A review on the multi-scaled structures and mechanical/thermal properties of tool steels fabricated by laser powder bed fusion additive manufacturing

The laser powder bed fusion(LPBF) process can integrally form geometrically complex and high-performance metallic parts that have attracted much interest,especially in the molds industry.The appearance of the LPBF makes it possible to design and produce complex conformal cooling channel systems in molds.Thus,LPBF-processed tool steels have attracted more and more attention.The complex thermal history in the LPBF process makes the microstructural characteristics and properties different from those of conventional manufactured tool steels.This paper provides an overview of LPBF-processed tool steels by describing the physical phenomena,the microstructural characteristics,and the mechanical/thermal properties,including tensile properties,wear resistance,and thermal properties.The microstructural characteristics are presented through a multiscale perspective,ranging from densification,meso-structure,microstructure,substructure in grains,to nanoprecipitates.Finally,a summary of tool steels and their challenges and outlooks are introduced.

Seismic Response Analysis of Steel Structure Isolation System Under Long-Period Seismic Motion

To analyze the seismic response of steel structure isolation systems under long-period seismic motion,a 9-story steel frame building was selected as the subject.Five steel structure finite element models were established using SAP2000.Response spectrum analysis was conducted on the seismic motion to determine if it adhered to the characteristics of long-period seismic motion.Modal analysis of each structural model revealed that the isolation structure significantly prolonged the structural natural vibration period and enhanced seismic performance.Base reactions and floor displacements of various structures notably increased under long-period seismic motion compared to regular seismic activity.Placing isolation bearings in the lower part of the structure proved more effective under long-period seismic motion.In seismic design engineering,it is essential to consider the impact of long-period seismic motion on structures and the potential failure of isolation bearings.

Fault Identification for Shear-Type Structures Using Low-Frequency Vibration Modes

Shear-type structures are common structural forms in industrial and civil buildings,such as concrete and steel frame structures.Fault diagnosis of shear-type structures is an important topic to ensure the normal use of structures.The main drawback of existing damage assessment methods is that they require accurate structural finite element models for damage assessment.However,for many shear-type structures,it is difficult to obtain accurate FEM.In order to avoid finite elementmodeling,amodel-freemethod for diagnosing shear structure defects is developed in this paper.This method only needs to measure a few low-order vibration modes of the structure.The proposed defect diagnosis method is divided into two stages.In the first stage,the location of defects in the structure is determined based on the difference between the virtual displacements derived from the dynamic flexibility matrices before and after damage.In the second stage,damage severity is evaluated based on an improved frequency sensitivity equation.Themain innovations of this method lie in two aspects.The first innovation is the development of a virtual displacement difference method for determining the location of damage in the shear structure.The second is to improve the existing frequency sensitivity equation to calculate the damage degree without constructing the finite elementmodel.Thismethod has been verified on a numerical example of a 22-story shear frame structure and an experimental example of a three-story steel shear structure.Based on numerical analysis and experimental data validation,it is shown that this method only needs to use the low-order modes of structural vibration to diagnose the defect location and damage degree,and does not require finite element modeling.The proposed method should be a very simple and practical defect diagnosis technique in engineering practice.

Deformation Mechanism of Bimodal Structured 2205 Duplex Stainless Steel in Two Yield Stages

A kind of micro/nanostructured 2205 duplex stainless steel(DSS)with uniform distribution of nanocrystals was prepared via aluminothermic reaction method.The analysis of stress-strain curve showed that the fracture strength and elongation of the specimen were 946 MPa and 24.7%,respectively.At present,the research on microstructure of bimodal 2205 DSS at room temperature(RT)mainly depended on scanning electron microscope(SEM)observation after loading experiments.The test result indicates that there are two different yield stages in stress-strain curve of specimen during tensile process.The microstructure of duplex bimodal structured stainless steel consists of two pairs of soft hard regions and phases.By studying deformation mechanism of bimodal structured stainless steel,the interaction between soft phase and hard phase are discussed.The principle of composition design and microstructure control of typical duplex stainless steel is obtained,which provides an important research basis for designing of advanced duplex stainless steel.

