Papers by Keyword: Precipitation Behavior

Abstract: In this study, we investigated the effect of Si and Mo on the sigma-phase precipitation in S32750 super duplex stainless steel slab. The activity for Mo with increasing Si and Mo was calculated by the Wagner formula, and the equilibrium solidification phase diagrams of S32750 duplex stainless steels with different Si and Mo contents were calculated using the thermo-calc software. The sigma phase precipitated mainly at ferrite/austenite phase boundaries and grew up towards the interior of ferrite phase in S32750 SDSS slab. The area fraction and size of the sigma phases significantly increased with increasing Mo content and Si content. Also, the increment in Mo and Si content affected the Mo concentration in sigma phase. The sample（Mo:3.4%,Si：0.3%） had a lowest sigma-phase area fraction of 2.84% and had lowest Mo content in σ phase.The calculation results showed that the increase of Mo and Si content increased the initial precipitation temperature and maximum precipitation amount of σ phase in S32750 SDSS equilibrium phase diagram. The activity of Mo also increased with increasing Si content and Mo content. That is, Mo and Si elements promoted the precipitation of σ phase.

Abstract: Pre-strain (PS) and pre-aging (PA) treatments are often applied during the preparation of Al-Mg-Si automotive aluminum alloy. In this study, the effect of combined PS and PA on the precipitation behavior and age hardening response for Al-Mg-Si alloys was investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile test, Vickers hardness test, and differential scanning calorimetry (DSC). It was found that the dislocations introduced by PS treatment and the cluster (2) formed during PA treatment effectively inhibited the cluster (1), which further strengthened the inhibition of natural aging hardening at room temperature (RT). The strength increment of the alloys was kept below 10.0 MPa during two weeks RT storage. The dislocations provided heterogeneous nucleation for the precipitates forming and the cluster (2) transformed into β″ strengthening phase during bake hardening treatment. With the acceleration response of the dislocations and the cluster (2), the age hardening response of Al-Mg-Si alloys obviously improved with the denser and larger β″ strengthening phase formed.

Abstract: Normalizing is an effective heat treatment in improving the microstructure and developing the mechanical properties of micro-alloyed steel. The normalizing parameters such as temperature and holding time are the main keys to microstructure and mechanical properties controlling. Therefore, obtaining an optimum combination of mechanical properties must be subjected to an ideal combination of these parameters. Furthermore, adjusting the optimum normalizing parameters must be considered for every chemical composition depending on the critical transformation temperatures. In this work, four micro-alloyed steel alloys containing V (0.008-0.1wt %) and Ti (0.002-0.072) were held on different normalizing temperatures for 30 minutes. The first holding temperature was carried out just above the Ac3 temperature and the second was carried out above the Ac3 by 100°C (Ac3+100°C). With the controlled normalizing condition, V-Ti-micro-alloyed steel alloy has produced an ultra-fine structure of grain size 2.2 microns and combined high strength of 725 MPa YS, 1058 MPa UTS and good ductility of 20%.

Abstract: The grain oriented silicon steel slabs were reheated at 1323K~1623K with different soaking time in this study. The precipitation behavior of the second phase particles in the steel slabs was investigated by using SEM/EDS combined with other analytic tools. The results show that the main precipitations in samples are MnS and AlN bearing a little of Cu_xS, and they frequently aggregate with each other by two or three types of precipitations. The quantities of the above three types of precipitations are up to 90% in each sample. These complex precipitations change into AlN bearing a little of Cu_xS or single AlN after being reheated. The shapes of MnS or AlN havent the common spherical or quadrilateral characteristic outline. There are a few of spherical or irregular shape inclusions such as SiO₂, Al₂O₃, TiO₂, CaO, CaS and their complex in the samples. Part of MnS or AlN can also grow around with these particles and then dissolves after being reheated. The sizes of the particles are main in the range of 0.05~0.2μm. There are no inclusions larger than 5.0μm in size observed in the fields of view. The volume density of the precipitations 0.05~0.2μm in size extracted from the quenched samples at 1523K using non aqueous solution electroextraction are 4.04×10⁴ /mm³, 4.73×10⁴ /mm³, 3.70×10⁴ /mm³, 3.33×10⁴ /mm³, 3.10×10⁴ /mm³ and 1.56×10⁴ /mm³ as the corresponding reheating time is 10min, 30min, 60min, 90min, 120min and 240min at 1523K respectively. The average size of them is 0.108μm. It is more precise to statistics the volume density of the particles in the steel by electrolysis extracting method than that of using metallogrphical analysis. No obvious microstructure variation was observed in the quenched samples after annealing different time. When the reheating temperature reaches 1523K and 1623 K, the grain size grows more easily and the grain boundary gets smoothly. The grain boundary of the samples becomes clear with the increase of the annealing temperature during the same annealing time. The reheating temperature in the present slab is chosen at 1523K and the reheating time is kept for 120 min, which will insure the particles in the slab to dissolve more completely and avoid grain growth abnormally at high temperature.

Abstract: The new type SFG HSS (super fine grain, high strength steel sheet) has been developed by adding solid-solution strengthening elements to conventional IF steel such as Si, Mn. The precipitation behavior of the second phase particles was observed and studied by a carbon extraction replica technique used transmission electron microscopy (TEM). The shape of the small second particles is similar to spherical and ellipse, the sizes of which are 10~30nm. It is seen that the particles are dispersed on the matrix. The precipitate composition of small particles is Nb (CN) and the precipitate composition of large particles is NbC examined by energy dispersive X-ray (EDX). It is noted that the yield strength is low as well as the tensile strength is high by the PFZ which is free of precipitate called precipitated free zone on the one side of the grain boundary. The results of microstructure shows that the second phase particles pinned on grain boundary not only can inhibit the grain growth, but also the grain can be fined.

Abstract: The precipitation behavior of the second phase particles was observed and studied by a carbon extraction replica technique used transmission electron microscopy (TEM) for Nb-bearing dual-phase steel. It is found that there are more second phase particles on the surface than that of in the center of sample. The mainly smaller particle which sizes of is below 20nm show the shape of spherical and ellipse. The sizes of larger particles are range of 20nm~40nm and the shape of them present rectangle or ellipse. The particles are dispersed on the matrix and the particle boundary is clear. The precipitate composition of particles are all Nb(C、N) examined by energy dispersive X-ray (EDX). The results of microstructure shows that the second phase particles pinned on grain boundary not only can inhibited the grain growth, but also the grain can be fined during the heating and cooling course.

Abstract: The precipitation behavior in an Al-Cu alloy isothermally annealed at 373K～493K was studied, using the Vickers microhardness tests, Differential scanning calorimetry(DSC) and Transmission electron microscope(TEM) observations. An additional endothermic peak was found in DSC measurements which may be attributed to the θ″-phase independent from the formations of the G.P.(Ⅱ) and the θ′-phase at the aging temperatures. High resolution transmission electron microscopy (HRTEM) revealed that the G.P.(Ⅰ) is formed at a very early stage of isothermal aging at 463K. Comparing the Vickers microhardness and TEM images, it was concluded that G.P.(Ⅱ) is mainly responsible for the peak hardness.