Spatial variation of residual stresses in a welded pipe for high temperature applications

Measurements of residual macro-stresses have been undertaken in a feature multipass circumferential single V butt-weld made from a P91 ferritic steel pipe over different spatial depths: (i) ≤10 μm by X-ray diffraction, (ii) ≤1 mm by incremental centre-hole drilling and (iii) through wall section using deep-hole drilling. The ability to make near-surface X-ray residual stress measurements on as-oxidised surfaces has been demonstrated and the implications for use in the evaluation of overall integrity are discussed. Each of the three measurement techniques provides complementary and consistent measurement of induced residual stresses for weld metal, heat affected zone and parent metal for the as-welded and the post-weld heat-treated conditions over the complete spatial range. The results are discussed with respect to the importance of the weld capping run in introducing near-surface compressive residual stresses, the through-wall profiles of the residual stresses measured at the weld metal position in hoop and axial directions and the presence of existing surface oxide.     

Creep damage prediction of the steam pipelines with high temperature and high pressure

Creep is the significant factor that caused failure of steam pipelines with high temperature and high pressure in the period of long-term service. In this paper, the creep tests were performed at serviced temperature of 520 °C for 1.25Cr–0.5Mo pipe material, and the creep and fracture constants were obtained by fitting the creep test data. Based on the modified Karchanov–Rabotnov constitutive equation, compiled the user subroutine computing the damage of the pipe element or 3D solid element, the creep damage prediction was carried out by finite element methods using ABAQUS codes for the steam pipelines with high temperature and high pressure, which serviced in a petro-chemical plant, the damage distribution and maximum damage location of the pipelines were obtained, which is testified by metallographic examination result. Furthermore, the local creep damage analysis of a tapered pipe serviced for 100,000 h was also carried out because tapered pipes used in the main steam pipeline is one of weakness in the piping system. Damage distribution and evolution in the analyzed tapered pipe were obtained. The location with the maximum damage value was determined, which is coincident with cracking position of the actual tapered pipe.     

某电厂锅炉高温再热器异种钢焊口断裂原因分析

分析了锅炉高温再热器焊口产生断裂的原因,锅炉停炉时产生交变应力,钢管产生裂纹,并发生脆性断裂。     

Melting of PCM around a horizontal cylinder with constant surface temperature

A numerical study of melting of phase change material (PCM) around a horizontal circular cylinder of constant wall temperature and in the presence of the natural convection in the melt region is presented. A two dimensional mathematical model is formulated in terms of primitive variables and a coordinate transformation technique is used to fix the moving front. The finite volume approach is used to discretize the system of governing equations to obtain a system of linear algebraic equations. An implicit scheme is used for the momentum and energy equations and an explicit scheme for the energy balance at the interface. The numerical predictions were compared with available results to establish the validity of the model and additional results are obtained to demonstrate the effects of Rayleigh and Stefan numbers as well as the wall temperature on the time for complete fusion and total melt volume.     

某企业自备热电厂高温高压机组节能改造研究

针对国内某洗涤用品企业自备热电厂锅炉热效率低、汽轮机汽耗率高以及企业实际用电情况,把原35t/h中温中压循环流化床锅炉及配套背压汽轮发电机组改为40t/h高温高压锅炉及配套反动式高转速汽轮发电机组(西门子技术)后,锅炉热效率提高了8个百分点,平均汽耗率降至8.3kg/k Wh,相同供热条件下,热电厂每年多供电1800多万k Wh,取得了较好的节能效果,为企业节约了燃煤及外购电成本。     

Strength design and life assessment of welded structures subjected to high temperature creep

Premature failures of weldments have taken a large percentage among the failure cases of high temperature components. The accurate prediction of the high temperature creep behavior of welded components is hence becoming increasingly important in order to realize an optimized design and management of a plant life. In the present paper, design and life assessment procedure for welded structures at elevated temperature is discussed. A concept of structural creep-rupture strength is proposed to define the weldment creep strength reduction factor. Multiaxial effect is taken into account. A damage mechanics approach is introduced to study the life reduction and ductility reduction due to the presence of a weld in structures at high temperature.     

