Different thermal fatigue behaviors between gray cast iron and vermicular graphite cast iron

The initiation and propagation of thermal fatigue cracks in gray cast iron and vemicular graphite cast iron were investigated by Uddeholm method to reveal the complex thermal fatigue behaviors of cast iron.Differences of thermal fatigue behaviors of gray cast iron and vemicular graphite cast iron were observed and analyzed.It is found that the observed differences are related to the combination of graphite morphology and the oxidization of matrix.More oxidized matrix is observed in gray cast iron due to its large specific surface area.The brittle oxidized matrix facilitates the propagation of microcracks along the oxidization layer.By contrast,the radial microcracks are formed in vermicular graphite at the edge of graphite due to fewer oxidization layers.It indicates that the thermal fatigue resistance of gray cast iron is dominated by graphite content and morphology while that of vermicular graphite cast iron strongly relates to the strength of the matrix.

Effect of Hot Deformation on Formation and Growth of Thermal Fatigue Crackin Chromium Wear Resistant Cast Iron

The formation and growth of thermal fatigue crack in chromium wear resistant cast iron was investigated, and the effect of hot deformation on the crack was analyzed by means of optical microscope and scanning electron microscope and high frequency induction thermal fatigue tester. The results show that eutectic carbide is the main location and passage for initiation and extension of thermal fatigue cracks, hot deformation can improve the eutectic carbiders morphology and distribution, inhibit the generation and propagation of thermal fatigue cracks. In the experiment, the propagation rate of thermal fatigue crack reduces with the quantity of hot deformation increasing, which was analyzed in the point view of the activation energy of crack propagation.

THERMAL FATIGUE AND FRACTURE MECHANICS ANALYSIS OF GREY CAST IRON

The in-phase and out-of-phase thermal fatigue,the C-P type and P-C type isothermal fa- tigue of grey cast iron were experimentally studied.The fatigue life was evaluated analytically by using the elastic-plastic fracture mechanics method(mainly J integral).The results of ex- periments and calculations showed that the lifes of in-phase and C-P type fatigue are longer than that of out-of-phase and P-C type fatigue respectively within the same strain range. This is in contrast to the results of other materials such as low carbon steel.On the other hand, the predicted lifes are consistent with experimental results.This suggests that J integral as a mechanics parameter for characterizing the thermal fatigue strength of grey cast iron and the mechanics model and the calculation method developed here are efficient.A parameter ΔW_1 was proposed from energy aspect to characterize the capacity of crack propagation. The isothermal fatigue life is the same as the thermal fatigue life for identical ΔW_1 values.

Study on the Thermal Fatigue Behavior of Hot Deformed Wear Resistance Cast Iron and Effect of Carbide

The thermal fatigue behavior of wear resistance cast iron with different quantity of deformation has been investigated. The results show that eutectic carbide is the main location and passage for initiation and extension of thermal fatigue cracks, approving that the more serious, the carbide breaks. The higher thermal fatigue resistance of wear resistance cast iron will be and thermal fatigue fracture belongs mainly to brittleness.

Effect of RE Modification and Heat Treatment on Formation and Growth of Thermal Fatigue Crack in Wear Resistant Cast Iron Containing Low Alloy

The formation and growth of thermal fatigue crack and the function of RE and heat treatment in wear resistance of cast iron containing low alloy were investigated,and it was analyzed in view of the activation energy for the crack′s propagation. The results show that the thermal fatigue cracks are mainly generated at eutectic carbides,and the cracks are grown by themselves spreading and joining each other. RE can improve the eutectic carbide′s morphology,inhibit the generation and propagation of thermal fatigue cracks,therefore,promote the activation energy for the crack′s propagation,and especially,which is more noticeable in case of the RE modification in combination with heat treatment.

Effect of Rare Earth Elements on Dynamics of Thermal Fatigue Crack′s Propagation in Low Alloy White Cast Iron

The effect of rare earth elements on dynamics of thermal fatigue crack′s propagation in low alloy white cast iron was studied. The results show that the generation and growth of the thermal fatigue crack can be restrained and the activation energy for the crack′s propagation can be increased by adding a certain amount of RE, and especially, the restraint for the thermal fatigue crack′s propagation is more evident under the combined action of RE and heat treatment at high working temperatures, which can be attributed to the segregation of RE to interfaces, the participation of granular carbides and the change of eutectic carbide morphology.