Discussion on Construction Technology and Welding Deformation of High-Rise Steel Frame Structure

Because of urbanization,land resources in China’s cities has become increasingly scarce.Therefore,modern buildings are becoming taller,making high-rise steel frame structures the new favorite of the construction industry.However,the construction of high-rise steel frame structures requires advanced technology.If the construction technology is effectively implemented and the welding techniques of the construction personnel align with the requirements for high-rise steel frame structures,it can help mitigate deformations in the steel structure,thus preserving the overall construction quality of high-rise steel frame structures.To enhance the applicability of steel frame structures in high-rise buildings,this paper focuses on analyzing the optimization path for the construction process of high-rise steel frame structures.It introduces a tailored approach to control welding-induced deformations in steel frame structures,aiming to make a valuable contribution to the advancement of China’s construction industry.

Analysis of Soundproof Technology of Assembled Steel Structure Houses

The problem of noise has always been highlighted in assembled steel structure houses.Therefore,it is necessary to use effective soundproof measures where steel beams intersect with the reserved line pipe openings,doors,windows,elevator shafts,and other locations.In this paper,we will investigate the areas with subpar soundproof performance in an assembled steel structure residential project and propose suitable noise control measures to address this issue.

The Formation of Colored Film on Stainless Steel and the Study for Surface Structure

本文介绍不锈钢化学着色膜的形成和制备，测定了膜层主要理化性能；利用ＳＥＭ、ＡＥＳ、ＡＦＭ、ＳＴＭ等方法研究着色膜的化学成分、表面形貌、结构特征。

Oxide and Sulfide Dispersive Precipitation and Effects on Microstructure and Properties of Low Carbon Steels

Electron microscopic investigation on low carbon steel strips produced by the CSP process has been carried out. Large number of oxide dispersive precipitates have been observed in the ferrite matrix of the steel strips. Dimension of them is about 10~20 nm. Electron diffraction study showed that the structure of these precipitates consists with cubic system spinel structure. Their lattice parameter is about 0.83 nm. The results implied that they should be complex oxides of Fe, Al et al. Small sulfide particles with 100-300 nm in size have also been observed. Remarkable strengthening and grain refinement effects can be obtained by the precipitations. The oxygen and sulfur in steels could play beneficial role under certain conditions.

Laser cleaning of steel structure surface for paint removal and repaint adhesion

Paint removal from steel structure is executed for shipyards of marine and offshore engineering.Due to environmental unfriendliness and unhealthy drawbacks of sand blasting technique, laser ablation technique is proposed as a substituting method.By absorbing high energy of the 1064 nm pulsed laser, the paint is vaporized quickly.The ablated debris is then collected by using a suction pump.Initial metal surface of the steel is exposed when laser beam irradiates perpendicularly and scans over it.The cleaned surface fulfills the requirements of surface preparation standards ISO 8501 of SA2.The adhesion is further characterized with pull-off test after carrying out painting with Jotamastic 87 aluminum paint.The repainting can be embedded onto the laser cleaned surface to bond much more tightly.The excellent adhesion strength of 20 MPa between repainted coating and the substrate is achieved, which is higher than what is required by shipyards applications.

Effect of Ce on the cleanliness, microstructure and mechanical properties of high strength low alloy steel Q690E in industrial production process

In order to improve the strength and toughness of Q690 E steel sheets,the effect of rare earth element Ce on the strength and toughness of Q690 E steel was studied by means of transmission electron microscopy,scanning electron microscopy,and metallographic microscope.The results showed that the addition of Ce in steel limited the combination of S with Mn and Ca,transformed Al2O3 inclusion into spherical CeAlO3 inclusion,and modified the precipitate form of some composite inclusions of TiN and sulfide oxides into TiN precipitation alone.The inclusions were spheroidizing.The size of inclusions was decreased from 3–5μm to 1–2μm,and the distribution was dispersed.Ce played a role in purifying molten steel through desulphurization and deoxidization.Meanwhile,the addition of Ce in steel effectively increased the nucleation particles in the liquid phase,improved the nucleation rate,enlarged the equiaxed grain refinement area,and limited the development of columnar crystals.The average grain size of slab decreased from 45.76 to 35.25μm,and the proportion of large grain size(>50μm)decreased from 40.41%to 23.74%.The macrostructural examination of slab was improved from B0.5 to C2.0,which realized the refinement of the solidified structure and reduced the banded structure of hot rolled plate.In addition,due to the inheritance of refined structure in the upstream,the recrystallization of deformed austenite and the growth of grain after recrystallization were restrained,and a refined tempered sorbite structure was obtained.When rare earth element Ce was added,the width of the martensite lath bundle was narrowed from about 500 nm to about 200 nm,which realized a remarkable grain refinement strengthening and toughening effect.Mechanical properties such as tensile,yield,and low-temperature impact toughness were significantly improved.