Effects of a stress relief heat treatment on the toughness of pressure vessel quality steels welded with high heat input processes

The research described in this paper is an extension of an earlier research programme on the influence of a post weld heat treatment on the toughness of welded joints in pressure vessel quality steels.¹ In this part of the work special attention was paid to the heat input of the welding process involved and the influence of the subsequent heat treatment. In addition to the qualification tests, several wide plate tests were carried out.The results indicate that, while for some steels the influence of a stress relief heat treatment on the toughness of the joint depends considerably on the heat input of the welding process, it is independent of the heat input of the welding process for other types of steel.     

Corrosion behaviour of construction materials for high temperature steam electrolysers

Different types of commercially available stainless steels, Ni-based alloys as well as titanium and tantalum were evaluated as possible metallic bipolar plates and construction materials. The corrosion resistance was measured under simulated conditions corresponding to the conditions in high temperature proton exchange membrane (PEM) steam electrolysers. Steady-state voltammetry was used in combination with scanning electron microscopy and energy-dispersive X-ray spectroscopy to evaluate the stability of the mentioned materials. It was found that stainless steels were the least resistant to corrosion under strong anodic polarisation. Among alloys, Ni-based showed the highest corrosion resistance in the simulated PEM electrolyser medium. In particular, Inconel^® 625 was the most promising among the tested corrosion-resistant alloys for the anodic compartment in high temperature steam electrolysis. Tantalum showed outstanding resistance to corrosion in selected media. On the contrary, passivation of titanium was weak, and the highest rate of corrosion among all tested materials was observed for titanium at 120 °C.     

Microstructure and high temperature tensile properties of wide gap brazed cobalt based superalloy X-40

Abstract

Wide gap brazing (WGB) of X-40 cobalt based superalloy was conducted in this study using BNi-9 braze alloy with X-40 and IN738 additive alloys. A groove was machined into X-40 bars with a nominal width of 6·35 mm before filler application. Following brazing at 1200°C for 15 min, the microstructure of the as brazed joints was examined using SEM, EDS and nanoindentation technique. Both WGB joints with X-40 and IN783 additive alloys contained primary matrix phase in addition to a number of boron containing phases which assumed either eutectic or discrete forms. Nanoindention testing revealed that these boron containing phases exhibited hardness values several times higher than the base alloy and matrix phase contributing to the embrittlement of the braze joint. Porosity was also observed in both types of WGB braze joints, the degree of which was greatest in the braze joints with IN738 additive alloy. Tensile testing at 950°C showed that the yield strength of both WGB joints was higher than that of the baseline specimens while the ultimate tensile strength of the WGB joints was lower than that of the baseline X-40. The ductility of the WGB joints was significantly inferior to that of the baseline X-40, particularly for WGB with IN 738 additive alloy.     

Experimental investigation of fatigue behaviour for welded joint with mechanical heterogeneity

Because of material and mechanical heterogeneity in welded joints, experimental studies of low-cycle fatigue (LCF) life and fatigue crack initiation behaviour were performed on weld metal, heat-affected zone (HAZ) and base metal of 16 MnR pressure vessel steel. A fatigue damage test was carried out by introducing a new damage variable. The fatigue damage evolution laws in each zone of a welded joint were obtained. A circular-notched compact tension specimen was designed and the whole process of fatigue damage, crack initiation and crack growth in each zone was investigated. The effect of mechanical heterogeneity in welded joints on the fatigue damage behaviour and fatigue crack initiation life was also investigated. The experimental and theoretical results indicate that the fatigue behaviour is markedly different in three zones of welded joints; there is a trend toward reduction in the fatigue life of HAZ as compared with the base metal. The weld metal does not have the same fatigue behaviour as confirmed by LCF life and crack growth rate. This study leads to the conclusion that fairly accurate estimates of fatigue life for welded joints should include the whole process of fatigue damage evolution and fatigue crack initiation as well as crack growth. Furthermore, a local experimental method should be carried out for weld metal, HAZ and base metal, respectively. Finally, the local fatigue failure criterion based on the weakest chain model has been presented.     

Limit and shakedown analysis of nozzle/cylinder intersections under internal pressure and in-plane moment loading

A simple technique called the elastic compensation method developed previously by Mackenzie & Boyle is used in combination with full three-dimensional finite-element analysis to obtain limit and shakedown interaction diagrams for nozzle/cylinder intersections subject to combined internal pressure and in-plane nozzle moment loading. The results are compared with solutions from the literature and also with detailed elasto-plastic thin shell finite-element analysis. It is found that the simple elastic compensation procedure can provide good estimates of plastic failure mechanisms for complex three-dimensional structures. A detailed discussion of various issues which arose relates to finite-element modelling and the measures taken to improve the results are also documented.     

