Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

The infamous type Ⅳ failure within the fine-grained heat-affected zone (FGHAZ) in G115 steel weldments seriously threatens the safe operation of ultra-supercritical (USC) power plants.In this work,the traditional thermo-mechanical treatment was modified via the replacement of hot-rolling with cold rolling,i.e.,normalizing,cold rolling,and tempering (NCT),which was developed to improve the creep strength of the FGHAZ in G115 steel weldments.The NCT treatment effectively promoted the dissolution of preformed M_(23)C_(6)particles and relieved the boundary segregation of C and Cr during welding thermal cycling,which accelerated the dispersed reprecipitation of M_(23)C_(6) particles within the fresh reaustenitized grains during post-weld heat treatment.In addition,the precipitation of Cu-rich phases and MX particles was promoted evidently due to the deformation-induced dislocations.As a result,the interacting actions between precipitates,dislocations,and boundaries during creep were reinforced considerably.Following this strategy,the creep rupture life of the FGHAZ in G115 steel weldments can be prolonged by 18.6%,which can further push the application of G115 steel in USC power plants.

Creep Rupture of Mismatched Welded Joints of Steels with Dissimil ar Creep Strengths

The finite element analysis of mismatched welded jo ints with a 30°groove angle was performed to study the mechanical behavior of D MWJs (dissimilar metal welded joints). It is concluded that the distribution of stress triaxiality in the DMWJs is uneven, especially near the fusion lines. The degree of creep strength mismatch has remarkable effect on the distribution. Th e higher the level of mismatch is, the more uneven the distribution is and the e asier for premature failure to occur in the joint.

On the microstructural origin of premature failure of creep strength enhanced martensitic steels

The creep strength enhanced martensitic steels are key material for the main power generating units in ultra-supercritical plants.Studies on the evaluation of their creep rupture life show there is an overestimation of rupture life after long-term creep,which is known as premature failure.However,the microstructural origin of the premature failure remains unclear.Here in this study,we have carefully investigated the microstructural transformations and their influences on creep rupture behavior,showing that the evolution of martensite and M_(23)C_(6) carbides as well as Laves phase are responsible for the premature failure.By using multi-step TTP-LMP method,we confirmed a three-stage creep rupture behavior under different stress regions.Further quantitative analysis showed that the coarsening of M_(23)C_(6) carbides and recovery of martensite exert equal and dominant effects on the premature failure in the medium stress region,while precipitation and coarsening of Laves phase are responsible for the premature failure in the low stress region.

The effects of DS blade's geometry features on material's creep strength

Because of the better creep performance,the directional solidification(DS)Nickel-based turbine blades have been widely used in advanced aero-engines.However,the DS turbine blade's different abrupt geometrical changes at different regions cause a variation of temperature field at those regions.Subsequently,the variable temperature field is very likely to lead to a different grain structures at those different region,and those different grain structure finally give rise to a variation in material's creep performance at different region in DS turbine blade.To study the variation in creep strength among different regions of a DS turbine blade,this article designed and manufactured three types of DS specimens to simulate the geometry features of platform,shroud and body part of a typical DS turbine blade.Creep tests on these specimens were conducted under the stress level of 608 MPa and temperature of 850。C,the creep rupture life of platform-like and shroud-like specimens are 93%and 73%of body-like specimens'respectively,which support the assumption that there exists a certain variation in material's creep strength among different regions in DS turbine blades.The fracture positions of these specimens also support above conclusion.It is suggested that the material's creep strength variation among different locations of DS turbine blades should be considered in future turbine blade life design and prediction.

TENSILE STRENGTH AND CREEP RESISTANCE OF Mg-9Al-1Zn BASED ALLOYS WITH CALCIUM ADDITION

Small amount of calcium addition to the Mg-9Al-0.8Zn-0.2Mn (AZ91) alloy resulted in obvious influence on mechanical properties. The yield strength of the alloys increased with the increase of Ca addition and the maximum strength was obtained from the alloy containing 0.15% of Ca. The creep resistance at the temperatures between 150-220°C was also significantly increased with Ca addition. The creep rate (at 200°C, 50 MPa) of the alloy with 0.15% Ca addition was one order of magnitude lower than that of the base alloy (AZ91). Microstructural observations revealed that the addition of calcium refined the microstructure and enhanced the thermal stability of the β precipitates, which accounted for the improvement of creep resistance at high temperatures.

