On post-instability processes in adiabatic shear in hot rolled steel

EXPERIMENTS carried out using a split Hopkinson torsional bar have shown that only one shear band develops in specimens of hot rolled steel which break during testing. We observed, however, that in specimens which were not deformed to failure, several fine shear bands appeared. We believe that these formed during the loading cycle before the appearance of the final shear band and were not due to the effect of unloading. So we developed a numerical model to study the evolution of shear banding from several finite amplitude disturbances (FADs) in both temperature and strain rate. This numerical model reveals the detailed processes by which the FADs evolve into a fully developed shear band and suggests that beyond instability, the so-called shear banding process consists of two stages: inhomogeneous shearing and true shear-banding. The latter is characterized by the collapse of the stress and an abrupt increase of the local shear strain rate.

Preparation and characterization of As-cast and hot-rolled Mg-3Al-0.5Mn-0.5Zn-1MM alloy

Mg-3Al-0.5Mn-0.5Zn-1MM alloy was prepared by metal mould casting method. The as-cast ingot was homogenized and then hot-rolled at 673 K with total thickness reduction of 65%. Microstructure and mechanical properties of the as-cast and hot-rolled samples were investigated. The results showed that the as-cast sample mainly consisted of alpha-Mg, beta-Mg17Al12, Al10Ce2Mn7, and Al11RE3 (RE = La and Ce) phases. The average grain size of the sample homogenized at 673 K was about 240 gm, and it was greatly refined to about 7 mu m by dynamic recrystallization for the hot-rolled sample.

Boundary Element Method (BEM) Analysis for Galvanic Corrosion of Hot Dip Galvanized Steel Immersed in Seawater

A numerical analysis of galvanic corrosion of hot-dip galvanized steel immersed in seawater was presented. The analysis was based on the boundary element methods (BEMs) coupled with Newton-Raphson iterative technique to treat the nonlinear boundary conditions, which were determined by the experimental polarization curves. Results showed that galvanic current density concentrates on the boundary of steel substrate and zinc coating, and the sacrificial protection of zinc coating to steel substrate results in overprotection of steel cathode. Not only oxygen reduction but also hydrogen reduction could occur as cathode reactions, which probably led up to the adsorption and absorption of hydrogen atoms. Flat galvanized steel tensile sample shows a brittle behavior similar to hydrogen embrittlement according to the SSRT (show strain rate test) in seawater.

Corrosion behaviour of hot dip zinc and zinc-aluminium coatings on steel in seawater

A comparative investigation of hot dip Zn-25Al alloy, Zn-55Al-Si and Zn coatings on steel was performed with attention to their corrosion performance in seawater. The results of 2-year exposure testing of these at Zhoushan test site are reported here. In tidal and immersion environments, Zn-25Al alloy coating is several times more durable than zinc coating of double thickness. At long exposure times, corrosion rate for the Zn-25Al alloy coating remains indistinguishable from that for the Zn-55Al-Si coating of similar thickness in tidal zone, and is two to three times lower than the latter in immersion zone. The decrease in tensile strength suggested that galvanized and Zn-55Al-Si coated steel suffer intense pitting corrosion in immersion zone. The electrochemical tests showed that all these coatings provide cathodic protection to the substrate metal; the galvanic potentials are equal to - 1,050, - 1,025 and - 880 mV (SCE) for zinc, Zn-25Al alloy and Zn-55Al-Si coating, respectively, which are adequate to keep the steel inside the immunity region. It is believed that the superior performance of the Zn-25Al alloy coating is due to its optimal combination of the uniform corrosion resistance and pitting corrosion resistance. The inferior corrosion performance by comparison of the Zn coating mainly results from its larger dissolution rate, while the failure of the Zn-55Al-Si coating is probably related to its higher susceptibility to pitting corrosion in seawater.

