锅炉用奥氏体不锈钢弯管内部氧化皮检测的新方法

以锅炉用奥氏体不锈钢管TP304H和TP347H为例,采用剩磁法和提升力法对奥氏体不锈钢弯管内部氧化皮的堆积量进行检测.结果表明:外部磁场激化后,奥氏体不锈钢弯管内部氧化皮的剩磁磁感应强度和提升力均随堆积量的增加而增大,在一定量后变化趋于平缓;剩磁磁感应强度和提升力还与奥氏体不锈钢管的规格有关,壁厚越大或内径越大,剩磁磁感应强度和提升力越小;V型弯管内氧化皮的剩磁磁感应强度和提升力大,U型弯管次之,L型弯管最小.剩磁法和提升力法是快速检测奥氏体不锈钢弯管内部氧化皮堆积量的一种新方法.     

锅炉管内壁氧化皮厚度测量影响因素分析

本文针对锅炉管内壁氧化皮厚度的超声波测量，研究了超声波在氧化皮中的传播速度、氧化皮分层、剥落、氧化皮与金属基体间隙等因素对测量准确性的影响规律，同时给出了典型情况下的测量波形。研究结果对氧化皮厚度超声波测量仪器的使用，提高测量准确性具有指导及参考意义。     

超(超)临界锅炉高温受热面蒸汽氧化皮的生长与剥落特性

阐述了超(超)临界锅炉高温受热面蒸汽氧化皮的晶粒特点,重点分析了铁素体钢和奥氏体钢蒸汽氧化皮的结构特征及其生成和脱落现律.指出高温受热面蒸汽氧化皮的产生是不可避免的,应进一步研究各种运行条件对氧化皮程度、结构形态和剥落时机的影响,以便从机组的运行方式、检修策略和控制理念等各方面来预测和防止氧化皮的危害.     

超临界锅炉奥氏体钢管氧化研究与预防

奥氏体钢(如SA-213TP347)以其良好的高温持久强度和高温抗氧化等性能被广泛使用在锅炉的过热器和再热器上;但是,随着运行时间加长,也发生了多起由于氧化皮剥落堵塞管子产生爆破,造成非正常停炉的事故;本研究通过跟踪在超临界状态下运行达20 000 h的电站锅炉受热面使用的奥氏体钢管(TP347H)内壁氧化状况,对生长的氧化皮和材料基体进行了取样,加工样品后,进行了宏观、体式镜和电镜能谱等实验检测,对比分析了奥氏体钢(TP347H)基体材料、剥落下的氧化皮和未剥落尚附着在基体上的氧化皮以及氧化皮内、外双层的实验检测数据和图像,详细阐明了氧化皮的微观形貌、组织结构及其化学成分的分布特征;阐述了高温水介质作用下奥氏体钢氧化腐蚀的机理,分析了奥氏体钢在高温水介质环境下氧化皮的产生、生长和剥落的影响因素提出了相应的防范措施。     

基于“环境破坏说”的氧化皮剥落理论

在“环境破坏说”的基础上阐释了锅炉高温受热面氧化皮剥落的理论．双层氧化皮内、外层界面存在的空穴是氧化皮剥落的内因,氧化皮承受的应力是氧化皮剥落的外因．氧化皮空穴的成因又可分为内因和外因,内因主要是金属含铬量,外因主要包括蒸汽参数和蒸汽含氧量等．主蒸汽含氧量与铬酸根含量之间具有显著的正相关性,而主蒸汽氢电导率是反映氧化皮铬蒸发的特征指标．外层氧化皮剥落以后,内层氧化皮会继续增厚,但难以剥落．对于已经发生了氧化皮大面积剥落的超（超）临界锅炉,如果检查确认高温受热面氧化皮剥落比较完全,剥落之氧化皮清理比较彻底,则在相当长时期内氧化皮剥落问题都将不再是运行忧患．     

