EFFECT OF Mg AND Zr ADDITION ON CREEP CRACK GROWTH BEHAVIOUR OF A Ni-BASE SUPERALLOY

The creep crack growth(CCG)and the time to carck initiation and rupture of specimen (t_(rc)and t_r)were measured by means of electrical potential method on single edge notched specimens at 700℃.The field near the crack tip under steady-state creep was represented by energy rate integral(C~*),and the CCG rate as a function of C~* has been obtained.The agreement between the predicted and observed t_(rc)values is quite good.It was found that the addition of small amount of Mg and Zr in the alloys causes t_(rc)and t_r of the specimens to in- crease significantly.

CREEP CRACK PROPAGATION IN CONVENTIONAL CAST AND DIRECTIONALLY SOLIDIFIED SUPERALLOY RENE 80

The creep crack propagation in superalloy René80 of two different microstructures,i.e., equiaxed grain structure by conventional casting and columnar grain by directional solidification,was investigated under static load at 1123 K.The creep crack growth rate, da/dt,seems to be correlated with the stress intensity factor,K.The creep crack growth rate in the directionally solidified alloy is lower than that in the conventional cast alloy,owing to the elimination of transverse grain boundaries.The effect of microstructure on creep crack propagation has also been discussed.

CREEP CRACK GROWTH BEHAVIOR OF ALLOY 718 DURING HIGH TEMPERATURE EXPOSURE

Alloy 718 is a precipitation strengthened nickel-based superalloy based on the precipitation of γ″-Ni3Nb (DO22 structure) and γ′-Ni3(Al,Ti) (L12 structure) phases. Creep crack growth rate (CCGR) was investigated after high temperature exposure at 593, 650 and 677℃ for 2000h in Alloy 718. In addition to the coalescence of γ′/ γ″ and the amount increasing of δ phase, the existence of a bcc chromium enriched α-Cr phase was observed by SEM, and the weight fraction of α-Cr and other phases were determined by chemical phase analysis methods. The CCGR behavior and regulation have been analyzed by means of strength and structure analysis approaches. The experimental results show higher the exposure temperature and longer the exposure time, lower the CCGR. This is probably attributed to the interaction of material softening and brittling due to complex structure changes during high temperature exposure. Therefore, despite α-Cr phase formation and amount enhancement were run in this test range. It seems to us a small amount of α-Cr will be not harmful for creep crack propagation resistance, which is critical for disk application in aircraft and land-based gas turbine.

EFFECT OF SULPHURIZING-OXIDIZING ENVIRONMENT ON CREEP CRACK GROWTH OF Ni-BASE SUPERALLOY GH30

An investigation was made on the creep crack growth behaviour under static load at 1023 K for Ni-base superalloy GH30 exposed to air and air+10% SO_2.The results showed that in the region of low stress intensity factor,the creep crack growth rate is higher in air+10% SO_2 than in air only,while in the high region,it is reverse.The fractograph of specimens has been analyzed,and the mechanism of creep crack growth together with the influences of sul- phur and oxygen has been discussed as well.

Carbide Effects on Creep Crack Growth of Ni-Cr Austenitic Steels

By controlling the carbon content of Fe-15Cr-25Ni alloys,three types of microstructures were obtained:single phase austenite (γ),γ+intergranular carbides, γ+intergranular carbides + intragranular carbides. Creep crack growth behaviour of the three alloys has been compared at 973 K and 1123 K. Intergranular carbides show higher creep crack growth resistance than intragranular carbides.Cav- ities nucleate at the triple junctions of grain bound- aries for single phase alloy,but at intergranular carbides for two-phase alloys.The precipitation of intergranular carbide not only changes the nucleation mechanism of cavities,but also inhibits the growth and coalescence of cavities.The precipi- tation of intragranular carbide obstructs the nucleation and growth of cavities furthermore.

EFFECT OF GRAIN BOUNDARY CARBIDE ON CREEP CRACK GROWTH IN Fe-Cr15-Ni25 ALLOY

Creep crack growth behavior of Fe-Cr15-Ni25 alloys with different grain boundary features has been studied.Cavities nucleate at triple junctions of grain boundaries for the single phase alloy and at grain boundary carbide for the alloy with grain boundary carbide.Grain bounda- ry carbide particles are obstacles to cavity growth and coalescence,and therefore increase the creep crack growth resistance greatly.

INFLUENCE OF GRAIN SHAPE AND CARBIDE ON CREEP CRACK GROWTH IN HK40

The influence of two different grain shapes and carbides in the HK40 alloy,the material for use of furnace tube,on the rate of creep crack growth has been investigated.The resistance to creep crack growth of the material with columnar grains is inferior than that with equiaxial grains when the load line is perpendicular to the columnar grain axis.The influence of secon- dary carbide on the rate of creep crack propagation depends upon the Brain shape.

