组织细化的控制相变技术机理研究

在Gleeble-1500热模拟机上模拟了不同的弛豫析出控制相变（RPC）工艺过程，结合金相、SEM、TEM以及EBSD技术研究了不同变形温度，变量量以及弛豫时间对含Nb,Ti低碳微合金钢最终组织细化的影响。此外Fe-Ni合金被用来研究弛豫过程的位错亚结构变化以析出行为。结果表明，经RPC工艺处理后，贝氏体束可得到明显的细化。弛豫过程中同时存在位 错弛豫，多边形化形成位错胞状亚结构以及应变诱导析出，这两者是引起最终贝氏体组织细化的主要原因。当两种因素互相协调并促进时，细化效果最佳，在本模拟条件下，最佳控制工艺为850℃变形后弛豫60－200s左右。     

高强度低碳贝氏体钢的工艺与组织细化

在超低碳贝氏体钢中，采用弛豫-析出-控制相变(RPC)技术可得到细化的中温转变组织，组织类型为细化的板条贝氏体及少量不规则粒状贝氏体或针状铁素体，与一般控轧空冷和调质处理组织比较，除细化外，所得贝氏体类型及形貌均有所不同，通过这种工艺细化的低碳贝氏体钢板其强度比控轧后空冷或轧后再加热-淬火(调质处理)钢有明显提高。在采用RPC工艺时，轧后弛豫时间长短对最终组织细化程度和形貌也有明显影响，从而造成性能有所差别，终轧后弛豫阶段形成并被应变诱导析出物钉扎的位错胞状组织或亚晶结构是细化相变组织、阻碍贝氏体生长的主要原因，冷却过程中，在贝氏体相变前形成的不规则粒状贝氏体或针状铁索体，分割了压扁的原奥氏体晶粒，同样限制了贝氏体板条柬的长度和宽度。     

低碳微合金钢贝氏体相变及组织细化的研究

针对低碳铌、钛微合金钢,采用Gleeble-1500热模拟机进行了加热、变形及冷却试验,对不同工艺制度下的显微组织进行了分析.结果表明,加热时间较长时,不利于随后冷却奥氏体向等轴铁素体相变,而发生贝氏体相变;在奥氏体区进行不同应变量的变形,随着奥氏体的形变量加大,相变贝氏体的组织细化;变形后适当增加弛豫时间,相变的贝氏体组织细化.     

X80管线钢变形弛豫后的显微组织及析出行为

应用Gleeble-1500D热模拟试验机、维氏硬度计和ZEISS Supra55型场发射扫描电子显微镜研究了不同弛豫时间下X80管线钢的显微组织、硬度及析出物的变化情况。研究结果表明,X80管线钢弛豫后的主要组织类型为贝氏体铁素体+条片状贝氏体,在弛豫过程中显微组织得到细化,并且显微组织的形态发生改变,片条状贝氏体数量增多。在弛豫时间200 s时,硬度出现峰值。随着弛豫时间延长,析出物的数量逐渐增多,并且弥散分布。     

变形对超高强贝氏体钢组织和力学性能的影响

采用低温奥氏体预变形+等温贝氏体相变相结合的工艺,研究了变形对中碳贝氏体钢相变和组织的影响,利用热模拟实验、SEM、TEM、XRD和拉伸实验等分析了变形影响残余奥氏体的微观机理及其对强塑性的影响规律.结果表明,过冷奥氏体在300℃变形20％,不仅可以加速随后等温贝氏体相变,细化贝氏体组织,同时还能增加室温组织中的残余奥氏体及其稳定性.残余奥氏体稳定性同时受C含量和位错密度影响,延长等温时间可以增加奥氏体中C含量;变形可以使奥氏体中位错密度增加,有利于获得稳定性较高的残余奥氏体,从而优化超高强贝氏体钢综合性能,制备的中碳超高强贝氏体钢抗拉强度为1733 MPa,延伸率达到15.7％.     

含Nb微合金钢时效过程中双硬化峰的形成

运用热模拟方法,结合萃取复型、硬度测量及透射电子显微镜观察(TEM)等方法,研究了含Nh微合金钢经850℃热变形并弛豫不同时间后、水淬并在525℃时效时的硬度变化及析出行为。结果表明：变形奥氏体弛豫一定时间淬火后,在时效过程中出现双硬化峰现象;而奥氏体变形后直接淬火或长时间弛豫(1000s)则没有第一个硬化峰出现。对比C—Mn钢的分析结果证明Nh是产生硬化峰的重要原因。电镜观察的初步结果显示,变形奥氏体中形成的细小析出在时效过程中逐渐长大,已粗化的析出颗粒在525℃等温时效时变化不明显;而经过γ→α相变后α相中过饱和的Nb在时效时会重新析出,形成第二个硬化峰。     

