Evaluation,degradation and life assessment of coatings for land based combustion turbines

Abstract

Use of metallic and thermal barrier coatings to protect hot section blades and vanes of combustion turbines for power generation has been common practice for the past three and one decades respectively. Because these coatings must be optimised with respect to both different forms of corrosion and modes of operation (base load versus peak load), their performance may be machine specific. Industrial end users generally do not have detailed knowledge of the failure mechanisms of the coatings and the basis for selecting coatings to suit specific requirements, topics the present review seeks to address. The evolution of protective coatings, coating failure mechanisms and a methodology for selecting machine specific coatings are described. The methodology, which can be used to rank and optimise coating systems and to predict the remaining life of coatings, forms the basis of a computer code known as COATLIFE. The ingredients of this methodology, i.e. degradation modelling and thermomechanical fatigue life prediction, are reviewed.     

Coupled effect of heat input and beam oscillation on mechanical properties of alloy 718 electron beam weldments

Abstract

Autogenous full penetration electron beam welds were made on alloy 718 with and without beam oscillation technique. Weldments were subjected to two types of post-weld heat treatments: direct aging (DA) and solution treatment at 980°C followed by aging (STA). When the welds were prepared using different heat inputs in the welding processes with and without beam oscillation, the influence of beam oscillation could not be studied in isolation but the coupled effect of heat input and beam oscillation was studied. Laves particles were finer in size and lower in amount in unoscillated welds compared with those in beam oscillated welds subjected to DA condition. δ phase needles were observed around Laves particles in the welds subjected to STA condition. The amount of Laves particles was less and that of δ phase was more in unoscillated welds compared with those in beam oscillated welds subjected to STA condition. Unoscillated weldments exhibited longer fatigue lives compared with beam oscillated weldments in both DA and STA conditions owing to less amount of Laves in the former. Weldments in STA condition had longer lives compared with those in DA condition. The role of δ phase needles in fatigue life could not be identified.     

Failure analysis of some petrochemical plant components

Abstract

Failure analysis of five sections of plant (the cooling elbow of a chlorination reactor, electrolyte feed pipelines, tubes from an ethylene cracking furnace, the upper pressure cover of the chlorination reactor and heat exchanger tubing) in a petrochemical factory were investigated. Their microstructures were observed using optical and scanning electronic microscopy with compositional analysis measured using energy dispersive X-ray analysis. Mass loss experiments in the laboratory using selected materials and the working environment were also conducted. The main causes of the corrosion of the parts are discussed. Based on the results of the failure analysis, methods to improve the life of the parts are suggested. For example, by selection of suitable substitute alloys the working life of the parts may be increased.     

Influences of post-weld heat treatment on fatigue crack growth behaviour of strength mismatched HSLA steel welds

Abstract

Welding of high strength low alloy steels (HSLA) involves usage of low, even and high strength filler materials (electrodes) compared with the parent material depending on the application of the welded structures and the availability of the filler material. In the present investigation, the influences of post-weld heat treatment (PWHT) on fatigue crack growth behaviour of under matched (UM), equal matched (EM) and over matched (OM) weld metals has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. The Shielded Metal Arc Welding (SMAW) process has been used to fabricate the single 'V' butt joints. Centre Cracked Tension (CCT) specimens have been used to evaluate the fatigue crack growth behaviour of the welded joints. Fatigue crack growth experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading (R = 0). From this investigation, it has been found that the fatigue performance of over matched joints is superior compared to under matched and equal matched joints. Moreover, PWHT reduced the magnitude of the tensile residual stress field in the weld region and subsequently enhanced the fatigue performance of the joints irrespective of weld metal strength mismatch.     

Isothermal and thermomechanical fatigue behaviour of a high temperature titanium alloy

Abstract

Isothermal and thermomechanical fatigue (TMF) behaviour (including cyclic stress response and number of cycles to failure) of a Ti – 5.6Al – 4.8Sn – 2.0Zr – 1.0Mo – 0.32Si – 0.8Nd (wt-%) hightemperature titanium alloy was examined. The purpose of the present investigation was to understand the effect of temperature fluctuation on the cyclic behaviour and fatigue life of this alloy and to test the suitability of lifetime prediction based on isothermal laboratory data. The results indicated that both the level of peak stress and fatigue life were decreasing with increasing test temperature from 400°C to 650°C in isothermal fatigue (IF) tests. In TMF tests run between 400°C and 600°C, the peak stresses corresponding to 600°C coincide well with that found in IF tests run at 600°C, while a slight increase in cyclic hardening was found for peak stress corresponding to 400°C compared to that found in a 400°C/IF test. This increase in cyclic hardening became more pronounced when the maximum temperature increased to 650°C. Fatigue life in 'out of phase' (OP) condition was found to be shorter than under an equivalent 'in phase' (IP) condition, and this gap increased with decreasing mechanical strain amplitude. The results indicate that lifetime prediction based on isothermal laboratory data may lead to non-conservative results if thermal fluctuations are present in components made of the present alloy.     