Creep crack growth in a Cr-Mo-V type steel: experimental observation and prediction

In this study, the creep crack growth （CCG） properties and fracture mechanism of a Cr-Mo-V steel at 566 C in compact tension （CT） specimens were investigated, and the CCG rate was predicted by using the NSW model. The results show that the CCG rate measured by CT specimens is much lower than that predicted by the NSW model under plane-strain state. This means that the NSW model prediction for the CCG rate of the steel is over-conservative. In addition, the CCG rate da/dt versus C measured by the experiments shows the piecewise linear relation on log-log scale instead of a single linear relation predicted by the NSW model. The main reasons for these results are that the actual creep fracture mechanism of the steel and the actual creep crack tip stress field in the CT specimens have not been fully captured in the NSW model. The experimental observation shows that the creep crack propagates in a discontinuous way （step by step） at meso-scale, and the cracks at micro-scale are usually formed by the growth and coalescence of voids on grain boundaries. The NSW model based on the creep ductility exhaustion approach may not correctly describe this creep fracture process. In addition, the opening stress and triaxial stress ahead of crack tips calculated by three-dimensional finite element method is lower than those predicted by the HRR stress field which is used in the NSW model under plane-strain state. The use of the high HRR stress field will cause high CCG rates. The change in the creep fracture mechanism at micro-scale in different ranges of C may cause the piecewise linear relation between the da/dt and C . Therefore, it is necessary to study the actual CCG mechanism in a wide range of C and the actual creep crack tip stress field to establish accurate CCG prediction models.

基于Zencrack的汽轮机叶片蠕变断裂分析方法研究

汽轮机高压级次的动叶片承受着高温和离心力的双重作用,叶片的工作环境及其恶劣。叶片在高温环境下长时间工作会产生蠕变,研究叶片的蠕变断裂对于分析和预防叶片事故具有重要意义。文章利用Zencrack模拟了带有初始裂纹的汽轮机叶片在高温和离心力作用下裂纹的扩展问题,并给出了裂纹的扩展方式和最终的形貌。

CRACK GROWTH BEHAVIOR AND FRACTURE FEATURE OF GH2132 UNDER CREEP-FATIGUE CONDITIONS

The superalloy GH2132 is equivalent to A286. The tests were carried out at 550°C under various cyclic frequencies (hold time) and load levels, and the fracture surfaces were examined by using a scanning electron microscope. It was shown that the fracture mode of creep-fatigue and the effect of cyclic loading on crack growth change with the growth of crack and the increase of net-section stress, and both are reversed when the net-section stress is up to the yield stress of material. When σn0.2, cracking is predominantly cyclic-dependent transgranular and cyclic loading accelerates creep crack growth, whereas when σn>σ0.2, the case is reversed.

CRACK GROWTH,FRACTURE FEATURE AND RUPTURE LIFE DURING CYCLIC CREEP OF SUPERALLOY GH2132

The effects of cyclic loading on crack growth, fracture feature and rupture life of superalloy GH2132 have been studied in the present paper. The crack growth tests were carried out at 550 on compact tension specimens under a wide range of load levels and various periods of hold time, and the fracture surfaces were examined on a scanning electron microscope. The results show that, for both static and cyclic creep, Stage-I of crack growth is related with initial test stress, during which the crack extension is about 0.5mm, and that the final effect of cyclic loading on rupture life depends on the counteraction of its two reversed effects on crack growth rate before and after the point n=0.2.

APPLICATION OF Ct-PARAMETER FOR CREEP-FATIGUE CRACK GROWTH WITH SLOW LOAD RISING CONDITION

Most of the assessment equations for Ct which is a wellknown fracture parameter characterizing high temperature crack growth rates, have limited applicability to constant load conditions after sudden loading. However, crack growth due to creep can also occur under load varying conditions when load rising time is so long that accumulated creep deformation near the crack tip is not negligible.In this paper, the estimation equation of the Ctparameter which can be applied to the case of slow load rising, i.e., (Ct)r, is explained. And the correlation between (Ct)r at the end of the load increasing period and Ct at the beginning of the succeeding load hold period is discussed. Finite element analyses of several cases with various loading conditions were performed and results were presented to show the effectiveness of the proposed Ct estimation scheme. The general applicability of the equation is also discussed.

ASYMPTOTIC ANALYSIS OF MODE Ⅱ STATIONARY GROWTH CRACK ON ELASTIC-ELASTIC POWER LAW CREEPING BIMATERIAL INTERFACE

A mechanical model was established for modeⅡinterfacial crack static growing along an elastic_elastic power law creeping bimaterial interface. For two kinds of boundary conditions on crack faces, traction free and frictional contact, asymptotic solutions of the stress and strain near tip_crack were given. Results derived indicate that the stress and strain have the same singularity, there is not the oscillatory singularity in the field; the creep power_hardening index n and the ratio of Young's module notably influence the crack_tip field in region of elastic power law creeping material and n only influences distribution of stresses and strains in region of elastic material. When n is bigger, the creeping deformation is dominant and stress fields become steady,which does not change with n. Poisson's ratio does not affect the distributing of the crack_tip field.

