MICROSTRUCTURAL CHARACTERISTICS AND MECHANICAL PROPERTIES OF SPRAY FORMED HIGH SPEED STEEL FOR WORK ROLL

The mechanical properties and microstructure characteristics of a high speed steel (HSS) for roll has been studied. As compared with the as-cast HSS, the spray-formed HSS have fine grain and segregation free microstructures. Carbides distribute uniformly. The morphology and types of the carbides in spray-formed HSS are different from those in as-cast HSS. The results of the high temperature tensile experiment show that, as to spray-formed HSS, there is a possibility of superplastic deformation in the range of 780-810℃.

Seismic performance of steel reinforced ultra high-strength concrete composite frame joints

To investigate the seismic performance of a composite frame comprised of steel reinforced ultra high-strength concrete （SRUHSC） columns and steel reinforced concrete （SRC） beams, six interior frame joint specimens were designed and tested under low cyclically lateral load. The effects of the axial load ratio and volumetric stirrup ratio were studied on the characteristics of the frame joint performance including crack pattern, failure mode, ductility, energy dissipation capacity, strength degradation and rigidity degradation. It was found that all joint specimens behaved in a ductile manner with flexural-shear failure in the joint core region while plastic hinges appeared at the beam ends. The ductility and energy absorption capacity of joints increased as the axial load ratio decreased and the volumetric stirIup ratio increased. The displacement ductility coefficient and equivalent damping coefficient of the joints fell between the corresponding coefficients of the steel reinforced concrete （SRC） frame joint and RC frame joint. The axial load ratio and volumetric stirrup ratio have less influence on the strength degradation and more influence on the stiffness degradation. The stiffness of the joint degrades more significantly for a low volumetric stirrup ratio and high axial load ratio. The characteristics obtained from the SRUHSC composite frame joint specimens with better seismic performance may be a useful reference in future engineering applications.

Graphitization of Spray-formed Ultrahigh Carbon Steels Containing Si during Hot Rolling

The graphitization behavior of ultrahigh carbon steels containing Si in hot rolling processes was investigated. The graphite stringers went mostly through the small pores and generally paralleled to the rolling direction. The influence of alloy elements on graphitization was estimated based on thermodynamics, which showed that Si content was important for graphitization. Graphite stringers nucleated at small pores and grew with carbon diffusion during hot rolling. Alloy contents, pores and hot deformation at γ+Fe3C phase range were the key factors for the formation of graphite. The probable effect of deformation on graphite formation during hot rolling was discussed in this paper.

Improving the Fatigue Performance of the Welded Joints of Ultra-Fine Grain Steel by Ultrasonic Peening

Contrast tests were carried out to study the fatigue performance of the butt joints treated by ultrasonic peening, aiming at the improvement of ultrasonic peening treatment(UPT) on welded joints of a new material. The material is a new generation of fine grain and high purity SS400 steel that has the same ingredients as the traditional low carbon steel. The specimens are in two different states:welded and ultrasonic peening conditions. The corresponding fatigue testing data were analyzed according to the regulation of the statistical method for fatigue life of the welded joints established by International Institute of Welding(IIW). Welding residual stress was considered in two different ways: the constant stress ratio R=0.5 and the Ohta method. The nominal stress-number (σ-N)curves were corrected because of the different plate thickness compared to the standard and because there was no mismatch or angular deformation. The results indicated that: 1) Compared with the welded specimens, when the stress range was 200 MPa, the fatigue life of the SS400 steel specimens treated by ultrasonic peening is prolonged by over 58 times, and the fatigue strength FAT corresponding to 106 cycles is increased by about 66%; 2) As for the SS400 butt joint (single side welding double sides molding), after being treated by UPT, the nominal S-N curve (m=10) of FAT 100 MPa(R=0.5) should be used for fatigue design. The standard S-N curves of FAT 100 MPa(R=0.5, m=10) could be used for fatigue design of the SS400 steel butt joints treated by ultrasonic peening.