锅炉高温受热面蒸汽侧氧化皮的形成及剥落机理研究进展

综合分析了各种类型的锅炉耐热钢管蒸汽侧氧化皮的微观形貌、氧化皮的层结构和成分组成。锅炉耐热钢蒸汽侧所生成的氧化皮有双层和3层结构,双层和3层结构的氧化皮的剥落位置发生在不同的亚层。对锅炉管内壁氧化皮剥落部位进行氧化皮生长的跟踪研究,可以得到氧化皮进一步生长的规律。氧化皮各层氧化物与基体金属间由于线性热膨胀系数差异所产生的热应力是导致氧化皮产生开裂和剥落的最根本原因,防治氧化皮大面积剥落的措施主要是抑制氧化皮生长速率和减少热应力的大幅度变化。由于实验室的试验条件难以模拟实际锅炉的蒸汽参数和应力变化,开发实验条件接近实际锅炉的蒸汽氧化试验是研究新型耐热钢内壁氧化皮生长及剥落机理的发展方向。     

超临界机组的合金管蒸汽侧氧化膜生长与剥落研究进展

超临界机组高温合金管氧化膜剥落问题是困扰机组安全与经济运行的难题,严重制约了机组蒸汽参数和效率的提高。特别是,在超临界机组采用给水加氧处理方式(OT)后,奥氏体不锈钢管内壁氧化膜大面积剥落事故屡见不鲜,尤以TP347H合金管为甚。本文总结了近年来国内外针对超临界机组合金管氧化膜研究的进展及相关成果,首先介绍了超临界蒸汽环境中合金管氧化机理和原子迁移机制,综述了铁素体和奥氏体合金表面氧化膜的形貌特征,分析了蒸汽溶氧对氧化膜生长速率、形貌和缺陷的影响。氧化膜完整性是决定合金抗腐蚀性能的重要因素,但在机组运行过程中氧化膜应力破坏了氧化膜完整性。进一步总结了国内外氧化膜应力和剥落研究的数值分析及实验研究情况,为我国超临界机组氧化膜剥落故障诊断研究提供参考。     

超临界机组锅炉滑停对氧化脱落的影响

当前由上海锅炉厂生产的350 MW超临界锅炉在进行滑停和正常停炉过程中,高温受热面奥氏体不锈钢材质,从高温降至低温时,对管壁内生成的氧化皮脱落情况进行对比分析发现:随着锅炉累计运行时间逐渐增长,锅炉高温受热面管壁内形成的氧化皮也逐步增厚,由于氧化皮与受热面管材的膨胀系数不同,温度变化率超出允许范围时,氧化皮会从管屏金属内壁上剥脱,一旦大量氧化皮集中剥落,就会在受热面弯头处堵塞管道引起爆管等等问题。文中就超临界锅炉在滑停过程中氧化皮脱落现象进行了探讨,并提出了锅炉滑停过程中的防控措施。     

超临界锅炉减温水引发氧化皮剥落问题的分析与防范

超(超)临界机组锅炉高温炉管内壁氧化皮剥落堵塞引起的过热爆管是近几年来困扰机组安全运行的突出问题之一。在对两起爆管事件进行失效分析过程发现,锅炉启动过程因减温水投用不当会大大促进氧化皮的集中剥落。通过列举两起爆管案例,查阅了减温水投用记录,进一步分析表明氧化皮的集中剥落与减温水投用不当有很强的关联性。针对减温水引发的过热爆管提出的防范措施可供生产实践借鉴。     

600MW超临界锅炉高温受热面蒸汽氧化初探

分析了600 MW超临界锅炉高温管屏蒸汽氧化及其氧化皮剥落的机理和影响因素,总结了氧化皮的产生、剥落和堆积堵塞对锅炉安全运行的危害,并从管材的选择及其热处理和锅炉设备的设计、运行等各环节提出了减缓氧化皮的产生、防止氧化皮集中脱落、避免因氧化皮堆积堵塞而导致锅炉高温管屏超温爆管的防控措施。     

Thermal environment effects on wave dispersion behavior of inhomogeneous strain gradient nanobeams based on higher order refined beam theory

In this article, an analytical model for the wave propagation analysis of inhomogeneous functionally graded (FG) nanobeam in thermal environment is developed based on nonlocal strain gradient theory, in which the stress accounts for not only the nonlocal elastic stress field but also the strain gradients stress field. The nanobeam is modeled through a higher order shear deformable refined beam theory which has a trigonometric shear stress function. The temperature field supposed to have a nonlinear distribution across the nanobeam thickness. Temperature-dependent material properties of nanobeams are spatially graded based on Mori–Tanaka model. The governing equations of the temperature-dependent functionally graded (FG) nanobeam are derived using the Hamilton’s principle. Numerical examples show that the characteristics of the wave propagation of FG nanobeam are influenced by various parameters such as nonlocality parameter, length scale parameter, gradient index, and temperature changes.     

