Influence of porosity on the sliding wear behavior of sintered Fe-1.5Mo-0.7C steels

Fe-1.5Mo-0.7C steels with different porosity can successfully be prepared by using traditional compacting, vacuum sintering, and in part Hot Isostatically Pressing (HIPing). Their dry sliding wear behavior in both as-sintered and heat treated states were investigated. When porosity is lower than 6.2%, further decreases of porosity have less influence on the wear coefficient of both as-sintered and heat treated steels. Pores in the sintered steels collect the debris during the rubbing process, and therefore the disadvantage in wear process due to the poor hardness and mechanical strength caused by high porosity is partly compensated for. During dry sliding the as-sintered steels have three types of wear mechanisms (i.e., oxidational wear, abrasive wear, and delamination wear), while oxidational wear and delamination wear are the main regimes in heat treated steels. Oxidation leads to the wear of sintered steels and in the meantime the oxides attached to the rubbing surface further lower intense wear of the rubbing system. Abrasive wear and delamination wear, which result in flake debris, are responsible for high wear coefficients.     

Influence of porosity on fatigue cumulative damage of sintered iron

Fatigue represents a common mechanical condition during service for materials used in structural components. This type of loading becomes particularly critical when alternating stress amplitude may be expected to vary, or change, in some way during the service life. Such variations and changes in load amplitude make the direct use of standard S-N curves inapplicable, because these curves are developed and presented for constant stress amplitude operation. In the present paper, results of cumulative damage under fatigue of porous materials are reported. The specimens used were produced by compacting and sintering iron powder. Their as-sintered porosity contents were P ₀ = 4.1 and 12.4 %. Uniaxial fatigue tests (tensile-compression) were carried out at room temperature on a hydraulic testing machine.     

Study of shrinkage porosity in spheroidal graphite cast iron

This study aims at identifying the relationship between the shrinkage cavities and the solidification structure in spheroidal graphite cast iron. Cast samples specially designed to contain shrinkage cavities were used. The solidification macrostructure was revealed using the Direct Austempering After Solidification method, while the solidification microstructure was revealed by using colour etching. At the midsection of the pieces, the shrinkage cavities and the solidification structure were observed jointly. The study showed that the classification of shrinkage porosity found in literature does not correspond to the ductile iron solidification model recognized by most of the scientific community. Early solidification models, and therefore shrinkage formation mechanisms, were proposed in instances when there was not a thorough knowledge of the morphology of the solid phases during solidification. Nowadays, defects formation mechanisms can be described with higher accuracy. Therefore, an updated classification of shrinkage porosity for spheroidal graphite iron is proposed.     

优化边界结构改善烧结Nd-Fe-B永磁材料的力学性能

在富稀土辅合金中添加Co,Nb,Ti,Ga,Al等元素,用双合金工艺制备一系列不同辅合金含量的烧结Nd-Fe-B磁体,对力学性能进行系统研究。结果表明,在主合金(NdDyTb)12.69(FeCoNb)84.01B6.00的基础上添加3.0wt%的(NdDyTb)25(FeCoNbTiGaAl)68B7富稀土辅合金,其抗弯强度从316MPa提高到344MPa,抗弯强度的改善来自于边界结构的改善以及一些新相的生成。当辅合金含量达到8wt%,合金的强度又下降为321MPa。还研究了热处理工艺对Nd-Fe-B永磁材料力学性能的影响,含3.0wt%辅合金的磁体抗弯强度由烧结态的291MPa提高到最佳回火态的344MPa,又下降为非最佳回火态的304MPa。     

Damage assessment in AISI/SAE 8620 steel and in a sintered Fe-P alloy by using acoustic emission

Mechanical behavior of an AISI/SAE 8620 steel and of a sintered Fe-P alloy has been investigated using acoustic emission signals. Four-point bending tests were carried out using strain gages to measure the deformation. The onset of plastic strain was determined through the acoustic signal. The acoustic signal of the sintered material was very strong and increased continually with external load. The correlation between microstructure and signal acoustic is discussed.     