Experimental study on the shear performance of quasi-NPR steel bolted rock joints

Quasi-NPR(negative Poisson’s ratio)steel is a new type of super bolt material with high strength,high ductility,and a micro-negative Poisson’s effect.This material overcomes the contrasting characteristics of the high strength and high ductility of steel and it has significant energy-absorbing characteristics,which is of high value in deep rock and soil support engineering.However,research on the shear resistance of quasi-NPR steel has not been carried out.To study the shear performance of quasi-NPR steel bolted rock joints,indoor shear tests of bolted rock joints under different normal stress conditions were carried out.Q235 steel and#45 steel,two representative ordinary bolt steels,were set up as a control group for comparative tests to compare and analyze the shear strength,deformation and instability mode,shear energy absorption characteristics,and bolting contribution of different types of bolts.The results show that the jointed rock masses without bolt reinforcement undergo brittle failure under shear load,while the bolted jointed rock masses show obvious ductile failure characteristics.The shear deformation ca-pacity of quasi-NPR steel is more than 3.5 times that of Q235 steel and#45 steel.No fracture occurs in the quasi-NPR steel during large shear deformation and it can provide stable shear resistance.However,the other two types of control bolts become fractured under the same conditions.Quasi-NPR steel has significant energy-absorbing characteristics under shear load and has obvious advantages in terms of absorbing the energy released by shear deformation of jointed rock masses as compared with ordinary steel.In particular,the shear force plays a major role in resisting the shear deformation of Q235 steel and#45 steel,therefore,fracture failure occurs under small bolt deformation.However,the axial force of quasi-NPR steel can be fully exerted when resisting joint shear deformation;the steel itself does not break when large shear deformation occurs,and the supporting effect of the jointed rock mass is effectively guaranteed.

Some Advances in the Application of Weathering and Cold-Formed Steel in Transmission Tower

Application of weathering and cold-formed steel in transmission lines can reduce steel consumption and environmental pollution. Some advances in the studies on the weathering and cold-formed steel in transmission tower are introduced. Firstly, corrosion-resistant tests of weathering steel samples under different simulating technical atmospheres were carried out separately for 240 hours. It shows that the corrosion degree of joint samples is higher than that of single chip samples, and the corrosion-resistant performance of weathering steel is superior to common carbon steel. The corrosion-resistance of weathering steel meets with the requirement of transmission tower. Secondly, experiments and finite element analysis for cold-formed angles and a 220kV prototype tower were completed, and the stability coeffi-cient fitting curves as well as the modification formulas of slenderness ratio for cold-formed members were determined. According to the structural characteristics of transmission towers, four sections of cold-formed angles with different sections and slenderness ratios were selected in this study. The finite element model well predicted the buckling behav-iour of the cold-formed members. Ultimate loads calculating by the fitting curve were well agreed to the experimental values, especially for the members with small slenderness ratios. Weight of the cold-formed steel tower can be reduced by more than 5 percent after considering the strength enhancement. Cost of the weathering and cold-formed steel transmission tower is nearly equivalent to that of hot-rolled steel tower with hot galvanizing.

Experimental study on the forming characteristics of 1.5 GPa ultrahigh-strength dual-phase steel

The DP1500 steel series successfully produced by Baosteel is a marked improvement over the cold-rolled hot-dip galvanized dual-phase steel series.Sufficient parameter data related to forming characteristics are needed for the successful application of dual-phase steel series in engineering structures.Therefore,differences in the mech-anical properties,forming limit,hole expansion ratio,and stretch bend limit of the 1.5 GPa ultrahigh-strength steel,including DP1500,QP1500,and MS1500,have been systematically studied.Results show that the DP1500 exhibits good plastic deformation performance and approximately 5% uniform elongation,and its true major strain minimum on the forming limit curve(FLC_(0)) value is approximately 0.083,which is higher and lower than the FLC_(0) values of MS1500 and QP1500 of the same strength grade,respectively.DP1500 also exhibits good flanging and pore expansion capabilities and superior performance to QP1500 and MS1500.The minimum radius-to-thickness(R/T) ratio(1.4) of DP1500 in the 90° bend tests transverse to the rolling direction is between the R/T ratios of MS1500 and the QP1500.Overall,the formability performance of DP1500 is between that of MS1500 and QP1500.Its excellent crash energy absorption and formability performance render it a suitable structural component,and it has been successfully tested and verified on a typical complex ultrahigh-strength steel skeleton structure.