Size-Dependent Thermal Buckling and Postbuckling of Functionally Graded Annular Microplates Based on the Modified Strain Gradient Theory

This article reports on the thermal instability of functionally graded (FG) annular microplates with different boundary conditions. The modified strain gradient elasticity theory is employed to capture size effects. The non-linear governing equations and boundary conditions are derived based on the first-order shear deformation theory (FSDT) and virtual displacements principle. The generalized differential quadrature technique is implemented so as to discretize. To obtain the critical buckling temperature, the set of linear discretized governing equations is solved as an eigenvalue problem. Also, the non-linear problem of thermal postbuckling is solved by the pseudo arc-length continuation method. The effects of boundary conditions, length scale parameter, and the variation of material through the thickness and geometrical properties on both critical buckling temperature and thermal postbuckling behavior are studied.     

Thermomechanical initial post-buckling of Timoshenko's beam on elastic foundations

The objective of this work is to propose an analytical solution for the thermomechanical initial post-buckling response of a thick beam resting on a linear elastic foundation and subject to a uniform temperature rise throughout its cross-section. The thermal strain is assumed to follow a linear law with the temperature rise and the material properties are considered temperature independent. The beam cross-section geometrical properties are constant along the beam length, and the formulation is consistent with the small strain assumption. The beam ends are assumed pinned and immovable, thermal expansion is not allowed and as a consequence compressive forces arise and the beam may buckle. If the temperature is increased further, the beam continues to deflect laterally, hence the problem is geometrically non-linear. In addition, the model is appropriate to describe the behavior of short beams as it takes into account transverse shear deformations. The governing equations are derived and made non-dimensional, and it is seen that three non-dimensional parameters control the thermomechanical initial post-buckling problem: The elastic foundation stiffness, the beam slenderness ratio and the beam cross-section shear coefficient. A classical perturbation method is applied to the non-linear set of differential governing equations, therefore the critical buckling temperatures (loads) and modes and the initial post-buckling behavior may be analytically determined. The change in length, reaction forces at the supports and geometric configurations are obtained as a function of temperature, the elastic foundation.     

Assessment of the limits to the protective effect of oxide scales in high temperature technology, set by deformation of the substrate material

At high temperatures materials have to stand corrosive attack by respective environments. The prerequisite for good high temperature corrosion resistance is the formation of dense protective oxide scales on the alloy surfaces which prevent increased corrosive attack. Due to their brittle nature, however, even low strains may crack the protective scales so that considerations on the limits of the protective effect become necessary when the substrate alloy is subjected to strains. These considerations are performed in the paper including aspects of scale cracking and healing and in the end a scale failure diagram is developed which shows the limits of the protective effect of oxide scales as a function of (substrate) strain and strain rate. It is also proposed to extend this diagram by the parameters of physical defect size in the scale and of scale thickness, which would lead to increased precision when characterizing the limits of the protective effect. The use of the scale failure diagram in design and service helps to avoid component damage resulting from the interaction of high temperature corrosion and mechanical stresses.     

Hygrothermal effects on static stability of embedded single-layer graphene sheets based on nonlocal strain gradient elasticity theory

Abstract

This article develops a nonlocal strain gradient plate model for buckling analysis of graphene sheets under hygrothermal environments. For more accurate analysis of graphene sheets, the proposed theory contains two scale parameters related to the nonlocal and strain gradient effects. Graphene sheet is modeled via a two-variable shear deformation plate theory needless of shear correction factors. Governing equations of a nonlocal strain gradient graphene sheet on elastic substrate are derived via Hamilton’s principle. Galerkin’s method is implemented to solve the governing equations for different boundary conditions. Effects of different factors such as moisture concentration rise, temperature rise, nonlocal parameter, length scale parameter, elastic foundation and geometrical parameters on buckling characteristics a graphene sheets are examined.     