Natural convection heat transfer from a horizontal isothermal elliptical cylinder with internal heat generation

This work examines the natural convection heat transfer from a horizontal isothermal cylinder of elliptic cross section in a Newtonian fluid with temperature dependent internal heat generation. The governing boundary layer equations are transformed into a non-dimensional form and the resulting nonlinear systems of partial differential equations are solved numerically applying cubic spline collocation method. Results for the local Nusselt number and the local skin-friction coefficient are presented as functions of eccentric angle for various values of heat generation parameters, Prandtl numbers and aspect ratios. Results show that both the heat transfer rate and skin friction of the elliptical cylinder with slender orientation are higher than the elliptical cylinder with blunt orientation. Moreover, an increase in the heat generation parameter for natural convection flow over an isothermal horizontal elliptic cylinder leads to a decrease in the heat transfer rate from the elliptical cylinder and an increase in the skin friction of the elliptical cylinder.     

Damage assessment method of P91 steel welded tube under internal pressure creep based on void growth simulation

Development of creep damage assessment methods for longitudinal welded piping of P91 steel is important and an urgent subject to maintain reliable operation of boilers in ultra super critical thermal power plants. Internal pressure creep tests were conducted on P91 steel longitudinal welded tubes to characterize the evolution of creep damage in a heat-affected zone (HAZ) of the longitudinal welded pipe. Failure occurred at a heat-affected zone without significant macroscopic deformation. It was found that initiation of creep voids had concentrated at mid-thickness region rather than surface. Three-dimensional finite element (FE) creep analysis of the creep tested specimens was conducted to identify stress and creep strain distribution within the specimen during creep. Finite element creep analysis results indicated that triaxial tensile stress yielded at the mid-thickness region of the HAZ. It was suggested that the triaxial stress state caused acceleration of the creep damage evolution in the heat-affected zone resulting in internal failure of the tube specimens. Void growth behavior in the heat-affected zone was well predicted with the previously proposed void growth simulation method by introducing void initiation function to the method. A “limited strain” was defined as rupture criterion and dependency of the maximum stress and multiaxiality on the “limited strain” was derived by the void growth simulation. Creep damage distribution in the HAZ under the internal creep test was calculated by proposed damage assessment method.     

Failure pressure of straight pipe with wall thinning under internal pressure

The failure pressure of pipe with wall thinning was investigated by using three-dimensional elastic–plastic finite element analyses (FEA). With careful modeling of the pipe and flaw geometry in addition to a proper stress–strain relation of the material, FEA could estimate the precise burst pressure obtained by the tests. FEA was conducted by assuming three kinds of materials: line pipe steel, carbon steel, and stainless steel. The failure pressure obtained using line pipe steel was the lowest under the same flaw size condition, when the failure pressure was normalized by the value of unflawed pipe defined using the flow stress. On the other hand, when the failure pressure was normalized by the results of FEA obtained for unflawed pipe under various flaw and pipe configurations, the failure pressures of carbon steel and line pipe steel were almost the same and lower than that of stainless steel. This suggests that the existing assessment criteria developed for line pipe steel can be applied to make a conservative assessment of carbon steel and stainless steel.     

Behaviour of a prestressed reactor pressure vessel for HTR under extremely high core temperature loading

The behaviour of a prestressed concrete reactor pressure vessel of a high temperature reactor at extremely high core temperatures ($\text{> 1000°}\text{C}$) is analysed. Such extreme temperatures may occur in the conditions of a hypothetical accident such as an unrestricted core heat up.The models used in previous experiments have replaced the top head, which actually consists of refuelling penetrations with steel liners, reinforcement and concrete, by a material having homogeneously distributed properties of these components. The difference between this and a realistic, detailed model of the top of a vessel of an HTR-1160 plant is analysed. The temperature fields are calculated taking into account the decomposition of the concrete and, in particular, the release of CO₂ from the limestone aggregates and the automatic spalling of material.The material properties were represented as a function of the temperature from room temperature up to melting point. The effect of the reinforcement net on the spalling rate of the concrete is examined.The calculations using the detailed model establish a later beginning and a much slower rate of decomposition of the top.