Microstructure and forming characteristics of spray-formed M42 high speed steel

M42 high speed steel （HSS）, a high grade alloy, was produced using the spray forming technology. Optical metallography （OM）,X-ray diffraction （XRD） and scanning electron microscope （SEM） were used to investigate the microstructures of the as-sprayed M42 HSS and the as-atomized powders. It is found that as-sprayed M42 HSS is composed of martensite, retained austenite and carbides. The grain size, as well as the morphology, size and distribution of the carbides have been greatly improved compared with those of the as-cast M42 steel （ a traditional process）. The size of the powder obtained at a high cold speed was ranged from 50 μm to 100μm in diameter. Its representative microstructures include dendrite, dendritic fragments, a mixture of dendrites, equiaxed grains and dendritic fragments,and equiaxed grains. The rapid solidification in the spray forming is a key factor to cause the refinement of the M42 HSS.

Microstructure of the spray formed Vanadis4 cold work die steel and its evolution during hot rolling and heat treatment

A high alloyed Vanadis4 （V4） steel was produced by spray forming process in Baosteel, and the microstructure of the as-sprayed steel and its evolution during hot rolling and annealing processes were characterized. It was found that the as-sprayed V4 steel has a homogeneous and fine scale rrficrostructure with uniform dispersion of carbides. The hot rolling temperature is the key factor in controlling the evolution of type, morphology and distribution of carbides, as well as the matrix microstructure of the as-rolled steels. The optimized processing parameters for the as-sprayed V4 steel are rolling at 1050℃ and then annealing at 900℃. The microstructural evolution mechanisms during hot rolling and annealing are determined according to the microstructural analyasis of the material at different stages.

Analysis on seismic performance of a new type of joint in steel structures

To examine the seismic performance of a newly fabricated weakened joint at the beam end position,four groups of energy-consuming steel plates with different weakening depths and thicknesses were subjected to horizontal cyclic reciprocating loading tests on beam ends.The tests were designed to evaluate the beams'hysteresis curve,skeleton curve,bearing capacity degradation curve,stiffness degradation curve,and ductility and the nodes'energy dissipation capacity.The test results show that a newly fabricated joint will not undergo brittle damage when the beam-column joint is welded at a displacement of 105 mm.Thus,the hysteresis curve will show an inverse S shape,and an obvious slip phenomenon will occur,which is mainly due to splicing.The diameter of the bolt connecting the slab to the beam flange is slightly smaller than the aperture.Due to the existence of slippage,the skeleton curve has no evident yield point.The joint ductility coefficient is less than 3.0,and the initial rotational stiffness of the joint is also small.The buckling of the splicing panel causes a rapid decrease in the joint bearing capacity.The main approaches,appropriately reducing the weakening depth and increasing the thickness of the splicing plate,can delay the occurrence of buckling and improve the ductility of the joint.

Feasibility of developing composite action between concrete and cold-formed steel beam

Cold-formed steel structures are steel structure products constructed from sheets or coils using cold rolling, press brake or bending brake method. These structures are extensively employed in building construction industry due to their light mass, ductility by economic cold forming operations, favorable strength-to-mass ratio and other factors. The utilization of cold formed steel sections with concrete as composite can hugely reduce the construction cost. However, the use of cold formed steel members in composite concrete beams has been very limited. A comprehensive review of developments in composite beam with cold formed steel sections was introduced. It was revealed that employing cold-formed steel channel section to replace reinforcement bars in conventional reinforced concrete beam results in a significant cost reduction without reducing strength capacity. The use of composite beam consisting of cold-formed steel open or close box and filled concrete could also reduce construction cost. Lighter composite girder for bridges with cold-formed steel of U section was introduced. Moreover, types of shear connectors to provide composite action between cold-formed steel beam and concrete slab were presented. However, further studies to investigate the effects of metal decking on the behavior of composite beam with cold-formed steel section and introduction of ductile shear connectors were recommended.

Performance analysis of the SAW joint of duplex stainless steel clad plate

Duplex stainless steel clad plate exhibits good performance and is relatively inexpensive,however,some difficulties must be overcome when welding different materials. In this study,submerged arc welding（ SAW） was used to weld Baosteel ship clad plates（ 2205 ＋ DH36）,and the performance of the welded joints was tested. The results indicate that the mechanical properties and the corrosion resistance of the welded joints meet the required specifications. The distribution of the main anti-corrosive elements and the phase ratio of the welded joint are analyzed,thereby indicating excellent uniformity and confirming that the welded joint is corrosion resistant.