Preliminary investigation of seismic damage to two steel space structures during the 2013 Lushan earthquake

Severe damage to steel space structures is rarely reported when compared to other structural systems damaged during past major earthquakes around the world. Two gymnasiums of steel space structures in downtown Lushan County that were damaged during the 2013 M7.0 Lushan earthquake in China were investigated and the observations are summarized in this paper. Typical damage to these two steel space structures ranges from moderate to severe. Moderate damage includes global buckling and dislocation of bolted connections of truss members, and inelastic elongation of anchor bolts and sliding of pedestal plates of supports. Severe damage includes member fracture caused by local buckling, and fracture failure of anchor bolts and welds. The distribution of structural damage to these two structures is described in detail and future research opportunities are suggested.

Structure Evolution and Solidification Behavior of Austenitic Stainless Steel in Pulsed Magnetic Field

To understand the solidification behavior of austenitic stainless steel in pulsed magnetic field, the solidification process is investigated by means of the self-made high voltage pulse power source and the solidification tester. The results show that the solidification structure of austenitic stainless steel can be remarkably refined in pulsed magnetic field, yet the grains become coarse again when the magnetic intensity is exceedingly large, indicating that an optimal intensity range existed for structure refinement. The solidification temperature can be enhanced with an increase in the magnetic intensity. The solidification time is shortened obviously, but the shortening degree is reduced with the increase of the magnetic intensity.

Methodology to Estimate Remaining Service Life of Steel Structure by Possibilistic Reliability Theory

Steel structure system of crane deteriorates over time due to environmental effects, material fatigue, and overloading. System structural reliability and remaining service life assessment methods are developed during the few decades. But until now estimating remaining service life methods of crane steel system by reliability theory begin to develop. Safety assessment of existing steel structure system requires the development of a methodology that allows for an accurate evaluation of reliability and prediction of the remaining life. Steel structures are the supporting elements in the special equipment such as hoisting machinery. Structure reliability and remaining service life safe assessment are important for steel structures. For finding the reason which caused the failure modes （such as fatigue strength failure, stiffness failure and stability failure）, incremental loading method based on possibilistic reliability is applied into dynamic structure failure path research. Through reliability analyzing and calculating for crane, it is demonstrated that fatigue damage is the most common failure mode. Fuzzy fatigue damage accumulation theory is used for basis theory and Paris-Eadogan equations are used for mathematical modeling. All fatigue parameter values of the welding box girder of bridge cranes are determined and fatigue remaining life formulas are deduced. After field test and collecting working parameters of numerous cranes, typical fatigue load spectrum was compiled for the dangerous point of box girders used in the area. Fatigue remaining life is assessed for different types and lifting capacities. Safety for steel structure system of bridge crane is assessed by two quantitative indexs： reliability and remaining life. Therefore, the evaluation means is more comprehensive and reasonable. The example shows that the two quantitative indexs are mutually correlated. Through analyzing the 120 t-22.5 m bridge crane of a certain enterprise, a new methodology to estimate remaining service life of steel structure by possibilistic reliability theory is introduced for safety evaluation of structure system.

Effects of hot top pulsed magneto-oscillation on solidification structure of steel ingot

Achieving a uniform structure with few defects in heavy steel ingot is of high commercial importance. In this present work, in order to verify the potential of pulsed magneto-oscillation(PMO) applied in the production of heavy ingot, an induction coil was located at the hot top of the steel ingot to develop a novel technique, named hot top pulsed magneto oscillation(HPMO). The influences of HPMO on the solidification structure, macro segregation and compactness of a cylindrical medium carbon steel ingot with the weight of 160 kg were systematically investigated by optical microscope(OM) and laser induced breakdown spectroscopy original position metal analyzer(LIBSOPA-100). The results show that HPMO not only causes significant grain refinement and promotes the occurrence of columnar to equiaxed transition(CET) but also can homogenize the carbon distribution and enhance the compactness of the steel ingot. Therefore, HPMO technique has the potential to be applied in the production of heavy steel ingots on an industrial scale.

