Metallurgical Concepts Research Articles

Steel grades 91 and 92 microstructure and precipitate evolution atlas and life assessment tool

ABSTRACT P91 and P92 steels are widely used in power plants and petrochemical industry for long-term service components. Due to high resistance to creep, fatigue and corrosion, the use of grade 91 and grade 92 steels allows ultrasupercritical conditions (600 °C, 300 bar). To achieve this goal the alloy design was based on the following metallurgical concepts: stable tempered martensitic microstructure, precipitation strengthening through M23C6 (Cr, Fe, Mo or W carbides) and fine MX (V, Nb carbides), solution hardening through elements as Mo or Mo/W and high Cr content. This study is focused on the microstructure evolution of grade 91 and grade 92 steels under aging and creep conditions. Three sets of laboratory-aged specimens heated in oven at 550°C, 600°C and 650°C were examined. Furthermore, the influence of stress on the microstructure was evaluated. The microstructures were characterized by several means of investigations and the results were compared to literature.

A precipitation strengthened high entropy alloy with high (Al+Ti) content for laser powder bed fusion: Synergizing in trinsic hot cracking resistance and ultrahigh strength

Additive manufacturing (AM) is a highly promising technique for producing near-net-shape high-value aerospace components with intricate geometries. However, due to the susceptibility to hot cracking, high-volume-fraction Ni3(Al,Ti)-type precipitation-strengthened Ni-base superalloys typically used for these applications are usually difficult or even infeasible to manufacture via AM. Previous metallurgical methods to mitigate hot-cracking in AM usually compromise strength, resulting in a trade-off between hot-cracking resistance and strength. Here, we overcome this trade-off in AM of a multicomponent Ni-rich precipitation-strengthened high-entropy alloy (MNiHEA) Ni46.23Co23Cr10Fe5Al8.5Ti4W2Mo1C0.15B0.1Zr0.02 (at%): synergizing remarkable hot-cracking resistance and ultrahigh strength. Crack-free MNiHEA is successfully fabricated via laser powder bed fusion (LPBF) without preheating, despite its high (Al+Ti) content of ∼7.4 wt%. This remarkable resistance to hot cracking arises from the comparatively low critical solidification range, the small average solidification cracking index, the suppression of the intermetallic phases in solidification, and the moderate hardening rate induced by the nanoprecipitates in aging, which are intrinsically imparted by thermodynamic and mechanical characteristics. The yield strength of the as-built-to-aged MNiHEA reaches ∼1.2 gigapascal, together with acceptable ductility. This ultrahigh strength outperforms its cast-to-aged counterpart by more than one-third and existing commercial superalloys CM247 and IN738LC. Such pronounced enhancement in strength is attained through multiple strengthening mechanisms, including solid-solution hardening, dislocation hardening, precipitation-hardening and grain-boundary strengthening. Our results demonstrate that intrinsic hot-cracking resistance can be utilized as a new metallurgical concept for mitigating hot-cracking without compromising strength in AM of high-temperature materials such as HEAs and superalloys.

Ablation technology applied to A356 alloys compared with conventional casting processes

Because ablation differs from conventional casting, some conventional metallurgical concepts, such as the relationship among secondary dendrite arm spacing (SDAS), microconstituents and tensile properties, should be revised.To understand the mechanisms governing ablation technology, the most popular of casting alloys, A356 alloys, were selected, and conventional melt treatments, such as degassing, grain refinement and Si modification with Sr, were applied. All ablated as-cast microstructural characteristics (SDAS, grain size, porosity, Si particle morphology and Fe-bearing phases) and the mechanical properties after T6 were compared with two conventional processes: permanent moulding and sand cast manufacturing with the same metal. SDAS values, which are commonly used to predict mechanical properties in conventional casting processes, are no longer valid for ablation, which presents coarse SDAS microstructures, similar to those obtained with the conventional sand casting process, with some isolated areas of fine SDASs. However, the high freezing rate in the last stage of the solidification process obtained by ablation technology generates fine eutectic Si and iron-bearing particles and low porosity. This makes it possible to obtain quality index values of Q = 480 MPa that are significantly higher than those of sand casting (Q = 426 MPa) and similar to those obtained for permanent moulds (Q = 480 MPa).Additionally, the potential advantages of ablation for T5 (Q = 419 MPa) and shorter solution time T6 treatments (Q = 479 MPa) were also investigated in terms of mechanical properties and Si particle size and shape.