Statistical model of magnetization reversal in Nd-Fe-B sintered magnets

Statistical model of magnetization reversal was used to simulate the magnetization reversal behavior in the sintered Nd-Fe-B magnets with double grain-size distributions due to the abnormal grain growth (AGG). The magnetic properties and mechanical properties due to the formation of AGG grains in Nd-Fe-B sintered magnets were tested. The results show that the magnetic properties, especially the rectangularity were severely deteriorated after the formation of the AGG grains and a step was shown on the demagnetization curve, and the occurrence of AGG mayaccount for the poor rectangularity and existence of the step on demagnetization curve according to the statistical model of magnetization reversal. The fracture toughness and bending strength are lowered because of the stress concentration in the AGG grains. The SEM images show that the formation of AGG grains is caused by the solid sintering due to the absence of RE-rich phase. Statistical model of magnetization reversal can qualitative by explain the dependence of the magnetization reversal behavior on the grain size in the Nd-Fe-B sintered magnets.     

The influence of cast structure on the austempering of ductile iron.: Part 2: Assessment of model calculations of microsegregation

Solute concentration line scan measurements are reported for two ductile irons of similar composition but different nodule counts. Si and Cu are shown to segregate towards graphite nodules and Mn and Mo towards intercellular boundaries. The degree of segregation is shown to decrease with increasing nodule count as predicted by the model described in Part 1. However, the complex profiles observed in the line scans preclude their use for quantitative assessment of the model predictions except for a few cases. A statistical analysis of the experimental data is applied to the present measurements to make the results independent of the analysis position in the microstructure. Solute concentration vs. normalised distance curves obtained from the analysis show good agreement with curves calculated using the model.     

The influence of chromium on mechanical properties of austempered ductile cast iron

An investigation was carried out to examine the influence of microstructure and chromium on the tensile properties and plane strain fracture toughness of austempered ductile cast iron (ADI). The investigation also examined the growth kinetics of ferrite in these alloys. Compact tension and round cylindrical tensile specimens were prepared from ductile cast iron with Cr as well as without Cr. These specimens were then given four different heat treatments to produce four different microstructures. Tensile tests and fracture toughness tests were carried out as per ASTM standards E-8 and E-399. The crack growth mechanism during fracture toughness tests was also determined. The test results indicate that yield strength, tensile strength, and fracture toughness of ADI increases with an increase in the volume fractions of ferrite, and the fracture toughness reaches a peak when the volume fractions of the ferrite are approximately 60% in these alloys. The Cr addition was found to reduce the fracture toughness of ADI at lower hardness levels (<40 HRC); at higher hardness levels (≥40 HRC), the effect of chromium on the fracture toughness was negligible. The crack growth mechanism was found to be a combination of quasi-cleavage and microvoid coalescences, and the crack trajectories connect the graphite nodules along the way.     

Effect of iron on microstructure and mechanical properties of primary AlSi7Mg0.3 alloy

Effect of Fe on microstructure and mechanical properties of the primary AlSi7Mg0.3 alloy and the potential of Mn addition to counteract any adverse effects was investigated in the present work. The primary AlSi7Mg0.3 is a better alloy than its counterpart with twice as much Fe. β platelets grow twice as big when the Fe concentration is doubled. This, in turn, increases shrinkage porosity and leads to a 3-fold decrease in the tensile elongation values. Adding an equal amount of Mn helps to modify the β platelets into more compact α particles and also reduces shrinkage porosity. While these structural changes are reflected by a modest improvement in the mechanical properties, Mn addition fails to offer a full recovery in the ductility of the AlSi7Mg0.3 alloy. Hence, limiting the Fe content of the primary AlSi7Mg0.3 alloy to 0.12 wt% is worthwhile and pays off with superior microstructural features and mechanical properties.     