Thermal Fatigue Behavior and Cracking Characteristics of High Si-Mo Nodular Cast Iron for Exhaust Manifolds

The thermal fatigue cracking behavior of high Si-Mo nodular cast iron （NCI） is investigated by means of optical microscope （OM）, scanning electron microscope （SEM） and energy dispersive spectroscope （EDS）, in order to find a new material used in exhaust manifolds in First Automotive Works （FAW） .Nodular cast irons with silicon content about 4.7% , in combination with up to 1.1% molybdenum , were produced by Jilin University and FAW. The repeated heating / cooling test was performed under cyclic heating at various maximum heating temperatures （Tmax） ranging from 800to 900℃.Experimental results indicate that the thermal fatigue cracking resistance of high Si-Mo NCI decreases with increasing the maximum heating temperature.The periods for crack initiation are 24-36 , 40-50and 70-90times associated with heating temperature of 900 , 850and 800℃ , respectively , when the holding time is about 10min at Tmax.When thermal fatigue cracking occurs , the cracking always initiates at the bigger surface of specimen.The major positions of cracks propagation are generally at the eutectic oxide boundary region and the region of the graphite disappearance.At the same time , the oxidation may accelerate crack initiation and propagation.On the other hand , micro-crack number varied from large to little because of shielding effect.As exhaust manifolds , the reasonable working temperature of high Si-Mo NCI is no more than 840℃ by test and analysis.

Effect of Rare Earth Elements on Thermal Fatigue of High Ni－Cr Alloy Cast Iron

The effect of RE on crack forming and developing of hot rolled high Ni-Cr alloy cast iron during thermal fatigue test was investigated.It shows that the serviceable life of hot rolled high Ni-Cr alloy cast iron can be improved by RE.The number of cycle before fracture was increased by 42%～163%.The optimum of RE addition is 0.05 wt%～0.15wt%.

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.

灰铸铁制动鼓石墨钝化生产工艺探讨

针对商用车在反复刹车时频繁的冷热循环易造成制动鼓开裂失效的问题,提出一种利用稀土元素、氮元素及钝化处理工艺钝化片状石墨端部的方法。在高碳当量成分条件下(CE≥3.9%),探讨灰铸铁制动鼓石墨钝化材质及生产工艺,提高制动鼓的常规力学性能和抗热疲劳性能,为商用车耐热及抗热疲劳制动鼓铸件的生产提供技术支持。

WC/铁基表面复合材料的热疲劳裂纹形成过程

为了对高热疲劳性能的表面复合材料的设计提供理论依据,采用热震试验方法对通过真空实型铸渗(V-EPC)方法制备的WC/铁基表面复合材料的热疲劳性能进行了研究,重点讨论了热疲劳裂纹的形成机制。研究结果表明,随着WC颗粒体积分数的增加,表面复合材料的热疲劳性能有所改善,当WC体积分数达到52%时,复合层表面在经过10次热循环后能保持完好。热疲劳裂纹扩展机制研究表明,陶瓷WC颗粒增强铁基表面复合材料的热疲劳裂纹的产生和扩展是由WC和基体本身的热应力和二者界面交变循环应力共同交互作用的结果,可以通过选用高热导率的陶瓷颗粒作为增强体、改善陶瓷颗粒本身的微观质量和采用与陶瓷颗粒热膨胀系数相接近的基体等方法提高复合材料的热疲劳性能。

稀土对高镍铬合金铸铁热疲劳性能的影响

研究了高镍铬合会铸铁热轧辊用材料的裂纹形成和在热循环４０和７０周次后裂纹的扩展过程。结果表明：稀土元素的加入，能明显提高高镍铬合金铸铁热轧辊用材料的抗热疲劳性能，使裂纹出现时冷热循环次数提高４２％～１６３％。同时给出稀土加入量以０．０５ｗｔ％～０．１５ｗｔ％为宜。

石墨形态对铸铁热疲劳性能影响的研究

采用板状带V型缺口的HT200、RuT340和QT600试样,通过对试样进行反复加热、冷却的方法,研究了石墨形态对铸铁热疲劳性能的影响,为金属锭模的合理选材提供依据。试验结果表明,在热循环过程中,微裂纹总是起源于石墨相;微裂纹产生后,主裂纹通常是沿着石墨和石墨间最近的基体扩展;在20-650℃间的热循环条件下,球墨铸铁的抗热疲劳性能最好,蠕墨铸铁次之,灰铸铁最差。

Sn对蠕墨铸铁组织与抗热疲劳性能的影响

研究了Sn含量对汽车制动鼓用蠕墨铸铁中蠕墨数量、形态、基体组织以及抗热疲劳性能的影响规律。结果表明，Sn可以明显改变蠕铁中的珠光体含量，对蠕化率无明显影响，可以改变蠕铁热疲劳性能。Sn含量为0．06％（质量分数，下同）时，珠光体含量最高为95％，片层间距为0．07μm。Sn含量增加时，石墨轴比率逐渐上升，石墨变得细长且分叉增多。在20～600℃循环条件下，含0．06％Sn时蠕铁的热疲劳裂纹长度最短，抗热疲劳性能最好。

