Creep fatigue crack development in dissimilar metal welded joints between steels and nickel based alloy

Abstract

Creep and strain controlled cyclic/hold creep fatigue tests have been performed at temperatures in the range of 550–575°C on specimens extracted from dissimilar metal welded (DMW) joints between two classes of steel and a nickel based alloy. The details and results of the tests are described. While crack development in the cyclic/hold creep fatigue test specimens tends to be creep dominated, the microstructural paths followed in the steels in the vicinity of their heat affected zones are not identical to those observed in creep rupture testpieces taken from the same DMW joint. In pure creep tests, cracking may occur adjacent to the fusion line and/or in the fine grain heat affected zone (FGHAZ), with rupture location being dependent on temperature stress and microstructural condition. In contrast, creep dominated creep fatigue cracking typically occurs in the intercritical heat affected zone/FGHAZ or the overtempered parent material on the steel side of such weldments, depending on the composition of the joint.     

Finite element simulation of welding residual stresses in martensitic steel pipes

Abstract

Finite element (FE) simulations of the welding of two high grade steel pipes are described. The first is a P91 steel pipe welded with a similar P91 weld consumable, and the second is a P92 steel pipe welded with dissimilar nickel–chromium based weld consumables. Both welds are multipass circumferential butt welds, having 73 weld beads in the P91 pipe and 36 beads in the P92 pipe. Since the pipes and welds are symmetric around their axes, the FE simulations are axisymmetric, allowing high FE mesh refinement and residual stress prediction accuracy. The FE simulations of the welding of the P91 and P92 pipes comprise thermal and sequentially coupled structural analyses. The thermal analyses model the heat evolution produced by the welding arc, determining the temperature history throughout the FE models. Structural analyses use the computed temperature history as input data to predict the residual stress fields throughout the models. Post-weld heat treatment (PWHT) of both pipes has also been numerically simulated by assuming that the FE models obey the Norton creep law during the hold time period at 760°C. The residual stresses presented here have all been validated by corresponding experimental measurements. Before PWHT, it has been found that, at certain locations in the weld region and heat affected zone (HAZ) in the pipes, tensile hoop and axial residual stresses approach the tensile strength of the material, presenting a high risk of failure. It has also been found that PWHT substantially reduces the magnitude of residual stresses by varying degrees depending on the material.     

Microstructure evolution in HAZ and suppression of Type IV fracture in advanced ferritic power plant steels

The effect of boron and nitrogen on the microstructure evolution in heat affected zone (HAZ) of 9Cr steel during simulated heating and on the Type IV fracture in welded joints has been investigated at 650 °C. Gr.92 exhibits a significant decrease in time to rupture after thermal cycle to a peak temperature near A_C3, while the creep life of Gr.92N, subjected to only normalizing but no tempering, and 9Cr-boron steel is substantially the same as that of the base metals. In Gr.92 after A_C3 thermal cycle, very few precipitates are formed along PAGBs in the fine-grained microstructure. In the P92N and 9Cr-boron steel after A_C3 heat cycle, on the other hand, not only PAGBs but also lath and block boundaries are covered by M₂₃C₆ carbides in the coarse-grained microstructure. It is concluded that the degradation in creep life in Gr.92 after the A_C3 thermal cycle is not caused by grain refinement but that the reduction of boundary and sub-boundary hardening is the most important. Soluble boron is essential for the change in α/γ transformation behavior during heating and also for the suppression of Type IV fracture in welded joints. Newly alloy-designed 9Cr steel with 160 ppm boron and 85 ppm nitrogen exhibits much higher creep rupture strength of base metal than P92 and also no Type IV fracture in welded joints at 650 °C.     

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A_c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.     

Investigation of embrittlement of step cooling heat treatment on pressure vessel steel

Abstract

The effects of thermal aging and step cooling embrittlement on the impact toughness of a reactor pressure vessel steel SA533B quenched and tempered (QT) with and without post-weld heat treatment (PWHT) have been studied. Charpy impact testings were conducted on the aged plates at 350°C for 5000 h to evaluate whether the embrittlement was induced by step cooling heat treatment. The results show that thermal aging increases the ductile–brittle transition temperature in both QT and PWHT states but dramatically decreases the upper shelf energy in QT state and has less effects on the PWHT state. By comparing the correlation between thermal aging embrittlement and step cooling embrittlement for both QT and PWHT states in steel, it is found that the step cooling heat treatment can obviously promote further embrittlement of the base metal in QT state but has little influence on the impact toughness in PWHT and thermal aged state. Further analysis indicates that the step cooling heat treatment cannot promote steel embrittlement at some heat treatment states. Finally, a new method is proposed to evaluate the degree of step cooling embrittlement of the pressure vessel steel.     