不同钛、铁含量对熟料烧结和溶出的影响

通过研究熟料烧结的生料配比及烧结工艺过程,分析熟料中钛、铁含量对熟料烧结及溶出的影响。指出在碱比一定的情况下,熟料中钛、铁含量对熟料烧结及溶出的影响。熟料中钛含量越高,熟料的溶出率越低,当碱比为0.82左右时,铁含量升高,熟料的溶出率也升高。     

Influence of shrinkage porosity on fatigue performance of iron castings and life estimation method

Shrinkage porosity exists more or less in heavy castings, and it plays an important role in the fatigue behavior of cast materials. In this study, fatigue tests were carried out on the QT400-18 cast iron specimens containing random degrees of shrinkage porosity defect. Experimental results showed that the order of magnitude of life scattered from 103 to 106 cycles when the shrinkage percentage ranged from 0.67% to 5.91%. SEM analyses were carried out on the shrinkage porosity region. The inter-granular discontinuous, micro cracks and inclusions interfered with the fatigue sliding or hindering process. The slip in shrinkage porosity region was not as orderly as the ordinary continuous medium. The shrinkage porosity area on fracture surface(SPAFS) and alternating stress intensity factor(ASIF) were applied to evaluate the tendency of residual life distribution; their relationship was fitted by negative exponent functions. Based on the intermediate variable of ASIF, a fatigue life prediction model of nodular cast iron containing shrinkage porosity defects was established. The modeling prediction was in agreement with the experimental results.     

The influence of cast structure on the austempering of ductile iron. Part 3. The role of nodule count on the kinetics,microstructure and mechanical properties of austempered Mn alloyed ductile iron

Austempering kinetic measurements and mechanical property measurements are reported for irons with different Mn contents and different nodule counts after austenitising at 870 °C and austempering at 375 °C. It is shown that increasing nodule count, which reduces segregation and changes the size and distribution of intercellular boundaries, increases the interphase boundary area between graphite and matrix and decreases the continuity of the unreacted austenite in the intercellular boundary. This accelerates the stage I reaction which broadens the heat treatment window and moves it to earlier austempering times. A high nodule count can be used to counter the delay of the stage I reaction caused by Mn additions used to increase the hardenability of the iron. A high nodule count produces a finer, more uniform ausferrite structure that increases the strength, ductility and impact energy of the austempered iron.     

铁粉焊条的焊接气孔分析

为了确定气孔的性质,采用悬浮熔融气相色谱仪,测定了气孔中气体的成分,并利用扫描电镜观察了内部气孔的形貌.结果表明,不论是酸性焊条还是碱性焊条,其焊缝气孔中的气体都是CO和H₂的混合气体,且在气孔的形成过程中CO起着更主要的作用.焊缝内部气孔的形状主要有两种,一种呈长虫状,多分布在焊缝厚度方向的中上部位;另一种呈海螺状,常分布在焊缝根部附近.试验还表明,提高焊条烘干温度时,碱性焊条的扩散氢量减少,气孔的数量也减少;酸性焊条则不同,焊缝中的扩散氢虽然减少了,但气孔数量反而增多了,故不能用扩散氢量的多少来度量气孔的产生与否.     

Effect of elongated graphite on mechanical properties of hot-rolled ductile iron

The mechanical properties of hot-rolled and then annealed ductile iron were evaluated. The deformation of this two-phase material and the effects of the elongated graphite spheres on the mechanical properties and the development of anisotropy of mechanical properties were studied. An attempt was made to describe the anisotropy of tensile strength in terms of the deformation of graphite spheres and of the root curvature, area fraction, and the interbridges that result in disproportional changes of stress concentration, loading capacity, and the tendency to break the interbridges and link the neighboring deformed graphite spheres.     

Microstructure and thermal stability of sintered pure tungsten processed by multiple direction compression

Multiple direction compression (MDC) was conducted on sintered pure tungsten (99.9%, mass fraction) with different reductions at 1423 K. The microstructure, microhardness and thermal stability of the MDC-processed samples were studied by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and differential scanning calorimetry (DSC) compared with those of the initial sintered tungsten. The results show that the dislocation density increases significantly with the reduction of MDC, ranging from 3.08×10¹⁴ m^–2 for the initial sintered tungsten to 8.08×10¹⁴ m^–2 for the tungsten after MDC with the reduction of 50%. The average grain size decreases from 83.8 to 14.7 μm and the microhardness value increases from HV_0.2 417 to HV_0.2 521. The recrystallization temperature for the tungsten samples processed by MDC is approximately constant at around 1600 K. The MDC of sintered tungsten results in a decrease of grain size concurrent with an increase of uniformly distributed nucleation sites, which leads to the improvement of the thermal stability.     