Design of Pipe Steel Hot Rolling Technological Process Based on System Analysis

Pipe steel sheet is manufactured by hot rolling technological process. Technological regimes of every technological operation can vary in a wide range affecting pipe steel sheet properties. It is shown that system analysis provides the effective way for searching out the basics for mathematical modeling of multi-variant technological processes. The detailed scheme of steel sheet hot rolling process is presented, determining its input and output parameters. Flows of material, energy, and information are presented for each technological operation. Metallurgical concept of pipe steel manufacturing is shown as the basics for competitive product manufacturing. It is proposed to analyze the hot rolling process as a set of target functions, which will make it possible to achieve the pipe steel sheet with the desired level of mechanical properties. The proposed approach based on system analysis allows to find tendencies for further development of hot rolling.

Mechanical Properties and Impact Toughness of Molybdenum Alloyed Ductile Iron

Grades of high silicon ductile iron offer excellent combinations of static strength and ductility as well as good machinability due to their fully ferritic, solution strengthened matrix. As a result of elevated silicon contents, however, the ductile-to-brittle transition temperature in the Charpy impact test is significantly increased. Thus, minimum required Charpy impact energies cannot be met for many applications by using high silicon ductile iron. Therefore, alloys with lower strength and higher toughness properties are commonly used for many technical applications. The enormous lightweight construction potential of high silicon ductile iron can therefore not be fully exploited. The present investigation pursues the metallurgical approach of partially substituting silicon with molybdenum as an alternative strengthening element in order to improve the toughness properties while maintaining similar static mechanical properties. Molybdenum serves as a carbide-stabilising element in ductile iron, while simultaneously promoting ferrite formation and is therefore regarded to be suitable alloying element. In Charpy impact tests, the ductile-to-brittle transition temperature could be reduced by about 55 °C by reducing the silicon content to 2.95 wt% and adding 0.21 wt% molybdenum compared to a high silicon alloy. Additionally, it was possible to mathematically describe the transition behaviour of the studied alloys using nonlinear regression functions and to achieve a sufficient correlation of empirically determined and calculated data. This present metallurgical concept offers a promising metallurgical tool for further improving the toughness properties of alloyed ductile iron.

Deformation mode and energy absorption of polycrystal-inspired square-cell lattice structures

Lattice structures are widely used in many engineering fields due to their excellent mechanical properties such as high specific strength and high specific energy absorption (SEA) capacity. In this paper, square-cell lattice structures with different lattice orientations are investigated in terms of the deformation modes and the energy absorption (EA) performance. Finite element (FE) simulations of in-plane compression are carried out, and the theoretical models from the energy balance principle are deve-loped for calculating the EA of these lattice structures. Satisfactory agreement is achieved between the FE simulation results and the theoretical results. It indicates that the 30° oriented lattice has the largest EA capacity. Furthermore, inspired by the polycrystal microstructure of metals, novel structures of bi-crystal lattices and quad-crystal lattices are developed through combining multiple singly oriented lattices together. The results of FE simulations of compression indicate that the EA performances of symmetric lattice bi-crystals and quad-crystals are better than those of the identical lattice polycrystal counterparts. This work confirms the feasibility of designing superior energy absorbers with architected meso-structures from the inspiration of metallurgical concepts and microstructures.

Circumventing huge volume strain in alloy anodes of lithium batteries

Since the launch of lithium-ion batteries, elements (such as silicon, tin, or aluminum) that can be alloyed with lithium have been expected as anode materials, owing to larger capacity. However, their successful application has not been accomplished because of drastic structural degradation caused by cyclic large volume change during battery reactions. To prolong lifetime of alloy anodes, we must circumvent the huge volume strain accompanied by insertion/extraction of lithium. Here we report that by using aluminum-foil anodes, the volume expansion during lithiation can be confined to the normal direction to the foil and, consequently, the electrode cyclability can be markedly enhanced. Such a unidirectional volume-strain circumvention requires an appropriate hardness of the matrix and a certain tolerance to off-stoichiometry of the resulting intermetallic compound, which drive interdiffusion of matrix component and lithium along the normal-plane direction. This metallurgical concept would invoke a paradigm shift to future alloy-anode battery technologies.

