An alternative approach in ceramic shell investment casting of AZ91D magnesium alloy: In situ melting technique

In this research, the possibility of ceramic shell investment casting of a magnesium alloy using in situ melting technique was explored. AZ91D granules were charged into shell investment mould and in situ melted under various processing parameters including heating temperature, flux application, shell mould thickness and permeability. Scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction techniques were used to characterise the cast samples. Thermal analysis was employed to further investigate the effect of mould thickness on the solidification behaviour of the metal. It was found that mixing flux with the granules not only reduced the temperature at which melting can be achieved, but it also contributed to produce castings with acceptable surface quality. The use of thinner mould provided higher solidification rate, which is believed to favour in situ melting of the granules. It enabled melting of the granules at 650 °C, which in turn helped to suppress the mould–metal reaction and produce castings with good surface quality. Shell mould permeability showed no influence on suppressing the mould–metal reaction at 650 °C.     

石墨炉原子吸收光谱法测定P91钢中Sb的含量

介绍了采用石墨炉原子吸收光谱法测定P91钢中Sb的含量。试样以盐酸、硝酸和高氯酸溶解,通过高氯酸冒烟除去氯化物。将试液移入石墨管中原子化,用Sb无极放电灯作光源,用原子吸收光谱仪于Sb217.6nm波长处测量其吸光度。     

Microstructural and textural evolution during large strain hot rolling (LSR) of Mg–Al (AZ31) alloy

Abstract

Grain refinement has been achieved through large strain rolling (LSR) in Mg AZ31 alloy. The evolution of microstructure and texture has been found to be dependent on the amount of reduction. After the critical amount of reduction, grain refinement proceeds through continuous dynamic recrystallisation (CDRX).     

The effect of biomass co-combustion in a CFB boiler on solids accumulation on surfaces of P91 steel tube samples

The paper describes the results of industrial tests associated with the co-combustion of biomass and agromass with coal and focused on diagnostics and assessment of the effect of fuel type on the accumulation of solids on the surface of P91 steel tube samples placed in the vicinity of steam superheater in a commercial large-scale (over 400 MW) circulating fluidized bed boiler.Analysis of samples indicated the effect of fuel type, as well as the location of the area on the steel tube on the rate of solids accumulation. Alkali compounds were quite uniformly distributed at both front and rear sections of the steel tube samples, while calcium and sulfur components were mainly found in the front area. The average solids accumulation rates calculated for the rear areas of P91 steel tube samples were roughly two times higher than those for the front ones. The highest solids accumulation rate on the front area (1.04·10⁻⁹ ms⁻¹) was calculated for the case of agromass co-combustion with coal.     

Using small punch test data to determine creep strain and strength reduction properties for heat affected zones

Abstract

The small punch (SP) test is a miniature technique that can provide information on creep performance of local features in welded components. However, the multiaxial stress and deformation history in SP test means that it can be a challenge to interpret the test results in a way that is comparable to uniaxial standard creep testing. This work aimed to compare SP test results from as new and service exposed P91 (9Cr–1Mo–V–Nb) base material (BM) and heat affected zones (HAZ) to uniaxial creep testing results from welded new and service exposed material. Two methods are proposed for predicting the uniaxial strain response for any zone of the weld: one alternative is to apply the SP rupture data and to accommodate the shape of the uniaxial base material creep curve to that of the zone of interest in the SP test. The other alternative is to use the SP deflection data and an appropriate translation function to the uniaxial creep curve. In both cases, the Wilshire creep equations have been used as the rupture model. The approach will also predict the stress reduction factors of welds and its constituent parts (including the subzones of HAZ). It is proposed that the approach is used to provide the local constitutive creep models for component assessment by finite element analysis (FEA).     