A Criterion for Crack Initiation and Propagation of Intact Rocks under Creep Condition

The mechanism and criterion of crack initiation and propagation of rocks were investigated by many researchers. And the creep behaviour of rocks was also theoretically and experimentally studied by some scientists and engineers. The characteristics of crack initiation and propagation of rocks under creep condition, however, are very important for rock engineering and still not paid enough attention by researchers. In this paper, the criterion and mechanism of crack initiation and propagation under creep condition were investigated using specimens collected from sandstone rock formations outcropping in the Emei Mountain, the Sichuan Province of China. Cuboid specimens under three point bending were used in this investigation. All specimens were classified into four sorts and used for Mode I fracture or creep fracture tests. The experimental result shows that due to creep deformation, rock crack will inevitably initiate and propagate under a load of K I , which is less than fracture toughness K IC but not less than a constant (marked as K IC2 ). K IC2 indicates the ability of rock to resist crack initiation and propagation under creep conditions and is less than fracture toughness K IC , defined as creep fracture toughness in this paper. K IC2 should be considered as an important parameter on design and computation of rock engineering. The microstructural mechanism for crack initiation and propagation of rock materials under creep condition was introduced based on competitive model between softening effect and hardening effect, and the validity of test result was explained. The test result was also verified in rheological theory. When K I is more than K IC2 but less than K IC , rock crack will initiate and propagate after a time interval of sustained loading under creep condition. In order to find the relation between duration of sustained loading, which can lead to crack initiation and propagation, and the initial stress intensity factor K I , an unequal interval time sequence forecasting and predicting model was introduced, and the relation was obtained for homogeneous and isotropic fine grained red sandstone. Finally a modified fracture toughness formula was given, in which the influence of fracture process zone(FPZ) was fully considered.

考虑蠕变三阶段的哈氏合金X蠕变-疲劳裂纹扩展模拟

结合应变硬化定律和连续损伤力学,提出了一种考虑蠕变3阶段的本构模型,对650℃下的哈氏合金X的CT试样进行了裂纹扩展有限元模拟分析.对比分析了不同蠕变阶段对纯蠕变裂纹扩展的影响,使用考虑蠕变3个阶段的本构模型进行了蠕变-疲劳裂纹扩展模拟,讨论了载荷幅值、载荷比和保载时间对裂纹扩展速率的影响,并分析了每种载荷条件对裂纹扩展期间损伤积累的贡献.结果表明:纯蠕变裂纹扩展模拟结果中,考虑所有蠕变阶段的本构模型计算结果与实验结果一致性最高.载荷从5 kN增加到7 kN,考虑蠕变第Ⅰ、Ⅱ阶段的裂纹扩展速率平均预测差异由0.57增至0.61,考虑蠕变第Ⅱ阶段和考虑蠕变第Ⅱ、Ⅲ阶段分别由0.67减至0.64、由0.16减至0.07.蠕变-疲劳裂纹扩展模拟结果显示,随着载荷幅值从5 kN增大至7 kN,载荷比从0.01增至0.50,裂纹扩展速率前者增大、后者减小,da/dt-C t,avg曲线斜率基本不变,同时蠕变损伤占比增大,疲劳和交互损伤占比减小,各加载情况结果的蠕变损伤均占主导,其次为交互损伤,疲劳损伤占比最小.随保载时间从1 s增加到1800 s,循环相关的裂纹扩展速率增大,时间相关的扩展速率减小,da/dt-C t,avg曲线斜率增大,蠕变损伤增大,疲劳损伤减小,交互损伤先增后减,保载时间为10 s时各损伤占比近似相等,保载时间为1 s时疲劳损伤为83%,保载时间为600 s时蠕变损伤为96%.

