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.

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。

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。

粗集料对超高强混凝土徐变的影响

徐变是混凝土固有的时变特性,对混凝土结构和构件的受力及长期服役性能有着重大影响。以粗集料为切入点,研究粗集料的掺量、粗集料最大粒径以及粗集料与基体性质协调性等因素对超高强混凝土的力学性能和徐变性能的影响。结果表明:掺粗集料的超高强混凝土徐变明显小于不掺粗集料的混凝土,掺粗集料可有效降低超高强混凝土的徐变,但减小粗集料的最大粒径对徐变无明显抑制效果。此外,超高强混凝土并非强度越高徐变越小,其徐变的大小主要受超高强混凝土强度与弹性模量的协同发展程度的影响。通过修正FIB MC2010模型,建立了适用于超高强混凝土的徐变预测模型。

微量Fe、Si对铝导电杆组织与性能的影响

采用电子背散射衍射(EBSD)、扫描电镜(SEM)、透射电镜(TEM)、扫描探针显微镜(SPM)、室温拉伸、单臂电桥和纳米压痕蠕变等方法,研究了微量Fe、Si对铝导电杆组织与性能的影响。结果表明:Fe、Si能够降低导电杆的晶粒尺寸,并使织构组分由弱〈001〉+〈111〉双织构向强〈111〉织构转变,产生沿挤压方向分布的Al-Fe和Al-Fe-Si相,且在第二相和(亚)晶界处出现位错塞积;Fe、Si通过粗颗粒促进再结晶和细颗粒抑制再结晶的共同作用,造成导电杆的显微组织沿径向出现周期性交替分布;Fe、Si能够在牺牲较小导电率的前提下,显著提高铝导电杆的强度和抗蠕变性能。

采用连铸坯制造的P92钢管性能试验研究

为评估采用连铸坯轧制的P92钢管的性能,对几家钢厂提供的3支采用连铸坯轧制的P92大直径厚壁钢管进行理化检测、625℃高温持久强度试验等。结果表明:采用连铸坯轧制的P92钢管的各项理化性能满足相关标准要求,625℃、10万h外推高温持久强度为93.1 MPa,高于规范推荐值。因此,连铸坯P92钢管已具备工程应用条件,可降低产业链成本。

硅灰和骨料对混凝土耐高温性能的影响

混凝土暴露在高温下,其组成材料经历不同的体积变化与损伤时就会产生裂纹使得耐久性能与力学性能降低,胶凝材料与粗骨料在其中起着重要作用。在此对比掺入10%硅灰和采用碎石、页岩陶粒对混凝土高温后强度损失的影响与形貌变化。高温后持续荷载下的徐变至关重要,所以还对比了不同骨料类型与荷载对混凝土徐变的影响。结果表明:400℃后混凝土强度明显下降,硅灰的掺入增大了强度降幅。轻骨料混凝土的残余抗压强度在高温下远高于常规混凝土,且内部由于热膨胀应力的缓解出现了较少的裂缝。当施加恒载时,500℃以下轻骨料混凝土变形大,超过500℃,普通混凝土变形高于轻骨料混凝土。

高温作用后的北山花岗岩时效变形特征研究

以甘肃高放废物重点预选场址的北山花岗岩为研究对象,开展不同热损伤和围压条件下的三轴分级蠕变试验,分析热力耦合作用下的北山花岗岩蠕变变形机理。试验结果表明,在单轴条件下,北山花岗岩的微裂纹密度随热处理温度升高而逐渐增加,进而导致轴向峰值应变持续增大。150℃和300℃温度荷载造成的轻微热开裂引发了裂纹尖端钝化,从而增强了热损伤花岗岩的长期强度;当热处理温度达到600℃后,石英相变产生的差异性热膨胀导致北山花岗岩的热裂纹密度显著增加,进而造成长期强度急剧降低。相比之下,在5 MPa和25 MPa围压的三轴蠕变试验中,静水压力能够愈合较低温度荷载造成的热开裂,使150℃和300℃热损伤花岗岩的轴向峰值变形和长期强度恢复至无损岩石水平。然而,上述静水压力未能完全愈合600℃热处理温度造成的热开裂,热损伤花岗岩的残余微裂纹导致轴向峰值应变增大,并造成了裂纹尖端的钝化效应,最终导致北山花岗岩的长期强度增大。

红砂岩多级卸荷蠕变试验及长期强度分析

在实际基坑和硐室的开挖过程中,岩体多处于环向卸荷的应力状态.不同于以往常规路径下的岩石蠕变特性研究,采用全自动岩石多场耦合三轴伺服仪,以红砂岩为试验对象着重研究了岩石环向卸荷路径下的蠕变力学特性.试验结果表明,与常规路径下的蠕变试验相似,岩石环向卸荷路径下的蠕变过程,同样由衰减蠕变到稳态蠕变再到非线性加速蠕变这一典型岩石流变三阶段所组成.随着围压的逐级卸载,轴向或者侧向蠕变变形,蠕变变形相对该级荷载下的瞬时变形的百分比,蠕变变形占总变形的比例,都整体呈现增大的趋势.侧向的蠕变速率、瞬时变形、蠕变变形在这一过程中逐渐超过轴向相应值,并最终达到轴向的2~3倍,出现明显的侧向扩容.基于经典弹塑性理论公式,绘制了塑性应变与偏应力水平的关系曲线,采用多种不同方式确定并比较了红砂岩卸荷蠕变过程中的长期强度.

