ADI的疲劳强度

论述了ADI的疲劳强度。ADI的疲劳强度优于普通球铁,具有与锻钢相当或比锻钢更好的疲劳强度。表面滚压或喷丸可以进一步增加ADI的疲劳抗力而与气体氮化钢和表面碳化处理钢相匹敌,使ADI完全可以在一定应用中替代碳钢和低合金钢铸件,锻钢件。     

Nb对灰铸铁热疲劳性能的影响

采用Uddeholm热疲劳方法测定了Nb对灰铸铁热疲劳性能的影响,试验结果表明：（1）Nb的加入能够提高灰铸铁的抗热疲劳性能,随着w（Nb）的增加,试样最大裂纹深度和宽度都逐渐减小;（2）由于Nb细化石墨使裂纹源减少和裂纹扩展途径变细,Nb元素能够改善材料表面经热疲劳处理后恶化的性能。     

Phase transformation and fatigue properties of austempered ductile irons

A series of studies were carried out regarding ausferrite transformation, the growth of ferrite from austenite, during isothermal holding. The mechanical properties associated with microstructures of alloyed and unalloyed austempered ductile irons (ADIs) were also investigated. Heat released from ausferrite transformation has been quantitatively measured in a previous study. The effect of alloying elements on the heat evolution rate and on the morphology of ausferrite has also been discussed. Mechanical property testing was carried out on specimens of ductile iron austempered at 593 K and 633 K for 2 hrs. Tensile properties and high-cycle fatigue life tests were also made. In addition, phase transformation of ADIs during ultrasonic vibration treatment was also investigated. SEM micrographs of deformed samples indicate and confirm the occurrence of stress-induced transformation of ADIs. The alloyed ADIs isothermally-held at 633 K develop an optimum tensile strength, elongation and superior impact toughness and good high-cycle fatigue strength. These properties are attributed mostly to a large amount of dispersed blocky austenite. This retained austenite may become work hardened during deformation.     

Estimation of pearlite fraction in ductile cast irons by eddy current method

The eddy current method has been used for estimation of the pearlite amount in ductile cast irons. Regression analysis has showed good assessment of the matrix structure of cast irons having different chemical compositions as well as graphite particle size and count by means of the eddy current method. Pearlite content of tested cast irons was shown to have paramount effect on the eddy current probe response from them and predictability of pearlite amount improves for specimens with similar composition or graphite shape.     

Effect of microstructure on tensile and fatigue properties of cast Mg–10Gd–2Y–0·5Zr alloy

Abstract

The microstructure and its effect on tensile properties and fatigue properties of a Mg–10Gd–2Y–0·5Zr (wt-%) cast alloy have been studied. The microstructures of as-cast, solution treated and T6 treated specimens were examined by optical and scanning electron microscopy (SEM). Tensile properties and fatigue properties of the specimens were determined and fractography was carried out. The SEM examination showed that the precipitates after T6 treatment were mainly distributed at grain boundaries, which accounts for the intergranular brittle fracture observed. The average grain size of the specimens measured after solution treatment varied from 87 to 128 μm. The mechanical tests showed that the tensile strength and low cycle fatigue strength increase with decreasing average grain size, whereas high fatigue strength is less sensitive to grain size. The fractography indicated that ductile and brittle fracture patterns coexist.     

Fatigue of Austempered Ductile Iron with Two Strength Grades in Very High Cycle Regime

In this study, Austempered ductile irons (ADIs) with two different strength grades were produced and the fatigue properties were measured at 10⁹ cycles. The results show that the S-N curves give a typical step-wise shape and there is no fatigue limit in the very high cycle fatigue regime. The two grades ADI have the similar fracture behaviors and fatigue failure can initiate from defects at specimen surface and subsurface zone. On the fracture surfaces of some specimens, the ‘granular-bright-facet’ area with rich carbon distribution is observed in the vicinity of the defect. The microstructure affects the crack behaviors at the early propagation stage. The ADI with upper and lower bainite shows higher fatigue strength compared with the ADI with coarse upper bainite.     