The Effect of Cold Working on Creep Rupture Strength and Microstructure of Ni-23Cr-7W Alloy

In order to clarify the reason why the creep rupture time of pre-strained Ni-23Cr-7W Alloy (HR6W) is longer than that of the non-pre-strained HR6W, microstructures of HR6W after a series of creep tests were investigated. The creep tests were conducted at 750°C, 90 and 100 MPa. In the pre-strained samples, the grain boundary shielding ratio by precipitates was larger than that of the non-pre-strained sample. In addition, in the pre-strained samples the size of the M23C6 carbide in the grains was finer than in the non-pre-strained sample. The W content in the M23C6 carbide in the pre-strained samples tended to be larger than in the non-pre-strained sample. Therefore, the Ostwald ripening of the carbide was delayed and the size of M23C6 carbide was thought to be fine for a long time. These observations show that creep strength in the pre-strained samples is higher than that of the non-pre-strained sample because of both precipitation strengthening inside of the grains and grain boundaries.

Development of a Nickel-base Cast Superalloy with High Strength and Superior Creep Properties

Derived from Russian alloy CHS88U, six experimental Ni-base alloys named as A to F in the Ni-Cr-Co-W-Ti-Al-Hf system are designed, evaluated and processed. One of these alloys, F, shows excellent high temperature tensile strength and ductility with superior creep rupture properties. As predicted by using modeling tools such as PHACOM and NEW PHACOMP, there is hardly the tendency for formation of topologically close-packed phase (TCP) phase in alloy F. Furthermore, through microstructural observation, it is also found that no TCP phase is formed in alloy F after long-time exposure at high temperature. So alloy F has well balance of phase stability and mechanical properties in view of application for gas turbines. It is proved that d-electron approach can be applied for design and development of nickel-base superalloys for gas turbine application.

Research on the creep damage and interfacial failure of dissimilar metal welded joint between 10Cr9Mo1VNbN and 12Cr1MoV steel

The mechanical properties, creep damage, creep rupture strength and features of interfacial failures of welded joints between martensite (SA213T91) and pearlite steel (12Cr1MoV) have been investigated by means of argon tungsten pulsed arc welding, high temperature accelerated simulation, creep rupture, mechanical property tests and scanning electronic microscope (SEM). The research results indicate that the mechanical properties of overmatched and medium matched joint deteriorate obviously, and they are susceptible to creep damage and failure after accelerated simulation operation 500 h, in the condition of preheat 250℃, and post welding heat treatment 750℃×1 h. However, the mechanical properties of undermatched joint are the best, the interfacial failure tendency of undermatched welded joint is less than those of medium and overmatched welded joint. Therefore, it is reasonable that low alloy material TR31 is used as the filler metal of weld between SA213T91and 12Cr1MoV steel.

MECHANICAL PROPERTIES OF FINE PITCH DEVICES SOLDERED JOINTS BASED ON CREEP MODEL

Nonlinear analyses of quad flat package （QFP） on printed circuit board （PCB） assemblies subjected to thermal cycling conditions are presented. Two different solders are considered, namely, Sn37Pb and Sn3.5Ag. The stress and strain response of fine pitch devices soldered joints was investigated by using finite element method based on Garofalo-Arrheninus model. The simulated results indicate creep distribution of soldered joints is not uniform, the heel and toe of soldered joints, the area between soldered joints and leads are the creep concentrated sites. The similar phenomena of stress curves simulated based on Garofalo-Arrheninus model and Anand equations is confirmed, and the creep strain value of Sn3.5Ag soldered joints is lower than that of Sn37Pb soldered joints. Thermal cycling results show that Sn3.5Ag strongly outperforms Sn37Pb for QFP devices under the studied test condition. This is well matched with the experimental outcome analyzed. In addition, the soldered devices were tested by micro-joints tester, the tensile strength of Sn3.5Ag soldered joints is found to be higher than that of Sn37Pb soldered joints. By analyzing the fracture microstructure of soldered joints, it is found that fracture mechanism of Sn3.5Ag soldered joints is toughness fracture, while fracture mechanism of Sn37Pb soldered joints includes brittle fracture and toughness fracture. The results of this study provide an important basis of understanding the mechanical properties of fine pitch devices with traditional Sn37Pb and Sn3.5Ag lead-free soldered joints.