Investigation into microstructure of spray formed V4 cold working mould steel and its roiling and annealing

The material selected for this work was the spray formed Vanadis4 high alloy cold working mould steel （abbreviated to V4 steel）. Its microstructure, hot rolling process, and annealing treatment have been investigated. Observed from the optical and electron microscopes, the as-sprayed V4 steel had the finer microstructure of uniform and equiaxial grains ,while after hot rolling for densification and spheroidized annealing, the V4 steel obtained an excellent spheroidized structure that is favorable to subsequent quenching and tempering treatment. The spheroidized structure and level of annealed hardness of the V4 steel are almost the same as expensive imported powder metallurgy the V4 steel. It is difficult to produce V4 steel with the conventional ingot metallurgical technique, so the multi-step and high-cost powder metallurgy method is generally used at present. Compared to the powder metallurgy technique, using the spray forming technique to produce the V4 steel has obvious advantages and potential market competitiveness in reducing production costs, simplifying working process, and shortening the production cycle.

Oxidation Resistance of the Aluminide Coating Formed on Carbon Steels

Low and medium carbon steels were aluminized by the pack aluminizing technique using halideactivated pure-Al and Fe-Al packs. The effect of mixture composition, aluminizing temperatureand time and C content of the steel substrate on the structure and thickness of the aluminidelayer, and on the oxidation resistance was investigated. The optimum oxidation resistance canbe achieved with a low carbon steel substrate when the intermetallic phases Fe3Al and FeAlform the surface of the aluminide layer. In this case, the Al concentration at the surface of thealuminide coating is at least ≥15 wt pct. Formation of high Al concentration phases (FeAl3 andFe2Al5) during aluminizing should be avoided as they tend to embrittle the aluminide layer andreduce its oxidation resistance.

Flexural Capacity Formula of Diaphragm-Through Joints of Concrete-Filled Square Steel Tubular Columns

Finite element analysis and parametric studies were performed to investigate the flexural capacity of the panel zone of diaphragm-through joints between concrete-filled square steel tubular columns and H-shaped steel beams. Through the comparisons of failure modes, load–displacement curves, and bearing capacity, it was found that the flexural capacity of the panel zone of diaphragm-through joints was determined by the tensile action and influence of the web of H-shaped steel beams, and the axial load should be taken into account. The steel tube and the diaphragm were the major parts of the joint that resisted the bending moment. The contribution of in-filled concrete had little influence on the flexural capacity of the panel zone of the joint and could be neglected. According to the results of these numerical studies, a formula that considered the influence of the web of H-shaped steel beams and the axial load was developed based on the yield lines in the diaphragm and the steel tube. The results of the proposed formula were in good agreement with the numerical data of this investigation. © 2017, Tianjin University and Springer-Verlag Berlin Heidelberg.

Double⁃Shear Experiments of Cold⁃Formed Steel Stud⁃to⁃Sheathing Connections at Elevated Temperatures

The behavior of cold⁃formed steel(CFS)stud⁃to⁃sheathing connections at elevated temperatures is an important parameter for the fire resistance design and modeling of mid⁃rise CFS structures.In this paper,three kinds of sheathings,namely,medium⁃and low⁃density calcium⁃silicate boards and oriented strand board,were selected for double⁃shear experiments on the mechanical properties of 253 screw connections at ambient and elevated temperatures.The effects of the shear direction,screw edge distance and the number of screws on the behavior of the connections were studied.The results showed that the shear direction and the screw edge distance more significantly influenced the peak deformation,while their impacts on the peak load varied with the type of sheathings.Compared with the single⁃screw connections,the peak loads of the specimens with double⁃screw connections obviously increased but did not double.Finally,a simplified load⁃displacement curve model of stud⁃to⁃sheathing connections at elevated temperature was generated first by establishing the prediction formula for characteristic parameters,such as the peak load,the peak deformation and the elastic stiffness,and then by considering whether the curves corresponded to stiffness increase phenomena.The present investigation provides basic data for future studies on the numerical modeling of CFS structures under fire conditions.