金属型对灰铸铁和球铁力学性能的影响研究

采用铜合金金属型和铸铁金属型浇注灰铸铁和球铁Y形试块，加入不同量的Mn、Ti、Cu和Sn，研究金属型材料和添加合金对灰铸铁和球铁抗拉强度和疲劳强度的影响。结果发现：与采用铸铁金属型浇注相比，未加合金、采用铜合金金属型浇注的灰铸铁和球铁的力学性能略高。采用铜合金铸型浇注时，由于冷速较快，使石墨和基体组织细化的作用较强，添加合金元素改善力学性能的作用较为明显，而且反复浇注时，铜合金金属型的热应力较小。因此，灰铸铁和球铁采用铜合金金属型铸造比采用铸铁金属型有利。     

Ni对高锰奥氏体球墨铸铁组织和性能的影响

为减少奥氏体铸铁中镍的添加量,研究了3种不同镍含量的Fe-13Mn-3.5C-3Si-3Cu-xNi高锰奥氏体球墨铸铁.铸铁经固溶处理后,奥氏体基体上的碳化物数量随镍含量的降低增多,抗拉强度、抗弯强度、伸长率以及冲击韧度均下降.镍含量为4％和8％时,铸铁表现出优异的综合力学性能,其较高的强度和塑性源于形变过程中的TWIP效应.随着镍含量从8％降为0,铸铁的摩擦系数和磨损率先增大后减小,其主要磨损机理由粘着磨损变为磨粒磨损.研究表明,为保证奥氏体球铁的综合性能,合金中需保留少量的镍元素.     

Thermal fatigue characteristics of plasma duplex treated nodular cast irons

The correlation between the thermal fatigue resistance and microstructure of untreated and plasma surface engineered nodular cast irons was investigated. Both the ferrite and pearlite matrix nodular cast irons were evaluated. During the tests, 60 mm long cylinders with 20 mm diameter were subjected to 1000 high frequency induction heating and water cooling cycles, lasting 7 and 5 s, respectively. The thermal fatigue damage was evaluated by analyzing the crack dimensions and distributions. The results showed that the plasma surface treatment increased thermal fatigue resistance. When thermal cracking occurred, the cracks always nucleated at the surface of the specimen. In addition, the nodular cast iron with a pearlite matrix had a better resistance to thermal fatigue than that of the ferrite matrix.     

Effects of austempering temperature on fatigue crack rate propagation in a series of modified (Cu, Ni, and/or Mo) nodular irons

Studies on austempered nodular cast irons were carried out to establish the optimum isothermic heat treatment at a given chemical composition that rendered the highest fatigue crack propagation resistance. Seven nodular iron chemical compositions with different concentrations of copper, nickel, and or molybdenum were tested at three austempering temperatures achieving ausferritic microstructures. Three-point bend tests for crack growth rates were performed at room temperature in a close loop servo hydraulic machine. Crack opening displacement measurements were performed using a controlled displacement telescope. A simple linear statistical analysis indicated that the lower the austempering isothermal temperature, the higher the fatigue strength of the alloys. Cu and Mo additions along with a good spheroidicity of graphite nodules in the iron favored this effect.     