Metallurgical Evaluation and Failure Analysis of Cracking in Wheel Rim SPFH590 Steel

Abstract Manufacturing wheel rims in the automobile industry demands high quality, high production, cost-effective and flawless wheels. At present, JIS G3138 SPFH 590 steel is widely used for manufacturing wheel rims for light and medium commercial vehicles. For high productivity, Flash butt welding is used for joining the edges of hot rolled strips. Cracking in wheel rims is a significant problem encountered in the production sectors. The present study is aimed at evaluating failure types in wheel rims by factors governing flash butt weld and subsequent cold forming processes. The microstructure of fractured specimens was examined by field emission scanning electron microscopy (FESEM) and an optical microscope. The mechanical characterization was analysed by tensile tests and crack tip opening displaced testing. The outcome of the results obtained was correlated with metallurgical concepts in order to find the cause of cracking failure.

Development of Direct Quenched Hot Rolled Martensitic Strip Steels

Metallurgical concepts for new ultra-high strength martensitic steels have been developed through direct quenching after hot rolling. In addition to the chemical composition, the hot rolling, quenching, and annealing parameters need to be optimized to fulfill the requirements for the demanding applications for which these steels are used. It is also shown that the welding behavior is influenced by the choice of alloying concept. Typical applications also require a high fatigue resistance, especially of formed components. For that reason, a dedicated set-up was developed that allows differentiation between materials, which is illustrated through the effect of inclusions on the fatigue performance of a bent test piece.

Fracture behaviour and microstructural evolution of structural mild steel under the multi-hazard loading of high-strain-rate load followed by elevated temperature

This paper presents the mechanical properties, microstructure evolution and fracture behaviour of structural mild steel subject to the multi-hazard loading scenario of post-impact-fire. Two-phase tensile tests were conducted on mild steel coupons to assess the coupling effect of strain-rate and subsequent temperature at three pre-determined deformation levels. Stress-strain characteristics of pre-damaged steel at different temperatures have been interpreted using well known metallurgical concepts. Scanning Electron Microscopy (SEM) fractographs have been utilized to detect pertinent microstructural alterations. Results indicate that the strength, energy absorption and ductility of steel material at elevated temperatures largely depend on the pre-deformation history of the material caused by high strain rate loading, with this effect dwindling at very high temperatures.

Metallurgical concepts for optimized processing and properties of carburizing steel

Carburized steel grades are widely used in applications where high surface near hardness is required in combination with good core toughness as well as high strength and fatigue resistance. The process of carburizing lower to medium carbon containing steel can generally provide this combination of properties and has been practiced for several decades. Such steel is essential in the vehicle power-train, machines and power generation equipment. However, the increasing performance demands by such applications as well as economical considerations forced steel producers to develop better alloys and fabricators to design more efficient manufacturing processes. The present paper describes recent concepts for alloy design optimization of carburizing steel and demonstrates the forthcoming beneficial consequences with regard to manufacturing processes and final properties.

A Novel Low-cost Hot Rolled High Strength Steel for an Automatic Teller Machine

A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. The low-cost C-Mn steel was microalloyed with 0.08 mass% – 0.12 mass% Ti rather than noble alloying elements, such as Nb, V, Mo, and Cu, etc. The novel steel had a good surface quality and welding property. After the hot rolled steel coils were leveled, the steel plates, the length of which was even down to 1500 mm, had an excellent flatness. The effects of hot rolling parameters on mechanical performance, microstructure and recrystallization behavior were studied. The metallurgical concept for the steel production was also discussed. The result shows that decreasing the finish rolling temperature, increasing cooling rate in the first cooling stage and decreasing the cooling rate in the last cooling stage, together with coiling at a modestly high coiling temperature all resulted in the refined grains and TiC precipitates, thereby improving the strength and toughness of this new steel greatly.