Numerical Optimization by Finite Element Method of Stainless Steel/Glass-Epoxy Composite Bolted Joint under Tension and Compression

The aim of this study was to optimize the geometry and the design of metallic/composite single bolted joints subjected to tension-compression loading. For this purpose, it was necessary to evaluate the stress state in each component of the bolted join. The multi-material assembly was based on the principle of double lap bolted joint. It was composed of a symmetrical balanced woven glass-epoxy composite material plate fastened to two stainless sheets using a stainless pre-stressed bolt. In order to optimize the design and the geometry of the assembly, ten configurations were proposed and studied: a classical simple bolted joint, two joints with an insert (a BigHeadR insert and a stair one) embedded in the composite, two “waved” solutions, three symmetrical configurations composed of a succession of metallic and composites layers, without a sleeve, with one and with two sleeves, and two non-symmetrical constituted of metallic and composites layers associated with a stair-insert (one with a sleeve and one without). A tridimensional Finite Element Method (FEM) was used to model each configuration mentioned above. The FE models taked into account the different materials, the effects of contact between the different sheets of the assembly and the pre-stress in the bolt. The stress state was analyzed in the composite part. The concept of stress concentration factor was used in order to evaluate the stress increase in the highly stressed regions and to compare the ten configurations studied. For this purpose, three stress concentration factors were defined: one for a monotonic loading in tension, another for a monotonic loading in compression, and the third for a tension-compression cyclic loading. The results of the FEM computations showed that the use of alternative metallic and composite layers associated with two sleeves gived low values of stress concentration factors, smaller than 1.4. In this case, there was no contact between the bolt and the composite part and the most stressed region was not the vicinity of the hole but the end of the longest layers of the metallic inserts.

Structure and mechanical properties of aluminum alloy/Ag interlayer/steel non-centered electron beam welded joints

Electron beam welding was carried out between aluminum alloy and steel with Ag interlayer. Seam morphology, structure and mechanical properties of the joints were investigated with different action positions of the electron beam spot. The results show that with the increment of the beam offset to the silver side from the interface between silver and steel, the seam morphology was improved, and the porosity in the Ag interlayer vanished. A transition layer mainly composed of Ag2Al and Al eutectic was formed at the interface between silver and aluminum, and became thin and spiccato as the beam offset increased. When the beam offset was too large, two IMC layers composed of FeAl and FeAl3 respectively were formed at the interface between steel and Ag interlayer. The optimal beam offset was 0.2 mm, and the maximum tensile strength of the joint was 193 MPa, 88.9% that of the aluminum alloy, and the fracture occurred at the interface between steel and Ag interlayer.

Effect of intermetallic compounds on heat resistance of hot roll bonded titanium alloy-stainless steel transition joint

The effect of intermetallic compounds on the heat resistance of transition joint was investigated. The experiment of post-weld heat treatment for the hot roll bonded titanium alloy-stainless steel joint using nickels interlayer was carried out, and the interface microstructure evolution due to heat treatment was presented. There was not found significant interdiffusion at stainless steel/nickel interface, when the specimens were heat treated in the temperature range of 600-800 °C for 10 and 30 min, while micro-cracks occurred at the stainless steel/nickel interface heat treated at 700 °C for 30 min. The thickness of intermetallic layers at nickel/titanium alloy interface increased at 600 °C, and micro-cracks occurred at 700 and 800 °C. The micro-cracks occurred between intermetallic layers or between intermetallic layer and nickel interlayer as well. The tensile strength of the transition joint decreased with the increase of heat treatment temperature or holding time.

EXPERIMENTAL RESEARCH AND NONLINEAR FINITE ELEMENT ANALYSIS ON NEW TYPE JOINT BETWEEN COLUMN AND STEEL BEAM OF CONCRETE-FILLED RECTANGULAR STEEL TUBULAR

Experimental results of new type joints between the column and the. steel beam of concrete-filled rectangular steel tubular （CFRT） under reversed cyclic loads are presented. The earthquake resistant capacity of the joint is influenced by infilled concrete, stiffener length and relative dimensions of column and beam. It is found that the hysteresis curves obtained in the experiment are full and the joints have a good energy dissipation capacity. The nonlinear finite element models are also used to analyze the hysteresis behavior of the joints under reversed cyclic loads using ANSYS 8.0. The influences of the stiffener length and the infilled concrete are analyzed. Analytical results show that the stiffener length and the infilled concrete are critical for the joints. Furthermore, the skeleton curves of the finite element models are in good agreement with those of experiments.