Elevated temperature endurance and creep properties of extruded 2D70 Al alloy rods

The elevated temperature performances of 2D70 Al alloy hot extrusion rods after two-stage homogenization and intensive deformation were studied by measuring the elevated temperature enduring strength and the creep ultimate strength. The fracture morphology of some selected samples after testing at different elevated temperatures was observed by scanning electron microscopy （SEM）. The results indicate that, as the test temperature increases, the elevated temperature enduring strength of 2D70 Al alloy decreases gradually. In a comparison between 150 C and 240 C, the notch enduring strength drops from 375 to 185 MPa and the smooth enduring strength drops from 337 to 130 MPa. Enduring strength is not sensitive to the notch. The notch sensitivity ratio （NSR） coefficient is in the range of 1.119 to 1.423 from 150 C to 240 C. The creep test results show that, as the test temperature increases from 150 C to 240 C, the creep ultimate strength of 2D70 Al alloy rods drops gradually from 312 to 117 MPa.

Effects of Manufactured-sand on Dry Shrinkage and Crccp of High-strength Concrete

The influences of natural sand, manufactured-sand （MS） and stone-dust （SD） in the manufactured-sand on workability, compressive strength, elastic modulus, drying shrinkage and creep properties of high-strength concrete （HSC） were tested and compared. The results show that the reasonable content （7%-10.5%） of SD in MS will not deteriorate the workability of MS-HSC. It could even improve the workability. Moreover, the compressive strength increases gradually with the increasing SD content,and the MS- HSC with low SD content （smaller than 7%） has the elastic modulus which approaches that of the natural sand HSC, but the elastic modulus reduces when the SD content is high. The influence of the SD content on drying shrinkage performance of MS-HSC is closely related to the hydration age. The shrinkage rate of MS-HSC in the former 7 d age is higher than that of the natural sand HSC, but the difference of the shrinkage rate in the late age is not marked. Meanwhile the shrinkage rate reduces as the fly ash is added; the specific creep and creep coefficient of MS-HSC with 7% SD are close to those of the natural sand HSC.

Magnesium nanocomposites:An overview on time-dependent plastic(creep) deformation

Magnesium(Mg) nanocomposites are created when nano-size particles are embedded into the Mg(or Mg alloy) matrix. The Mg nanocomposites, cited as high-strength energy-saving materials of future, are a group of emerging materials with excellent combination of strength and ductility and superior specific strength property(strength-to-weight ratio). Having said this, Mg nanocomposites are considered as promising replacement for other structural alloys(i.e. aluminum and titanium) wherever low density and high strength are required, i.e. transportation, aerospace, defense, etc. To be able to apply this group of materials for real components, different failure mechanisms at ambient and elevated temperatures under static and dynamic loading condition must be well documented. Compared with other metals and alloys,rate-dependent plastic deformation(creep), at ambient and elevated temperatures, of these novel materials is not yet well studied which seems a tangible lack of knowledge. This is required since the materials in service are often exposed to medium and elevated temperatures and/or static loads for long duration of time and this encourages creep failure on them. To this end, the information and the controlling mechanisms on time/temperature-dependent response of the material need to be developed to be able to predict the response of the Mg nanocomposites where the materials are under creep conditions. This paper aims at providing an overview on(i) creep-resistant Mg alloys(as matrix) and their chemical compositions, and(ii) responses of the Mg nanocomposites at different creep conditions(time and temperature). The controlling mechanisms contributing to the strength and ductility of the Mg nanocomposites due to the presence of the nanoparticles have been reviewed briefly in the present article. In this paper both traditional(uniaxial) and depth-sensing indentation creep of Mg nanocomposites are reviewed. Also, some fundamental questions and possible explanations have been raised on the creep characteristics of Mg nanocomposites and the contribution of micro structural features(i.e.grain boundaries, twins, precipitates, nanoparticles). This overview article provides a comprehensive summary to understand one of the failure modes(creep) at ambient and elevated temperature in the energy saving Mg nanocomposites that would be of interest for those in academia who explore novel nanocomposites.

