重型卡车铸铁制动鼓开裂失效行为的机理研究

制动装置对于车辆的安全行驶起着至关重要的作用。目前,载重货车所采用的制动装置主要为灰铸铁制动鼓,当汽车在山路或坡道连续刹车行驶时,制动鼓通常会出现裂纹,影响制动效果,甚至引发交通事故。本文以某铸造公司生产的HT250制动鼓为研究对象,对未服役和服役失效制动鼓试样的微观组织和显微硬度进行了分析和比较。结果表明,制动鼓失效后工作面的石墨片变得粗大,且石墨与珠光体界面出现裂纹,石墨片部分有灰色相生成,而侧面的组织无明显变化。此外,失效制动鼓的硬度有所提高。灰铸铁制动鼓开裂失效行机理主要为经多次冷热循环后,石墨与基体间产生微裂纹,经扩展后导致制动鼓开裂失效。     

重载汽车蠕墨铸铁制动鼓的失效分析

对失效的制动鼓工作面进行宏观分析和金相检验,结果表明:蠕墨铸铁制动鼓失效的主要形式是龟裂,龟裂以工作面居多,非工作面基本无裂纹;失效后制动鼓工作面的石墨体积变粗大,部分石墨部位出现裂纹;失效后的蠕墨铸铁金相组织中珠光体分解,发生组织相变,出现大量尺寸较大的马氏体、贝氏体的混合组织;相变后的组织由于膨胀系数不同,在交变热应力的作用下基体部分出现微裂纹,这些基体微裂纹和石墨裂纹是制动鼓开裂的两大裂纹源。     

重型汽车制动鼓开裂分析

重型汽车的制动鼓材料为灰口铸铁,在使用中容易开裂。为此,对开裂的制动鼓进行了宏观检查、金相检验、化学成分分析、硬度测试及断口分析,以揭示制动鼓开裂的原因。结果表明,由于石墨与基体的结合强度弱,且石墨的强度、膨胀系数等与基体存在明显差异,导致裂纹起源于石墨片,并在拉应力、摩擦力的作用下沿制动鼓轴向扩展。     

汽车制动鼓的失效分析

随着汽车载重和速度的增加,使得所需要的制动鼓材料应当发展同时具有高强度、高抗热疲劳性和良好的硬度.通过对失效的江淮某汽车制动鼓取样分析,其失效原因:①材料强度、硬度过低,制动鼓出现磨痕台阶和轴向裂纹;②在制动时发生组织转变,进一步促使网状裂纹的产生.     

汽车制动鼓断裂分析

通过对HT250制动鼓的断裂分析,认为制动鼓在使用过程中的失效是由于制动鼓在铸造时底部存在夹渣,或由于制动鼓在使用过程中产生了热疲劳裂纹,裂纹失稳扩展而导致了断裂.     

消除汽车制动鼓表面斑点的研究

通过对失效的汽车制动鼓取样分析,认为汽车制动时,制动鼓发生了组织转变（相变）。相变的结果导致在制动鼓工作表面出现硬质斑点及裂纹。解决的办法是提高灰铸铁含C量和在灰铸铁中添加微量合金元素。     

客车制动鼓开裂失效分析及对策

对典型开裂失效形式的客车制动鼓化学成分、显微组织和力学性能等进行了检验与分析.结果表明,制动鼓表面出现的黑色硬点、裂纹是成分不合理和制动时摩擦热作用引起的相变所致.     

重卡制动鼓失效分析及铸造工艺研究北大核心

通过对重型载货汽车失效制动鼓进行裂纹失效及热-力耦合仿真分析，发现HT250材料制动鼓由于实际工况最高应力值远大于其疲劳极限经验值，而产生热疲劳裂纹开裂失效；为此，将制动鼓材料优化为RT450，同时进行了铸造工艺优化，获得了内部质量优异的铸件，蠕铁制动鼓服役寿命超过12个月。     

钒钛对灰铸铁制动鼓组织性能的影响

钒、钛都是强碳化物形成元素。研究了钒、钛元素的加入,对灰铸铁制动鼓组织性能的影响。通过对普通制动鼓和加入0.17wt%钒、0.08wt%钛的钒钛制动鼓的金相及力学性能检测,结果表明,碳元素的扩散受到了阻碍,从而抑制了石墨的长大,使得A型石墨含量减少,D型石墨含量增多,石墨的长度变短,削弱了A型石墨对基体的割裂作用。同时,钒和钛生成的碳化物能起到细化晶粒和强化基体组织的作用,显著提高了灰铸铁的抗拉强度及耐磨性,使制动鼓的使用性能大大提高。     

重型车长寿命制动鼓的开发

对重型车制动鼓开裂问题进行了分析,研究表明:(1)重型车制动鼓产生开裂的原因是制动鼓工作温度高,摩擦部位截面存在大的热应力,以及制动鼓材料导热性不足;(2)铸件材料牌号达到HT250并且铸件本体强度190～230 N/mm2,制动鼓强度已足够,使用中不会因为制动鼓强度不足产生开裂问题。试验表明:HT250制动鼓的w(C)量从3.30%提高到3.40%～3.70%,适当加入合金元素,大大改善了制动鼓的抗热裂能力,提高了制动效能,解决了重型车制动鼓使用过程出现的开裂、摩擦面硬斑等失效问题,提高了汽车的安全性。     

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.