Effects of Casting Size on Microstructure and Mechanical Properties of Spheroidal and Compacted Graphite Cast Irons: Experimental Results and Comparison with International Standards

The aim of this research was to investigate the effects of casting size (10-210 mm) on the microstructure and mechanical properties of spheroidal (SGI) and compacted (CGI) graphite cast irons. A comparison of the experimental mechanical data with those specified by ISO standards is presented and discussed. The study highlighted that the microstructure and mechanical properties of SGI (also known as ductile or nodular cast iron) are more sensitive to casting size than CGI (also known as vermicular graphite cast irons). In particular, in both types of cast iron, hardness, yield strength and ultimate tensile strength decreased, with increasing casting size, by 27% in SGI and 17% in CGI. Elongation to failure showed, instead, an opposite trend, decreasing from 5 to 3% in CGI, while increasing from 5 to 11% in SGI. These results were related to different microstructures, the ferritic fraction being more sensitive to the casting size in SGI than CGI. Degeneration of spheroidal graphite was observed at casting size above 120 mm. The microstructural similarities between degenerated SGI and CGI suggested the proposal of a unified empirical constitutional law relating the most important microstructural parameters to the ultimate tensile strength. An outstanding result was also the finding that standard specifications underestimated the mechanical properties of both cast irons (in particular SGI) and, moreover, did not take into account their variation with casting size, at thicknesses over 60 mm.     

FATIGUE PROPERTIES OF AUSTEMPERED DUCTILE IRON （ADI） IN WATER ENVIRONMENT

The acicular ferrite in austempered ductile iron (ADI) matrix around graphite was corroded preferentially in wet condition, promoting crack origination and propagation and resulting in the disappearance of ADI fatigue limit. ADI fatigue strength was gradually reduced with increasing the time of test and was reduced by 50% in wet condition at 10^7 cycles compared with the fatigue limit in dry condition.The fatigue strength variation of ferritic ductile iron in wet condition was similar to that of ADI.The ferritic ductile iron, however, has better corrosion resistance so that the fatigue strength was lowered only by 10^7 in wet condition at 10^7 cycles compared with the fatigue limit in dry condition. On the other hand, the fatigue limits of ADI and ferritic ductile iron were dropped by 32% and 25% in tap water dipping 480h/dry condition respectively compared with those in dry condition.The reduction of fatigue limit was attributed to corrosion pits formation correlated with stress concentration, resulting in origination and propagation of fatigue crack.     

Effects of solidification structure on fatigue crack initiation and fatigue strength in Al−Si−Mg cast alloys

Rotating-bending uniaxial fatigue tests and micro-fatigue crack initiation tests were carried out using a permanent mold cast (PMC) and semi-solid die cast (SDC) with Al−7%Si−0.35%Mg composition in order to examine the relationship between solidification structures and fatigue behaviors. The crack length was measured using a replication method. Fatigue strength was improved in SDC, which was almost consistent with the predicted fatigue strength using the size of Si particle cluster. Resistance to fatigue crack initiation and fatigue strength were improved in SDC owing to the finer Si cluster and to higher ultimate tensile strength. Fatigue crack in PMC was preferentially initiated at pores. For SDC, the fatigue crack was initiated at the Si particle/matrix interface, and then sucessively grew along eutectic cell boundaries.     

Solidification behavior of water-cooled and subzero-chilled cast iron on mechanical properties

Hypoeutectic cast iron specimens cast using chills that are water-cooled and liquid-nitrogen-cooled (subzero chilling) were compared with specimens of the same chemical composition, which were sand-cast without any chill. The solidification behavior, number of eutectic cells, grain size, and effects of these on the mechanical properties such as strength and fracture toughness were recorded and analyzed in this paper. It is revealed from the above investigation that subzero and water-cooled chilled cast irons exhibit severe under cooling as compared to that of normal sand-cast iron. Thus, it is concluded from the investigation that nucleation conditions are completely altered during solidification, which is considered to be responsible for the variation in eutectic cell size, grain size, microstructure, and hence mechanical properties of the cast iron.     