New experience in field of application of corrosion resistance protection in flue gas desulphurisation plant absorbers

Abstract

Absorbers form the main part of the flue gas desulphurisation plants for 360 MW units at the Belchatow Power Station in Poland. The plant working cycle and service life are dependent on the grades of the corrosion resistance, protection materials used as an absorber lining in individual zones. The objective of manufacturers activities in this field has been to extend the service life of the flue gas desulphurisation plant. This has been implemented by testing 'wallpapering' technology in the absorber cylindrical part, using plate sheets of Hastelloy nickel alloys, 0·5 to 1·6 mm in diameter. Experiments involving Hastelloy nickel alloys, grades C-22 and C-2000, among others, were used to evaluation the resistance of the base material, 'wallpaper' plate sheet, welded joints and heat affected zones (HAZ) towards corrosion. The corrosion tests were executed for three different conditions of corrosive environment: (a) actual boiler outlet environment: chemical composition of boiler outlet flue gases, with addition of 10 wt-%K₂SO₄; a testing temperature of 70°C; and a testing time of up to 1000 h; (b) actual boiler outlet environment: chemical composition of boiler outlet flue gases, with addition of 6 wt-%K₂SO₄ and 4 wt-%NaCl; a testing temperature of 70°C; and a testing time of up to 1000 h; (c) simulated waste incineration environment, produced during thermal utilisation of wastes with the following chemical composition: N₂–9%O₂–0·08%SO₂–0·2%HCl; testing temperatures of 450 and 600°C; and a testing time was up to 1150 h. For the purpose of comparison, corrosion resistance tests were carried out on boiler steel types T22, E911, HCM12 and TP347H. This paper includes results of the evaluation of welded joints structure and HAZ, joint corrosion resistance characteristics, corrosion product test results, as well as requirements and recommendations with regard to fabrication of welded joints, the preparation degree of lining plate join surfaces, and requirements in field of weld joints.     

The potential for heat pumps in drying and dehumidification systems II: An experimental assessment of the heat pump characteristics of a heat pump dehumidification system using R114

An experimental heat pump dehumidifier is described. Actual coefficients of performance (COP)_A are plotted against the gross temperature lift (T_CO - T_EV) for various bypass ratios and air velocities. Interpolated values of (COP)_A for a specified temperature lift were obtained by fitting each set for various dry bulb temperatures of air leaving the humidifier using a linear equation. These values of (COP)_A are plotted against the linear velocity of the air stream approaching the evaporator at different dry bulb temperatures. The curves show a maximum of (COP)_A at approach velocities in the region of 1·6 ms^?1.     

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.     

Creep fracture behavior of dissimilar weld joints between T92 martensitic and HR3C austenitic steels

The creep fracture of T92/HR3C dissimilar weld joints is investigated. HR3C austenitic steel is welded with T92 martensitic steel to obtain a T92/HR3C weld joint. After welding, creep tests are carried out at 625 °C in the stress range 110–180 MPa. The results indicate that the creep fracture mechanism is dependent on stress. When stresses ≥140 MPa, the fracture location is at the T92 base material and the connection of adjacent dimples results in transcrystalline fracture. For stresses <140 MPa, the fracture location is at the T92 coarse-grained heat affected zone and growth of M₂₃C₆ particles as well as Laves phase (Fe₂(W, Mo)) precipitation on the grain boundaries leads to intergranular fracture.     

Conjugate heat transfer analysis of a heat generating vertical plate

This paper mainly deals with conjugate heat transfer problem pertinent to rectangular fuel element of a nuclear reactor dissipating heat into an upward moving stream of liquid sodium. Introducing boundary layer approximations, the equations governing the flow and thermal fields in the fluid domain are solved simultaneously along with two-dimensional energy equation in the solid domain by satisfying the continuity of temperature and heat flux at the solid–fluid interface. The boundary layer equations are discretized using fully implicit finite difference scheme so as to adopt marching technique solution procedure, while second-order central difference scheme is employed to discretize the energy equation in the solid domain and the resulting system of finite difference equations are solved using Line-by-Line Gauss–Seidel iterative solution procedure. Numerical results are presented for a wide range of parameters such as aspect ratio, A_r, conduction–convection parameter, N_cc, heat generation parameter, Q, and flow Reynolds number, Re. It is concluded that there exist an upper or a lower limiting value of these parameters above or below which the temperature in the fuel element crosses its allowable limit. It is also found that an increase in Re results in considerable increase in overall heat dissipation rate from the fuel element.     