Combinatorial Approach Based on Interdiffusion Experiments for the Design of Thermoelectrics: Application to the Mg2(Si,Sn) Alloys

Thermoelectric materials are ranked by their ability to meet a criterion of excellence that is related to their conversion efficiency. This dimensionless figure of merit, zT = (σS2T)/κ, measures how well a candidate material can convert heat to electricity where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. This antagonist combination of properties controlling the performance of thermoelectrics makes it difficult to design these materials. Notwithstanding, it has been established that functional properties are controlled by both composition and microstructural parameters and that refined microstructures promote better thermoelectric properties.(1) This has led to new approaches inspired by metallurgical concepts that rely on the use of thermal treatments to generate and control microstructures that give the desired transport properties.(1-5) The combination of alloy composition and time and temperature that can be applied in a thermal treatment is nearly infinite; thus, identification of the optimal composition and treatment is very time-consuming using one-composition-at-a-time synthesis and property testing. Designers need the knowledge of thermodynamics and kinetics, as well as an understanding of the composition-microstructure-property relationships to better identify the most promising materials...

Development of Cu-bearing bake-hardenable steel sheets for automotive exposed panels

Recently, newly developed bake-hardenable (BH) steel sheets strengthened by copper sulfide (CuS) have been successfully employed in commercial production lines that supply automotive outer panels. The metallurgical concepts governing fabrication of these new BH steel sheets require keeping carbon content as low as 0.0015 wt.% without any additional amount of titanium and/or niobium for solute carbon scavenging. The role of CuS precipitates has turned out to raise the yield strength acting as a barrier against dislocation movement. In this paper, we studied the effects of chemical compositions and manufacturing process variables on the microstructure and mechanical properties of newly developed BH steel sheets. We found that the control of carbon and nitrogen showed a good balance between bake-hardenability (BH) and yield point elongation (YP-El). We identified the crystallographic relationship between the nano-size CuS precipitates and the ferrite matrix of (001)sulfide//(001)α-Fe and [001]sulfide//[001]α-Fe. We also found that the BH and YP-El were affected by the formation of aluminium nitride (AlN) and the annealing temperature.

CURRENT CAPABILITIES OF THE THERMO-MECHANICAL MODELLING OF WELDING PROCESSES

Some fundamental principles and advanced applications of thermo-mechanical modelling techniques used for industrial welding processes are described. The paper covers a range of modelling procedures comprising welding simulation and residual stress analysis of multi-pass, thick-walled ferritic steel pipes; welding simulation and distortion analysis of nickel-based superalloy thin plates; and inertia friction welding of dual alloy drive shafts. A number of pertinent mechanical and metallurgical concepts are discussed, including material behaviour models and material properties, microstructural evolution, solid state phase transformation and post-weld heat treatment. The paper emphasises the general methodologies of thermal modelling procedures and their role in the holistic process of the life assessment and design improvement of power plant piping systems and aeroengine casings and shaft components, operating under high temperature creep and fatigue service conditions.

Drawing of CCCT diagrams by static deformation and consideration deformation effect on martensite and bainite transformation in NiCrMoV steel

Abstract In this article, three Compression Continuous Cooling Transformation (CCCT) diagrams of NiCrMoV steel were considered that indicate induced transformations by compression during continuous cooling of austenite. CCCT diagrams of this steel were compared with its Continuous Cooling Transformation (CCT) diagram. In comparison, it was deduced that in all three CCCT, M d and B d have increased, the probable nose of bainite has shifted to left and the bainite area has increased. The phenomenon of enhancing M d and B d in CCCT was justified by thermodynamical and metallurgical concepts. Thermodynamical consideration indicated that the induced stress at higher temperatures results in to increasing mechanical driving force and decreasing total free energy of transformation. Metallurgical considerations specified that if the deformation temperature is selected so that the strength of austenite after deformation increases, the essential shear for martensite transformation (MT) and bainite transformation (BT) will get more difficult; therefore, M d and B d decrease. But if during or after deformation, recrystallization or full recovery process is onset, the structure of austenite gets softer and so the essential shear for MT and BT gets easier and M d and B d increase. In addition, it was characterized that by deformation accompanying dynamic recrystallization (DRX) or static recrystallization (SRX), the grains get finer, thus in finer grains, nucleation of bainite gets easier than nucleation of martensite. Thus, bainite becomes more preferable than martensite. So the probable nose of bainite in CCCT diagrams shifts to left relative to CCT and the bainite area increases.