Effect of surface rolling on fatigue behavior of a pearlitic ductile cast iron

Surface rolling is a mechanical treatment usually used in parts fabricated with steel and ductile cast iron, specifically in stress concentration regions, to improve fatigue properties. This process hardens and introduces compressive residual stresses to the surface of the material through the application of controlled strains, thus provoking a reduction of resulting tensile stress at its surface under cyclic loading. This work deals with the effect of surface rolling on high cycle fatigue behavior of a pearlitic ductile cast iron used in crankshafts by the automotive industry. Rotating bending fatigue tests were performed in both smooth and notched specimens, the latter either with or without a surface rolling treatment. Compressive residual stresses and heavy plastic deformation imposed on the surface grains due to cold work made difficult the nucleation and propagation of the crack at the rolled surface of the notch. As a consequence, surface-rolled notch testpieces presented a higher endurance limit (478 MPa) than both smooth (299 MPa) and notched (166 MPa) testpieces did. The surface rolling apparatus developed for this work proved to be very efficient and simple, providing good control of parameters involved in the process (i.e., rolling load, frequency, and number of revolutions).     

42CrMoVNb细晶高强钢的疲劳行为

研究了不同热处理制度下得到的3种晶粒尺寸的42CrMoVNb细晶高强钢的疲劳性能.结果表明，它们的光滑疲劳试样S-N曲线在10^6—10^7周期范围内无平台出现，疲劳极限消失．SEM断口观察表明，光滑试样疲劳裂纹起源位置与寿命长短密切相关，测量了长寿命试样疲劳断裂源夹杂物的位置及尺寸．通过对实验数据的初步分析，给出了夹杂物及奥氏体晶粒尺寸应控制的水平．     

Nb对灰铸铁高温抗拉强度和抗氧化性的影响

研究了高温条件下,w（Nb）量为0.037%和0.11%对灰铸铁抗拉性能和抗氧化性能的影响,试验结果表明：w（Nb）量为0.037%时,试样为脆性断裂;w（Nb）量为0.11%时,产生了韧性断裂,试样断口出现了韧窝。铸铁加Nb后,石墨和珠光体得到细化,与此同时,富Nb相镶嵌于基体上,有利于提高灰铸铁的抗拉强度;Nb使石墨细化的作用也使氧原子难以进入基体内部,阻止氧化反应,有利于提高灰铸铁的抗氧化性。     

Mechanical properties of malleable iron after isothermal quenching by various regimes

In recent years the range of application of cast irons in a heat treated state has widened. Isothermal quenching of irons that provides a favorable combination of strength and ductile parameters commonly inherent in steels is the most promising. The present work concerns the effect of regimes of isothermal quenching on the mechanical properties of cast iron. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 29–31, December, 1998.     

Effect of nodularity on mechanical properties and fracture of ferritic spheroidal graphite iron

Ferritic spheroidal graphite irons with nodularity from 72% to 96% were prepared. The relationship between the nodularity and the mechanical properties of the ferritic spheroidal graphite iron was investigated. The effect of nodularity on the mechanical properties and tensile fracture of the cast iron were studied. Results showed that the tensile strength Rm, yield strength R_(p0.2), elongation to failure A_5, and impact energy KV_2 of the cast iron had a good linear relationship with its nodularity. Nodularity and annealing treatment would obviously affect the fracture characteristics of ferritic spheroidal graphite iron. The annealed ferritic spheroidal graphite iron with 93% nodularity showed a completely ductile rupture. With the decrease of nodularity from 93% to 72%, the cleavage fracture area ratio increased gradually from 0% to 8.3%. Compared with as-cast ferritic spheroidal graphite iron, annealing treatment reduced the cleavage fracture area of the ferritic spheroidal graphite iron.     

不同循环载荷下54SiCr6钢的疲劳强度

对高强弹簧钢54SiCr6在3种循环加载条件下(旋转弯曲,超声和拉压)的疲劳性能进行了测试和比较.由于疲劳试样存在应力梯度和尺寸差异,3种循环载荷下对应的疲劳强度有较大差别.断口分析表明,3种疲劳样品也具有不同的断口形貌.通过计算高应力截面积,可以得到不同载荷下疲劳强度之间的定量关系,从而为评估不同载荷下的不同试样的高强钢疲劳强度提供依据.