolution, distortions, and residual stresses. Experimental work was conducted to validate numerical predictions and included temperature measurements, EBSD/XRD microstructural examinations. Additional test data from the published literature was used to validate melt pool sizes and unfused powder predictions. We conclude that the continuum-level internal state variable approach proposed here can be calibrated with a reasonable amount of effort and can be used directly to link processing conditions to microstructural features. Upcoming work investigates the influence of microstructure features on mechanical performance to address the process-structure-property-performance relationship in SLM additive manufacturing.

Influence of heat input and radius to pipe thickness ratio on the residual stresses in circumferential arc welded pipes of API X46 steels

The present work aims to study residual stresses caused by circumferentially welding of two similar API X46 steel pipes by means of finite element modeling. Considering the metallurgical phase transformations and through thermal-mechanical uncoupled analysis, the 3D modeling was carried out by SYSWELD software. Materialistic thermal and mechanical properties of all phases were defined in terms of temperature as well as phase transformation properties. Residual stress was measured through hole-drilling method. The obtained results were used to verify the finite element model. By means of full factorial experiment designing method, effects of heat input and radius to pipe thickness ratio on maximum values of hoop and axial residual stresses were investigated. The effect of each factor was studied in 3 levels and by 9 experiments. Results of statistical analysis revealed that increase in heat input and radius-thickness ratio would lead to higher values of maximum hoop and axial residual stresses. However, interactions of high level of heat input and a low level of radius-thickness ratio increased inter-pass temperature and consequently caused a sudden raise in maximum values of residual stresses.

Metallurgical Properties and Phase Transformations of Barium-Strontium Modifier

Metallurgical properties and phase transformations of barium-strontium modifier were tested in laboratory conditions resembling steel processing in furnace and ladle. When heating barium-strontium modifier start of melting, kinetics of decomposition, phase and structure transformation were studied. The concentrate under consideration has been revealed to be a complex mineral compound containing barytocalcite, calcite, calciostrontianite, dolomite and siderite. The reaction kinetics of decomposing mineral components of barium-strontium modifier to oxides does not considerably affect slag formation in conditions of out-of-furnace steel processing.

On a New Kinetic Modelling Approach of the Irreversible Quasi-Surface Metallurgical Phase Transformations

Irreversible quasi-surface metallurgical phase transformations are the specific response of some metallic materials—such as metals and alloys—subjected to high thermomechanical loads applied very near their surface during the manufacturing processes or after being put into operation. These solid/solid phase transformations can be observed, for example, on the tread of many rails in railroad networks frequented by freight trains. The severe thermal and mechanical loads imposed on the surface of the rails and in the immediate vicinity of the surface by the wheel/rail contact often result in highly localized irreversible metallurgical transformations. A new kinetic model based on a previous study is presented here, which accounts more realistically for the nucleation and growth of these irreversible solid/solid phase transformations resulting from high thermomechanical loads. This metallurgical behavioral model was developed in the framework of continuum thermodynamics with gradients of temperature and internal variables.

Influence of the Impact Angle and the Gravity Direction on Heat Transfer during the Cooling of a Cylinder by a Free Planar Subcooled Impinging Jet

Cooling from impinging jet is nearly compulsory in steel industry processing especially in Run Out Table processing and steel tubes production because of the high heat transfer rates provided by the boiling of the subcooled water jet. As far as metallurgical phase transformations, residual stresses and deformations in the workpiece are concerned, the temperature drop during cooling must be perfectly controlled thanks to a fully understanding of the heat transfer mechanisms. In a previous study [1] the effect of surface to jet velocity ratio on heat transfer has been characterized and it has been shown that this parameter has a significant influence on shoulder of flux collapse. In order to understand the effect of more industrial quench process parameters, an innovative experimental quenching device has been designed and built. It allowed us to make heat transfer measurements at the surface of a hot nickel cylinder impinged by a subcooled water jet, according to several angles of impact and three jet directions against gravity. The results clearly highlight an effect of these two parameters on the heat transfer mechanisms at the surface of the tube. These results allow a better understanding of the origins of temperature heterogeneities inside the tube during the quench.

Nd:YAG Laser Welding of Sheet Metal Assembly: Transformation Induced Volume Strain Affect on Elastoplastic Model

This study presents the effect of transformation induced volume strain on the thermo-elastoplastic model in the simulation of Nd: YAG laser welding process applied for thin sheet metal dual phases steel DP600. The metallurgical phase transformations during heating and during cooling are used as the thermal expansion dependent parameters. The effect of transformation induced volumetric strain was identified where the comparisons of the elastoplastic model with and without metallurgical effect are based on the global distribution of residual stresses such as the longitudinal residual stresses and the transverse residual stresses.

Cooling of a Rotating Cylinder by a Subcooled Planar Jet - Influence of the Surface Velocity on Boiling Regime

Cooling from impinging jet is nearly compulsory in steel industry processing especially in Run Out Table processing and steel tube production because of the high heat transfer provided by the boiling of the subcooled water jet. As far as metallurgical phase transformations, residual stresses and deformations in the workpiece are concerned, the temperature drop during cooling must be controlled thanks to a full understanding of the heat transfer mechanisms. One of the main characteristic using jet impingement is that the transition boiling regime may exist for very high superheat and thus the Leidenfrost temperature is higher than in pool boiling; consequently, boiling curves generally have a particular shape in the transition boiling regime which is usually called “shoulder of flux”. In this study, an innovative experimental quenching device has been used for analyzing the effect of the wall velocity of the surface to be cooled on the boiling curves (i.e. heat transfer) and we especially point out that the “shoulder of flux” (i.e. transition boiling regime) is strongly dependent on the surface to jet velocity ratio (r*). We found that a very small increase of the wall velocity has a high influence on shoulder of flux collapse.

Failure of truck brake discs: A coupled numerical–experimental approach to identifying critical thermomechanical loadings

Premature failure is the major problem encountered in the operation of truck brake discs. The present study was carried out with the aim of developing brake discs with increased lifespans, so as to improve the safety and reliability of these components. To this end, two aspects, namely design and material, were investigated. In order to understand the different damage modes, we studied the behaviour of a grey cast iron brake disc combined with two semi-metallic pads under extreme conditions. A coupled numerical–experimental approach enabled us to determine the critical thermomechanical loadings associated with braking-induced metallurgical phase transformations.

A perspective on residual stresses in welding

Welding induced residual stress occurs due to non-uniform simultaneous heating and cooling, local variation in shrinkage because of variable cooling rates in different regions of the weld, and strains associated with metallurgical phase transformations. The residual stress in a welded joint can augment the externally applied load and cause structural failure. Prediction and mitigation of residual stresses still remain important issues in welding. The purpose of this special issue of Science and Technology of Welding and Joining is to present recent research on calculation, measurement and alleviation of residual stresses in welding.