WC_p/钢基表面复合材料热疲劳裂纹萌生及扩展机理

为了对经受热疲劳的表面复合材料的设计提供理论依据,采用热震试验方法对通过真空实型铸渗(V-EPC)方法制备的WC/钢基表面复合材料的热疲劳性能进行了研究,重点讨论了表面复合材料热疲劳裂纹萌生及扩展的影响因素。结果表明,在本实验条件下Ni6025WC体积分数为15%时的热疲劳性能较5%时得到较大改善。表面复合材料的热疲劳裂纹的萌生和扩展的影响因素有以下几个方面:WC颗粒本身微观质量和热导率、复合层与基材中主要元素以及WC颗粒的平均热膨胀系数、复合层中WC颗粒之间的间距、由热震而产生的界面交变循环应力等。通过改善以上影响因素可以提高复合材料的热疲劳性能。

合金元素对球墨铸铁抗热疲劳性能的影响

研究了合金元素对球墨铸铁抗热疲劳性能的影响。结果表明,就抗热疲劳性能而言,在本研究范围内,耐热球墨铸铁模具材料的较佳成分配比w为:5.0%Si、0.5%Mn0、.5%Mo、1.2%Cr。Si是影响材料热疲劳抗力的重要元素,对耐热球墨铸铁模具材料的抗热疲劳性能起着决定性作用,它的含量直接决定了耐热球墨铸铁模具材料热疲劳抗力的总体水平。Mo元素的加入能够使晶粒细化,其含量越多,则晶粒越细小,材料对热疲劳裂纹萌生和扩展的抗力就越强。

球铁铝锭模材料热疲劳性能的研究

模拟铝锭模的使用条件,研究了不同基体组织球墨铸铁的热疲劳性能。结果表明,退火铁素体铝铸锭模使用寿命最长。该铝锭模安装在包头铝业公司的2台铝锭连续铸造机上使用,平均每天铸锭48t／台,连续使用410天。而普通材质的球铁铝锭模使用寿命仅为120到150天左右。

石墨形态及合金元素对铸铁抗热疲劳性能的影响

对石墨形态、合金元素等对铸铁抗热疲劳性能的影响进行了研究。加入提高抗氧化性的合金元素可显著提高铸铁的抗热疲劳性能。从首次产生裂纹的循环次数、裂纹扩展速度及变形量等综合考虑，蠕铁的抗热疲劳性能最好。

灰口铸铁低频热疲劳载荷下的微观组织与拉伸性能

实验分析了灰口铸铁经周期性的高温氧化处理后,其抗拉强度、基体组织、断口形貌与化学成分的变化规律,并探讨了热疲劳载荷下灰口铸铁失效的微观-宏观机制。结果表明:灰口铸铁的抗拉强度和抗热疲劳性能在微观上与石墨形貌、基体组织和晶粒尺寸有关,宏观上受温度、氧化条件、载荷周期和化学成分的影响。随着高温氧化处理次数增加,试样脱碳越来越明显,珠光体逐步减少直至消失;同时,短小而零碎的新石墨不断形成,断口处石墨暴露率逐步增大,基体连续性显著恶化;此外,试样基体铁素体化后,尽管脱碳速率降低,但基体晶粒逐步粗化、二次渗碳体析出使晶界弱化,各温度下抗拉强度随碳含量减小而显著下降。

蠕墨铸铁铁道车辆闸瓦摩擦磨损性能研究

随着铁路车辆向高速和重载方向发展，人们开展了抗热疲劳剥落能力优异的蠕墨铸铁在火车刹车闸盘上的应用研究。然而研究内容只涉及其耐磨性，在摩擦性能方面还很少有研究成果公开发表。因此，以含磷蠕墨铸铁为研究对象，系统地考察了石墨形态和磷含量变化对材料摩擦磨损性能的影响，并且依据研究结果制造火车闸瓦进行了实际装车使用性能的试验研究。结果表明，在给定的试验条件下于几种不同形态石墨的铸铁中，蠕墨铸铁既有优良的耐磨性，又有高的摩擦系数，故其是适合制造铁道车辆闸瓦的材料之一；随着磷含量的提高，蠕墨铸铁的摩擦系数快速增大，而磨损量则明显减小，而且蠕墨铸铁闸瓦的制动火花性能也明显改善；在同车对比运行试验以后，含磷蠕墨铸铁闸瓦摩擦表面的裂纹数量和尺寸都远比高磷灰铸铁闸瓦摩擦表面的小，而与中磷灰铸铁闸瓦摩擦表面的相当，可见使用含磷蠕墨铸铁闸瓦能够保证车辆安全运行。