RHEOLOGICAL BEHAVIOUR FOR AN ANHYDRITE SPECIMEN AND DETERMINATION OF ITS LONG TIME STRENGTH

Based on the method of torsional creep, the creep laws of ananhydrite specimen are studied in this paper. When a shearing stressapplied to the specimen is less than a value, only the primary stagetakes place. How- ever, when the shearing stress is more than anothervalue, all the three stages of a creep curve, i. e. primary, steady-state and accelerated are exhibited.

卸荷损伤砂岩的分级加载三轴蠕变力学特性试验研究

为探究含初始卸荷损伤砂岩的三轴蠕变力学特性,借助MTS816岩石力学试验系统通过控制卸荷量制备了不同初始卸荷损伤程度的砂岩试样,开展了卸荷损伤砂岩分级加载三轴蠕变力学试验,分析了初始卸荷损伤程度对砂岩应变特征、蠕变速率、变形模量、长期强度及破坏模式的影响规律。研究结果表明:(1)分级蠕变条件下,不同初始卸荷损伤程度砂岩的轴向和环向应变均呈阶梯式增长,轴向瞬时应变随载荷水平的增加呈近线性增长趋势,而环向瞬时应变呈非线性增长趋势,且随初始卸荷损伤程度的加剧,增加单位荷载引起的轴向瞬时应变增量不断增加;(2)卸荷损伤砂岩在非破坏阶段经历了瞬时变形、减速蠕变和稳态蠕变3个阶段,稳态蠕变速率随初始卸荷损伤程度的增加呈指数增长趋势;砂岩在破坏阶段经历了瞬时变形、减速蠕变、稳态蠕变和加速蠕变4个阶段,其中加速蠕变阶段用时极短,却能产生较大的扩容变形,导致岩样发生失稳破坏;(3)砂岩蠕变总时长、平均变形模量和长期强度均随初始卸荷损伤程度的增加呈非线性减小趋势,当初始卸荷损伤程度超过70%后,砂岩蠕变总时长、平均变形模量和长期强度降幅急剧增加,最高可达24.77%、33.28%和21.79%;(4)随着初始卸荷损伤程度的增加,砂岩蠕变破坏模式由单一的剪切破坏逐渐向复杂的张拉-剪切混合式破坏转变。

Aging Coefficient, Creep Coefficient and Extrapolating Aging Coefficient from Short Term Test for Sealed Concrete

Aging coefficient and creep coefficient are important parameters for creep analysis of any structure. With the aim to obtain those parameters, an experimental investigation is carried out on sealed concrete. Altogether ten specimens were tested, out of which four were for creep, two for shrinkage and remaining four for relaxation test. A year of relaxation test and two years of creep test results were analyzed to compute aging coefficient and creep coefficient. From regression analysis of observed and calculated aging coefficient using formula of Bazant and Kim, modification for aging coefficient is performed and extrapolation equations are generated. Instead of ACI model, B3 model has been used to obtain compliance function as the parameters of B3 model seems more relevant to creep mechanism of concrete compared to that of ACI model.

黄土卸荷蠕变特性与典型开挖型黄土滑坡机理研究

开挖卸荷是黄土滑坡的主要诱发因素之一,为深入研究黄土的卸荷蠕变特性与开挖型黄土滑坡的机理,本文选择典型开挖型黄土滑坡——榆林市上庄滑坡黄土作为研究对象,进行了一系列三轴卸荷蠕变试验,分析了卸荷对黄土蠕变特性的影响,并结合试验成果和数值模拟方法对开挖型黄土滑坡机理进行了揭示。三轴卸荷蠕变试验结果表明在卸荷条件下,卸荷应力水平越大,试样蠕变变形越大且达到稳定蠕变状态所需的时间就越长。初始固结应力越大,试样蠕变破坏所需卸荷值占围压的比例越小。利用等时曲线法求取了试样长期强度并计算出研究区黄土的长期黏聚力与长期内摩擦角值,引入长期强度折损率,并对瞬时强度与长期强度的规律进行了对比分析。数值模拟计算结果显示在坡脚开挖处形成了应变集中区,坡面由上至下的位移逐渐增大。相较于未开挖情况下,开挖后土体中卸荷应力明显增大,导致坡体蠕变变形增大,最终造成了坡体滑移。研究成果可为此类滑坡防治提供理论借鉴。