Improved approach for predicting weld creep strength factors of ferritic steels

Abstract

The cross-weld (CW) creep strength of ferritic steels is typically lower than that for parent metal (PM), and in the past the ratio of CW to PM creep strength (weld strength factor – WSF) was assumed to be limited to ～80%. For newer Cr steels WSF can be significantly lower for a typical design life of 100 000 h or more. The possibility of low WSF is also accommodated in the current design codes such as EN 13445, but no suggested WSF values are given for guidance. Assuming a too high WSF for such welds obviously results in an unsafe (too long) predicted creep life. Unfortunately, as a further complication the WSF of the newer Cr steels can decrease when the operating temperatures are increased for improved efficiency of future power plants. It is hence important that reliable and sufficiently high values of WSF can be guaranteed. However, there is often much less extensive data on the creep strength of welds than on parent steel, and also the extrapolation to long term values of WSF can add more relative uncertainty than what is expected in extrapolating the long term creep strength of parent steel. Here an improved approach is proposed to predict WSF using the Wilshire creep model to obtain the relationship between the CW creep strength and the corresponding parent material (PM) strength. The Wilshire model directly provides the WSF value for each CW data point, when the expected normalised stress is based on the CW time to rupture at stress and temperature. The corresponding master curve parameters are those for PM, when the PM hot tensile strength is also known. The WSF data points for each CW test can then be fitted for temperature and stress dependence. This approach avoids fitting distortion in WSF, unlike the traditional assessment where a master curve is first obtained for the CW creep strength. As an example, WSF of welded P91 steel at 100 000 h is here predicted in the temperature range of 550–650°C.     

自动机床刀具的自动保护

介绍用单片机与逻辑控制电路自动测量和控制刀具的切削温度和切削力，从而保证刀具在切削加工中长久正常地工作，以达到提高刀具寿命、减少刀具消耗的目的。     

Management Issues of a Mobile Agent-Based Service Environment

This paper introduces the concepts of a servicemanagement system for mobile agent-based services. Incontrast to several other approaches in whichagent-based applications are used to handletraditional network or service managementaspects, this paper describes a way to manage thedistributed agent environment itself. The proposedapproach was designed and developed by using OSI SystemsManagement concepts as a starting point, which are modified andadjusted in order to fulfill the new requirementsassociated with the mobile agent paradigm. However, thispaper does not cover integration or co-existence of the introduced agent management system (AMS)and traditional OSI management systems. Instead, theentire AMS environment is based on mobile agenttechnology. At present, this approach is validated inthe context of the European research project MobileAgent Environments in Intelligent Networks(MARINE).     

A new epoxy/episulfide resin system for electronic applications — 2: pot life and prepreg storage life evaluation

A new epoxy/episulfide thermosetting resin was evaluated, using a dicyandiamide (DICY) curing agent. The system was found to exhibit a longer pot life and prepreg storage life than the standard epoxy system. The properties are attributable to the change in reaction mechanism caused by the addition of episulfide. The reaction mechanisms were studied using a mono-functional model compound system; the extent of episulfide-amine reaction is relatively high in the epoxy/episulfide system relative to epoxy-amine. A tert-amine salt is probably formed which does not act as a catalyst for the epoxy ring opening, resulting in a greater pot life.     

Study on Design Techniques of a Long Life Hot Forging Die with Multi-Materials

A new design technique for the long life hot forging die has been proposed. By finite element analysis, the reason for the failure of hot forging die was analyzed and it was concluded that thermal stress is the main reason for the failure of hot forging die. Based on this conclusion, the whole hot forging die was divided into the substrate part and the heat-resistant part according to the thermal stress distribution. Moreover, the heat-resistant part was further subdivided into more zones and the material of each zone was reasonably selected to ensure that the hot forging die can work in an elastic state. When compared with the existing techniques, this design can greatly increase the service life because the use of multi-materials can alleviate the thermal stress in hot forging die.