Computational and statistical analysis on the U-tube heat exchanger with different passes configuration: Taguchi method

In the present study, the passive technique of heat transfer in which single pass and double passes are included in a simple U-tube heat exchanger is analyzed. The computational fluid dynamics (CFD)-based parametric analysis is carried out to optimize the parameters affecting the temperature drop and heat transfer achieved from the U-tube heat exchanger. ANSYS Fluentv20 is used for the CFD analysis, and the RNG k-ɛ model and energy equation were considered to define the turbulence and heat transfer phenomena. The Taguchi method is used to formulate the experimental work and analyze the working parameters of the U-tube heat exchanger, such as hot and cold mass flow rate and hoRenew Energyt inlet temperature and cold inlet temperature. For the U-tube heat exchanger, four operating parameters are considered at four different levels in the Taguchi method. The best combination of parameters for achieving a maximum temperature drop is A₄B₁C₂D₃, and it is A₃B₄C₁D₂ in case of heat transfer. A U-tube single-pass heat exchanger is more effective as compared with other U-tube heat exchangers (zero- and double-pass). Experimental results are provided to validate the suitability of the purpose of the approach.     

Heat and fluid flow characteristics of liquid sodium flowing past a nuclear fuel element with non-uniform energy generation

The prime objective of the present study is to analyze numerically the steady state fluid flow and heat transfer characteristics of liquid sodium as a coolant flowing past over a rectangular nuclear fuel element having non-uniform volumetric energy generation. Accordingly, employing stream function-vorticity formulation and using finite difference schemes, the equations governing the flow and thermal fields in the coolant are solved simultaneously with energy equation for the fuel element by satisfying the conditions of continuity of temperature and heat flux at the solid–fluid interface. Keeping Prandtl number Pr = 0.005 for liquid sodium as constant, numerical results are presented and discussed for a wide range of aspect ratio A_r, conduction–convection parameter N_cc, total energy generation parameter Q_t and Reynolds number Re_H. It is concluded that the rate of heat dissipation from the fuel element to the coolant is independent of A_r, N_cc and Q_t, whereas it increases in proportion to the increase in Re_H. It is also found that for a given material of the fuel element, there is an upper limiting value of N_cc and Re_H beyond which decrease in coolant temperature is negligibly small.     

Forced convection heat transfer inside an anisotropic porous channel with oblique principal axes: Effect of viscous dissipation

An analytical study on laminar and fully developed forced convection heat transfer in a parallel-plate horizontal channel filled with an anisotropic permeability porous medium is performed. The principal axis of the anisotropic porous medium is oriented from 0 to 90 degrees. A constant heat flux is applied on the outer wall of the channel. Both clear (Newtonian) fluid and Darcy viscous dissipations are considered in the energy equation. Directional permeability ratio parameter A¹ is defined to combine both the effect of the dimensionless permeability ratio parameter K¹=(K₁/K₂) and orientation angle φ into one parameter. The effects of the parameter A¹, the Darcy number Da and the modified Brinkman number Br¹ on the heat transfer and fluid flow characteristics in the channels are investigated and presented in graphs. The obtained results show that the parameters A¹, Da and Br¹ have strong effects on the dimensionless normalized velocity and temperature profiles as well as on the Nusselt number. It is found that for a particular value of A¹, called as critical value A_cr¹, the external heat applied to the surface of the channel is balanced by the internal heat generation due to viscous dissipation and the bulk mean temperature approaches the wall temperature. Hence, the Nusselt number approaches infinity for the critical values A_cr¹.     