Microstructure development and thixoextrusion of magnesium alloy prepared by repetitive upsetting-extrusion

Thixoextrusion involves processing alloys with a spheroidal microstructure in the semi-solid state. Before thixoextrusion, repetitive upsetting-extrusion (RUE) is introduced into the strain induced metal activation (SIMA) process to predeform AZ80 magnesium alloy. Microstructure evolution of RUE formed AZ80 magnesium alloy during partial remelting is studied at temperatures for times. Tensile mechanical properties of thixoextruded components are determined and compared with those of AZ80 magnesium alloy thixoextruded from starting material produced by casting. The results show that with increasing number of RUE passes solid grain size decreases and the rate of liquation is improved. Prolonged holding time results in grain coarsening and the improvement of degree of spheroidization. The variation of the solid grains with holding time obeys the Lifshitz, Slyozov and Wagner law. Increasing the heating temperature is favorable for the formation of spheroidal solid grains. The tensile properties for AZ80 magnesium alloy thixoextruded from starting material produced by RUE are better than those of AZ80 magnesium alloy thixoextruded from starting material produced by casting.     

提高热连轧板板形质量的方法与措施

通过分析楔形、凸度、平直度等板形控制机理,结合莱钢热连轧工艺装备和板形质量特点,优化原始辊型、换辊周期、负荷分配等工艺,制定出与产品质量相关联的换辊制度、换辊模式、轧制计划安排等控制策略,改善了产品断面形状,板凸度由-20～250μm改善到30～70μm,板形异常卷控制在0.25%以下,薄规格比重提高到总产量的30%～40%。     

流变铸造AZ91-Sn合金的组织演化、拉伸行为及磨损性能

研究了不同Sn含量流变铸造AZ91合金的组织演化、拉伸行为及磨损性能。结果表明：Sn的合金化改变了Al在镁基体中的固溶度,并且显著细化了微观组织。加入0.8%（质量分数,下同）的Sn后,合金平均晶粒尺寸从105.0 μm降至42.1 μm。高熔点的金属间化合物为析出相提供了异质形核点,这些弥散析出的第二相在流变凝固过程中有效地细化了镁基体。弥散分布的第二相抑制了枝晶组织生长,从而进一步提升了合金的力学性能。随着Sn含量的增加,合金磨损率显著降低,磨粒磨损逐渐消失。3.0%Sn合金化的流变铸造AZ91合金具有最高的抗拉伸强度以及最好的耐磨损性能。     

均匀化退火处理对AZ91镁合金轧制成形行为的影响

为改善铸态AZ91镁合金组织不均匀性,提高其轧制成形能力,本文研究了均匀化退火处理对AZ91镁合金轧制变形前后微观组织及力学性能的影响。实验结果表明：均匀化退火处理可以有效改善合金组织中第二相分布;经400℃、多道次轧制后,沿晶界附近分布的细长条状Mg₁₇Al₁₂相数量显著减少,部分脆性Mg₁₇Al₁₂相发生断裂,以小颗粒状弥散分布于晶界附近和基体内部。均匀化后轧制组织比原始轧制组织强度略有提高,而伸长率提高达50%。轧制后的拉伸断口形貌也显示合金塑性得到明显改善。这为后续进一步研究AZ91镁合金在不同工艺参数条件下的轧制成形奠定基础。     

AZ31镁合金热轧边裂预判模型研究

采用等压法,通过等温热压缩实验获得了AZ31镁合金变形温度和应变速率分别在473~673 K和0.005~5 s-1条件下对临界断裂应变的影响规律,以及Zener-Hollomon表达式,据此针对AZ31建立了临界断裂应变与变形温度和应变速率间的基本模型;在此基础上,基于镁合金轧制边裂的基本机理,引入CockcroftLatham断裂准则,建立了含有材料变形激活能和基本轧制工艺参数的AZ31镁合金轧制边裂预判模型;并通过相同条件下有限元模拟和热轧试验分别得到沿板宽方向损伤值和边部裂纹深度,以此对所建立的边裂预判模型进行验证,结果显示所建立边裂预判模型的预测值和实测值平均误差为11.3%。