不同浸润时间下煤岩蠕变力学模型研究

以煤岩为研究对象,进行不同浸润时间下的单轴压缩蠕变试验。试验表明:煤岩在轴向应力加载瞬间,产生瞬时弹性应变,接着出现衰减、稳定蠕变,当加载应力高于长期强度时,表现出加速蠕变行为;通过等时应力-应变曲线求拐点的方式,确定煤岩长期强度,随着浸润时间的增长而递减。基于煤岩蠕变的黏弹塑性,确定基础蠕变模型,利用考虑损伤的Hooke体描述煤岩瞬时弹性应变,分数阶黏滞体表征黏弹性应变,分数阶黏塑性体反映黏塑性应变,构建一个新的改进后的煤岩蠕变力学模型。结果表明:改进后模型对煤岩蠕变曲线具有良好模拟效果,能较为准确反映煤岩在不同浸润时间下的蠕变力学行为。

干湿循环作用下吹填固化轻质土蠕变特性研究

采用新型固化技术对高含水率吹填土进行固化处理,通过三轴流变仪,考虑干湿循环次数、围压和初始静偏应力等因素研究其对吹填固化轻质土蠕变变形和抗剪强度指标的影响规律。试验结果表明:吹填固化轻质土具有密度小、质量轻及强度高等优点。在一定程度上,增大围压可有效缓解吹填固化轻质土的蠕变变形,而初始静偏应力对其影响较小,前7次干湿循环对土体破坏应力的影响最大,而后基本保持不变。抗剪强度指标c、φ值均随初始静偏应力的增加而逐渐降低,随干湿循环次数的增加呈现出先减小而后逐渐稳定的趋势,且前7次干湿循环,二者均呈指数函数降低;7次以后,变化均较小并逐渐稳定。基于c、φ值与时间关系曲线,分别构建了不同偏应力作用下吹填固化轻质土c、φ值的预测公式。根据蠕变曲线发展态势,通过回归分析,引入围压、初始静偏应力和干湿循环次数等因素,建立了吹填固化轻质土蠕变变形预测模型,并验证了其可行性。研究结果可为实际工程场地提供理论依据。

含水排土场蠕变特性及长期强度研究

为揭示排土场岩土体蠕变特性及长期强度,采用ZLB-1型三联蠕变直剪蠕变仪,以分级加载形式开展一系列试验,研究不同含水率和固结压力下,岩土体蠕变特性及长期强度变化规律。试验结果表明:15%~39.2%(饱和)含水率范围内,排土场岩土体显示为非线性蠕变,且随着剪应力增加,蠕变曲线分别呈现衰减蠕变、非稳定蠕变及加速蠕变,直至试样发生破坏;施加剪应力瞬间发生较大瞬时应变,并随剪应力增加而变大;长期强度指标与含水率呈现指数递减趋势;对比15%含水率试样,饱和含水率下排土场岩土体长期强度指标中黏聚力降低66%,内摩擦角降低69%,其变化规律满足指数拟合公式。研究成果可为同类型含水排土场稳定性计算提供基础数据。

花花岩组滑坡堆积体蠕变特性研究

花花岩组滑坡是一种堆积体滑坡,其滑体由松散的土石堆积体组成,在滑坡缓慢变形中具有明显的蠕动特点。为研究该滑坡堆积体的蠕变特性,确定滑坡堆积体的长期强度指标,对滑坡堆积体进行了直剪蠕变试验。试验结果表明:滑坡堆积体在直剪蠕变试验中表现出明显的减速蠕变和稳态蠕变特征,在较大水平荷载作用下会进入加速蠕变状态;采用Burgers模型以及改进后的Burgers模型对蠕变曲线进行拟合,其结果表明,改进的Burgers模型对非线性的加速蠕变过程拟合效果更好,拟合曲线更接近实际蠕变曲线;通过过渡蠕变法确定了滑坡堆积体的长期强度范围,滑坡堆积体的长期强度为其瞬时抗剪强度的39%~59%。

冻土动力学研究综述

冻土动力学是寒区岩土工程所要考虑的重要内容,是研究冻土抗震和抗长期循环荷载的理论基础。本文主要介绍了冻土动力学参数的测试方法,回顾了冻土动力学参数、冻土动强度、冻土动蠕变破坏特征和冻土动蠕变强度的研究进展,并对部分冻土动态本构模型和动蠕变模型进行了简单的介绍,最后对冻土动力学的发展趋势进行了展望。