Laboratory creep tests for time-dependent properties of a marble in Jinping Ⅱ hydropower station

In order to investigate the time-dependent behaviors of deep hard rocks in the diversion tunnel of Jinping II hydropower station, uniaxial creep tests were carried out by using the triaxial testing machine RC-2000. The axial compressive load was applied step by step and each creep stage was kept for over several days. Test results show that： （1） The lateral deformation of rock specimens is 2-3 times the axial compressive deformation and accelerates drastically before damage, which may be employed as an indicator to predict the excavation-induced instability of rocks. （2） The resultant deformation changes from compression to expansion when the Poisson＇s ratio is larger than 0.5, indicating the starting point of damage. （3） In the step-loading stages, the Poisson＇s ratio approximately remains constant; under constantly imposed load, the Poisson＇s ratio changes with elapsed time, growing continuously before the specimen is damaged. （4） When the applied load reaches a certain threshold value, the rock deteriorates with time, and the strength of rocks approximately has a negative exponent relation with time. （5） The failure modes of the deep marble are different in long- and short-term loading conditions. Under the condition of short-term loading, the specimen presents a mode of tensile failure; while under the condition of long-term loading, the specimen presents a mode of shear failure, followed by tensile failure.

考虑温度和时间双重影响的混凝土蠕变模型构建

为了研究温度对混凝土结构蠕变特性的影响,开展不同温度作用下混凝土三轴蠕变试验,分析了不同温度作用下混凝土结构的轴向蠕变特性。在温度作用下,混凝土的内部损伤与非均匀程度参数有关,建立非均匀程度参数与温度之间的关系;假定混凝土蠕变模型是关于温度和时间的函数,结合上述损伤变量和微元体强度分布概率模型,建立了考虑温度和时间双重影响的混凝土蠕变模型。基于最小二乘法和试验数据对模型的准确性进行验证,并采用西原模型对其优越性作进一步验证。结果表明:不同温度作用下蠕变损伤模型与试验数据的吻合度都较好,且相关性系数在0.95以上,说明了所建立模型的正确性;同时,该模型与试验数据的吻合度要优于西原模型,并且在描述加速蠕变方面具有一定的优越性。该理论模型为地下工程中混凝土支护结构的长期变形预测研究提供理论依据。

考虑高温蠕变损伤的服役金属屈服强度模型探析

在役设备长期承受高温高压且无法预测其重要力学性能指标,有必要建立考虑高温蠕变损伤的屈服强度模型。传统屈服准则未考虑温度的影响以及材料有效杨氏模量随损伤下降的趋势。在阐述金属温度相关性屈服强度理论模型和K-R蠕变损伤本构方程的基础上,通过建立服役杨氏模量与蠕变损伤内变量的内在联系,推导出考虑高温蠕变损伤的服役金属屈服强度模型。以SA508-Ⅲ钢材料为例,开展了蠕变中断+高温拉伸试验,理论计算与试验结果呈现一致性。

预时效对7B50铝合金应力时效行为及力学性能的影响

采用扫描电镜和透射电镜观察了7B50铝合金预时效及蠕变时效后的组织,采用电子蠕变试验机进行了蠕变时效试验,采用拉伸试验机测试了合金力学性能,分析了预时效对蠕变时效铝合金组织和力学性能的影响。研究结果表明,140℃3 h预时效的7B50铝合金,在160℃160 MPa蠕变时效8 h,力学性能显著提高,屈服强度可达593.4 MPa,有望成为适合蠕变时效成形的板材。与固溶处理后常温放置的试样相比,预时效可以减少空位浓度,降低合金蠕变应变,还可以促进原子扩散,调控晶格畸变和蠕变应变。提高预时效温度可以减少空位浓度,抑制η′相向η相的转变,从而获得η′相密度高且弥散分布的理想显微组织,显著提高了合金的强度。140℃3 h预时效使合金在蠕变时效的屈服强度曲线上产生了一个峰值保持区,有利于延长时效时间并改善蠕变行为而不牺牲合金强度。