Study on growth factors of intermetallic layer within hot-dipped 25% Al-Zn alloy coating on steel

25%Al-Zn alloy coating performs better than hot dip galvanized coating and 55%Al-Zn-Si coating with regard to general seawater corrosion protection. This study deals with the interfacial intermetallic layer's growth, which affects considerably the corrosion resistance and mechanical properties of 25%Al-Zn alloy coatings, by means of three-factor quadratic regressive orthogonal experiments, The regression equation shows that the intermetallic layer thickness decreases rapidly with increasing content of Si added to the Zn-Al alloy bath, increases with rise in bath temperature and prolonging dip time. The most effective factor that determined the thickness of intermetallic layer was the amount of Si added to Zn-Al alloy bath, while the effect of bath temperature and dip time on the thickness of intermetallic layer were not very obvious.

Corrosion resistance of steel with hot-dipped galvanized zinc and zinc alloy coatings in seawater

Hot-dipped galvanized zinc and zinc alloy coatings were used as the hot-dipped low alloy zinc coatings (aluminum content less than protective metallic coatings for steel structures in seawater in Chi- or equal to 10 wt%) is equal to or even lower than that of the pure na. Corrosion of the two coatings immersed in sea water in Qingdao zinc sheet, while the performance of the hot-dipped high alloy zinc and Xiamen for 6 years were introduced and analyzed, which pro-coatings is higher than that of the pure zinc sheet. The hot-dipped vides a basis for further development and applications of these coat- high alloy zinc coatings can be further developed for optimal tings in China. Tests proved that the anti-corrosion performance of formance in the future.

Formation and structure of composite coating of HDA and micro-plasma oxidation on A3 steel

Composite coatings were obtained on A3 steel by hot dipping aluminum(HDA) at 720 ℃ for 6 min and micro-plasma oxidation(MPO) in alkali electrolyte. The surface morphology, element distribution and interface structure of composite coatings were studied by means of XRD, SEM and EDS. The results show that the composite coatings obtained through HDA/MPO on A3 steel consist of four layers. From the surface to the substrate, the layer is loose Al2O3 ceramic, compact Al2O3 ceramic, Al and FeAl intermetallic compound layer in turn. The adhesions among all the layers are strengthened because the ceramic layer formed at the Al surface originally, FeAl intermetallic compound layer and substrate are combined in metallurgical form through mutual diffusion during HDA process.Initial experiment results disclose that the anti-corrosion performance and wear resistance of composite coating are obviously improved through HDA/MPO treatment.

Effect of diffusion on coating microstructure and oxidation resistance of aluminizing steel

The effect of diffuse treatment on coating microstructure and oxidation resistance at high-temperature of hot-dip aluminum were studied by means of TEM, SEM and XRD. The results show that, the diffusion temperature has significant effect on structure of coatings and its oxidation resistance. After diffusion at 750 degreesC, the coating consists of thick outer surface layer (Fe2Al5+ FeAl2), thin internal layer (FeAl + stripe FeAl2), and its oxidation resistance is poor. After diffusion at 950 degreesC, the outer surface layer is composed of single FeAl2 phase, the internal layer is composed of FeAl phase, and its oxidation resistance declines due to the occurrence of early stage internal oxidation cracks in the coating. After diffusion at 850 degreesC, the outer surface layer becomes thinner and consists of FeAl2 Fe2Al5(small amount), the internal layer becomes thicker and consists of FeAl+spherical FeAl2, and the spheroidized FeAl2 phase in the internal layer and its existing in FeAl phase steadily improve the oxidation resistance of the coating.

Finite element analysis on stresses field of normalized layer thickness within ceramic coating on aluminized steel

Multilayer ceramic coatings were fabricated on steel substrate using a combined technique of hot dipping aluminum(HDA) and plasma electrolytic oxidation(PEO). A triangle of normalized layer thickness was created for describing thickness ratios of HDA/PEO coatings. Then, the effect of thickness ratio on stresses field of HDA/PEO coatings subjected to uniform normal contact load was investigated by finite element method. Results show that the surface tensile stress is mainly affected by the thickness ratio of Al layer when the total thickness of coating is unchanged. With the increase of A] layer thickness, the surface tensile stress rises quickly. When Al2O3 layer thickness increases, surface tensile stress is diminished. 'Meanwhile, the maximum shear stress moves rapidly towards internal part of HDA/PEO coatings. Shear stress at the Al2O3/Al interface is minimal when Al2O3 layer and Al layer have the same thickness.