Effect of liquid steel flow pattern on slab quality and the need for dynamic electromagnetic control in the mould

This paper addresses surface/subsurface quality (slivers, pipe, pinholes, blowholes, inclusion content) and diversion rate (slab casting abnormality codes) of products cast on high or low throughput thick slab machines. Examples from LTV, NKK and TKS pertaining to curved mould and vertical/bending casters are reviewed, which make apparent the important effect of steel flow stability and steel flow pattern in the mold on defect occurrence. The first conclusion, which pertains mostly to slow speed or wide slab machines, is that creating or maintaining a stable double roll flow is key to eliminating slab defects. The second conclusion, relating mostly to high speed casters, is that the intensity of the double roll flow pattern must not be excessive. With this in mind, the efficiency of an existing versatile electromagnetic based technology that allows accelerating, slowing down, and stirring the liquid steel was examined. The analysis shows good response of the liquid steel to various imposed magnetic fields, presents corresponding slab/coil quality improvements, and demonstrates the relevance of the proposed hydrodynamic and metallurgical concepts.

Design of a creep resistant nickel base superalloy for power plant applications: Part 1 - Mechanical properties modelling

Models have been developed and used as tools to design a new 'made to measure' nickel base superalloy for power plant applications. In Part 1, Gaussian processes are used to model the tensile and creep rupture properties of superalloys as a function of their composition and processing parameters, making use of large databases on existing alloys. The models are able to estimate the actual influence of alloying elements on the mechanical properties over a wide range of temperature and stress. They have been used, in conjunction with general metallurgical concepts and industrial requirements, as a basis for the design of a new Ni-Cr-W-Al-Ti-Fe-Si-C-B superalloy with desirable properties. It is estimated that the proposed forgeable and weldable alloy should have a creep rupture life at 750° C of 100 000 h under a stress of 100 MPa, with a huge reduction in price compared to existing commercial alloys with similar properties. In a following paper, Part 2, an attempt is made to design against the formation of undesirable phases and chemical segregation, using phase diagram calculations. Preliminary experimental results are presented in Part 3.

Tensión superficial, viscosidad y densidad de algunas escorias CaO-Al<sub>2</sub>O<sub>3</sub>

The metallurgical concepts with regard to the structures and properties of calcium aluminate melts have been based upon analogies within ternary CaO-Al2O3-SiO2 systems. In this work the results of surface tension, density and viscosity of some calcium aluminate slags, in the temperature range of 1500 to 1600°C are presented and the results are discussed based upon recent structural characterisation results of crystalline aluminates. The compositional range investigated was from 45 to 53 weight-% alumina. The results indicate a reduction in density as the molar ratio CaO-Al2O3 decreases. Surface tension falls on increasing either the molar ratio CaO-Al2O3 or temperature. Conversely, viscosity increases with increasing molar ratio CaO-Al2O3 and decreasing temperature. The compositional dependence of both surface tension and viscosity data may be associated with the presence of some aluminium ions in octahedral co-ordination, and a concept of surface behaviour is proposed which involves surface activity of aluminate anions containing aluminium ions in a reduced valence state, such as Al2+.

Fundamental Prediction of Steel Weld Metal Properties

Empirically derived expressions are commonly used to predict specific steel weldment properties. These expressions usually consider only constitutional considerations and are limited to their ability to predict the influence of the thermal experience (heat input, weld preparation, and plate thickness). Carbon equivalent and basicity index are examples of such predictive expressions. This paper reviews some of the existing expressional forms and introduces new functional forms that are based on metallurgical engineering concepts. Forms for equations which can predict weld metal properties and behavior as a function of composition are proposed. Methodology for including cooling rate in the predictive equations is also suggested. The concept of developing iso-property diagrams that allow better selection of welding consumables with variations in heat input is introduced and discussed.