改进型无限冷硬铸铁轧辊回火组织转变研究

通过对改进型无限冷硬铸铁轧辊不同回火温度和时间的系列回火试验,用扫描电镜(SEM)、能谱仪(EDS)、X射线衍射仪(XRD)等分析手段,研究回火对铸态组织中亚稳残余奥氏体转变的影响,分析铸态下贝氏体在组织回火中的分解及碳化物析出行为。试验结果表明：改进型无限冷硬铸铁350℃以下温度回火后奥氏体仍然稳定存在;425～450℃温度问回火在下贝氏体针内部析出合金渗碳体,大部分奥氏体转变为上贝氏体组织;500℃回火后得到回火索氏体组织。     

低碳20Mn2SiVB钢贝氏体相变及其TRIP效应

用光学显微镜、扫描电子显微镜及X射线衍射仪对20Mn2SiVB钢在贝氏体区不同温度等温不同时间所获得的组织和形态进行了研究。试验表明,20Mn2SiVB钢在贝氏体等温转变时,首先在奥氏体晶界析出贝氏体铁素体,随着等温时间的延长,铁素体板条增多,分割奥氏体晶粒,形成贝氏体铁素体和其板条间的富碳奥氏体岛;在920 ℃奥氏体化,420 ℃贝氏体区等温不同时间后空冷所获得组织为：无碳化物贝氏体、粒状贝氏体、残留奥氏体和马氏体,各相的体积分数随着保温时间的不同有所变化。在920 ℃奥氏体化420 ℃等温5 min后,试样可获得较好的综合性能,具有一定的TRIP效应,其R_m≈ 1090 MPa;A。≈ 15.4%     

弛豫及冷却过程对大变形管线钢组织及性能的影响

利用Gleeble-3800模拟X70大变形管线钢奥氏体再结晶区和非再结晶区的双道次变形、弛豫及加速冷却过程.采用金相显微镜、SEM等试验方法,研究了空冷驰豫时间及冷却速度对显微组织的影响.结果表明:从770℃弛豫到700℃后开始快冷的冷速为8.1℃/s时,驰豫时间(53.8～100 s)对组织、硬度的影响较小;不同弛豫时间对应的组织均为双相组织,马氏体/贝氏体呈岛状分布在铁素体基体上,铁素体晶粒大小不一,碳化物多且密集;从770℃弛豫到700℃后采用水冷时,随着驰豫时间延长,双相组织中铁素体增多,硬度降低;相同驰豫时间及水淬方式冷却试样的硬度及马氏体/贝氏体数量明显高于8.1℃/s冷却的试样.     

等温淬火时间对贝氏体球铁组织和性能的影响

将球墨铸铁铸件高温奥氏体化后于300℃等温淬火并保温不同时间,得到贝氏体球墨铸铁,研究了保温时间对贝氏体球铁的组织和力学性能的影响。结果表明,当保温时间较短时,奥氏体不稳定,冷却至室温或者经过机加工后会部分转变为马氏体,硬度值较高,但强度及塑韧性较差。适当延长保温时间,会得到稳定的奥氏体+贝氏体组织,具有较好的综合力学性能。保温时间继续延长时,会导致高碳稳定的奥氏体析出碳化物,使贝氏体球铁的强度和塑韧性下降。     

Effects of Post-weld Heat Treatment on Microstructure,Mechanical Properties and the Role of Weld Reinforcement in 2219 Aluminum Alloy TIG-Welded Joints

In as-welded state, each region of 2219 aluminum alloy TIG-welded joint shows diff erent microstructure and microhardness due to the diff erent welding heat cycles and the resulting evolution of second phases. After the post-weld heat treatment, both the amount and the size of the eutectic structure or θ phases decreased. Correspondingly, both the Cu content in α-Al matrix and the microhardness increased to a similar level in each region of the joint, and the tensile strength of the entire joint was greatly improved. Post-weld heat treatment played the role of solid solution strengthening and aging strengthening. After the post-weld heat treatment, the weld performance became similar to other regions, but weld reinforcements lost their reinforcing eff ect on the weld and their existence was more of an adverse eff ect. The joint without weld reinforcements after the post-weld heat treatment had the optimal tensile properties, and the specimens randomly crack in the weld zone.     

The First Phase of the SRIF Industrialized Base in ＂West Area＂ Has Been Put into Operation

After nearly two years＇ tense construction, the first phase of industrialized base of Shenyang Research Institute of Foundry （SRIF）, located at the Tiexi Casting and Forging Industrial Park in the west of Tiexi District, has now been completed and formally put into operation.     