Lessons from the seismic behavior of a steel grid roof structure heavily damaged in Lushan earthquake

The seismic behavior of a school gymnasium, whose steel grid roof was heavily damaged during the Mw6.6 Lushan earthquake in 2013, is simulated through nonlinear dynamic analysis. The simulated damage is compared with field observations to validate the numerical model, based on which a parametric study was performed to provide insight into the failure process and damage patterns of steel grids. The results suggest that the grid damage is strongly related to roofsubstructure interactions. These include not only the substructure's amplification of the vibration, but the uncoordinated displacement of the substructure's columns which support the grid also play an equally important role. In particular, the latter effect may significantly alter the internal force distribution in the steel grid and lead to unexpected buckling of members that are proportioned as tension-only members. While such interactions are generally not accounted for in the design practice for grid structures in China, similar seismic damage may be expected for other existing grid roofs in future earthquakes. As is also demonstrated in this study, seismic isolation of the roof is a promising solution to protect grid roof structures by mitigating the detrimental effects of roof-substructure interactions.

Microstructure of Steel Fiber Reinforced Polymer-cement-based Composites

Mercury intrusion porosimetry was used to measure the pore structure of steel fiber reinforced polymer-cement-based composite. The results indicate that the large pore volume decreases by 57. 8% - 51.2% and by 87. 1% - 88% with the addition of steel fibers and polymers respectively. When both steel fibers and polymers are simultaneously added, the large pore volume decreases by 88.3% - 90.1% . As a surface active material , polymer has a favorable water-reduced and forming-film effect, which is contributed to the decrease of the thickness of water film and the improvement of the conglutination between the fibers and the matrix. Polymers could form a microstructure network. This network structure and the bone structure of cement hydration products penetrate each other and thus the interpenetrating network with sticky aggregate and steel fiber inside forms.

Electro-Pulse on Improving Steel Ingot Solidification Structure

Pulse Electric Discharging (PED) is a novel technique that can modify solidifying structure and reduce grain size. ItS effectsapplied to the high carbon liquid steel were presented here. The macrostructure and microstructure of the high carbon alloy steel werealso observed. Results show that (1 ) the length of columnar crystal at the edge of ingot without PED treatment is much longer than thatwith PED, and (2) the perlite lamellae of steel billets after deposed by PED are twisted and shortened. An explanation of those experimental results is given.

Calculation and Analysis of Valence Electron Structure of Mo_2C and V_4C_3 in Hot Working Die Steel

Taking the hot working die steel （HWDS） 4Cr3Mo2NbVNi as an example, the phase electron structures （PES） and the biphase interface electron structures （BIES） of Mo2C and V4 C3 , which are two kinds of important carbides precipitated during tempering in steel were calculated, on the basis of the empirical electron theory of solids and molecules and the improved TFD theory. The influence of Mo2 C and V4 C3 on the mechanical properties of HWDS has been analyzed at electron structure level, and the fundamental reason that the characteristic of the PES and the BIES of carbides decides the behavior of them has been revealed.

Seismic behavior of concrete filled steel tubular arch structures

Shaking table tests of a 1:10 scale arch model performed to investigate the seismic behavior and resistance of concrete filled steel tubular (CFT) arch structures are described in this paper. The El-Centro record and Shanghai artificial wave were adopted as the input excitation. The entire test process can be divided into three stages depending on the lateral brace configurations, i.e., fully (five) braced, two braces removed, and all braces removed. A total of 46 tests, starting from the elastic state to failure condition, have been conducted. The natural vibration frequencies, responses of acceleration, displacement and strain were measured. From the test results, it is demonstrated that the CFT arch structures are capable of resisting severe ground motions and that CFT arches offer a credible alternative to reinforced concrete arches, especially in regions of high seismic intensity.

Effect of Cr/Mn segregation on pearlite–martensite banded structure of high carbon bearing steel

The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling.With the use of an optical microscope, scanning electron microscope, transmission electron microscope, and electron probe microanalyzer, the segregation characteristics of alloying elements in cast billet and their relationship with hot-rolled plate banded structure were revealed.The formation causes of an abnormal banded structure and the elimination methods were analyzed.Results indicate the serious positive segregation of C, Cr, and Mn alloy elements in the billet.Even distribution of Cr/Mn elements could not be achieved after 10 h of heat preservation at 1200℃, and the spacing of the element aggregation area increased, but the segregation index of alloy elements decreased.Obvious alloying element segregation characteristics are present in the banded structure of the hot-rolled plate.This distinct white band is composed of martensitic phases.The formation of this abnormal pearlite–martensite banded structure is due to the interaction between the undercooled austenite transformation behavior of hot-rolled metal and the segregation of its alloying elements.Under the air cooling after rolling, controlling the segregation index of alloy elements can reduce or eliminate the abnormal banded structure.