热处理保温时间对焊接接头性能的影响

本文通过对不同热处理保温时间的焊接接头进行拉伸、弯曲和冲击试验,得到不同保温时间对焊接接头性能的影响,可以在生产过程中优化焊接和热处理工艺,在保证得到优质可靠的焊接接头性能的前提下,减少能源消耗,提高生产效率。     

Effects of aspect ratio on mixed convection in a horizontal rectangular duct with heated and cooled side walls

This paper describes the effect of aspect ratio on mixed convection in a horizontal rectangular duct with heated and cooled side walls numerically and experimentally. In the numerical analysis, fluid flow and temperature distributions for Ri=1.61, Pr=6.99, Re=100 and aspect ratio, Ar=0.2–10, were obtained by solving dimensionless governing equations using the SIMPLE procedure. The QUICK scheme was applied to the convective term of these equations. In the experimental analysis, the flow behavior for A_r=0.5–2 was visualized by the dye‐injection method. Numerical results showed that the swirl flow was generated along the flow direction, and its pitch length was influenced by A_r. The pitch length was the shortest when A_r=0.5–1, and this tendency was the same in numerical results and experimental results. The heat transfer behavior was also discussed corresponding to the flow, and the heat transfer ratio was highest at A_r=1 in 0.2 ≤ A_r ≤ 10. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20391     

Numerical modeling of conjugate heat transfer through concrete hollow bricks

The purpose of this study is to determine the thermal conductance of concrete hollow bricks, which is necessary for the evaluation of the energy efficiency of a building. The three varieties of hollow concrete bricks that are often used to build walls in Morocco are the subject of this study. A computational model created using the finite volume method is used to evaluate the conjugate heat transfer through concrete hollow bricks. According to the results, the use of hollow brick type A_h3 reduces the heat flux by approximately 86% compared with type A_h1. It is undeniable that hollow bricks type A_h3 with a thermal conductivity of 1 W/m K can improve the thermal characteristics of building walls.     

Performance of repair welds on aged 2.25Cr–1Mo boiler header welds

In order to clarify the performance of repair welds on power boiler, thick parts such as header and steam piping, an ex-service aged 2.25Cr–1Mo header was repaired using SMAW with postweld heat treatment and the mechanical properties of the repair welded joints were experimentally evaluated.Creep rupture life of the repair welded joint was almost same as that of service-degraded base metal and heat-affected zones. It was proved that the life reduction would not be caused by repair welding. In creep–fatigue tests with strain holding, some type of repair welded joints was fractured at the heat affected zone caused by repair welding. This may be caused by strain concentration at the heat-affected zone under strain holding. Charpy impact toughness of the simulated heat affected zone due to repair welding was much higher than that of service-degraded base metal. It was proved that the toughness would be restored by repair welding.     

Local uniform heat treatment and its effects on the toughness of welded C-Mn pressure vessel steel

The present paper gives the results of a research programme on the influence of a local postweld heat treatment on the toughness of welded joints in a C-Mn pressure vessel steel. The welded test plates, 50 mm thick, were locally heat treated using electrical resistance heating over a width of about 200 mm. The influence of the local heat treatment on the toughness of the welded joints was evaluated using wide plate test specimens. The results indicate that an important improvement in toughness is obtained by the local heat treatment, resulting in toughness levels which are comparable to those obtained by a furnace stress relief heat treatment.     

Fin performance in an oscillating temperature environment

An analytical solution is presented to describe the performance of a straight convecting fin operating in an oscillating temperature environment. The solution involves the amplitude parameter, A, and the frequency parameter, B, in addition to the steady-state convection-conduction parameter, N. The effects of these parameters on the amplitude and phase angle of spatial temperature and base heat flux are discussed. For most practical finned surfaces, i.e. N < 2, the time-average fin efficiency is found to be practically independent of B in the range 0·01–10 but increases as A increases. This is in contrast with the previously reported study of periodic variation of base temperature wherein, for N < 1, the time-average fin efficiency was found to reduce significantly with the increase of parameters A and B in the same range.     

Effect of heat pre-treatment temperature on isolation of hydrogen producing functional consortium from soil

A functional hydrogen producing consortium was isolated from soil by heat pre-treatment technique and hydrogen production at different substrate concentration was evaluated. The forest soil was heat pre-treated at 65, 80, 95, 105 and 120 °C temperature for 1 h. As revealed by PCR-DGGE analysis and hydrogen yield, the hydrogen producing microbial community changed with increase in heat pre-treatment temperatures giving potential hydrogen producing consortium at 95–105 °C soil pre-treatment. The maximum hydrogen production rate, hydrogen yield and cumulative hydrogen with 15–20 g glucose were 1390–1576 mL/L/day, 1.83–1.93 mol H₂/mol glucose, and 2966–3146 mL H₂/L, respectively. The metabolic pathways shifted from ethanol-type to acetate–formate type as soil pre-treatment temperature increased from 65 to 120 °C. The soil heat pre-treatment approach is effective for isolating hydrogen producing natural Clostridium consortium from the soil as enumerations of the functional strains need specific temperature range to florish.