Thermodynamics Study on the Decomposition of Chromite with KOH

Institute of Process Engineering, Chinese Academy of Sciences, China, has proposed a method for oxidative leaching of chromite with potassium hydroxide. Understanding the mechanism of chromite decomposition, especially in the potassium hydroxide fusion, is important for the optimization of the operating parameters of the oxidative leaching process. A traditional thermodynamic method is proposed and the thermal decomposition and the reaction decomposition during the oxidative leaching of chromite with KOH and oxygen is discussed, which suggests that chromite is mainly destroyed by reactions with KOH and oxygen. Meanwhile, equilibrium of the main reactions of the above process was calculated at different temperatures and oxygen partial pressures. The stable zones of productions, namely, K2CrO4 and Fe2O3, increase with the decrease of temperature, which indicates that higher temperature is not beneficial to thermodynamic reactions. In addition, a comparison of the general alkali methods is carried out, and it is concluded that the KOH leaching process is thermodynamically superior to the conventional chromate production process.     

Influence of Heat Treatment on Damping Behaviour of the Magnesium Wrought Alloy AZ61

The effect of isochronal heat treatments for 1h on variation of damping, hardness and microstructural change of the magnesium wrought alloy AZ61 was investigated. Damping and hardness behaviour could be attributed to the evolution of precipitation process. The influence of precipitation on damping behaviour was explained in the framework of the dislocation string model of Granato and Lücke.     

Effect of Rare Earth Doping on Impedance,Modulus and Conductivity Properties of Multiferroic Composites:0.5(BiLa_xFe_(1-x)O_3)–0.5(PbTiO_3)

The Lanthanum-doped bismuth ferrite–lead titanate compositions of 0.5(Bi LaxFe1-xO3)–0.5(Pb Ti O3)(x = 0.05,0.10,0.15,0.20)(BLxF1-x-PT) were prepared by mixed oxide method.Structural characterization was performed by X-ray diffraction and shows a tetragonal structure at room temperature.The lattice parameter c/a ratio decreases with increasing of La(x = 0.05–0.20) concentration of the composites.The effect of charge carrier/ion hopping mechanism,conductivity,relaxation process and impedance parameters was studied using an impedance analyzer in a wide frequency range(102–106Hz) at different temperatures.The nature of Nyquist plot confirms the presence of bulk effects only,and non-Debye type of relaxation processes occurs in the composites.The electrical modulus exhibits an important role of the hopping mechanism in the electrical transport process of the materials.The ac conductivity and dc conductivity of the materials were studied,and the activation energy found to be 0.81,0.77,0.76 and 0.74 e V for all compositions of x = 0.05–0.20 at different temperatures(200–300 °C).     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

Growth Kinetics of Single Inclusion Particle in Molten Melts

On the basis of the single-particle framework, a new theory on inclusion growth in metallurgical melts is developed to study the kinetics of inclusion growth on account of reaction and collision. The studies show that the early growth of inclusion depends on reaction growth and Brawnian motion collision, and where the former is decisive, the late growth depends on turbulence collision and Stokes＇ collision, and where the former is dominant; collision growth is very quick during the smelting process, lessened in the refining process, but nearly negligible in the continuous casting process.     

Theoretical Analysis for the Motion and Temperature of Melt Droplets in Spray Degassing Process

The motion of melt droplets in spray degassing process was analyzed theoretically. The height of the treatment tank in spray degassing process could be determined by the results of theoretical calculation of motion of melt droplets. To know whether the melt droplets would solidify during spraying process, the balance temperature of melt droplets was also theoretically analyzed. Then proof experiments for theoretical results about temperature of melt droplets were carried. In comparison, the experimental results were nearly similar to the calculation results.     

Using Finite Element and Contour Method to Evaluate Residual Stress in Thick Ti-6Al-4V Alloy Welded by Electron Beam Welding

This work was to reveal the residual stress profile in electron beam welded Ti-6Al-4V alloy plates(50 mm thick) by using finite element and contour measurement methods.A three-dimensional finite element model of 50-mmthick titanium component was proposed,in which a column–cone combined heat source model was used to simulate the temperature field and a thermo-elastic–plastic model to analyze residual stress in a weld joint based on ABAQUS software.Considering the uncertainty of welding simulation,the computation was calibrated by experimental data of contour measurement method.Both test and simulated results show that residual stresses on the surface and inside the weld zone are significantly different and present a narrow and large gradient feature in the weld joint.The peak tensile stress exceeds the yield strength of base materials inside weld,which are distinctly different from residual stress of the thin Ti-6Al-4V alloy plates presented in references before.     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.