Control of the casting process of grey cast iron by means of the wedge-penetration test

The graph of force versus penetration-depth from a wedge-penetration test gives information on the structure of the phases of the material under test. The simplicity of this test makes it ideal for on-line quality control of cast pieces. When strength is not satisfactory, knowledge about this structure permits a fast correction of the solidification conditions, increasing production capacity. Finally, it is proposed to adopt wedge strength as a standard, instead of converting this strength to tensile strength.     

球墨铸铁行走轮的低温正火工艺

对一种Cu微合金化球墨铸铁行走轮进行了低温正火工艺试验,获得了850 ℃和860 ℃正火后行走轮轮缘和轮辐的显微组织和力学性能数据。分析了合金元素Cu对球墨铸铁组织和性能的影响,以及对奥氏体化及珠光体形成相变温度区间带来的变化。结果表明,试验含Cu的球墨铸铁行走轮850 ℃正火就可以获得95%珠光体,达到完全正火的状态,力学性能符合QT650-4牌号的要求。比较了850 ℃和860 ℃正火后组织和性能,虽然都能满足要求,但860 ℃正火后表面硬度较850 ℃正火的高,机械切削性能相对较差。说明试验行走轮正火工艺温度窗口较窄,因此,选择850 ℃正火工艺为佳。该热处理工艺已在工程上得到应用,生产的行走轮已批量出口。     

金属纤维烧结毡力学本构关系的实验研究

本文实验研究了316L不锈钢纤维烧结毡的变形能,杨氏模量,强度等力学性能与材料相对密度之间的本构关系。研究表明金属纤维烧结毡沿面内方向的拉伸断裂能和压缩变形能与相对密度分别成线性关系和幂大于1的抛物线关系。金属纤维烧结毡沿面内方向拉伸或压缩时的模量和强度与材料相对密度均成线性关系。这反映了金属纤维烧结毡受面内方向应力时结点间纤维骨架以拉伸(压缩)变形为主。     

微合金化铸铁同质焊条焊接工艺

应用微合金化铸铁同质焊条,采用小电流打底、大电流连续焊工艺,研究了预热温度与焊缝组织及性能之间的相关性.结果表明,微合金化铸铁焊条石墨化能力强,焊缝白口倾向小;小电流打底、大电流连续焊工艺可有效地减小熔深,在很大程度上抑制了熔合区白口的产生.微合金化铸铁焊条可实现常温焊接.预热温度小于200℃即可获得组织和性能与母材一致的同质焊缝.随着焊件预热温度的升高,焊缝中的石墨形态由细小的点状逐渐向菊花状、片状过渡,铁素体含量增多,焊缝硬度减小.焊件预热至200℃所获得的焊缝组织由珠光体、铁素体和细片状石墨及菊花状石墨组成,熔合区则由珠光体、少量碎块状铁素体及过冷石墨片组成,接头力学性能良好.     

Fluidity and solidification microstructures of flake graphite cast iron in thin sections

Fluidity determinations have been undertaken using a silica tube as the fluidity channel, metal being drawn in to the test tubes by means of a regulated vacuum system. Fe-Si and Ca-Si inoculants were added to a hypoeutectic cast iron melt which was then tested for fluidity. A linear relationship between fluidity length and tube diameter was obtained. Fluidity length increased with increasing suction pressure and with increasing temperature of the molten iron. Fluidity length is proportional to the square root of effective suction pressure. Structures within the fluidity specimens have been classified, from the tip backwards, into three zones, containing ledeburite, mottled and flake graphite structures. The length of each structural zone increases with increasing suction pressure. The total length of the ledeburite zone, plus the mottled zone, is constant for increasing tube diameter. Thus, the flake graphite zone increases linearly with tube diameter. The effects of inoculation on the structure and its fading were evaluated by the length of the flake-graphite zone.