Structure and mechanical properties of ceramic coatings fabricated by plasma electrolytic oxidation on aluminized steel

Ceramic coatings were formed by plasma electrolytic oxidation (PEO) on aluminized steel. Characteristics of the average anodic voltages versus treatment time were observed during the PEO process. The micrographs, compositions and mechanical properties of ceramic coatings were investigated. The results show that the anodic voltage profile for processing of aluminized steel is similar to that for processing bulk Al alloy during early PEO stages and that the thickness of ceramic coating increases approximately linearly with the Al layer consumption. Once the Al layer is completely transformed, the FeAl intermetallic layer begins to participate in the PEO process. At this point, the anodic voltage of aluminized steel descends, and the thickness of ceramic coating grows more slowly. At the same time, some micro-cracks are observed at the Al2O3/FeAl interface. The final ceramic coating mainly consists of gamma-Al2O3, mullite, and alpha-Al2O3 phases. PEO ceramic coatings have excellent elastic recovery and high load supporting performance. Nanohardness of ceramic coating reaches about 19.6 GPa. (c) 2007 Elsevier B. V. All rights reserved.

Formation and Structure of Composite Coating of HAD and Micro-Plasma Oxidation on A3 Steel

Composite coatings were obtained on A3 steel by hot dipping aluminum(HAD) at 720 degreesC for 6 min and micro-plasma oxidation (MPO) in alkali electrolyte. The surface morphology, element distribution and interface structure of composite coatings were studied by means of XRD, SEM and EDS. The results show that the composite coatings obtained through HAD/MPO on A3 steel consist of four layers. From the surface to the substrate, the layer is loose Al2O3 ceramic, compact Al2O3 ceramic, At and FeAl intermetallic compound layer in turn. The adhesions among all the layers are strengthened because the ceramic layer formed at the At surface originally, FeAl intermetallic compound layer and substrate are combined in metallurgical form through mutual diffusion during HAD process. Initial experiment results disclose that the anti-corrosion performance and wear resistance of composite coating are obviously improved through HAD/MPO treatment.

The Effect of the Thickness of Fe2 Al5 Phase Layer at Fe/Al Interface on the Mechanics Behavior of Hot Dip Aluminizing Coating

Hot Dip Aluminized Coatings with different thickness were prepared on Q235 steel in aluminum solutions with different temperature for certain time. Through tensile tests and in-situ SEM observations, the effect of the coating's microstructure on the tensile strength of the samples was studied. It was disclosed at certain aluminum solution temperature,transaction layers mainly composed of Fe2 Al5 phase got thicker with time prolonging, and this changed initial crack's extending direction from parallel with to vertical with stretching direction. The change in crack direction decreased tensile strength of samples, thus made the coating easy to break. It was concluded that the existence of thick Fe2 Al5 phase layer was the basic reason for the lowering of tensile strength of the coating.

Electron Beam Surface Treatment. Part I: Surface Hardening of AISI D3 Tool Steel

The effects of electron beam surface hardening treatment on the microstructure and hardness of AISI D3 tool steel have been investigated in this paper. The results showed that the microstructure of the hardened layer consisted of martensite, a dispersion

Monte-Carlo simulation of surface crack growth rate for offshore structural steel E36-Z35

The Monte- Carlo method is used to simulate the surface fatigue crack growth rate for offshore structural steel E36-Z35, and to determine the distributions and relevance of the parameters in the Paris equation. By this method, the time and cost of fatigue crack propagation testing can be reduced. The application of the method is demonstrated by use of four sets of fatigue crack propagation data for offshore structural steel E36-Z35. A comparison of the test data with the theoretical prediction for surface crack growth rate shows the application of the simulation method to the fatigue crack propagation tests is successful.

Numerical Simulation of Transient Thermal Features in the Remelting Processing of Stainless Steel

应用有限元方法对层流等离子体射流不锈钢表面重熔工艺中的瞬态热物理现象进行了数值模拟研究.针对不同加热距离,确定了材料熔化和凝固过程中的瞬态温度场、温度梯度和凝固率的时间和空间分布特征.通过引入等效温度面积密度概念,研究了不锈钢重熔热处理的适合条件.结果表明,9～13mm的范围是较为适宜的加热距离,该结果与试验观察基本符合.