P92钢接头Ⅳ型开裂蠕变孔洞临界值的分析及应用

【目的】分析P92管道接头热影响区发生Ⅳ型开裂的蠕变孔洞临界值的影响因素,得到蠕变孔洞临界值计算方法,为评估接头开裂风险提供依据。【方法】收集服役后发生Ⅳ型开裂的P92接头,在未开裂处取样进行610℃下不同应力条件的高温持久蠕变试验,然后使用光学显微镜、电子显微镜观察服役开裂接头和持久断裂接头热影响区的蠕变孔洞分布形貌,并借助图像分析软件测量蠕变孔洞的面积分数,同时使用硬度仪测试热影响区软化情况,最后综合测试结果分析细晶区软化程度和接头应力水平对蠕变孔洞临界值的影响,并拟合应力、显微硬度、孔洞临界值三者之间的函数关系。【结果】研究发现,P92钢接头细晶区相比于接头其它区域软化速度更快,但软化作用并非其发生Ⅳ型开裂的必要条件,当应力水平较高时,细晶区没有发生明显软化时就可以触发蠕变开裂;得到接头轴向应力水平、蠕变孔洞面积分数临界值和显微硬度的函数关系式。【结论】引起Ⅳ型开裂的蠕变孔洞面积分数临界值与相对应力水平(轴向应力与硬度的比值)有关,相对应力水平越高,蠕变孔洞临界值越小,接头安全性就越低;通过测量接头细晶热影响区的蠕变孔洞面积分数和显微硬度,可以估算开裂接头的轴向应力水平,也可以评估在役未开裂接头的Ⅳ型开裂风险。

国产加钒钢焊接接头不同区域的蠕变裂纹扩展行为

以国产2.25Cr-1Mo-V加钒钢焊接接头为研究对象,在550℃和不同初始应力强度因子(19.04,17.57,16.40,14.64,13.47MPa·m^(0.5))下对母材、焊缝和热影响区进行蠕变裂纹扩展试验,研究了焊接接头不同区域的蠕变裂纹扩展行为,探究了应力强度因子和C^(*)参量表征蠕变裂纹扩展速率的适用性,并观察了断口形貌。结果表明:随着初始应力强度因子的增加,母材、焊缝和热影响区的蠕变断裂寿命缩短,归一化稳态蠕变裂纹扩展速率增大。在相同初始应力强度因子下,焊缝的蠕变断裂寿命最长、归一化稳态蠕变裂纹扩展速率最小,说明焊缝抵抗蠕变裂纹扩展的能力最强,蠕变性能最佳;热影响区的蠕变断裂寿命最短、归一化稳态蠕变裂纹扩展速率最大。接头不同区域的归一化蠕变裂纹扩展速率与应力强度因子的变化曲线都呈类似“V”字形状,对应蠕变裂纹的萌生、稳定扩展和加速扩展阶段;应力强度因子和归一化蠕变裂纹扩展速率的分布具有显著的分散性;C^(*)参量与归一化蠕变裂纹扩展速率具有更强线性关系,且C^(*)参量和归一化蠕变裂纹扩展速率的分布集中。随着初始应力强度因子的增加,母材、焊缝和热影响区的断裂模式由沿晶断裂或穿晶断裂向沿晶/穿晶混合断裂转变。

残余应力与拘束对GH4169合金疲劳及蠕变-疲劳裂纹扩展速率影响研究

为深入理解残余应力与拘束在疲劳载荷下的交互作用,以镍基高温合金GH4169为研究对象,选用紧凑拉伸(CT)试样,对不同拘束CT试样的上方施加不同大小的预加载荷从而在裂尖产生不同应力。将该应力作为残余应力,系统研究残余应力和拘束交互作用下GH4169合金的疲劳和蠕变–疲劳裂纹扩展速率。结果表明:随着裂尖残余压应力的增加,不同拘束下GH4169合金的疲劳和蠕变–疲劳裂纹扩展速率均降低。与低拘束试样相比,高拘束试样的疲劳和蠕变–疲劳裂纹扩展速率对残余应力的变化更加敏感,这主要与裂尖Mises应力和垂直于裂纹扩展方向的正应力有关。与疲劳裂纹扩展速率相比,蠕变-疲劳裂纹扩展速率对残余应力的变化更加敏感。

多级冷压合与高温蠕变应力松弛TC4钛合金微观组织演变机理研究

采用多级冷压合与高温蠕变应力松弛成形工艺制备TC4钛合金薄壁构件,研究了不同蠕变工艺参数对微观组织的影响规律。通过光学显微镜和电子背散射衍射技术分析了晶粒尺寸及取向、位错密度和裂纹萌生扩展。结果表明,在低塑性变形条件下,较高的蠕变温度可以促进α相晶粒长大。然而在大塑性变形条件下,微观组织受到蠕变温度和塑性变形共同影响。较高的温度加快位错回复,引起位错密度降低,从而减小了晶粒生长的驱动力,最终导致在较高的蠕变温度下α相晶粒反而更小。与600℃蠕变工艺相比,750℃蠕变后的TC4板材出现更多的裂纹,可能是β相粗化所致。

Influence of Hot Corrosion on High Temperature Creep Rupture for Single Crystal Superalloy DD3

nfluence of hot corrosion on the creep rupture behavior for both single crystal Ni-based superalloy DD3and specimens coated with Pt-Al was studied. In mixed salt, Pt-Al coating can improve creep properties of thealloy. The low melting point eutectic products lead to premature failure of alloy. This article proposes a newfracture model to explain the creep fracture behavior of DD3 at elevated temperature.