蒸汽侧氧化膜对超临界机组T92钢管壁温的影响

通过建立T92钢受热管及其蒸汽侧氧化膜的数值分析模型,定量分析了不同厚度氧化膜对T92钢受热管壁温的影响,并在此基础上得出了不同受热条件下导致T92钢管超温运行的氧化膜的临界厚度值.结果表明:随着氧化膜厚度的增加,管壁平均温度和平均温度升高幅度均呈近似线性增加;且管内壁温度越高、管外热流密度越大,氧化膜的临界厚度值越小.因此,当T92钢应用于高温受热面时,需综合考虑壁面热负荷和管内壁温度对氧化膜临界厚度的影响.     

高拱坝建基面相对损伤面积敏感性分析

基于损伤本构关系的非线性有限元能较为真实地模拟拱坝的实际工作性态,但用于高拱坝安全度评价时水压力和温度荷载、坝体混凝土的弹性模量和抗拉强度、地基岩石的弹性模量等均具有随机性。以某高拱坝为例,以温降荷载组合作为基本研究工况,根据地形地质条件和坝体体形及施工过程,建立高拱坝非线性有限元计算模型,通过正交试验法建立18种计算方案,得到不同方案下上下游面、建基面相对损伤面积,再运用无量纲化和多元线性回归分析的方法计算建基面相对损伤面积与各影响因素之间的关系。结果表明,建基面相对损伤面积与水荷载、温降荷载呈正相关,与地基岩石弹性模量、坝体混凝土弹性模量、坝体混凝土抗拉强度呈负相关。对建基面相对损伤面积的影响程度从大到小依次为水荷载、地基岩石弹性模量、坝体混凝土弹性模量、坝体混凝土抗拉强度、温降荷载。     

Crack arrest in modern steels and their weldments—Comparison between small and large scale experiments

The results of a range of different small scale material characterisation tests were correlated with the crack arrest temperature at the yield strength of a number of modern ferritic steels and their weldments. The crack arrest temperature was determined experimentally using large scale structurally representative double tension tests. The comparison of small and large scale tests indicates that a safe estimate of the crack arrest temperature for the steels and welds investigated can be obtained using the nil-ductility transition temperature plus 40°C. The 50% fracture appearance transition temperature or the 20% fracture appearance transition temperature plus 20°C obtained from full thickness drop weight tear test are also reliable measures of the lower bound crack arrest temperatures.     

Transient Piezothermoelasticity of a Two-Layered Composite Hollow Cylinder Constructed of Isotropic Elastic and Piezoelectric Layers Due to Asymmetrical Heating

This article is concerned with the theoretical treatment of transient piezothermoelastic problem involving a two-layered hollow cylinder constructed of isotropic elastic and piezoelectric layers due to asymmetrical heat supply. The transient two-dimensional temperature is analyzed by the method of Laplace transformation. By using the exact solutions for piezoelectric hollow cylinder and isotropic hollow cylinder, the theoretical analysis of transient piezothermoelasticity is developed for a two-layered composite hollow cylinder under the state of plane strain. As an example, numerical calculations are carried out for an isotropic elastic hollow cylinder made of steel, bonded to a piezoelectric layer of cadmium selenide. Some numerical results for the temperature change, the stress and the electric potential distributions in a transient state are shown in figures. Furthermore, the influence of thickness of the piezoelectric layer or the isotropic elastic layer upon the temperature change, stresses and electric potential is investigated.     

Numerical simulation on the slag flow and heat transfer characteristics of the cyclone barrel for a cyclone-fired boiler

A numerical simulation coupled with the user-defined function (UDF) is conducted on a cyclone-fired boiler. The thermal boundary condition is established for the cyclone barrel. The effects of the temperature of critical viscosity and the refractory material thickness on slag behavior are investigated. The results show that the temperature of critical viscosity is closely related to the location where the slag begins to form in the cyclone barrel. The refractory material thickness remarkably influences the surface temperature of the refractory material, and the solid slag thickness adapts to the variation of the refractory material thickness.